U.S. patent number 3,910,261 [Application Number 05/478,429] was granted by the patent office on 1975-10-07 for end-tidal gas analysis apparatus for respirators.
This patent grant is currently assigned to Bourns, Inc.. Invention is credited to Charles W. Ragsdale, James Weigl.
United States Patent |
3,910,261 |
Ragsdale , et al. |
October 7, 1975 |
End-tidal gas analysis apparatus for respirators
Abstract
Intermittently operated gas analysis apparatus and a method for
monitoring the proportion of particular gases carried in the
end-tidal portion of breathing. An analysis sample is obtained,
following a predetermined time delay, when a patient expires air
into a respirator. The apparatus is rapidly deactuated at the
beginning of inspiration, and remains off until the patient again
expires breath. By an appropriate setting of the time delay, a gas
sample that accurately represents the end-tidal condition may be
obtained, making the analyzer particularly suitable for monitoring
the CO.sub.2 content of expired breath.
Inventors: |
Ragsdale; Charles W.
(Riverside, CA), Weigl; James (Rialto, CA) |
Assignee: |
Bourns, Inc. (Riverside,
CA)
|
Family
ID: |
23899896 |
Appl.
No.: |
05/478,429 |
Filed: |
June 11, 1974 |
Current U.S.
Class: |
600/532;
73/863.01; 422/84 |
Current CPC
Class: |
A61B
5/0836 (20130101) |
Current International
Class: |
A61B
5/083 (20060101); A61B 5/08 (20060101); A61B
005/00 () |
Field of
Search: |
;128/2.07,2.08,2R,2C
;73/421.5,23 ;23/254C,254R |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
829,409 |
|
Mar 1960 |
|
GB |
|
261,798 |
|
May 1968 |
|
OE |
|
1,915,959 |
|
Oct 1970 |
|
DT |
|
Other References
"Auto. Measurement of Lung Function" The Lancet, Sept. 18, 1965,
pp. 573-574..
|
Primary Examiner: Gaudet; Richard A.
Assistant Examiner: Cohen; Lee S.
Attorney, Agent or Firm: Koppel; Richard S. Becker; William
G.
Claims
What is claimed is:
1. End-tidal gas analysis apparatus suitable for use with a
respirator of the type having an air source actuated in response to
patient demand and a conduit system which includes inspiratory and
expiratory conduits for respectively delivering air to and
receiving air from a patient, said apparatus comprising:
means for analyzing gas contained in said conduit system,
means for supplying gas from said conduit system to said analyzing
means,
actuator means adapted to sense the beginning of patient expiration
and to initiate operation of said gas supplying means after a
predetermined delay period following the beginning of said patient
expiration, and
deactuator means adapted to sense the beginning of patient
inspiration and rapidly terminate operation of said gas supplying
means in response thereto,
said actuator means and deactuator means cooperating to operate
said gas supplying means for substantially the full interval
between the end of said delay period and the beginning of patient
inspiration, and to inhibit operation of said gas supplying means
during substantially the full remainder of the time the analysis
apparatus is in use.
2. Apparatus according to claim 1, wherein said deactuator means
includes a flow rate sensor adapted to sense gas flows associated
with the beginning of inspiration.
3. Apparatus according to claim 1, wherein said gas supplying means
includes means for extracting a gas sample from said respirator
conduit system for delivery to said analyzing means, the operation
of said extracting means being controlled by said actuator means
and deactuator means.
4. Apparatus according to claim 1, wherein said deactuator means
includes a gas flow rate sensor disposable in the inspiratory
conduit, said sensor being adapted to produce a signal in response
to a gas flow through said inspiratory conduit exceeding a
predetermined threshold level, and a processing circuit means for a
signal produced by said sensor.
5. End-tidal gas analysis apparatus suitable for use with a
respirator of the type having an air source actuated in response to
patient demand, a conduit system which includes inspiratory and
expiratory conduits for respectively delivering air to and
receiving air from a patient, and gas flow control valves disposed
respectively in said inspiratory and expiratory conduits, said
apparatus comprising:
means for analyzing gas contained in said conduit system,
means for supplying gas from the portion of said conduit system
between said flow control valves to said analyzing means,
actuator means adapted to sense the beginning of patient expiration
and to initiate operation of said gas suppling means after a
predetermined period of time following the beginning of said
patient expiration, and
deactuator means adapted to sense the beginning of patient
inspiration and rapidly terminate operation of said gas supplying
means in response thereto,
said actuator means and deactuator means cooperating to operate
said gas supplying means for substantially the full interval
between the end of said delay period and the beginning of patient
inspiration, and to inhibit operation of said gas supplying means
during substantially the full remainder of the time the analysis
apparatus is in use.
