U.S. patent application number 12/905230 was filed with the patent office on 2011-02-17 for method and apparatus for providing positive airway pressure to a patient.
This patent application is currently assigned to RIC INVESTMENTS, LLC. Invention is credited to Mark C. Estes, John Fiore, Jeff Kepler, Douglas M. Mechlenburg, Heather Ressler.
Application Number | 20110036352 12/905230 |
Document ID | / |
Family ID | 32392433 |
Filed Date | 2011-02-17 |
United States Patent
Application |
20110036352 |
Kind Code |
A1 |
Estes; Mark C. ; et
al. |
February 17, 2011 |
METHOD AND APPARATUS FOR PROVIDING POSITIVE AIRWAY PRESSURE TO A
PATIENT
Abstract
A system including methods and apparatus for treatment of a
medical disorder such as obstructive sleep apnea or congestive
heart failure. The system includes a gas flow generator and a
patient interface. The system delivers a flow of gas to the patient
using the gas flow generator and the patient interface. The system
determines that the patient is experiencing an apnea and, in
response, performs a pressure change maneuver. This maneuver
includes increasing a pressure of the flow of gas for a period of
time, and decreasing the pressure of the flow of gas after the
pressure increase. The system then monitors the change in flow rate
for the flow of gas resulting from the pressure change maneuver, if
any, and determines whether the airway of the patient is open or
closed based on the monitored change in the flow rate.
Inventors: |
Estes; Mark C.; (SYLMAR,
CA) ; Fiore; John; (MONROEVILLE, PA) ;
Mechlenburg; Douglas M.; (MURRYSVILLE, PA) ; Ressler;
Heather; (NEW ALEXANDRIA, PA) ; Kepler; Jeff;
(EXPORT, PA) |
Correspondence
Address: |
PHILIPS INTELLECTUAL PROPERTY & STANDARDS
P.O. BOX 3001
BRIARCLIFF MANOR
NY
10510
US
|
Assignee: |
RIC INVESTMENTS, LLC
MURRYSVILLE
PA
|
Family ID: |
32392433 |
Appl. No.: |
12/905230 |
Filed: |
October 15, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11926867 |
Oct 29, 2007 |
7810496 |
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12905230 |
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11182387 |
Jul 15, 2005 |
7296573 |
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11926867 |
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10304590 |
Nov 26, 2002 |
6932084 |
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11182387 |
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09610733 |
Jul 6, 2000 |
6609517 |
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10304590 |
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09041195 |
Mar 12, 1998 |
6105575 |
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09610733 |
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08679898 |
Jul 15, 1996 |
5794615 |
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09041195 |
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08253496 |
Jun 3, 1994 |
5535738 |
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08679898 |
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Current U.S.
Class: |
128/204.23 ;
128/205.25 |
Current CPC
Class: |
A61M 2016/0027 20130101;
A61M 2016/0036 20130101; A61M 16/024 20170801; A61M 16/0051
20130101; A61M 16/205 20140204; A61M 16/204 20140204; A61M
2016/0039 20130101; A61M 16/0066 20130101; A61M 16/0069 20140204;
A61M 2016/0021 20130101 |
Class at
Publication: |
128/204.23 ;
128/205.25 |
International
Class: |
A61M 16/00 20060101
A61M016/00; A61M 16/06 20060101 A61M016/06 |
Claims
1. A method of delivering pressurized gas to an airway of a
patient, comprising the steps of: providing a gas flow generator
and a patient interface adapted to couple the gas flow generator to
an airway of a patient; delivering a flow of gas to such a patient
using the gas flow generator and the patient interface; determining
that the patient is experiencing an apnea; performing a pressure
change maneuver responsive to determining that the patient is
experiencing the apnea, wherein the pressure change maneuver
includes increasing a pressure of the flow of gas for a period of
time, and decreasing the pressure of the flow of gas after the
pressure increase; monitoring a change in flow rate for the flow of
gas resulting from the pressure change maneuver; and determining
whether the airway of the patient is open or closed based on the
monitored change in the flow rate.
2. The method of claim 1, further comprising increasing the
pressure of the flow of gas delivered to the patient in response to
determining that the airway of the patient is closed.
3. The method of claim 1, wherein the step of delivering the flow
of gas to the patient includes generating a flow of gas using a
blower, and wherein the step of increasing the pressure of the flow
of gas includes changing an operating speed of the blower.
4. The method of claim 1, wherein the step of delivering a flow of
gas to the patient includes generating a flow of gas using an
blower, and wherein the step of increasing the pressure of the flow
of gas comprises: (a) controlling a pressure controller valve
provided downstream of the blower, (b) changing an operating speed
of such a blower, or (c) controlling the valve and changing an
operating speed of the blower.
5. The method of claim 1, wherein the step of delivering the flow
of gas to the patient comprises: differentiating between an
expiratory phase and an inspiratory phase of a respiratory cycle of
the patient; establishing a reduced pressure profile for a pressure
of the flow of gas provided to the patient, wherein the reduced
pressure profile has a contour that is predetermined prior to
initiation of the expiratory phase, and wherein the contour defines
a changing pressure over a period of time; and controlling the gas
flow delivered to the patient so that (1) the pressure of the flow
of gas is provided at a first pressure level during at least a
portion of the inspiratory phase, and (2) the pressure of the flow
of gas is provided according to the reduced pressure profile during
at least a portion of the expiratory phase of the respiratory
cycle.
6. The method of claim 1, further comprising selecting a first
gain, and wherein the step of delivering the flow of gas to the
patient comprises: monitoring a characteristic associated with the
flow of gas; and controlling the flow of gas such that the flow of
gas is delivered to the patient at (1) a therapy pressure during at
least a portion of an inspiratory portion of a respiratory cycle,
wherein the therapy pressure is a pressure sufficient to counter
airway collapsing forces, and (2) based on the first gain and the
monitored characteristic during at least a portion of an expiratory
phase of a respiratory cycle
7. An apparatus adapted to deliver a flow of gas to an airway of a
patient, the apparatus comprising: (a) a gas flow generator adapted
to generate a flow of gas; (b) a conduit adapted to carry the flow
of gas generated by the gas flow generator to the airway of the
patient; (c) a sensor coupled to the gas flow generator or the
conduit and adapted to monitor a characteristic associated with the
flow of gas; and (d) a processor adapted to: (1) determine that the
patient is experiencing an apnea based on an output of the sensor,
(2) control the gas flow generator so as to increase a pressure of
the flow of gas for a period of time responsive to determining that
the patient is experiencing the apnea, and to decrease the pressure
of the flow of gas after the pressure increase, (3) monitor a
change in flow rate for the flow of gas based on the pressure
increase, and (4) determine whether the airway of the patient is
open or closed based on the monitored change in flow rate.
8. The apparatus of claim 7, wherein the processor is further
adapted to control the gas flow generator so as to increase the
pressure of the flow of gas delivered to the patient in response to
determining that the airway of the patient is closed.
9. The apparatus of claim 7, wherein the gas flow generator is a
blower, and wherein the increases in the pressure of the flow of
gas includes changing an operating speed of the blower.
10. The apparatus of claim 7, wherein the gas flow generator is a
blower, and wherein the increase in the pressure of the flow of gas
comprises: (a) controlling a pressure controller valve provided
downstream of the blower, (b) changing an operating speed of such a
blower, or (c) controlling the valve and changing an operating
speed of the blower.
11. The apparatus of claim 7, wherein the processor is adapted to:
differentiate between an expiratory phase and an inspiratory phase
of a respiratory cycle of the patient; establish a reduced pressure
profile for a pressure of the flow of gas provided to the patient,
wherein the reduced pressure profile has a contour that is
predetermined prior to initiation of the expiratory phase, and
wherein the contour defines a changing pressure over a period of
time; and control the gas flow delivered to the patient so that (1)
the pressure of the flow of gas is provided at a first pressure
level during at least a portion of the inspiratory phase, and (2)
the pressure of the flow of gas is provided according to the
reduced pressure profile during at least a portion of the
expiratory phase of the respiratory cycle.
12. The apparatus of claim 7, further comprising an input device
adapted to select a first gain, and wherein the processor is
adapted to: monitor a characteristic associated with the flow of
gas based on an output of the sensor; and control the gas flow
generator so that the flow of gas is delivered to the patient at
(1) a therapy pressure during at least a portion of an inspiratory
portion of a respiratory cycle, wherein the therapy pressure is a
pressure sufficient to counter airway collapsing forces, and (2)
based on the first gain and the output of the sensor during at
least a portion of an expiratory phase of a respiratory cycle.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is continuation and claims priority under
35 U.S.C. .sctn.120 from U.S. patent application Ser. No.
11/926,867, filed Oct. 29, 2007, which claims priority under 35
U.S.C. .sctn.120 from U.S. patent application Ser. No. 11/182,387,
filed Jul. 15, 2005, now U.S. Pat. No. 7,296,573, which is a
continuation and claims priority under 35 U.S.C. .sctn.120 from
U.S. patent application Ser. No. 10/304,590, filed Nov. 26, 2002,
now U.S. Pat. No. 6,932,084, which is a continuation-in-part and
claims priority under 35 U.S.C. .sctn.120 from of U.S. patent
application Ser. No. 09/610,733 filed Jul. 6, 2000, now U.S. Pat.
No. 6,609,517, which is a continuation and claims priority under 35
U.S.C. .sctn.120 from U.S. patent application Ser. No. 09/041,195
filed Mar. 12, 1998, now U.S. Pat. No. 6,105,575, which is a
continuation-in-part and claims priority under 35 U.S.C. .sctn.120
from U.S. patent application Ser. No. 08/679,898 filed Jul. 15,
1996, now U.S. Pat. No. 5,749,615, which is a continuation-in-part
and claims priority under 35 U.S.C. .sctn.120 from U.S. patent
application Ser. No. 08/253,496 filed Jun. 3, 1994, now U.S. Pat.
No. 5,535,738.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates generally to methods and
apparatus for treating breathing and/or cardiac disorders and, more
particularly, to methods and apparatus for providing a pressure to
an airway of a patient during at least a portion of the breathing
cycle to treat obstructive sleep apnea syndrome, chronic
obstructive pulmonary disease, congestive heart failure, and other
respiratory and/or breathing disorders.
