dsm wrote:Assuming the pressure (SWS says it is maintained at the impeller, I believe it is maintained at the pressure transducer which is at the air exit port) remains constant, then the flow will vary - as you breathe out you are pushing against the 10 CMs being fed in & as you explained before the pressure transducer detects a change in pressure at the air exit port & lowers the blower speed - this control in older models was not exactly precise - but then as you breathe in the flow swings back strongly towards you the user.
Well, generally, all feedback transducers are calibrated so that
system output is what is being accurately read by that feedback transducer.
In the various patent descriptions system-output pressure is usually referenced at either the mask or at the machine. In that latter case, machine-side output is typically referenced as impeller output by patent descriptions. So the transducer can be calibrated for that system-side output typically referenced at the impeller whenever proximal sensing at the mask is not employed. Still, there's a negligible pressure gradient or loss between those two near-points in the system (the feedback transducer and impeller near points).
DSM wrote:SWS has pointed out that while the pressure in one part of the system (human connected by a 6 foot tube to a fluctuating pump) can remain constant, it won't neccesarily be the same at the other end of the system at that point in time. It can vary by 2-3 CMs in this particular description (at 10 CMs pressure).
Well, that's a very good point. That's kind of the "middle point"---with the human diaphragm at one far end contributing pressure, and the CPAP impeller at the other far end also contributing pressure.
Each of those far-end contributors will incur a pressure gradient or gradual loss toward that middle point. We might want to let the thought just ruminate in our minds a bit---that we used to have only the diaphragm (of those two) before we went on CPAP. Let's also take note of what the pressures used to be at our mouths back then: if we had measured those pressures at the mouth, we would have measured alternating positive and negative pressures...
Enter CPAP and its highly descriptive acronym: it's a
Continuous
Positive Air
Pressure machine. Two characteristics about the machine's delivery pressure worth noting are that it's both continuous and positive---despite our respiration still requiring that our airway pressures alternate with respect to atmospheric pressure. But that
"continuous positive" part of the acronym describes what happens back at the machine's system output side---not what the alternating pressures happen to be inside the human airway. Inside the human airway we still need to alternate between negative and positive pressures if we expect to alternately reverse our airflow so that we breathe both in and out.
So despite that machine continuously sourcing positive-pressure at the impeller, the pressure at the mask is going to be a resultant of both pressure-delivering contributors and their respective gradient inefficiencies or losses along the way toward that middle point (with the respiratory drive now adapting a bit as it factors CPAP's inspiratory assistance and expiratory back pressure). The human diaphragm is really the biggest pressure-delivering contributor between those two. Despite a CPAP impeller maintaining a constant delivered pressure at its far end, the human diaphragm is going to cause mask pressure to go positive during inhale and negative during exhale (referenced at the machine---reverse that if you want to reference at the diaphragm). Reference it where the airway used to collapse, and you have a bit of extra positive stenting pressure (thanks to the CPAP machine) that now holds the airway open.
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Anyway, Resmed EasyBreathe is going to deliver nothing but positive pressures, even if EPR or BiLevel is turned on. But at least with EPR turned on, EasyBreathe at least has a positive-pressure differential between IPAP and EPAP with which to mimic the alternating pressures provided by the human diaphragm. If you leave EPR turned off, then you have a lot less pressure differential to mimic.
When this thread relied on abstract terms only to describe EasyBreathe's "natural" pressure variations, that qualitative description left almost no reason to have an EPR=off ("CPAP modality") and EPR=on (intermittent "Bilevel modality"). Once you get to placing those abstract or qualitative terms aside, and work the quantitative end a bit, you're stuck with the realization that: 1) that machine will never alternate positive and negative pressures as the human diaphragm would, 2) it's a positive-pressure template that the system targets by design, and 3) you have to move that positive pressure up and down to mimic human respiration (thus that mimicking fares better with EPR on---since you need some pressure leeway to move those pressures up and down for that natural mimicking act).
The best that machine's design can do with BiLevel disabled (EPR=off) is probably to work on that front-edge expiratory pressure transition by: 1) avoiding additive pressure spikes (via a more tightly controlled servo loop in the time domain), and/or briefly drop the pressure during that front-edge of the human inhale-to-exhale transition. That last technique is what patented C-Flex does. There may or may not be room for a patent-infringement in court if Resmed employs that softening during that same leading-edge transition...
But making the curve nice and gradual during BiLevel or EPR=on moments, is a more difficult patent-infringement beef IMO. That's why I initially thought that Resmed may have reserved all EasyBreathe techniques (except a tightly-controlled servo loop) strictly for BiLevel modality. Regardless, they may very well be softening that same leading-edge expiratory transition in CPAP mode as Respironics does---only better perhaps.
