Note: Descriptions are shown in the official language in which they were submitted.
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METHOD AND APPARATUS FOR OPTIMIZING THE
CONTINUOUS POSITIVE AIRWAY PRESSURE FOR
TREATING OBSTRUCTIVE SLEEP APNEA
BACKGROUND OF THE INVEI~iTION
This invention relates to a method and apparatus for
adjusting the positive airway pressure of a patient to an optimum
value in the treatment of obstructive sleep apnea, and more
particularly to a breathing device which maintains constant
l0 positive airway pressure and method of use which analyzes an
inspiratory flow waveform to titrate such a pressure value.
Obstructive sleep apnea syndrome (OSAS) is a well recognized
disorder which may affect as much as 1-5% of the adult
population. OSAS is one of the most common causes of excessive
daytime somnolence. OSAS is most frequent in obese males, and
it is the single most frequent reason for referral to sleep
disorder clinics.
OSAS is associated with all conditions in which there is
anatomic or functional narrowing of the patient s upper airway,
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and is characterized by an intermittent obstruction of the upper
airway occurring during sleep. The obstruction results in a .
spectrum of respiratory disturbances ranging from the total
absence of airflow (apnea) to significant obstruction with or
without reduced airflow (hypopnea and snoring), despite continued
respiratory efforts. The morbidity of the syndrome arises from
hypoxemia, hypercapnia, bradycardia and sleep disruption
associated with the apneas and arousals from sleep.
The pathophysiology of OSAS is not fully worked out.
However, it is now well recognized that obstruction of the upper
airway during sleep is in part due to the collapsible behavior
of the supraglottic segment during the negative intraluminal
pressure generated by inspiratory effort. Thus, the human upper
airway during sleep behaves as a Starling resistor, which is
defined by the property that the flow is limited to a fixed value
irrespective of the driving (inspiratory) pressure. Partial or
complete airway collapse can then occur associated with the loss
of airway tone which is characteristic of the onset of sleep and
may be exaggerated in OSAS.
Since 1981, continuous positive airway pressure {CPAP)
applied by a tight fitting nasal mask worn during sleep has
evolved as the most effective treatment for this disorder, and
is now the standard of care. The availability of this non-
invasive form of therapy has resulted in extensive publicity for "
apnea and the appearance of large numbers of patients who
previously may have avoided the medical establishment because of
the fear of tracheostomy. Increasing the comfort of the system,
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which is partially determined by minimizing the necessary nasal
pressure, has been a major goal of research aimed at improving
patient compliance with therapy. Various systems for the
treatment of obstructive sleep apnea are disclosed, for example,
in "Reversal of Obstructive Sleep Apnea by Continuous Positive
Airway Pressure Applied Through The Nares", Sullivan et al,
Lancet, 1981, 1:862-865; and "Reversal Of The 'Pickwickian
Syndrome' By Long-Term Use of Nocturnal Nasal-Airway Pressure";
Rapoport et al., New England Journal of Medicine, October 7,
1982. Similarly, the article "Induction of upper airway
occlusion in sleeping individuals with subatmospheric nasal
pressure", Schwartz et al., Journal of Applied Physiology, 1988,
64, pp. 535-542, discusses various polysomnographic techniques.
Despite its success, limitations to the use of nasal CPAP
exist. These mostly take the form of discomfort from the mask
and the nasal pressure required to obliterate the apneas.
Systems for minimizing the discomfort from the mask are
disclosed, for example, in U.S. Pat. Nos. 4,655,213, Rapoport et
al, and 5,065,756, Rapoport, as well as in "Therapeutic Options
For Obstructive Sleep Apnea", Garay, Respiratory Management,
Jul/Aug 1987, pp. 11-15; and "Techniques For Administering Nasal
CPAP", Rapoport, Respiratory Management, Jul/Aug 1987, pp: 18-21.
Minimizing
the necessary pressure remains a goal of the preliminary testing
of a patient in the sleep laboratory. However, it has been shown
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that this pressure varies throughout the night with sleep stage
and body position. Furthermore, the therapeutic pressure may
both rise or fall with time in patients with changing anatomy
(nasal congestion/polyps), change in weight, changing medication
or with alcohol use. Because of this, most sleep laboratories
currently prescribe the setting for home use of nasal CPAP
pressure based upon the single highest value of pressures needed
to obliterate apneas during a night of monitoring in the sleep
laboratory. Retesting is often necessary if the patient
complains of incomplete resolution of daytime sleepiness, and may
reveal a change in the required pressure.
~UMIrLATtY OF THE INVEUmvnrr
The invention is therefore directed to a method and
apparatus, in a system for the treatment of obstructive sleep
apnea, for optimizing the controlled positive pressure to thereby
minimize the flow of air from a flow generator while still
ensuring that flow limitation in the patient s airway does not
occur. In particular, the invention relates to a breathing
device and method of use to adjust a controlled positive pressure
to the airway of a patient by detecting flow limitation from
analysis of an inspiratory flow waveform.
In accordance with the invention, an apparatus for the
treatment of obstructive sleep apnea is provided, comprising a
source of air, and means for directing an air flow from said
-source to a patient. This part of the system may be of the type
disclosed, for example, in U.S. Pat. No. 5,065,756. In addition,
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means are provided for sensing the waveform of said airflow, to
detect deviations therein that correspond to flow limitation in
the air supplied to the patient. Such deviations may be, for
example, deviations from a substantially sinusoidal waveform,
flattening, or the presence of plateaus, in the portions of the
waveform corresponding to inspiration of the patient. In
response to such variations in said airf low, the system of the
invention increases or decreases the pressure to the patient.
In accordance with the method of the invention, the
controlled positive pressure to the patient is increased in
response to the detection of flow waveform portions corresponding
to flow limitations in the patient airway. Such pressure
increases may be effected periodically. Similarly, the
controlled positive pressure may be periodically decreased in the
absence of such flow limitation. The system may be provided with
a program that periodically decreases the controlled positive
pressure in the absence of detection of flow limitations in the
patient airway, and that periodically increases the pressure in
the presence of detection of such flow limitations.
The method for determining whether to increase or decrease
the controlled positive pressure is comprised of several steps.
The first step is to detect the presence of a valid breath and
store an inspiratory waveform of that breath for further
analysis. Next, the waveform of the stored breath is analyzed
regarding its shape for presence of flow limitation. Whether
f low limitation is present is in part determined by flow
limitation parameters calculated from the shape of the waveforms
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of the current breath and of the immediately preceding breath.
Once the presence of flow limitation has been analyzed, the ,
system determines an action to take for adjustment of the
controlled positive pressure. The pressure setting is raised,
lowered or maintained depending on whether f low limitation has
been detected and on the previous actions taken by the system.
The preferred breathing device or apparatus consists of a
flow generator, such as a variable-speed blower, a flow sensor,
an analog to digital converter, a microprocessor, and a pressure
controller, such as a blower motor speed control circuit, a
patient connection hose, a nasal coupling, such as a nose mask
or similar fitting, and, optionally, a pressure transducer.
