I had a major (and stupid) unit conversions problem in this post. I've taken the time to correct it here.
(02-27-2014, 03:46 PM)PaytonA Wrote: This makes some sense to me except for one thing. The machine (mine is a ResMed) must supply enough air to satisfy 3 things. 1-Air for the vent. 2-air for the breath. 3-air for the leak.
Air for the vent and air for the leak are the same thing. The machine does NOT know where or why air is leaking out of the system. And it is irrelevant (from the blower's point of view) if the system needs 35 L/min of air added to maintain pressure because:
- the mask's intentional leak (vent) rate is 30 L/min and there's a small leak in the cushion of 5 L/min
- the mask's intentional leak (vent) rate is 35 L/min
- the mask's intentional leak (vent) rate is 25 L/min and there's a modest leak through the corner of the mouth of 10 L/min
In all three cases, the machine has to blow into the system a grand total of 35 L/min to maintain the desired pressure.
Only the RESMED machines bother to try to distinguish between whether all the leak is intentional
(and so the reported leak is 0.0) or whether some of the leak is unintentional
. And since all you do is set a generic "mask type" on the Resmed, all the machine is doing is subtracting off a fixed number for the expected leak rate for that mask type at that particular pressure. But individual masks of the same make and model have (slightly) different leak rates when used by real PAPers night after night, and so regardless of what the Resmed machines use for the intentional leak rate for the mask, it is an approximation
of the real intentional leak rate for that particular user on that particular machine when the mask is actually in use. When you use a PR machine you get a feel for what your intentional leak rate actually runs, and it can be as much as 5 L/min below or above the mask's advertised "intentional leak rate" at your pressure, but over time you do notice that your baseline leak rate usually stays within a 5 L/min range that is "normal" for you and your mask. (For example, my leak rate is almost always about 7 L/min what the charts say it should be. It's always been that way and I think it's just normal for me
and my machine.)
As for "air for the breath:" The critical idea is is that our inhalations
and our exhalations
are roughly the same size. And the time needed to exchange the air is really not that important when the machine is calculating the leak rate for a one minute period
. The leak rate graph is NOT the instantaneous leak rate, it is an average leak rate and as such, the data gathered for it is at intervals that are much less frequent than that needed for the wave flow.
And relative to the amount of air the machine is blowing into the system to keep it pressurized, the size of our individual inhalations and exhalations is not that much.
(MUCH irrelevant detail ommited)
On a minute-by-minute basis: We typically have a minute ventilation of 5-8 L/min. Hence there's already way more air being blown into the system (at least 20-30 L/min) that we can possibly ever breath into our lungs over the course of the minute: So most of the air the CPAP has to blow into the system is actually used to compensate for the (intentional) leaks built into the system to make sure that the air we exhale into the system is vented out of the system instead of being rebreathed.
And all the air we inhale
from the system is (almost immediately) replaced by the same amount of exhaled
air, which is immediately vented out of the mask's exhalation vents.
In other words:
During one minute, the machine blows enough (new) air into the system to maintain pressure. Since all
the air in the system is replaced each minute we can assume that of the (new) air that's blown in, we inhale some of it, but all of the rest of it is vented through the leaks (both intentional and unintentional). Over the course of a minute, the air we inhale is replaced by the air we exhale, and so we get the following calculation concerning the total leak rate and the total amount of air pumped into the system by the blower unit:
- Total (new) air pumped into the system in one minute in order to maintain pressure
= (new air we inhale in one minute) + (new air we don't inhale during that minute)
= (air we exhale in one minute) + (new air that is vented or leaked during that minute)
= total amount of air leaked out of the system during one minute.
In other words, if we make two modest assumptions:
- 1) The total amount of air we exhale during one minute is equal to the air we inhale in one minute
2) The machine blows enough air into the system to replace all (or virtually) all of the air in the hose and mask (i.e. the machine is blowing at least 30 L/min)
then the fact that our exhaled air is immediately vented out of the system is enough for us to conclude that total leak rate and the rate the machine adds air to the system are equal to each other.
Quote:The machine knows how much air (at least approximately) is required for the vent.
No. The typical CPAP doesn't have a mask setting and has no idea how much air is needed for the intentional venting for the mask. Only the Resmeds have a mask setting and the mask setting simply tells the machine what number to subtract from the total leak rate to report as "leak".
Quote:That leaves 2 unknowns unless the machine is able to detect the flow at a point where neither inhalation nor exhalation is occurring.
Detecting inhalations and exhalations is actually done through measuring very subtle differences in "back pressure" at the blower's end of the hose. When you inhale the back pressure is reduced very slightly; when you exhale the back pressure is increased very slightly. These differences in back pressure are not enough to affect the rate the machine is adding air into the system, but they are enough to detect how much air is going into or out of the lungs---i.e. the wave flow. Detecting when NO air is flowing into or out of the lungs is critically important for detecting apneas.