Abstract:
A system and a method for circuit compliance compensated volume control in a patient respiratory ventilation system having a flow regulated feedback servo control loop, a volume delivery controller, and a patient volume observer. In the flow regulated feedback servo control loop, an estimate of patient volume is used for feedback control, such that a tidal volume is achieved upon servo regulation, and the peak inspiratory flow is modulated based on volume error between the set tidal volume and the estimated patient volume. Thereby, a constant inspiratory time and a constant I:E ratio can be maintained. In the volume delivery control, the feedback volume error is normalized to a volume error percentage, and the gain of the controller is dynamically changed based on the volume error percentage, such that the controller effort can be minimized when the volume target is approached. The patient volume observer is operative to estimate the patient delivered volume based on the estimated circuit volume and the measured net delivered volume, while the measured net delivered volume includes effects of leaks and valve dynamics and is synchronously captured with true patient breathing.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     Not Applicable  
       STATEMENT RE: FEDERALLY SPONSORED RESEARCH/DEVELOPMENT  
       [0002]     Not Applicable  
       BACKGROUND  
       [0003]     The present invention relates in general to a system and a method for circuit compliance compensated volume control in a patient respiratory ventilation system, and more particularly, to a respiratory ventilation system suitable for use in all ages and sizes of patients by effectively and accurately estimating and compensating for the patient circuit compliance  
         [0004]     In order to deliver an accurate set tidal volume to a patient in a respiratory ventilation system, the patient circuit compliance has to be compensated. The compensation of patient circuit compliance is especially crucial for neonatal patients whose lung compliance can be as small as about one thirteenth of the circuit compliance. Without compensating the circuit compliance, inaccurate volume and inadequate flow will be delivered to the patient. Therefore, various designs and algorithms have been proposed to facilitate the patient circuit compliance compensation in the respiratory ventilation system. Currently, the settings or approaches in many of the circuit compliance compensation designs or algorithms actually impact the ability of exhaling delivered tidal volume for the patient and consequently causing gas trapping and auto PEEP. Therefore, most of the ventilators available in the market do not allow the circuit compliance compensation designs applied to neonatal patients due to the stringent precision requirement on volume delivery. The burden of achieving accurate volume delivery is thus left for the clinician.  
         [0005]     Currently, two algorithms that directly add an estimate of patient circuit volume to a set tidal volume are commonly used. In one of the commonly used algorithms, an estimate of patient circuit volume is directly added to a set tidal volume by extending the inspiratory time with a specific peak flow. The patient circuit volume is computed using the peak airway pressure (measured by an expiratory pressure transducer) and an estimate of the patient circuit compliance. As understood, the extension of inspiratory time often impacts the ability of the patient to exhale the delivered tidal volume; and consequently, results in gas trapping and auto PEEP. Such adverse effects are much more significant for young pediatric or neonatal patients whose lung compliance is comparative to or as small as only 1/13 of the patient circuit compliance. Therefore, patient circuit compliance compensation based on the first algorithm is not suitable for those patients with small lung compliance. In addition, such algorithm is not responsive when changes in airway resistance and/or lung compliance occur.  
         [0006]     In the second approach, an estimate of patient circuit volume is added to set tidal volume by increasing the preset peak inspiratory flow, which ultimately causes the increment of the average peak airway pressure. The patient circuit volume is computed using the average peak airway pressure of previous (four) mandatory/machine breaths and an estimate of the patient circuit compliance. The patient circuit volume is thus continuously elevated breath after breath. Due to positive feedback of average peak airway pressure, the second algorithm can establish a runaway (not converge) condition on neonatal patient size where the ratio of circuit compliance to patient (lung) compliance is as high as 13:1. Moreover, this algorithm is not robust in cases where airway resistance is high due to effects such as gas compression which occurs as a result of positive feedback of peak airway pressure. Therefore, this algorithm is only effective on adults and some pediatrics patient sizes, and it is not responsive when changes in airway resistance and/lung compliance occur either.  
         [0007]     It is therefore a substantial need to develop a system and a method operative to provide circuit compensated volume control in a patient respiratory ventilation system without any of the above adverse effect and clinically acceptable for all patient sizes.  
       BRIEF SUMMARY  
       [0008]     A system and a method for circuit compliance compensated volume control in a patient respiratory ventilation system clinically acceptable for patients at all sizes and ages are provided. The system and method as provided allow the patient to receive an accurate inspiratory flow while maintaining a constant ratio of inspiratory time versus expiratory time (I:E ratio) throughout volume delivery. As the constant I:E ratio is maintained, gas trapping and auto PEEP is prevented. The existing on-board sensors are used for estimating the volume required for compensating the patient circuit compliance and determining the accurate inspiratory flow, such that no additional device is required for implementing the system and method as provided. The operation of the system and the method is designed based on the governing physics of the patient and the ventilation system, such that the leakage through the expiratory limb during volume delivery by the ventilator and access volume delivery due to valve dynamic of the ventilator are accounted for. Therefore, the system and method are robust against changes in airway resistance and patient compliance.  
         [0009]     The system for circuit compliance compensated volume control in a patient respiratory ventilation system can be divided into three main subsystems, including a flow regulated feedback servo control loop, a volume delivery controller, and a patient volume observer. In the flow regulated feedback servo control loop, an estimate of patient volume or a measured patient volume is used for feedback control, such that delivery of the set tidal volume to the patient can be achieved. The inspiratory flow is modulated based on volume error between the set tidal volume and the estimated patient volume. Thereby, a constant inspiratory time and a constant I:E ratio can be maintained. In the volume delivery control, the feedback volume error is normalized to a volume error percentage, and the feedback volume error is weighed by a gain which is dynamically determined based on the volume error percentage. Thereby, the desired tidal volume can be obtained with the minimized controller effort. The patient volume observer is operative to estimate the patient volume based on the estimated circuit volume and the measured machine delivered net volume, while the volume affected by leakages of expiratory limb and valve dynamics is synchronously captured with true patient breath.  
         [0010]     The method for circuit compliance compensated volume control includes the steps of estimating a patient volume based on an estimated circuit volume and a measured machined delivered net volume; regulating the machine delivered net volume based on a feedback of the estimated patient volume; and modulating the inspiratory flow. The estimated circuit volume is obtained by a relationship between the circuit volume and the circuit pressure estimated based on the circuit compliance. The machine delivered net volume is regulated with a dynamic gain scheduling. More specifically, a gain is dynamically adjusted upon a normalized volume error defined as the ratio of volume differential between the set tidal volume and the estimated patient volume to the set tidal volume. Thereby, the desired inspiratory flow can be modulated and the patient volume can be estimated while a constant inspiratory time and I:E ratio can be maintained.  
         [0011]     A ventilation system that incorporating the above volume control system is also provided. The ventilation system includes a ventilator for supplying inspiratory gas to the patient and receiving the expiratory gas exhaled from the patient. The system further comprises a patient circuit, preferably a Y-circuit for delivering the inspiratory and expiratory gas to and from the patient, respectively. Sensors and transducers are provided to measure the inspiratory and expiratory flows, the Y-circuit pressure and the PEEP. By the reading of existing flow sensors and pressure transducers and computation of the readings, the circuit compliance compensated volume control system is operative estimate the circuit volume and the patient volume based on the measured results and the estimated patient volume, so as to provide a circuit compliance volume compensation factor to regulate the machine delivered net volume, and thus delivered to modulate the insipratory flow, so as to delivery a desired tidal volume to the patient. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     These and other features and advantages of the various embodiments disclosed herein will be better understood with respect to the following description and drawings, in which like numbers refer to like parts throughout, and in which:  
         [0013]      FIG. 1  illustrates a respiratory circuit diagram of a patient who is receiving machine ventilation;  
         [0014]      FIG. 2  shows a graph of circuit compliance obtained from empirical data and an estimated circuit compliance approximated from the empirical data;  
         [0015]      FIG. 3  illustrates a block diagram showing a ventilation system that incorporates a system for circuit compliance compensated volume control;  
         [0016]      FIG. 4  snows a block diagram of the system for circuit compliance compensated volume control; and  
         [0017]      FIG. 5  illustrates a block diagram of a volume delivery controller of the system as shown in  FIG. 4 .  
     
