blarg wrote: I know on CPAP at 15, the pressure causes me to not exhale fully. That's why it feels weird for a few minutes, and then I adapt. There's a baseline of air that sits there that isn't there at ambient pressure. This is the "inflation" I'm talking about...
Right. That CPAP at 15 cm is static pressure. That static pressure goes toward airway inflation to free airway obstructions.
However, that static pressure makes exhalation more difficult---simply because there is now less pressure differential with respect to atmospheric pressure. And yet, amazingly your otherwise healthy physiology acclimates and compensates to the point that you may not notice that 15 cm as much (if at all). That's because your own "adaptable" neurological functions and pulmonary functions adjust. They not only adjust your muscular efforts to compensate with more diaphragmatic effort upon exhalation, but they also adjust to that altered pressure-based sensory input. That's all assuming you have "adequately adaptable" and otherwise healthy physiology.
But that was all about static or inflationary pressure at 15 cm. There was no ventilatory assistance there by the way of machine-sourced pressure transitions. You simply inflated your airway to clear obstructions with static pressure. Now let's throw BiLevel at 18/15 into the equation. What's your static pressure there? It's still 15 cm. There you have an alternating 3 cm pressure wave form superimposed on 15 cm of static pressure. In this case you receive a 3 cm measure of pressure-based ventilatory assistance by virtue of that 18/15 pressure-wave oscillation. Here pressure support (or PS=3) accounts for that added measure of ventilatory assistance during inspiration---but it does
not facilitate expiration any more than your CPAP=15 (recall that both are instantaneously the same with respect to atmospheric pressure during expiration).
Now let's put both that 15 cm static pressure and that wave-transitional PS=3 together into a simple balloon analogy or model. Think of blowing an elongated balloon half way up. Now hold that pressure. There's your 15 cm of static pressure. The elongated balloon is now partially inflated, so that narrow airway occlusions are now free. But for the purposes of our model, hold that balloon pressure at exactly 15 cm. If you do, there's no air moving in or out, toward achieving an equalization of pressure. If you change that static pressure of 15 cm one iota, then you're going to get some airflow because of pressure equalization.
Continuing the above model, now repeatedly inflate that balloon to 18 cm, then drop back down to 15 cm. Repeat again and again. You get some airflow because of the 18/15 cm transition. That 15 cm is still static or inflationary pressure maintained at all times. That 3 cm fluctuation is what causes the airflow. That is what a PS=3 or ventilatory "pressure support" of 3 cm is all about. But it's an inspiratory effort benefit. It's not at all an exhalation-related pressure benefit for you compared to your CPAP=15 (in biology and biophysics it might be, but not in ordinary physics). That 3 cm alternating pressure wave rides on top of your 15 cm static pressure. Only in this case you were the CPAP machine and the balloon was the patient's airway. But even in the case of central dysregulation the BiLevel patient does the vast majority of the ventilatory work in that match up of human meets machine.
Those are only the
most basic inanimate physics involved in xPAP inflation and ventilatory pressure support, Blarg. And it's really no more hypothesis than gravity causing an apple to fall downward from a tree is hypothesis. But that's all physics and not necessarily
biophysics. Regardless, as I pointed out earlier, there's no significant "extra" lung inflation when a centrally "well-regulated" and "fully adaptable" OSA patient uses BiLevel. If that "extra" lung inflation happens, as it sometimes does, it's considered both unintentional and undesirable. I should probably again point out that Bilevel considerations for central and various non-OSA related pulmonary conditions are a different set of considerations than BiLevel for purely obstructive apnea. And just for good measure I'll reemphasize that human physiology is where machine meets an entire variety of aditional challenges. I'm absolutely no expert in those areas.
I'm all out of explanations, Blarg. So please don't get me to defending that apples fall to earth, as if that were some kind of "hypothesis". .
