Abstract:
A ventilator that delivers air at different pressures to a patient during inspiratory and expiratory cycles of breathing and that cycles from inspiratory to expiratory operation when the patient&#39;s respiratory flow passes a threshold level. The threshold generally increases form the beginning of inspiration to the end of inspiration. The increase can be linear over all or only a proton of the inspiratory cycle, and the threshold can be adjusted so that cycling is prevented during the initial portion of an inspiratory cycle. The minimum and maximum levels may both be functions of peak flow and the threshold may increase as a function of elapsed inspiratory time. The rate at which the threshold increases from a minimum level to a maximum level may be adjustable for individual patient needs and may be determined from previous breaths.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of U.S. patent application Ser. No. 11/571,068, filed Dec. 21, 2006, which is a national phase entry under 35 U.S.C. §371 of International Application No. PCT/AU2005/000895, filed Jun. 22, 2005, published in English on Jan. 5, 2006, as International Publication No. WO 2006/000017, which claims priority of U.S. Provisional Patent Application No. 60/582,580, filed Jun. 23, 2004, which are all incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to methods and apparatus for the provision of ventilatory assistance synchronized with a subject&#39;s respiratory cycle. More specifically, the invention relates to methods for synchronizing the ventilator to cycle its pressure response in conjunction with the patient&#39;s respiration cycle. 
     BACKGROUND OF THE INVENTION 
     The present invention is applicable to any form of ventilation in which respiratory flow is used for cycling, although it is primarily useful in noninvasive pressure-support ventilation. The invention also finds use in invasive ventilation, especially of the pressure support variety. The invention finds greatest use in the presence of higher levels of pressure support, with patients who typically do not have sleep apnea but do have respiratory insufficiency or failure, where ventilatory assistance is typically provided mainly at night but may well be provided during some of the daytime. Cycling becomes a prominent issue when respiratory mechanics are abnormal, especially in chronic obstructive pulmonary disease (COPD). 
     In a ventilator, it is often necessary to have a control process to detect when the patient&#39;s actual respiratory cycle switches from inspiration to expiration or vice versa so that the ventilator can implement an appropriate ventilation response. For example, some ventilatory devices provide a process for determining when to trigger an inspiratory pressure for purposes of delivering an appropriate pressure during the inspiratory portion of the patient&#39;s respiratory cycle. Similarly, such a device may also have a process for determining when to cycle to an expiratory pressure for purposes of delivery of an appropriate machine-patient response during the patient&#39;s expiration. Such processes serve to synchronize the ventilator with the patient&#39;s actual respiratory cycle. Those skilled in the art will recognize that “triggering” is the event associated with the initiation of the pressure levels intended for the patient&#39;s inspiration and “cycling” is the event associated with switching to the pressure levels intended for the patient&#39;s expiration. 
     For example, a bi-level ventilator provides a higher pressure level during the inspiratory portion of the patient&#39;s breathing cycle, a so-called IPAP, and a lower pressure level during the expiratory portion of the breathing cycle, a so-called EPAP. Traditionally, the switching may be accomplished by monitoring the respiratory flow or pressure and defining a threshold level, such as zero or a percentage of peak flow. When the measured respiratory flow value falls below the threshold, the device will deliver the EPAP. Another alternative to such switching may involve recorded respiration rates and the monitoring of elapsed time from the start of inspiration; the machine may switch to the expiratory portion of the respiratory cycle after reaching a time that is the expected time for the inspiratory portion of the respiratory cycle. 
     A goal of these processes for cycling is to make a ventilator device more comfortable for a user because if respiratory events are not properly synchronized, the device may be quite uncomfortable for a patient. A considerably more important goal is to optimize gas exchange and, especially in COPD with severe expiratory flow limitation, to prevent prolonged inspiratory times which lead to dynamic hyperinflation. Current methods for cycling can sometimes improperly detect expiration and result in an improper pressure change. For example, if the ventilator cycles into expiration too early, less support will be provided to the patient during inspiration when it is needed. Thus, there is need to improve such processes by minimizing improper synchronization. 
     BRIEF DESCRIPTION OF THE INVENTION 
     It is an objective of the invention to improve synchronization by providing a variable cycling threshold that changes as a function of time. 
     It is a further objective of the invention to provide a synchronization expiratory threshold that changes to become more sensitive during inspiration as the cycle advances. 
     Additional objectives will be apparent to those skilled in the art upon consideration of the following description of the invention. 
     In the invention, a synchronization threshold is calculated that is compared to a measure of flow for purposes of determining whether the ventilator should advance from inspiration to expiration. In a preferred embodiment, the threshold is a function of time that permits it to change within a single inspiratory cycle from a less sensitive threshold during early portions of the cycle to a more sensitive threshold during latter portions of the cycle. In one embodiment of the invention, the threshold is provided with a refractory period to prevent a transition to expiration during certain times within a cycle when such a transition is not desired or unlikely to accurately reflect the patient&#39;s actual advance to expiration. Additional aspects of the invention are described in more detail in the following specification. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts the structure of the preferred ventilator apparatus for implementing the methods of the current invention; 