6. End-tidal gas analysis apparatus suitable for use with a
respirator of the type having an air source actuated in response to
patient demand and a conduit system which includes inspiratory and
expiratory conduits for respectively delivering air to and
receiving air from a patient, said apparatus comprising:
a gas analysis device;
a pump means for pumping gas from said conduit system to said
analysis device;
a signal processing circuit means controlling the operation of said
pump means; and
first and second gas flow sensors disposable respectively in said
expiratory and inspiratory conduits, said first and second sensors
being respectively connected to said signal processing circuit
means to provide input signals when the sensed gas flows exceed
predetermined threshold rates;
said signal processing circuit means including means to actuate
said pump means after a predetermined period of time following
receipt of an input signal from said first sensor, and further
including rapid response means to deactuate said pump means in
response to an input signal from said second sensor.
7. End-tidal gas analysis apparatus suitable for use with the air
supply of a respirator, comprising:
a conduit system including inspiratory and expiratory conduits for
respectively delivering air from said air supply to and receiving
air from a patient,
gas flow control valves disposed respectively in said inspiratory
and expiratory conduits,
means for analyzing gas contained in said conduit system,
means for supplying gas from said conduit system to said analyzing
means, including means for extracting a gas sample from the portion
of said conduit system between said flow control valves,
actuator means adapted to sense the beginning of patient expiration
and to initiate operation of said extracting means after a
predetermined delay period following the beginning of said patient
expiration, and
deactuator means adapted to sense the beginning of patient
inspiration and rapidly terminate operation of said analyzing means
in response thereto.
8. The method of analyzing end-tidal gas expired by a patient into
a respirator conduit system, comprising the steps of:
providing a gas analysing means and a means for supplying gas
thereto from said conduit system;
sensing an expiratory gas flow into the conduit system from the
patient;
actuating the gas supplying means after a predetermined period of
time following the beginning of expiration sensed in the conduit
systems:
sensing an inspiratory gas flow from the conduit system to the
patient; and
rapidly deactuating the gas supplying means in response to the
beginning of inspiratory gas flow sensed in the conduit system
9. The method of claim 8, suitable for measuring the end-tidal
CO.sub.2 content of an expiratory breath, wherein said
predetermined time period is at least sufficient to reach the end
tidal portion of expiration.
Description
BACKGROUND OF THE INVENTION
This inention relates to a gas analysis method and apparatus, and
more particularly to a method and apparatus for analyzing the
end-tidal portion of gas expired by a patient into a
respirator.
It is often desirable to analyze the expired breath of a respirator
patient to determine, for example, the arterial CO.sub.2 content.
This measurement is complicated by the non-uniformity of the
CO.sub.2 concentration, which begins at a small amount and
gradually builds up until a plateau is reached. The CO.sub.2 level
at the plateau, which is commonly referred to as the end-tidal
region, is the true indicator of arterial condition. Gas samples
obtained at other times, either prior to the beginning of the
end-tidal region or after expiration has ended and the patient is
inhaling, can produce an artificially low CO.sub.2 reading.
One popular gas analysis system employs a sensor in the expiratory
path of a respirator to measure the air flow issuing from a patient
and control the operation of the analysis mechanism in response
thereto. The sensor detects the beginning of expiration, which is
characterized by a large outflow of air, and produces a signal to
begin sampling the expired gas. A timer is provided to delay the
sampling long enough for the end-tidal region to be reached, after
which sampling takes place for as long as the expiratory air flow
remains strong enough to sustain a signal from the sensor. Sampling
is terminated when the air flow falls below this level, and the
sampling system is reset for the next breath cycle.
While this system generally restricts sampling to the expiratory
phase of breathing, a problem exists in that the expiratory air
flow may fall below the threshold rate for terminating sampling
near or even before the beginning of the end-tidal region. If
sampling does terminate before the end-tidal region is reached, any
CO.sub.2 reading will not accurately reflect the arterial
condition. Even if sampling terminates after the end-tidal region
has begun, the possibility of premature sampling may encourage the
choice of time delay for beginning sampling that runs out
considerably before the end-tidal region, again producing an
erroneous arterial CO.sub.2 indication. In any event, the very
small expiratory flow which characterizes the latter portion of the
end-tidal region has rendered attempts to sample for the full
end-tidal period quite difficult.