[0004] 2. Description of the Related Art
[0005] During obstructive sleep apnea syndrome (OSAS), the airway
is prone to narrowing and/or collapse while the patient sleeps.
Continuous positive airway pressure (CPAP) therapy seeks to avoid
this narrowing by supplying pressure to splint the airway open.
With CPAP, this splinting pressure is constant and is optimized
during a sleep study to be sufficient in magnitude to prevent
narrowing of the airway. Providing a constant splinting pressure,
i.e., CPAP, is a simple solution to the problem posed by the
collapsing airway. However, this approach exposes the patient to
pressures that are higher than the pressures needed to support the
airway for most of the breathing cycle.
[0006] During inspiration, the pressure created within the lungs is
lower than the pressure at the nose. This pressure difference
drives the flow of air into the lungs. This pressure difference
creates a pressure gradient in the airway connecting the lungs with
the nose. That is to say, the nose is typically at ambient pressure
while the lungs and airway of the patient are at sub-ambient or
negative pressures. This negative pressure acts upon the airway and
contributes to its collapse. CPAP levels are typically set to raise
the pressure level in the entire respiratory system to the level
required to both eliminate the sub-ambient pressures generated by
inspiration and overcome any mechanical collapsing forces that
result from the structure of the airway tissues, muscle tone, and
body position. The inspiratory pressures, i.e., inspiratory
positive airway pressure or "IPAP," in bi-level positive airway
pressure systems are set in a similar manner.
[0007] During exhalation, a positive pressure gradient exists
between the interior of the lungs and the exterior of the body.
This positive pressure gradient helps to support the airway during
exhalation. At the end of exhalation, the pressure gradient is
essentially zero; flow is likewise zero and the airway is
unaffected by respiratory efforts. Any collapse of the airway at
the end of exhalation is purely a function of the structure of the
airway tissues, muscle tone, and body position. Bi-level devices
seek to supply the expiratory pressure required to support the
airway at the end of exhalation.
[0008] It should be noted that over the course of a breathing
cycle, the pressure gradients between the lungs and the exterior of
the body are not constant. The inspiratory pressure gradient falls
from zero at the start of inspiration to a peak negative value and
then rises back to zero at the end of inspiration. The expiratory
pressure gradient rises from zero at the start of exhalation to a
peak value and then falls back to zero as exhalation ends. Because
the pressure gradient varies over the breathing cycle, the pressure
necessary to overcome airway collapse should ideally vary over the
breathing cycle.
[0009] Traditional CPAP therapy ignores these variations in
pressure requirements and provides therapy at one pressure level.
Conventional CPAP is rather crude and offers far from optimal
therapy since the CPAP pressure is based solely on a worst-case
treatment parameter, i.e., the peak pressure requirements during
inspiration.
[0010] Representing an advancement over conventional CPAP, bi-level
positive airway pressure (bi-level PAP) therapies seek to take
advantage of the different pressure requirements to lower the
pressure during exhalation. Nevertheless, bi-level therapies also
fail to afford optimal treatment because the inspiratory positive
airway pressure (IPAP) of bi-level PAP is again based on the
patient's peak needs encountered during inspiration and remains
constant over the entire inspiratory phase respiration. Also,
during bi-level treatment, the expiratory position airway pressure
(EPAP) remains constant and is related solely to the support needs
at the end of exhalation.
[0011] In addition to OSAS, positive airway pressure therapy, such
as bi-level PAP therapy, has been applied in the treatment of other
breathing disorders, such as chronic obstructive pulmonary disorder
(COPD). One of the problems with this mode of treatment, however,
is that the patient has difficulty stopping inspiratory flow. This
phenomenon arises due to the disparity between applied IPAP and the
pressure needed to overcome the patient's respiratory resistance at
the end of inspiration. As the former pressure typically exceeds
the latter, the "surplus" IPAP at the end of inspiration leads to
uncomfortable and potentially harmful hyperinflation of the patient
lungs.
[0012] Conversely, in order to begin inspiratory flow, a COPD
patient must reduce the pressure inside his lungs to a pressure
that is less than the ambient pressure at the inlet of his
respiratory system. Due to the condition commonly known as
"Auto-PEEP," the pressure in the patient's lungs is typically above
ambient pressure at the end of exhalation. The patient's breathing
muscles thus must perform additional work to expand the lungs and
thereby reduce lung pressure below ambient before flow into the
lungs can occur. Auto-PEEP is typically treated with a form of
resistive counter pressure known as PEEP (positive end expiratory
pressure). PEEP is set at a level just below the patient's
Auto-PEEP level, thereby reducing the amount of breathing work
required to initiate inspiratory flow.
[0013] With conventional treatments, such as pressure support, CPAP
or bi-level therapy, PEEP is achieved by applying the same pressure
over the entire phase of expiration, e.g., the EPAP phase of
bi-level PAP therapy. It should be noted that EPAP is not
synonymous with PEEP. EPAP indicates a constant pressure delivered
to the patient throughout exhalation, while PEEP indicates positive
end expiratory pressure. By definition, the PEEP pressure is only
required at the end of exhalation. As such, the administration of
EPAP throughout the expiratory cycle to assure that satisfactory
PEEP is maintained undesirably contributes to the breathing work
that a patient must perform during exhalation.
[0014] In addition to CPAP and bi-level PAP, other systems have
been proposed for clinical research and/or therapeutic application,
including treatment of OSAS, COPD and other breathing disorders,
that offer an assortment of methods and apparatus by means of which
a subject's respiratory efforts may be induced, superseded,
assisted and/or resisted. Some of these systems perform their
prescribed functions responsive to one or more parameters
associated with a subject's respiratory activity including, but not
limited to, inspiratory and/or expiratory flow, inspiratory and/or
expiratory pressure, tidal volume and symptoms indicative of airway
obstruction, e.g., snoring sounds. Some achieve their objectives
transthoracically while others deliver air at positive or negative
pressure directly to the subject's airway.
[0015] An early example of such a system, commonly referred to as
an "iron lung," is disclosed in a publication entitled "Mechanical
Assistance to Respiration in Emphysema, Results with a
Patient-Controlled Servorespirator," authored by James R. Harries,
M.D. and John M. Tyler, M.D., published in the American Journal of
Medicine, Vol. 36, pp. 68-78, January 1964. The iron lung proposed
in that publication is a respirator designed to apply and remove
transthoracic pressure to and from the exterior surface of the body
of a subject who sits in a large pressurizable chamber in order to
assist the patient's respiratory efforts (i.e., the iron lung
applies negative pressure during inspiration and either ambient or
positive pressure during expiration). Sophisticated for its day,
the apparatus continually controlled the internal chamber pressure
in response to the patient's spontaneous respiration, specifically
in response to detected respiratory flow or volume. Indeed, a
signal obtained from a strain gauge pneumograph fastened around the
patient's chest was electrically separated into three components:
one proportional to volume, another to inspiratory flow and a third
to expiratory flow. Each component was assigned a separate gain
control. The component signals are then recombined to control the
pressure in the chamber by means of an electrically driven variable
valve situated between a blower and the chamber.
[0016] Although effective for their intended purposes, this and
other iron lung devices have generally fallen into disfavor because
of their bulk, inconvenience, cost and limited application. That is
to say, because of their size and cost such equipment is purchased
and maintained essentially exclusively by medical facilities such
as hospitals and clinics. Further, iron lungs do not lend
themselves to treatment of OSAS and related disorders where comfort
and unobtrusiveness are critical for patient compliance and
treatment efficacy. This is because negative pressure applied
during inspiration compounds the factors that operate to collapse
the airway during an inspiratory phase.
[0017] An essay entitled, "An Apparatus for Altering the Mechanical
Load of the Respiratory System," authored by M. Younes, D. Bilan,
D. Jung and H. Krokes, and published in 1987 by the American
Physiological Society, pp. 2491-2499, discloses a system for
loading and unloading of a subject's respiratory efforts to effect
various respiratory responses. The system may load or unload during
inspiration, expiration, or both, to assist or resist a subject's
spontaneous respiratory activity. The system may apply a continuous
positive or negative pressure directly to the subject's airway and
loading or unloading occurs via a command signal generated by
detected respiratory flow, volume, applied voltage, an external
function, or other source.
[0018] A drawback to this system, however, is that a single
resistive gain is chosen for resistive loading or unloading. This
single gain is applied to a "half-wave" of the respiratory cycle
(either inspiration or expiration) or the "full-wave" thereof (both
inspiration and expiration). In other words, under full-wave
respiratory loading or unloading, a single chosen gain value is
employed during both inspiration and expiration. Thus, a gain that
may produce favorable results in regard to reducing breathing work
during inspiration, for example, may cause less than desirable or
even detrimental consequences during expiration. The converse is
true for a gain selected specifically for optimizing expiratory
work reduction.
[0019] In addition, the Younes et al. system operates as a closed,
leak-proof system. Hence, to predict its ability to function in an
open, leak-tolerant system would be problematic. As such, whether
it may be adapted to OSAS treatment, which invariably involves some
degree of known and unavoidable unknown system leakage, is
suspect.
[0020] U.S. Pat. No. 5,107,830 to Younes essentially reiterates all
of the "breathing assist" (unloading) disclosure that is covered in
the Younes, et al. American Physiological Society publication
discussed above. In the system disclosed in U.S. Pat. No.
5,107,830, however, the adjustable pressure gain is only realized
during inspiration because pressure output is set to zero during
exhalation. Additionally, output pressure is calculated as a
function of both detected patient inspiratory flow and volume.
Furthermore, the system is applicable to COPD but not OSAS
therapy.
[0021] An article entitled "A Device to Provide
Respiratory-Mechanical Unloading," authored by Chi-sang Poon and
Susan A. Ward and published in March 1987 in IEEE Transactions on
Biomedical Engineering, Vol. BME-33, No. 3, pp. 361-365, is
directed to an apparatus which functions somewhat similar to one
mode of operation described in both Younes disclosures. That is,
the Poon, et al. device may operate to unload a subject's
breathing, but only during inspiration. Poon, et al. provide their
inspiratory assistance by establishing a positive mouth pressure
throughout inspiration in a constant proportion to instantaneous
flow. The constant proportion is achieved by (1) selecting a
desired gain for a detected positive mouth pressure signal, (2)
calculating the ratio of the gain-modified mouth pressure signal
over a detected signal reflecting instantaneous flow, (3) comparing
the calculated ratio to a selected reference ratio to generate a
valve motor control signal, and (4) using the valve motor control
signal to operate a motor that drives a servo valve to control the
positive pressure applied to the subject's airway. Thus, the
apparatus output pressure is determined as a function of both
detected pressure and flow. Further, the pressure must be output at
a value sufficient to maintain a constant ratio of pressure to
flow.
