Adaptive control system for a medical ventilator

A control system for a single-valve controlled medical ventilator adaptively invokes separate flow delivery or flow exhaust control functions in response to the sensed dynamic state of the ventilator without any dependence on measured patient parameters. The control system selects either the flow delivery control system or flow exhaust control system based on a determination of the mode in which the ventilator is operating. The determination is made using a sensed ventilator operational parameter, such as the ratio of manifold pressure to flow output, to select the appropriate control function. A hysteresis routine is provided to prevent "hair triggering" between the flow delivery and flow exhaust control systems.

BACKGROUND OF THE INVENTION 
The invention relates to medical ventilators and control systems therefor. 
Specifically, the invention relates to an apparatus for adaptive control 
of the flow and pressure of gases in a medical ventilator. 
Medical ventilators provide respiratory support and anesthesia to patients 
undergoing medical treatment. The primary function of the ventilator is to 
maintain suitable pressure and flow of gases inspired and expired by the 
patient. Ventilators function in a variety of respiratory control modes, 
each depending on patient status and the judgement of the physician and 
anesthesiologist. Each modal application places different demands on the 
dynamic characteristics of the ventilator. For many applications, 
including intensive care unit (ICU) and anesthesia delivery applications, 
it is important for the ventilator to respond in a timely fashion to quick 
changes in target breathing patterns. In order to provide this degree of 
responsiveness for wide ranges of patient cases, ventilator systems must 
be adaptable to the variations in patient and ventilator dynamics that 
occur over the course of a single breath. Ventilator control systems thus 
require wide adaptability to provide adequate responsiveness throughout 
different modes of operation and during the changes in system dynamics 
that occur over the course of patient breathing. 
In the past, mechanical implements have been utilized for the control of 
the flow and pressure of gases delivered to the patient. Ventilators 
incorporating such pneumatic hardware offered only limited modes of 
operation and frequently required many independently controlled valves and 
pneumatic circuits. Efforts to increase the adaptability of the ventilator 
to more modes of operation, or to increase the responsiveness of the 
ventilator control system, resulted in an increase in cost and complexity. 
There has thus developed a desire to implement relatively simple and 
inexpensive control systems that provide adequate responsiveness to quick 
changes in patient breathing patterns. 
U.S. Pat. No. 5,315,989, issued to the present inventor discloses a medical 
ventilator with a single-valve control system for the flow and pressure of 
inspiratory and expiratory gases throughout the respiratory cycle. The 
single-valve control system is advantageous in reducing the complexity and 
cost of the ventilator. The valve is controlled via microprocessor for 
closed-loop minimization of the error between a sensed parameter, i.e., 
flow or pressure, and a predetermined reference signal provided by a 
waveform generator. The sensed parameter is thereby made to track the 
desired waveform selected in accord with patient breathing patterns and 
status. 
Prior art systems of the type described in U.S. Pat. No. 5,315,989 work 
well when used in ventilation modalities consisting of bi-state valve 
control. For example, in performing Volume Ventilation, the flow valve of 
the prior art system is turned on to a specified level for the inspiratory 
period, then it is turned off to allow exhalation to occur. However, in 
modes of ventilation, such as Pressure and Positive End Expiration 
Pressure (PEEP) control, which require modulation of the flow valve, 
system responsiveness may be insufficient to achieve the response 
requirements of state of the art ICU and anesthesia ventilators. In these 
applications, required response times are typically less than 150 ms to 
achieve 63% full-scale output. Without this level of control 
responsiveness, the ventilator will be unable to generate sharp pressure 
waveforms and may even impose additional Work of Breathing (WOB) on 
patients respiring spontaneously. 
One of the difficulties in designing a control system for ventilators of 
the type described in U.S. Pat. No. 5,315,989, lies in properly adjusting 
the control system to the vastly different dynamic characteristics of the 
ventilator as it repetitively passes from inspiratory to expiratory phases 
of a normal breath cycle. During the inspiratory phase of a patient 
breath, patient parameters, such as patient lung resistance and 
compliance, become a significant part of the patient-ventilator system and 
significantly affect ventilator response. In contrast, during the 
expiratory phase of a patient breath, patient parameters do not 
appreciably affect ventilator dynamics, which are then largely a function 
of the components of the ventilator itself. 