In another system known to the art gas samples are obtained at a
location comparatively remote from the patient, downstream from the
valve which shuts off the expiratory conduit during inspiration,
and sampling is not begun until the expiratory phase has terminated
and inspiration has been detected. While this system endeavors to
eliminate the problems associated with sampling before the
end-tidal region has been reached, the advantages of obtaining gas
samples directly as they are expired from the patient are lost.
SUMMARY OF THE INVENTION
In view of the above-stated problems existing in the prior art, it
is an object of the present invention to provide a novel and
improved system and method for analyzing the end-tidal portion of
expired breath.
Another object is the provision of a novel and improved system and
method by which gas samples are obtained only during the end-tidal
portion of expiration, and which permit samples to be taken over a
substantial amount of that portion.
Still another object is the provision of novel and improved
apparatus and a method for analyzing the end-tidal portion of
expiration in which the acquisition of gas samples is controlled in
response to large gas flows.
In the accomplishment of these and other objects, an end-tidal gas
analysis apparatus and method is provided by the present invention
for use with a respirator of the type having an air source actuated
in response to patient demand, and a conduit system including
inspiratory and expiratory conduits for respectively delivering air
to and receiving air from the patient. Broadly described, the
apparatus includes a gas analyzing device capable of testing gas
within a respirator conduit system, an actuator adapted to initiate
operation of the analyzing device a predetermined period of time
following the beginning of expiration, and a deactuator which
senses the beginning of patient inspiration and rapidly terminates
operation of the analyzing device in response thereto. With an
appropriate interval between the beginning of expiration and
analysis, the analysis period may be set to occur substantially
entirely within the end-tidal period, and to extend over a large
portion thereof.
According to more particular features of the invention, gas flow
rate sensors are disposed in both the inspiratory and expiratory
conduits and deliver input signals to a signal processing circuit
when gas flows are detected. The processing circuit includes an
amplifier, a threshold device adapted when its threshold is
exceeded to terminate operation of the analyzing device, A.C.
coupling between the amplifier and threshold device, and a D.C.
restoration circuit between the A.C. coupling and the threshold
device. D.C. bias is substantially removed by the restoration
circuit from the signal applied to the threshold device, thereby
enabling the threshold level to be rapidly exceeded and analysis
terminated when inspiration begins.
According to another feature, a means such as a pump is provided to
extract gas samples from the respirator conduit system for delivery
to the analysis device. By sampling between flow control valves in
the inspiratory and expiratory conduits, expired gas may be
obtained directly from the patient while expiration is still going
on.
Further objects, features, and advantages of the invention will
become apparent to those skilled in the art from the following
detailed description of a particular embodiment, along with the
drawings in which:
FIG. 1 is a somewhat idealized graph of a respirator patient's
breathing cycle, with the CO.sub.2 concentration in the respirator
conduit system indicated in dashed lines;
FIG. 2 is a functional diagram of the analysis apparatus shown in
conjunction with a respirator system;
FIG. 3 is a block diagram of electrical circuitry employed in the
invention; and
FIG. 4 is a schematic diagram of a processing circuit for
controlling the operation of the gas analysis apparatus in response
to patient inspiration and expiration.
DETAILED DESCRIPTION
Referring first to FIG. 1, a typical air-flow pattern to and from a
patient assisted by a respirator is shown, with inspiration
indicated by a negative flow level and expiration by a positive
flow level. At the end of inspiration (coinciding with the
beginning of expiration), designated time T.sub.1 in the figure, a
large flow of air is expelled by the patient into the respirator.
The outward air flow gradually tapers off to a very low level,
indicated by reference numeral 2, followed by a pause with
essentially no air flow until the patient again draws breath at
time T.sub.3. Thereafter the breathing sequence continues in a
repetitive sequence of inspirations and expirations.
