Abstract:
A feedback controller for regulating respiratory gas in a mechanical breathing aid system has a comparator means for periodically generating, in a current breathing cycle, an error signal representing the difference between a value of a gas parameter measured for gas within the system and a target value of the gas parameter, and a control signal generator for processing the error signal in accordance with a control function to generate a control signal usable in the regulation of the respiratory gas. The controller has a variable value integral gain stage which provides an input to an integrator element. An adaption unit determines, for the current breathing cycle an extreme value of the periodically generated error signal and varies the value of the integral gain used in the integral gain stage for a next breathing cycle dependent on a rate of change of the value of the extreme error signal with value of the integral gain.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an apparatus and method for the feedback control of respiratory gas flow within a mechanical breathing assist apparatus and in particular to an apparatus and method for the adaptive feedback control of the gas flow. 
     2. Description of the Prior Art 
     Feedback controllers are used within a mechanical breathing assist apparatus, such as a ventilator system, to adjust gas flow rates based on a measurement of a system gas parameter, for example gas pressure, rise time, or flow rate, in order to achieve and maintain the value of that variable at or within an operating range of a target value. These controllers are usually operably connected to a flow control regulator, such as a solenoid valve, to provide a control signal used to adjust the opening of the valve. How the adjustment is made to reach the target value depends not only on the measured value of the system flow parameter that is fed back to the controller, but also on additional parameters known as control parameters. These control parameters directly affect the performance and stability of the controller and their optimal values may change with time as system properties, such as compliance and resistance, vary. A mechanical breathing assist apparatus is particularly problematical to control in this manner since its pneumatic system includes (or is connected in use to) a patient&#39;s respiratory system, including lungs, the compliance and resistance of which can change unpredictably with time and with patient. 
     In order to overcome this problem it is known to provide controllers having control parameters which automatically vary or “adapt” with changes in properties of the ventilator system. One such controller which provides an adaptation for a next breath that is based on the analysis of gas delivery in previous breaths is disclosed in U.S. Pat. No. 5,271,389. This controller has a comparator for periodically generating in a current breathing cycle, an error signal representing the difference between a value of a gas parameter measured for gas within the system and a target value of the gas parameter, a control signal generator for processing the error signal in accordance with a control function having a variable value control parameter to generate a control signal usable in the regulation of the respiratory gas, and adaption means for varying the value of the variable value control parameter responsive to the error signal. The adaption means operates by summing the error signal in a particular period with all past error signals for the corresponding period of past breaths to provide a cumulative error signal which is used to vary the control parameter for the same period of the next breath. Thus the correction will “improve” as the error values from more breaths are added to the cumulative signal. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an adaptive feedback control of gas flow within a mechanical breathing aid system in which the adaption is made on a breath-by-breath basis without the need to rely on a cumulative error signal. 
     The above object is achieved in accordance with the principles of the present invention in a feedback controller for regulating respiratory gas flow in a mechanical breathing assist apparatus, having a comparator which periodically generates, in a current breathing cycle, an error signal which represents a difference between a value of a gas parameter that was measured for gas within the system, and a target value for this gas parameter, a control signal generator which processes the error signal using a control function having a control parameter with a variable value, to generate a control signal used to regulate the respiratory gas, and an adaptation unit for varying the value of the control parameter dependent on the error signal, by determining, for the current breathing cycle, an extreme value of the periodically generated error signal and by varying the value of the control parameter for a next breathing cycle dependent on the rate of change of the extreme value of the error signal relative to the value of the control parameter in the current breathing cycle. 
     By providing for the adaptive variation of a variable control parameter for a subsequent breathing cycle which is based on the change of an extreme error signal value (that is a maximum or a minimum value depending on how the error signal is derived and which phase of the breathing cycle is being controlled) with the value of the control parameter, then disturbances in the breathing assist apparatus system are automatically compensated, based on the past performance of the system and typically based on the performance of the system over consecutive breathing cycles, without the need to establish a cumulative error signal. 
