U.S. patent number 3,896,800 [Application Number 05/383,422] was granted by the patent office on 1975-07-29 for method and apparatus for triggering the inspiratory phase of a respirator.
This patent grant is currently assigned to Airco, Inc.. Invention is credited to Anthony B. Cibulka.
United States Patent |
3,896,800 |
Cibulka |
July 29, 1975 |
Method and apparatus for triggering the inspiratory phase of a
respirator
Abstract
A method and apparatus are herein disclosed for triggering the
inspiratory phase of a respirator in response to an inhalation
attempt on the part of a patient being "breathed." The invention is
used on respirators where a positive end expiratory pressure is
provided the patient at the end of his exhalation, that is, the
patient is prevented from exhaling completely by the respirator
retaining a net positive pressure within the patient's lungs. To
sense an inhalation attempt in such respirators and to
simultaneously distinguish such an attempt from a leak in the
patient's breathing line, a patient trigger system is provided
which generates an output signal in response to differential
pneumatic inputs including a variable reference pressure input and
a breathing line pressure input. The reference pressure input
comprises a delayed breathing line pressure which is communicated
to the triggering device during a portion of a patient's exhalation
phase so that the reference pressure closely approaches and tracks
the breathing line pressure toward the end of the exhalation phase.
At this point, an inhalation attempt even on the part of a feeble
patient is operable to trigger the system which, in turn, shifts
the respirator into an inspiratory assistor mode. The net operative
result of such a triggering system is the generation of an
appropriate output signal only in response to breathing line
pressure decay "rates" associated with a patient's attempt to
inhale.
Inventors: |
Cibulka; Anthony B. (Poynette,
WI) |
Assignee: |
Airco, Inc. (Montvale,
NJ)
|
Family
ID: |
23513068 |
Appl.
No.: |
05/383,422 |
Filed: |
July 27, 1973 |
Current U.S.
Class: |
128/204.26 |
Current CPC
Class: |
A61M
16/00 (20130101); A61M 16/022 (20170801) |
Current International
Class: |
A61M
16/00 (20060101); A61M 016/00 () |
Field of
Search: |
;128/145.8,142,142.3,142.2,142.4,145.5,145.6,146.4,146.5,188,196,197,202 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Gaudet; Richard A.
Assistant Examiner: Recla; Henry J.
Attorney, Agent or Firm: Rathbun; Roger M. Bopp; Edmund W.
Mathews; H. Hume
Claims
I claim:
1. A method for triggering a respirator from an expiratory phase
into an inspiratory phase to force gas to a patient through a
breathing line comprising the steps of:
monitoring the pressure within the patient breathing line during
the expiratory phase;
sensing the pressure within the patient breathing line during a
period which commences a predetermined time period after
commencement of the expiratory phase, and
triggering the respirator into the inspiratory phase upon said
patient's attempting to breathe while preventing triggering due to
pressure declines generated by leaks from said system, by effecting
said triggering when a drop in said sensed pressure over a
sub-interval of said period exceeds a minimum predetermined value;
and disabling said triggering irrespective of the level of said
sensed pressure where the average rate of pressure decline over
said subinterval is less than a second pre-determined value.
2. A method according to claim 1 wherein said step of sensing the
rate of pressure drop is carried out during a period commencing 0.2
- 0.5 seconds after commencement of the expiratory phase and ending
at the time the respirator is actuated into the inspiratory
phase.
3. A method for triggering a respirator from an expiratory phase
into the inspiratory phase to force gas to a patient through a
patient breathing line comprising the steps of:
monitoring the pressure within the patient breathing line to
provide a first monitored pressure,
modifying the first monitored pressure with respect to time to
provide a modified pressure whereby the modified pressure
substantially tracks the first monitored pressure at low rates of
change in the first monitored pressure, and
sensing a predetermined minimum pressure differential between said
first monitored pressure and said modified pressure to actuate the
respirator into an inspiratory phase.