[0022] A publication entitled "Servo Respirator Constructed from a
Positive-Pressure Ventilator," by John E. Remmers and Henry
Gautier, which was published in August, 1976 in the Journal of
Applied Physiology, Vol. 41, No. 2, pp. 252-255, describes a
modified ventilator that may function as a "demand" respirator
generating a transthoracic pressure proportional to phrenic
efferent respiratory discharge. Phrenic efferent respiratory
discharge is an indication of the outgoing brain signal to the
phrenic nerve, which controls diaphragm function. A phrenic
efferent respiratory discharge signal causes the diaphragm to
contract whereby the subject exerts an inspiratory effort. The
phrenic efferent respiratory discharge serves as the apparatus
command signal and is processed to produce a moving time average
(MTA) and the subject's tracheal pressure serves as a negative
feedback signal. Like the Poon et al. device, the Remmers et al.
apparatus provides respiratory assistance only during
inspiration.
[0023] An apparatus for automatically regulating the flow and
pressure output of a respirator is disclosed in U.S. Pat. No.
3,961,627 to Ernst et al. Like the aforementioned Poon et al.
device, however, the Ernst et al. apparatus relies upon an unduly
complicated scheme dependent upon detected respiratory pressure and
flow in calculating delivered output flow and pressure. More
particularly, Ernst et al. propose regulating the delivered flow
and pressure of a respiration gas in a respirator during the
respiration cycle in which the actual flow and pressure of the
respiration gas are measured via a measuring device arranged
proximate a patient interface. The measured values are converted
into electrical signals and the flow and pressure of the
respiration gas are controlled during the inspiration and
expiration portions of the respiration cycle via a valve arranged
between a respiration gas source and the measuring device. The
method for regulating the flow and pressure output comprises (1)
measuring the actual flow of respiration gas proximate the patient,
(2) measuring the actual pressure of respiration gas proximate the
patient, (3) calculating nominal values of flow and pressure from
preselected fixed values and the actual values, (4) comparing the
actual values measured for the flow and pressure with the nominal
values, and (5) obtaining from the comparison a control signal for
modulating the valve and thereby regulating the flow and pressure
of the respiration gas.
[0024] Additionally, apart from its utilization of two detected
respiratory parameters (flow and pressure) and the complex manner
in which these and other variables are reiteratively processed to
produce apparatus flow and pressure output, the Ernst et al.
system, although capable of delivering a base pressure equivalent
to a patient's required end expiratory pressure, is nevertheless
unable to deliver any pressure less than the base pressure.
Consequently, the Ernst et al. apparatus requires the patient to
perform more breathing work than is necessary to satisfy his
respiratory needs, especially in the expiratory phase of a
respiration cycle, thereby deleteriously affecting the patient's
comfort and likelihood of continued compliance with the
treatment.
[0025] In addition to the treatment of breathing disorders,
positive airway pressure therapy has been applied to the treatment
of congestive heart failure (CHF). In using CPAP on CHF, the effect
of the CPAP is to raise the pressure in the chest cavity
surrounding the heart. This has the impact of reducing the amount
of pressure the heart has to pump against to move blood into the
body. By reducing the pressure the heart works against, the work
required of the heart is reduced. This allows the sick heart to
rest and potentially to get better.
[0026] The pressure in the chest cavity is also impacted by
respiration effort. With inspiration, the pressure in the chest is
reduced (negative relative to resting pressure) due to inspiratory
effort. This forces the heart to pump harder to move blood into the
body. With expiration, the pressure in the chest is slightly
increased (positive relative to resting pressure) due to the
elastic properties of the chest. This allows the heart to decrease
its efforts to pump blood. While conventional CPAP can help the
heart rest, it has negative aspects for the patient such as
increased work of exhalation and discomfort from the pressure.
SUMMARY OF THE INVENTION
[0027] It is an object of the present invention to provide an
uncomplicated system operable to deliver pressurized air to the
airway of a patient and readily adaptable to the treatment of OSAS,
COPD and other respiratory and/or pulmonary disorders that does not
suffer from the disadvantages of conventional pressure application
techniques. This object is achieved by providing an apparatus for
delivering pressurized breathing gas to an airway of a patient. The
apparatus, which is referred to below as a "proportional positive
airway pressure" or "PPAP" apparatus, includes a gas flow
generator, a patient interface that couples the gas flow generator
to the patient's airway, a sensor that detects a fluid
characteristic associated with a flow of gas within the patient
interface, a pressure controller that regulates the pressure of
breathing gas provided to the patient, and a control unit that
controls the pressure controller.
[0028] In an exemplary embodiment, the control unit controls the
pressure controller so that the breathing gas is delivered using
the gas flow generator and the patient interface. The system
determines that the patient is experiencing an apnea and, in
response, performs a pressure change maneuver. This maneuver
includes increasing a pressure of the flow of gas for a period of
time, and decreasing the pressure of the flow of gas after the
pressure increase. The system then monitors the change in flow rate
for the flow of gas resulting from the pressure change maneuver, if
any, and determines whether the airway of the patient is open or
closed based on the monitored change in the flow rate.
[0029] These and other objects, features and characteristics of the
present invention, as well as the methods of operation and
functions of the related elements of structure and the combination
of parts and economies of manufacture, will become more apparent
upon consideration of the following description and the appended
claims with reference to the accompanying drawings, all of which
form a part of this specification, wherein like reference numerals
designate corresponding parts in the various figures. It is to be
expressly understood, however, that the drawings are for the
purpose of illustration and description only and are not intended
as a definition of the limits of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] FIG. 1 is a functional block diagram of an apparatus
according to the instant invention;
[0031] FIG. 2 is a functional block diagram of a further embodiment
of an apparatus according to the instant invention;
[0032] FIGS. 3A and 3B are flow and pressure diagrams,
respectively, graphically representing the general manner in which
an apparatus according to the instant invention outputs pressurized
breathing gas in a proportional relation to the patient flow in
both the inspiratory and expiratory phases of a single respiratory
cycle;
[0033] FIGS. 4A and 4B are flow and pressure diagrams,
respectively, similar to FIGS. 3A and 3B, exemplifying a number of
apparatus output pressure curves that are achieved through
selective adjustment of inspiratory and expiratory gain setting
controls of the proportional positive airway pressure circuitry of
the instant invention;
[0034] FIGS. 5A and 5B are flow and pressure diagrams,
respectively, similar to FIGS. 3A and 3B, contrasting a pressure
output curve typical of an apparatus according to the instant
invention with pressure output curves of a conventional respiratory
assistance apparatus;
[0035] FIGS. 6A and 6B are flow and pressure diagrams,
respectively, similar to FIGS. 3A and 3B, depicting alternative
pressure profiles that are employed at the beginning of an
inspiratory phase of respiration to facilitate the onset of
inspiration;
[0036] FIGS. 7A and 7B are flow and pressure diagrams,
respectively, similar to FIGS. 3A and 3B, illustrating a resultant
apparatus pressure output curve according to a further embodiment
of the present invention;
[0037] FIGS. 8A and 8B are flow and pressure diagrams,
respectively, similar to FIGS. 3A and 3B, showing a resultant
apparatus pressure output curve achieved by combing a conventional
bi-level positive airway pressure therapy with the proportional
positive airway pressure therapy according to the instant
invention;
[0038] FIGS. 9A and 9B are flow and pressure diagrams,
respectively, similar to FIGS. 3A and 3B, reflecting a further
resultant apparatus pressure output curve achieved by combining a
conventional bi-level positive airway pressure therapy with
proportional positive airway pressure therapy according to the
instant invention;
[0039] FIG. 10 is a functional block diagram of a further
embodiment of an apparatus according to the instant invention;
[0040] FIGS. 11A and 11B are flow and pressure diagrams,
respectively, similar to FIGS. 7A and 7B, illustrating a resultant
apparatus pressure output curve according to a further embodiment
of the present invention that utilizes a simplified pressure
profile generating technique;
[0041] FIG. 12 is a pressure diagram illustrating the occurrence of
oscillations in the pressure provided to the patient during
exhalation;
[0042] FIG. 13 is a pressure diagram illustrating a first technique
for reducing the oscillations illustrated in FIG. 12;
[0043] FIG. 14 is a pressure diagram illustrating a second
technique for reducing the oscillations illustrated in FIG. 12;
and
[0044] FIG. 15 is a pressure and flow diagram illustrating a
further technique for handling pressure oscillations during the
expiratory phase of the breathing cycle.
DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS
[0045] There is generally indicated at 10 in FIG. 1 a proportional
positive airway pressure apparatus according to a presently
preferred embodiment of the instant invention and shown in the form
of a functional block diagram. Apparatus 10 is operable according
to a novel process to deliver breathing gas, such as air, oxygen or
a mixture thereof, at relatively higher and lower pressures (i.e.,
generally equal to or above ambient atmospheric pressure) to a
patient 12 in proportion to the patient's respiratory flow for
treatment of OSAS, snoring, CHF, COPD and other respiratory or
cardio-respiratory disorders.
[0046] Apparatus 10 includes a gas flow generator 14, such as a
conventional CPAP or bi-level PAP blower, i.e., a centrifugal
blower, that receives breathing gas from any suitable source, e.g.,
a pressurized bottle 16 of oxygen or air, the ambient atmosphere,
or a combination thereof. The gas flow from flow generator 14 is
passed via a delivery conduit 18 to a breathing appliance or
patient interface 20 of any suitable known construction that is
worn by patient 12. In an exemplary embodiment of the present
invention, the conduit 18 is a large bore flexible tube and the
patient interface 20 is either a nasal mask or a full face mask, as
shown. Other breathing appliances that may be used in lieu of a
mask include a mouthpiece, a nasal seal, nasal prongs or cannulae,
an endrotracheal tube, a trachea adapter or any other suitable
appliance for interfacing between a source of breathing gas and a
patient. Also, the phrase "patient interface" can encompass more
that the interface worn by the patient. For example, the patient
interface can include delivery conduit 18 and any other structures
that connect the source of pressurized breathing gas to the
patient.