    
     DETAILED DESCRIPTION  
       [0018]     In an electric circuit, the electric current I flows from a high potential level to a low potential level. When the electric current I flows through a passive circuit element such as a resistor, an inductor, a capacitor or a load, a voltage drop ΔV is created across such element. When two or more of the passive circuit elements of the electric circuit are connected to each other in parallel, the total electric current is split into two smaller currents distributed flowing through the respective elements. The magnitudes of the currents depend on the characteristic values, such as the resistance, the conductance, and the capacitance of the elements. In a patient respiratory circuit, the gas flow Q circulates from a high pressure level to a low pressure level in a way similar to the electric current I, and the gas flow Q through a circuit element such as an airway resistance causes a pressure drop ΔP similar to the voltage drop ΔV in the electric circuit.  FIG. 1  illustrates a circuit diagram of a patient respiratory circuit. As shown, the patient respiratory circuit typically comprises a patient circuit for circulating gas between a ventilator and a patient. The ventilator is operative to provide an inspiratory gas flow Q INSP  and receive an expiratory gas flow Q EXP  to and from the patient through the patient circuit, respectively. Ideally, the flow differential between the inspiratory flow Q INSP  and the expiratory flow Q EXP , that is, the net flow Q NET , is all to be delivered to the patient, so as to provide the tidal volume required thereby. However, in real practice, the volume loss within the patient circuit is inevitable due to the distensibility at least partially attributed to the circuit compliance C T  thereof. The circuit compliance C T  is in parallel flow communication with the lung compliance C L  and behaves similarly to a capacitor in an electric circuit.  
         [0019]     Without the circuit compliance compensation, the machine delivered net volume V NET  integrated by the net gas Q NET  flow Q NET  is equivalent to the tidal volume delivered to the patient. However, as discussed above, as a portion of the net gas flow Q NET  is offset by the circuit compliance C T n, the volume actually delivered to the patient&#39;s lung is actually smaller than the required tidal volume. Further, as the circuit compliance C T  is defined as a ratio of the volume offset by the patient circuit to the pressure across the patient circuit. The offset volume is proportional to the circuit compliance C T . Therefore, when the circuit compliance C T  is much larger than the lung compliance C L , a majority of the net flow Q NET  will be distributed to the patient circuit instead of being supplied to the patient&#39;s lung.  
         [0020]     In this embodiment, the patient circuit is connected to a ground pressure level PEEP. Therefore, the pressure across the patient circuit is thus the pressure differential between the pressure measured at a patient piece of the patient circuit P Y  and PEEP. In this embodiment, an airway resistance R L  exists in the patient&#39;s airway, such that the pressure applied to the patient&#39;s lung will be reduced by a factor of Q L   2 R L . The pressure at the patient circuit P Y  and the lung pressure P L  can be expressed by the following equation:
 
 P   Y   =P   L   +Q   L   2   R   L   (1).
 
 By definition, the circuit compliance C T  and the lung compliance C L  can be expressed as:  
                 C   L     =       V   TID         P   L     -   PEEP         ;   and           (   2   )                   C   T     =       V   CC         P   Y     -   PEEP         ,           (   3   )             
 
 where V TID  is equivalent to V L , which is the actual gas volume delivered to the lung of the patient, and V CC  is the gas volume offset by the circuit compliance. The gas volumes V TID  and V CC  can be derived by integrating the gas flow Q T  and Q L  flowing through the patient circuit and the patient&#39;s lung L. Therefore, the sum of the gas volumes V CC  and V L  is equal to the machine delivered net volume V NET  as expressed by Equation (4), which can be integrated from the net gas flow Q NET .
 