         FIG. 2  is a graph of one embodiment of an expiratory synchronization threshold that varies with time within a cycle; 
         FIG. 2B  is a graph of a sensitivity function for varying an expiratory synchronization threshold with time within a cycle; 
         FIG. 3  is a graph of another embodiment of an expiratory synchronization threshold that varies with time within a cycle; 
         FIG. 4  is a graph of one embodiment of an expiratory synchronization threshold that includes a refractory period within a cycle; 
         FIG. 5  is a graph of one embodiment of an expiratory synchronization threshold that varies with time within a cycle and includes a refractory period; and 
         FIG. 6  is a graph of another embodiment of an expiratory synchronization threshold that varies with time within a cycle. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In reference to  FIG. 1 , the pressure delivery device includes a servo-controlled blower  2 , a mask  6 , and an air delivery conduit  8  for connection between the blower  2  and the mask  6 . Exhaust gas is vented via exhaust  13 . Optionally, a flow sensor  4   f  and/or pressure sensor  4   p  may also be utilized in which case mask flow may be measured using a pneumotachograph and differential pressure transducer or similar device to derive a flow signal F(t), and mask pressure is measured at a pressure tap using a pressure transducer to derive a pressure signal P mask (t). The pressure sensor  4   f  and flow sensor  4   p  have only been shown symbolically in  FIG. 1  since it is understood that those skilled in the art would understand how to measure flow and pressure. Flow F(t) and pressure P mask (t) signals are sent to a controller or microprocessor  15  to derive a pressure request signal P request (t). Alternatively, a flow signal f(t) and pressure signal P mask (t) may be estimated or calculated in relation to the blower motor by monitoring current supplied to the motor and/or the speed of the motor as disclosed in U.S. Pat. Nos. 5,740,795, 6,332,463 or 6,237,593 without the provision of flow and pressure sensors as described above. Optionally, the blower motor speed may be held generally constant and pressure changes in the mask may be implemented by controlling an opening of a servo-valve that may variably divert or deliver airflow to the mask. 
     A controller  15  or processor is configured and adapted to implement the methodology described in more detail herein and may include integrated chips, a memory and/or other instruction or data storage medium. For example, programmed instructions with the control methodology may be coded on integrated chips in the memory of the device or loaded as software. 
     Preferably, the device delivers varying pressure levels of continuous positive airway pressure which are generally higher during inspiration than expiration. However, consistent with the control principles of the invention as described herein, other types of ventilatory pressure treatment may be implemented in the apparatus, such as other more comfortable variants of natural patient-synchronized pressure changes. 
     In accordance with the principles of my invention, the synchronization threshold is varied within a single inspiratory breathing cycle as a function of time. In other words, the threshold does not remain constant during the cycle. Rather, the threshold increases over time to make it more sensitive during the inspiratory cycle and thus render the threshold more likely to result in the cycling of the ventilator as the inspiratory cycle advances to expiration. For example, a variable cycling threshold may be continuously calculated by the device as the inspiratory time lapses and it can be changed during that time period until expiration is detected by the flow falling below the threshold. Another way of looking at the invention is that the ventilator becomes increasingly sensitive to the initiation of cycling as inspiration progresses. 
     One example of this invention is illustrated in  FIG. 2 . In the figure, the cycling threshold function (shown as a dashed line) for detecting expiration is changed during the course of inspiration from less sensitive to more sensitive. This increase in sensitivity improves synchronization because with such an in-cycle varying threshold, the device is less likely to inadvertently switch into expiration in an early stage of inspiration but, as inspiration advances, the threshold becomes more likely to cause the ventilator to switch into expiration. In other words, it is more difficult to cycle during the early parts of inspiration and easier during the latter portions of inspiration. 
     In the embodiment, the threshold varies as a function of time, T max , which preferably is the expected time of the inspiration. Moreover, the threshold may optionally be restricted to vary between a minimum threshold value and/or a maximum threshold value. In one embodiment, the maximum and minimum thresholds may be a function of peak flow, such as a proportion or percentage of a previous breath&#39;s peak flow, e.g., 50% and 10% respectively. Alternatively, the maximum and minimum thresholds may be some predefined fixed amount above a zero based flow measure. A clinician or physician may select these percentages or amounts. 
     Between the minimum and maximum, the threshold is preferably an increasing function such as an increasing function of elapsed inspiratory time. For example, the increasing function may be a ratio of elapsed inspiratory time (T insp   _   elapsed ) to expected inspiratory time T max  (e.g., k*T insp   _   elapsed /T max ), where k is a preset sensitivity constant that can be adjusted higher for patients that need a faster increase in sensitivity to permit quicker cycling to expiration for patients that need faster cycling. In one embodiment, the T max  or expected inspiratory time is determined from previous breaths, for example, as the average inspiration time from the previous five normal breaths (excluding coughs etc.). Alternatively, T max  may be a preset maximum inspiration cycle time. The elapsed inspiration time T insp   _   elapsed  counter can be set to 0 during expiration and begin counting at the beginning of inspiration. For example, the threshold may be set as follows: 
     
