U.S. patent application number 10/575197 was filed with the patent office on 2007-06-14 for methods and apparatus for heart failure treatment.
Invention is credited to David John Bassin, Michael Berthon-Jones, Glenn Richards, Anthony John Ujhazy, Jonathan Cadwell Wright.
Application Number | 20070135724 10/575197 |
Document ID | / |
Family ID | 36581245 |
Filed Date | 2007-06-14 |
United States Patent
Application |
20070135724 |
Kind Code |
A1 |
Ujhazy; Anthony John ; et
al. |
June 14, 2007 |
Methods and apparatus for heart failure treatment
Abstract
Methods and apparatus for assessing the condition of and
treating patients for heart failure by the delivery of continuous
positive airway pressure are disclosed. Treatment of obstruction
due to reflex vocal cord closure often experienced by heart failure
patients is distinguished from treatment of upper airway
obstruction typically associated with Obstructive Sleep Disorder.
Treatment may also be implemented by delivering synchronized
cardiac pressure oscillations superimposed on a respiratory
pressure level to provide assistance for the heart. Heart treatment
pressure dose indicator may be calculated for prescribing and
monitoring the delivery of treatment. The apparatus may also
generate data to track heart failure condition that may be
indicative of the degree of severity of heart failure based upon
breathing patterns to assist in the diagnosis and management of
heart failure patients.
Inventors: |
Ujhazy; Anthony John; (New
South Wales, AU) ; Wright; Jonathan Cadwell; (New
South Wales, AU) ; Richards; Glenn; (New South Wales,
AU) ; Bassin; David John; (New South Wales, AU)
; Berthon-Jones; Michael; (New South Wales, AU) |
Correspondence
Address: |
GOTTLIEB RACKMAN & REISMAN PC
270 MADISON AVENUE
8TH FLOOR
NEW YORK
NY
100160601
US
|
Family ID: |
36581245 |
Appl. No.: |
10/575197 |
Filed: |
October 15, 2004 |
PCT Filed: |
October 15, 2004 |
PCT NO: |
PCT/AU04/01420 |
371 Date: |
July 31, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60512553 |
Oct 17, 2003 |
|
|
|
Current U.S.
Class: |
600/529 |
Current CPC
Class: |
A61M 2230/40 20130101;
A61M 2016/0036 20130101; A61M 16/024 20170801; A61M 16/06 20130101;
A61M 16/0051 20130101; A61B 5/4836 20130101; A61M 2016/0021
20130101; A61M 16/0069 20140204; A61B 5/0826 20130101; A61M 2230/10
20130101; A61B 5/7282 20130101; A61M 16/0057 20130101; A61M
2230/005 20130101; A61M 2205/3344 20130101; A61M 16/0066 20130101;
A61M 2230/63 20130101; A61M 2230/04 20130101; A61M 2230/435
20130101; A61B 5/087 20130101; A61M 2230/18 20130101; A61B 5/746
20130101 |
Class at
Publication: |
600/529 |
International
Class: |
A61B 5/08 20060101
A61B005/08 |
Claims
1-120. (canceled)
121. An apparatus for evaluation of heart failure in a patient
comprising: a means for supplying a controllable level of
breathable gas to a patient at a pressure above atmospheric; a flow
sensor to generate a flow signal indicative of the patient's
airflow; and a controller to process said flow signal and control
said means for supplying wherein said controller is adapted and
configured for: delivering breathable gas at a pressure above
atmospheric to a patient with said means for supplying; and
calculating a heart failure indicator from said flow signal, said
heart failure indicator representing information about the
patient's condition.
122. The apparatus of claim 121 wherein said calculating includes
analyzing said airflow to determine an extent of Cheyne-Stokes
breathing of the subject.
123. The apparatus of claim 122 wherein the controller is further
configured and adapted for prompting for heart failure monitoring
characteristics and recording said heart failure monitoring
characteristics and said heart failure indicator in a memory.
124. The apparatus of claim 123 wherein one of said heart failure
monitoring characteristics is a level of B natriuretic peptide.
125. The apparatus of claim 122 wherein the controller is further
configured and adapted for controlling a step of identifying
subsequent heart failure treatment based at least in part upon said
heart failure indicator.
126. The apparatus of claim 125 wherein said subsequent heart
failure treatment is an increase in the pressure of the breathable
gas.
127. The apparatus of claim 122 wherein the controller is further
configured and adapted for controlling comparing said heart failure
indicator to a prior heart failure indicator determined during a
previous treatment session.
128. The apparatus of claim 122 wherein the controller is further
configured and adapted for reducing said pressure during a detected
episode of Cheyne-Stokes breathing for a predetermined period of
time to permit a determination of said heart failure indicator from
said predetermined period of time such that a pattern of
Cheyne-Stokes breathing can emerge without significant influence
from treatment pressure.
129. The apparatus of claim 122 wherein said calculating comprises
analyzing said airflow to determine a duration of a waxing and
waning cycle.
130. The apparatus of claim 125 wherein said controller is further
configured and adapted for analyzing said heart failure indicator
as a function of a threshold value.
131. The apparatus of claim 125 wherein said indicator is a
function of a measure of ventilation.
132. The apparatus of claim 125 wherein said controller is further
configured and adapted for analyzing said heart failure indicator
to determine a change in said heart failure indicator over
time.
133. The apparatus of claim 132 wherein said change is a difference
between a previous heart failure indicator and a subsequent heart
failure indicator.
134. The apparatus of claim 128 wherein said change is a ratio of a
previous heart failure indicator and a subsequent heart failure
indicator.
135. The apparatus of claim 128 wherein said controller is further
configured and adapted for generating a warning signal as a
function of said change from said step of analyzing.
136. The apparatus of claim 135 wherein said warning signal
triggers an audible alarm in said device.
137. The apparatus of claim 122 wherein said calculating comprises
a frequency analysis of said airflow in a range of frequencies
indicative of Cheyne-Stokes breathing cycle.
138. The apparatus of claim 137 wherein said frequency analysis of
said airflow is in a range of about 1/20 hertz to 1/90 hertz.
139. The apparatus of claim 138 wherein said heart failure
indicator includes a magnitude of a component of said airflow at a
frequency in said range.
140. The apparatus of claim 139 wherein said heart failure
indicator is a sum of magnitudes of components of said airflow in a
sub-range of frequencies in said range.
141. The apparatus of claim 140 wherein said frequency analysis of
said airflow is performed with data sampled from a measure of
ventilation derived from said airflow.
142. The apparatus of claim 141 wherein said measure of ventilation
is a minute volume.
143. The apparatus of claim 139 with further instructions for
controlling a step of comparing said magnitude with a threshold
value.
144. The apparatus of claim 143 wherein said threshold value is a
magnitude derived from a previous frequency analysis.
145. The apparatus of claim 122 wherein said indicator is a measure
of ventilation.
146. The apparatus of claim 145 wherein said measure of ventilation
is a threshold of about 15 L/min.
147. The apparatus of claim 122 wherein said indicator is a ratio
of a minimum ventilation and a maximum ventilation.
148. The apparatus of claim 147 wherein the minimum ventilation and
maximum ventilation are derived from a measure of minute
ventilation.
149. The apparatus of claim 147 wherein the minimum ventilation and
maximum ventilation are derived from a measure of tidal volume.
150-178. (canceled)
Description
FIELD OF THE INVENTION
[0001] The invention relates to methods and apparatus for
diagnosing, managing and treating congestive heart failure.
BACKGROUND OF THE INVENTION
[0002] It has been estimated that in the United States alone that
almost five million people suffer from congestive heart failure.