[0047] The apparatus also includes a sensor, such as a flow
transducer 22 or similar flow sensing element, situated within or
near the breathing circuit, i.e., the patient interface 20, conduit
18 or gas flow generator 14. Flow transducer 22 may be any suitable
gas flow meter, such as, for example, a bidirectional dynamic mass
flow sensor. Preferably, however, the flow transducer is a pressure
responsive sensor for detecting the magnitude of the pressure
gradients between the inlet of the patient's airway and his lungs.
Within the scope of the present invention, flow and respiratory
pressure gradient are highly correlated.
[0048] It is to be understood that the flow sensor need not be
coupled directly to conduit 18. On the contrary, the present
invention contemplates the use of any sensor or a plurality of
sensors that can measure the flow of breathing gas in the conduit.
For example, flow in the system can be measured at the patient
interface device or can be measured or estimated from the motor or
piston speed, power, current, torque, or feedback control used to
provide the elevated pressure by flow generator 14. Thus, the
sensor need not be a direct flow measurement, but can be determined
indirectly from measurements associated with the flow generator. In
short, the present invention contemplates any conventional
technique for measuring the flow of gas delivered to the patient as
flow transducer 22.
[0049] In accordance with a presently preferred embodiment, the
flow transducer 22 is interposed in line with conduit means 18,
most preferably downstream of a pressure controller 24. The flow
transducer generates output signals that are provided, as indicated
by reference numeral 26, to PPAP circuitry 28 described in greater
detail hereinafter. The output signals include first flow rate
signals indicative of inspiration by the patient and second flow
rate signals indicative of the patient's expiration. The signals
are continuously transmitted and correspond to the instantaneous
flow rate of breathing gas within conduit means 18.
[0050] In addition, the output from flow transducer 22 is also
desirably provided, as indicated by reference numeral 30, to an
optional leak detecting system 32. A suitable leak detector for
present purposes is that disclosed in U.S. Pat. No. 5,148,802, the
disclosure of which is incorporated herein by reference. However,
other techniques for substantially instantaneously calculating
system leakage, including both known leakage, such as that
discharged through a mask exhaust port 34, and unknown leakage,
such as that at various conduit couplings or at the patient contact
site of the patient interface 20, are acceptable. With any
non-invasive embodiment of the present invention, i.e., not
involving an endrotracheal tube or trachea adapter, the patient
flow must be estimated taking into account the aforesaid known and
unknown system leaks.
[0051] The output signal from the leak detecting system 32 is
provide, as at 36, to PPAP circuitry 28. In this way, the PPAP
circuitry logic continuously compares the output from flow
transducer 22 with that from leak detecting system 32 to
discriminate that portion of system flow associated with the
patient's respiration from that caused by system leakage. As a
result, PPAP circuitry 28 more precisely controls the output of the
pressure controller 24 as a function of patient respiratory flow,
rather than overall system flow.
[0052] If formed as a mask, as illustrated, patient interface 20
commonly includes, as mentioned above, a suitable exhaust system,
schematically indicated at 34, to exhaust breathing gases during
expiration. Exhaust system 34 preferably is a continuously open
port that imposes a suitable flow resistance upon exhaust gas flow
to permit pressure controller 24, located in line with conduit 18
between flow generator 14 and patient interface 20, to control the
pressure of air flow within the conduit and thus within the airway
of the patient. For example, exhaust port 34 may be of sufficient
cross-sectional flow area to sustain a continuous exhaust flow of
approximately 15 liters per minute at a system pressure of 10 cm
H.sub.2O. The flow via exhaust port 34 is one component, and,
typically, the major component of the overall system leakage, which
is an important parameter of system operation. In an alternative
embodiment, it has been found that a non-rebreathing valve may be
substituted for the continuously open port.
[0053] Pressure controller 24 controls the pressure of breathing
gas within conduit 18 and thus within the airway of the patient.
Pressure controller 24 is located preferably, although not
necessarily, downstream of flow generator 14 and may take the form
of an adjustable, electronically-controlled valve. Of course, other
method of pressure control are contemplated by the present
invention, such as controlling the operation of the pressure
generator, either alone or in combination with a pressure control
valve. For example, one embodiment of the present invention
contemplates that the pressure generator is a blower, which
includes an impeller mounted in a housing and rotated by a motor.
In this embodiment, the pressure controller is a control system
that controls the operating speed of the motor in the blower
without the need for a dedicated flow control valve.
[0054] Apparatus 10 also desirably includes a safety circuit,
preferably comprising an adjustable maximum pressure setting
control 38 and an adjustable minimum pressure setting control 40
operatively connected to pressure controller 24. The safety circuit
allows the manufacturer, the patient or his overseeing health care
professional to selectively establish minimum and maximum system
output pressures below and above which the system will not dispense
pressurized gas. The minimum pressure will, of course, be at least
zero and, preferably, a threshold pressure sufficient to maintain
pharyngeal patency during expiration. The maximum pressure, on the
other hand, will be a pressure somewhat less than that which would
result in over-inflation and perhaps rupture of the patient's
lungs. The safety circuit functions differently than the pressure
controls which determine, for instance, the CPAP prescription
pressure or the IPAP and EPAP prescription pressures used in
bi-level PAP therapy. That is, instead of establishing lower and
upper prescription pressures to be administered during normal usage
of the apparatus (subject to the influence of the PPAP circuitry
28), the maximum and minimum pressure setting controls 38 and 40
set absolute minimum and maximum fail-safe output pressure limits
which are not to be exceeded. Thus, the danger of potential
physical harm to the patient in the even of malfunction of other
system components, e.g., the prescription pressure controls, is
effectively eliminated.
[0055] PPAP circuitry 28, according to the present invention, is
subject to the influence of additional essential controls,
including a base pressure control 42, an inspiratory gain setting
control 44, and an expiratory gain setting control 46. The base
pressure control 42 establishes a base pressure (Pbase), usually
greater than or equal to zero and conceptually equal to the EPAP
level in bi-level therapy, sufficient to maintain airway patency at
the beginning and end of exhalation. The inspiratory gain setting
control 44 permits selection of a resistive gain (Gain.sub.Insp) to
be applied to the detected inspiratory flow. Similarly, the
expiratory gain setting control 46 enables selection of a resistive
gain (Gain.sub.Insp) to be applied to the detected expiratory
flow.
[0056] In a broad sense, PPAP therapy and the PPAP apparatus 10
constitute a novel system providing pressure to a patient via
nasal, nasal/oral, oral, or trachea interface to treat OSAS, COPD
and other breathing disorders. The pressure delivered to the
patient is a function of the patient flow rate. The function can be
described as follows:
Pdelivered=Pbase+Gain*Flow
where: "Pdelivered" is the pressure delivered to the patient
interface; "Pbase" is the base line pressure (greater than or equal
to zero and conceptually equal to EPAP); "Flow" is the estimated
patient flow rate determined by the flow transducer; and "Gain" is
the constant used to augment pressure based on the flow rate. The
gain constant can further be refined to allow one constant for
inspiration (positive flow) and a different constant for exhalation
(negative flow).
[0057] FIGS. 3A and 3B represent flow and pressure diagrams,
respectively, graphically depicting the manner in which apparatus
10 outputs pressurized breathing gas in proportional relation to
patient flow, as detected by flow transducer 22, in both the
inspiratory and expiratory phases of a respiratory cycle. The
pressure curve of FIG. 3B reflects a situation where the same gain
is chosen for both inspiratory and expiratory flow. Conceivably,
essentially the same pressure curve may be generated by the
apparatus disclosed in the aforementioned essay entitled "An
Apparatus for Altering the Mechanical Load of the Respiratory
System," by Younes, et al. which may use a single resistive gain
applicable to both inspiration and expiration.
[0058] With PPAP apparatus 10, however, separate and independent
gains may be chosen for inspiration and expiration, whereby gains
best suited to optimizing performance, i.e., minimizing breathing
work, may be precisely matched with each of the inspiratory and
expiratory phases. Thus, the function of the apparatus described in
the Younes et al. article corresponds to a special and relatively
limited application of the present invention where the selected
inspiratory and expiratory gains are identical.
[0059] As is far more often the case, however, an optimum
inspiratory gain is not the optimum expiratory gain and vice versa.
Thus, the pressure output of the PPAP apparatus 10 is more
accurately described according to the following functions, which
functions can be encoded into the PPAP circuitry 28.
Pinhalation=Pbase+Gain.sub.Insp*Flow,
and
Pexhalation=Pbase+Gain.sub.Exp*Flow,
where: "Gain.sub.Insp" is the constant used during inspiration
(positive flow) to boost pressure based on the flow rate; and
"Gain.sub.Exp" is the constant used during exhalation (negative
flow) to reduce pressure based on the flow rate.
[0060] The gain typically selected has a range of about 0 to 10 cm
H.sub.2O/liter/second for inspiration. The gain chosen for
exhalation is normally lower than the inspiratory gain, e.g.,
values in the range of 0 to 4 cm H.sub.2O/liter/second, although
higher gain values may be chosen for inspiration and/or expiration,
if such is desired or necessary.
[0061] Regardless of the chosen gain values, applying a flow signal
derived from a normal respiratory pattern will result in a pressure
rise above Pbase during inspiration and will drop below Pbase
during exhalation. When patient flow is near zero, i.e., at the
beginning and end of inspiration, as well as the beginning and end
of exhalation, the output pressure approaches Pbase.
[0062] FIGS. 4A and 4B perhaps most clearly exemplify the effect
that the selection of different gains for both the inspiratory and
expiratory phases of a respiratory cycle has upon the pressure
output curve. Gain.sub.Insp(a), Gain.sub.Insp(b), Gain.sub.Insp(c)
and Gain.sub.Insp(d) represent, in descending order, several of an
infinite range of gain values that may be applied during
inspiration. Similarly, Gain.sub.Exp(e), Gain.sub.Exp(f),
Gain.sub.Exp(g) and Gain.sub.Exp(h) indicate increasing expiratory
gain values. With different gain settings, any number of wave forms
can be generated. For example, a high setting may be established
for Gain.sub.Insp and a low setting for Gain.sub.Exp, or vice
versa, or the gain settings for inspiratory flow and expiratory
flow may be the same.
[0063] In one embodiment of the present invention, PPAP therapy
seeks to provide only the pressure that is necessary to prevent
airway collapse at any given moment during the breathing cycle.