Alternative patient circuits may be employed, such as those
disclosed in U.S. Pat Nos. 4,655,213 and 5,065,756. For example,
a positive pressure breathing gas source may be connected to a
pressure control valve proximate the breathing gas source and
connected to a nasal mask having a venting means.
In the preferred embodiment, the blower supplies air through
the flow sensor to the patient via a hose and nasal coupling.
The microprocessor obtains the flow waveform from the digitized
output of the flow sensor. Using the method of the present
invention described herein, the microprocessor adjusts the speed
of the blower via the motor control circuit to change the air
pressure in the patient supply hose. A pressure transducer may
be provided to measure the actual pressure in the patient hose.
In addition, the microprocessor may store measured pressure and
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flow waveform values in its data memory to provide a history
for real-time or off-line processing and analysis.
Other features and advantages of the present
invention will become apparent from the following detailed
description, taken in conjunction with the accompanying
drawings, which illustrate, by way of example, the
principles of the invention.
According to one aspect of the present invention,
there is provided a breathing device for optimizing positive
airway pressure to a patient, comprising: means for applying
an initial level of positive airway pressure of a breathing
gas to the patient; means for storing data values
representative of the inspiratory flow of breathing gas to
the patient; means for determining whether the stored data
values indicate a flow limitation in the patient including
processing means for determining a plurality of peak
inspiratory flow values for flow limited breaths, for
determining a plurality of peak inspiratory flow values for
non-flow limited breaths, for calculating the ratio of the
area of the inspiratory waveform to the area of a pure sine
wave to provide a first index, for correlating the stored
data values with a pure sine wave to create a second index,
for comparing a regression fit of the stored data values
with a regression fit of a pure sine wave to create a third
index, for comparing a peak value of the stored data values
with a peak value of a derivative of the stored data values
to create a fourth index, and for comparing a peak value of
the stored data values with an average of the plurality of
peak flow values for flow limited breaths and with an
average of the plurality of peak flow values for non-flow
limited breaths to create a fifth index; and means for
increasing the positive airway pressure when the stored data
values indicate a flow limitation in the patient.
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According to another aspect of the present
invention, there is provided a breathing device for
optimizing positive airway pressure to a patient,
comprising: a source of breathing gas at controlled positive
pressure to the airway of the patient; a flow sensor
configured to generate first data values representative of
an inspiratory flow of breathing gas to the patient;
computer memory configured to store the first data values
generated by said flow sensor; a microprocessor including
means for calculating the area of the inspiratory waveform
from said first data values and calculating the area of a
pure sine wave to generate a ratio of said areas and
configured to generate a first signal when said ratio
indicates a flow limitation in the patient; and a pressure
controller responsive to the first signal from said
microprocessor and coupled to said source of breathing gas
for increasing the positive pressure to the airway of the
patient.
According to still another aspect of the present
invention, there is provided an apparatus for providing
breathing gas to the airway of a patient, comprising: a
source of breathing gas at positive pressure to the airway
of the patient; a sensor configured to generate data values
representative of inspiratory flow of breathing gas to the
patient; processing means configured for identifying a flow
limitation based on a comparison between an area of a sine
wave and an area of an inspiratory waveform calculated from
said data values; and a pressure controller in operative
relationship with said source of breathing gas, said
pressure controller responsive to said processing means so
as to increase the positive pressure to the airway of the
patient in the event a flow limitation is defined.
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According to yet another aspect of the present
invention, there is provided a method for detecting flow
limitations in the airway of a patient, comprising the steps
of: a) providing a nasal fitting in fluid communication with
the airway of the patient, the nasal fitting having means
for measuring the inspiratory flow of ambient air to the
patient; b) detecting the onset of an inspiratory flow of
air to the airway of the patient; c) storing data values
representative of the inspiratory flow of breathing gas to
the patient; and d) determining whether the stored data
values indicate a flow limitation in the patient; e)
repeating steps b) through d) for a plurality of patient
inspirations to create a plurality of peak flow values for
flow limited breaths and to create a plurality of peak flow
values for non-flow limited breaths; f) calculating the
ratio of the inspiratory waveform to the area of a pure sine
wave to create a first index; g) correlating the stored data
values with a pure sine wave to create a second index; h)
comparing a regression fit of the stored data values with a
regression fit of a pure sine wave to create a third index;
i) comparing a peak value of the stored data values with a
peak value of a derivative of the stored data values to
create a fourth index; and j) comparing a peak value of the
stored data values with an average of the plurality of peak
flow values for flow limited breaths and with an average of
the plurality of peak flow values for non-flow limited
breaths to create a fifth index.
BRIEF DESCRIPTION OF THE DRAWING
FIGURE 1 is the waveform of the airflow of
a 30 second epoch to a sleeping patient from a CPAP
generator, with a CPAP pressure of 10 cm H20.
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FIG. 2 is the waveform of the airflow of
a 30 second epoch to the sleeping patient of FIG. 1, from a
CPAP generator, with a CPAP pressure of 8 cm H20.
FIG. 3 is the waveform of the airflow of
a 30 second epoch to the sleeping patient of FIG. 1, from a
CPAP generator, with a CPAP pressure of 6 cm H20.
FIG. 4 is the waveform of the airflow of
a 30 second epoch to the sleeping patient of FIG. 1, form a
CPAP generator, with a CPAP pressure of 4 cm H20.
FIG. 5 is the waveform of the airflow of
a 30 second epoch to the sleeping patient of FIG. l, from a
CPAP generator, with a CPAP pressure of 2 cm H20.
FIG. 6 is a simplified cross sectional view of a
Starling resistor.
FIG. 7 is a simplified block diagram of an
experimental setup employing a Starling resistor.
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FIG. 8 is a set of waveforms generated by use of the setup
of FIG. 7.
FIG. 9 is a simplified block diagram of a system in
accordance with the invention.
FIG. 20 is a flow diagram-illustrating one technique for
adjusting the CPAP pressure, in accordance with the invention.
FIG. 11 is a transition diagram of a three phase state
machine with states corresponding to the phases of respiration.
FIG. 12 is a plot of a total flow signal depicting the state
transitions shown in FIG. 11.
FIG. 13 is a set of waveforms used to correlate an
inspiratory wave with a sinusoidal half wave.
FIG. 14 shows a regression fit to a mid-third of an
inspiratory wave and to a sinusoidal half wave.
FIG. 15 is a plot of a total flow signal and a derivative
of an inspiratory waveform depicting a respiratory effort index.
FIG. 16 contains a table of the probability factors used to
modify the f low limitation parameters.
FIG. 17 is a flow diagram illustrating one technique for
determining whether and how to adjust the controlled positive
pressure, in accordance with the invention.
FIG. 18 is a detailed block diagram of a therapeutic
apparatus in accordance with the invention.
FIG. 19 is a detailed block diagram of a diagnostic system
in accordance with the invention.
FIG. 20 is a perspective view of a nose fitting for
diagnostic use with the method of the present invention.