Statistics from the American Heart Association also suggest that
new cases of heart failure are diagnosed at a rate of about 500,000
each year. Of the newly diagnosed patients fifty percent are likely
to die within five years from the initial diagnosis. Of course,
these numbers do not account for the number of patients in other
countries who also suffer from heart failure. Given these numbers,
it is clear that congestive heart failure is a significant human
crisis.
[0003] Heart failure is a condition that is characterized by a
reduced ability of the heart to circulate blood through the body.
Typically, an underlying disease, such as high blood pressure
(e.g., hypertension), clogged arteries (e.g., coronary artery
disease), heart defect (e.g., cardiomyopathy, or valvular heart
disease) or some other problem (e.g., diabetes, hyperthyroidism, or
alcohol abuse) will lead to a decrease in circulation over time. As
the heart works less efficiently, its capacity to circulate blood
decreases and the body's requirements for oxygen are not met. The
cardiac muscle tends to enlarge as the heart works harder over time
to compensate for the decrease in efficiency.
[0004] Heart failure may be identified by the phase of the heart
cycle that is particularly associated with the nature of the
circulatory problem. By this identification, two types of heart
failure are known as systolic and diastolic. In systolic heart
failure, the cardiac muscles' ability to contract decreases. This
loss of contraction results in a decrease in the ability of the
heart to force blood through the circulatory system of the body. In
diastolic heart failure, the cardiac muscles' ability to relax
diminishes. As the heart muscles become stiffer, the heart does not
sufficiently fill with blood and thus each subsequent contraction
circulates a lower volume of blood.
[0005] Alternatively, heart failure may be characterized by whether
it stems from problems with the left or right side of the heart.
Left-sided heart failure occurs when the left ventricle does not
sufficiently pump oxygenated blood to the body. Right-sided heart
failure occurs when the right ventricle does not pump adequately,
which leads to fluid build-up in the veins.
[0006] Although the phrase "congestive heart failure" is often used
to describe all types of heart failure including the above listed
types, congestive heart failure is more accurately descriptive of a
symptom of heart failure relating to pulmonary congestion or fluid
buildup in the lungs. This congestion is more commonly a symptom of
systolic and left-sided heart failure. As the efficiency of the
pulmonary system declines, increased blood volume near the input
side of the heart changes the pressure at the alveolar arterial
interface, an interface between the lung capillaries and the
alveolar space of the lungs. The change in pressure at the
interface causes blood plasma to push out into the alveolar space
in the lungs. Shortness of breath ("dyspnea") and general fatigue
are typical perceived manifestations of congestive heart
failure.
[0007] Congestive heart failure ("CHF") is currently classified by
severity. Class I patients have no apparent symptoms and no
physical activity limitations. Class II patients experience some
symptoms during moderate to severe physical activity. Class III
patients suffer symptoms at mild levels of physical activity. Class
IV patients experience symptoms with any form of physical activity
as well as at rest.
[0008] While the only cure to CHF is heart transplant, there are a
number of drug and surgical treatments directed at reducing the
underlying problem that led to the heart failure and/or to
alleviate the symptoms of heart failure. For example, the use of
diaretics is intended to reduce congestion by depleting the body of
fluids. Vasodilators such as ACE inhibitors are used to expand
blood vessels and reduce resistance to blood circulation. Beta
blockers seek to reduce the work load on the heart by normalizing
the rhythm of the heart. Cardiotonic drugs are directed at
increasing the force of the heart's contractions. Surgical
procedures include physical manipulation in an attempt to increase
the internal size of constricted arteries, for example, by balloon
angioplasty or stenting.
[0009] As previously noted, as a consequence of heart failure there
is a decreased flow of oxygen in the circulatory system. This
decease in oxygenated blood through the body has an impact on the
body's respiratory controller. Thus, there appears to be a
relationship between congestive heart failure and respiratory
conditions known as Sleep Disordered Breathing ("SDB"). For
example, it has been noted that 50-60% of heart failure patients
have SDB. In this category of patients, approximately 29% may be
classified as suffering from obstructive sleep apnea, a breathing
condition associated with the cessation or limitation of airflow
due to occlusion usually at the level of the tongue or soft palate.
In addition, 33% of the patients may be classified as suffering
from (a) Cheyne-Stokes respiration, a breathing condition
characterized by waxing and waning breathing patterns or (b)
central sleep apnea, a condition involving a cessation of airflow
due to a cessation of patient respiratory effort. For those
patients suffering from Cheyne-Stokes breathing, there is a greater
degree of concern. These patients have a higher mortality rate then
heart failure patients without Cheyne-Stokes breathing.
[0010] Sleep disordered breathing has long been treated by
application of Continuous Positive Airway Pressure ("CPAP"). CPAP
was invented by Sullivan and taught in U.S. Pat. No. 4,944,310.
That patent describes continuous positive airway pressure being
applied to a patient, through the patient's nares, to treat
breathing disorders, including obstructive sleep apnea. It has been
found that the application of pressure which exceeds atmospheric
pressure, typically in the range 4 to 15 centimeters of H.sub.2O,
is useful in treatment. The pressure acts as a pneumatic splint to
maintain upper airway patency to ensure free flow of air while the
patient sleeps.
[0011] In one form, nasal CPAP treatment of Obstructive Sleep Apnea
("OSA") involves the use of an automated blower, such as the
AUTOSET T.TM. device or AUTOSET SPIRIT.TM. available from ResMed
Ltd., to provide a constant supply of air or breathable gas at
pressures in the range of 4 to 20 cm H.sub.2O to the airway of a
patient via a mask. Examples of suitable nasal CPAP masks are the
MIRAGE.TM. nasal mask and the MIRAGE.TM. full face mask also
available from ResMed Ltd. The AUTOSET T.TM. device continuously
monitors the state of the patient's airway and determines an
appropriate pressure to treat the patient, increasing it or
decreasing it as necessary. Alternatively, bilevel pressures are
delivered to the patient as in the VPAP II.TM. devices also
available from ResMed Ltd. Some of the principles behind the
operation of the AUTOSET T.TM. and VPAP II.TM. devices are
described in U.S. Pat. No. 5,704,345. The entire disclosure of U.S.
Pat. No. 5,704,345 is incorporated herein by reference.
[0012] One form of pressure treatment is delivered in accordance
with a smooth pressure waveform template and a continuous phase
variable to provide comfortable pressure support substantially in
phase with the patient's respiratory cycle. The device is the
subject of International Publication No. WO 98/12965. The device is
also the subject of U.S. patent application Ser. No. 08/935,785,
now U.S. Pat. No. 6,532,957, the entire disclosure of which is
hereby incorporated by reference.
[0013] Another form of pressure treatment is directed at treatment
of Cheyne-Stokes breathing. In a device designated AUTOSET CS.TM.,
also provided by ResMed Ltd., pressure support is varied in phase
with patient respiration in such a manner to oppose the waxing and
waning changes in patient respiration that characterize
Cheyne-Stokes breathing. The device is the subject of International
Publication No. WO 99/61088. The device is also the subject of a
U.S. patent application Ser. No. 09/316,432, now U.S. Pat. No.
6,532,959, the entire disclosure of which is incorporated herein by
reference.
[0014] At present, there are no known devices with features
designed to treat a range of symptoms of heart failure patients
through application of pressure as opposed to devices that might
only incidentally provide such benefits. U.S. Pat. No. 5,794,615
teaches a device to provide a level of pressure support to reduce
cardiac pre-load and after load. However, the device is only taught
to provide one continuous level of pressure during inspiration and
another level during expiration. The patent does not suggest the
provision of a waveform of cardiac pressure oscillations in phase
with a patient's cardiac rhythm, a feature of the present invention
as described below. Moreover, the device provides no assistance
directed to alleviating Cheyne-Stokes breathing or distinguishing
between obstructions due to vocal cord reflex and obstructions from
typical OSA due to collapse of the upper airway during sleep.