 V   NET   =V   CC   +V   TID   (4).
 
 From Equations (3) and (4), an estimate of patient volume can be expressed as:
 
 {circumflex over (V)}   TID   =V   NET   −V   CC   =V   NET   −C   T ( P   Y   −PEEP )  (5).
 
 From Equations (1) and (2),  
                 P   Y     -   PEEP     =         V   TID       C   L       +       Q   L   2     ⁢       R   L     .                 (   6   )             
 
 From Equations (5) and (6), an estimate of the net volume that the ventilator needs to deliver is:  
                 V   NET     =       V   TID     +         C   T     ⁡     (       P   Y     -   PEEP     )       ⁢           ⁢   or         ⁢     
     ⁢           V   NET     -     V   TID     +       C   T     ⁡     (         V   TID       C   L       +       Q   L   2     ·     R   L         )         ;     and   ⁢           ⁢   therefore       ,     
     ⁢         V   ^     NET     =         (     1   +       C   T       C   L         )     ⁢     V   TID       +       C   T     ·     Q   L   2     ·       R   L     .                     (   7   )             
 
 From Equation (7), the machine delivered net volume V NET  can be computed if the lung compliance C L , the circuit compliance C T , the airway resistance RL, the desired patient flow Q L , and the desired patient flow V TID  are known. It will be appreciated that, as the volume parameters in Equation (7) are integrations of the corresponding gas flows, the pressure parameter P Y  typically indicates the peak pressure at the patient circuit. 
 
         [0021]     In Equation (7), since the lung compliance C L  and the airway resistance R L  can vary with time or condition, it would be very difficult to accurately determine the appropriate machine delivered volume as desired on a real-time basis to achieve a desired patient tidal volume. This appears to be the major reason that the conventional circuit compliance compensation design is only applicable to the patients having specific sizes of lungs. Therefore, as provided in this embodiment, the system and method for circuit compliance compensated volume control estimate the patient circuit volume and a patient tidal volume using the existing sensors in the ventilator. The actual inspiratory gas flow Q INSP  and the expiratory gas flow Q EXP  are measured by the existing sensors of the ventilator, and the machine delivered net flow Q NET  can be derived from the difference thereof. In such manner, various volume variables can be obtained based on the true inhalation and exhalation of the patient.  
         [0022]     By definition, the positive end expiratory pressure PEEP is the pressure P Y  measured at the end of the expiratory phase. Therefore, before the patient is receiving the machine ventilation, the ground pressure level of the patient circuit is zero or other preset values. The circuit compliance C T  can be predetermined by supplying known volumes to the patient circuit and measuring the responsive circuit pressure at each specific circuit volume. The circuit compliance C T  for a specific patient circuit can thus be expressed by an empirical relationship between the circuit volume V CC  and the circuit pressure ΔP Y (=P Y ). When the patient circuit is applied to circulating gas between the ventilator and the patient as shown in  FIG. 1 , one can thus derive the circuit volume V CC  by providing the circuit pressure ΔP Y (P Y −PEEP) to the circuit compliance relationship. In this embodiment, the circuit pressure P Y  is defined as the pressure measured at the expiratory limb of the patient circuit during the inspiratory phase, that is, P Y =P EXP  during I-phase; or P Y  is the average pressure between P EXP  and the pressure measured at the inspiratory limb of the patient circuit during the expiratory phase P INSP , that is, P Y =(P INSP +P EXP )/2 during E-phase.  
         [0023]      FIG. 2  shows the relationship between the circuit volume V CC  and the pressure differential ΔP Y  obtained from empirical data. As shown, the empirical data show a nearly linear relationship between the circuit volume V CC  and the differential circuit pressure ΔP Y . Therefore, a linear line with a slope CKT_CMP SLP  that reflects the circuit compliance C T  can be drawn from the graph, and the circuit volume V CC  can be presented by the equation as:
   {circumflex over (V)}   CC   =CKT   —   CMP   SLP ·( P   Y   −PEEP )+ CKT   —   CMP   INT   (8), 
 where CKT_CMP INT  is the intercept with the V CC  axis. It will be appreciated that, in addition to mathematical formula as provided in Equation (8), a lookup table in which the empirical data of the responsive circuit volumes for various pressures can also be used to estimate the circuit volume V CC . In addition, according to specific condition, a non-linear relationship between the circuit volume and the pressure may also be obtained and utilized for estimating the circuit volume. 
 
         [0024]     When the circuit volume of the patient circuit is obtained, the tidal volume delivered to the patient can be estimated from Equation (4), that is,
 
 {circumflex over (V)}   TID   =V   NET   −{circumflex over (V)}   CC   (9)
 
 As mentioned above, the net volume delivered by the machine V NET  can be derived by integrating the net flow delivered by the machine Q NET , that is, the difference between the inspiratory and expiratory gas flows Q INSP  and Q EXP  as:  
               V   NET     =       ∫     Start   ⁢           ⁢   of   ⁢           ⁢   I   ⁢     -     ⁢   phase         Q   NET     ⁢   crosses   ⁢           ⁢   0       ⁢       (       Q   INSP     -     Q   EXP       )     ⁢       ⅆ   t     .                 (   10   )             
 
 In this embodiment, the circuit volume V CC  will not be updated until the differential gas flow, that is, the net gas flow Q NET  (=Q INSP −Q EXP ), crosses zero; and therefore, the calculation or computation of the net machine delivered volume V NET  is integrated over the differential gas flow Q NET  from the start of the inspiratory phase to the time when the net flow Q NET  crosses 0. In the case that the net flow Q NET  crosses 0 before the inspiratory phase is complete, the circuit volume V CC  and the tidal volume V TID  are estimated at the end of the inpsiratory phase. 
 