       
         
               
               
             
           
               
                   
               
             
             
               
                   
                 Temp_Threshold = (k*T insp _elapsed/T max ) * max_threshold 
               
               
                   
                 If Temp_threshold &lt; min_threshold then 
               
               
                   
                  Set Threshold to the min_threshold 
               
               
                   
                 Else if Temp_threshold &gt; max_threshold then 
               
               
                   
                  Set Threshold to max_threshold 
               
               
                   
                 Else 
               
               
                   
                  Set Threshold to the temp_threshold 
               
               
                   
               
             
          
         
       
     
     Other alternative schemes may be utilized for varying the cycling threshold as a function of inspiratory time. For example, the synchronization threshold may be set (a) to the minimum threshold during a first portion of the expected inspiratory period, e.g., about 25% of the expected cycle time, (b) to ramp from the minimum threshold to the maximum threshold during a middle portion of the expected inspiratory period, (e.g., about the next 50% of the expected inspiratory cycle time), and (c) to the maximum threshold for the last portion of the expected inspiratory period (e.g., about the last 25% of the expected cycle time). Such a function may be implemented by a threshold multiplier or threshold sensitivity function as illustrated by the graph of  FIG. 2B . Utilizing the function of  FIG. 2B , a synchronization threshold for cycling may be calculated by the following formula:
 
Threshold=Ts( t )*Peak_Flow
 
where:
         Ts(t) is a threshold sensitivity function of inspiratory cycle time that results in a multiplier between 0 and 1 as illustrated by the graph in  FIG. 2B ; and   Peak_Flow is the peak inspiratory flow.       

     In the illustrated graph of  FIG. 2B , during the first portion of the inspiratory cycle, the multiplier may be 0 or a low value (e.g., 0.10), and as the cycle advances the multiplier can ramp to a maximum level (e.g., 0.60). Those skilled in the art will recognize different forms of the multiplier graph that can be produced for the threshold sensitivity function for increasing the sensitivity of the threshold over time within a cycle to implement effective cycling thresholds. In all embodiments of my invention, the threshold generally increases from the beginning of inspiration to the end of inspiration. In one form the rate of increase is constant. 
     In another embodiment, as illustrated in  FIG. 3 , the threshold may ramp as a monotonically increasing function. The ramp may start at 0 flow at the beginning of inspiration and increase as the inspiratory portion of the respiratory cycle advances. This embodiment may be implemented by the formula described with regard to the embodiment of  FIG. 2  by setting the minimum threshold to 0 and the sensitivity constant to 1. The maximum threshold may be set to some desired percentage of peak flow. In this way, as the time of the patient&#39;s inspiration approaches the expected time or some predetermined maximum time, the cycling threshold will gradually ramp up to the preset maximum percentage of peak flow. Alternatively, with regard to the sensitivity function of  FIG. 2B , the graph of the multiplier may start at 0 and increase to a higher level. It is clear that in the case of timed breaths, since the flow is typically zero or negative at the start of a timed breath, this algorithm will cause immediate cycling, since the actual flow is less that the cycling threshold of 0. Hence this cycling threshold function is applicable as stated only with triggered breaths, but may be used in combination with an absolute refractory period (minimum inspiratory time before cycling can occur) which allows time for the pressure support delivered by the ventilator to cause the flow to be significantly positive by the end of the absolute refractory period. (Alternatively, cycling can be controlled to take place when the flow crosses the threshold only in the downward direction.) 
     In one embodiment, a refractory period is implemented to prevent cycling to expiration during a certain time period. For example, such an implementation is illustrated in the graph of  FIG. 4 . In  FIG. 4 , a constant threshold is illustrated with a refractory period that is a function of inspiratory time. In the illustrated example, the cycling threshold is inoperable at the beginning of inspiration, thus preventing cycling during the refractory period. Although the cycling threshold can be at a desired percentage of a peak flow that will cycle the ventilator properly, during the refractory period it may be set to a level that will prevent cycling, (e.g., −4*peak flow). However, after the lapsing of the refractory period, the threshold steps up to return to the desired operable threshold level. For example, the threshold may be set to be inoperable during the first 25% of the expected time for the inspiratory cycle (T max ) as follows: 
     
       
         
               
               
               
             
           
               
                   
               
             
             
               
                   
                   
                 If (T insp _elapsed &lt; 0.25 * T max ) then 
               
               
                   
                   
                  Threshold = −4 * peak flow 
               
               
                   
                   
                 Else 
               
               
                   
                   
                  Threshold = 0.25 * peak flow. 
               