An indication of the patient's arterial CO.sub.2 concentration may
be obtained by analyzing his expired breath; this in turn can be
accomplished by measuring the CO.sub.2 level in the respirator
conduit or air tube network. This latter quantity is indicated by a
dashed line in FIG. 1. The CO.sub.2 concentration during
inspiration, indicated by reference numeral 4, is generally that of
the ambient atmosphere, or about 0.03%. During expiration the
concentration climbs from the ambient level to a plateau 6 at a
peak of about 5.4% for healthy patients. The plateau 6, which
extends through approximately the final third of expiration, is
referred to as the end-tidal level and is the true indicator of
arterial CO.sub.2 concentration. It will be noted from the drawing,
however, that the end-tidal region may be reached near or even
after the time the expiration flow has fallen to a low level 2. If
this flow rate is used to indicate the end of expiration, at which
time sampling is terminated, the samples obtained may represent a
period prior to the end-tidal region and an artificially low
CO.sub.2 reading will result.
Briefly stated, the present invention achieves an accurate
end-tidal sample by sensing the beginning of patient expiration at
T.sub.1, initiating sampling during the end-tidal region at a time
T.sub.2 after expiration begins, and rapidly terminating sampling
when the next inspiration is sensed at time T.sub.3. Sampling may
be conducted during a large portion of the end-tidal region, and
control functions need be operated only in response to large air
flows.
A respirator system employing gas analysis apparatus that achieves
this result is shown in block diagram form in FIG. 2. The
respirator includes an air supply device 8, which provides a
desired air mix and pumps the air through a check valve 10 into an
inspiratory conduit such as plastic tube 12 in response to patient
demand. Expired air is routed through an expiratory tube 14 to a
chamber 16, and thence out of the respirator system through a
continuation expiratory tube 18. Air is channeled to and from the
patient through a tap 20, while an inflatable bladder valve 22 is
alternately expanded to close off the exit from expiratory tube 14
during inspiration, and relaxed during expiration to permit the
discharge of expired air. Thus far, the described system is well
known to the art, and the construction details thereof may be
tailored in any of several convenient ways according to the needs
of the user.
The novel gas analysis apparatus includes an air pump 24 that is
tapped by a tube 26 into the conduit system at a location between
valves 10 and 22 where expired air can be extracted, a gas analysis
device 28 such as a thermal conductivity cell adapted to receive
air from pump 24, and an electronic processing circuit 30
controlling the operation of pump 24. A first air flow sensor such
as termistor 32 is inserted into continuation expiratory tube 18 to
produce a signal for delivery to processing circuit 30 when an
expiratory air flow is detected. A second sensor such as thermistor
34 is inserted into the inspiratory tube 12 to sense the beginning
of patient inspiration and in response deliver a second signal to
the processing circuit 30. The triggering flow rate for each of the
thermistors 32 and 34 is generally from one to two liters per
minute, although the trigger levels may be varied as desired. For
respirators which incorporate an inspiratory flow sensor to control
a breath assist mechanism, this sensor may also be used in the
analysis apparatus instead of thermistor 34.
Referring now to FIG. 3 for a block diagram of processing circuit
30, a bias circuit 36 supplies a voltage sufficient to establish a
heating current through the thermistors comprising expiratory
sensor 32 and inspiratory sensor 34. The expiratory sensor 32 is
connected via an A.C. coupling circuit 38 to a threshold comparator
40, where the signal is compared with a reference threshold signal
and an output produced for delivery to the set input of a flip-flop
42 when the threshold is exceeded. The output of the flip-flop is
in turn connected to the drive mechanism for pump 24.
Inspiratory sensor 34, also heated by bias circuit 36, is connected
to a D.C. amplifier 44, having a gain factor of 8.5, and thence to
an A.C. coupling and D.C. restoration circuit 46, where the A.C.
component of the amplified sensor signal undergoes a D.C. shift.
The output of circuit 46 is applied to a threshold device such as
comparator 48, which is turn has an output connected to reset the
flip-flop 42. The aforesaid D.C. shift is such that threshold
device 48 is rapidly triggered at the beginning of inspiration to
produce an output which resets the flip-flop 42.
A delay circuit 49 receives the set output of flip-flop 42 and,
after a predetermined delay period, emits a pulse to energize the
drive mechanism for pump 24. The delay is set at about two-thirds
of the expiratory period, or generally in the area of 7.5 seconds.
The pump drive energizing signal is rapidly terminated, without
appreciable delay, when flip-flop 42 is reset.