         [0025]      FIG. 3  illustrates a patient respiratory ventilation circuit or system incorporating the system for circuit compliance compensated volume control as discussed above. As shown in  FIG. 3 , the ventilation system includes a ventilator  10 , a patient circuit  20  for circulating the inspiratory gas and expiratory gas between the ventilator  10  and a patient, the system for circuit compliance compensated volume control  30 , and a servo control subsystem  40  for controlling operations of the ventilator  10 . The ventilator  10  typically includes a user interface such as a monitor  12  for displaying various conditions and parameters of the patient and the ventilation system, and an input device (not shown) allowing the operator or user to input the required settings and parameters. The input device may include buttons or any adjusting devices built on the front panel or other devices including keyboard, mouse or remote controls allowing the user to input setup information to the ventilator  10 . Alternatively, the monitor  12  may be in the form of a touch screen in which both the display and input device are integrated. Based on the input data or information, the processor is operative to control the ventilator  10  for performing the desired operations. The ventilator  10  further includes an inspiratory port  14  and an expiratory port  16  through which the inspiratory gas to the expiratory gas are supplied and received to and from the patient through the patient circuit  20 , respectively. An inhalation flow control valve or orifice is typically installed at the inspiratory port  14  for controlling the inspiratory flow Q INSP , and an exhalation valve is preferably installed at the expiratory port  16  for controlling the open/close condition of the expiratory port  16 . In this embodiment, inspiratory and expiratory flow sensors  11  and  13  are installed at the inspiratory and expiratory ports  14  and  16  for measuring the inspiratory Q INSP  and expiratory flow Q EXP , respectively. In addition, an inspiratory pressure transducer  15  and an expiratory pressure transducer  17  may also be installed to measure the inspiratory and expiratory pressure P INSP  and P EXP , respectively.  
         [0026]     As shown, the patient circuit  20 , such as a Y circuit, is used to connect the ventilator  10  to the patient, so as to construct the respiratory circuit for circulating gas between the ventilator  10  and the patient. The Y circuit  20  includes an inspiratory limb  22  with one end connected to the inspiratory port  14  and an expiratory limb  24  with one end connected to the expiratory port  16  of the ventilator  10 . The other ends of the inspiratory port  14  and the expiratory port  16  merge at one end of a patient piece  26 , of which the other end is applied to the patient. Other accessories or component devices such as filters may also be installed in various part of the Y circuit  20 . To directly measure the gas flow Q Y  delivered to the patient, a flow sensor  28  is preferably installed at the patient piece  26 . It will be appreciated that the inspiratory and expiratory flow sensors  11  and  13  and the inspiratory and expiratory pressure transducers  15  and  17  may also be installed on the inspiratory limb  22  and expiratory limb  24 , respectively. Preferably, the measurable process variables, including the inspiratory flow Q INSP , the expiratory flow Q EXP , the inspiratory pressure P INSP , the expiratory pressure P EXP  and the PEEP are sampled by a predetermined frequency. For example, in one embodiment, these processes are sampled every 2 msec. The ventilator  10  may further comprise a sensor processor  18  operative to process the measured process parameters, including Q INSP , Q EXP , P INSP , P EXP  and other sensor readings before outputting to the circuit compliance compensated volume control system  30 . The sensor processor  18  may includes an individual sensor in communication with the sensors  11 ,  13 ,  15 ,  17  and  28  and the circuit compliance compensated volume control system  30 . Alternatively, the sensor processor  18  may be integrated into the above-mentioned processor of the ventilator  10  that control the operations of the ventilator  10 .  
         [0027]     Referring to  FIGS. 3 and 4 , the system for circuit compliance compensated volume control  30  includes a circuit compliance estimator  31 , a patient circuit volume estimator  32 , a patient volume observer  33 , a volume delivery controller  34 , and a volume to flow converter  35 . As discussed above, the circuit compliance C T  of the patient circuit  20  can be estimated by measuring the pressure differential ΔP Y  at various given circuit volumes V CC  before the patient is receiving the machine ventilation. In this embodiment, the circuit compliance estimator  31  is operative to estimate the circuit compliance C T , such that the relationship between the circuit volume V CC  and the pressure differential ΔP Y , including the slope CKT_CMP SLP  and the intercept CKT_CMP INT , can be also obtained. The slope CKT_CMP SLP  and the intercept CKT_CMP INT  of the circuit compliance estimator  31  are then output to the patient circuit volume estimator  32 . The circuit volume estimator  32  is also connected to the ventilator  10  for receiving the Y circuit pressure P Y  and the PEEP measured by the pressure transducer  17 , such that the pressure differential ΔP Y  can be computed. Based on ΔP Y , the slope CKT_CMP SLP  and the intercept CKT_CMP INP , the patient circuit volume V CC  can be estimated by Equation (8) and denoted as VOL CKT     —     EST  output to the patient volume observer  33 . Again, it will be appreciated that, in addition to Equation (8) that mathematically expresses the relationship between the circuit volume V CC  and the responsive pressure differential ΔP Y , the circuit compliance C T  may also be in the form of a lookup table which provides the responsive pressure differentials for the circuit volumes based on empirical data.  
         [0028]     The patient volume observer  33  is operative to receive the measured machine delivered net volume VOL NET , that is, the machine delivered net volume derived by integrating the net flow Q NET , and the estimated circuit volume VOL CKT     —     EST  obtained by the circuit volume estimator  32 . By subtracting the estimated circuit volume VOL CKT     —     EST  from the measured machine delivered net volume VOL NET , the patient volume, that is, the estimated tidal volume VOL TID     —     EST  actually delivered to the patient, is provided by the patient volume observer  33 . Preferably, the estimated circuit volume VOL CKT     —     EST  and the estimated patient volume VOL TID     —     EST  are updated according to the timing when the net flow Q NET  crosses zero instead of the timing when the machine breath cycles from inspiratory phase to expiratory phase. The update timing for the volume variables will be further discussed later.  
         [0029]     In this embodiment, when a patient circuit disconnected is detected or when any type of circuit integrity alarm is activated, the volume variables will not be updated until the patient circuit is reconnected or the alarm is off. That is, the machine delivered net volume VOL NET , the estimated patient volume VOL TID     —     EST  and the estimated circuit volume VOL CKT     —     EST  freeze at previously computed values as:
 