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FIG. 21 is a partial cross-sectional view of a nose fitting
for diagnostic use with the method of the present invention.
FIG. 22 is a flow diagram illustrating a method of
diagnosing and treating a patient in accordance with the
invention.
FIG. 23 is a flow diagram illustrating a method of
diagnosing a system disconnect and distinguishing between central
and obstructive apnea.
DETAILED DI~30LOEURE OF THE INVENTION
FIGS. 1-5 illustrate the waveforms of flow from a CPAP
generator, obtained during the testing of a patient, in sleep
studies. In these tests, the patient was wearing a CPAP mask
connected to an air source, in the manner illustrated in U.S.
Pat. No. 5,065,765. Each of these tests illustrate an epoch of
30 seconds, with the vertical lines depicting seconds during the
tests. FIGS. 1-5 depict separate sweeps that were taken from 1
to 2 minutes apart, and with different pressures from the source
of air.
FIG. 1 illustrates a "normal" waveform, in this instance
with a CPAP pressure of 10 cm H20. This pressure was identified
as corresponding to apnea free respiration. It is noted that
this waveform, at least in the inspiration periods, is
" substantially sinusoidal. The waveforms of FIGS. 2-5 illustrate
that, as the controlled positive pressure is lowered, a
predictable index of increasing collapsibility of the airway
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occurs, prior .to the occurrence of frank apnea, periodic
breathing or arousal. .
When the CPAP pressure was decreased to 8 cm HBO, as
illustrated in FIG. 2, a partial flattening of the inspiratory
flow waveform, at regions 2a, began to occur. This flattening
became more definite when the controlled positive pressure was
decreased to 6 cm H2o, as illustrated by the reference numeral 3a
in FIG. 3. The flattening becomes even more pronounced, as seen
at the regions 4a of FIG. 4, when the controlled positive
pressure was reduced to 4 cm. Reductions in the CPAP pressure
from the pressure of apnea free respiration resulted in snoring
by the patient. When the controlled positive pressure was
reduced to 2 cm H20, as illustrated in FIG. 5, there was
virtually zero inspiratory flow during the inspiratory effort,
as seen at the portions 5a. Shortly after the recording of the
waveform of FIG. 5, the patient developed frank apnea and
awakened.
The waveforms of FIGS. 1-5 are consistent with experiments
wherein the collapsible segment of the air passage is simulated
by a Starling resistor. A Starling resister 10, as illustrated
in FIG. 6, is comprised of a rigid external tube 11 supporting
an internal collapsible tube 12. Water is introduced into the
space between the outer tube 11 and inner tube 12, for example,
through a tube connected to a water column 13 of adjustable
height to enable variation of the external pressure applied to
the collapsible tube 12. With reference to FIG. 7, in this
experiment, a commercial CPAP flow generator 14 is coupled to the
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"distal" end of the Starling resistor 10, and "respiration" is
simulated by a sinusoidal pump 15 coupled to the "proximal" or
"intrathoracic" end of the resistor 10. A volume reservoir 16
is coupled to the proximal end of the Starling resistor, to
provide a capacitive volume that prevents excessive negative
pressure from developing during total system occlusion (apnea).
The flow tracing of FIG. 8 was generated using the system
of FIG. 6, with the level of water in the column 13 set between
5 and 15 cm H20. The airflow from the CPAP flow generator was
started at a pressure of 14 cm HZO, then sequentially decreased
to 12 em, 11 cm, 8 cm and 6 cm HzO, and finally returned to 13 cm
H20. In FIG. 8, the upper curve shows the waveform of the
airflow, the middle curve shows the waveform of the proximal
pressure (i.e., at the port of the sinusoidal generator 15, and
the lower curve illustrates the CPAP pressure. The gradations
at the top of FIG. 8 denote seconds. FIG. 8 thus reflects the
large increase in resistance across the Starling resistor, and
mimics the increasingly negative intrathoracic pressure routinely
seen in patients with an apnea, snoring and any increased upper
airway resistance syndrome.
In accordance with the invention, waveforms of the flow of
air, of the type illustrated in FIGS. 1-5, are employed in order
to control the flow of air from a CPAP generator, to thereby
' minimize the flow of air from the generator while still ensuring
that flow limitation does not occur.
In one embodiment of the invention, as illustrated in FIG.
9, a CPAP mask 20 with leak port 19 is connected via tube 21 to
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receive air from a CPAP flow generator 22. These elements may
be of the type disclosed in U.S. Pat. No. 5,065,756, although the -
invention is not limited thereto, and any conventional CPAP
system may alternatively be employed. A conventional flow sensor
23 is coupled to the tube 21, to provide an electric output
signal corresponding to the waveform of the airflow in the tube
21. This signal is applied to a signal processor 24, which
detects the existence in the waveforms of conditions that
indicate flow limitation. The signal processor 24 outputs a
signal to a conventional flow control 25 for controlling the
pressure applied by the flow generator to the tube 21. It is of
course apparent that, depending upon the type of flow generator
22, the signal processor may directly control the flow generator,
instead of controlling a flow control device 25.
One method for adjusting the CPAP pressure in accordance
with the invention is illustrated in FIG. 10. After the CPAP
mask has been fitted to a patient and the CPAP generator has been
connected to the mask (step 40), the CPAP pressure is set at a
starting pressure. This pressure is determined by patient
preference to ease the patient in falling asleep. It may be
either a low pressure to minimize discomfort or the patient's
previous therapeutic level for those used to a higher pressure
at sleep onset. In addition, a time based hold at this pressure
may be incorporated. After a settling period of about 30 seconds
(step 41), the flow signal is analyzed (step 42).
If it is determined that flow limitation has occurred (step
43) and that the CPAP pressure is less than the maximum allowed
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(step 44), then the CPAP pressure is increased by 0.5 cm H20
(step 45) and the method returns to the settling step 41 for
further processing. If at the pressure comparing step 44 the
pressure was not less than the maximum allowed CPAP pressure,
then the method returns to the settling step 41 without
increasing the CPAP pressure.
If it was determined that a flow limitation was not present
(step 43), then a determination is made (step 46) whether a
predetermined time has elapsed following the last change in the
CPAP pressure. The predetermined time may be, for example, two
minutes. If the predetermined time has not elapsed, then the
method returns to the settling period step 41. If the
predetermined minimum time has elapsed, it is determined whether
the CPAP pressure is greater than the minimum allowed pressure
25 (step 47). If it is greater than the minimum allowed pressure,
then the CPAP pressure is decreased by 0.5 cm H20 (step 48), and
the method returns to the settling step 41. Otherwise, the
returns to the settling step 41 without decreasing the CPAP
pressure.
While the above described example of the method of the
invention employed CPAP pressure change steps of 0.5 cm H20, it
is apparent that the invention is not limited to pressure changes
of this magnitude. In addition, the pressure changes may not
' necessarily be equal throughout the range of adjustment.