     Preferably an integral gain control parameter is varied assuming a linear relationship between an extreme pressure error signal and the value of the integral gain parameter. In a feedback controller, such as a PID or PI controller in which respectively a “proportional-integral-derivative” or a “proportional-irtegral” control function is implemented, the integral gain parameter that is found to be highly sensitive to disturbances in the pneumatic system such as changes in lung resistance and compliance. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a section of a ventilator system including a feedback controller according to the present invention. 
     FIG. 2 shows a flow chart of the operation of an adaption means of the feedback controller of FIG.  1 . 
     FIG. 3 shows a flow chart for the calculation of the rate of change of I gain by the adaption means the operation of which is shown in FIG.  2 . 
     FIG. 4 shows a flow chart for the calculation of I gain from the rate of change calculated according to the steps of FIG.  3 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Considering now FIG. 1, a feedback controller  1  is shown which is adapted to control one or both of an inspiration gas flow control valve  2  and an expiration gas flow control valve  2 ′. These valves  2 ,  2 ′ are respectively disposed in an inspiration gas flow path  3  and an expiration gas flow path  3 ′ of a respiration gas which flows between a ventilator unit  4  and a patient&#39;s respiratory system  5 . A sensor unit  6  is also provided within the flow path  3  (and also the flow path  3 ′) to measure a gas pressure, and its output is supplied periodically (typically at a sample frequency of several kHz) as an input  7  to the controller  1 . Also provided as an input to the controller  1  is a signal  8  that is representative of a desired or target gas pressure. 
     The feedback controller  1  includes a comparator  9  which receives the input target  8  and actual flow parameter signals  7  for a particular breathing cycle and periodically generates an error value signal  10  representative of their difference; and a control signal generator  11  which receives the error value signal and uses it to establish a control signal  12  for periodically controlling one or both of the valves  2 ,  2 ′ during the breathing cycle. 
     The control signal generator  11  includes a proportional gain unit  13  which receives the error value signal  10  and amplifies it by a predetermined amount to produce a proportional signal  14  component of the control signal  12 ; an integration unit  15  for producing an integral signal  16  component of the control signal  12 ; and a differential unit  17  for producing a differential signal  18  component of the control signal  12 . The feedback controller  1  of the present embodiment is thus of a type commonly referred to as a PID controller. 
     The feedback controller  1  also includes an adaption unit  19  which receives the error signal  10 , and first determines a maximum value of the error signals  10  that have been generated periodically during a predetermined portion of the breathing cycle and then, dependent on the so determined maximum value, determines a gain parameter for use in the next breathing cycle. This gain parameter is based on a calculation within the adaption unit  19  of the rate of change of the maximum value relative to the value of the gain parameter from previous breathing cycles that is stored within a memory (not shown) of the adaption unit  19 , as will be discussed in greater detail below. 
     The integrator unit  15  includes an integral gain stage  20  which receives the periodic error signal  10  and amplifies it by an amount dependent on the value of the gain parameter passed from the adaption unit  19  before passing it to an integrator element  21  where it is integrated to provide the integral signal component  16  of the control signal  12 . A summing element  22  sums the proportional signal component  14 , the integral signal component  16  and the differential signal component  18  and emits the sum as an output for use as the control signal  12 . 
     The following discussion assumes, for the present embodiment, that the controller  1  is adapted to control the inspiration valve  2  during an inspiration phase of a breathing cycle. In controlling the flow valve  2  it is desirable to provide a small initial overshoot (O) of the target pressure since this will result in a shorter rise time. 
     However the overshoot (O) should not be too large since this may cause discomfort and even injury to a patient&#39;s respiratory system  5 . The overshoot (O) is therefore intended to be controlled to lie within upper (a) and lower (b) limits. The maximum value of the error signal  10  then is a measure of this overshoot (O) and may be a negative value, which in this case would represent an undershoot. By arranging for the integral gain (I), used in the integral gain stage  20 , to adapt its value depending on the size of this overshoot (O), the feedback controller  1  will be responsive to the type of lung  5  connected to the flow path  3  as well as to changes within the flow path  3  itself. This is because the magnitude of the integral gain (I) is highly dependent on the mechanical resistance and compliance of the pneumatic system  3 , 5 . 
     Now, assuming a linear relationship between the overshoot (O) and the integral gain value (I) which is used in the integration unit  15 , the desired integral gain value ( 13 ) required to provide a satisfactory overshoot (O 3 ) in a next breath is given by 
     
