(07-31-2015, 03:59 PM)justMongo Wrote: Since the blower is an impeller type, it does indeed speed up when one inhales. Has to speed up to keep constant pressure with the higher outflow. I can hear it speed and slow on my S9.
I am not sure it would matter whether it is an impeller or not. Also, we are talking about a small volume of air in an otherwise partly-closed (vented) system. If it were speeding up and slowing down to maintain precise
total pressure (a function of static pressure combined with velocity pressure), a change in fan speed would probably be imperceptible in that small volume at these relatively weak pressures, because the change in static (and therefore in the total) pressure as you inhale and exhale is very minimal. But it probably is not maintaining precise
total pressure anyway, because there is no therapeutic reason for it to do that.
The pressure is based on cm of H20 (how far a column of water will rise when static pressure combined with velocity pressure is applied to a pressure tap in the airflow), so these are tiny differential pressures compared to atmospheric pressure or an increase of a couple of PSI in a tire.
The CPAP bases pressure settings on
velocity pressure, which is set to be constant, even though the
total pressure in the system might vary slightly through the respiration cycle due to slight changes in
static pressure when you breathe in and out. Compare the airflow of a normal exhale to the airflow of a xPAP putting out 10-15 cm, and the difference is pretty great.
The
static pressure varies slightly as you breathe in and out, so the
total pressure does vary slightly, but not enough that the
velocity pressure must be modulated by the xPAP to keep the
total pressure constant. There is no real therapeutic advantage to having the tiny change in
static pressure variance compensated for by modulating the
velocity pressure.
Also, the pressure of CPAP is therapeutic only during inhalation; there is no need to have that pressure maintained all that precisely during exhalation, and for the few patients where significant EPAP control is necessary, a CPAP is not prescribed anyway.
OSA is a problem with airway collapse on inhale. The airway does not generally collapse on exhale. So whether the total pressure in the system goes up during exhale is not relevant to the therapy, and compensating for that change in total pressure during exhalation is not needed.
IOW, the
total pressure does not need to be maintained that precisely, only the
velocity pressure to assist inhalation. The therapeutic difference between a pressure of 10 and a pressure of 10.1 is probably not even discernable medically, which is probably why the granularity is not any greater than that.
That applies to CPAP fixed-brick pressure. If you have an auto machine, or an ASV, VPAP, or a Bi-PAP, or if you are using pressure relief for inhale, certainly
velocity pressure and therefore
total pressure will vary, and you might be able to notice that. But a properly functioning S9 AutoSet should be absolutely silent regardless what
velocity pressure it is putting out or how much it may be varying.