4. A method according to claim 3 including the step of providing a
pressure differential responsive means operatively associated with
the respirator and wherein:
said step of monitoring the first pressure within the patient
breathing line includes the step of communicating the patient
breathing line pressure with the pressure differential responsive
means as a first input thereto,
said step of modifying the first monitored pressure to provide a
modified pressure includes the step of communicating the patient
breathing line pressure with the pressure differential responsive
means through a restricted passageway, as a second input thereto
during a portion of the expiratory phase, and
said step of sensing a predetermined minimum pressure differential
to actuate the respirator into the inspiratory phase comprises the
step of triggering the respirator by the operation of the pressure
differential responsive means when the predetermined minimum
pressure differential is sensed between the first input and the
second input.
5. A pressure differential triggering system for shifting a
respirator from an expiratory phase into an inspiratory phase in
response to a patient's effort to inhale, said system
comprising:
a patient breathing line providing communication between the
patient and the respirator,
first and second enclosed chambers, a flexible diaphragm positioned
between said first and second chambers and adapted to be flexed in
response to a minimum predetermined pressure differential between
the pressure in said first chamber and the pressure in said second
chamber,
means for communicating the pressure in said patient breathing line
to said first chamber,
modifying means adapted to sense the pressure in said patient
breathing line and communicate as a reference pressure to said
second chamber, a modified pressure which substantially tracks the
patient breathing line pressure communicated to said first
chamber,
whereby said modified pressure in said second chamber lags the
patient breathing line pressure in said first chamber with respect
to time and rapid changes in pressure in said patient breathing
line cause said minimum predetermined pressure differential between
said first and second chambers to flex said diaphragm, and
means to sense the flexing of said diaphragm to provide an
inspiratory phase triggering signal to the respirator.
6. The improvement according to claim 5 wherein said modifying
means communicates the modified pressure to the second chamber only
during a portion of the expiratory phase and comprises a switching
valve:
said switching valve being operated by a valve actuating means,
and
said valve actuating means being operable to place said switching
valve into a position communicating the modified pressure with the
second chamber during a period commencing 0.2 - 0.5 seconds after
commencement of the expiratory phase and terminating upon the
commencement of the next inspiratory phase.
7. The improvement according to claim 6 wherein said modifying
means includes a pneumatic resistor restricting communicating the
pressure sensed in the breathing line with the second chamber.
8. In a respirator for supplying gas to a patient and having
inspiratory and expiratory phases, the respirator being adapted to
retain a predetermined positive gas pressure within the patient at
the end of the expiratory phase and having switching means to
switch the respirator from the expiratory phase to the inspiratory
phase, the improvement comprising:
a patient breathing line for supplying gas from the respirator to
the patient;
means to monitor the pressure in the patient breathing line during
the expiratory phase,
means to sense the pressure in said patient breathing line during a
period which commences a predetermined time period after
commencement of the expiratory phase; and
means to trigger the respirator into the inspiratory phase upon
said patient's attempting to breathe, said means preventing
triggering due to pressure declines generated by leaks from said
system by effecting said triggering where a drop in said sensed
pressure over a sub-interval of said period exceeds a minimum
predetermined value; and said means further, disabling said
triggering irrespective of the level of said sensed pressure where
the average rate of pressure decline over said sub-interval is less
than a second predetermined value.
9. Apparatus for triggering a respirator from an expiratory phase
into an inspiratory phase of a breathing cycle in reponse to an
effort on the part of a patient to inhale comprising, in
combination:
a patient breathing line communicating between the respirator and
the patient,
pressure differential responsive means,
means for communicating the pressure in said breathing line to said
pressure differential responsive means as a first input
thereto,
means for modifying the pressure in said patient breathing line and
communicating the modified pressure to said pressure differential
responsive means only during a portion of an expiratory phase, as a
second input to said pressure differential responsive means,
said modifying means retarding any pressure variations of the
second input with respect to time, whereby the modified pressure
comprising the second input lags out of phase with respect to the
pressure in the breathing line comprising the first input, and
means responsive to a predetermined minimum pressure differential
between said first input and said second input in said pressure
differential responsive means to trigger the respirator from the
expiratory phase into the inspiratory phase.
