-SWS wrote:the EPR pressure-delivery waveform itself shows a downward pressure-average skewing rather than an upward skewing (that skewing being relative to the reference value mentioned by DreamStalker: comprised of IPAP's setting and EPAP's setting, both summed then divided by two).
I suppose one could get picky here, and say rather than an average, you would actually take the mean, which takes into account the variable of time-- the spike seen at IPAP
peak is short, whereas most of the time the pressure is closer to baseline (which, for purposes of calculation, is EPR pressure level). So at a pressure of 10 cmH2O, EPR 3.0 cmH20, the mean pressure is probably something like 7.8-8.0 cmH2O vs an average of 8.5 cmH2O, as represented here:
PTAF/CPAP represents patient breathing, upward inflection is inspiration, downward exhalation. Real time pressure measurement in the PRESSURE waveform.
So where does that "upward skewing" of the pressure-average come from then? I have to assume it comes largely from the total amount of time the EPR algorithm safely holds EPAP at the higher value of IPAP. Or in other words, I suspect it largely comes from the minority amount of time the EPR algorithm decides to behave like a CPAP machine (with its higher IPAP/EPAP average) rather than behaving like a BiLevel machine.
In fiddling around with this thing, I've not noted a lot of things that suspends EPR, or when that occurs, that EPR is suspended for very long. So the question would follow, if an expected mean pressure is not seen, then why would that be. I would offer as a possible explanation that resistance to exhalation created a brief pressure surge and forces IPAP
peak > CPAP
set, and the net P
mean is now not only greater than the expected mean, but perhaps even greater than the expected average as well. This is seen in the following example:
This waveform set was generated during full face mask utilization at a pressure of 10 cmH2O, EPR 3.0 cmH20. Every breath exceeds CPAP
set, with IPAP
peak caused by exhalation against resistance (literally and figuratively, if flow cannot compensate quickly enough, the only variable left is pressure). This also gives the perception of an initial resistance to exhalation, quite contrary to the design of EPR.
The criteria for the latter scenario is not clear. Aside from high exhaled flow rates, I would think that differences in mask properties will result in differences in end-result pressure therapy. Masks with reduced leak rate could create the phenomenon in waveform set 2, as back pressure is created during exhalation through a restricted orrice, perhaps creating the spike supra-therapeutic CPAP. This was apparently demonstrated during observation of waveforms while using a whisper swivel, with a leak rate of 25 LPM @ 10 cmH2O. On the other hand, the actual generation of waveform 2 was done with ResMed UltraFFM, and its leak rate of 37 LPM should have been plenty. Perhaps turbulence based on the location of the exhalation ports, or an increase in dead space had bearing on measurements.
Or maybe it's something a heckuva lot more obvious...
Also of note is the additional negative pressure seen immediately prior to breaths 2, 4, and especially 6. In the first waveform set, EPR termination occurs so quickly that the transition from expiration to inspiration appears seamless, and being at a sub-therapeutic pressure level at the point of inspiration may be academic. In the second, however, EPR is quite lax in termination and an increased negative inspiratory effort is seen during sub-therapeutic pressure. This is the scenario whereby EPR might create a problem in an unstable airway, and perhaps explain increased AHI in some individuals when the only variable is EPR.
SAG
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