 VOL   NET     K     =VOL   NET     K−1   
 
 VOL   TID     —     EST     K     =VOL   TID     —     EST     K−1   ; and
 
 VOL   CKT     —     EST     K     =VOL   CKT     —     EST     K−1     (11),
 
 where K is an index indicating the sampling number of the above volume variables. The sampling interval for these volume variables is determined based on factors such as the individual ventilator settings and the patient conditions. 
 
         [0030]     When the flow sensor  28  is installed in the Y circuit  20 , the patient flow Q Y  can also be measured. The measured patient flow Q Y  can be used to compute a measured patient volume VOL TID     —     Y  for facilitate volume limit of the circuit delivery controller  34 , so as to prevent an excessive circuit compliance compensation volume factor VOL TID     —     CTL  from being generated. The measured patient volume VOL TID     —     Y  can also used to replace the estimated patient volume VOL TID     —     EST  for computing the circuit compliance compensation volume factor VOL TID     —     CTL . In addition, the inspiratory flow Q INSP  may also be integrated to obtain the inspiratory volume VOL INSP . The applications of the measured patient volume VOL TID     —     Y  and the inspiratory volume VOL INSP  will be discussed in details later in this specification. Similarly to the volume variables expressed in Equation (11), computation of both the measured patient volume VOL TID     —     Y  are frozen whenever the patient circuit disconnect is detected or the alarm is activated as:
 
 VOL   TID     —     Y     K     =VOL   TID     —     Y     K−1   ;
 
 VOL   INSP     K     =VOL   INSP     K−1     (11-1).
 
         [0031]     Preferably, at the start of every inspiratory phase, or whenever any user setup value of the ventilator  10  varies, the measured machine delivered net volume VOL NET , the measured patient volume VOL TID     —     Y , and the inspiratory volume VOL INSP  are reset to an initial value (0 in this embodiment) and updated from the initial value every sampling interval (2 msec in this embodiment) as:
 
 VOL   NET     K−1   =0,  VOL   NET     K   =( Q   NET     K   /60)*0.002
 
 VOL   TID     —     Y     K−1   =0,  VOL   TID   Y     K   =( Q   Y     K   /60)*0.002
 
 VOL   INSP     K−1   =0,  VOL   INSP     K   =( Q   INSP     K   /60)*0.002  (12)
 
         [0032]     During the inspiratory phase, the net flow Q NET , the patient delivered flow Q Y  and the inspiratory flow Q INSP  are continuously monitored. When the inspiratory phase has started for at least a predetermined time (for example, TIME INSP &gt;50 msec) and the net flow Q NET  cross zero (that is, when Q NET     K   &lt;0 and Q NET     K−1   &gt;0), a zero-crossing net flow Q NET  is detected and flagged, while the machine delivered net volume VOL NET , the measured patient volume VOL TID     —     Y , and the inspiratory volume VOL INSP  are continuously updated as:
 
 VOL   NET     K     =VOL   NET     K−1   +( Q   NET     K   /60)*0.002 ,VOL   NET     K   =max( VOL   NET     K   , 0)
 
 VOL   TID     —     Y     K     =VOL   TID     —     Y     K−1   +( Q   Y     K   /60)*0.002,  VOL   TID     —     Y     K   =max( VOL   TID     —     Y     K   , 0)
 
 VOL   INSP     K     =VOL   INSP     K−1   +( Q   INSP     K   /60)*0.002.  (13)
 
         [0033]     If the net flow Q NET  has been detected to cross zero during the inspiratory phase, the estimated circuit volume VOL CKT     —EST    and the estimated patient volume VOL TID     —     EST  are updated at the start of the expiratory phase following the inspiratory phase as:
 
 {circumflex over (V)}OL   CKT     —     EST     K     =CKT   —   CMP   SLP ·( P   Y     K     −PEEP   K )+ CKT   —   CMP   INT 
 
 {circumflex over (V)}OL   TID     —     EST     K     =VOL   NET     K     −{circumflex over (V)}OL   CKT     —     EST     K   
 
 {circumflex over (V)}OL   TID     —     EST     K   =max( {circumflex over (V)}OL   TID     —     EST     K   , 0)  (14);
 
 and the machine delivered net volume VOL NET  and measured patient volume VOL TID     —     Y  are reset to the initial setup values and, again, updated from the initial values as:
 
 VOL   NET     K−1   =0 , VOL   NET     K   =( Q   NET     K   /60)*0.002 , VOL   NET     K   =min( VOL   NET     K   , 0)
 
 VOL   TID     —     Y     K   =0 , VOL   TID     —     Y     K   =( Q   Y     K   /60)*0.002 , VOL   TID     —     Y     K   =min( VOL   TID     —     Y     K   , 0)  (15).
 
