U.S. patent application number 11/421710 was filed with the patent office on 2006-09-14 for ventilatory stabilization technology.
Invention is credited to Eric A. Hajduk, Ronald S. Platt, John E. Remmers.
Application Number | 20060201505 11/421710 |
Document ID | / |
Family ID | 34807545 |
Filed Date | 2006-09-14 |
United States Patent
Application |
20060201505 |
Kind Code |
A1 |
Remmers; John E. ; et
al. |
September 14, 2006 |
Ventilatory Stabilization Technology
Abstract
A system for reducing central sleep apnea (CSA) is described in
which certain methods of increasing a patient's rebreathing during
periods of the sleep cycle are used. By increasing rebreathing
during periods of overbreathing, the over-oxygenation which
typically results from the overbreathing period can be reduced,
thus reducing the compensating underbreathing period and
effectively reducing the loop gain associated with the central
sleep apnea. Nasal occlusion and a leak resistant oral interface
provide control for gas leaks from a patent interface.
Inventors: |
Remmers; John E.; (Calgary,
CA) ; Hajduk; Eric A.; (Calgary, CA) ; Platt;
Ronald S.; (Calgary, CA) |
Correspondence
Address: |
THOMPSON LAMBERT;SUITE 703D, CRYSTAL PARK TWO
2121 CRYSTAL DRIVE
ARLINGTON
VA
22202
US
|
Family ID: |
34807545 |
Appl. No.: |
11/421710 |
Filed: |
June 1, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10762979 |
Jan 23, 2004 |
7073501 |
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11421710 |
Jun 1, 2006 |
|
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09498504 |
Feb 3, 2000 |
6752150 |
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10762979 |
Jan 23, 2004 |
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60118616 |
Feb 4, 1999 |
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Current U.S.
Class: |
128/204.21 ;
128/201.26; 128/201.28; 128/202.22; 128/204.18; 128/204.23;
128/204.26 |
Current CPC
Class: |
A61M 2016/0042 20130101;
A61M 2016/0036 20130101; A61M 2016/103 20130101; A61M 16/026
20170801; A61M 16/085 20140204; A61M 16/0069 20140204; A61M 16/0045
20130101; A61M 2210/0625 20130101; A61M 2230/432 20130101; A62B
18/006 20130101; A61M 2210/0618 20130101 |
Class at
Publication: |
128/204.21 ;
128/204.18; 128/204.23; 128/204.26; 128/201.28; 128/202.22;
128/201.26 |
International
Class: |
A61M 16/00 20060101
A61M016/00; A62B 18/08 20060101 A62B018/08 |
Claims
1. A method comprising: providing an apparatus comprising a blower
and a leak resistant patient interface adapted to be fit on a
patient's airway, the leak resistant patient interface operably
connected using a tube to the blower, the leak resistant patient
interface having an exit; fitting the leak resistant patient
interface to the patient's airway; detecting a breathing disorder
in the patient; and treating the breathing disorder with the
apparatus.
2. The method of claim 1 in which treating the breathing disorder
comprises adjusting the apparatus.
3. The method of claim 2, wherein the adjusting step comprises
adjusting the apparatus to treat obstructive sleep apnea.
4. The method of claim 3, wherein the adjusting step further
comprises adjusting the apparatus to treat central sleep apnea.
5. The method of claim 4 in which, during the adjusting step of
treating central sleep apnea, gas flow from the blower is less than
that used to treat obstructive sleep apnea.
6. The method of claim 5, wherein the adjusting step of treating
central sleep apnea is such that gas pressure from the blower is
set below four cm H.sub.2O pressure.
7. The method of claim 6, wherein the adjusting step of treating
central sleep apnea is such that gas pressure from the blower is
set at two cm H.sub.2O pressure or below
8. The method of claim 4 wherein the adjusting step of treating
central sleep apnea is such that during periods of increased
breathing associated with the breathing disorder, some exhaled
gasses flow from the patient retrograde into the tube.
9. The method of claim 8, wherein the adjusting step of treating
central sleep apnea is done such that during an initial exhale
portion of increased breathing associated with the breathing
disorder, some of the patient's exhaled gasses flow retrograde into
the tube towards the blower and away from the exit and wash flow
out of the tube such that during a next inhale portion some
rebreathing occurs.
10. The method of claim 8, wherein the adjusting step of treating
central sleep apnea is done such that during normal breathing
periods little rebreathing occurs.
11. The method of claim 10, wherein the adjusting step of treating
central sleep apnea is done such that during normal breathing
periods some retrograde flow occurs but wash flow is sufficient to
remove exhaled air before a next inhale portion.
12. The method of claim 8, wherein the retrograde flow into the
tube is influenced by gas pressure from the blower and by the size
of the exit, and gas flow rate from the blower is varied without
significantly affecting leak resistant patient interface
pressure.
13. The method of claim 4, wherein the obstructive sleep apnea
treating step occurs before the central apnea treating step.
14. The method of claim 13 in which the central sleep apnea
treating step occurs after a reduction in obstructive sleep
apnea.
15. The method of claim 1 in which the leak resistant patient
interface comprises a dental appliance and a nasal occlusion
device, and fitting the leak resistant patient interface to the
patient comprises: fitting the dental appliance to the mouth of the
patient; and blocking the patient's nose with the nasal occlusion
device.
16. A method comprising: providing an apparatus comprising a blower
and a leak resistant patient interface adapted to be fit on a
patient's airway, the leak resistant patient interface operably
connected using a tube to the blower, the leak resistant patient
interface having an exit, the resistance of the exit being set that
during treatment of a breathing disorder in the patient, expiratory
air from the patient flows through the tube towards the blower and
away from the exit; fitting the leak resistant patient interface to
the patient's airway; treating obstructive sleep apnea with the
apparatus; and adjusting the apparatus to treat the breathing
disorder.
17. The method of claim 16 in which the leak resistant patient
interface comprises a dental appliance and a nasal occlusion
device, and fitting the leak resistant patient interface to the
patient comprises: fitting the dental appliance to the mouth of the
patient; and blocking the patient's nose with the nasal occlusion
device
18. The method of claim 16 wherein the adjusting step is such that
during periods of increased breathing associated with the breathing
disorder, some of the patient's exhaled gasses flow retrograde into
the tube.
19. The method of claim 18, wherein the adjusting step is done such
that during an initial exhale portion of increased breathing
associated with the breathing disorder, some exhaled gasses from
the patient flow retrograde into the tube and wash flow out of the
tube such that during a next inhale portion some rebreathing
occurs.
20. The method of claim 18, wherein the adjusting step is done such
that during normal breathing periods little rebreathing occurs.
21. The method of claim 20, wherein the adjusting step is done such
that during normal breathing periods some retrograde flow occurs
but wash flow is sufficient to remove exhaled air before a next
inhale portion.
22. The method of claim 16, wherein treating obstructive sleep
apnea comprises supplying blower pressure greater than eight cm
H.sub.2O.
23. The method of claim 16, wherein treating obstructive sleep
apnea occurs before the adjusting step.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of application Ser. No.
10/762,979, filed Jan. 23, 2004, now United States patent no. which
is a continuation-in-part of application Ser. No. 09/498,504 filed
Feb. 3, 2000, now U.S. Pat. No. 6,752,150, which claimed the
benefit of U.S. provisional application No. 60/118,616 filed Feb.
4, 1999.