[0015] Any reference herein to known prior art does not, unless the
contrary indication appears, constitute an admission that such
prior art is commonly known by those skilled in the art to which
the invention relates, at the priority date of this
application.
SUMMARY OF THE INVENTION
[0016] It is an objective of the invention to provide methods and
apparatus for managing the treatment of respiratory disorders in
congestive heart failure patients.
[0017] It is a further objective to provide methods and apparatus
that assist in the identification or diagnosis of heart failure to
assist with treatment of the patient.
[0018] Other objectives will be apparent to those skilled in the
art from a review of the description of the invention as contained
herein.
[0019] The invention provides methods and apparatus for detecting
reflex vocal cord closure. The vocal cord closure detector derives
a measure indicative of the closure. Preferably, the measure is
indicative of a state of sleep and may be derived from respiratory
airflow of the patient as a function of a minute ventilation and an
elapsed time. The delivery of positive airway pressure treatment is
controlled as a function of the measure. In the preferred
embodiment of the invention, any apnea or obstructive event
detected before about 30 minutes of sleep is determined to be vocal
cord closure and treatment levels are not increased. Apneas
detected after these thresholds are met and treated as a non-vocal
cord obstructive apnea by an increase in pressure. Alternatively,
reflex vocal cord closure may be detected by distinguishing an
incident of vocal cord closure from another type of airway
obstruction based on a derived measure indicative of vocal cord
closure. The step of distinguishing may include detecting an
obstructive event and conditioning an increase in treatment
pressure in response to the detected obstructive event by an
analysis of the derived measure of the closure. This analysis may
include a comparison of the derived measure with a time limit of
about 30 minutes. The invention further includes methods and
apparatus for providing a synchronized cardiac waveform to perform
some work of the cardiac organ. The cardiac waveform may be a
square wave or sinusoidal wave in phase with detected cardiac
rhythm and is preferably superimposed with continuous, bi-level or
other oscillatory respiratory treatment pressure that supports the
patient's respiration or maintains an open airway. The cardiac
waveform is preferably delivered with amplitudes in a range of
approximately 1 to 2 cm H.sub.2O.
[0020] The invention also includes a means for calculating a heart
treatment index to regulate or measure the dose of treatment to the
heart from the synchronized oscillations. The measure determines
the index as a function of duration and delivered pressure and
preferably accounts only for time that the treatment oscillations
are actually delivered to the thorax by excluding treatment during
closed airway apnea or periods of high leak. In one embodiment of
the invention, the index is the product of an average pressure and
the duration of treatment.
[0021] Finally, the invention includes methods and apparatus for
assessing heart failure in a patient by calculation or
determination of a heart failure indicator or index. Such an
indicator may be determined from respiratory airflow by assessing
an extent of Cheyne-Stokes breathing in the patient. Alternative
embodiments of the indicator include measures of the duration of
waxing and waning cycles or frequency analysis of components of a
measure of airflow in a range of frequencies associated with
Cheyne-Stokes breathing. In addition, an appropriate indicator may
be a measure of minute ventilation compared with a threshold of
about 15 L/min. or alternatively a ratio of a minimum and maximum
of a measure of ventilation, such as a minute ventilation or a
tidal volume. Changes in such indicators taken by comparing or
analyzing indicators from a current session with assigned
thresholds or predetermined threshold values including indicators
from prior sessions provide a diagnostic tool for assessing
improvement or deterioration of the patient's condition.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] An embodiment or embodiments of the present invention will
now be described, by way of example only, with reference to the
accompanying drawings, in which:
[0023] FIG. 1 shows apparatus according to the invention;
[0024] FIG. 2 is a flow chart of an embodiment of the invention for
detecting vocal cord closure;
[0025] FIG. 3 is a flow chart of an embodiment of the invention for
delivering treatment based on the detection of vocal cord
closure;
[0026] FIG. 4 is a graph of one form of a synchronized cardiac
pressure oscillations in accordance with the invention;
[0027] FIG. 5 is a graph of a respiratory treatment pressure
waveform in accordance with the invention;
[0028] FIG. 6 is a graph of superimposed cardiac pressure
oscillations with respiratory treatment pressure;
[0029] FIG. 7 depicts several graphs relating to detection of
Cheyne-Stokes breathing in a patient;
[0030] FIG. 8A depicts a graph of minute ventilation determined
from a flow signal from a patient experiencing Cheyne Stokes
breathing and a graph of a frequency spectrum of the minute
ventilation;
[0031] FIG. 8B depicts a graph of minute ventilation from a patient
experiencing normal breathing and a graph of a frequency spectrum
from the minute ventilation;
[0032] FIG. 9 illustrates a treatment protocol for distinguishing
between vocal cord closure and upper airway obstruction;
[0033] FIG. 10 illustrates a method of detecting obstruction from a
measure of pressure and ventilation;
[0034] FIG. 11 is a flow chart illustrating steps in a methodology
for determining a positive pressure dose measure of the
invention;
[0035] FIG. 12 is a flow chart illustrating steps in a methodology
for determining a heart failure indicator or index.
DETAILED DESCRIPTION OF THE EMBODIMENT OR EMBODIMENTS
[0036] In reference to FIG. 1, the heart failure treatment
invention involves an apparatus that includes a blower 2, a flow
sensor 4f, pressure sensor 4p, a mask 6, and an air delivery
conduit 8 for connection between the blower 2 and the mask 6.
Exhaust gas is vented via exhaust 13. Mask flow is preferably
measured using a pneumotachograph and differential pressure
transducer to derive a flow signal F(t). Mask pressure is
preferably measured at a pressure tap using a pressure transducer
to derive a pressure signal Pmask(t). The pressure sensor 4f and
flow sensor 4p have only been shown symbolically in FIG. 1 since it
is understood that those skilled in the art would understand how to
measure flow and pressure. Flow F(t) and pressure Pmask(t) signals
are sent to a controller or microprocessor 15 to derive a pressure
request signal PRequest(t). The controller or processor is
configured to implement the methodology described in more detail
herein and may include integrated chips, a memory and/or other
instruction or data storage medium. Programmed instructions may be
either coded on integrated chips in the memory of the device or may
be loaded as software.
[0037] The apparatus further includes a communication port or
module 10, for example, a wireless communication transceiver and/or
a network card, for communication with other devices or computers
such as hand-held display and control devices 12. The apparatus
optionally includes an oximeter in the main blower housing. There
is a sense tube 14 connected to the main housing of the blower to
the mask that allows the apparatus to sense oxygen concentration
and pressure levels in the mask 6. The apparatus may further
include additional communications interface 16 for connection to
additional diagnosis devices. The diagnosis unit optionally
includes a pulse oximeter 20, respiratory movement sensors 22, EEG
& ECG 24 and/or EOG 25. The unit may also include a set of
electrodes 28 for detecting cardiac rhythm.
[0038] While this apparatus is described as a single unit, it is
understood that a combination of devices and/or computers linked by
any available communications method may be used to accomplish the
goals of the invention. For example, the apparatus can interface
with a variety of hand-held devices such as a Palm Pilot via
wireless communication. With such a device, a physician may, for
example, remotely monitor, analyze or record the status or data
history of a patient or diagnose the severity of the patient's
condition using the device. For example, remote devices may store
heart failure indicators, such as in a database of patient heart
failure recovery information for one or more patients, from data
generated by use of the apparatus. Furthermore, the treatment
program that is being run on the patient can be monitored and
changed remotely. In the event patient data is transmitted over
open networks, the data may be encrypted for purposes of patient
confidentiality.
[0039] The apparatus incorporates various treatment protocols. One
protocol is intended for treating obstructive apneas. Another is
for treating central apneas. An additional protocol is for treating
Cheyne-Stokes breathing. As described in more detail below, the
apparatus determines treatment automatically.