Similarly, the flow limitation determination step 43 may
involve any of a number of waveform analysis procedures. For
example, the signal corresponding to the airflow waveform may be
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differentiated in the portions thereof corresponding to
inspiration. A markedly peaked result from such differentiation
indicates the presence of flow limitation, as is evident from an
analysis of the differentials of the waveforms of FIGS. 1-5.
Alternatively, the waveform may be analyzed for the presence of
harmonics of the cyclic rate of the waveform in the inspiration
period thereof, since the presence of a significant amplitude of
harmonics of the cyclic rate (i.e., the breathing rate) indicates
the present of a waveform indicative of flow limitation. It is
evident that analyses of this type may be effected by
conventional hardware or software. The invention, however, is
not limited to the above specific techniques for determining
divergence of the waveform from the normal non-flow limited
waveform to a waveform indicating the presence of flow
limitation.
The optimizing method for determining whether to increase
or decrease the controlled positive pressure is comprised of
several steps. The first step is to detect the presence of a
valid breath and store data values corresponding to an
inspiratory flow waveform of that breath for further analysis.
Alternatively, flow data values may be stored for the entire
breath. Next, the stored breath waveform is analyzed regarding
its shape for presence of flow limitation. Whether flow
limitation is present is in part determined by flow limitation
parameters calculated from the shape of the waveforms of the
current breath and of the immediately preceding breath. Once the
presence of flow limitation has been analyzed, the system
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determines an action to take for adjustment of the controlled
. positive pressure. The pressure setting is raised, lowered or
maintained depending on whether flow limitation has been detected
and on the previous actions taken by the system.
The optimizing method has several .input parameters which are
used in the determination of the action to be taken during the
automatic adjustment mode. For example, the initial controlled
positive pressure, or "start value," must be available for use
when power-on occurs in the breathing device. Similarly, the
method requires a "therapeutic level" of controlled positive
pressure to return to whenever an exception condition is
detected, such as high constant flow. If the method cannot
determine with reasonable certainty that breathing is present,
it returns the controlled positive pressure to the prescribed
therapeutic level. Also, a "low limit" and a "high limit" are
required to determine the minimum and maximum controlled positive
pressure level the system will generate when operating in the
automatic adjustment mode. The method cannot cause the
controlled positive pressure to exceed the maximum or minimum
limits of pressure. A prescription pressure can be set which can
modify the pressure response based on the relationship between
this prescription pressure and the actual currently generated
pressure. This serves to bias pressure changes toward the
therapeutic pressure.
The method for optimizing the controlled positive pressure
will now be described in more detail. The first step in the
optimizing method is the detection of a valid breath. A valid
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breath is determined by a cyclical fluctuation in the respiratory
signal superimposed on the constant system leak. This detection
is implemented using a three phase state machine with states
corresponding to the phases of patient respiration. The transi-
tion diagram of this state machine is shown in FIG. 11 and
described below. As is well known in the art, the logic for the
state machine may be programmed into the software of a micro-
processor or similar computer hardware.
The total flow signal present within the positive pressure
flow generator is used as a basis for the breath detection method
steps. The breath detection method produces measured data
corresponding to the inspiratory flow waveform. Similarly, the
breath detection method estimates the constant leak flow and
determines several breath description parameters, which are
Z5 described in more detail below. These measured and calculated
data form the input to the flow limitation detection step of the
optimizing method.
As shown in FIG. 12, the state machine uses the actual f low
signal from the controlled positive pressure source and two
derived reference flow signals to determine state transitions.
The first state of the state machine is the inspiratory state
(INSP). The second state is the expiratory state (EXP). In the
third state (PAUSE), the state machine is in transition from INSP
to EXP, or from EXP to INSP. The onset of an INSP state defines
the beginning of a valid breath. Likewise, the onset of the next
INSP state defines the end of a valid breath.
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The four state changes (C1, C2, C3, and C4), are shown in
FIGS. 11 and 12. The first state change (C1), is determined by
the state machine moving from the PAUSE state to the INSP state.
This transition denotes the completion of the preceding breath,
which is processed before proceeding to the next breath. The
data collected and calculated for a breath is discarded if it
does not meet certain preprogrammed minimal time and amplitude
criteria. The first transition occurs whenever the system is in
PAUSE and the total flow signal exceeds the sum of a calculated
average leak value (ALV) plus a calculated safety value {SAFE)
used as a dead-band. In addition, the derivative of the flow
signal must be greater than a minimum set value. This criteria
enables the system to differentiate between the onset of
inspiration and mere changes in flow leakage in the breathing
device.
The average leak value (ALV) is a calculated running average
of the actual flow signal modified to reflect the possible
absence of an expiratory signal. The estimate of the average
leak flow is updated during each of the three phases INSP, EXP,
PAUSE. The safety reference value (SAFE) is the level of
fluctuation in the flow signal which is considered noise. This
is calculated as an average of a fraction of the peak flow in
each breath. Alternatively, the total flow signal may be first
differentiated and then integrated to remove the constant DC
off set component (leak flow) and the value of ALV set to zero.
Also, the method steps may be applied to the estimated flow
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signal output of a CPAP generator (which has the constant leak
value subtracted out) and the ALV set equal to zero. .
In the second transition (C2), the machine state changes
from the INSP state to the PAUSE state. This transition occurs
when the system is in the INSP state and the total flow signal
drops below the ALV. In the next transition (C3), the state
machine changes from the PAUSE state to the EXP state. This
transition occurs when the system is in the PAUSE state and the
total flow signal drops below the ALV minus SAFE reference value.
Lastly, the state machine transitions from the EXP state to the
PAUSE state (C4). This transition occurs when the system is in
the EXP state and the total flow signal rises above the ALV.
The system performs certain calculations during the phase
states (INSP, EXP, PAUSE) and phase transitions (C1, C2, C3, C4).
During the inspiratory phase (INSP), the system accumulates and
stores measured data of total flow, e.g., in a flow buffer. Also
during the inspiratory phase, the system determines the maximum
inspiratory flow value and the maximum derivative value for the
total flow signal. During the expiratory phase (EXP), the system
determines the maximum expiratory flow value.
During the first transition (C1), the system determines
whether the current breath meets the valid criteria for time and
size. At the same time, the system calculates a new safety value
(SAFE) as a fraction of the breath size. During the second
transition (C2), the system determines the inspiratory time and
calculates the running average of the maximum derivative. During
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the fourth transition (C4), the system calculates the expiratory
time.
The determination of the degree of flow limitation present
is based on four shape detection parameters, the sinusoidal
index, the flatness index, the respiratory effort index and the
relative flow magnitude index. The sinusoidal parameter or index
is calculated as a correlation coefficient of the actual total
inspiratory flow wave (filtered) to a reference sinusoidal half
wave. As shown in Fig. 13, a half sinusoidal template 50 is
compared to the actual total inspiratory flow data, for example,
using a standard Pearson product moment correlation coefficient
or by calculating a ratio of the area under the template and the
area under the actual inspiratory total flow data curve. The
correlation coefficient is an index ranging from 1 (sinusoidal
or not flow limited) to 0 (not sinusoidal).