BACKGROUND
[0002] Central sleep apnea is a type of sleep-disordered breathing
that is characterized by a failure of the sleeping brain to
generate regular, rhythmic bursts of neural activity. The resulting
cessation of rhythmic breathing, referred to as apnea, represents a
disorder of the respiratory control system responsible for
regulating the rate and depth of breathing, i.e. overall pulmonary
ventilation. Central sleep apnea should be contrasted with
obstructive sleep apnea, where the proximate cause of apnea is
obstruction of the pharyngeal airway despite ongoing rhythmic
neural outflow to the respiratory muscles. The difference between
central sleep apnea and obstructive sleep apnea is clearly
established, and the two can co-exist. While central sleep apnea
can occur in a number of clinical settings, it is most commonly
observed in association with heart failure or cerebral vascular
insufficiency. An example of central sleep apnea is Cheyne-Stokes
respiration.
[0003] The respiratory control system comprises a negative feedback
system wherein a central pattern generator creates rhythmic bursts
of activity when respiratory chemo-receptors sensing carbon
dioxide, oxygen and pH are adequately stimulated (FIG. 1). While
this neural output of the brainstem central pattern generator to
the respiratory muscles derives from a neural rhythm generated
intrinsically by the central pattern generator, the generator
becomes silent if the feedback signals, related to arterial
P.sub.CO2 and P.sub.O2, are not sufficiently intense. In other
words, the respiratory rhythm is generated by a conditional central
pattern generator which requires an adequate input stimulus derived
from peripheral chemoreceptors sensing arterial P.sub.CO2 and
P.sub.O2 from central chemoreceptors sensing brain P.sub.CO2/pH.
Furthermore, the intensity of neural activity generated by the
respiratory central pattern generator depends directly upon the
arterial P.sub.CO2 inversely on the arterial P.sub.O2. Thus, the
central and peripheral chemoreflex loops constitute a negative
feedback system regulating the arterial P.sub.O2 and P.sub.CO2,
holding them constant within narrow limits (FIG. 1).
[0004] This normal regulation of arterial blood gases is
accomplished by a stable ventilatory output of the respiratory
central pattern generator. By contrast, central sleep apnea
represents an instability of the respiratory control system. The
instability can arise from one of two mechanisms, namely: (1)
intrinsic failure of the respiratory central pattern generator in
the face of adequate stimulation by respiratory chemoreceptors; or
(2) lack of adequate stimulation of the central pattern generator
by respiratory chemoreceptors. The former is referred to as the
"intrinsic instability" and the latter is referred to as the
"chemoreflex instability." Theoretically, both mechanisms can
co-exist. The common form of central sleep apnea is thought to be
caused by the chemoreflex instability mechanism.
[0005] The chemoreflex control of breathing might exhibit
instability either because the delay of the negative feedback
signal is excessively long or because the gain of the system is
excessively high. Current evidence indicates that the latter
constitutes the principal derangement in central sleep apnea caused
by heart failure. Specifically, the overall response of the control
system to a change in arterial P.sub.CO2 is three-fold higher in
heart-failure patients with central sleep apnea than in those
having no sleep-disordered breathing. This increased gain probably
resides within the central chemoreflex loop; however, high gain of
the peripheral chemoreflex loop cannot be excluded. Accordingly,
the fundamental mechanism of central sleep apnea is taken to be
high loop gain of the control system, which results in feedback
instability during sleep.
[0006] Central sleep apnea causes repeated arousals and
oxyhemoglobin desaturations. Although firm evidence linking central
sleep apnea to morbidity and mortality is lacking, a variety of
evidence leads to the inference that central sleep apnea may
promote cardiac arhythmias, strokes, or myocardial infarctions. The
repeated nocturnal arousals are likely to impair daytime cognitive
function and quality of life. No treatment has become established
as being effective for central sleep apnea. Stimulating drugs such
as theophyline may be helpful, and carbonic anhydrase inhibitors
may relieve central sleep apnea in normals sleeping at high
altitude. Nasal continuous positive airway pressure may directly or
indirectly improve ventilatory stability. Increasing inspired
fractional concentration (F) of oxygen in the inspired gas
generally does not eliminate central sleep apnea, whereas
increasing inspired F.sub.CO2 (F.sub.|CO2=0.01-0.03) promptly
eliminates central sleep apnea. However, long-term exposure to high
F.sub.1CO2 would appear to be an undesirable long-term therapy.
SUMMARY
[0007] In an aspect of a breathing disorder treatment system, there
is provided a method for varying the efficiency of pulmonary gas
exchange by using a controlled amount of rebreathing during certain
periods of a respiration cycle so as to counteract the effects of
the transient excessive ventilation on the level of carbon dioxide
and oxygen in the lungs and in the arterial blood. In effect, this
strategy is an attempt to stabilize breathing by minimizing
oscillations in the feedback variables.
[0008] In an aspect of a breathing disorder treatment system, the
use of the system counteracts periodic breathing due to central
sleep apnea by decreasing loop gain of the respiratory control
system, and is also applied to the treatment of obstructive sleep
apnea. In one embodiment, the breathing disorder treatment system
dynamically modulates efficiency of pulmonary gas exchange in
relation to pulmonary ventilation. When pulmonary ventilation is
stable at resting values, the performance of the system is
unchanged. However, during a period of hyperpnea, i.e. when
ventilation increases transiently to supra-normal levels, the
system is made more inefficient, thus decreasing loop gain and
stabilizing the system.
[0009] Rebreathing can be used to increase the inspired percentage
carbon dioxide and reduce the inspired percentage oxygen just
before or during the period of overbreathing. In one embodiment,
the patient's ventilation is continuously monitored and analyzed in
real time so that the ventilation periodicities of the central
sleep apnea breathing can be detected and the inspired carbon
dioxide and oxygen concentrations adjusted appropriately by varying
the amount of exhaled gas that is reinspired.
[0010] In another embodiment, a rebreathing apparatus is used as
part of a nasal continuous positive airway pressure (CPAP) system.
The use of continuous positive airway pressure may have a
beneficial effect on cardiac function in patients with congestive
heart failure. In the future it is likely that patients with
congestive heart failure will receive nasal CPAP for treatment of
the heart failure. Central sleep apnea may not immediately
disappear upon administration of conventional nasal CPAP therapy as
central sleep apnea respiration is basically of a non-obstructive
origin. However, over a period of about four weeks the degree of
heart failure improves; thus, the resulting central sleep apnea
respiration may be relieved by the continuous positive airway
pressure. This is described in the papers, Naughton, et al.,
"Effective Continuous Positive Airway Pressure on Central Sleep
Apnea and Nocturnal Percentage Carbon Dioxide in Heart Failure"
American Journal Respiratory Critical Care Medicine, Vol. 1509, pp
1598-1604, 1994; Naughton, et al., "Treatment of Congestive Heart
Failure and Central Sleep Apnea Respiration during Sleep by
Continuous Positive Airway Pressure," American Journal of Critical
Care Medicine, Vol. 151, pp 92-97, 1995; and, Naughton, et al.,
"The Role of Hyperventilation in the Pathogenesis of Central Sleep
Apneas in Patients with Congestive Heart Failure," American Review
of Respiratory Diseases, Vol. 148, pp 330-338, 1993.
[0011] It is desirable to have a prompt elimination of the central
sleep apnea respiration because the resulting daytime sleepiness
and impaired cognition resulting from repeated arousals impair the
patient's quality of life. Immediately relieving central sleep
apnea breathing during the CPAP treatment would have the advantage
that the patient would experience a better sleep and would be more
rested. This in turn would enhance compliance with the CPAP
treatment program. Conventional nasal CPAP provides no immediate
relief of central sleep apnea respiration and resulting
arousals.