This will generally result in supplying, at appropriate times,
maximum pressure only when peak negative airway inspiratory
pressures are detected and minimum pressure only when peak positive
airway exhalation pressures are detected. At all other times during
the breathing cycle, the PPAP apparatus delivers air at a variable
pressure responsive to the patient's respiratory efforts in a range
between the maximum and minimum pressures. As mentioned above, PPAP
therapy also involves the administration of a base pressure of zero
or greater to which the product of a selected gain times
instantaneous flow (inspiratory and expiratory) is continuously
added to produce the instantaneous output pressure of the PPAP
apparatus. An identical gain may be selected for inspiration and
expiration, or different gain values may be independently selected
for inspiration and expiration. The base pressure will be the
pressure necessary to overcome any mechanical collapsing forces
that result from the structure of the airway tissues, muscle tone,
and body position. In other words, the base pressure is generally
equivalent to the expiratory positive airway pressure or "EPAP"
typically used in bi-level PPAP therapy.
[0064] In this connection, FIG. 5B illustrates the pressure output
curve generated by the PPAP apparatus 10 vis-a-vis conventional
CPAP and bi-level PAP apparatus over a single respiratory cycle. So
long as the appropriate inspiratory and expiratory splint pressures
are applied at point 48 (peak inspiratory flow), point 50
(beginning of exhalation) and point 52 (end of exhalation), less
pressure may be provided at all other times during the breathing
cycle than is normally supplied by conventional CPAP or bi-level
PAP therapy. This reduced output pressure is represented by the
"PPAP" curve of FIG. 5B. The hatched areas of that figure reflect
the difference in pressures provided by PPAP and the IPAP and EPAP
phases of bi-level PAP during a typical respiratory cycle. The
hatched areas may be conceptualized as the respiratory work or
effort savings that are attributed to PPAP. This work savings, as
would be expected, translates to greater comfort for the PPAP
assisted patient and increased compliance with the respiratory
treatment. According to the present invention, PPAP therapy thus
represents a novel respiratory disorder treatment by which patient
comfort (and, therefore, treatment compliance) exceed that offered
by either CPAP or bi-level PAP therapy.
[0065] Referring again to FIG. 1, it will thus be appreciated that
pressure controller 24 is continuously governed by and outputs
variable pressure responsive to a command signals 54 from PPAP
circuitry 28. Command signals 54, in turn, are the product of the
influences of one or more of the outputs from flow transducer 22,
leak detection system 32, base pressure control 42, inspiratory
gain setting control 44, expiratory gain setting control and, in an
alternative embodiment, a pressure profile control 56 discussed
below.
[0066] In normal breathing, a negative pressure gradient must be
generated before flow can begin. Hence, the negative pressure
waveform generated in the airway must precede and thereby induce
inspiratory flow at the start of inspiration. In an unstable
airway, which is characteristic of OSAS, for example, this
asynchronous relationship of negative pressure gradient and
inspiratory flow onset would, if not accommodated by suitable
compensatory measures, lead to a situation where the PPAP therapy
would not generate sufficient pressure (due to low flow) to
overcome the negative pressure in the airway, whereby total or
partial airway collapse may result. This problem can be solved by a
number of methods. For instance, a higher PPAP base pressure can be
used to provide additional pressure to support the airway at the
beginning of inspiration. Alternatively, however, as demonstrated
by FIGS. 6A and 6B, a temporary pressure increase can be added at
the start of inspiration to support to the airway until sufficient
flow is generated to drive the PPAP process. The present invention
offers several viable approaches by means of which pressure can be
added during the initial phase of inspiration to support the airway
as inspiratory flow increases.
[0067] Temporary pressure increases may be effected using pressure
profile control 56 in operative connection with PPAP circuitry 28
to select a desired elevated pressure waveform in the early stages
of inspiration. In this regard, pressure profiles may be used as
minimum values for the output pressure at the outset of
inspiration, thereby giving rise to the following alternative
equations for available output pressure during inspiration.
Pinhalation=greater of:
Pbase+Gain.sub.Insp*Flow
or
Pbase+Pprofile
where: "Pinhalation" is the pressure delivered to the patient
interface during inspiration "Pbase" is the base line pressure
(conceptually equal to EPAP); "Flow" is the estimated patient flow;
"Gain.sub.Insp" is the constant used during inspiration (positive
flow) to boost pressure based on the flow rate; and "Pprofile" is a
function that generates a pressure profile to support the airway at
the start of inspiration. Such pressure profile functions may be
constant, e.g., a step profile as shown by the dotted line
identified by numeral 58 in FIG. 6B, time based (for instance, a
backwards ramp profile as shown by the dotted and dashed line
identified by numeral 60 in FIG. 6B), or any other functional
shape.
[0068] Alternatively, pressure profiles can be used exclusively to
control the output pressure for a predetermined initial segment of
inspiration. The following equations represent system output
pressure during inspiration under such control conditions.
Pinhalation=Pprofile from start of breath to X
and
Pinhalation=Pbase+Gain.sub.Insp*Flow from X to start of
exhalation
where: "Pinhalation" is the pressure delivered to the patient
interface during inspiration; "Pbase" is the base line pressure
(conceptually equal to EPAP); "Flow" is the estimated patient flow;
"Gain.sub.Insp" is the constant used during inspiration (positive
flow) to boost pressure based on the flow rate; and "Pprofile" is
any function that generates a pressure profile to support the
airway at the start of inspiration. Such functions could be
constant, such as, for example, a step profile, or time based, such
as a backwards ramp profile, or any other functional shape. "X" is
a preselected transition point determined by time, or analysis of
the flow signal, such as curvature, percent drop from peak flow
rate, integration, derivative, analysis of prior breaths or a
combination of flow analysis and time.
[0069] The PPAP apparatus 10 also has the capacity to measure and
store in a memory 62 (FIG. 1) the following parameters: tidal
volume, inspiratory time, expiratory time, peak pressure, peak
flow, 0.sub.2 saturation (as a voltage input from an external
source), plural pressure (as a voltage input from an outside
source), mask pressure, estimated leakage, and system parameters,
e.g., Pbase, Auto Gain.sub.Insp, Gain.sub.Insp, Gain.sub.Esp, IPAP
and EPAP. It is to be understood that this list is not exclusive;
other parameters can be stored in memory 62.
[0070] A further method by which the present system addresses the
problem presented by the changing needs of the patient is to
combine the beneficial features of PPAP with a more controlled
therapy such as CPAP, as is shown in FIGS. 7A and 7B.
[0071] With CPAP, a single pressure is generated and delivered
throughout the sleeping session. PPAP can be advantageously joined
with CPAP to lower the pressure provided to the patient during
exhalation. The resulting equations for pressure delivered under
combined PPAP-CPAP are as follows:
Pinhalation=CPAP
and
Pexhalation=CPAP+Gain.sub.Exp*Flow
where: "Gain.sub.Exp" is the constant used during exhalation
(negative flow) to reduce pressure based on the flow rate.
[0072] It should be noted that the CPAP pressure for this
embodiment can be fixed for all time during the therapy session or
it can vary over the course of the treatment session. For example,
it is known to provide a patient with an auto-titrating CPAP
pressure, where the CPAP pressure is varied based on the monitored
condition of the patient, the condition of the pressure support
system, or both. Thus, the present invention contemplates combining
PPAP during exhalation with an auto-titrating CPAP pressure during
inhalation, so that the CPAP pressure is determined using any
conventional auto-titration technique, such as that taught by U.S.
Pat. Nos. 5,203,343; 5,458,137 and 6,087,747 all to Axe et al. or
U.S. Pat. No. 5,645,053 to Remmers et al.
[0073] It has further been discovered that patient comfort can be
increased when using the PPAP-CPAP combination. It can happen when
using conventional triggering techniques, that the trigger from
inspiration to expiration is detected before the patient's flow
reaches zero. In other words, the patient's expiratory effort
begins, not at the zero flow point, but at a period of time
slightly before that point. For example, at point "p" in FIG. 7A
the pressure support system may consider the patient to be in the
expiratory phase of the breathing cycle.
[0074] If Pexhalation is calculated at point "p", where the patient
flow is positive, the patient would receive an increase in
pressure, when they should be getting a decrease in pressure or
"pressure relief". This can be avoided by delaying the calculation
of Pexhalation until the flow actually goes below zero. However,
the patient may perceive a delay in the pressure relief. Therefore,
a further embodiment of the present invention contemplates
calculating Pexhalation at the trigger from inspiration to
expiration as follows:
Pexhalation=CPAP+Gain.sub.Exp*(Flow-FlowOffset),
where "Flow" is the current patient flow and "FlowOffset"
corresponds to the last value of patient flow (if positive) during
inspiration. In essence, the FlowOffset value allows Pexhalation to
be calculated at point "p" in FIG. 7A, but accounts for the fact
that the flow is positive at this point by subtracting out an equal
or greater amount of flow using the FlowOffset value. Pexhalation
is calculated using the FlowOffset only if the Flow is positive.
Otherwise, the FlowOffset can be removed or set to zero.
[0075] The value of FlowOffset must be decreased during the
expiratory phase to ensure that the offset reaches zero by the end
of expiration, i.e., when the patient's flow increases back to
zero, so that the pressure relief returns to zero at that time. The
decrease on FlowOffset can be done periodically or continuously
throughout the expiratory phase.
[0076] FIGS. 8A and 8B demonstrate that PPAP can also be combined
with bi-level PAP therapy in a number of ways to produce effective
therapeutic pressure waveforms. One application, generally similar
to the aforementioned PPAP-CPAP scenario, is to use PPAP to lower
the pressure during exhalation. The resulting equations for the
delivery of composite PPAP-bi-level PAP pressure are as
follows:
Pinhalation=IPAP
and
Pexhalation=EPAP+Gain.sub.Exp*Flow
where: "Gain.sub.Exp" is the constant used during exhalation
(negative flow) to reduce pressure based on the flow rate.