Control systems tailored to produce acceptable responsiveness during the 
inspiratory phase of ventilator operation, which usually requires high 
gain, may become unstable when used to control ventilator dynamics during 
the expiratory phase of ventilator operation. On the other hand, control 
systems that are well-suited for expiratory phase of ventilator operation 
are too sluggish and lack sufficient response for adequate control during 
the inspirational phase of operation. Prior art attempts to solve these 
problems focused on independent mechanical functionality for control of 
the inspiratory and expiratory phases of operation. Such modifications add 
to the cost and complexity of the ventilator apparatus and do not easily 
lend themselves to application in the context of single-valve control 
systems. 
Another drawback of prior art ventilator control systems like those 
described in U.S. Pat. No. 5,315,989, is that the these systems do not 
recognize or adapt to changes in the ventilator-patient system dynamics 
which may occur over the course of a single respiratory breath. For 
example, in pressure control modes, at the end of the inspiratory phase of 
a patient breath, the patient's lung will already have attained the target 
pressure of the ventilator. There will be no gas flow into the patient 
lung. Under these circumstances, the dynamic response of the ventilator 
will be equivalent to the dynamic response during the expiratory phase of 
operation, even though the patient is still in the inspiratory phase of 
the breath. Ventilator control systems which provide control based on 
timed inspiratory and expiratory periods, rather than the actual dynamic 
state of the ventilator, erroneously assume an inspiratory phase control 
model and provide inappropriate over-responsiveness under these 
conditions. 
While prior art patient parameter-based control systems normally produce 
acceptable performance during the lung filling stages of inspiration, they 
may produce unstable performance during the latter stages of the 
inspiratory period where the lung is completely filled. 
A closer inspection of the control problem as outlined reveals that an 
ideal control system for a ventilator such as that described in U.S. Pat. 
No. 5,315,989 must adaptively determine and adjust for variations in 
dynamic operation depending on the mode of operation of the ventilator, 
i.e., whether the ventilator is in a "flow delivery" or "flow exhaust" 
state of operation. As previously noted, these modes of operation do not 
necessarily coincide with the inspiratory and expiratory breath phases of 
the patient. This is particularly evident when the ventilator is used in 
an anesthesia application where a continuous amount of fresh gas flow is 
added to the breathing circuit throughout the inspiratory period. In this 
application the ventilator must transition to its "flow exhaust" state 
during the later stages of inspiration in order to prevent this fresh gas 
flow from raising the pressure of the full lung above its 
clinician-specified target level. Conversely, the ventilator may achieve a 
"flow delivery" dynamic state during the expiratory phase of a breath 
where fresh gas must be delivered to the breathing circuit in order to 
compensate for leakage in the ventilator-patient system or a spontaneous 
respiration. 
Adaptive control schemes for medical ventilators have been disclosed in the 
prior art, but none to date are suited for implementation in ventilators 
utilizing single-valve control. Moreover, prior art systems do not control 
adaptation based on changes in the dynamic state of the ventilator. For 
example, U.S. Pat. No. 5,303,698 describes an adaptive control system for 
medical ventilator. The ventilator utilizes independent control of the 
inspiratory and expiratory branches of the ventilator to achieve high 
speed control of the pressure within a patient's mouth in accordance with 
a selected waveform. The system invokes separate inspiratory and 
expiratory control operations based on measured patient parameters. Hence, 
these prior art systems do not provide adaptive control that is based on 
the dynamic state of the ventilator. Moreover, while such systems may be 
adaptable to single-valve ventilators, they do not solve the control 
problem of managing the changing dynamic characteristics of "flow 
delivery" and "flow exhaust" states of operation. 