An area ratio provides an index of the truncation of the
breath that results from flow limitation and ranges from near 0
(extreme flow limitation) to > 1 (no flow limitation). The
template is a pure half sine wave such that its period matches
the duration of the actual inspiratory total flow data curve and
its amplitude is such that the derivative of the template at its
positive going zero crossing matches the initial derivative of
the actual inspiratory total flow data curve at its zero
crossing.
A typical non-flow limited shape 52 and flow limited shape
54 are shown in FIG. 13 for comparison. The comparison may be
applied to an entire inspiratory waveform (halfwave) or to the
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mid-portion whose shape is most characteristic of either normal
or flow limited breaths. The preferred section of the inspira- -
tory waveform is that which is most discriminate between normal
and flow limited behavior, for example, the mid-portion of the
inspiratory flow data.
The flatness parameter is a representation of the degree of
flatness (or curvature) present in the total inspiratory flow
signal. This index is calculated as a variance ratio of the
actual signal around a calculated regression line (actual
curvature) and an ideal half sinusoidal signal around the same
regression line (curvature standard). As shown in FIG. 14, the
regression (REGR) is calculated using the mid-portion of the
inspiratory flow data 56, for example, from the end of the first
third of the inspiratory portion of the breath TO to the
beginning of the last third of the inspiratory portion of the
breath T1. This regression is calculated using least squares
techniques. The variance of the actual total inspiratory flow
data 56 around this regression line is then calculated for the
mid-portion of the inspiration. Likewise the variance of the
mid-portion of a pure half sinusoidal template with matching
period and amplitude around the regression line is also
calculated. The ratio of these two variances produces the
flatness parameter or index which ranges from 1 (sinusoidal) to
0 (flat) . '
The system calculates the respiratory effort index as the
ratio of peak derivative (rate of change of flow with respect to
time) of the early inspiratory waveform to the peak flow value
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of the inspiratory waveform. FIG. 15 shows the peak (A) of the
total inspiratory flow waveform as plotted against the peak (B)
of the waveform for the derivative of the inspiratory flow. The
ratio of the peak values (B/A) is also known as the "effort
index." This parameter is useful to detect flow limitation in
a patient, because an increased respiratory effort is manifested
in an increased slope of the inspiratory flow waveform.
The system calculates the relative flow magnitude index as
the peak flow of the inspiratory flow waveform minus the peak
flow of the previous inspiratory flow waveforms showing flow
limitation divided by the running average of the peak flows of
the non-limited breaths minus the average of the flow-limited
breaths. This parameter is calculated as:
MINMAX - FLAW-MIN
MAX-MIN
WHERE: FLOW is the peak flow rate of the current breath
MIN is an average of the peak flow of the 20
most recent flow limited breaths.
MAX is an average of the peak flaw of the 20
most recent normal breaths.
This results in a parameter or index which ranges from 0 (flow
limited) to 1 (normal).
The four shape detection parameters described above are
calculated for the current valid breath and the values are
combined using a mathematical function, such as a logistic
' regression sum. Similarly, weighting factors may be used,
wherein the weight given to one or more of the indexes may be
zero, positive or negative. The combined values provide a flow
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limitation parameter which has a value between 0 and 1 that
characterizes the likelihood that the current breath has a shape .
characteristic of flow-limitation. The value of the flow
limitation parameter is further modified based on the value of
the preceding breaths' flow limitation parameters used as a prior
probability, allowing calculation of a posterior probability.
The four shape detection parameters (sinusoidal index,
flatness index, respiratory effort index and relative flow
magnitude index) are used in a mathematical function to determine
a likelihood of flow limitation using a logistic regression
equation:
p - ef cx~
1+ef c">
Where "p" is the probability of flow limitation; "e" is the base
of the natural logarithms; X1, X2, X3 and X4 are the shape
detection parameters; B0, B1, B2, B3 and B4 are the weighting
coefficients (which may include zero) and
f {X) - Bo + B1*X1 + B2*XZ + B3*X3 + B4*Xq.
The probability of flow limitation (p) has a limited range from
0 {flow limitation) to 1 (normal) and is valid for all values of
the function f {x) .
FIG. 16 shows the prior probability factor which is applied
to the initial value of the flow limitation parameter calculated
from the shape parameters to yield a final value for the current
valid breath. The prior probability factors are used to modify
the flow limitation parameter based on previous breath's value
for flow limitation. The underlined value is an estimate of the
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best value to be used as a multiplicative or additive to the
index. Thus, the flow limitation parameter is made more
important when other flow limited breaths have been detected.
Similarly, the index is made less "flow limited" if the present
occurrence is an isolated incident.
If the flow limitation parameter is between 1 and a
predetermined normal reference value, e.g., 0.65-0.8, then the
breath is classified as "normal." If the flow limitation
parameter is between 0 and a predetermined flow limited reference
value, e.g., 0.4, then the breath is classified as "flow
limited." If the flow limitation parameter is between the normal
and flow limited reference values, then the breath is classified
as "intermediate."
The probability of flow limitation is then compared to the
area ratio index. If the probability index classifies a breath
as normal then the breath remains classified as normal. If the
probability index classifies a breath as flow limited or
intermediate the final classification will be determined by the
area ratio index. If this ratio is less than some specified
value the breath will be classified as flow limited and if the
ratio is greater than or equal to the specified value the breath
will be classified as normal. As each valid breath is
identified, its likelihood of being flow limited is calculated.
' The flow limitation parameter approaches a value of 1 for a
normal breath and 0 for a flow limited breath. In the method of
the present invention, a decision is made as to whether to adjust
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the controlled positive pressure. This decision is dependent on
three factors:
1) the value of the flow limitation parameter for the
current breath;
2) the value of the flow limitation parameters in the
preceding interval (several breaths);
3) whether the controlled positive pressure has been
adjusted (and the direction) in the preceding interval
of time.
Generally, if flow limitation is detected, the controlled
positive pressure will be raised. Similarly, if no flow
limitation is detected for an interval of time, then the
controlled positive pressure is lowered to test for the
development of flow limitation. The desired effect of the method
of the present invention is for the controlled positive pressure
to remain slightly above or below the optimal positive pressure
despite changes in the optimal therapeutic level of pressure
which may occur over time.
As shown in the flow chart of FIG. 17, the method of the
present invention uses a decision tree to determine whether to
change the controlled positive pressure to the airway of the
patient. The steps of the method may be programmed in the
software of a microprocessor or similar computer. As part of
the decision process, the system calculates a time weighted
majority function (MF) from the flow limitation parameter values
for a certain number of previous breaths, e.g., three, five or
ten breaths depending on the type of current breath. Depending
on the combination of parameters, the controlled positive
pressure is raised or lowered a large (1.0 cm) or small (0.5 cm)
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step, returned to the value prior to the last change or left
. unchanged from the last value.