               
                   
               
             
          
         
       
     
     Similarly, such a refractory period may be implemented for the increasing functions of the earlier embodiments disclosed in  FIGS. 3 and 4 . For example, as illustrated by the cycling threshold of  FIG. 5 , the increasing function that is restricted between minimum and maximum thresholds has a refractory period during an early portion of inspiration and only becomes effective thereafter. 
     Various aspects of the previously described implementations may be combined to create other dynamic thresholds for synchronizing a ventilator. One such combination is illustrated in  FIG. 6 . In the graph, during a first time portion of the cycle, a refractory period is enforced. In the next time portion of the cycle (A), a minimum threshold is applied. In the following time portion (B), the threshold is ramped from the minimum threshold to a second tier threshold. The second tier threshold is applied during a fourth time period (C). Finally, in the final period D, the threshold is an increasing function of a maximum from the second tier threshold. 
     The respiratory mechanics of timed and triggered breaths differ. In a timed breath, the respiratory flow may be significantly negative at the start of the breath, and patient effort may occur only some time after the start of the breath or not at all. With a low pressure support level, early cycling may occur, particularly if the cycling threshold is a fixed value or is the maximum of a proportion of peak flow and some fixed minimum (positive) cycling threshold. In the case of a pressure support servoventilator, in which the pressure support level may be quite low during a period of spontaneous breathing, the consequence may be that the first one or two breaths after a transition from triggered to timed breaths may be relatively ineffective, and certainly less effective than if premature cycling had not occurred. For this and other reasons there is an advantage in having different cycling threshold algorithms for timed and triggered breaths. 
     Under some circumstances, typically during a timed breath, there is an advantage in having a moderately negative cycling threshold for some proportion of inspiration. This means that the inspiration can be terminated by a patient effort, but will not be terminated passively. At the end of the period of negative cycling threshold, the cycling threshold may change abruptly or smoothly towards a more positive value. 
     Cycling threshold during part or all of the inspiratory time may be a function both of some predetermined constant values and some function of respiratory flow, such as (instantaneous) peak respiratory flow. 
     In a preferred embodiment a number of these features are combined so that the cycling algorithm during a timed breath is as follows: 
     There is a maximum inspiratory time TiMax, at which cycling will occur regardless of any other conditions. TiMax is set to a value that is reasonable for timed breaths; for example, if the timed respiratory period (the reciprocal of the backup rate) is TTotTimed, TiMax is equal to a reasonable inspiratory fraction for the patient in question (typically between 0.25 and 0.4) multiplied by TTotTimed. 
     For inspiratory time in [0, TiMax/6) cycling cannot occur. 
     For inspiratory time in [TiMax/6, TiMax/3) the cycling threshold is some small negative value, QCycMin, such as −0.1 l/s. 
     Let QPeak(Ti) denote instantaneous peak flow at inspiratory time Ti. 
     Let QPeakPos (Ti)=max (instantaneous peak flow QPeak(Ti), 0) 
     For inspiratory time Ti in [TiMax/3, TiMax*⅔) the cycling threshold QCyc is given by interpolation between the value QCycMin at the start of this inspiratory time interval and a threshold equal to a proportion FCyc of QPeakPos (Ti) at the end of this inspiratory time interval, in particular by:
 
QCyc=(Ti−TiMax/3)/(TiMax*⅔−TiMax/3)*(FCyc*QPeakPos(Ti)−QCycMin)+QCycMin
 
     For inspiratory time Ti in [TiMax*⅔, TiMax) the cycling threshold QCyc is given by a proportion FCyc of QPeakPos (Ti). 
     Although the invention has been described with reference to various embodiments as described in this entire disclosure, it is to be understood that these embodiments are merely illustrative of an application of the various principles of the invention. Numerous modifications, in addition to the illustrative embodiments of the invention discussed herein may be made and other arrangements may be devised without departing from the spirit and scope of the invention. For example, although cycling thresholds have generally been graphically illustrated in this disclosure, analogous thresholds may be implemented as triggering thresholds for inspiration.