[0077] Another approach to merging PPAP with bi-level therapy is
shown in FIGS. 9A and 9B where IPAP is applied to the patient for a
first portion of the inspiratory cycle and PPAP is applied for the
remainder of the breathing cycle. Gain.sub.Insp is automatically
calculated for each breath based on IPAP and the flow rate as
follows:
Pinhalation (t.sub.0 to t.sub.1)=IPAP
and
Pinhalation (t.sub.1 to t.sub.2)=Pbase+AutoGain.sub.Insp*Flow
and
Pexhalation=Pbase+Gain.sub.Exp*Flow
where: "Flow" is the estimated flow rate; "t.sub.0" is the time at
the start of breath; "t.sub.1" is the time when the estimated flow
rate is a predetermined percentage of peak inspiratory flow rate;
"t.sub.2" is the time at the start of exhalation; "IPAP" is a
continuously applied inspiratory positive airway pressure;
"Pinhalation (t.sub.0 to t.sub.1)" is the pressure delivered to the
patent from t.sub.0 to t.sub.1; "Pbase" is a continuous base
pressure; "AutoGain.sub.Insp" equals (IPAP-Pbase)/Flow at t.sub.1;
"Pinhalation (t.sub.1 to t.sub.2)" is the pressure delivered to the
patient from t.sub.1 to t.sub.2; "Gain.sub.Exp" is the constant
used during exhalation to reduce pressure delivered to the patient;
and "Pexhalation" is the pressure delivered to the patient during
exhalation.
[0078] It is to be understood that the flow and PPAP pressure
output curves of FIGS. 3A through 9B represent the apparatus output
pressure and flow during the inspiratory and expiratory phases of a
single respiratory cycle. The PPAP and flow curves can, of course,
be expected to vary somewhat from respiratory cycle to respiratory
cycle depending on the patient's respiratory requirements,
particularly under fully automated PPAP therapy described
hereinafter. Furthermore, somewhat greater variations will likely
occur between the respiratory cycles associated with different
stages of an extended treatment session, especially during OSAS
treatment.
[0079] FIG. 2 represents a further preferred embodiment of a PPAP
apparatus pursuant to the present invention, designated herein by
reference numeral 10'. Apart from the addition IPAP/EPAP (bi-level
PPAP) circuitry 64, PPAP apparatus 10' is identical in structure
and function to PPAP apparatus 10. According to this embodiment,
output 66 from flow transducer 22 is fed to bi-level PAP circuitry
64. Bi-level PAP circuitry 64 may assume any conventional form such
as, for example, that described in U.S. Pat. Nos. 5,148,802;
5,433,193; and 5,632,269, the contents or which are incorporated
herein by reference. Output 68 from bi-level PPAP circuitry 64 is
transmitted to the PPAP circuitry 28. Output 68 consists of an IPAP
signal if the patient is inhaling and an EPAP signal in the event
the patient is exhaling. The logic of the PPAP circuitry 28
utilizes this input according to a preselected one any of the
aforementioned combinations of PPAP-bi-level therapy to generate a
desired pressure command signal 54.
[0080] Pursuant to the present invention, the pressure delivered to
the patient is determined by the base pressure, the flow rate and
the gain (and the pressure profile if used). For a given patient
condition, these settings can be adjusted as necessary to stabilize
the airway. In OSAS, a patient's periodic and, to a lesser extent,
instantaneous condition is variable with sleep state and body
position. Thus, settings that may work well in during one portion
of a sleeping session may not work as well at a different time. In
other words, settings that support the airway at its most unstable
state may cause pressures that are higher than necessary during
more stable times. Likewise, settings that work well at one point
in the session may be insufficient at another time.
[0081] The present invention proposes several methods to minimize
the impact of the patient's changing needs on the optimization of
PPAP therapy. One such method is to automatically adjust the gain,
pressure profile and baseline pressure to meet the patient's
demands. This adjustment can be based on analysis of patient
parameters related to flow, e.g., magnitude, shape, derivative,
integral (volume), pressure, snoring, arterial oxygen saturation,
exhaled C0.sub.2, airway diameter, or other parameters.
[0082] Using one or more of these parameters the system may adjust
the Gain.sub.Insp to prevent partial airway obstruction (hypopnea).
The goal of such systems is to increase Gain.sub.Insp responsive to
any of the following patient conditions:
[0083] decreased inspiratory flow;
[0084] decreased inspiratory volume;
[0085] increased airway resistance, as determined by flow or
pressure signal analysis;
[0086] airway instability, as indicated by pressure or sound
variations;
[0087] drops in arterial oxygen saturation; or
[0088] decreases in airway diameter.
[0089] The apparatus according to the invention may also maintain
minimal Gain.sub.Insp in the absence of these conditions.
[0090] The present system may also adjust the base pressure (Pbase)
to prevent complete collapse of the airway (apnea) or severe
collapse (severe hypopnea). Apnea can be detected by analysis of
the flow signal and/or by using reflected pressure waves, or a
combination of pressure and flow to determine airway patency.
Moreover, it may be important to determine if the apnea is caused
by airway collapse or by a lack of respiratory drive. If an
obstructive event is detected the base pressure can therefore be
increased to open the airway. A further capability of the present
system is to maintain a minimum Pbase in the absence of these
conditions.
[0091] The system may also adjust the pressure profile (Pprofile)
to prevent apnea or hypopnea at the onset of inspiration. As such,
the system may increase Pprofile in response to decreased
inspiratory flow, decreased respiratory volume, flow waveform shape
analysis that indicates increasing airway resistance, pressure or
sound variations indicative of airway instability, drops in
arterial oxygen saturation, decreases in airway diameter or a
change in exhaled C0.sub.2. Commensurate, therewith, the present
invention also functions to maintain the minimum pressure profile
in the absence of these conditions.
[0092] FIG. 10 reveals a presently preferred embodiment of a fully
automated PPAP apparatus 10'' constructed according to the present
invention. Generally similar in structure and function to PPAP
apparatus 10 of FIG. 1, PPAP apparatus 10'' additionally
incorporates a microprocessor or central processing unit (CPU) 70
that preferably utilizes an output signal 72 from flow transducer
28 as a continuous feedback signal to enable the CPU to
continuously adjust Pbase, Pprofile, Gain.sub.Insp, and
Gain.sub.Exp as necessary. The CPU may, however, be configured to
effect its continuous system control functions responsive to any of
the aforementioned patient parameters related or unrelated to
respiratory flow.
[0093] Apparatus 10'' also has the capability to detect hypopnea,
as evidenced by decreases in peak flow and/or tidal volume for a
given period of time, and the occurrence of apneas, as manifested
by very little flow for a given period of time. To detect hypopnea,
for example, the CPU 70 may be programmed to make a comparison
between a short term average of peak inspiratory flow rate or tidal
volume (e.g., a 3 breath average) and a long term average of peak
flow rate or tidal volume (e.g., greater than 20 breaths). If a
decrease of greater than 25% is detected the system determines a
hypopnea to be present. This determination is desirably made only
if the leakage is well estimated and stable. Thus, large changes in
leak or initiation of a leak recovery will cause data to be
ignored.
[0094] The invention further includes a method for determining if
the airway is open (central apnea) or obstructed (obstructive
apnea) during an apnea. Once an apnea of significant duration is
detected the system, under the direction of CPU 70, automatically
increases Gain.sub.Insp by 2 cm H.sub.2O, waits approximately 1
second and decreases the pressure back to the original value. If
there is a significant change in flow during this pressure change,
the system concludes that the airway is open (central apnea). If
there is no significant change in flow the system determines that
the airway is obstructed (obstructive apnea). The system will
continue to monitor each apnea for its entire duration at periodic
intervals to determine the nature of the apnea.
[0095] In accordance with a preferred embodiment, the PPAP
apparatus 10'' controls are automatically adjusted as follows. In
the event of a hypopnea, Gain.sub.Insp is increased by 2
cm/liter/second. In the event of an obstructive apnea, Pbase is
increased by 1 cm H.sub.2O. The device will continue to increase
Pbase as long as an obstructive apnea of significant duration is
detected. The device will not increase Gain.sub.Insp again, if
necessary, until 5 breaths have passed. If no hypopnea or apneas
occur over a period of 30 breaths, Gain.sub.Insp is decreased by 1
cm/liter/second. If no hypopnea or apneas occur over a period of 50
breaths, Pbase is decreased by 1 cm H.sub.2O. In addition, the
apparatus may control the delivery of 0.sub.2 while patient flow is
greater than zero, if such desired or necessary.
[0096] Although not illustrated, still further embodiments of the
present invention contemplate the incorporation of fully automated
PPAP with CPAP and/or bi-level PAP therapy. In these cases CPAP or
IPAP may be controlled using the same logic that controls
Gain.sub.Insp in the above-described fully automated PPAP system.
Likewise, Pbase may be controlled in a similar manner to that
described in connection with fully automated PPAP.
[0097] The fully automated PPAP-CPAP or PPAP-bi-level PAP systems
may also adjust Pprofile to prevent apnea or hypopnea at the start
of inspiration. Such systems may therefore increase CPAP (or IPAP)
or Pprofile in the face any of the following patient
conditions:
[0098] decreased inspiratory flow;
[0099] decreased inspiratory volume;
[0100] increased airway resistance, as determined by flow or
pressure signal analysis;
[0101] airway instability, as indicated by pressure or sound
variations;
[0102] drops in arterial oxygen saturation; and
[0103] decreases in airway diameter.
[0104] It will be understood that CPAP or IPAP would be maintained
at minimal levels in the absence of these conditions.
[0105] Using PPAP therapy, therefore, it is additionally possible
to employ PPAP in response to expiratory flow to reduce pressure
applied during expiration to less than the patient's PEEP level
throughout all but the end of the expiratory phase in a manner
similar to that described for lowering the pressure below Pbase
during exhalation in the treatment of OSAS. This lowering of
applied pressure to less than PEEP during the expiratory phase
diminishes breathing work and enhances patient comfort when
compared to the constant expiratory phase pressure applied during
EPAP. Indeed, PPAP can be adapted to any ventilation mode that uses
PEEP. Such applications may include pressure support with PEEP, PAV
with PEEP or other applications of PEEP in respiratory assistance
therapy.
[0106] As noted above, the present invention contemplates providing
different inspiratory gains or expiratory gains depending on the
pressure support needs of the patient. See, e.g., FIG. 4B and the
associated text. A further embodiment of the invention contemplates
allowing the patient to select the gain (inspiratory or expiratory)
so that the user can try different gains to determine which gain
provides the best pressure support therapy. It can be appreciated
that this embodiment is best suited for allowing the patient to
select the expiratory gain, such as when using the PPAP-CPAP
combination, because it is commonly accepted that the inspiratory
positive pressure, and, hence, the inspiratory gain, should be set
by a physician and should not be altered by the patient.