10. Apparatus according to claim 9 wherein said pressure
differential means comprises a housing, a flexible diaphragm
separating said housing into a first chamber for receiving said
first input and a second chamber for receiving said second input,
and wherein said flexible diaphragm flexes in response to the
predetermined rate of pressure drop in said first input to trigger
the respirator into an inspiratory phase.
11. Apparatus according to claim 10 wherein said means for
modifying the pressure in said patient breathing line and
communicating the modified pressure to said pressure differential
responsive means operates only during a portion of an expiratory
phase and comprises a switching valve:
said switching valve being operated by a valve actuating means,
and
said valve actuating means being operable to switch said switching
valve into a position communicating the second input with said
pressure differential responsive means during a period commencing
0.2 - 0.5 seconds after commencement of the expiratory phase and
terminating upon the commencement of the next inspiratory
phase.
12. Apparatus according to claim 10 wherein said modifying means
includes a pneumatic resistor restricting a gas passageway formed
by said means for providing the second input to said pressure
differential responsive means.
13. Apparatus according to claim 11 wherein said modifying means
includes a pneumatic capacitor comprising a variable chamber in
communication with said second chamber, said variable chamber
adapted to be reduced in volume in response to a decrease in said
second pressure input.
Description
BACKGROUND OF THE INVENTION
The present invention relates to respirators and, more
specifically, to a method and apparatus for triggering the
inspiratory phase of a respirator of the type which retains an
exhalation positive plateau pressure in the patient's lungs at the
end of expiration only in response to a patient's attempt to
inhale.
The intensive care units of most medical institutions, today,
include equipment for assisting or sustaining the critical
functions of patients who are too feeble, sick or injured to
sustain their own functioning. One such critical function, of
course, is breathing.
For some time now, respirators have been used to assist or even to
entirely control, through forced breathing, a patient's breathing
cycle. In particular, a gas including oxygen or pure oxygen may be
communicated with a patient's lungs, under pressure, in response to
that patient's attempt to inhale. Typically, the patient's
breathing line is provided with a pressure sensor which actuates a
means for communicating pressurized gas to the patient's lungs in
response to the sensing of a predetermined increment of negative
pressure in the breathing line.
For most patients, a respirator having a pressure sensor which
responds to a negative or slight vacuum pressure in the patient
breathing line is satisfactory, however, in some patients it is
necessary to retain a positive pressure within the patient's lungs
for reasons such as preventing alveolar collapse. In such instances
the respirator should be triggered by a pressure drop in the
patient line when the patient attempts to inhale, however, the
pressure within the patient line may not drop below atmospheric
pressure but should trigger at some positive pressure. The basic
triggering sensors, heretofore used, are thus ineffective to sense
a pressure drop occurring wholly above atmospheric pressure,
particularly in patients that are too feeble to overcome the
positive plateau pressure in order to draw a vacuum to actuate the
sensor and shift the respirator into an inspiratory phase.
Prior efforts in overcoming these problems have included various
means for biasing the trigger so that a patient can shift the
respirator into an inspiratory phase without having to draw a
vacuum but merely by making some attempt at inhalation.
However, one drawback of such biased trigger systems has been that
the biasing force must be pre-set and bears no relationship to a
varying patient breathing line pressure which variation may be
caused by a slow or moderate leak. Accordingly, an inherent
disadvantage accompanies such systems, in that a slow or moderate
leak, which would not be detected by a low pressure alarm system,
may produce a pressure drop increment which would be sensed by the
triggering system as a patient's effort to inhale. This, in turn,
would cause the premature shifting of the respirator into an
inspiratory phase.
Therefore, it would be advantageous if a method and apparatus were
provided for discriminately shifting a respirator into an
inspiratory phase only in response to a patient's effort to inhale
and independent of any slow or moderate leaks which may occur in
the patient's breathing line.