If there has been no change (NC) in the controlled positive
pressure for the past interval, the present breath is normal (N)
and the majority function is normal, then the controlled positive
pressure is lowered by a large step {LOWR LG}. If, however, the
present breath is intermediate (I) and the majority function is
intermediate or flow limited (FL), then the controlled positive
pressure is raised by a small step (RAISE SM). Similarly, if the
present breath is flow limited, then the controlled positive
pressure is raised a small step if the majority function is
intermediate and by a large step (RAISE LG) if the majority
function is flow limited. Else, no change is made to the
controlled positive pressure.
If the controlled positive pressure has been lowered in the
past interval, the present breath is normal and the majority
function is normal, then the controlled positive pressure is
lowered by a large step (LOWER LG) . If, however, the present
breath is intermediate or flow limited and the majority function
is intermediate or flow limited, then the controlled positive
pressure is raised to the previous level (RAISE PV). Else, no
change is made to the controlled positive pressure.
If the controlled positive pressure has been raised in the
past interval, no action is taken for a period of time, e.g., 10
breaths. Then if the present breath is normal and the majority
function is normal, the controlled positive pressure is lowered
by a small step {LOWR SM). Conversely, if the present breath is
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intermediate or flow limited, then the controlled positive
pressure is raised by a small step if the majority function is -
intermediate and by a large step if the majority function is flow
limited. Else, no change is made to the controlled positive
pressure.
In addition, the detection of apnea is used to initiate the
decision to raise the controlled positive pressure. Apnea is
detected as the absence of fluctuations in flow that are of
sufficient amplitude to represent breaths. As shown in FIG. 23,
if an apnea of sufficient duration 184 occurs then the algorithm
first determines whether this represents a true patient apnea or
a patient disconnect from the pressure generator. If the average
flow rate 186, as described in the section on breath detection,
is greater than some predefined value 290 then a patient
disconnect condition has occurred and the pressure is changed to
some absolute, predefined level and the algorithm then waits for
the resumption of breathing. If the average flow rate is below
the threshold value then this represents a true apnea. Once an
apnea has been detected it can be further classified as either
obstructive or central. This classification is based on the
presence (central apnea) or absence (obstructive apnea) of
regular, small-amplitude flow pulsations with a frequency in the
range of the cardiac frequency 192. These pulsations can be
detected from the flow signal after it is appropriately filtered
and transformed to magnify their amplitude. The signal ,
transformation function (which preferentially magnifies the
amplitude of the signal near its average value) may include, but
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not be limited to, non-linear mathematical functions (e. g. square
root) and look up tables. These periodic fluctuations are then
detected in the transformed signal with variance and/or period
amplitude techniques which identify fluctuations at a frequency
similar to that of a cardiac cycle (e. g. 40-120/min). If cardiac
frequency oscillations in the flow signal are detected then the
apnea is classified as central 194. In response to a central
apnea CPAP pressure may be increased by a different algorithm
than that used for obstructive apnea, or allowed to remain
unchanged 196. If cardiac frequency oscillations are not
detected then the apnea is classified as obstructive 198 and the
controlled positive pressure is raised 200. The controlled
pressure is then held at or above this new increased pressure for
a predefined period of time. After the defined time period has
elapsed the pressure may decrease below this new pressure if
indicated by the absence of flow limitation and apnea. An
additional apnea occurring within a predefined time window of a
previous apnea will also increase the controlled positive
pressure and may set a longer time period during which pressure
may not drop below the new controlled pressure.
Alternatively, the controlled positive pressure may be
continuously adjusted at a rate set by a slope parameter, e.g.,
0.1 cm per two seconds. The slope parameter, both its magnitude
and sign, are updated breath by breath or every other breath
based on the classification of the breath as normal or flow
limited and previous controlled pressure changes. This allows
for continuous adjustment of the controlled positive pressure.
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The system may prevent decreases in pressure for a predefined
period of time if some threshold number of successive increases -
in the controlled pressure occur, e.g., if increases have been
made on the previous 5 breaths. In no event can the controlled
positive pressure be set below the low limit or above the high
limit reference values. An additional modification to the
adjustment of CPAP pressure control may be based on the
relationship between the currently applied CPAP pressure and the
prescription CPAP pressure to bias changes in such a way as to
favor changes toward the prescription pressure and against
changes away from it. For instance, when the actual CPAP
pressure is greater than the prescription pressure, a limit may
be placed on the magnitude of the pressure increase allowed
during a specified unit of time. When the actual CPAP pressure
is lower than the prescription pressure then no limit is placed
on the rate of increase. Likewise the rate of pressure decrease
may be modified by this relationship.
FIG 18. shows an alternative therapeutic apparatus in the
spirit of the present invention. The breathing device 70 is
composed of a flow sensor circuit 72 which senses the flow rate
of the breathing gas in the tubing or hose 74 leading to the
patient. The flow sensor produces an analog output voltage
proportional to the breathing gas flow rate which is conveyed via
multiplexer 76 to an analog to digital converter circuit 78 which
produces a digital output value which is proportional to the
analog voltage output from the flow sensor.
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A microprocessor 80 with associated memory 81 and other
peripheral circuits executes computer programs which implement
the optimizing methods heretofore described. The microprocessor
or similar computing device uses the digital output values from
a multiplexes 76 and an analog-to-digital converter 78. The
microprocessor produces a speed control signal which adjusts a
motor speed control circuit 82 which controls the speed of a
blower motor 84. Similarly, the variable-speed motor drives the
fan blades of a blower 86 which supplies the air flow to the
patient through or past the air flow sensor 72. The speed of the
blower determines the pressure in the patient circuit. Thus, the
microprocessor is able to adjust the pressure of the patient
ei~~ti~t 7 Q- 3yi -~~5pc~ns8 to-- t he - data-- v~ iues f i o~'n tile f ~.~w -
-se~c5or-.-
The breathing device 70 may also incorporate a pressure
sensor circuit 90 to allow the microprocessor 80 to obtain a
direct measurement of the pressure in the patient tubing 74 via
the analog to digital converter circuit 78. Such a configuration
would allow the microprocessor to maintain the pressure within
the maximum and minimum pressure limits established by the
prescribing physician. The actual operating pressure levels can
be stored in the memory 81 of the microprocessor every few
minutes, thus providing a history of pressure levels during the
hours of use when the stored data values are read and further
' processed by a separate computer program.
A signal representative of the speed of the blower could be
stored in memory instead of the pressure data values; however,
such speed values do not change as rapidly as measured pressure
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values. If the blower pressure versus speed characteristics are
suitable, i.e., approximately constant pressure at a given speed
regardless of the air flow rate, then the pressure sensor circuit
may be eliminated, thereby reducing the cost to produce the
apparatus and making it affordable by a greater number of
patients. Alternatively, a patient circuit having a positive
pressure breathing gas source and pressure control valve, as
disclosed in U.S. Pat. No. 5,065,756, may be used.