There is thus a need for a medical ventilator control system which is 
adaptable to ventilators using single-valve control of inspiratory and 
expiratory flow and pressure and which provides adaptive control which is 
independent of measured patient parameters. Moreover, there is desired a 
ventilator control system which can sense the dynamic state of the 
single-valve ventilator and adapt its control parameters accordingly so as 
to provide ventilation response characteristics in all modes of operation 
which meet or exceed those of typical state-of-the-art anesthesia and ICU 
ventilators. 
SUMMARY OF THE INVENTION 
The present invention solves the problems of the prior art by providing an 
adaptive control system which selects a flow delivery control function or 
a flow exhaust control function depending on the dynamic state of the 
ventilator. Selection is based on a sensed operational parameter, such as 
the ratio of gas pressure to gas flow in the breathing circuit. Switching 
logic is provided with a hysteresis routing to prevent "hair triggering" 
between the flow delivery and flow exhaust control functions. Selection of 
the appropriate control system thus occurs as a function of the sensed 
dynamic state of the ventilator, without any dependence on measured 
patient parameters. The system is readily adaptable to a ventilator 
utilizing single-valve control.

DETAILED DESCRIPTION 
FIG. 1 schematically illustrates a medical ventilator apparatus suitable 
for implementing a preferred embodiment of the present invention. The 
mechanical aspects of the ventilator apparatus are similar to those 
disclosed in U.S. Pat. No. 5,315,989, the disclosure of which is 
incorporated herein by reference. It will be noted, however, that the 
mechanical embodiment of FIG. 1 differs from that described in FIG. 4 of 
U.S. Pat. No. 5,315,989 in that, for example, in accordance with the 
advantages of the present invention, the safety functionality previously 
provided by a safety valve to regulate the pressure provided to the 
expiratory valve is now simply achieved using on/off solenoid valve (not 
shown) disposed upstream of the control valve and controlled by the 
microprocessor. Other differences will be evident from the description 
which follows. 
Ventilator 8 comprises a gas source 10, which typically provides 
pressurized gas at 50 psi, communicates through a primary regulator 12 
with source conduit 14 which supplies flow control valve 4 with breathing 
gas at approximately 26 psi. Flow control valve 16 is preferably a 
proportional solenoid valve and controls the magnitude of gas flow into 
conduit 18. Conduit 20 communicates with conduit 18 and provides an 
inspiratory flow branch to ventilator connection 22. An expiratory flow 
branch is provided by conduit 24, which functions to convey gas from 
ventilator connection 22 to exhaust valve 26. Check valve 27 is located in 
conduit 20 to prevent flow from conduits 24 and 20 into conduit 18 during 
expiration of gas from patient connection 22. 
Expiratory valve 26 controls the pressure and flow through conduit 24. 
Expiratory valve 26 is preferably a diaphragm or balloon type valve which 
is capable of controlling the pressure in conduit 24 according to a 
reference pressure. Reference control pressure is provided to expiratory 
valve 26 via pressure control conduit 28. A flow restrictor 29 is provided 
on vent conduit 27 to provide a control bleed from pressure control 
conduit 28. When pressure in expiratory conduit 24 exceeds the reference 
pressure in conduit 28, gas is exhausted from expiratory conduit 24 to the 
atmosphere. Thus, the pressure in expiratory conduit 24 is controlled by 
the reference pressure in pressure control conduit 28, which is in turn 
controlled by the flow control valve 16. 
Ventilator connection 22 may be made to include a bellows assembly 23, as 
illustrated in FIG. 1, where conduit 20 communicates with bellows outer 
chamber 26 to actuate bellows 25. In this application, the patient's 
breathing tract is in communication with the interior of bellows 25 and 
thus isolated from the gas in ventilator 8. Alternatively, in an ICU 
application, bellows assembly 23 is omitted and ventilator connection 22 
communicates directly with the breathing tract of the patient. Thus, in an 
ICU application, ventilator 8 provides breathing gas directly to the 
patient. 