         [0034]     Under the condition that the net flow Q NET  does not cross zero during the inspiratory phase, the machine delivered net volume VOL NET  and the measured patient volume VOL TID     —     Y  will not be reset at the start of the expiratory phase. That is, the machine delivered net volume VOL NET  and the measured patient volume VOL TID     —     Y  are continuously updated during the expiratory phase as,
 
 VOL   NET     K     =VOL   NET     K−1   +( Q   NET     K   /60)*0.002 , VOL   NET     K   =max( VOL   NET     K   , 0)
 
 VOL   TID     —     Y     K     =VOL   TID     —     Y     K−1   +( Q   Y     K   /60)*0.002,  VOL   TID     —     Y     K   =max( VOL   TID     —     Y     K   , 0)
 
 VOL   INSP     K     =VOL   INSP     K−1   +( Q   INSP     K   /60)*0.002.  (16)
 
         [0035]     When the zero-crossing net flow Q NET  is detected within a predetermined period of time such as 100 μsec after the machine has cycled to the expiratory phase (that is, when TIME EXP &gt;100 μsec and Q NET     K−1   &gt;0 and Q NET     K   &lt;0); or alternatively, when the expiratory has lasted over the predetermined period of time such as 100 μsec before the zero-crossing net flow Q NET  is detected (that is, TIME EXP &lt;100 msec and Q NET     K   &gt;0), the estimated circuit volume VOL CKT     —     EST  and the estimated patient volume VOL TID     —     EST  are updated as Equation (14), and the machine delivered net volume VOL NET  and the measured patient volume VOL TID     —     Y  are reset to the initial setup values and updated therefrom as:
 
 VOL   NET     K−1   =0 , VOL   NET     K   =( Q   NET     K   /60)*0.002 , VOL   NET     K   =min( VOL   NET     K   , 0)
 
 VOL   TID     —     Y     K   =0 , VOL   TID     —     Y     K   =( Q   Y     K   /60)*0.002 , VOL   TID     —     Y     K   =min( VOL   TID     —     Y     K   , 0)  (17).
 
         [0036]     In this embodiment, the measured machine delivered net volume VOL NET  and the measured patient volume VOL TID     —     Y  are reset according to the timing of zero-crossing net flow Q NET  instead of the phase of the machine breath phase. This allows calculation upon the estimated circuit volume VOL CKT     —     EST  and the estimated patient volume VOL TID     —     EST  synchronized with true patient inhalation and exhalation. Thereby, a more accurately real patient volume can be computed. The estimated patient volume VOL TID     —     EST  is thus updated according to the timing of the zero-crossing net flow Q NET , such that all the machine delivered net volume VOL NET  can be accounted when the patient breath and the machine breath are out of phase, that is, when the net flow Q NET  does not cross zero at the time the machine breath is cycling to the expiratory phase.  
         [0037]     At the beginning of every inspiratory phase, the estimate of the patient volume VOL TID     —     EST  is subtracted from a set tidal volume VOL TID     —     SET  to obtain a volume error VOL TID     —     ERR  reflecting the error of tidal volume between the setup value and the actual value as estimated. The volume error VOL TID     —     ERR  can thus be used to compute an estimated circuit compliance volume compensation factor VOL TID     —     CTL  by the volume delivery controller  34  to regulate the desired machine/system delivered net volume VOL SYS , so to modulate the inspiratory flow Q 1     —     SET  of the ventilator  10 . In this embodiment, an initial output of the volume delivery circuit  34  is predetermined at the beginning of the computation, that is, the circuit compliance volume compensation factor VOL TID     —     CTL  is initialized as:
 
 VOL   TID     —     CTL =INI —   CKT   —   VOL.   (18).
 
 The circuit compliance volume compensation factor VOL TID     —     CTL  will be reset to the initial value INI_CKT_VOL when the user settings of the ventilator  10  are changed. That is, any time when a new set of parameters is input to the system, the circuit compliance volume compensation factor VOL TID     —     CTL  will be reset to the initial value INI_CKT_VOL and updated for every breath. 
 
         [0038]     In the embodiment as shown in  FIG. 5 , the volume delivery controller  34  further comprises an error percentage converter  341 , a gain scheduler  342 , and a volume integrator  344  for generating the circuit compliance volume compensation volume factor VOL TID     —     CTL     K    for the current breath K. The error percentage converter  341  is used to compute a ratio of the feedback volume error VOL TID     —     ERR     K    to the set tidal volume VOL TID     —     SET     K    as:  
               VOL     PCT_ERR   K       =              VOL     TID_ERR   K              VOL     TID_SET   K         ×   100.             (   19   )             
 
 The error percentage VOL PCT     —     ERR     K    provides a useful indication of the ratio between the circuit compliance C T  and the lung compliance C L  of the patient. That is, when the error percentage VOL PCT     —     ERR     K    is larger, it indicates that a majority of the measured machine delivered net volume VOL NET  is distributed to the patient circuit  20  instead of being supplied to the patient&#39;s lung. Under such circumstance, a larger amount of volume may be required to compensate for the circuit compliance C T  in order to provide the correct machine delivered net volume VOL SYS , so such sufficient volume can be delivered to the patient&#39;s lung. Therefore, the volume delivery controller  34  further comprises a gain scheduler  342  which receives the error percentage VOL PCT     —     ERR     K    and provides a gain K VTID  according to the error percentage VOL PCT     —     ERR     K    for dynamically weighing the feedback volume error VOL TID     —     ERR     K   , so as to according to the error percentage VOL PCT     —     ERR     K   . A product of the gain K VTID  and the volume error VOL PCT     —     ERR     K    is then obtained by a multiplier  343 . The product of the gain K VTID  and the volume error VOL TID     —     ERR     K   , that is, the weighted volume error, is then added to the circuit compliance volume compensation factor VOL TID     —     CTL     K−1    computed in the previous breath in the integrator  344 , and the circuit compliance compensated patient volume VOL TID     —     CTL     K    for the current breath can be estimated as:
 
 VOL   TID     —     CTL     K     =K   VTID   *VOL   TID     —     ERR     K     +VOL   TID     —     CTL     K−1     (20).
 
