Posted by - Sleepy Coote on February 13, 2003 at 22:09:21:

In Reply to: Re: Optimum pressure caveats, indeed!! posted by Rebecca on February 13, 2003 at 09:51:03:

Hi Rebecca. My understanding is that, for those of us with purely obstructive sleep apnea, a certain baseline rate of closures or AHI occurs when untreated. Throughout an in-lab titration study air pressure is gradually and experimentally increased as our AHI is measured. Since AHI in this case measures obstructive closures, our AHI goes down as increasing levels of air pressure experimentally stint the airway open. A lower air pressure, say 5 cm hypothetically, will usually stint the airway open for some minority of our obstructive sleep events. The next incrementation, 6 cm to continue the example, would adequately take care of even more obstructive sleep events. Eventually an optimum pressure is encountered that manages to cover most of our obstructive events. At this point our AHI is usually below 5---but note this lowered AHI is comprised of obstructive events.

If the optimum pressure were progressively exceeded, central apneas would progressively begin to appear in many patients, as you point out. At this point the AHI would actually start to increase---but now being comprised of central apneas instead of obstructive. I believe most within the medical community attribute these pressure-induced central apneas to the "over pressure" in the lungs and a resulting inhibitory mechanism within the brain.

Rhetorically speaking, why is there no central apnea problem in this hypothetical patient at optimum his pressure or below? Likely because this suitably low air pressure is potential energy that has been largely dissipated or spent in order to thrust the airway open. The lungs do not over-inflate in the ideal pressure scenario, as there is little or no remaining potential energy to over inflate the lungs---thus "optimal pressure". After this near-perfect expenditure of energy our lungs are left with near normal air pressure.

However, since this biomechanical airway stinting only requires so much potential energy, significant surplus air pressure would actually over-inflate the lungs. After all, that surplus energy yielded by high air pressure must be dissipated somehow! Thus the over inflation of our lungs is precisely how the majority of this surplus energy is finally spent. With that said, people with a high enough pressure, yet a low enough AHI would likely spend more time over inflated than not on constant pressure CPAP. Recall that CPAP is really a potential energy CONSTANT in a physics equation. And that constant is sometimes not so handily offsetting a VARIABLE----that variable being our apnea-based airway closures.

A constant being used to offset a variable is really not an ideal. This physics equation will at times yield surplus of energy in the form of surplus air. In fact, I believe that my constant 10 cm of pressure left me with an air pressure surplus the vast majority of time given my relatively low AHI. Even your pressure of 7 cm might leave you with a significant air pressure surplus the majority of time as you sleep on fixed pressure CPAP. The value of the delivered air pressure is not nearly as significant here as the value of the surplus. You just might run a 17 cm pressure and still achieve an ideal surplus value of 0 cm. By contrast you might run a 7 cm pressure and yet still manage to achieve an air pressure surplus of several cm at any given moment in time. This is not a static situation although it is almost always described as such. It is a very dynamic situation comprised of many moments in time, each moment yielding its own surplus value of air pressure. The dynamics of this situation can yield small quantities of time running at an air pressure surplus or very large quantities of time running at surplus. But it is really wrong to think of it as a static equation.

On the subject of treating an airway variable with an air pressure constant: why do we do it? We do it because it works exceptionally well in the majority of cases. Most people can tolerate this very dynamic air-pressure surplus situation very well, within limits. Additionally, a fixed pressure flow generator is a real "no-brainer" to design compared to an auto-Pap. It is much more accurate at going about its simple business of delivering constant air pressure.

Indeed, auto-Pap, theoretically, would constitute a therapeutic ideal in most cases. After all, the design objective of an auto-Pap is to treat an airway-closure variable with an exact countering, or offsetting, variable. That is just theory, however. The reality of implementation still falls short. While sensor and transducer technology is now superb, today's biggest auto-Pap design challenge remains algorithmically detecting and responding to complex breathing patterns in real time. People with "common denominator" breathing patterns seem to be at an advantage. By contrast, people with uncharacteristic breathing patterns seem to be at a real disadvantage where today's auto-Paps are concerned. Regardless, today's auto-Paps are significantly more accurate and applicable than previous generations.

But, yes, I definitely agree with you Rebecca. I think people like us with spotty air demand needs can be at a big therapeutic disadvantage. Constant pressure CPAP can certainly leave us with a significant potential energy surplus. So we then experimentally resort to auto-Paps in hopes of treating a minimal variable with a minimal variable. Then we simply hope the auto-Pap we selected can properly recognize our particular breathing needs.

Follow Ups:

• Re: Optimum pressure caveats, indeed!! Rebecca 10:01 2/14/03 (6)
  • Re: Optimum pressure caveats, indeed!! - Sleepy Coote 11:13 2/14/03 (5)
    • Re: Optimum pressure caveats, indeed!! Rebecca 12:01 2/14/03 (4)
      • Re: Optimum pressure caveats, indeed!! - Sleepy Coote 12:52 2/14/03 (3)
        • Re: Optimum pressure caveats, indeed!! Rebecca 13:19 2/14/03 (1)
          • Re: Optimum pressure caveats, indeed!! - Sleepy Coote 13:33 2/14/03 (0)
        • Re: Optimum pressure caveats, indeed!! - Sleepy Coote 12:56 2/14/03 (0)

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