Pressure sensor 30 communicates with the interior of conduit 18 and 
provides a signal, indicative of the pressure in conduit 18, to processor 
32 via signal line 33. The pressure in conduit 18 is hereinafter referred 
to as manifold pressure or P.sub.man. Processor 32 includes a 
microprocessor connected via an electronic bus to read only memory (ROM) 
and random access memory (RAM) in a known digital computer configuration. 
Waveform generator 34 provides a desired pressure waveform to processor 
32. Flow control valve solenoid 16 is controlled by processor 32 via 
control signal line 5 to track the desired pressure waveform as will be 
described below. Proximal airway pressure sensor 35, which is located at a 
point having a pressure that represents the pressure of the patient's 
airway, also provides signals to processor 32 via signal line 37. 
Conduits 18, 20 and 24 define a ventilator circuit which communicates with 
the ventilator connection 22. During most of the inspiratory phase of a 
patient breath, the ventilator operates in a flow delivery mode whereby 
flow is delivered from gas source 10 through the flow control valve to 
conduits 18 and 20 and finally to the patient connection 22. During most 
of the expiratory phase of a patient breath, check valve 27 prevents flow 
from conduit 20 to conduit 18 and gas flows via conduit 24 to expiratory 
valve 26 where it is exhausted to the atmosphere. The ventilator thus 
operates in a flow exhaust mode. 
It should be noted, however, that the operational modes of the ventilator 
do not necessarily coincide with the different phases of a patient breath. 
The ventilator may operate in a flow exhaust mode during the late stages 
of the inspiratory phase of a patient breath because the patient lungs may 
have already filled with breathing gas. The balance of gas delivered to 
conduit 20 will thus pass through conduit 24 and be exhausted through 
expiratory valve 26. Moreover, the ventilator may also operate in a flow 
delivery mode during the expiratory phase of a patient breath where flow 
input is necessary to compensate for leakage in the ventilator-patient 
system or the patient's spontaneous respiration effort. 
FIG. 2 represents the components of a control system according to a 
preferred embodiment of the present invention. Sensor 30 provides a signal 
on line 33 which represents the value of the pressure within conduit 18 
(FIG. 1). The pressure value is input to the switching logic, represented 
by block 300, which provides a means for adaptively selecting either the 
flow delivery control algorithm 100 or the flow exhaust control algorithm 
200. The selection is made based on an operational parameter which is 
indicative of the mode of operation of the ventilator. Preferably, the 
operational parameter is the ratio of the pressure to the flow of gas in 
conduit 18. The flow of gas in conduit 18 is provided to switching logic 
300 by way of the command signal currently being issued to the flow 
control valve solenoid 16. As represented by the dotted line, switching 
logic 300, flow delivery control algorithm 100, and flow exhaust control 
algorithm 200 are implemented using software instructions to processor 
unit 32. 
Referring to FIG. 3, the adaptive selection of the appropriate control 
function by switching logic 300 incorporates a hysteresis routine to 
prevent hair triggering between the two control functions. At block 310, 
the operational parameter, i.e., the pressure/flow ratio, is determined 
using the signal from sensor 33 (FIG. 1) and the flow command signal being 
issued to the flow control valve solenoid. At 312, the logic determines 
whether the operational parameter is within a first tolerance, preferably 
less than +5%, of a value corresponding to the control bleed value of the 
parameter. The control bleed value is predetermined and corresponds to the 
operational parameter value, i.e. pressure/flow ratio that occurs when all 
of the flow output of the flow valve exits the ventilator through 
restrictor 29. If the operational parameter is within this tolerance of 
the control bleed values, the flow exhaust control function is invoked at 
block 314. The ventilator may already be operating in the flow exhaust 
mode, in which case the ventilator control function remains unchanged. 
If the operational parameter falls outside of the 5% range of the control 
bleed value, the switching logic delays selection of the flow delivery 
control function. Decision block 316 causes the switching logic to loop 
back to the start of the routine, represented by terminal "A" until the 
operational parameter falls outside a second tolerance, preferably greater 
than +7% of the control bleed value, as represented by block 318. The two 
tolerances thus represent a "dead-band" wherein the switching logic delays 
selection of the flow delivery control function until the operational 
parameter falls outside of the deadband. This prevents "hair triggering" 
or rapid cycling between inspiratory and flow exhaust control functions 
which may occur during low inspiratory flows or transitions in the dynamic 
state of the ventilator. Hair triggering may occur, for example, when the 
switching module triggers selection flow exhaust control system and 
immediately causes the operational parameters to fall out of tolerance. 