SUMMARY OF THE INVENTION
The foregoing drawbacks in present respirator triggering methods
and apparatus are overcome by providing a respirator which is
capable of maintaining a positive end expiratory pressure in the
patient's lungs at the end of exhalation and yet which can safely
and effectively be triggered by an attempt by the patient to inhale
to shift the respirator into the inhalation mode, where the
pressure in the patient line need not go below atmospheric pressure
but may be retained at a positive pressure, yet a gradual depletion
of pressure in the patient line, such as may be caused by a leak in
that line, will not cause triggering of the respirator.
In the present method and apparatus, triggering is achieved in a
positive end expiratory pressure respirator by the provision of a
patient triggering system having differential inputs for detecting
the rate of pressure decay. A first input represents the patient's
breathing line pressure while a second input represents a variable
reference pressure consisting of a delayed breathing line pressure
obtained during most of the patient's exhalation phase. The delayed
breathing line pressure gradually approaches the breathing line
pressure even when a leak exists in the breathing line.
Accordingly, the system automatically adjusts for breathing line
pressure decay and will actuate the inspiratory cycle only in
response to a predetermined rate of drop in breathing line pressure
with respect to the variable reference pressure.
BRIEF DESCRIPTION OF THE DRAWINGS
While the invention is particularly pointed out and distinctly
claimed in a concluding portion of the specification, a preferred
embodiment is set forth in the following detailed description which
may be best understood when read in connection with the
accompanying drawings in which:
FIG. 1 is a graph illustrating a patient's breathing cycle under a
"controller mode" of a respirator wherein the inspiratory phase is
initiated independently of the patient's efforts to inhale;
FIG. 2 is a graph illustrating a patient's breathing cycle under an
"assistor mode" wherein an inspiratory phase is initiated by a
patient's efforts to inhale;
FIG. 3 is a diagrammatic illustration of a prior art,
non-differential patient trigger and respiratory system which may
be utilized to produce the patient's breathing cycle illustrated in
FIG. 2;
FIG. 4 is a graph illustrating a patient's breathing cycle when
assisted by a respirator providing positive pressure to the
patient's lungs at the end of an expiratory phase and triggered by
the patient's efforts to inhale;
FIG. 5 is a diagrammatic illustration of a prior art, mechanically
biased, non-differential patient trigger utilized to produce the
patient breathing cycle set forth in FIG. 4;
FIG. 6 is a diagrammatic illustration of a prior art, pneumatic,
differential patient trigger device having means for pre-setting a
reference pressure input;
FIG. 7 is a graph illustrating a patient's breathing pressure cycle
utilizing the triggering devices shown in FIGS. 5 and 6 when a
small leak exists in the patient's breathing line;
FIG. 8 is a graph of a patient's breathing cycle utilizing the
patient trigger devices shown in FIGS. 5 and 6 when a relatively
large leak exists in the patient's breathing line;
FIG. 9 is a diagrammatic illustration of apparatus according to the
present invention as arranged during an inspiratory portion of an
operating cycle;
FIG. 10 is a diagrammatic illustration of the switching valve
portion of the apparatus of FIG. 9 as arranged during an expiratory
portion of an operating cycle;
FIG. 11 is a graph illustrating a patient's breathing cycle during
the operation of the triggering device of the present invention
illustrated in FIGS. 9 and 10;
FIG. 12 is a graph illustrating a patient's breathing cycle
utilizing the apparatus according to the present invention with a
breathing line having a small leak; and
FIG. 13 is a graph of a patient's breathing cycle utilizing the
apparatus and method according to the present invention in
association with a breathing line having a relatively large
leak.
THE PRIOR ART
Referring now to the drawings which like numerals are used to
indicate like parts throughout the various views thereof, FIG. 1
presents a graph illustrating a patient's breathing cycle while a
respirator is in a "controller mode" and the patient's inspiratory
phase is initiated independently of any effort to inhale.
