sleepydave wrote:When the coming of Expiratory Pressure Relief (EPR) was first announced, I had some questions as to whether this modality would offer relief on active expiration only during the CPAP mode, and perhaps address the issues that other expiratory adjuncts were having, or if the drop in expiratory pressure were carried out all the way to the next inspiration, relying on inspiration as the trigger to terminate EPR, and thus essentially operate in a BiPAP mode.
The following waveform analyses were performed on the EPR mode at 10 cmH2O with an EPR setting of 3 cmH2O. The breath rate is approximately 12.
The first graph shows the breathing waveform on top, inspiration being an upward deflection and expiration downward, while the bottom graph is measuring pressure. The pressure settings are seen as faint numbers at the left of the pressure waveform. You can see that the EPR, reflected as a drop in the therapeutic pressure on the pressure waveform down to about 7 cmH2O, is carried out all the way to the point of inspiration, and the inspiratory effort therefore takes place at a sub-therapeutic pressure. The baseline pressure returns to 10 cmH2O, but not until after inspiration has begun. In other words, inspiration is the trigger to terminate EPR, and instead of a CPAP pressure of 10 cmH2O with an expiratory adjunct, we are effectively left with BiPAP of 10/7:
This might not make a clinical difference if the patient ends up with the same results on BiPAP 10/7 that he would have on CPAP 10 cmH2O (which could be the case if there were only flow limitations, snores, or hypopneas). But if the new EPR-defined EPAP is below the apnea threshold, then there could be a problem.
In the second graph, the waveforms are superimposed to show more clearly that inspiration is occurring at a sub-therapeutic level:
There is a time limitation associated with the termination of EPR. In the next graph, you can see how the EPR eventually terminates and returns to baseline. In this instance, the breath rate was approximately 6, so the time to EPR termination was appoximately 5 seconds. The first arrow represents EPR termination, while the second signifies patient breath:
And here again, the graphs are superimposed to show the return to baseline relative to inspiration:
This means that eventually, there will be a return to baseline CPAP if in fact, an apnea occurs, and at the most, only one breath would be missed. Is the net result clinically relevant? I'm not sure either way. But if you're generating negative intrathoracic pressure or creating arousals, then there could be an issue.
In re: putting EPR in the AutoSet mode, that could be an effective way to overcome this supposed shortcoming of EPR. If events were to start occurring at, in this case, "10 cmH2O of CPAP". then baseline pressure could be raised, theoretically, to "13 cmH2O of CPAP", or effectively BiPAP 13/10. Course now we're right back where we started. The only outstanding question would be if the 13/10 format was better tolerated than the straight 10.
But how would the algorithm work? If the apnea identification in AutoCPAP is 10 seconds, and the EPR terminates at (in this case) 6 seconds, how would it know to increase the CPAP (really the "EPAP" segment of EPR) to address apneas? I would assume that flow limitations would be properly addressed with CPAP increase (because you're really raising the "IPAP" segment of EPR).
As an aside, therein lies the problem once you start talking about
Auto-BiPAP. Do you increase the IPAP, and keep EPAP fixed, or do you vary the EPAP as well, looking to address apneas. You're gonna need two totally separate algorithms, and they can't interfere with each other.
There are a couple of options available with EPR. It can be used during the ramp period only, which would offer significant patient comfort during a time where the perhaps the greatest period of patient difficulty occurs. After the ramp period is over, it returns to the set pressure.
Before you select full-time EPR, though, and carry EPR throughout the night, you should consider how your particular situation might respond to this modality. And I think the key to EPR, AutoCPAP with EPR (should that ever come about) and AutoBiPAP will be how apneas are addressed. You might be OK dealing with hypopneas, snores and RERAs if you're of the belief that BiPAP can properly address these issues. And that's a whole 'nother discussion.
sleepydave