[0012] A conventional CPAP system is modified in one embodiment of
a breathing disorder treatment system to allow a controlled amount
of rebreathing during a portion of the central sleep apnea
respiration cycle. In one embodiment, a valve is used to control
the amount of rebreathing. When the valve is closed, rebreathing
occurs and when the valve is open no rebreathing occurs. A computer
connected to a flow meter can be used to detect periodicities in
the central sleep apnea respiration cycle. The computer can then
control the valve to open and close. Nasal occlusion in combination
with an oral appliance may be used to guarantee controlled
re-breathing.
[0013] Another embodiment of abreathing disorder treatment system
concerns use of a passive low-bias-flow device for treating
obstructive and central sleep apnea. This apparatus includes a
gas-supply means, such as a blower, and a leak free patient
interface that is fitted to a patient's airway. The gas-supply
means is adjusted so that air flow from the gas-supply means is
such that for the patient's normal breathing, the gas flow supplied
by the gas-supply means is sufficient to prevent a significant
amount of the patient's exhaled gases from flowing retrograde into
a tube between the gas-supply means and the leak free patient
interface. The device may first be used to treat obstructive sleep
apnea. During periods of increased breathing preceding or following
central sleep apnea, the preset air flow is such that some of the
patient's exhaled gases flow retrograde into the tube. Some of the
exhaled gases flowing retrograde into the tube will be rebreathed
by the patient. Thus, during periods of overbreathing associated
with central sleep apnea, there will be some rebreathing of gases
containing a higher F.sub.CO2 and a lower F.sub.O2 than room air.
Note that conventional CPAP systems are set such that there is no
retrograde air flow any time in the sleep cycle.
[0014] Yet another embodiment of a breathing disorder treatment
system is a method for adjusting an apparatus comprising a
gas-supply means, a leak free patient interface and a tube between
the leak free patient interface and the gas-supply means. In this
method, the leak free patient interface is fitted to the patient's
airway. The supply of gas from the gas-supply means is set high
enough that during the patient's normal breathing, the gas flow
supplied by the gas-supply means is sufficient to prevent a
significant amount of the patient's exhaled gases from flowing
retrograde into the tube, but set low enough that during periods of
increased breathing increased with central sleep apnea, some of the
patient's exhaled gases flow retrograde into the tube.
[0015] Still another embodiments of a breathing disorder treatment
system concerns a method for treating obstructive and central sleep
apnea wherein the supply of gas from a gas-supply means has a
varying gas pressure that changes at different times during the
patient's sleep cycle. In this way, rebreathing can be increased.
For example, in one embodiment, the gas pressure from the blower is
decreased during periods of increased breathing associated with
central sleep apnea so that some of the patient's exhaled gases
flow retrograde between the leak free patient interface and the
blower. This approach is less advantageous because users often find
the varying leak free patient interface pressure to be annoying.
Also, varying of the leak free patient interface pressure can
affect the internal dead space in a manner counter to the
rebreathing effect.
[0016] The general approach is that the blower pressure is set at a
minimum level that eliminates all evidence of upper airway
obstruction, or at a level deemed appropriate for treating heart
failure. The bias flow is then reduced to a level that eliminates
central sleep apnea without increasing the external dead space
during unstimulated breathing. The bias flow can then be fixed at
this level or varied systematically within or between cycles of
periodic breathing.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] There will now be described a breathing disorder treatment
system, with reference to the drawings, in which:
[0018] FIG. 1 is a diagram illustrating central sleep apnea;
[0019] FIG. 2 is a diagram illustrating one embodiment of a
breathing disorder treatment system;
[0020] FIG. 2A is a diagram illustrating use of the embodiment of
FIG. 2 with a dental appliance;
[0021] FIGS. 2B and 2C and 2D illustrate two embodiments of an oral
appliance of FIG. 2A;
[0022] FIG. 3 is a diagram illustrating central sleep apnea
respiration;
[0023] FIG. 4A is a diagram of one embodiment of a breathing
disorder treatment system using a passive loop gain modulation for
ventilization stabilization using a single pre-set gas flow
pressure from a blower;
[0024] FIG. 4B is a diagram of an alternate embodiment of the
system of FIG. 4A using a flow meter and a computer;
[0025] FIG. 5 is a diagram of one embodiment of a breathing
disorder treatment system which uses computer control of the blower
pressure to modify the vent pressure from the blower during certain
periods of a sleep cycle;
[0026] FIG. 6 is a diagram of an embodiment of a breathing disorder
treatment system which uses computer control of a dead space
attached to valves so as to cause rebreathing during certain
periods of a sleep cycle;
[0027] FIG. 7 is a diagram of one embodiment of a breathing
disorder treatment system using a recirculator to increase
rebreathing during certain periods of a sleep cycle;
[0028] FIGS. 8A-8F are diagrams depicting air flow accorded in
tubing connecting between the blower and the mask;
[0029] FIG. 9 depicts the changes in V.sub.ret and V.sub.wash that
occur when pulmonary ventilation is stimulated by increasing
arterial P.sub.CO2;
[0030] FIGS. 10, 11 and 12 are diagrams that illustrate the
dependence of V.sub.ret, V.sub.ED and T.sub.FRAC on V.sub.E;
[0031] FIG. 13 is a diagram that illustrates the relationship of
V.sub.A and V.sub.E at the four levels of V.sub.B;
[0032] FIG. 14 is a diagram illustrating the general dependence of
the loop gain on the ratio log V.sub.E/V.sub.A;
[0033] FIG. 15 is a diagram that illustrates the breathing air flow
in the tube of a conventional CPAP system;
[0034] FIG. 16 is a diagram that illustrates the normal breathing
flow in the tube of the embodiment of FIG. 4A;
[0035] FIG. 17 is a diagram that illustrates overbreathing flow in
the tube in the embodiment of FIG. 4A;
[0036] FIG. 18A is a diagram of an embodiment of a breathing
disorder treatment system in which the size of the exit tube of the
mask is varied slowly over the patient's sleep cycle;
[0037] FIG. 18B is a graph illustrating one example of changing of
the exit hole size during the night, for the apparatus of FIG. 18A;
and
[0038] FIG. 19 is a diagram of an alternate embodiment using the
blower output as an active control device to adjust the level of
rebreathing by a patient.
DETAILED DESCRIPTION
[0039] FIG. 2 is a diagram illustrating the rebreathing apparatus
of one active control embodiment of a breathing disorder treatment
system. In this embodiment, a continuous positive airway pressure
apparatus including blower 20, tube 22 and patient interface 24 is
used. Patient interface 24, for example a mask or oral interface,
preferably produces an airtight tight seal to the face for use in
the continuous positive airway pressure treatment. A discussion of
continuous positive airway pressure and a preferred continuous
positive airway pressure apparatus is described in Remmers, et al.
U.S. Pat. No., 5,645,053, "Auto-CPAP Systems and Method for
Preventing Patient Disturbance Using Airflow Profile Information."
In conventional CPAP, a blower is used to maintain a relatively
high constant pressure in a mask and to provide a bias flow of
fresh air from the blower out the mask.
[0040] In one embodiment of a breathing disorder treatment system,
tube 26 is connected to the exhaust port 31 of the patient
interface and conducts gas to the variable resistor 28.
Alternatively, the valve can be located on the exhaust port of the
patient interface. Tube 22 is used as a dead space for rebreathing
during some periods of the central sleep apnea respiration. When
the valve 28 is open, no rebreathing occurs because all the exhaled
gas is carried out tube 26 through valve 28 by the bias flow before
inspiration occurs. When valve 28 is closed, the bias flow ceases
and no expired air is conducted through tube 26. In this case, some
partial rebreathing occurs because the expired air is conducted
retrograde up tube 22 to the blower. The gases in the tube 26 have
a higher concentration of carbon dioxide and a lower concentration
of oxygen than room air. When the patient inspires, gas is
conducted from the blower to the patient and the previously expired
gases are inhaled by the patient.