As can be seen, the controller mode induces forced breathing to a
patient and is especially helpful when a patient is virtually
unable to initiate or sustain breathing on his own. Of course, the
breathing cycle induced under the controller mode is not
necessarily synchronized with the natural breathing cycle of the
patient and may make the patient hypoxic or may cause
hyperventilation. Therefore, it is desirable to have the patient's
breathing cycle controlled by the patient's natural demand for air
and in accordance with the patient's normal inspiratory and
expiratory characteristics.
For this reason, most respirators provide the above-mentioned
controller mode only during such times as the patient is incapable
of generating minimum breathing activity. Accordingly, a patient
actuated trigger mechanism is generally provided for triggering an
inspiratory phase upon the sensing of an attempt on the part of the
patient to inhale.
FIG. 2 shows a patient's breathing cycle while under a respirator
operating in an assistor mode. Generally, the patient trigger is
preset so that the sensing of a certain incremental negative
pressure s is operable to actuate the trigger, which, in turn,
shifts the respirator into an inspiratory phase.
It will be observed, that the cycle shown in FIG. 2 requires that
the patient draw a negative pressure, i.e., a vacuum, before the
respirator initiates an inspiratory phase. Had the patient not been
able to draw the vacuum required, the increment s would not have
been produced and the machine would have operated in the controller
mode illustrated by FIG. 1 by means of apparatus well known in the
art. In this sense, the controller mode overrides the assistor mode
when the patient is unable to generate the necessary increment s to
initiate the inspiratory phase of his breathing cycle.
FIG. 3 sets forth a diagrammatic representation of prior art
apparatus which may be utilized to achieve the patient breathing
pressure curve set forth in FIG. 2. Essentially, the apparatus
includes a housing 10 defining a breathing line pressure input
chamber 12 which is enclosed by a flexible diaphragm member 14. A
gas line 16 communicates the chamber 12 with the patient's
breathing line 18 which, in turn, communicates with the respiratory
unit 20. As used herein, the term "patient's breathing line"
includes any gas line in communication with the patient's lungs for
delivery of gas thereto, and may also include any further gas line
adapted to track the pressure in the patient's lungs.
A respirator control unit 22 may be provided and is actuated in
response to the movement of the flexible diaphragm 14. Any one of
various different devices may be used to detect the movement of the
diaphragm 14 in response to the patient's drawing of a vacuum in
the chamber 12. Such movement detectors include sensitive
electrical switches, direct electrical contact of points (one of
which moves with the diaphragm), light source and photo-cell
arrangements, and fluidic detectors.
For the purpose of diagrammatic illustration only, a member 24 is
shown to extend from the flexible diaphragm 14 into electrical
switching relationship with the electrical controller unit 22.
While the above described triggering scheme is satisfactory for
most applications, it is sometimes desirable to keep a positive
pressure in the patient's lung alveoli at all times. In such cases,
a positive pressure is maintained in the patient's lungs at all
times, including the entire expiratory phase.
FIG. 4 is a graph of a patient's breathing cycle wherein positive
pressure is maintained in the lungs at all times. Pressure curve 26
represents the minimum positive pressure to be maintained within
the patient's lungs except for a momentary decrease in pressure
occasioned by the patient's effort to inhale. The pressure level 26
is generally referred to as the positive end expiration pressure
and hereinafter will be referred to as the PEEP.
It can be seen by reference to FIG. 4, that when a PEEP is
maintained in a "breathed" patient, an apparatus such as that
illustrated in FIG. 3 is ineffective in that it would be difficult
for even a healthy patient to draw sufficient vacuum within the
chamber 12 to initiate the inspiratory phase of his breathing
cycle. Therefore, a prior art apparatus such as that illustrated
diagrammatically in FIG. 5 is sometimes utilized to assist the
patient in initiating an inspiratory phase.