Circuit details are shown in FIG. 4 of the processing circuit block
diagram functions illustrated in FIG. 3. A bias voltage of -15
volts is applied to an input terminal 50, which is connected to a
resistor 52. The expiratory an inspiratory thermistors 32 and 34
are connected between ground and circuit nodes 53 and 54,
respectively, and thereafter through resistors 55 and 56 to the
other side of resistor 52 from terminal 50. The thermistors and
their associated resistors are paralleled by a zenor diode 57 and
normally carry a current of about 5.3 ma for still air. The
expiratory thermistor 32 is also A.C. coupled to an operational
amplifier which serves as comparator 40 through an RC circuit
comprising a series connected resistor 58 and capacitor 60 and a
further resistor 62 between ground and capacitor 60, the thermistor
output being applied at the signal input 64 to comparator 40. The
reference input 66 of comparator 40 receives a reference signal
from a voltage divider circuit which comprises resistors 68 and 70
connected between input terminal 50 and ground. The output of
comparator 40 is connected via appropriate coupling circuitry to
the set input of flip-flop 42, which consists of a pair of
cross-coupled NAND gates 72 and 74.
The signal path associated with the inspiration thermistor 34 will
now be traced. The thermistor 34 is connected for amplification to
a standard noninverting D.C. amplifier circuit 44, shown enclosed
in dashed lines, the output 76 of which leads to A.C. coupling and
D.C. restoration circuit 46, also shown enclosed in dashed lines.
This latter circuit is formed from a series connected resistor 78
and capacitor 80, together with a diode 82 which provides a
conductive path to ground on the far side of the series circuit
from amplifier 44. The circuit output is connected to the signal
input 84 of comparator 48, the reference input 86 of which is
supplied with a reference voltage from a voltage divider circuit
which comprises resistors 88 and 90 connected between input
terminal 50 and ground. The output of comparator 48 is connected
via appropriate coupling to the other input of flip-flop 42.
A monostable multivibrator 92 located at the output of flip-flop 42
introduces a delay to the output signal, the duration of the delay
being selectively adjustable by means of an adjustment mechanism
94. The output of flip-flop 42 and the multivibrator 92 are fed to
an AND gate 96, the output of which is applied to the drive
mechanism for air pump 24.
In operation, the respirator functions in the usual manner with
inspiratory air flows supplied on patient demand, followed by an
expiration period. Immediately before expiration begins, there is
little or no air flow past expiratory thermistor 32, and its
resistance is relatively low. A steady state voltage of
approximately-1.5 volts is thereby established at node 53. The air
flow at the beginning of expiration cools thermistor 32, increasing
its resistance so that a negative pulse of from about 0.2 to 1.0
volts for the average person is superimposed onto the steady state
voltage at node 53. This pulse triggers comparator 40 to set
flip-flop 42, delivering an input to multivibrator 92. After the
predetermined delay period the multivibrator produces an output
which, together with the continuing output from flip-flop 42,
triggers AND gate 96. This actuates the pump 24, which now begins
to extract air from the respirator tube network for delivery to the
analyzer 28.
The extraction of a gas sample from the respirator tubes continues
until the beginning of inspiration, at which time the inspiratory
air flow cools the thermistor 34, increasing its resistance so that
a negative voltage pulse is produced at node 54. Absolute voltage
levels at node 54 are approximately the same as at node 53 or a 1.5
volt D.C. level with a 0.2 to 1.0 volt A.C. component. The
inspiratory signal is amplified by amplifier 44, then A.C. -coupled
by capacitor 80 to the comparator 48. Without diode 82, the A.C. -
coupled signal from capacitor 80 would be shifted in the positive
direction, introducing a delay before the signal level could fall
to a level low enough to trigger the comparator 48. Diode 82 avoids
this problem by becoming conductive during the intervals between
inspiration signals, holding the voltage at its anode to about 0.7
volts and charging the capacitor 80. At the appearance of an
inspiration pulse, the threshold level of comparator 48 is rapidly
exceeded (in a negative direction) so that it produces an output
that resets flip-flop 42, thereby removing one of the inputs to AND
gate 96 and very rapidly deactuating pump 24.
Intermittent pumping continues in this fashion during successive
breathing cycles, with the pump 24 operating only during the
end-tidal period. End-tidal gas is accumulated in analysis device
28, and an accurate indication of the arterial CO.sub.2 level or
other end-tidal characteristic may be obtained.
While a particular embodiment of the invention has been shown and
described, numerous additional modifications and variations are
possible in light of the above teachings. It is therefore intended
that the scope of the invention be limited only in and by the terms
of the appended claims.
* * * * *