[0041] Normally, the bias flow of gas from the blower through the
patient interface and out port 30 would be adequate to completely
purge the system during the expiratory phase of the respiratory
cycle so that no gas expired by the patient remains in the system.
Thus, the gas inspired by the patient had a composition of room air
(O.sub.2 concentration 21%; CO.sub.2 concentration about 0%).
Conversely, if the bias flow is reduced to zero by completely
occluding port 30 with valve 28, the gas exhaled by the patient
would fill the tube 22 connecting the patient interface to the
blower. Such expired gas would typically have a carbon dioxide
concentration of 5% and an oxygen concentration of 16%. Upon
inhalation, the patient would first inspire the high carbon
dioxide, low oxygen mixture filling the tube, followed by
inhalation of room air from the blower. Depending upon the length
of the tubing this mixture could amount to rebreathing of 20 to 60
percent of the tidal volume. By varying the exhaust port outflow
resistance, the degree of rebreathing between these limits can be
varied and the inspired concentration of carbon dioxide and oxygen
can be manipulated. In one embodiment, flow meter 32 connected to
computer 34 is used to detect the flow of gases to and from the
blower 20. The computer 34 is used to identify the periodicities in
pulmonary ventilation caused by the central sleep apnea respiration
and to control the valve 28 to cause rebreathing during certain
periods of the central sleep apnea cycle.
[0042] The gas flow from the blower comprises the bias flow
(patient interface exit flow+leak flow) plus the respiratory
airflow. The computer monitors this flow and calculates the bias
flow, leak flow, retrograde flow, retrograde expired volume and
wash volume.
[0043] A computer 34 can detect the amplitude of the central sleep
apnea cycle and to adjust the resistance of the valve 28 according.
For example, if there are large variations in pulmonary ventilation
during the central sleep apnea cycle, the valve 28 can be
completely closed during the overbreathing period. If there are
small variations in pulmonary ventilation during the central sleep
apnea cycle, the valve 28 can be partially open during the
overbreathing period. Thus, a higher level of rebreathing will
occur when the variation in pulmonary ventilation during the
central sleep apnea cycle is high than will occur when the
variation in pulmonary ventilation during the central sleep apnea
cycle is low.
[0044] Because of the low impedance of the CPAP blower 20,
variations of the resistance in the outflow line cause very little
change in patient interface pressure. Accordingly, the full range
of variations in outflow resistance can be made without producing
significant deviations in the desired CPAP patient interface
pressure.
[0045] The flow meter 32 and computer 34 can quantitate the level
of pulmonary ventilation. For example, the ratio of breath volume
to breath period gives an indication of the level of the
instantaneous pulmonary ventilation. Other indices such as mean or
peak inspiratory flow rate could also be used.
[0046] FIG. 3 shows an idealized diagram of the periodicities of
the overbreathing and underbreathing during central sleep apnea
respiration. This diagram shows the regions of overbreathing 50a
and the regions of underbreathing 50b compared to the moving time
average of ventilation. The computer system will be able to
determine the periodicities of the central sleep apnea breathing.
Typically, there is about a 50-60 second periodicity to the
overbreathing and underbreathing in the central sleep apnea
breathing.
[0047] A number of techniques are used to control the degree and
timing of rebreathing with the valve 28 in order to eliminate
central sleep apnea. One way of controlling rebreathing so as to
reduce the central sleep apnea respiration is to anticipate the
different cycles in the central sleep apnea respiration. For
example, looking at FIG. 3, at time A, the system will anticipate a
period of overbreathing and thus begin rebreathing by closing valve
28 as shown in FIG. 2. By the time overbreathing portion 50a
occurs, there is some level of rebreathing. Because of this,
pulmonary gas exchange becomes less efficient during the period of
overbreathing and, thereby, the resulting rise in lung oxygen and
fall in lung carbon dioxide will be less. As a result, the level of
oxygen in the blood does not get too high and the level of carbon
dioxide does not get too low. This stabilizes the oxygen and carbon
dioxide pressures in the arterial blood and thus will reduce the
amplitude of subsequent underbreathing or the length of the apnea.
At time B, the system will anticipate an underbreathing cycle by
opening the valve 28 and rebreathing will no longer occur. The
apparatus of a breathing disorder treatment system can reduce
central sleep apnea rebreathing (line 50) to a lower level as shown
in dotted line 60 in FIG. 2. Time A and time B for the beginning
and end of the rebreathing can be determined by the computer 34
shown in FIG. 2.
[0048] FIG. 4A is a diagram that illustrates a passive loop gain
modulation system for use in a breathing disorder treatment system.
FIG. 4A depicts a system using a gas-supply means such as the air
blower 60 connected to a length of input tubing 62 and then to a
patient interface 64. This system uses a simple fixed exit port for
the patient interface. A tubing volume greater than that normally
used with obstructive sleep apnea can be used with a breathing
disorder treatment system. For example, a ten-foot rather than
six-foot tubing can be used. Thee blower 60 preferably has a very
low impedance. That is, changes in the air flow do not
significantly change the air pressure supplied by the blower. This
can help maintain a relatively stable patient interface pressure
even as the tube flow becomes retrograde.
[0049] Additionally, in one embodiment, the air blower is able to
supply air pressure much lower than conventional CPAP blowers. In
one embodiment, the air blower can be adjusted to supply pressures
below 4 cm H.sub.2O (preferably 2 cm H.sub.2O or below). The
ability to supply such small pressures allows for the retrograde
flow as discussed below. The patient interface is fitted about the
patient's airway. During normal breathing, the air supplied from
the blower 60 and tube 62 to the patient interface 64 does not
cause any rebreathing because any exhaled air will be flushed
before the next inhale period. During periods of heavy breathing,
the preset gas flow pressure is set so that enough exhaled air
flows retrograde into the tube such that during the next inhale
period some expired gas is rebreathed. In this embodiment, the
overbreathing occurs during certain periods of the sleep cycle
associated with central sleep apnea. Rebreathing during periods of
overbreathing during central sleep apnea tends to reduce the
resulting spike in the blood oxygen level. Thus, the period of
underbreathing following the overbreathing in the central sleep
apnea sleep cycle will also be reduced.
[0050] The alternating periods of under- and overbreathing are
reduced by the rebreathing which takes place during the periods of
overbreathing. The rebreathing attenuates the arterial blood oxygen
spike and the reduction in arterial P.sub.CO2 caused by the
overbreathing. Thus, there is less underventilation when the blood
reaches the chemoreceptors. Thus, the amplitude of the periodic
breathing is reduced.
[0051] The embodiment of FIG. 4 is different than the conventional
CPAP in that the preset gas flow pressure is lower and/or the
patient interface exit hole is smaller than that used with
conventional CPAP systems. By reducing the gas flow pressure from
the typical CPAP gas flow pressures, and/or reducing the patient
interface exit hole size, the retrograde flow during the
overbreathing periods is produced.
[0052] The system of FIG. 4A has the advantage that it does not
require active control of the blower pressure. The patient can be
checked into a sleep center and the correct blower pressure and
patient interface exit hole size set. Thereafter, the system can be
placed on the patient's airway every night without requiring an
expensive controller-based system. The preset blower gas pressure
depends upon the air flow resistance caused by the patient
interface 64, the normal exhale pressure and the overbreathing
exhale pressure. If the gas-supply pressure system is an air blower
60, then by modifying the revolutions per minute of the air blower,
the preset gas flow pressure can be set.