More specifically, FIG. 5 illustrates a modification of the
apparatus shown in FIG. 3 wherein a mechanical bias is applied to
the flexible member 14. The bias is of the same polarity as the
force created by a patient's attempt to inhale so as to assist the
patient in moving the flexible diaphragm against the positive
pressure within chamber 12. Accordingly, a spring 28, or the like,
may be operatively associated with the diaphragm 14 to urge the
diaphragm toward an inspiratory phase initiating position. The
magnitude of the bias force applied by the spring 28 may be
adjusted by a lead screw 30, or the like, so as to pre-set the
pressure drop increment necessary to move the diaphragm 14.
The apparatus of FIG. 6 operates essentially in the same manner as
the apparatus of FIG. 5. A pneumatic bias is applied to the
diaphragm 14 by supplying pressurized gas to a reference chamber 32
formed in the overall housing 10 of the trigger assembly which
reference pressure may be adjusted by means of a regulator valve
34. By applying a positive reference pressure to the chamber 32,
the flexible diaphragm 14 may be moved in the actuating direction
in response to a pressure drop in chamber 12 which causes the
pressure therein to be reduced a predetermined pressure increment
below the reference pressure.
While the operation of the triggers illustrated diagrammatically in
FIGS. 5 and 6 is generally satisfactory when a positive pressure is
maintained within a patient's lungs under ideal conditions, if a
leak should develop in the patient's breathing line, such prior art
triggers are likely to initiate an inspiratory phase
prematurely.
In particular, by reference to FIG. 7, it can be seen that when a
small leak develops in the patient's breathing line, the pressure
in the patient's breathing line may fall the s increment beneath
the reference pressure (which in this case is PEEP) and initiate an
inspiratory phase prematurely. FIG. 8 illustrates the condition
that would exist if a larger leak were to develop within the
patient's breathing line which leak may not be sufficiently large
to actuate a low pressure alarm or other alarm system, typically
installed in existing prior art respirators.
It can further be seen by reference to FIG. 8, that a larger leak
would initiate the inspiratory phase entirely too early putting the
respirator machine into a free running condition which is liable to
cause hyperventilation in the patient.
Clearly, such a condition is highly undesirable.
THE PREFERRED EMBODIMENT OF THE PRESENT INVENTION
Referring now to FIGS. 9 and 10, apparatus according to the present
invention is shown diagrammatically to include a housing 10 having
a flexible diaphragm 14 mounted therein to define a breathing line
pressure chamber 12 and a variable reference pressure chamber 32. A
gas line 16 communicates the chamber 12 with the patient's
breathing line 18 which, in turn, communicates with the respiratory
unit 20.
The reference pressure chamber 32 is connected with the patient's
breathing line 18 via lines 16, 17 and 36. A pneumatic resistor 38
and a pneumatic capacitor 40 are operatively connected within the
lines 17 and 36, respectively, to delay the pressure build-up in
the reference pressure chamber 32 when the chamber 32 is
communicated with the pressure of breathing line 18. A switching
valve 42 is operable to vent the reference pressure chamber 32 to
atmosphere through tube 46 when in the position illustrated in FIG.
9 and to communicate the reference pressure chamber 32 to the
patient's breathing line 18, when in the position illustrated in
FIG. 10. When in this last mentioned position, both the reference
pressure chamber 32 and the compliant pneumatic capacitor 40 are
gradually pressurized to the pressure existing in the line 18.
The system of the present invention functions by varying the
reference pressure in response to variations in the pressure of the
breathing line 18 so that the trigger will actuate in response to a
pressure drop at "rates" which are consistent with the rates
produced by efforts of patients to inhale but will not actuate an
inspiratory phase in response to pressure drop at rates consistent
with small or moderate leaks in the overall system supplying the
patient.
For illustration purposes, FIG. 11 shows a pressure curve (solid
line) for a patient's breathing cycle while the desired PEEP
comprises a broken line curve 44. The reference pressure curve
showing the pressure in chamber 32 is indicated as dotted line 46.
It should be kept in mind that an inspiratory phase will be
initiated when the solid pressure curve falls s below the reference
pressure curve 46.
During the inspiratory phase, the valve 42 is switched to vent the
reference pressure chamber to atmosphere through the exhaust port
46.