The control logic depicted in FIG. 3 operates in a continuous loop, as 
denoted by the connection terminals "A". Typically, the loop can be 
executed within 4 milliseconds by a digital computer, which provides many 
iterations during a single patient breath. 
It is preferable to control the selection of the flow exhaust control mode 
using the +5% tolerance on the pressure/flow ratio discussed above. 
However, the selection of the flow delivery control mode is preferably 
based on the proximal air pressure sensed by sensor 35. It will be 
understood that other forms of hysteresis or "dead-bands" may be 
implemented in place of those described above. For example, time-based 
hysteresis may be provided where a switch back to the previous control 
mode is disabled for a given time such as 300 mS. 
FIGS. 4 and 5 depict, in block diagram form, models of the flow delivery 
and flow exhaust control systems in conjunction with the response 
characteristics of the ventilator apparatus. Both control systems comprise 
negative feedback loops for minimizing the error between a target pressure 
signal, P.sub.in from pressure waveform generator 34 and a feedback signal 
corresponding to airway pressure P.sub.aw. FIG. 4 represents a preferred 
embodiment of a flow delivery control system according to a preferred 
embodiment of the invention. Referring to FIG. 4, waveform generator 34 
provides an input pressure signal, P.sub.in to summation block 110 where 
feedback signal P.sub.m is subtracted to provide a pressure error signal, 
P.sub.err to the flow delivery control filter 112. Flow delivery control 
filter 112 implements the s-domain flow delivery control function: 
##EQU1## 
here K is the control gain, nominally set to value of 5, T.sub.1 is a 
control lead constant and T.sub.2 is a control lag constant. 
Blocks 114 and 116 represent the response of the flow valve and ventilator 
system. Applicants have found that satisfactory control is achieved by 
using a generic model of the flow delivery system lumped parameter 
response 116 by the equation: 
##EQU2## 
where R.sub.b is the bleed resistance through resistor 29; R.sub.p is the 
patient airway resistance; C.sub.c is the overall ventilator circuit 
compliance including that of bellows 25; C.sub.p is the patient lung 
compliance; Q.sub.v is the valve flow and P.sub.aw is the airway pressure. 
Control filter 112 generates a valve command signal based on the error 
signal P.sub.err. A feedback signal is provided through airway pressure 
transducer 118 and an analog anti-alias filter 120, which are represented 
by the transfer functions shown in FIG. 4. 
FIG. 5 represents a preferred embodiment of the flow exhaust control system 
according to the present invention. It is important to note that the 
dynamic operational state represented by FIG. 5 does not always coincide 
with the exhalation phase of patient breathing. Rather, the state may 
occur during the inspiratory phase of a patient breath. For example, 
during a patient breath, the patient lung will reach a state where it is 
completely filled with breathing gases. The flow of gases from the 
breathing circuit delivery system to the patient is zero. However, in an 
anesthesia application, fresh gas is still being supplied to the system 
and must be released to maintain the airway pressure. As recognized by 
applicant, use of the flow delivery control system designed for optimal 
ventilator response during the inspiratory phase, such as that represented 
in FIG. 4, during this zero-flow inspiratory period may be inappropriate 
due to the higher gain characteristics of the system response. That is, a 
very small change in control valve output, i.e. flow, causes a relatively 
large change in pressure at the patient airway. 
It will be understood that FIGS. 4 and 5 depict control systems that are 
optimized for use in a bellows-equipped anesthesia application. Different 
control models will apply to ICU applications. 
Flow exhaust control system 200 comprises an inner control loop 217, which 
operates on the manifold pressure signal, P.sub.man and an outer control 
loop 219 which operates on airway pressure P.sub.aw. Inner control loop 
217 comprises the flow exhaust control filter 212, flow valve response 
214, manifold system response 216, manifold pressure transducer 218 and 
anti-alias filtering 220. Summing block 222 combines the manifold pressure 
feedback signal and target pressure signals, P.sub.m1 and P.sub.in 
together with the manifold pressure correction signal P.sub.corr. 
The outer control loop 219 is necessary to compensate for the exhalation 
valve dynamics. The outer control loop comprises inner control loop 217, 
anti-alias filter 294 and airway pressure transducer 210. The outer 
control loop 219 includes a proportional control stage, comprising a 
proportional gain 282 and an airway/manifold offset correction 284, which 
is adjusted in an integral fashion over several breaths. The outer control 
loop feeds a correction for manifold pressure to the inner loop's control 
target pressure based on airway pressure information. Exhalation 
valve/breathing circuit RC response, represented by block 290, is also 
included in the outer control loop of the system model. Summing block 292 
is provided with a feedback signal resulting from the transformation of 
airway pressure through the airway pressure transducer 210 and anti-alias 
filter 294. 
Flow exhaust control filter 212 is represented by the equation: 
##EQU3## 
where K is the gain, nominally (0.033) and T.sub.1 is control lead 
(0.120). 
Manifold system response 216 is represented by the equation: 
##EQU4## 
where C.sub.m is pneumatic manifold compliance (upstream of drive gas 
check valve); R.sub.b is the bleed resistance; Q.sub.v is the valve flow 
and P.sub.m is the airway pressure. 
The exhaust circuit dynamics during release of gas from ventilator in the 
flow exhaust mode is represented by block 290 and the equation: 
##EQU5## 
Where .tau. is a function of the ventilator circuit compliance, tidal 
volume, patient parameters and exhalation valve resistance and is 
empirically determined. 
FIG. 6 represents the flow and pressure response achieved by a preferred 
embodiment of the invention. It will be understood that pressure is 
maintained at the desired level in a continuous fashion, despite the 
variations in flow that occur over the course of a patient breath. The 
solid line 310 represents flow response and the solid line 312 represent 
the pressure response achieved with a preferred embodiment of the present 
invention. 
Point A represents a transition in the ventilator control dynamics from the 
flow exhaust mode to the flow delivery mode. In this illustration, the 
transition coincides with the onset of patient inspiration, represented by 
the rapid increase in flow. Flow rapidly increases and then decreases as 
the lungs become filled. Point B represents the point at which the patient 
lung is filled. Here, the ventilator control transitions from the flow 
delivery mode to the flow exhaust mode. The constant flow subsequent to 
point B is the control bleed flow, which operates to maintain the lung in 
a filled condition. The control bleed flow remains nearly constant until 
patient expiration begins at point C. 
The flow response characteristic of prior art control systems is 
represented by the dotted line 314. It can be seen that the prior art flow 
response is much slower than that achieved by the present invention. This 
is due to the fact that prior art flow control systems required a 
relatively slow control function so as to maintain stability when 
performing in flow exhaust mode. The effect of this limitation is seen in 
prior art pressure response 316 which requires a significant time to rise 
to the target pressure during patient inspiration. In contrast, the 
pressure response 312 achieved by the present invention closely tracks the 
desired square waveform target pressure. 
During the patient expiratory periods, the pressure and flow responses of 
the prior art and present invention are the same. This is because the 
control schemes of the present invention during the expiratory periods are 
similar to those of the prior art which provide adequate response 
characteristics during expiration. In accordance with the present 
invention, however, the transition to these schemes is based on the 
ventilator dynamic state instead of the respiratory periods of the 
patient. 
Although particular embodiments of the present invention have been shown 
and described, many other embodiments incorporating the inventive 
teachings may be easily constructed by those skilled in the art. The 
foregoing description is intended to illustrate rather than limit the 
scope of the present invention which is defined by the claims that follow. 
Specifically, the disclosed control algorithms need not be utilized as 
other control functions may be used without departing from the scope of 
invention.