[0053] The air supply pressure for patients with central sleep
apnea but without obstructive sleep apnea can be set at a
relatively low level such as below 4 cm H.sub.2O. The normal
patient interface exit holes produce the desired effect at these
pressures. The end-tidal F.sub.CO2 and inspired F.sub.CO2 can be
monitored by a CO.sub.2 meter 65 with an aspiration line connected
to the patient interface. Importantly, all mouth leaks should be
eliminated by using a leak resistant patient interface in order to
have expired gas move into the tubing 62. This can be achieved by
applying a chin strap, or by using an oral appliance 25 (FIG. 2A)
such as a full arch dental appliance applied to the upper and lower
teeth, or both. An alternative approach to difficult mouth leaks is
to use a full face mask covering the mouth as well as the nose.
This means that expired gas emanating from the nose or the mouth
will travel retrogradely up the tubing 62 toward the blower. While
it is important that leaks between the patient interface and the
patient be minimized, it is also important that as much as possible
of the exhaled air of the patient be conserved and made available
for re-breathing. Hence, if the patient interface connects to the
nose, then the mouth passageway should be blocked, and if the
patient interface connects to the mouth, then the nasal passageway
should be blocked. In either case, leaks through the unused
passageway should be minimized.
[0054] Examples of an oral appliance 25 are illustrated in more
detail in FIG. 2B and FIG. 2C and FIG. 2D. In FIG. 2B, the oral
appliance 25 is a dental appliance. In FIG. 2B, the oral appliance
25 is designed for fitting within the teeth and has an upper tray
25A that fits between the lips and teeth of a patient, and a lower
tray 25B that provides a sealing surface for the lips to rest on.
An opening 25C in the center of the oral appliance 25 of FIG. 2B
communicates with a CPAP hose connector 25D to provide CPAP
pressure delivery. The oral appliance 25 of FIG. 2C is fitted to a
patient's mouth directly onto the lips, without using the teeth.
The oral appliance 25 of FIG. 2C and FIG. 2D is held on a patient
with a mask 27 that fits around a patient's airway and is secured
with the use of straps and a pad 29A at the back of the patient's
head. A tube 29B with normal bias ports 29C blocked, and low-flow
bias flow port 29D, connects to the CPAP apparatus through CPAP
connection 29E. The length of the tube 29B allows for a controlled
amount of rebreathing.
[0055] A feature of the mode of action of the technology described
in this patent document relates to the behaviour of the system
during hyperventilatory periods. At these times, when such a
hyperventilatory phase occurs, the patient generates a large tidal
volume and short duration of expiration. Together, these induce
rebreathing of expired gas that has flowed retrogradely into the
CPAP conduit 29B connecting the CPAP blower to the patient
interface such as oral appliance 25. Patients with central sleep
apnea using Low Flow CPAP nightly in the home may find that, during
periods of hyperventilation, mouth leaks may occur of sufficient
magnitude to vitiate the rebreathing of exhaled gasses. For such
patients, it is preferable to use a dental appliance 25 to apply
CPAP pressure to the mouth together with nasal occlusion to
eliminate leaks from the nose. Data from studies on patients using
a dental appliance and nasal occlusion revealed that the therapy
was effective in resolving the central sleep apnea and that during
hyperpnic phases no leak of exhaled gasses occurred. For effective
application of Low Flow CPAP an oral interface, such as the oral
appliance 25, should be used in combination with nasal occlusion.
Nasal occlusion may be obtained through plugs inserted in the
nostrils or an external U-shaped clamp 29F (FIG. 2D) similar to
what would be used by a swimmer.
[0056] If the patient has an element of obstructive sleep apnea,
the patient interface pressure is increased progressively until all
evidence of upper airway obstruction is eliminated. If the patient
is receiving nasal CPAP as treatment for heart failure, patient
interface pressure is set at the desired level (typically 8-10 cm
H.sub.2O). The bias flow (patient interface hole size) can then be
reduced until central sleep apnea is eliminating without adding
dead space.
[0057] For patients with heart conditions, the patient interface
pressure can be set at the valve suggested by the literature
(typically about 10 cm H.sub.2O). Then the bias flow is
adjusted.
[0058] The flow through tube 62 depends upon the difference in
pressure between the blower pressure (i.e., pressure at the outlet
of the blower) and patient interface pressure. Blower pressure is
set by the revolutions per minute (RPM) of the blower and will be
virtually constant because the internal impedance of the blower is
very low. When no respiratory airflow is occurring (i.e., at the
end of expiration), patient interface pressure is less than blower
pressure by an amount that is dictated by the flow resistive
properties of the connecting tube and the rate of bias flow. This
is typically 1-2 cm of water pressure difference when bias flow is
at 0.5-1.5 L/sec. When the patient interface is applied to the
patient and the patient is breathing, patient interface pressure
varies during the respiratory cycle depending upon the flow
resistance properties of the connecting tube and the airflow
generated by the patient. During inspiration the patient interface
pressure drops, typically 1-2 cm of water, an during expiration
pressure may rise transiently a similar amount. During quiet
breathing the peak-to-peak fluctuations in patient interface
pressure are less than during heavy breathing or hyperpnea.
[0059] Thus, during quiet breathing the patient interface pressure
rises during exhalation and this reduces the driving pressure
difference between the blower and the patient interface, thereby
reducing flow in the tube. If the expired tidal volume increases,
however, peak expiratory flow will increase and this will be
associated with a further increase in patient interface pressure.
If patient interface pressure increases to equal blower pressure,
flow in the tube will stop. When patient interface pressure exceeds
blower pressure, flow in the tube will be in a retrograde
direction, i.e., from the patient interface to the blower. Such
retrograde airflow will first occur early in expiration and the
volume of air which moves into the connecting tube will be washed
out later in expiration as patient interface pressure declines and
flow from the blower to the patient interface increases. However,
if bias flow is low and the tidal volume is large, a large amount
of retrograde flow will occur and a large volume of expired gas
will move into the tube. Because the bias flow is small, the wash
flow purging the tube will be small. In such a case, not all of the
retrograde volume will be washed out before the next inspiration.
As a consequence, the overall inspired gas will have a somewhat
reduced oxygen concentration and an elevated carbon dioxide
concentration.
[0060] FIGS. 15-17 illustrate the flow in the tube between a blower
and a patient interface. FIG. 15 is a graph that illustrates
breathing air flow in the tube of a conventional CPAP system. Note
that during the exhale portion, the flow from the blower to the
patient interface always overpowers the exhale pressure such that
there is no retrograde flow into the tube. This is typically done
by setting the air blower pressure and exhaust port resistance such
that bias flow out of the patient interface is relatively high and
the possibility of retrograde flow is avoided. This normal flow
occurs even for the overbreathing associated with central sleep
apnea.
[0061] FIGS. 16 and 17 are diagrams that illustrate the effect of
breathing in systems of a breathing disorder treatment system in
which the blower pressure and bias flow out of the exit hole of the
patient interface are set such that there is retrograde flow during
portions of overbreathing associated with central sleep apnea.
[0062] FIG. 16 illustrates the situation in which there is normal
breathing. Even with normal breathing, there is some retrograde
flow during the period 202. Later in the exhale period the
retrograde volume is washed from the tube by the normal flow that
occurs during period 204. Thus there is little or no rebreathing
during the normal breathing periods. The system of a breathing
disorder treatment system does not add dead space during the normal
breathing periods. This is important because the addition of dead
space can increase the concentration of carbon dioxide that is
supplied to the bloodstream. It is assumed that if the increased
carbon dioxide level persists for multiple days, the body will
readjust the internal feedback system an undesirable manner.
[0063] FIG. 17 illustrates an embodiment showing overbreathing
along with the apparatus of a breathing disorder treatment system.
