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Sleepyhead - Pressure vs Mask Pressure Graphs
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Hi everyone,

I ran across this old thread while researching the issue of mask pressure and wanted to add my two cents.

Because there is no pressure sensor at the mask, mask pressure is almost certainly a computed value based upon other inputs.  

The reason that the mask pressure doesn't always stay consistent is that the CPAP is not a perfect machine.  It cannot read your mind and respond in advance.  Instead, it has to respond to your behavior, and it can take a few milliseconds to do that.

When you breath in, or breath out, the CPAP has to respond to your inputs in order to keep the pressure where it is supposed to be.  The moment you start to breath in, the pressure in the system starts to drop.  The CPAP sees the pressure start to drop and then ramps up the fan to accommodate and push the pressure back to where it is supposed to be.  The reverse happens when you start to breath out:  The pressure starts to increase when you breath out, but the CPAP detects that and them ramps the fan speed down to bring the pressure back where it is supposed to be.  Likewise, if you have a leak, the mask pressure will drop until the fan can ramp up in an attempt to push more air back in and get the pressure back to the set level.  If the leak resolves, the pressure will increase until the CPAP instructs the fan to ramp back down and the excess air bleeds off through the mask vent.  These things all happen in a matter of milliseconds, and that's why you only see these weird jumps as a blip in the graph.

In sum, the CPAP's goal is to maintain a constant pressure, but all of our breathing and whatnot requires it to make constant adjustments to fan speed to do so.  And for a few milliseconds here and there between when we start doing something different and when the CPAP responds to our different actions, the mask pressure will be higher or lower than it is supposed to be.  

Mask pressure will also vary based upon EPR settings or if you have a bi-level, as demanded by EPAP and IPAP.  But, that's another story, never mind..  Smile
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