CPAP Pressure vs. Volume

[PaulaO2]

Click the image to see a larger version of it. Attachments are usually quite small.

[JumpStart]

Jumpstart, I think the idea Zimlich had about not being able to inflate a balloon (or a dirigible, or the lungs) is valid. The missing variable, I believe, in the Boyle's Law conundrum, is that with CPAP, there is an ever-flowing air exhaust port on the mask interface, so the increase in the lung pressure is nil, or volume is dependent only on the person's inspiratory effort (not the CPAP), whereas in mechanical ventilation, there is an endotracheal intubation tube that seals off the trachea by way of an inflatable balloon. That is how the ventilator is able to "blow the lungs up" like a balloon.

The exhaust port is, as I understand it, used to avoid the otherwise deadly CO2 buildup which would occur if our exhalation did not have an escape mechanism. It does not, as far as I know, reduce the pressure provided at the airway - if it did, cpap would be rather useless in providing a "splint", IMO. I believe that if the exhaust port did as you suggest, ie, allows no pressure to the airway, that I am wasting my time on this infernal machine! Unless I have pressure in the airway, there is no method for holding tissue or tongue, and that pressure must go to the lungs, unless I have a leak somewhere between my airway and lung. :grin: But the issue is NOT that a ventilator provides a higher pressure - of course it does. The most recently discussed issue seems to be whether a cpap provides any pressure at the lungs - and I contend it does.

I seem to recall in another recent thread that someone determined that 20 cm/H2O is significantly less than 1 psi - more like .250 in round (and possible poorly recalled) numbers. If so, the ventilator which runs to 80 cmH20 would roughly go to an amazing 1 psi.

[Bazinga]

So, this raises a question for me as a total newbie, that someone here might answer: Does the CPAP machine adjust its flow output depending upon resistance encountered (whether by the balloon, or by the nose, mouth and throat) to maintain a constant pressure with varying topography or body "architecture"? The physics of CPAP is fascinating, but certainly not my forte!

[greatunclebill]

So, this raises a question for me as a total newbie, that someone here might answer: Does the CPAP machine adjust its flow output depending upon resistance encountered (whether by the balloon, or by the nose, mouth and throat) to maintain a constant pressure with varying topography or body "architecture"? The physics of CPAP is fascinating, but certainly not my forte!

short answer is no. cpap = constant pressure. normally your pressure is high enough to push thru all apneas by opening the airway. if it encounters one above your set pressure it won't open. it will not add more pressure to open the airway. this is why setting the correct pressure is so important. now in an automatic cpap (called apap), the machine starts at a low set pressure and raises enough to open the airway within the pressure range you set. once again why it is so important to get the right pressure. with the balloon, the pressure needed to inflate it was more than 20 so it didn't inflate just like the throat wouldn't open if your pressure was too low. does that help?

welcome to the forum.....

[PaulaO2]

A 'straight' CPAP will push out it's set pressure no matter what. So if the setting is 12, it will push 12. I don't think it cares if there is resistance. I don't think it pushes harder when it finds resistance.

An 'auto' CPAP will increase the actual pressure in response to a blockage. So if the setting is 12, it will slowly increase the pressure (13, 14, etc) until the airway is clear.

I think.

[greatunclebill]

i did some quick research. the average toy balloon inflated is 1 or 2 psi. 1 psi = 27.7"/ h2o. so the balloon would require 70.4 - 140.8 cm/h2o pressure to inflate. no wonder 20 didn't affect it.

[JumpStart]

I found the thread I was looking for. For those wishing to do there own pressure trials,

According to the calculator here, a pressure of 20 cm/h2o translates to only .284 psi.

And, 80cmH20 is only 1.138 psi!

[Sleepster]

So, this raises a question for me as a total newbie, that someone here might answer: Does the CPAP machine adjust its flow output depending upon resistance encountered (whether by the balloon, or by the nose, mouth and throat) to maintain a constant pressure with varying topography or body "architecture"? The physics of CPAP is fascinating, but certainly not my forte!

Yes. It has to make huge adjustments to the flow rate to keep the pressure constant, maintaining that pressure splint is what it's all about when it comes to CPAP therapy.

[TetraMan]

Your reasoning is correct - in a static situation.  Any air pressure exerted against a closed/rigid container will be constant from one machine to the next.  But lungs are not a closed/rigid container.  Airflow capability of the machine, e.g., in cfm (cubic feet per minute) does come into play.