In the embodiment of FIG. 17, the overbreathing is such that there
is some retrograde flow of exhaled gases, which remain in the tube
at the time of the next inhale portion. This means that at the next
inhale portion, the patient will reinspire some exhaled gases with
the resultant higher concentration of carbon dioxide. Note that in
FIG. 17, the initial exhale region 206 is greater than the exhale
region 208.
[0064] In one embodiment of a breathing disorder treatment system,
the retrograde flow volume and wash volume for the normal breathing
can be used to set the operation of a breathing disorder treatment
system. In one embodiment, the retrograde volume region 202 should
be one-half the size of the wash flow region 204 for normal
breathing. Other rules of thumb such as the comparisons of the
aveolar ventilation to the bias flow out of the patient interface
and/or comparisons of the washout time to the duration of
expiration could also be used to set the operations of the system
of a breathing disorder treatment system.
[0065] FIG. 4B shows the device of FIG. 4 with the addition of a
computer 67 and flow meter 69. The flow meter 69 is used to detect
the desired air flow in the tube 62. The blower can then be
adjusted so that there is retrograde flow during periods of
overbreathing and no retrograde flow otherwise. The device of FIG.
4B can be used to calibrate the device of FIG. 4A for an individual
patient.
[0066] FIG. 18A and 18B illustrate an embodiment in which the
patient interface exit size is slowly changed over the course of
the night. In this embodiment, the blower 210 supplies airflow at a
selected pressure. Flowmeter 212 is connected into the tube 214
which allows the flow in the tube 214 to be determined along with
additional parameters of the system including the aveolar volume,
bias flow, and the like. The processor 216 slowly changes the size
of the exit hole using the variable air resistance apparatus 218.
Unlike the system of FIG. 2, the size of the variable output
resistance 218 is modified slowly over the night.
[0067] Looking at FIG. 18B, if the patient has obstructive sleep
apnea as well as central sleep apnea, at the beginning of the night
the output valve can be set relatively large, increasing the bias
flow out of the patient interface and thus reducing any effect of
retrograde into the tube 214. Once the obstructive sleep apnea is
reduced, the valve diameter can be slowly decreased, which can
cause an increase of retrograde flow into the tube 214 during the
overbreathing portion of central sleep apnea and thus can cause
rebreathing which can reduce the central sleep apnea. Additional
adjustments in the patient interface valve opening can be made
based upon calculations made by the processor 216.
[0068] FIG. 5 is an embodiment of a breathing disorder treatment
system in which the blower 70 is dynamically controlled. A flow
meter 72 is placed in the tube 74 between the blower 70 and patient
interface 76. A flow meter can also be placed near the patient's
face. The system of FIG. 5 allows computer control to decrease the
blower pressure during certain periods of a sleep cycle. Thus,
during periods of heavy breathing, the blower pressure can be
reduced to facilitate retrograde flow and rebreathing. This
embodiment is less advantageous because of the mixed effects of
changes in the patient interface pressure. By modifying the gas
supply pressure supplied by the blower 70, the retrograde flow into
the tube 74 can be increased and decreased, as desired.
[0069] FIG. 6 is an alternate embodiment of a breathing disorder
treatment system. In this embodiment, the patient interface 82 is
connected to dead space 84 by computer-controlled valves 86 and 88.
The amount of rebreathing during certain period of the sleep cycle
can be modified by changing bias flow by opening and closing the
valves 86 and 88, thus reducing the central sleep apnea.
[0070] FIG. 7 is an embodiment using a recirculator 90. During
certain portions of the sleep cycle, the recirculator 90 allowing
exhaled air to be drawn in by the recirculator 90 recirculated and
supplied to the user at the patient interface 92. In this manner,
the central sleep apnea can be reduced by increasing the
rebreathing at selected portions of the sleep cycle.
Technical Description
[0071] One embodiment of the breathing disorder treatment system is
applied in the setting of nasal continuous positive pressure (CPAP)
therapy. The loop gain of the negative feedback respiratory control
system is reduced principally by increasing the volume of external
dead space (V.sub.ED), the common airway through which gas is
conducted during inspiration and expiration. The external dead
space constitutes an extension of the internal dead space
(V.sub.ID) comprising the airways of the lung and the upper airway.
The total dead space (V.sub.D) equals the sum of the internal and
external dead spaces. V.sub.D=V.sub.ED+V.sub.ID (Equation 1)
[0072] This volume represents an obligatory inefficiency of the
control system in that it reduces the portion of the tidal volume
(V.sub.T) that participates in gas exchange within the lungs.
Specifically, the tidal volume is the sum of two components
V.sub.T=V.sub.D+V.sub.A (Equation 2)
[0073] where V.sub.A represents the "alveolar" portion of the tidal
volume, i.e. the volume that participates in respiratory gas
exchange. Also, V.sub.E=V.sub.A+V.sub.D, where the symbols V.sub.E,
V.sub.A, and V.sub.D signify the products f.V.sub.T, f.V.sub.A and
f.V.sub.D (f represents respiratory frequency). In the negative
feedback loop of the respiratory control system (FIG. 1), V.sub.E
represents the output of the respiratory central pattern generator
and V.sub.A is a variable which influences arterial blood gas
pressures. The link between V.sub.E and V.sub.A is, of course,
V.sub.D which is the primary variable manipulated in dynamically
controlling loop gain.
[0074] Dynamic control of the rebreathing volume is achieved when
the patient is breathing through a nasal CPAP apparatus. When using
conventional nasal CPAP the nose is covered by a mask which is
connected to a pressure-generating source by a length of tubing.
The nose mask is flushed continuously by a stream of gas flowing
from the pressure source and exiting the exhaust port of the mask.
This will be referred to as the bias flow (V.sub.B). When using
nasal CPAP for its traditional application, i.e., treatment of OSA,
the rate of exhaust flow is relatively high so that virtually all
the expired gas which enters the mask from the nose flows into the
mask and out the exhaust port. Because of the relatively high
V.sub.B the mask is completely washed out before the next
inspiration occurs. Thus, the gas inspired from the mask has a
composition equal to that flowing from the blower (typically room
air: F.sub.|O2=0.293; F.sub.|CO2=0.0003). In this situation,
typical for OSA treatment, the nose mask adds no external dead
space. The breathing disorder treatment system in this embodiment
dynamically increases V.sub.ED by using a lower value of V.sub.B
and this, in turn, dynamically reduces V.sub.A (Equation 2). Thus,
the component of pulmonary ventilation effective in gas exchange,
alveolar ventilation (V.sub.A), is altered on a moment-to-moment
basis. Since V.sub.A determines the values of the feedback
variables, arterial P.sub.O2 and P.sub.CO2, V.sub.D directly
influences loop gain (FIG. 1). Thus, the loop gain (L.G.) of the
system can be manipulated as below:
.dwnarw.V.sub.B.fwdarw..uparw.V.sub.ED.fwdarw..dwnarw.V.sub.A.fwdarw..dwn-
arw.L.G. (Equation 3)
[0075] Importantly, the increase in V.sub.ED occurs only during
periods of hyperpnea, as described below. Thus, during normal
breathing, no dead space is added to the system.
[0076] As secondary strategies, a breathing disorder treatment
system utilizes changes in CPAP pressure to change lung volume and,
thereby, influence loop gain of the respiratory control system. In
particular, in increase in lung volume decreases loop gain by
decreasing the dynamic change in feedback variables (arterial
P.sub.CO2 and P.sub.O2) when alveolar ventilation changes
dynamically. As well, such an increase in lung volume decreases the
end-expiratory length of inspiratory muscles, thereby decreasing
their force generation during inspiration. Together, both effects
of nasal CPAP decrease the loop gain. When CPAP pressure is
dynamically varied in synchrony with the periodic breathing cycle,
both effects dynamically modulate loop gain. However, experience
indicates that, over the range of CPAP pressure of 1-10 cm
H.sub.2O, these produce a smaller decrease in loop gain than
varying V.sub.D. Additionally, dynamic changes in V.sub.D are less
likely to disturb the sleeper than changes in CPAP pressure.