The methodology for detecting flow limitation can be applied
by an automated or manual analysis of the inspiratory flow
waveform from the positive pressure generator or from any
measurement of the inspiratory flow waveform_ Thus the method
and apparatus heretofore described may be used for diagnostic
purposes in a hospital, sleep lab or the home. Detection and
measurement of inspiratory and expiratory flow can be from a
standard CPAP system with a flow signal output or by a diagnostic
system 100 as shown in FIG. 19. Data values representative of
the measured inspiratory and expiratory flow can be logged by a
microprocessor 110 in various forms of computer memory 114.
As shown in FIGS. 20 and 21, the detection and measurement
of breathing gas flow is made from a tight sealing nose fitting
102 (mask or prongs) configured with a resistive element 106
inserted in the flow stream as breathing gas exits from and
enters into the fitting. The nasal fitting is further provided
with a port 108 for connection to a flow or pressure transducer
104. The resistive element causes a pressure difference to occur
between the upstream side and the downstream side when air flows
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through the element. The magnitude of the pressure difference
is proportional to the magnitude of the flow of the air through
the resistive element. By continuously measuring the pressure
difference, the measurement of the air flow through the resistive
element is effectively accomplished. In the preferred
embodiment, the pressure measurement is made between the inside
of the nose fitting and the ambient pressure in the room.
Additional details regarding the construction of such a nose
fitting may be found in U.S. Pat. No. 4,782,832.
An alternative nose fitting may consist of a tight fitting
nasal mask such as that disclosed in U.S. Pat No. 5,065,756. An
improved mask seal may be achieved by using a ring of dual sided
adhesive tape formed in a ring or oval along the perimeter of the
mask where the nasal mask contacts the patient. In addition, the
perimeter of the nasal mask may be configured with a pliable
material which would conform to the shape of the face of the
patient. A vent and flow restrictor may be configured in the
mask and placed in fluid communication with a flow and/or
pressure sensor or transducer.
In FIGS. 20 and 21, a nasal prong 102 has been configured
with a mesh screen resistor 106 at the air inlet, which creates
a pressure signal within the nasal prong proportional to the air
flow through the nasal prong. Although the figures show an
external pressure transducer 104 coupled to the nose fitting by
flexible tubing 108, the pressure transducer could be embedded
within the structure of the nose fitting, thereby sensing the
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pressure difference between the inside and outside of the nose
fitting. Pressure and flow data values may be continuously
measured and recorded on a data logging device such as a micro-
processor 11o having program memory 111 and a storage medium 1i4.
Thus, the recorded flow signal may be analyzed during or after
collection to categorize breaths as described heretofore.
Such an analysis can be tabulated in several ways, which
permit either diagnosis of subtle elevations of upper airway
resistance (not resulting in frank apnea) or to adjust a single
prescription pressure of CPAP in a well standardized manner
either in the laboratory or on the basis of home studies.
Possible tabulations of percent time or numbers of breaths with
normal, intermediate, and flow limited contours may include time
of night, patient position (which can be recorded simultaneously
with a position sensor 122 in the mask or on the patient's body),
sleep stage {as recorded separately) and controlled positive
pressure.
The controlled positive pressure could be constant through
out the night or varied in several ways to gain diagnostic and
therapeutic information of relevance to a patient's condition.
For example, the controlled positive pressure could be changed
throughout a night manually in the sleep laboratory by a
technician. Similarly, the controlled positive pressure could
be changed automatically via an automated system, either in
response to feedback control or using pre-set ramps or steps in
the controlled positive pressure throughout the night (in
laboratory or at home). Likewise, the controlled positive
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pressure could be changed on multiple individual nights, e.g.,
at home.
As shown in FIG. 18, the flow waveforms may be recorded in
a recording device, such as a microprocessor 80 with associated
memory 81. As heretofore described, data values may be recorded
while the patient is using a self-adjusting controlled positive
pressure apparatus described herein. Similarly, data values may
be recorded while the patient is connected to a constant-pressure
air supply having a flow sensor. Such flow waveforms are
20 obtained at positive airway pressures above ambient pressure.
However, to determine the frequency and severity of flow
limitations and apnea in a patient who is not receiving therapy,
it is necessary to obtain flow waveforms when the patient is
breathing at ambient pressure.
Figures 19, 2o and 21 illustrate a device of the present
invention wherein a nose fitting 102 is used without connection
to a breathing gas supply for obtaining flow data values at
ambient pressure. The nose fitting is connected to a pressure
or flow sensor 104 which supplies data values to a microprocessor
100 via a multiplexer 166 and analog-to-digital converter 118.
Software for storing and analyzing the data may be stored in
read-only program memory 111, while the data values are stored
in random-access memory or non-volatile memory 114.
Additionally, an oximeter 120 and/or similar diagnostic devices
may be connected to the patient and multiplexer for generating
additional data values for use and storage by the microprocessor.
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An alternative nasal connection could be achieved by using
a "standard'° nasal cannula commonly used for supplying
supplemental oxygen therapy to patients. Such a cannula does not
provide a seal between the nasal prong and the naris, so it does
not capture all the air flowing to and from the patient.
However, it does capture the small pressure fluctuations in the
nares and transmit them to an attached pressure sensor to provide
a signal representative of the actual flow waveform shape.
The recording device may be configured with a microprocessor
120 which uses a sample-and-hold circuit, and an analog-to
digital converter 118 to digitize samples of analog voltage
signals at a suitable rate and resolution to capture relevant
waveform detail, e.g., fifty samples per second rate and
resolution of one part in 256 ("eight bit") for breathing flow
waveforms. The digitized samples are then stored in time-
sequential order in a non-volatile memory device 114, e.g.
magnetic disk drive, "flash" memory, or battery-backed random-
access memory.
In order to record more than one signal, e.g. flow and
pressure waveforms and position signal, in time-correlated
sequence, the individual signals can be repetitively sequentially
connected to the sample-and-hold circuit by a multiplexer circuit
116. All of these recording device circuit and devices are well
known to one skilled in the art of electronic circuit design, and
can readily be obtained commercially.
In order to enhance the diagnostic potential of this flow
waveform sensing and analyzing technique, the flow sensor 104
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could be combined with a position sensor 112 to determine the
- dependence of position attributes of the flow limitation.
Likewise, adding pulse oximetry 120, which measures the
oxyhemoglobin saturation level in the patient's blood, to the
flow and position measurements would provide a very useful
combination of diagnostic signals, adequate to diagnose and
document the severity of the upper airway obstructions.
The following describes a method of diagnosing and treating
obstructive sleep apnea and upper airway resistance syndrome
using the methods and apparatus for determining flow limitation
in a patient as heretofore described. At present, a patient
seeking physician treatment has symptoms of excessive sleepiness
and possibly snoring, in spite of the patient apparently spending
enough time in bed to provide adequate sleep. Such symptoms may
or may not be indicative of obstructive sleep apnea and require
further analysis, typically in an overnight stay in a sleep lab.