         [0039]     The volume delivery controller  34  further comprises a volume restrictor  345  to prevent a negative circuit compliance volume compensation factor VOL TID     —     CTL     K    from being output. More specifically, the volume restrictor  345  restricts the output of the volume delivery controller  34  between a maximum value and zero as:
 
 VOL   TID     —     CTL     K   =max.( VOL   TID     —     CTL     K   , 0)  (21)
 
         [0040]     As discussed above, the measured patient volume VOL TID     —     Y  can be used as a volume limit to prevent the volume delivery controller  34  from generating an excessive volume factor to compensate for the circuit compliance. To this extent, the system for circuit compliance compensated pressure control  30  further comprises a volume limiter  37  operative to receive the measured patient volume VOL TID     —     Y  and compare the measured patient volume VOL TID     —     Y  to the set tidal volume VOL TID     —     SET . Before the measured patient volume VOL TID     —     Y  reaches a set tidal volume VOL TID     —     SET  preset by the user, that is, when VOL TID     —     Y &lt;VOL TID     —     SET , the volume delivery controller  34  operates normally to generate the circuit compliance volume compensation factor VOL TID     CTL    based on Equation (20). When the measured patient volume VOL TID     —     Y  reaches the set tidal volume VOL TID     —     SET , the volume error VOL TID     —     ERR  is set as zero:
 
VOL TID     —     ERR =0  (22), and
 
 the output of the volume delivery controller  34 , that is, the circuit compliance volume compensation factor VOL TID     —     CTL  is frozen at the value computed in the previous breath as:
 
VOL TID     —     CTL     K   =VOL TID     —     CTL     K−1     (23)
 
 Effectively, the volume limiter  37  is operative to switch on or activate operation of the volume delivery controller  34  when the measured patient volume VOL TID     —     Y  is smaller than the set tidal volume VOL TID     —     SET , and to switch off or inactivate operation of volume delivery controller as soon as the measured patient volume VOL TID     —     Y  is equal to or exceeds the set tidal volume VOL TID     —     SET . 
 
         [0041]     Table I shows an exemplary gain K VTID  set up according to the error percentage VOL PCT     —     ERR :  
                                         TABLE I                                   K VTID     VOL PCT   —ERR                                          1   0           2    25%           2.5    50%           4   100%           4   150%                      
 
 According to Table I, when the error percentage VOL PCT     —     ERR     K    is 100% and 150%, the gain K VTID  is set at 4, such that four times of the feedback volume error VOL TID     —     ERR     K    is added to the previously estimated circuit compliance volume compensation factor VOL TID     —     CTL     K−1   . When the error percentage VOL PCT     —     ERR  drops to 50%, 25% and 0, the gain K VTID  is consequently reduced to 2.5, 2, and 1, respectively. The empirical data shows that gain K VTID  varies with the error percentage VOL PCT     —     ERR  effectively reconciles the desired machine/system delivered net volume such that the desired tidal volume can be achieved within four breath cycles. 
 
         [0042]     The output VOL TID     —     CTK     K    of the volume delivery controller  34  is then converted into the a circuit compliance flow compensation factor as Q TID     —     CTL     K    by the volume-to-flow converter  35 , such that the inspiratory gas flow Q INSP  can be updated to provide the accurate volume to the patient as computed above. To convert the volume factor VOL TID     —     CTL     K    into the flow factor Q TID     —     CTL     K   , the inspiratory time T INSP     —     EST     K    is estimated first. As it is known that the set tidal volume VOL TID     —     SET     K    can be computed by the integration of a predetermined peak inspiratory flow Q PEAK     —     SET     K    time t throughout the inspiratory phase. Therefore, when the predetermined peak inspiratory flow Q PEAK     —     SET     K    and the set tidal volume VOL TID     —     SET     K    are known, the inspiratory tome T INSP     —     EST  can be estimated by such relationship. In this embodiment, the predetermined peak inspiratory flow Q PEAK     —     SET     K    is equal to a preset peak flow Q PEAK     —     USER     K    when a square waveform of the inspiratory flow is selected. In the case that a decelerating waveform is selected, the predetermined peak inspiratory flow Q PEAK     —     SET     K    is a function of the preset peak flow Q PEAK     —     USER     K    and the time t into the inspiratory phase. Therefore, dependent on the waveform as selected, the inspiratory time T INSP     —     EST  can be estimated as:  
                 T   ^       INSP_EST   K       =     {                 VOL     TID_SET   K         (       Q   PEAK_USER     /   60     )       ,           square   ⁢           ⁢   waveform                     (     4   /   3     )     ·     VOL     TID_SET   K           (       Q   PEAK_USER     /   60     )       ,           decelerating   ⁢           ⁢   waveform           ,               (   24   )                 Q     PEAK_SET   K       =     {               Q   PEAK_USER     ,           square   ⁢           ⁢   waveform                 f   ⁡     (       Q   PEAK_USSER     ,   t     )       ,           decelerating   ⁢           ⁢   waveform           .               (   25   )             
 
 where 
 
 The circuit compliance flow compensation factor Q TID     —     CTL     K    can thus be converted into the flow Q TID     —     CTL     K    as:  
               Q     TID_CTL   K       =     60   ·       (       VOL     TID_CTL   K           T   ^       INSP_EST   K         )     .               (   26   )             
 
 Therefore, the required inspiratory flow Q INSP     —     SET     K    can be computed by:
 
 Q   I     —     SET     K     =Q   PEAK     —     SET     K     +Q   TID     —     CTL     K     (27),
 
 while the overall commanded volume to be used by the servo-control subsystem  40  and to be used by breath control for cycling based on volume, that is, the desired machine/system delivered net volume VOL SYS  is updated as:
 
 VOL   SYS     K     =VOL   TID     —     SET     K     +VOL   TID     —     CTL     K     (28).
 