Accordingly, the use of increase in CPAP pressure to decrease lung
volume and, thereby, decrease loop gain, represents a supplementary
strategy of a breathing disorder treatment system.
[0077] The patient with central sleep apnea or combined central and
obstructive sleep apnea sleeps with a nasal CPAP mask sealed to the
face (FIGS. 2, 4A, 4B, 5, 6, 7) Mouth leaks, if present, are
eliminated by a chin strap and/or an oral appliance combined with
nasal occlusion. If this is not adequate, the nose mask is replaced
with a full face mask. The patient interface is connected to a
positive pressure outlet of a low impedance blower by a tubing, in
one embodiment typically 2-3 cm in diameter and 1.5 m long. The
bias flow exits the patient interface either through an orifice of
fixed, selectable size (FIGS. 4A, 4B, 5, 6, 7) or through a tubing,
in one embodiment (1.5 m long, 1 cm in diameter) connected to a
computer-controlled variable resistor (FIG. 2). In such a system,
the patient interface pressure is determined by blower RPM, and the
rate of bias flow V.sub.B is the resultant of patient interface
pressure and patient interface outflow resistance. The apparatus
shown in FIGS. 2 and 4B includes a pneumotachagraph for measuring
flow from the blower. This device is suitable for initial titration
or for nightly therapeutic use. Also, a CO.sub.2 meter can be added
with a sampling catheter connected to the patient interface. This
allows monitoring of end-tidal and inspired F.sub.CO2. The device
shown in FIG. 4A is a simpler version of that shown in FIG. 4B and
is suitable for nightly use.
[0078] The dynamically variable bias flow device (FIG. 2) allows
moment-to-moment adjustment of bias flow with negligible changes in
patient interface pressure. The exhaust resistor can be controlled
by an independent observer during a polysomnographic study, or it
can be automatically controlled by a computer algorithm. The
control of external dead space volume (V.sub.ED) is either
passively adjusted with the exhaust resistance being constants, or
actively adjusted with exhaust resistance being varied in time. In
the passive adjustment implementation, bias flow is constant in
time since a fixed exhaust orifice is used. In the active
adjustment, bias flow changes in time owing to the change in
resistance of the bias flow resistor.
[0079] FIG. 8 depicts airflow recorded in the tubing which connects
the blower to the patient interface. Positive values signifying
airflow from the blower to the patient interface, and negative
values indicate airflow from the patient interface to the blower.
The former is referred to as "wash" airflow since it eliminates
expired gas from the patient interface; the latter is referred to
as "retrograde" airflow since it represents expired air flowing in
the reverse direction to that which normally occurs during CPAP
administration. As shown in FIG. 8A (top panel), airflow in the
tubing is equal to the sum of two air flows, V.sub.B and
respiratory airflow. The former is constant and the latter varies
with the respiratory cycle. Inspiratory airflow produces an upward
deflection in V and expiratory airflow produces a downward
deflection in V. At the end of expiration (upward arrow in FIG.
8A), respiratory airflow equals zero and tubing airflow equals bias
airflow which is chosen to be 1.0 L/sec in this example. Peak
expiratory airflow occurs early in expiration (downward arrow in
FIG. 8A) and equals 1.0 L/sec in this example. At this time, tubing
airflow is zero because peak expiratory airflow equals V.sub.B.
[0080] FIGS. 8A, 8B, 8C, 8D, 8E and 8F depict the changes in tubing
airflow that occur as V.sub.B is progressively reduced from 1.0
L/sec (FIG. 8A) to 0.15 L/sec (FIG. 8F). Respiratory airflow is
held constant throughout. As V.sub.B is reduced from 1.0 to 0.5,
0.35, 0.25, 0.20 and 0.15 L/sec (FIGS. 8A, 8B, 8C, 8D, 8E, 8F),
retrograde airflow appears during expiration and becomes
progressively larger. The volume of air which moves retrogradely
during expiration (V.sub.ret, hatched area) increases progressively
as V.sub.B is decreased. Conversely, the volume of air which moves
from the blower to the patient interface during expiration
(V.sub.wash, stippled area) decreases as V.sub.B is decreased.
[0081] The volume of air resident in the patient interface and
tubing at the end of expiration (downward arrow, FIG. 8A) is
referred to as residual volume (V.sub.R). V.sub.R can be estimated
as the difference between V.sub.RET-V.sub.WASH.
V.sub.R=V.sub.RET-V.sub.WASH (Equation 4)
[0082] In the first five examples shown in FIG. 3, V.sub.R is
negative or equal to zero (FIGS. 8A, 8B, 8C, 8D, 8E), signifying
that with this respiratory pattern, there is no added dead space
(V.sub.ED=0). However, if pulmonary ventilation were to increase,
V.sub.RET would increase and V.sub.R would become positive.
Similarly, if the duration of expiration (T.sub.e) were to
decreases, V.sub.WASH would decrease and V.sub.R would become
positive. When breathing is stimulated by an increase in arterial
P.sub.CO2 and a decrease in arterial P.sub.O2, tidal volume
increases and T.sub.e decreases. Accordingly, if V.sub.B is
relatively low (0.35 and 0.25 in this example), chemical
stimulation will cause V.sub.R to assume a positive value so that
higher levels of pulmonary ventilation will be associated with
greater values of V.sub.R.
[0083] The presence of a positive value for V.sub.R indicates that
V.sub.ED will assume a finite value (FIG. 8F). However, V.sub.R
does not equal V.sub.ED. During inspiration, gas resident in the
patient interface and tubing flows to one of two places, namely:
out the exhaust port or into the respiratory tract. Only the latter
constitutes V.sub.ED. Accordingly, a fraction of V.sub.R will be
inspired, that fraction depending on the value of V.sub.B relative
to the inspiratory flow rate. Use of a high value of V.sub.B will
minimize V.sub.ED. Thus, chemical stimulation of breathing causes
three changes in the respiratory pattern, an increase in expiratory
air flow rate, a decrease in Te, and an increase in inspiratory air
flow rate, each of which acts independently to augment V.sub.ED.
Together, they cause a sharp rise in V.sub.ED when V.sub.E
increases by chemical stimulation if the V.sub.B is relatively low.
FIG. 8B illustrates the time in expiration when the tubing and
patient interface are flushed by fresh, room air. This time is
expressed as a fraction of T.sub.e and referred to as T.sub.FRAC.
T.sub.FRAC increases progressively as V.sub.B decreases. When
T.sub.FRAC equals 100%, a critical value of V.sub.B has been
reached; further decreases in V.sub.B will produce a finite value
of V.sub.ED.
[0084] To calculate V.sub.ED, the following relationship is used:
V.sub.ED=V.sub.R-(V.sub.B)(t) (Equation 5)
[0085] where t defines the time required for V.sub.R to be
eliminated from the patient interface and conducting tubing as
shown in FIG. 9. V.sub.ED can be calculated by progressively
incrementing inspiratory time (t) from zero (the onset of
inspiration) and calculating V.sub.SUM inspired volume plus exhaust
port volume, i.e.,
V.sub.SUM=.intg..sub.oV.sub.1+.intg..sub.oV.sub.B (Equation 6)
[0086] where V.sub.1 represents inspiratory flow rate, i.e., total
flow minus bias flow during inspiration. The incrementing procedure
continues until V.sub.SUM equals V.sub.R.
