Does the amount of air change with pressure?

[digitaleagle]

Velbor - Thank you very much for the comprehensive and well thought out explanation. That helps a lot.

I'm sure a lot of what I experienced the first couple of days was psychological apprehension to using the new device.

I seem to be getting more comfortable with it now.

[idamtnboy]

PRESSURE. CPAP machines maintain a positive pressure by pushing more air into our mask than the vents let out. But this pressure, regardless of how it feels, is quite small. Think of a drinking straw. Put about 8 inches (that's about 20cm) into a glass of water. Blow bubbles. That's the highest pressure provided by most CPAP's. Not very much. Certainly not enough to "force" air into your lungs.

A higher pressure DOES result in a higher flow rate of air into the mask. But ANY CPAP flow rate at ANY pressure still provides more air than you lungs "suck in" on their own. The VOLUME of air available for you to breathe in is ALWAYS sufficient with CPAP to let you lungs suck in as much as they otherwise would from room air. But you breathe in NO MORE air than you would without the CPAP (assuming an open airway).

I'm confused. You say the CPAP does not increase the volume of air that enters the lungs. But this portion of one of my graphs seems to indicate otherwise. The Resmed data management guide has this in it.

Minute Ventilation Minute ventilation is the volume of air breathed in (or out) within any 60-second period.
Flow Flow is an estimate of the airflow entering the lungs. It is derived by taking the total flow and then removing the leak and mask vent flow components.

Looking at the graph below you can see the leak rate jumped from 0 to ~15 L/min, and then dropped back to 0. Correspondingly at the same times, the flow went from ~23 L/min to ~18 L/min and back to ~23 L/min. These are +/- peak values. The Minute Ventilation went from ~7.5 L/min to ~5 L/min and then back up to ~7 L/min (eyeball average). The CPAP pressure is constant at 9. It sure looks to me like when the leak is less, more air is moved into the lungs. At least according to how I understand Resmed's definitions and the numbers on the graphs. What am I missing?

[Velbor]

What am I missing?

I think that what is missing is a) a link between cause and effect, and b) an appreciation of machine measurement and estimation limitations.

First, as to "flow", while ResMed's definition is correct as far as it goes, it is a bit of an understatement. As an "estimate" of airflow entering the lungs, it is more precisely the "measured" airflow leaving the machine less the "expected" venting of the mask. It does not include the machine's estimate of "unintended leak". There is no physiological measurement of actual lung airflow.

Second, "minute ventilation" also needs a somewhat more nuanced definition, since there are no sensors at your mouth and nose taking measurements. It too reflects the measured airflow leaving the machine, with some adjustments. These adjustments are principally expected venting and estimated leak, so we would expect to see some correlation between MV and leak; they are "incestuously" related. Note that while the definition is for "any 60-second period", the time gradations on your graph show variations at a much finer level.

Most important is to realize that MV is never perfectly constant. The depth of breaths will change with yawning, positional changes, dreaming, and myriad other unmeasurable factors, none of which are necessarily causally related to CPAP itself. My "eyeballing" of the MV line leads me to smooth out the undecipherable bumps, and what I see is that your overall minute ventilation for the timeframe shown is quite stable and normal. No hypopnea, no apnea, which is what the data line is really there to reflect. Reading much beyond that is really reading into the data factors which aren't there.

Blessings, Velbor

[jnk]

I enjoy reading Velbor's stuff. It always makes me think. I have learned a lot from him over the last few years.

My experience has been that I need plenty of pressure in order to feel that I am getting enough air when I use PAP. I think that may be for two reasons: (1) the resistance inherent in the mask and hose and (2) the need for my airway to be expanded by the pressure.

My reasoning is that it is hard for me to breathe when I put on a mask and hose that are not connected to a machine. It is hard for me to get air in and out of my lungs. It seems to take more work of breathing. However, once sufficient pressure has been added by a machine, that pressure helps to overcome the resistance of the hose and mask.

I also believe that it is easier to breathe through an airway that is sufficiently expanded. All you have to do is lightly lay your fingers on the outside of the sides of your nostrils and push them in less than a millimeter to see what constricted airflow feels like. (Breathe-right strips help some people breathe better based on that principle.) If PAP expands a part of the airway by a small amount inside, it makes it easier to breathe.

Maybe part of it is perception. But I believe there is some science behind the perception.

Bottom line for me is that some nights I have to raise my minimum pressure to make up for airway issues in order to feel comfortable using the mask. It takes at least 8 cm of pressure for me to feel comfortable going to sleep most nights. And some nights during allergy season, I need at least 10 cm of pressure.

I sometimes wonder if I would have been able to deal with this therapy at all had I been handed a machine set up with a 4-cm ramp engaged and no way to figure out how to change that. That would have made my start of PAP therapy an absolute nightmare!

-jeff