Boy, I wish I had as much time for this as you do! (Truly! And respectfully!
) I'll just say this:
1) The entire purpose of bi-level CPAP is to improve patient compliance. (Unless you have a different idea about what "compliance" is.) And it has done that admirably. It may not apply to all patients, and it may only help a smaller percentage of CPAP users, but that doesn't mean it doesn't hold the key to addressing the general problem. What is a fact, though, is that bi-levels are more expensive, and more complex, as they need to cover the needs of a smaller population (e.g., higher pressures). It saved my life - the fact that 23 years ago, a pulmonologist knew enough to switch my constant-level CPAP to a bi-level. And that the particular machine he prescribed (an early Respironics BiPAP Pro with Bi-Flex) happened to have some hardcoded algorithms that suited my needs. "Compliance" or something else, it made the difference between me abandoning CPAP and not being around by now, and being a vigorous, healthy individual today.
2) I love simple arithmetic, but it doesn't tell the whole story when it comes to the complex physiology and psychology of a sapient human. Vital capacity is more than the sum of those three volumes, their proportionality to the total notwithstanding (and good enough argument by itself). One aspect of it is that - yes, tidal volume is important, but when that one is larger than usual (most people), then the CPAP machine has a harder task in the case of people with larger such capacity (and I've posted about that recently). Just like the typical CPAP user has a hard time explaining to someone without OSA why the machine is so important to them, the non-typical OSA+UARS+Central+WhoKnowsWhatElse patient has hard times explaining why such a convoluted airflow requirements is so crucial to them. More on that in the next paragraph.
3) I'll try to explain my situation, without having any idea what percentage of CPAP users are like me (and to what extent). Let's start with the simple case: soft tissue in the pharynx collapses when lying down and unconscious, so we apply some continuous pressure to keep it open - case closed! If the airways are relatively wide, and the soft tissue is "compliant" enough to be easily pushed back (i.e., with lower pressures), then it is indeed that simple. But, how about if the airways are really narrow and tight (to take it in a sequential order - from machine to lungs)? Now the pressure in the pharynx (which is all you care about) will differ significantly from the pressure at the source (where the machine sensor is) - simple Bernoulli's principle. How does the CPAP account for that? It has no sensors inside our body to close a feedback loop, so - heuristics? Or - not at all. Then, what do we do? Increase the pressure. With all of its downsides. It's where the bi-level comes, but not only. I was not able to use the Respironics System One bi-level. The reason - its gradual transition from EPAP to IPAP. Luckily, in some ResMed machines, it's adjustable (rise time). And here is where most people are shocked: to me, the shorter the rise time - the better; it's more comfortable. I don't need reduction of pressure as it rises, I need an increase! And before you think that I'm a rare animal - there's a reason those machines are designed with the ability to reduce rise time to as fast as the blower will go - they knew something! Now, as to why that is - here is where vital capacity comes in. Years of untreated OSA, while being a big, athletic man, has trained my mind to have the ability, to need to "know" that if it asks for enough air to fill the full capacity - regardless of the fact that while sleeping (or trying to!) I "should" only be aiming to fill my tidal volume - it
can do that. My data shows exactly that - flow increases significantly as I'm trying to get extra air, and then... I awake. It may be at the root of all UARS cases, but it certainty is in mine. So, that's what I meant when saying that I wish there weas a CPAP machine that takes into account the pressure drop after all restrictions, and the flow demand of the patient. It is impossible to account for all that just by looking at the pressure sensor inside the machine, but pending some invention where we can place pressure sensors and flow meters inside our airways, we may just need some powerful heuristic algorithms; and more adjustability by the patients (which we all know how much the CPAP manufacturers love doing!)
4) You are obviously a very smart and compassionate person, and I don't mean to ask you to solve any problems of mine, or in general. I know I am not providing nearly enough detail and information, so - please, don't worry about it. Maybe one day I'll have enough time to actually delve deep into this, but knowing that laws and regulations will never allow CPAP to be open-srouced, I don't know that I would. But maybe bringing some awareness is good enough at this time?
McSleepy
So, if you're a physician, you certainly would understand that: