(11-15-2012 11:25 PM)Sleepster Wrote: The mask pressure is a tad higher, I'd guess, than the pressure? It's probably an attempt to account for the extra resistance added to the pressure by the mask, so the more restrictive your mask is (nasal pillows as opposed to full face, for example) the bigger the difference.
(11-16-2012 12:29 AM)PaulaO2 Wrote: Yes, the mask pressure is higher by a lot.
(11-16-2012 12:52 AM)archangle Wrote: I think the pressure reading is the "set" pressure, i.e. the pressure it's trying to apply at the mask. The "mask pressure" is the machines best estimate of what the actual pressure is at the mask, considering airflow, air drag, fluid dynamics, etc. If you have EPR turned on, "mask pressure" also appears to show the change in pressure as it switches from the inhale pressure to exhale pressure. It would also probably show bilevel pressure changes as well.
The machine can take measurements of the airflow and of the pressure at its outlet (at the inlet of the humidifier and hose).
Based on these two measurements, the software in the machine will need to estimate the pressure reaching the mask. It must use the calculated pressure reaching the mask to calculate the intentional leak through the mask vent holes, based on whatever it assumes (or is told) is the resistance to airflow of the mask vent holes. And the unintentional leak would need to be calculated as being the difference between the measured airflow at the inlet of the humidifier, less the calculated intentional leak of the mask vent holes.
Mask Pressure can be calculated by starting with the pressure measured at the output of the machine and subtracting the pressure dropped across the humidifier and hose.
The pressure dropped across the humidifier and hose would need to be calculated from the airflow at the machine's outlet. The machine's software would need to have a look-up table representing the pressure loss across the humidifier as a function of the measured airflow, and another lookup table representing the pressure loss across the hose as a function of the airflow (and therefore the machine needs to assume or be told what type of hose is being used).
Conceptually speaking, the actual Mask Pressure must be the same or lower than the pressure the machine measures at its outlet, because the resistance to airflow of the humidifier and hose can only cause a pressure loss across the humidifier and hose, never a pressure gain. The greater the measured airflow at the machine's outlet, the greater the pressure loss across the humidifier and hose, and the lower the pressure at the mask.
Likewise, if there is no airflow, there would be no pressure loss or difference across the humidifier and hose. So, if all the vent holes in the mask were to be closed off (making the resistance to airflow of the mask very high or infinite), and if all the unintentional leaks were stopped, there would be no airflow in the humidifier and hose, and therefore the pressure measured at the machine's outlet would be the same as the pressure in the mask.
Instead, of course, the mask vents allow intentional leak through the mask, but the higher the mask resistance, the lower the intentional leak through the mask vents, and therefore the lower the airflow in the hose (assuming no unintentional leaks), and therefore the lower the pressure loss across the humidifier and hose, and therefore the lower the measured pressure needs to be at the machine for the mask pressure to be at the perfect set pressure (the target therapy pressure at the mask).
Therefore, if the machine is not capable of being told what type of mask is being used, and the mask being used has higher resistance than whatever resistance the machine is assuming the mask has, the machine could be set a little lower in pressure, to compensate for the mask resistance being higher than the machine is assuming. However, the pressure difference would probably be small and negligible, especially if using a standard (fat) hose and if the set pressure (target therapy pressure) is not extremely high.