[0040] In one anode, the device provides a generally constant
pressure throughout a breathing cycle, but may vary the pressure in
accordance with indications of partial or complete obstruction of
the airway. One technique for accomplishing this using a
combination of flow limitation and snore measurements is described
in U.S. Pat. No. 5,704,345 (Berthon-Jones). Other known alternative
methods to vary the pressure for delivering CPAP treatment to a
patient to treat obstructive apneas would be recognized by those
skilled in the art and may be utilized as operating modes in the
device.
[0041] In another mode, the apparatus provides a higher pressure to
the mask during the inspiratory portion of the breathing cycle, a
so-called IPAP (inspiratory positive airway pressure), and a lower
pressure to the mask during the expiratory portion of the breathing
cycle, a so-called EPAP (expiratory positive airway pressure). This
may be accomplished by monitoring the respiratory flow to the
patient and defining threshold levels to distinguish between
inspiration and expiration. When flow exceeds a threshold then the
device delivers IPAP. Below a threshold, the device delivers
EPAP.
[0042] Alternatively, the treatment delivered by the apparatus will
smoothly vary in accordance with patient respiration to provide a
smooth pressure waveform. For example, the device calculates a
continuous phase variable to provide support in phase with the
patient's breathing cycle and calculates the pressure to be
delivered in accordance with a pressure waveform template. The
delivery of such pressure is disclosed in U.S. patent application
Ser. No. 08/935,785. Alternatively, pressure may be supplied in
proportion to patient respiratory airflow.
[0043] In another form, pressure support is varied in phase with
patient respiration in such a manner to oppose the waxing and
waning changes in patient respiration that characterize
Cheyne-Stokes breathing. The methodology for such treatment is
disclosed in U.S. patent application Ser. No. 09/316,432.
[0044] While the blower 2 may alternately generate different
pressure levels in accordance with the varying pressure delivery
methods just described, in an alternative version, a near-constant
speed of blower 2 can be maintained and the pressure drops are
achieved by venting with the inclusion of a controllable release
valve. The same apparatus can be used for many different therapies
simply by adjusting the equation that is used to set the speed of
the blower or to manipulate the venting with the release valve.
[0045] In providing these treatment methodologies, an accurate
determination of respiratory airflow is important. Thus, the flow
rate of air to the patient is adjusted to account for the effect of
leak. To this end, leak airflow may be determined by using a method
such as taught in U.S. Pat. No. 6,152,129 (Berthon-Jones), the
entire disclosure of which is incorporated herein by reference.
Other known methods for determining leak may also be used by the
device.
[0046] With such a device, positive pressure ventilation can be
applied in the treatment of heart failure patients as further
described herein. Positive pressure addresses the symptoms of heart
failure patients by (a) providing increased airflow to assist in
drying fluid from the lungs; (b) reducing fluid transfer to the
lungs by increasing the pressure in the alveolar space to offset
the pressure differential across the alveolar arterial interface;
(c) performing some work of the heart to assist with circulation by
reducing the size of the heart to allow the heart to operate more
efficiently as a result of increased pressure in the thoracic
cavity adjacent to the heart or by providing a contracting
assistance or oscillating force in the thoracic cavity, (d)
supporting respiration to provide ventilatory assistance that
compensates or prevents the waxing and waning cycles associated
with Cheyne-Stokes breathing while also providing support to
prevent or treat obstructive events; and (e) performs some portion
of the work of breathing. Further objectives will be apparent to
those skilled in the art based upon the disclosure herein.
[0047] A. Reflex Vocal Cord Closure Detection
[0048] One of the complexities of the design of the operation of
such a device relates to the nature of the breathing difficulties
experienced by CHF patients. As previously noted, CHF patients are
likely to experience Cheyne-Stokes breathing, central apneas and/or
obstructive events. However, the treatment protocol for each may be
distinct. Therefore, a device of the invention is configured to
automate a change of the treatment protocol based upon the
likelihood of the occurrence of the various respiratory and airway
abnormalities particularly associated with heart failure
patients.
[0049] To this end, it has been observed that CHF patients may
experience Cheyne-Stokes breathing while a patient is awake.
Patients may also experience Cheyne-Stokes breathing or central
apneas in earlier stages of sleep particularly stage 1 and stage 2
sleep but not typically during REM sleep. Patients also experience
obstructive apneas due to upper airway collapse. Such collapse is
typically a result of the relaxed state of the patient induced by
sleep. Therefore, these obstructive apneas may occur during the
latter stages of sleep and are more likely to occur during REM
sleep.
[0050] Methods for the detection of obstructive apnea, airway
obstruction and central apnea are disclosed in detail in U.S. Pat.
No. 5,704,345 and are otherwise known in the art. Obstructive
events may be determined by analysis of patient flow to determine
shape factors, flow flattening indices, roundness indices, etc.
Moreover, with a detected apnea, (e.g., a calculated variance
falling below a threshold value) it can be determined whether the
apnea constitutes airway obstruction or an absence of respiratory
effort (i.e., central apnea). In one such technique, when an apnea
is detected as occurring, the apparatus applies an oscillatory
pressure waveform of known frequency and magnitude and assesses the
patency of the airway from the flow that is induced in the airway.
In one form, if the airway is patent during an apnea, then the
apnea is judged to be central. However, if the airway is closed
during an apnea, then the apnea is judged to be obstructive. In
another technique, when an apnea is detected as occurring, the
apparatus monitors the airflow for the presence of a signal of
cardiac origin. If a cardiac signal is detected, then the airway is
judged to be patent and the apnea classified as central. If no
cardiac signal is detected, then the airway is judged to be closed
and the apnea classified as obstructive.
[0051] Other methods for distinguishing between central and
obstructive apneas include monitoring chest movement to detect
physical respiratory effort using respiratory bands or monitoring
the movement of the suprasternal notch, for example, as taught in
International Patent Application WO 01/19433 (Berthon-Jones et
al.), also taught in U.S. patent application Ser. No. 08/396,031,
the disclosure of which is hereby incorporated by reference. When
there is no respiratory effort when an apnea is detected, it may be
considered a central apnea rather than obstructive.
[0052] An alternative method for determining the existence of
obstructive events involves an analysis of the relationship between
a measure of ventilation and changes in pressure support. For
example, if a measure of minute ventilation does not increase or
remains the same when support pressure is increased, this would
indicate that the patient is experiencing an obstructive event.
Thus, the failure of the measure of ventilation to increase in
relation to increases in pressure support would tend to indicate
that the patient's airway is obstructed. In such a methodology, the
measure of minute ventilation is monitored to detect a decrease in
the minute ventilation. When pressure ventilation is increased to
compensate for the decrease in the measure of minute ventilation,
if the measure of minute ventilation does not increase in this
general time frame, the device interprets the condition as
detecting an incident of obstruction. This method is illustrated in
the graphs of FIG. 10. The graphs plot a continuously determined
minute ventilation (e.g., the volume of air inspired by the patient
during the previous 60 seconds) and pressure delivered at the mask
with respect to a common time scale. An obstructive event is
determined when, after a decrease in minute ventilation shown at
100, there is no increase in minute ventilation corresponding to an
increase in pressure 102. While the graph illustrates the method
during delivery of a relatively constant CPAP pressure, those
skilled in the art will recognize that the method may be
accomplished in the presence of bi-level treatment or other
pressure support which synchronizes smooth and comfortable pressure
changes with the patients respiratory cycle by monitoring changes
in end expiratory pressure.