A solenoid 48 may be connected to the switching valve 42 so that
the valve 42 is maintained in the venting position shown in FIG. 9
during a patient's inspiratory phase. The solenoid may be activated
at some time after the start of the expiratory phase. However, the
switching of the valve 42 to the position shown in FIG. 10 must be
delayed for a time period T.sub.D. This period should generally be
in the range of 0.2 to 0.5 seconds.
During the time period T.sub.D, the pressure in a patient's lung
and airway decreases to approximately the ideal PEEP level 44. At
the end of period T.sub.D, the switching valve 42 is switched to
the position as shown in FIG. 10, thereby connecting the trigger
reference chamber 32 and the pneumatic capacitor 40 to the
patient's airway and lungs by way of the patient breathing line 18
and the pneumatic resistor 38. The resulting exponential increase
in pressure in the pneumatic capacitor 40 and trigger reference
chamber 32 is gradual because of the time delaying effect of the
capacitor 40 and the pneumatic resistor 38. The capacitor 40 may be
formed with an elastic compliant wall 50 so as to provide the
damping effect of a large capacitor without requiring the large
physical dimensions of a rigid wall capacitor having an equivalent
damping or time delaying effect.
As can be seen by reference to FIG. 11, the reference pressure
curve 46 gradually approaches the pressure level in the patient's
lungs (solid curve) during the expiratory phase. When the patient
attempts to inhale, the pressure in the input pressure chamber 12
drops at a fairly rapid rate. The pressure in the reference chamber
32 cannot decrease as rapidly due to the time delaying action of
the pneumatic resistor 38 and thus the pressure in chamber 12 soon
decreases to a pressure s below the pressure in the reference
chamber 32 and thereby lifts the diaphragm 14 upwardly to trigger
the main respiratory unit 20 to the inspiratory phase.
Referring briefly to FIG. 12, a patient's breathing cycle is
illustrated during the condition where the patient's breathing line
has developed a small leak. It can be seen that the reference
pressure 46 tends to track the decaying pressure in the breathing
line 18 so as to prevent premature initiation of the inspiratory
cycle in response to such a leak.
FIG. 13 shows the condition of FIG. 12 wherein a larger leak has
developed in the breathing line 18 which leak may be insufficient
to actuate an alarm system but may be sufficiently large to
eliminate the positive pressure within the patient's lungs at the
end of the expiratory phase. Even under such an extreme condition,
however, it will be seen that the reference pressure nearly tracks
the decaying pressure in the patient's breathing line and an
inspiratory phase is not initiated until the patient attempts to
inhale when the breathing tube pressure line (solid line) falls s
with respect to the variable reference pressure 46.
In order to insure such operation, the pneumatic resistor 38 should
be selected so that the variable reference pressure curve 46 has
the correct response time.
It can thus be seen that through the synergistic cooperation of the
elements comprising the present invention, the apparatus of the
preferred embodiment is uniquely uncomplicated so as to be
relatively inexpensive in manufacture and safe in operation.
The present invention represents a technical advance in the field
in that, through the operation thereof, a respirator operating with
PEEP may be discriminately controlled so that the inspiratory phase
of a patient's breathing cycle will be initiated only in response
to the patient's attempt to inhale. Therefore, the system is not
adversely affected by breathing tube leaks.
While the preferred embodiment is set forth in the foregoing
paragraphs, it is of course to be understood that various
modifications and changes may be made therein without departing
from the invention.
For example, the pressure inputs may be converted into other
parameters, such as electrical voltages, which may then be phase
oriented as indicated in FIGS. 11-13 and directed as inputs to an
electrical triggering device in a manner well known in the
electrical arts. Similarly, it is not necessary that the variable
reference pressure precisely follow the existing breathing line
pressure. All that is required is that the reference pressure
substantially track the existing pressure so that some lateral
spacing of the two pressure lines is permissible.
Accordingly, it is intended to cover in the following claims all
such modifications and changes as may fall within the true spirit
and scope of the present invention.
* * * * *