[0087] FIG. 9 depicts the changes in V.sub.RET and V.sub.ED that
occur when pulmonary ventilation is stimulated by increasing
arterial P.sub.CO2. V.sub.B is assumed to equal 0.25 L/sec in all
cases, and is approximately two times resting V.sub.A(5.7 L/min).
FIG. 8D depicts the respiratory pattern under unstimulated, resting
conditions (V.sub.E=8.0 L/sec). When ventilation is mildly
stimulated (V.sub.E=15.0 L/sec, FIG. 9A), V.sub.RET increases and
V.sub.WASH decreases so that V.sub.ED equals 0.26 L. Further
stimulation of breathing (FIG. 9B) results in V.sub.ED equal to
0.47 L when V.sub.E equals 19.5 L/sec, V.sub.ED equal to 0.79 L
when V.sub.E equals 25.7 L/sec (FIG. 9C) and V.sub.ED equal to 1.19
L when V.sub.E equals 36.7 L/sec (FIG. 9D). Note that T.sub.FRAC
increases progressively as V.sub.E increases for a constant
V.sub.B.
[0088] The dependence of V.sub.RET, V.sub.ED and T.sub.FRAC on
V.sub.E is shown in FIGS. 10, 11 and 12, respectively, for all four
values of V.sub.E. Each plot shows a family of V.sub.B isopleths.
V.sub.RET, V.sub.D and T.sub.FRAC show a quasi-linear increase as
V.sub.E increases (FIGS. 10, 11 12 and 13).
[0089] FIG. 13 illustrates the relationship between V.sub.A and
V.sub.E at the five levels of V.sub.B. For values of 1.0 L/sec and
greater, all points lie on a monotonically ascending curve.
However, for lower values of V.sub.B, the relationship is shifted
downward, indicating that an increment in V.sub.E caused by an
increase in chemical stimulus will cause a smaller increment in
V.sub.A. This implies a reduction in loop gain which can be
quantitated as the change in slope of this relationship. Note that
at values of V.sub.E equal to 0.35 L/sec less, V.sub.A becomes
constant for values of V.sub.E greater than 15 L/sec. In other
words, the breathing disorder treatment system clamps V.sub.A at
some maximal value.
[0090] FIG. 14 illustrates the overall dependence of loop gain on
the ratio, log V.sub.E/V.sub.A. This ratio, calculated for resting
breathing, provides a normalized index of V.sub.E for any patient.
The relationship is plotted over the range of log V.sub.E/V.sub.A
from 0 to 1, i.e. over the range of variation in V.sub.E where
V.sub.ED is less than zero under resting conditions. Note that the
loop gain decreases steeply as resting V.sub.RET/V.sub.WASH
decreases from 0.5 to 0. For this reason, we select a ratio value
of 0.3 for usual application of the method in treating central
sleep apnea. In this situation, V.sub.B is approximately two times
V.sub.A and T.sub.FRAC equals 80%. This value results in a 50%
decrease in loop gain while providing more than adequate washout of
expired gases from the apparatus under resting conditions.
Accordingly, loop gain is reduced to a value that stabilizes
breathing for many patients with central sleep apnea without any
risk of adding external dead space when the patient is breathing
normally and having no central sleep apnea.
[0091] The goal of the passive dead space method is to apply nasal
CPAP with a V.sub.B sufficient to produce V.sub.ED=0 under resting
conditions, but such that the V.sub.ED will increase with
increasing V.sub.E sufficient to reduce the loop gain and stabilize
breathing. Specifically, during hypopnea or normal breathing, the
apparatus produces no gas exchange inefficiency in breathing.
However, during hyperpnea, V.sub.ED increases progressively as
V.sub.E rises above normal. The net effect is that V.sub.D is
dynamically adjusted in keeping with variations in V.sub.E such
that the periodic fluctuation in V.sub.A is attenuated. This means
that fluctuations in arterial P.sub.O2 and P.sub.CO2 are reduced,
so that loop gain of the system is reduced. This acts to stabilize
breathing.
[0092] The advantage of the passively adjusting dead space device
is that loop gain can be reduced by a relatively simple apparatus
requiring no active algorithmic, dynamic adjustment in V.sub.B.
Once the effective V.sub.B has been determined, this can be
achieved by permanent adjustment of the resistance of the exhaust
port of the patient interface, thereby eliminating the need for an
exhaust tubing and computer-controlled exhaust resistor. However,
if the loop gain of the patient's respiratory control system is
very high, the passive apparatus may not reduce the loop gain
sufficiently to stabilize breathing. In that case, a dynamically
adjusting V.sub.ED apparatus is employed. In the embodiment that
dynamically adjusts V.sub.B, the indicator variables (V.sub.RET,
V.sub.WASH, T.sub.frac and V.sub.B/V.sub.A) are calculated on line.
Periodic breathing is detected either by the recurrence of apneas
or by autoregressive analysis. V.sub.B is reduced progressively
until evidence of central sleep apnea is eliminated or until the
indicator variables reach their critical limits (V.sub.RET,
V.sub.WASH=0.8, T.sub.frac=80% and V.sub.B/V.sub.A=2).
[0093] FIG. 19 depicts another embodiment of the breathing disorder
treatment system. The patient with central sleep apnea wears a full
face mask 220 which can be loose fitting, but should be leak
resistant such as by using an oral interface with nasal occlusion.
The mask 220 is purged by a bias flow from a high-impedance blower
222 which supplies a constant rate of airflow to the mask. This
bias flow is selectable and rapidly adjustable by the controlling
computer 224. The bias flow exits to the atmosphere through a
low-resistance reservoir tubing 226. The respiratory airflow (both
inspiration and expiration) occurs through this reservoir tubing.
Because of the tubing's low resistance, the mask pressure remains
near atmospheric pressure. A pneumotachograph (flow meter 228) in
the reservoir tubing allows monitoring of bias flow and respiration
airflow and calculation of wash volume during expiration and
expired tidal volume.
[0094] Under resting conditions or when no central sleep apnea
respiration is detected, bias flow is held relatively high so that
wash volume exceeds the volume of gas expired into the tube.
Accordingly, when inspiration begins, the reservoir tube has been
washed completely with bias flow, and the patient inspires room
air. Thus, no external dead space has been added when the patient
is breathing normally and no ventilatory periodicity is detected by
the computer. When the computer 222 detects ventilatory
periodicity, bias flow is varied in synchrony with the periodicity.
Specifically, when instantaneous ventilation is greater than the
moving average, bias flow is reduced so that wash volume is less
than expired tidal volume. This causes rebreathing and decreases
loop gain of the system. During periods of underbreathing, bias
flow is maintained at high values so that no rebreathing occurs.
The volume of gas resident in the reservoir tubing 226 at the end
of expiration (i.e., the rebreathing volume) is calculated on line
and is adjusted to be proportional to the difference between
instantaneous ventilation and moving average ventilation. Thus,
dead space increases progressively as overbreathing occurs thereby
minimizing the effect of the excessive ventilation on arterial
blood gases. This, in turn, minimizes the duration of the apnea or
magnitude of hypopnea that follows the overbreathing and stabilizes
ventilation.
[0095] It will be appreciated by those of ordinary skill in the art
that the breathing disorder treatment system can be implemented in
other specific forms without departing from the spirit or central
character thereof. The presently disclosed embodiments are
therefore considered in all respects to be illustrative and not
restrictive. The scope of the breathing disorder treatment system
is indicated by the appended claims rather than the foregoing
description, and all changes which come within the meaning and
range of equivalence thereof are intended to be embraced herein.
Accordingly, the above description is not intended to limit the
breathing disorder treatment system, which is to be limited only by
the following claims.
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