[0053] Heart failure patients suffering from Cheyne-Stokes
breathing are likely to suffer from reflex vocal cord closure in
the initial stages of sleep when PCO2 in the blood is low,
approximately between 1 and 30 minutes into sleep. As stable sleep
is entered and partial pressure of carbon dioxide (PCO2) increases,
the likelihood of these events diminishes. Vocal cord closure may
generally be considered an obstructive event that may be detected
in the manner that upper airway occlusions at the level of the
tongue or soft palate are detected. Existing methods of obstruction
detection as previously mentioned will detect both upper airway
collapse and vocal cord closure but they cannot distinguish between
these events. Due to the dangerous and counter-productive levels of
pressure that would be required to open vocal cord closure (about
60 to 70 cm H.sub.2O or higher), the vocal cord event preferably is
not treated like that of typical obstructive apneas, i.e., by
increasing pressure, such as the end expiratory pressure, until the
obstruction is opened. While vocal cord events may be treated by
the same levels of pressure as other obstructive events, the
treatment levels would not likely open the closure. Rather, such
treatment is only likely to disturb the patient's sleep and prevent
development of more stable deeper sleep. For these reasons, no
increase in the treatment of reflex vocal cord closure is
preferred.
[0054] Accordingly, in determining the appropriate treatment
protocol, a device of the invention preferably estimates or
approximates whether the patient is awake or asleep, e.g., in some
stage of sleep, and thus distinguishes between vocal cord closure
and more typical obstructive events associated with sleep apnea.
General steps in such a methodology are summarized in the flow
chart of FIG. 2. In a delivering step 30, a controlled supply of
breathable gas at a pressure above atmospheric is supplied to the
patient. In a deriving step 32, a flow derived measure indicative
of a vocal cord closure in a patient is determined. In a detecting
step 34, an incident of vocal cord closure is detected as a
function of the indicator.
[0055] With such an indicator, the device then selects between
different treatment regimes. For example, initially, Cheyne-Stokes
breathing and central apneas are treated while the patient is awake
or in the early stages of sleep by delivering variations in
pressure in phase with patient respiration to meet a target
ventilation. During this treatment period obstructive events are
ignored. Alternatively, obstructive events may be detected by
observing an absence of or substantial decrease in airflow but
those obstructions that are likely to be vocal cord closure are
preferably not treated according to the indicator. After satisfying
a threshold comparison with the indicator, subsequent obstructive
events that are detected will be treated by an increase in
pressure, such as, by increasing end expiratory pressure. Such a
methodology is illustrated in the flow chart of FIG. 3. In a
detection step 36, airway obstruction is detected. In an evaluation
step 38, an indicator of vocal cord closure is determined. Finally,
in a treatment step 40, treatment is determined as a function of
the indicator. If vocal cord closure is detected then pressure is
maintained at the current level or decreased. If vocal cord closure
is not detected then pressure is increased as in the case of a
typical obstruction. Appropriate pressure responses to typical
obstructive events is described in more detail in U.S. Pat. No.
5,704,345.
[0056] One such indicator relates to a measure of ventilation, for
example, a minute ventilation, i.e., the volume of measured airflow
over the course of a minute. When a ventilation measure falls below
a certain threshold, the indicator may suggest that the patient is
in a later stage of sleep. To this extent, it will serve as an
indicator to distinguish between obstructive events of vocal cord
closure as opposed to tongue and soft palate closure. For example,
if the minute ventilation, preferably averaged over a period of
time, e.g., five minutes, is in a range of about 5 to 10 liters per
minute, the patient is likely in a later stage of sleep. The
accuracy of the indicator may be improved by making the threshold
determination an additional function of time. For example, if the
measure of ventilation is below a certain level and a period of
time has elapsed, such as about 30 minutes, the patient is more
likely to be in a later stage of sleep. In the preferred
embodiment, if the mask has been on the patient for more than about
30 minutes and the minute ventilation averaged over a period of
about five minutes is less than about 12 liters per minute, a later
stage of sleep is indicated. This would also indicate that any
detected obstructive events are of the more typical upper airway
collapse of traditional obstructive sleep apnea rather than reflex
vocal cord closure.
[0057] Another alternative indicator may be based upon the pressure
swing. Swing is the difference between inspiration and expiration
pressure levels as delivered by the apparatus in keeping with the
effort of the patient's respiratory cycle. Typically, pressure
swing is in the range of about 3 to 10 cm H.sub.2O. Higher pressure
swings in the range near about 10 cm H.sub.2O may be indicative of
Cheyne-Stokes breathing. Thus, if swing is lower than that range
for a period of time, e.g. about ten minutes, it would tend to
indicate that the patient has settled into a later stage of sleep.
Therefore, as the swing stabilizes, i.e., approaches a threshold,
for example, about 8 cm H.sub.2O or less, and maintains that
threshold for a certain time, the swing will be indicative of a
later stage of sleep.
[0058] In a more simplified embodiment, a measure of time may serve
to detect vocal cord closure by distinguishing between obstructive
apneas and vocal cord closure based on this measure. In this
embodiment, any detected obstructive events prior to the expiration
of a period of time are considered to be vocal cord closure and not
treated. For example, for any obstructive event detected before
expiration of a time period of about 30 minutes from some start
event, such as the initiation of a treatment session (e.g., when
the machine is turned on or when the mask pressure first raises
above ambient pressure) or by some other resetting or starting of a
timer, these detected events would be considered vocal cord
closure. Those events that are detected after expiration of the
time period would then be considered treatable upper airway
obstruction. In view of the preference to treat upper airway
obstruction, rather than vocal cord closure, an alternative
methodology may simply abstain from detecting obstructive events
prior to the expiration of the time period. In this methodology,
all events after the expiration of the time period are determined
to be treatable upper airway obstructive events.
[0059] A preferred treatment protocol for responding to obstructive
events in a manner that distinguishes between upper airway
obstructive events and vocal cord closure is illustrated in FIG. 9.
During a period of early sleep, e.g. about 30 minutes from starting
use of the apparatus, obstructive events detected by any method are
determined to be vocal cord closure. No increase in pressure from
an initial pressure setting is permitted. During the latter stages
of sleep, e.g. after about 30 minutes, detected obstructive events
are treated by a step up in end expiratory pressure (EEP), for
example, about 1-2 cm H.sub.2O, which may be fixed or user
selectable. Optionally, additional increases in pressure for
detected obstructive events may be limited by the expiration of
additional time periods. For example, a subsequent step in the EEP
upon detection of another obstructive event would not be permitted
until after about 10 minutes. Furthermore, increases in the EEP
would not be permitted beyond a maximum pressure level. In the
preferred embodiment, the initial EEP pressure is 5 cm H.sub.2O and
increases in pressure step about 2 cm H.sub.2O and these steps are
permitted after about 30 minutes have elapsed from the time that
the patient has begun using the mask. Additional steps are
permitted at about ten minute intervals thereafter. The limitation
imposed by the additional intervals can be phased out after a
sufficient time has elapsed which will substantially suggest that
the patient is fully asleep, e.g., after about 1 to 2 hours of
sleep, preferably after 100 minutes.
[0060] While vocal cord closure may also be determined by an
insertable camera proximate to the patient's vocal cords, due to
issues of comfort and equipment cost it is preferred to determine
an indicator of closure as described above from a measure of
respiratory airflow as a function of time. Of course, additional
indicators may be based upon sleep data from EEG signals
(electroencephalography) and/or EOG signals (electro-oculographic)
or any other equipment used to determine sleep. Those skilled in
the art would understand the nature of data from these devices for
the purpose of distinguishing between the various stages of sleep
that a subject will experience.
[0061] B. Positive Pressure Dose Measure
[0062] In order to manage administration of the treatment of the
heart in the presence of positive pressure, for example, by
delivering bilevel CPAP treatment, the preferred device calculates
an index that represents the dose of treatment for the heart. With
such a dose index, a physician may prescribe a certain quantity of
treatment. Compliance then can be monitored by the pressure
treatment apparatus so that the physician and patient can be
certain that a treatment regimen is being satisfied.