[0107] For example, the present invention contemplates having a
user select a first expiratory gain setting and use the pressure
support system at that gain. The efficacy, comfort, or both of the
pressure support system operating at the first gain, can then be
monitored using any conventional technique, such as by having the
patient complete a questionnaire. See, for example, published PCT
application publication no. WO00/18347, which teaches a technique
for providing a pressure support system and an efficacy
questionnaire. In a subsequent therapy session, the user selects a
second expiratory gain different than the first. The efficacy,
comfort, or both of the pressure support system operating at the
second gain can then be monitored and compared against the results
garnered using the first gain. This process can be repeated using
still other gains. In this manner, the optimal gain can be
determined. The present invention also contemplates allowing the
user to adjust the base pressure to determine its optimum level
using the above-described technique.
[0108] The present invention further contemplates that the
above-described patient-feedback determination of the optical gain
or base pressure can be performed automatically. For example, the
pressure support system can be programmed to automatically provide
different gains or base pressures during different therapy
sessions. After each session the user can be prompted to take a
survey to gauge the effectiveness, comfort, or both of that therapy
session. After sufficient data is collected the system can then
automatically select the gain or base pressure based on the survey
results that is optimum for that patient.
[0109] Furthermore, the administration of oxygen in phase with
inspiration may also easily be included with PPAP therapy for the
treatment of COPD patients requiring supplemental oxygen.
[0110] The present invention also contemplates that the pressure of
the gas being provided to the patient can be controlled so as to
vary over time. For example, in one embodiment of the present
invention, the pressure provided to the patient increases from a
first minimum pressure to a desired therapy pressure over a period
of time. This ramp increase in pressure provides the patient with
time to fall asleep under relatively low pressure that is increased
to the therapy pressure over time. Thereafter, the pressure
increases so that the therapy pressure is being applied after the
patient is asleep. A reverse process can be performed in the
morning, with the pressure being decreased from the therapy
pressure shortly before the patient intends to wake up. Ramp
control 120 in FIG. 10 schematically illustrates a manually
actuated controller that provides commands to PPAP circuitry 28 to
cause the pressure to be provided according to a ramp cycle.
Furthermore, the ramp control may be adjusted according to the
output of CPU 70. Ramp control 120 can be used to set the
parameters associated with the ramp function, such as the ramp
period, ramp start time, ramp stop time, and ramp shape.
[0111] Examples of techniques for controlling the pressure level
provided to the patient via one or more ramp functions, as well as
other methods for controlling the patient pressure, are disclosed
in U.S. Pat. Nos. 5,492,114; 5,551,418 and RE 35,295, the contents
of each are incorporated herein by reference. Many of the
techniques taught by these patents can be incorporated into the
present apparatus and method to provide the optimum therapy
necessary to treat the patient.
[0112] In a still further embodiment of the present invention, an
alarm 122 is coupled to PPAP circuitry 28 and/or CPU 70. Alarm 122
can be controlled so as to be actuated as a result of a variety of
circumstances. However, in a preferred embodiment of the invention,
alarm 122 is actuated responsive to an automatically determined
gain falling outside a predetermined range of values.
[0113] The present invention also contemplates limiting a value for
an automatically determined gain, such as AutoGain.sub.Insp
discussed above, to prevent the automatically determined gain from
exceeding predetermined limits, for example, from exceeding limits
that may result in an excessively high pressure being provided to
the patient. The limits on the amount that the gain can themselves
be altered so that these limits vary over a predetermined period of
time. Also, the amount of change that may take place in the
automatically determined gain over a predetermined period of time
can also be controlled, thereby preventing the automatically
determined gain from changing by more than a predetermined amount
over the predetermined period of time.
[0114] In using PPAP to treat CHF, the present invention reduces
mean pressure and work of exhalation while still providing the same
level of rest to the heart. By applying a positive base pressure
substantially equivalent to a pressure needed to reduce cardiac
preload and afterload (preferably in the range of 5-10 cm
H.sub.2O), the present invention helps the heart reduce its
efforts. With additional positive pressure during inspiration in
proportion to respiratory effort, one can overcome the effect of
negative pressure being produced during inspiration. PPAP is
particularly appropriate in CHF patients in that the typical CHF
patient has normal lung compliance. In these patients, much of the
respiratory loading can be inferred from the flow signal. By
reducing the pressure below the base pressure during exhalation,
one can reduce the work of exhalation without, reducing the benefit
to the heart. The net effect will be the same benefit to the heart
with reduced work of breathing and lower mean pressure.
[0115] Similar to PPAP therapy's use for preventing airway
collapse, PPAP therapy for the treatment of CHF delivers only the
minimum amount of pressure needed to reduce cardiac preload and
afterload. This will result in supplying a base pressure to the
exterior of the heart equivalent to the pressure needed to reduce
cardiac preload and afterload in the absence of respiratory loading
and a varying pressure which is needed to overcome the impact of
respiratory loading on cardiac preload and afterload while
minimizing the work of breathing.
[0116] Supplying positive pressure to the exterior of the heart via
the respiratory system has two benefits firstly, the positive
pressure will reduce the enlarged heart of a CHF patient to a size
closer to normal. This return to normal size, allows the muscles of
the heart to work more effectively. Secondarily, the positive
pressure in the chest cavity reduces the amount of pressure the
heart must overcome to pump blood to the rest of the body.
[0117] The heart and chest cavity are at the same pressure.
Typically this pressure fluctuates about ambient pressure due to
the impact of respiratory loading. The circulatory system has a
working pressure that varies as the heart pumps but averages 100 mm
HG in normal-tensive patients. The heart must supply the power to
force blood from the chest cavity into the pressurized circulatory
system. Increasing the pressure in the chest cavity reduces the
amount of pressure the heart must over come to pump blood. A
pressure in the chest cavity of 10 cm H.sub.2O or approximately 10
mm Hg will reduce the load on the heart by 10 mm Hg/100 mm Hg or
roughly 10%.
[0118] The impact of respiratory effort on the heart is as follows:
during inspiration, the pressure in the chest (and thus surrounding
the heart) becomes more negative relative to the rest of the body.
This increased negative pressure increases the amount of pressure
the heart must generate to pump blood from the chest cavity to the
body. By providing pressure in excess of the base pressure during
inspiration, PPAP is able to offset this decrease in chest cavity
pressure and maintain a relatively constant pressure in the
chest.
[0119] During exhalation the pressure in the chest becomes less
negative relative to the rest of the body. By reducing the pressure
during exhalation, PPAP is able to offset the increase in chest
cavity pressure and maintain a relatively constant pressure in the
chest.
[0120] By minimizing the decrease in pressure and taking advantage
of the increased pressure during exhalation, the variable portion
of PPAP allows a lower baseline to be set relative to using a
constant pressure with the same benefits to the heart. This lower
baseline and reduced pressure during exhalation also reduces the
work of breathing and increases patient comfort.
[0121] It is further desirable to implement a version of PPAP
similar to that discussed above with respect to FIG. 7 on existing
CPAP devices. Providing a version of PPAP on existing CPAP devices
enhances the patient comfort and, hence, compliance without the
significant financial and other burdens appurtenant to
manufacturing and introducing a new CPAP device that includes a
PPAP mode. Providing a PPAP therapy on a CPAP device can be
accomplished in a variety of ways, such as that discussed above
with respect to FIGS. 7A and 7B.
[0122] A version of PPAP can be implemented on a CPAP system in a
more cost effective manner if the reactive component used to
generate the reduced pressure curve during exhalation in FIG. 7 is
replaced with a defined reduced pressure profile. This pressure
profile replaces the constant CPAP pressure otherwise applied by
the CPAP device during the expiratory phase of the patient's
breathing cycle. In an preferred embodiment of the present
invention, the defined pressure profile has a shape that generally
corresponds to a patient's normal flow.
[0123] FIGS. 11A and 11B are flow and pressure diagrams similar to
FIGS. 7A and 7B illustrating a resultant apparatus pressure output
curve according to a further embodiment of the present invention
that utilizes a simplified pressure profile generating technique.
Flow signal 80 in FIG. 11A illustrates the patient's inspiratory
phase 82 and expiratory phase 84. As shown in FIG. 11B, a
continuous CPAP pressure 86 is delivered during the inspiratory
phase 82. During the expiratory phase, the pressure support device
is controlled to deliver a reduced pressure following a
predetermined pressure profile 88. The resulting equations for
pressure delivered under the combined CPAP and PPAP are as
follows:
Pinhalation=CPAP
and
Pexhalation=CPAP-Predetermined Pressure profile.
[0124] The predetermined pressure profile, which is used to reduce
the CPAP pressure, has a magnitude M, which is typically selected
by a respiratory therapist in a range of 0-4 cmH.sub.2O, and a
duration D that, unlike the pressure curve in FIG. 7B, is not
directly determined base on the patient's instantaneous flow or
volume. The magnitude M represents the drop in pressure from the
constant CPAP value. The duration D value is preferably a fraction
of an average expiration period of the patient.
[0125] Multiple predefined pressure profiles, having different
magnitudes, durations or both can be stored in a CPAP/PPAP device
and provided to the patient. FIG. 11B illustrates three
predetermined pressure profiles P1, P2 and P3, having magnitudes
M1, M2 and M3 and durations D1, D2 and D3, respectively. In a
preferred embodiment of the present invention, the pressure
profiles are selected so that the pressure provided to the patient
during exhalation roughly correspond to the contour generated from
flow or volume based PPAP, as shown, for example, in FIGS. 3A and
3B. As shown in FIG. 11B, the pressure drops off quickly at the
start of expiration then rises slowly toward the baseline CPAP
pressure.
[0126] Because this embodiment for a CPAP/PPAP device does not
control the flow and/or pressure provided to the patient based on
the flow or pressure signal from the patient, as is the case with
the PPAP devices and techniques discussed above, but instead,
merely detects the start or expiration and/or inspiration, the
sensor required by this embodiment need not be as accurate as in
the previous embodiments. For example, a thermister or thermocouple
could replace the costly pneumotach flow meter to determine the
inspiratory and expiratory state. Also, the pressure profile
provided to the patient during the expiratory phase can be
generated using motor speed control, thereby avoiding the use of a
pressure control valve.