         [0043]     The inspiratory volume VOL INSP  integrated from the measured inspiratory flow Q INSP  can be used to determine the breath phase of the machine. As summarized by Equation (29), when the measured inspiratory volume VOL INSP  is smaller than the updated or desired machine delivered net volume VOL TID     —     SYS , that is, the set tidal volume VOL TID     —     SET  compensated with the circuit compliance volume compensation factor VOL TID     —     CTL  (VOL TID     —     SET+VOL   TID     —     CTL ), the machine breath remains at the inspiratory phase. However, when the measured inspiratory volume VOL INSP  is equal to or larger than the updated machine delivered net volume VOL TID     —     SYS , the machine breath enters or have entered the expiratory phase, respectively.  
             phase   =     {             cycle   ⁢           ⁢   to   ⁢           ⁢   E   ⁢     -     ⁢   phase     ,       VOL     INSP   ⁢           ⁢   K       ≥     (       VOL     TID_SET   K       +     VOL     TID_CTL   K         )                     remain   ⁢           ⁢   in   ⁢           ⁢   I   ⁢     -     ⁢   phase     ,       VOL     INSP   K       &lt;     (       VOL     TID_SET   K       +     VOL     TID_CTL   K         )                         (   29   )             
 
         [0044]     As shown in  FIG. 4 , the system of circuit compliance compensated volume control  30  further comprises a plurality of adders/subtractors  301 ,  302 ,  303 , and  304 . As shown, the adder/subtractor  301  is operative to receive the inspiratory flow Q INSP  and the expiratory flow Q EXP , so as to calculate the net flow Q NET  defined as the flow differential therebetween. The adder/subtractor  302  has two inputs to receive the set tidal volume VOL TID     —     SET  and the estimated patient volume VOL TID     —     EST  from the patient volume observer  32 . Thereby, the difference between the set tidal volume VOL TID     —     SET  and the estimated patient volume VOL TID     —     EST , that is, the feedback volume error VOL TID     —     ERR     K    defined as the volume differential thereby can be derived and input to the volume delivery controller  34 . The adder/subtractor  304  bypasses the volume-to-flow converter  35  to calculate the desired machine delivered net volume VOL SYS  based on the set tidal volume VOL TID     —     SET  and the circuit compliance volume compensation factor VOL TID     —     CTL  computed by the volume delivery controller  34 . The output of the adder/subtractor  304  is connected to the servo control subsystem  40  as well as a phase detector  36 , which has another input connected to an integrator  311  for integrating the inspiratory flow Q INSP  into the inspiratory volume VOL INSP . By comparing the inspiratory volume VOL INSP  and the output of the adder/subtractor  304 , the phase detector  36  is operative to determine the current breath phase of the machine according to Equation (29). The adder/subtractor  303  has an input connected to the volume-to-flow converter  35 , the other input for receiving the predetermined peak inspiratory flow Q PEAK     —     SET , and an output connected to the servo control sub-system  40 . By the adder  303 , the desired inspiratory flow Q INSP     —     SET  can be computed and input to the sub-system servo control system  40 . In addition to the integrator  311 , other integrators  312  and  313  can also be installed to compute the machine delivered volume VOL NET  and the measured patient volume VOL TID     —     Y  when the patient flow Q Y  is measured, respectively.  
         [0045]     As discussed above, the measured machine delivered net volume VOL NET  is input to the patient volume observer  33 , and the measured tidal volume VOL TID     —     Y  can be used in a volume limiter  37  for controlling the maximum output of the volume delivery controller  34 . In one embodiment, the measured tidal volume VOL TID     —     Y  can also be used to replace the estimated patient volume VOL TID     —     EST  for estimating the circuit compliance volume compensation factor VOL TID     —     CTL  To estimate the circuit compliance compensated volume based on the measured tidal volume VOL TID     —     Y , a switch  38  is inserted to selectively connect the adder/subtractor  302  to the integrator  313  or the patient volume observer  33 . By simply operating the switch  38 , the measured patient volume VOL TID     —     Y  or the estimated patient volume VOL TID     —     EST  can be selected as feedback for estimating the circuit compliance volume compensation factor VOL TID     —     CTL .  
         [0046]     The above adders/subtractors  301 - 304  and integrators  311 - 313  may also be formed as individual in the system  30 ; or alternatively, they can also be integrated into the respective devices instead. For example, the integrators  311  and  312  may be integrated into the phase detector  36  and the patient volume observer  33 , respectively, and the adder/subtractors  301 ,  302  and  303  may be integrated as a portion of the patient volume observer  33 , the volume delivery controller  34 , and the servo control sub-system  40 , respectively. Alternatively, the adder/subtractor  303  may also be integrated at the output of the volume-to-flow converter  35 , while the adder/subtractor  304  may also be integrated into the output of the volume delivery controller  34 . In addition, the circuit compliance compensated volume control system  30  may be implemented by individual hardware or a processor integrated into the ventilator  10 . The circuit compliance compensated volume control system  30  may also be implemented by a software executable by a personal or laptop computer connected to the ventilator or by the processor of the ventilator  10  directly.  
         [0047]     As shown in  FIGS. 3 and 4 , the desired inspiratory flow Q I     —     SET  and the desired machine delivered volume VOL TID  are input to the servo control subsystem  40 , which, according to the desired inspiratory flow Q I     —     SET , generate a flow-control valve command signal FCV D/A  to control the orifice of the inspiratory port  14 , so as to command the ventilator  10  to deliver the desired inspiratory flow Q I     —     SET . In addition to the flow-control valve command signal FCV D/A , the servo control subsystem  40  is also operative to generate an exhalation valve command signal EV D/A  to control opening or closing status of the exhalation port  16 .  
         [0048]     The above description is given by way of example, and not limitation. Given the above disclosure, one skilled in the art could devise variations that are within the scope and spirit of the invention disclosed herein, including various ways of implementing the circuit compliance compensated volume control systems. Further, the various features of the embodiments disclosed herein can be used alone, or in varying combinations with each other and are not intended to be limited to the specific combination described herein. Thus, the scope of the claims is not to be limited by the illustrated embodiments.