[0063] Steps in such a methodology are illustrated in the flow
chart of FIG. 11. In a delivery step 1102, a supply of breathable
gas is delivered at a pressure above atmospheric to the airway of
the patient. In a control step 1104, the pressure is controlled to
perform work of the heart of the patient. In a determination step
1106, a heart treatment index representative of a dose of heart
treatment experienced by the patient's heart is determined.
[0064] It is desirable to have such an index as a function of time
and a function of pressure to assess the number of pressure hours
received by the patient. Thus, the index may be viewed as having a
pressure component and a time component. The preferred pressure
component of the index is an average value of the pressure. Thus,
the average pressure taken over the time period of treatment
multiplied by the length of the time period can serve as a measure
of dose. For example, if the average pressure delivered to the
patient during a 5 hour treatment period is 7 cm H.sub.2O, the dose
is 35 cm H.sub.2O--hours. In this embodiment of the does index,
higher treatment pressures are weighted more than lower treatment
pressures for purposes of determining compliance. Thus, a patient
receiving an average 9 cm H.sub.2O of support 7 pressure would
satisfy the 35 cm H.sub.2O --hours dose in approximately 3.9
hours.
[0065] Due to the breathing patterns experienced by CHF patients
(i.e., Cheyne-Stokes breathing, partial obstruction, etc.) and the
likely swings in pressure that are a result of treatment of such
events, the index may be derived as the root mean square value of
the pressure delivered during the treatment period. Such an index
will more accurately provide an indicator of the pressure delivered
given the pressure swings when compared with, for example, a median
pressure.
[0066] With regard to the time component of the index, since CHF
patients are likely to experience airway obstruction either due to
a typical obstructive apnea or reflex vocal cord closure, the total
time duration that the device delivers pressure support is not
necessarily sufficient to accurately measure the treatment actually
experienced by a subject's heart. Rather, it is preferred to
consider the time period during which pressure is actually
delivered to the thorax by excluding periods of closed airway
obstruction. To this end, the preferred heart failure treatment
dose index is a measure of delivered pressure during a time period
that excludes duration of obstructive apneas and reflex vocal cord
closures.
[0067] Accordingly, in the preferred embodiment, the dose index is
a product of the average of the delivered pressure and an estimate
of the number of hours that pressure is delivered to the thorax.
Thus, the total number of hours of machine use is reduced to
exclude the total time associated with airway obstructive events as
these events are detected during treatment. For this purpose, the
device quantifies periods of detected obstruction from the start of
each obstruction through a point when the obstruction is
alleviated. For example, an obstruction timer may commence when an
obstructive apnea is detected if a calculated airflow variance
falls below a threshold value. The obstruction timer will continue
to track time until the calculated variance rises again above the
threshold value. Of course, the time of the event may be excluded
if it is determined to be a central apnea since the thorax will
still be treated during a central apnea. Those skilled in the art
will recognize other methods for quantifying the time period of the
obstructive events that prevent pressure treatment of the thorax.
Similarly, during the period of obstructive apnea, the
determination of the average pressure may be suspended so that the
pressure readings during this period do not affect the average
pressure calculation.
[0068] Optionally, time periods of high or significant mask leak
may similarly be excluded from the computation of thorax treatment
time and average pressure. Such periods may be considered
ineffective treatment. For example, the volume of measured airflow
taken over a single breath cycle may be compared to a threshold
value to determine if significant or high leak exists. Thus, the
time period from such a comparison indicating high leak until the
time that the comparison no longer indicated high leak could be
excluded from the total treatment time and average pressure
calculation. Those skilled in the art will recognize other methods
for detecting leak for purpose of excluding periods of leak from
the dose computation.
[0069] With such a device, treatment can be prescribed and
delivered based on the dose index. For example, a pressure
treatment device may optionally be implemented to accept as input a
prescribed dose as a set-point index or prescribed threshold before
use of the device. Then during treatment, the actual delivered dose
is calculated and compared to the prescribed threshold to assess
whether the actual delivered does satisfies the prescribed
threshold. When the threshold is reached, the device may
automatically cease delivery of pressure treatment. Preferably, the
extent of the compliance with the prescribed dose may be recorded
in the device for use by or transmission to the patient's
physician. In one embodiment, the device may indicate that the dose
has been achieved by generating a message, warning or alarm to the
user or physician to advise the user that the dose has been reached
and no further treatment is required.
[0070] C. Cardiac Pressure Oscillations
[0071] As previously noted, positive pressure can be applied to a
subject's respiratory system to provide an oscillating force in the
thorax proximate to the wall of the heart to assist with the work
of the heart. To this end, positive pressure may be supplied to the
subject's lungs to cause compression on the heart by increasing
pressure during the systolic phase when the cardiac muscles
contract. By reducing delivered pressure during the diastolic
phase, treatment will more readily allow the cardiac muscles to
relax.
[0072] These changes in pressure amplitude are chosen to have some
effect on the heart without causing the subject discomfort. In the
preferred embodiment, a cardiac pressure waveform with peak
amplitude in the chosen range is superimposed on a level of
positive pressure delivered in accordance with CPAP, bi-level
pressure support or other pressure support variant such as the
pressure delivered in accordance with a smooth pressure waveform
template as disclosed in U.S. patent application Ser. No.
08/935,785. The cardiac treatment pressure waveform cycles to
increase and decrease in phase with the cycle of the heart. One
such synchronized cardiac waveform is illustrated in FIG. 4.
Preferably, the peak pressure amplitudes associated with the
systolic phase are in a range of about 1 to 4 cm H.sub.2O. Although
FIG. 4 depicts a clipped sinusoidal waveform, other waveform shapes
may be used. For example, an oscillatory square wave may also serve
to provide support for the contractions of the heart.
[0073] In order to synchronize the oscillations, the device
determines the phase of the heart and triggers the cardiac pressure
oscillations. In the preferred embodiment, cardiac rhythm is
detected from an output signal from either an electrocardiogram
(ECG) or a set of electrodes. Such electrodes each include a signal
wire from the device which is attached to a metallic or otherwise
conductive skin contact that can detect an electric charge.
Electrical current generated by the heart in a person's chest flows
to the surface and at the skin produces differences in electrical
voltage which can be measured between pairs of electrodes placed at
two points on the skin. Data from such electrodes is analyzed to
time the delivery of each increase in pressure to generate the
cardiac pressure waveform. Alternatively, cardiac rhythm may be
determined from an airflow signal as described in U.S. Pat. No.
5,704,345 or otherwise estimated from the patient's pulse by an
automated pulse rate detector on the wrist or finger of the
patient.
[0074] An example of the superposition of a cardiac pressure
waveform and a respiratory pressure level or waveform is
illustrated with reference to FIGS. 4, 5 and 6. The waveform shown
in FIG. 5 depicts a hypothetical bi-level respiratory treatment
pressure waveform with an IPAP generated for the inspiratory
portions of a subject's respiratory cycle and an EPAP generated for
the expiratory portions of the subject's respiratory cycle. The
waveform depicted in the graph of FIG. 6 illustrates the
superposition of the synchronized cardiac treatment pressure
waveform of FIG. 4 with the respiratory treatment pressure waveform
of FIG. 5.
[0075] D. Congestive Heart Failure Indicator
[0076] Another difficulty involving the treatment of heart failure
patients relates to disease management. There are currently no
known methods for accurately and continuously assessing a degree of
severity or a degree of change in the patient's condition to assist
care providers in predicting whether the patient is improving or
not in response to a particular treatment regime. For example, when
a physician treats heart failure with a particular drug, there is
often insufficient information concerning whether the prescribed
dose is particularly effective for the patient.