[0127] While the above embodiment has been described above with
respect to the use of a predetermined pressure profiled used to
reduce a CPAP pressure during expiration, the same technique can be
applied to a bi-level pressure support device to achieve a pressure
curve shown, for example, in FIG. 8B by reducing the bi-level EPAP
pressure by the predetermined pressure profile.
[0128] It has been observed that in some cases where PPAP is
implemented and the gain is set above a certain amount, for example
beyond the range of 2-3 cm/liter/sec, there is a tendency for the
pressure generated by the device to become unstable. More
specifically, as shown in FIG. 12, oscillations 90 occur in the
pressure waveform 92 applied to the patient during portions of the
patient's expiratory phase 94. These oscillations typically occur
after the initial pressure drop following the onset of the
expiratory phase and are believed to be generated as a result of
the interaction of the patient's flow and the resulting pressure
decrease.
[0129] Despite the chance of such oscillations occurring, it is
still preferable to provide a relatively large decrease in the
pressure being provided to the patient at the onset of expiration
while maintaining the pressure profile as smooth as possible during
the remainder of the expiratory phase. This is accomplished
according to one embodiment of the present invention by providing
pressure to the patient during expiration and after the maximum
pressure reduction according to the following equations:
Pexhalation=the greater of:
Pbase+Gain.sub.Exp*Flow
or
a Current Pressure,
where the "Current Pressure" is the pressure being provided to the
patient at that time during the expiratory phase.
[0130] FIG. 13 illustrates a pressure profile 92' similar to
profile 92 illustrated in FIG. 12, except that the oscillations
occurring during the expiratory phase have been removed using the
above described technique. For example, at point 96, which is after
the maximum reduction in pressure at point 95, the pressure begins
to decrease from the immediately previous pressure due to a
pressure oscillation. The above oscillation prevention technique
prevents this decrease by substituting the current pressure, i.e.,
the pressure at point 96 at all points thereafter where the
calculated pressure, i.e., Pbase+Gain.sub.Exp*Flow, is less than
the current pressure, thereby creating a plateau section 98 that
corresponds to the pressure at point 96, i.e., the current
pressure, until a point 100 where the calculated pressure becomes
greater than the current pressure. It should be noted that for
purposes of this invention Flow is considered to be negative during
the expiratory phase.
[0131] By ensuring that the pressure provided to the patient is
always the greater of the current pressure and the calculated
pressure, the pressure received by the patient during expiration
does not oscillate. If the pressure to be provided to the patient
begins to decrease below the current pressure, the device will not
use the calculated pressure, but will continue to provide the
patient with the current pressure, thereby preventing a pressure
decrease below the current pressure.
[0132] It can happen, however, that after a first minimum flow is
reached, the flow will again drop below that first minimum. For
example, FIG. 15 illustrates an example where the patient flow
reaches a first minimum in region 150, increases in region 152 and
then falls below the first minimum in region 154. If this occurs,
the pressure to be delivered to the patient should not be clamped
at the current pressure Pcurrent. Instead, the pressure to be
delivered should be calculated using the PPAP technique, i.e.,
Pexhalation=Pbase+Gain.sub.Exp*Flow. Thus, in region 154, where the
patent flow has again fallen to a level below the previous minimum
level, the pressure delivered is calculated as
Pexhalation=Pbase+Gain.sub.Exp*Flow. Of course, the present
invention contemplates that Pbase can be a CPAP or IPAP pressure,
as noted above.
[0133] In a second embodiment of the present invention, the
pressure oscillations are avoided by using an entirely different
calculation for determining the pressure to be provided to the
patient during the expiratory phase. Instead of basing the
calculation of the pressure to be provided to the patient based on
the patient flow multiplied by a gain, which is selected either
manually or automatically, as in the previous embodiments, the
calculation of the pressure to be provided to the patient is based
on the volume of gas still contained in the lungs, referred to as
the volume to be exhaled. The volume to be exhaled corresponds to a
difference between a current volume of gas in the patient and a
volume of gas in the patient at rest. The volume of gas currently
in the lungs can be readily estimated from the flow signal. The
volume of gas in the patient at rest is determined using
conventional techniques, and can be updated on a periodic base to
ensure the accuracy of the calculation.
[0134] According to this embodiment, the pressure output by the
PPAP device at least during a portion of the expiratory phase is
described by the following function, which can be encoded into the
PPAP circuitry:
Pexhalation=Pbase-(Volume.sub.to be exhaled*Gain.sub.Exp)
where: "Pbase" is the base line pressure (greater than or equal to
zero and conceptually equal to EPAP); "Volume.sub.to be exhaled" is
the difference between the current volume of gas in the patient
less the volume of gas in the patient at rest; and "Gain.sub.Exp"
is the constant used during expiration (negative flow) to reduce
pressure.
[0135] The pressure output during the inspiratory phase is
determined using the techniques discussed above. FIG. 14
illustrates a pressure curve 102 generated using the above equation
to determine the pressure to be provided to the patient. It can be
appreciated from FIG. 14 that pressure curve 102 accomplishes the
functions of lowering the pressure during expiration and returning
the pressure to the baseline at the end of the expiratory phase,
preventing airway collapse.
[0136] Because the volume of gas to be exhaled (Volume.sub.to be
exhaled) is relatively large at the onset of the expiratory phase,
the pressure drop at the beginning of exhalation can be quite
large. It is preferable to smooth the large drop at the onset of
the expiratory phase by including a dampening factor in the
calculation of the pressure to be provided to the patient.
[0137] There are many techniques that can be used to dampen the
initial pressure drop at the start of the expiratory phase.
However, according to a preferred embodiment of the present
invention, the pressure provided to the patient during the
expiratory phase is described according to the following
equation:
[0138] from the start of the expiratory phase (t.sub.0) to X:
Pexhalation = Pbase - t X ( Volume to be exhaled * Gain Exp )
##EQU00001##
[0139] and from X to the end of the expiratory phase (t.sub.1):
Pexhalation=Pbase-(Volume.sub.to be exhaled*Gain.sub.Exp)
where: "t" is a current time following the start of the expiratory
phase; and "X" is a predetermined transition point after the start
of the expiratory phase determined by time, or analysis of the flow
signal, such as curvature, percent drop from peak flow rate,
integration, derivative, analysis of prior breaths or a combination
of flow analysis and time.
[0140] As shown in FIG. 14, the value of X is chosen so that the
pressure provided to the patient during the initial period from
t.sub.0 to X is calculated taking into consideration the dampening
factor t/X, thereby reducing the pressure drop at the onset of
exhalation. Thereafter, the dampening factor is not taken into
consideration and the exhalation pressure to be applied to the
patient is calculated according to the second of the above two
equations. Thus, this embodiment of the present invention provides
a smooth pressure curve throughout the expiratory phase while
ensuring that the initial pressure drop at the start of the
exhalation is within expectable parameters.
[0141] It is to be understood that any other dampening technique
for smoothing the size of the initial pressure drop at the start of
the expiratory phase can be used in this embodiment. Thus, the
present invention is not limited to the dampening technique
discussed above.
[0142] It can be appreciated that, in one embodiment of the present
invention, the pressure to be delivered to the patient
(Pinhalation, Pexhalation) determined using the above-described
techniques, is substantially continuously determined during the
patient's breathing cycle. Thus, the system is continuously
calculating a new setpoint pressure to be delivered to the patient
and controlling the pressure support system via the pressure
controller to reach that setpoint pressure. In a further embodiment
of the present invention, the setpoint pressures (Pinhalation,
Pexhalation) determined using the above-described techniques are
filtered using a low pass filter. This is done to smooth the
transition from one calculated setpoint pressure to the next. In a
preferred embodiment of the present invention, the low pass filter
has a 50 msec time constant.
[0143] In a still further embodiment of the present invention, the
rate of change for the setpoint pressure delivered to the patient
is monitored and controlled to prevent sharp changes in the
delivered pressure. During PPAP therapy, the limits on rate of
change of the setpoint are limited to keep changes in the pressure
to be delivered to the patient more comfortable for the patient.
Rapid changes in the setpoint pressure may impact the patient's
natural waveform. By limiting this impact, the result is a more
comfortable therapy, especially in the PPAP-CPAP combination. In
addition, increasing the pressure to be delivered to the patient
too quickly may cause the undesirable effect of an apparent early
transition from expiration to inspiration.
[0144] The present invention contemplates controlling the rise
rate, i.e., any increase in pressure, and/or the fall rate, i.e.,
any decrease in pressure, so that the rise rate and/or the fall
rate is held within set limits. For example, the present invention
contemplates that increases in the setpoint pressure to be
delivered to the patient be made slower at the end of expiration.
Thus, the rise rate is more limited at the end of the expiratory
phase than the rate at which fall rate is limited at the start of
the expiratory phase.
[0145] In an exemplary embodiment of the present invention, the
rate of change is controlled as follows. For positive changes in
pressure, if the current calculated setpoint pressure is greater
than the previous calculated setpoint pressure plus a rise rate
limit, the current setpoint pressure is set to the previous
setpoint pressure plus the rise rate limit, otherwise the current
calculated pressure is used as the setpoint pressure. For negative
changes in pressure, if the current calculated setpoint pressure is
less than the previous setpoint pressure plus a fall rate limit,
the current pressure to be delivered to the patient is set to the
previous setpoint pressure plus the fall rate limit. Otherwise, the
calculated setpoint pressure is used. This feature of the present
invention can be applied to the calculated Pinhalation setpoint
pressure or the calculated Pexhalation setpoint pressure to ensure
that pressure changes during either or both phase of the breathing
cycle do not occur too sharply.
[0146] It can be appreciated that the setpoint pressure
(Pinhalation or Pexhalation) need not be calculated continuously.
For example, the setpoint pressure can be calculated using the
above-described PPAP techniques during different stages of the
respiratory cycle. In which case, filtering other rate of change
control techniques can be used to smooth the transition from one
setpoint pressure to the next.
[0147] Although the invention has been described in detail for the
purpose of illustration based on what is currently considered to be
the most practical and preferred embodiments, it is to be
understood that such detail is solely for that purpose and that the
invention is not limited to the disclosed embodiments, but on the
contrary, is intended to cover modifications and equivalent
arrangements that are within the spirit and scope of the appended
claims.
* * * * *