(12-05-2013, 12:02 AM)apdtap Wrote: Thanks for the reply. My main point is whether those high end machines can find the best EPAP IPAP quickly. I will try and inform after a few more days. My experience in the morning so far is that they cannot?
I think that you have some major misconceptions about how the Auto algorithms for Auto Bi-level machines work.
First and formost: Plain old auto bi-level PAP machines are NOT ventilators. They do NOT try to force you to breath during the middle of an apnea. (There are PAP machines that can act as non-invasive ventilators----they are the bi-level ST machines, the ASV machines, and the AVAPS machines. But the plain old Resmed S9 VPAP Auto and the PR System One BiPAP Auto do NOT have a "T" mode and cannot try to trigger an inhalation when you stop breathing.)
Second: These machines do NOT "quickly" find "the best" EPAP or IPAP pressure. Rather, their auto algorithms are designed to raise the pressure response to events after
the events happen. They typically raise the IPAP/EPAP pressures somewhere between 1 and 4 or 5 cm over a period of time that might be as short as 30 seconds or so (under some circumstances for the Resmed S9) to as long as 5 or 6 minutes (for the PR System One.) Moreover, once they raise the pressure, they do NOT quickly lower the pressure back down to the minimum settings
. Moreover, auto bi-level PAP machines do not necessarily raise the pressures after every single apnea: Rather, they adjust the pressures after
the end of a cluster of two or more events in order to prevent more events from happening. And the machine will start to lower the pressures only after the machine is happy with the shape of the wave flow (i.e. the machine thinks you are breathing satisfactorily.) And once the machine starts to lower the IPAP/EPAP pressures, it will lower them in a gradual fashion over the course of several minutes. (The S9 will typically start lowering the pressures sooner, but it will take longer to lower them all the way back to "baseline".)
Third: These machines also raise the pressures in response to snoring and flow limitations. Both machines respond pretty aggressively to snoring and flow limitations since these are thought to be precursors to a potentially collapsing airway. And the idea is to provide more air pressure before
the airway actually collapses. A flow limitation is defined as a change in the shape of the inspiration part of the wave flow---the trace of the air flow into and out of your lungs with each and every breath you take. Certain changes in the shape of the inspiration part of the wave flow are thought to indicate that the airway is just barely beginning to collapse. The airflow is not yet restricted enough to count as a hypopnea and nowhere near restricted enough to count as an apnea, but because the airway might be beginning to collapse
, the machines increase the pressure to make it harder for the airway to collapse enough to cause a real hypopnea or a real apnea.
Fourth: The machines' apnea, hypopnea, and flow limitation algorithms are based on the premise that the person attached to the machine is likely asleep
. And normal sleep breathing is usually much more regular and quite a bit more shallow than normal wake breathing. When we are awake, we take the occasional deep cleansing breath (or even a series of them). We may consciously slow our breathing down as a way to relax ourselves. We may take a super deep, long breath preparing to sigh. We'll temporarily hold our breath while concentrating on something----sometimes it's something as simple as turning over in bed with a six foot hose attached to our nose. And we may not even be aware that we're momentarily holding our breath for several seconds while we're awake. The point is this: Normal wake breathing patterns are often irregular enough to fool APAPs into thinking that we're exhibiting signs of sleep disordered breathing because the breathing pattern does NOT resemble normal sleep breathing patterns. It's not uncommon, for example, for a full efficacy data machine to record some "false" events as your breathing settles down from deep, irregular wake breathing right after putting on the mask to shallower, slower breathing as you try to relax enough to go to sleep. If the air flow on those shallower breathes is 30-40% less than the series of DEEP inhalations right after you put the mask on, the machine may very well score a hypopnea or two. And if it scores a pair of closely spaced events while you are settling down in bed? The machine's auto algorithm will kick in and increase the pressure because of the cluster of two events.
Finally: The PR BiPAP Auto has an algorithm that proactively tests increases of IPAP pressure during the night to see if a small increase in IPAP pressure will lead to a better shape in the inspiration part of the wave flow pattern. If the increase in IPAP pressure does improve the shape of the wave flow, then a new, higher
"base line" IPAP pressure setting is established. If the machine increases the IPAP (or the IPAP and EPAP) in response to some combination of hypopneas, apneas, flow limitations, snoring, and RERAs, after the machine is happy enough with the shape of the wave flow it will then first start testing small decreases in pressure down to the last "baseline" IPAP pressure. If any deterioration in the shape of the wave flow is detected, the IPAP pressure increased back up to the current setting. What this means is that the PR System One BiPAP can be very reluctant to reduce the IPAP pressure all the way back down to the minimum IPAP pressure setting during the night. You really have to look at what's driving the increase in IPAP pressure to understand when or whether the System One will manage to decrease the IPAP all the way back down to the minimum IPAP setting.