       
           I   3 = I   2 +[( I   1 − I   2 )×( O   3 − O   2 )/( O   1 − O   2 )]  ( 1 ) 
       
     
     wherein I 1  and O 1  are respectively the gain value and the overshoot associated with a previous breath (preferably the immediately preceding breath); and I 2  and O 2  are respectively the gain value and the overshoot associated with the current breath. 
     If the value of the desired overshoot (O 3 ) for the next breath is selected to lie midway between the limits a,b of an acceptable overshoot, then equation ( 1 ) may be re-written as 
     
       
           I   3 = I   2 +[( I   1 − I   2 )×((( a+b )/2)− O   2 )/( O   1 − O   2 )]  ( 2 ) 
       
     
     With (O 1 −O 2 )/(I 1 −I 2 ) written as dO/dI, the rate of change of overshoot with gain value, then equation ( 2 ) can be expressed as 
     
       
           I   3 = I   2 +[(( a+b )/2)/( dO/dI )]  ( 3 ) 
       
     
     Considering now FIG. 2, FIG.  3  and FIG. 4, flow charts for the operation of the adaption means  19  are shown. The first step  23  is to calculate within a specified time period from the beginning of an inspiration phase (which is typically of the order of 100 ms for neonates and 200 ms for adults) a maximum error signal, Emax, from periodically determined error signals  10  entered during this period as an error value E. This first step  23  includes a step  24  of comparing the currently input error value E with a stored value of Emax obtained during the specified period of the current breathing cycle and either replacing (step  25 ) the current value of Emax with the value E of the current error signal  10  or maintaining (step  26 ) the stored value of Emax. After the specified period, step  27  is performed making the last stored value of Emax the value of the overshoot (O 2 ) for the current breath. In step  28  a decision is made as to whether a new value of the integral gain should be provided as the integral gain control parameter (I 3 ) for the next breath. If the overshoot (O 2 ) for the present breath falls outside the predetermined limits a,b, then a new value of integral gain control parameter (I 3 ) is determined (step  29 ) for use in the next breath. 
     The step  29  of determining the gain control parameter (I 3 ) includes a step  30  (FIG. 3) for calculating a rate of change of overshoot with integral gain control value (dO/dI) and a step  31  (FIG. 4) wherein, based on this value, the gain control parameter (I 3 ) is calculated for use in the integral gain stage  20  of the feedback controller  1 . The step  29  of gain control determination may need to be carried out iteratively until the overshoot (O) lies within the desired upper (a) and lower (b) limits since the linear relationship is only an approximation which becomes better for consecutive breaths. 
     In calculating the value dO/dI (step  30  of FIG. 3) it is first determined (step  32 ) whether the overshoot O 2 , associated with the current breath, is equal to that overshoot O 1 , associated with a previous, preferably immediately preceding, breath. If it is, or if it is not but it is determined (step  33 ) that the current gain control value  12  and the previous one I 1  are the same, then dO/dI maintains its previous value (step  34 ) when the new gain value I 3  is calculated (step  31 ). If the overshoots O 2  and O 1 , and the integral gain control parameters I 2  and I 1 , differ, then the value (dO/dI) is calculated (step  35 ). If this value lies within limits (step  36 ) that are selected to discriminate against inaccuracies in the measurements, which have been made, then this value is used in the calculation of the new I gain (step  31 ) I 3 . If the value dO/dI lies outside these limits then dO/dI is set to MAX (step  37 ) and this value used in calculating the new integral gain (step  31 ) I 3 . The lower limit used at step  36  is here chosen as 0 since a change in I gain is expected to provide a change in overshoot and the upper limit as a maximum allowable value MAX above which an unexpectedly change indicative of a spike or “glitch” is considered to have occurred. 
     The step  31  (FIG. 4) of calculating the gain value I 3  includes a first step (step  38 ) of determining whether a patient is connected to the ventilator  4 . 
     If a patient has been disconnected for some reason, for example for the removal of secretion from a patient&#39;s throat, then the new gain value I 3  is set to the current gain value I 2  (step  39 ). This is done in order to prevent a “runaway” gain value I 3  being set, which may lead to a patient being exposed to dangerous pressure levels when reconnected. Otherwise a new gain value  13  is calculated (step  40 ) using equation ( 3 ). If the difference between this new gain value I 3  and the current gain value I 2  lies within preset limits (Imin and Imax in step  41 ), selected to ensure that a pressure which is not too extreme can be delivered to the patient, then the value of I 3  that was calculated at step  40  is provided for output (step  42 ) for use within the integral gain stage  20  of the integration unit  15 . If this difference, calculated at step  41  is less than the lower limit, Imin, then I 3  is set to I 2 −Imin (step  43 ). If this difference, calculated at step  39 , is larger than the upper limit, Imax, then I 3  is set to I 2 +Imax (step  44 ). The new value of the integral gain control parameter I 3  is then supplied as an output at step  42  for use within the feedback controller  1  in the regulation of the valve  2  in the inspiration phase of the next breathing cycle. 
     It will be appreciated that expiration pressure within the gas flow path  3 ′ may be controlled, for example to maintain a pre-determined PEEP level, in a manner similar to that described above with respect to inspiration pressure regulation. In this case the expiration valve  2 ′ is controlled by the feedback controller  1 , modified to provide adaptive regulation of the expiration pressure. The adaption unit  19  will operate principally according to the flow charts shown in FIGS. 2 and 3 but using different limits and a different time period which an extreme error value E will be determined. Typically this time period begins upon detection that an error signal (defined as PEEP value —pressure measured at sensor  6 ) is larger than 0.5 cm H 2 O and that the derivative of the error signal is negative. The time period ends a predetermined time, typically in the range of 100-200 ms, after it begins. 
     Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventors to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of their contribution to the art.