Under the present invention, the physician provides the patient
with a diagnostic device for use during sleep at home. The
diagnostic system records flow and pressure waveforms as
previously described, using a nose mask, nasal cannula or similar
nasal fitting having a flow restrictor and a pressure and or flow
transducer. While the patient is using the diagnostic device
at home, the digitized waveforms are stored in nonvolatile memory
such as flash memory, floppy or hard disk, or battery-powered
random-access memory (RAM). One or two additional measurements
may optionally be recorded: patient sleeping position from a
position sensor on the patient, and blood oxyhemoglobin
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saturation level (in percent Sa02) from a device such as a pulse
oximeter. Since the value of these two measurements do not
change relatively rapidly (one value per second for each
additional measurement versus fifty values per second for flow),
the memory storage requirements would not be increased
significantly.
After using the diagnostic device to record the desired
parameters while sleeping for one or more nights, the patient
returns the device or data storage unit, e.g., a disk or non-
l0 volatile memory card, to the physician. The physician extracts
the data from the storage, and analyzes it to determine the
amount of flow limitation and apnea present, along with the other
two parameters, if they were recorded. The resulting analysis
is used to determine whether the patient needs a more detailed
sleep study (in a sleep lab or in the home), or whether therapy
should be started without further studies.
If the decision is to start therapy because sufficient flow
limitation and/or apnea is present, the patient is provided with
a self-adjusting therapy device for home use of the method of the
present invention described heretofore. The home therapy device
also incorporates a recording component which records flow,
pressure and one or two optional parameters as described above.
After using this therapy device during sleep for one or more
nights, the data is returned to the physician. The physician
analyzes it to document the reduction of flow limitation and
apnea achieved by the therapy device, to document the reduction
in Sao2 desaturations if the optional parameter was recorded, and
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to determine whether the patient's condition could be effectively
treated by a less expensive therapy device which is not self-
adjusting, for example standard continuous positive airway
pressure.
The patient is then given the appropriate therapy device,
or, if anomalies in the breathing pattern are observed during the
recorded therapy nights, the patient may be referred for a more
extensive sleep study. After the patient has been using the
therapy device for several weeks or months, a repeat use of the
self-adjusting therapy device with recording component for a
follow-up study should be accomplished. The data are analyzed
as above, and the appropriate actions taken.
FIG. 22 shows a method of diagnosing and treating a patient
who reports excessive sleepiness and perhaps also snoring.
Initially at step 150, the patient reports being excessively
sleepy and possibly having snoring episodes, perhaps raucous,
raspy snoring with abrupt interruptions of the snoring sounds
characteristic of obstructive apneic episodes. At step 152, the
patient is instructed how to use the diagnostic device and how
to position the sensor(s). The diagnostic device collects flow
data, and optionally, position and/or oximetry data. The data
is collected at a rate sufficiently high to capture the details
of each waveform. For flow, a rate of fifty samples per second
is appropriate, while position and oximetry only require one
sample per second each. The data file also contains information
to correlate the data with time and date, so sleeping patterns
can be ascertained.
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Prior to the period of diagnostic sleep, the patient puts
on the sensor (s) , turns on the diagnostic device, and goes to
sleep. When the patient awakes in the morning, the patient turns
the diagnostic device off. During the period of sleep, the diag-
nostic device collects the data as described above. If the
diagnostic procedure is to be a multi-night period of sleep, then
the patient repeats this data collection phase for however many
nights are required. At step 154, and after the required number
of nights of data collection, the patient returns the diagnostic
device with the stored data. If the study will be extended, then
the patient removes the data storage module and returns only the
module to the physician.
The stored data are analyzed at step 156 to determine the
amount and severity of flow limitation, and the number and
severity of apneas, if any. This analysis can be performed
either manually, or preferably by an automated process such as
the methods for determining flow limitation as described
heretofore. At step 158, a decision is then made as to whether
the patient has flow limitation. If there is no evidence of flow
limiting or apnea in the stored data, then the patient is
referred at step 160 to a sleep lab for a more comprehensive
study to determine whether the patient has other problems such
as restless legs syndrome, etc.
If flow limitation is present, or apneas are found, then the
patient is instructed how to use the self-adjusting controlled
positive pressure therapy device, step 162. The therapy device
is equipped with a module which collects and stores flow and
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pressure data, along with the (optional) position and oximetry
data, as described above. Note that this step includes
collecting data from a pressure sensor measuring the pressure at
the airflow outlet of the therapy device. Such pressure data
could also be obtained from a pressure sensor connected to the
patient attachment device (ADAM shell or nasal mask, etc.).
Although less desirable because of the uncertainty of actual
pressure at the patient, the pressure data could be replaced by
data representing the blower speed, if the therapy device adjusts
pressure by changing blower speed. The patient sleeps at home
with the therapy device for the required number of nights.
During each night, the therapy device collects and stores the
data as described above.
At step 164, the patient returns the therapy device or its
data storage module for analysis of the stored data after the
required number of nights of data collection. Then, the stored
flow data are analyzed at step 166 to determine the amount and
severity of flow limitation, and the number and severity of
apneas, if any exist. The flow limitation analysis can be
performed either manually, or preferably by an automated process
such as by the methods described heretofore. The stored pressure
data are analyzed at step 168 to determine the pressure required
to alleviate flow limitations and apneas, and the distribution
' of required pressures during the diagnostic period of sleep.
When the pressure data values are collected from the outlet of
the therapy device, then the data values can be corrected to
reflect the estimated pressure at the nasal fitting if the
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resistance of the hose or tubing between the outlet and the nasal
fitting is known. For example, Pnose = Pout~et - FLOW * RESISTANCE.
At step 170, a decision is made whether the analysis
determines that there are still a significant number of sleep
s disordered breathing episodes during the night, and/or that the
therapy device is incapable of alleviating such episodes at its
highest pressure limit. Such a limit may have been selected by
the prescribing physician at a level less than the maximum
capability of the therapy device. If the therapy device did not
restore normal breathing, then the patient is referred to a sleep
lab for a more comprehensive study, step 160.
If the therapy device restores normal breathing patterns for
the patient, the pressure data are reviewed at step 172 for the
proper prescription of a controlled positive pressure therapy
device. If the peak therapy pressures fluctuate significantly,
then the patient is provided with a prescription for a self-
adjusting controlled positive pressure therapy device for
continued home use, step 174. To reduce patient cost, such a
device would not necessarily incorporate the data storage
capabilities of the therapy device used for the previous steps
in this method. If, however, the pressure data show consistent
night-to-night peak pressures, and the maximum pressure used to
alleviate sleep-disordered breathing events is relatively low,
e.g., eight centimeters of water pressure or less, then the
patient would be prescribed conventional CPAP (non-self-
adjusting) therapy, step 176.
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While several particular forms of the invention have been
illustrated and described, it will be apparent that various
modifications can be made without departing from the spirit and
scope of the invention. For example, references to materials of
construction and specific dimensions are also not intended to be
limiting in any manner and other materials and dimensions could
be substituted and remain within the spirit and scope of the
invention. Accordingly, it is not intended that the invention
be limited, except as by the appended claims.