[0077] Accordingly, the preferred device determines or calculates
one or more heart failure indicators or indices to indicate a
change in heart failure condition or to rate a degree of severity
of the heart failure condition. The changing value of such an
indicator or index may provide a diagnostic tool for the physician
to assess the state of the patient's health. For example, the
indicator may provide information to inform the physician that the
dosage of pharmacological agents given to the patient ought to be
changed. If the index indicates that heart failure was stabilizing
then it may be appropriate to reduce or maintain the dosage in use.
Optionally, in accordance with an assessment of the heart failure
indicator in which the indicator suggests that the patient is
destabilizing, the device may begin to provide a specific dose of
superimposed cardiac oscillations to perform some work of the
patient's heart as previously described.
[0078] Alternatively, the index may be used to monitor the efficacy
or effectiveness of a drug protocol. For example, the index may be
monitored for a group of patients. This may include the storing of
multiple indices in a database of patient information. By an
analysis of such data, it may be determined that a drug is safe
and/or appropriate as a treatment for heart failure in general.
[0079] Moreover, an index determined in accordance with the
invention may be used by a physician in conjunction with other
known methods of analyzing the health of the patient. For example,
an index or indicator in accordance with the invention may be used
in conjunction with changes in weight, medication dosage or lung
fluid levels to detect changes in a subject's condition. Such an
index may also be part of a battery of indicators for diagnosing
whether or not a patient is suffering from heart failure in the
first instance. For example, levels of B Natriuretic Peptide (BNP),
a protein present in the blood that is secreted by heart muscle
that is failing, which may be determined by a blood test and
recorded by the apparatus and associated with the periods of use of
the device as well as the indicators determined by the device.
[0080] Optionally, the apparatus prompts for input from a user so
that the user can enter weight changes, medication dosage, number
of apneas or hypopneas, or other heart failure monitoring
characteristics, such as levels of BNP. Thus, the device also
serves as a database for recording heart failure monitoring
characteristics. The device then may periodically transmit such
data relating to the patient's condition to a centralized system
for physician analysis. Alternatively, transmissions of such data
may be performed on physician prescribed times or intervals or
based on certain event criteria being met, such as a transmission
trigger based upon recorded data meeting certain thresholds (e.g.,
the total number of apneas or hypopneas exceeding a certain
threshold level or a change in a heart failure indicator compared
with prior indices.)
[0081] The heart failure indices are determined from an analysis of
the patient's breathing characteristics or by the machines'
responses to the patient's breathing patterns. The indices may be
determined in conjunction with a protocol for delivering treatment
pressure or without such treatment pressure, for example, by simply
monitoring patient respiratory airflow. Such indices serve as heart
failure indicators to show patient improvement or relapse as
detailed below. Such indicators may be recorded over various
sessions with the device. In subsequent sessions, a current
indicator or an average of such indicators may be compared with an
indicator or average of such indicators from one or more prior
sessions to analyze changes in the indicators.
[0082] Steps in such a methodology for evaluating heart failure in
a patient are illustrated in the flow chart of FIG. 12. In a
delivery step 1202, breathable gas is delivered to the patient at a
pressure above atmospheric. In a measuring step 1204, the
respiratory airflow of the patient is measured. In a determining or
calculating step 1206, a heart failure indicator representing
information about the patient's heart condition is derived from the
respiratory airflow.
[0083] One such index is based upon a measure of the extent of the
subject's Cheyne-Stokes breathing or a so-called "Cheyne-Stokiness"
of the patient. As previously described, heart failure patients
typically experience periods of the waxing and waning defined as
Cheyne-Stokes breathing. The cycle of the waxing and waning
typically will vary in the range of about 30 to 90 seconds, or even
as high as 120 seconds. By examining variations or changes in the
cycle, such as periodicity, it can serve as an indicator of the
degree of severity or change in heart failure condition.
[0084] For example, by measuring the number Cheyne-Stokes cycles
and determining increases or decreases in this number.
Alternatively, the envelope period or duration of each waxing and
waning cycle may be measured and compared to the time for one or
more previous cycles or an average of prior cycles. A decrease may
suggest that the patient's heart failure condition is improving.
Similarly, an increase may suggest that the patient's heart failure
condition is destabilizing. Thus, a current cycle time would be
compared to a threshold, e.g., based on previously determined cycle
times. Alternatively, the difference between current and prior
measurements or a ratio of current and prior measurement may also
serve as such an indicator.
[0085] One method for determining the duration of Cheyne-Stokes
breathing is to determine the start time of a hyperapnea or
hyperventilation and the end time of a subsequent apnea or hypopnea
as shown in respiratory graph 70 of FIG. 7. In one embodiment, the
start of the cycle may be indicated by an increase in a short-term
measure of ventilation, for example, an instantaneous ventilation
or a volume of airflow measured over a period of several seconds or
less. Alternatively, it may be determined from an increase in peak
values of airflow. The duration would then include the time period
of the increase, the time period of a subsequent decrease in the
short-term ventilation measure or decrease in peak airflow, and may
include the duration of a possible period of cessation of airflow.
Other methods for detecting each of these events, i.e., hyperapnea
or hyperventilation, hypopnea and central apnea, are known in the
art.
[0086] Alternatively, taking into account the nature of
Cheyne-Stokes breathing, duration may be determined by identifying
a particular point in one cycle and the similar point in the
subsequent cycle. One such method involves a measure of
instantaneous ventilation and a measure of recent average
ventilation as disclosed in U.S. patent application Ser. No.
09/316,432. By monitoring a measure of instantaneous ventilation
relative to a measure of recent average ventilation or target based
on the average ventilation measure, an estimate of cycle time for
Cheyne-Stokes breathing may be determined. For example, by
monitoring the intersections of an instantaneous ventilation and an
average ventilation and determining the time between two similar
intersections or otherwise measuring the time between two such
intersections, the duration of a Cheyne-Stokes cycle may be
estimated. This method is illustrated in FIG. 7. The figure shows
three graphs. The first graph of respiratory airflow depicts a
number of cycles of Cheyne-Stokes breathing. The second graph shows
a plot of one instantaneous ventilation (Instantaneous Ventilation
1) as measured in accordance with U.S. patent application Ser. No.
09/316,432. The third graph is a plot of another measure of
instantaneous ventilation (Instantaneous Ventilation 2). As shown,
the duration of the Cheyne-Stokes cycle on the respiratory graph 70
may be determined by measuring the duration on the ventilation
graph 72. Thus, a timer may be reset to zero at the moment
instantaneous ventilation changes from less than to greater than
the recent average ventilation and the time elapsed is assessed
when that condition is again true. Alternatively, the time of these
points may be recorded and cycle time maybe derived by calculation
to determine the difference between the recorded times.
[0087] In another embodiment, the heart failure indicator may be an
indication of Cheyne-stokes breathing determined from an average of
a flow signal adjusted to remove leak. For example, if a measure of
ventilation which, for preference, is an average minute ventilation
or inspired volume over a one minute period, exceeds about 15
L/min, it is likely that the patient is experiencing an episode of
Cheyne-Stokes breathing. The total number of these events can be
logged. The changing value of such a heart failure indicators or
indices may provide a diagnostic tool for the physician to assess
the state of the patient condition. Thus, the information may be
recorded over several sessions and increases or decreases in this
number or durations from prior sessions, or averages from prior
sessions, may then be determined, recorded and transmitted to the
physician for review and analysis.
[0088] In one embodiment, the heart failure indicator may be a
measure of ventilation variability. Such a measure may be a ratio
of a maximum and minimum of a measure of ventilation. In patients
experiencing Cheyne-Stokes breathing, a measure of minute
ventilation will vary from a low of about 0 L/min to a high of
about 25 L/min. For non-Cheyne Stokes breathing, patients typically
will only experience changes in minute ventilation in a range of
about 4 to 8 L/min. Accordingly, a heart failure indicator may be
the ratio of the minimum to the maximum, preferably, as follows:
Heart Failure Indicator=Minimum
(Ventilation)/Maximum(Ventilation)
[0089] Notably, as the minimum measure of ventilation may be near
0, it is preferred that the minimum be divided by the maximum to
avoid a divide by zero operation. Moreover, as an alternative, the
indicator may be based on the ratio of minimum and maximum tidal
volumes (i.e., a measure of ventilation of the patient taken during
the inspiratory portion of a respiratory cycle.) For patients
experiencing Cheyne-Stokes breathing, the tidal ventilation may
vary from 0 up to 1.5 or as much as 2.5 liters. Another method for
determining an appropriate heart failure indicator, which is
intended to assess the extent of Cheyne-Stokes breathing, is to
measure a volume of airflow, e.g., a minute ventilation or a
filtered measure of airflow with a time constant of about 10
seconds, and record the resulting waveform or sampled data
representing the waveform over time. At the conclusion of a period
of measurement, such as one treatment episode or various intervals
during one such episode, the data or waveform is analyzed to
isolate the frequencies typically associated with Cheyne-Stokes
waxing and waning periods. This frequency analysis involves a
Fourier Transform such as a DFT or FFT to examine a frequency range
of about 5.times.10-2 Hz to 1.1.times.10-2 Hz since the period for
a typical Cheyne-Stokes cycle is in the range of about 20 to 90
seconds in duration. With such an analysis preferably with emphasis
in a sub-range of 1/30 Hz to 1/60 Hz, a quantitative measure of the
extent of Cheyne-Stokes breathing may be extracted. Since normal
patient respiratory cycles would be reflected at much higher
frequencies, the chosen range would be indicative of Cheyne-Stokes
cycles. An example of a frequency spectrum 80 is shown in FIG. 8A,
depicting a frequency analysis of components of a measure of
airflow in the range of frequencies generally associated with
Cheyne-Stokes breathing. The bottom graph of FIG. 8A illustrates
the existence of Cheyne-Stokes breathing in the minute ventilation
signal of the top graph of FIG. 8A. The bottom graph of FIG. 8B
illustrates the lack of Cheyne-Stokes breathing in the minute
ventilation signal of the top graph of FIG. 8B.
[0090] By observing different distributions or different spreads at
different peaks in the band or by comparing shifting frequency
bands in the frequency range of interest over different periods,
changes in the resultant frequency spectrum will indicate changes
in Cheyne-Stokes breathing. Thus, an embodiment of the invention
quantifies components in the range of Cheyne-Stokes frequencies as
one or more indices and/or generates a visual frequency spectrum of
the Cheyne-Stokes frequencies to show a graphical pattern of the
patient's Cheyne-Stokes breathing. Such indicators may be based
upon analyses or quantifications of the spectrum that include a
determination of a total power, for example, a sum of amplitudes or
magnitudes of components at all frequencies in the entire band of
frequencies or a portion thereof, a determination of magnitudes or
a measure of a peak value at one or more separate frequencies, a
determination of a measure of central tendency or standard
deviation, or similar.
[0091] By monitoring changes in the resulting quantification and/or
pattern shifts the indicators may then serve as indicators of a
change in heart failure condition. For example, if the magnitudes
or patterns indicate that the patient's Chenye-Stokes breathing is
increasing in duration, becoming more frequent or otherwise
increasing in intensity, it may indicate that the patient's heart
failure condition is destabilizing and a different treatment
approach (medicinal or otherwise) to the patient's condition may be
warranted. Alternatively, a decrease in the duration, frequency or
intensity of such breathing may indicate that the current treatment
approach is appropriate. Thus, these quantifications (e.g.,
duration, a magnitude, sum of magnitudes, or standard deviation)
derived during a current session may be compared to a threshold
value. The threshold may be a pre-determined acceptable level or a
quantification derived from a previous analysis in a current
session or from a previous treatment session with the device.
[0092] In one embodiment of the device based upon an analysis of
any of the various indicators previously mentioned or as a function
of a change in such indicators, a warning such as an audible alarm
or other visual signal is generated or this information may be
transmitted to a system for access by a physician. For example, if
the current heart failure indicator including a duration is greater
than a prior indicator or a desired threshold of change in the
indicator, a warning is generated. The alarm or warning is intended
to alert a care provider or user to the potential destabilization
of the patient if a change in the indicator suggests such
destabilization. Alternatively, indicators may be recorded over
time to provide long-term statistical analysis of the patient's
condition subsequent to treatment with the device. For example,
such indicators may be recorded over various sessions with the
device in a database or other information storage medium.
[0093] In one embodiment of the invention, the device may respond
to the onset of one or more periods of Cheyne-Stokes breathing by
reducing pressure support to permit data from a Cheyne-Stokes
breathing event to be recorded in the absence any significant
treatment pressure that might impact or change the nature of the
pattern of breathing. For example, if a Cheyne-Stokes breathing
event is detected, the amplitude of pressure support may be reduced
by about 50% for a particular period of time, for example, about 10
minutes, so that data during the event may be recorded. A limit on
the number of these untreated data collection sessions may also be
implemented so that the patient's Cheyne-Stokes breathing does not
go completely untreated during any treatment session with the
apparatus. For example, the device may record data for three
Cheyne-Stokes events during any single treatment session. This
reduced treatment data collection process may be performed
consecutively for each consecutively detected Cheyne-Stokes event
up to the optional limit. Alternatively, a certain number of
detected Cheyne-Stokes events, one or more, may be fully treated by
an appropriate support pressure response before a subsequently
detected event will be evaluated in a reduced treatment data
collection process. Thus, a full treatment process and reduced
treatment data collection process may alternate over a number of
detected Cheyne-Stokes events.
[0094] In addition to providing a measure for a qualitative
analysis of the patient's heart failure condition, the indicator
may be utilized as an input to an automated pressure adjustment
algorithm that can serve to provide additional treatment for the
heart. For example, the indicator may be utilized to increase end
expiratory pressure. In one embodiment, when the minute ventilation
exceeds a 15 L/min threshold, the pressure may be increased by a
small quantity, for example about 1 cm H.sub.2O. Optionally, any
additional changes to the treatment pressure based on the indicator
would not take place until expiration of a predetermined time
period, such as about 10 minutes.
[0095] Similarly, in the case of the indicator that is determined
as a ratio of minimum and maximum ventilation measures, based on a
comparison of the indicator with a threshold value, the pressure
may be increased by a small degree, e.g., about 1 cm H.sub.2O. For
example, the pressure may be increased as follows: If MIN(tidal
volume)/MAX(tidal volume)<a threshold in a range of about 0.04
to 0.25, preferably about 0.1 to 0.2, for example, about 0.15,
after a period of about 10 minutes then increase the pressure by
about 1 cm H.sub.2O.
[0096] The aforementioned indicators or the measures on which they
are based may be filtered with a time constant of about 5 or 10
minutes. Such an operation permits the filtering out of the
consequences of short term changes in these continuously determined
measures.
[0097] While the invention has been described with various
alternative embodiments and features, it is to be understood that
the embodiments and features are merely illustrative of the
principles of the invention. Those skilled in the art would
understand that other variations can be made without departing with
the spirit and scope of the invention as defined by the claims.
[0098] Where ever it is used, the word "comprising" is to be
understood in its "open" sense, that is, in the sense of
"including", and thus not limited to its "closed" sense, that is
the sense of "consisting only of". A corresponding meaning is to be
attributed to the corresponding words "comprise", "comprised" and
"comprises" where they appear.
[0099] It will be understood that the invention disclosed and
defined herein extends to all alternative combinations of two or
more of the individual features mentioned or evident from the text
or drawings. All of these different combinations constitute various
alternative aspects of the invention.
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