Easy questions first:
Quote:Also, how do you make your machine, in my case the ResMed S9 Elite begin the time that is ACUTAL rather than where it is, it begins at 18:30 which is 6:00 pm. I don't go to bed until 11:00, so when trying to see where these things occur, the time is off. Is there a way to set the internal clock?
Since you are using a RESMED S9, the machine's clock can be set by going into the
clinical set up menu.
Quote: How do you know how long an apnea lasts, and what is really happening when reading the Sleepyhead stats, is that the time in parentheses to the right of the time that the incident occurred?
That number in the parenthesis is a good estimate how long the apnea lasted.
Now for the harder one:
Quote:I watched a You Tube video from another site and the guy was saying that some apneas that show up are not really apnea at all, it depends on the time the apnea flags, is that right?
The short answer is: Yes, it really does depend on the time the apnea is flagged and what you were doing at that time, which is NOT recorded by the machine."
The long answer is: Unlike Santa Claus, our machines cannot know when we're sleeping and when we're awake. This is because our machines do not have any EEG data to work with. And for events to be
real, they must occur when we are
asleep when they start.
Now add in the fact that wake breathing is not as regular as sleep breathing. Even though we are not usually aware of it, wake breathing is, in fact, controlled by our voluntary nervous system. (Sleep breathing is controlled by the automatic (involuntary) nervous system that also controls our heart.) While awake, we do things like sigh every now and then; we do deep breathing for a while and then breath (much more) shallowly and/or much more slowly for a while; and we momentarily hold our breath while concentrating on something (sometimes something as simple as turning over in bed). All this natural variation in WAKE breathing can look like SLEEP DISORDERED breathing to our machines, and hence it is not uncommon for a PAP machine to score some false events during the times you are awake because the machine has no way to tell that you are awake when the events happen. What kind of events seems to depend on the person. For many people, these fake events will more likely be scored as CAs or Hs, but when I've run the experiment of consciously holding my breath at the beginning or end of the night, the fake apnea is just as likely to be scored as an OA as a CA; it may have something to do with
how a particular individual holds their breath.
Now add in another phenomenon: The transition to sleep involves handing off control of breathing from the voluntary nervous system to the involuntary nervous system. And part of that "handing off the breathing" also involves a resetting of the CO2 levels needed to trigger an inhalation. The net result is that it is also not uncommon for there to be some short pauses in breathing while that CO2 trigger is being reset. These are almost always central in nature, and on an in-lab sleep test, CAs that are clearly sleep transition CAs are NOT scored as sleep disordered breathing unless they are so numerous that it is clear that they are preventing the person from smoothly transitioning from WAKE to SLEEP. Our machine scores these events because it has no EEG data to use to determine that a particular CA actually is a perfectly normal transition to sleep CA that would not be scored on an in-lab sleep test.
It's important to note that the machine's reported AHI is an
approximation of the true AHI:
Machine reported AHI = (number of events detected)/(time machine was running)
True AHI = (number of events that occurred while asleep)/(time spent in Sleep)
For people who have very little WAKE time while using their machine, the
machine run time and the
sleep time will be close enough to not make much of a difference. And the number of events scored while awake will (usually) be small (because the wake time is small), and hence they won't really affect the overall AHI all that much.
For people who have a lot of WAKE time while using their machine, there are two competing factors at work that influence the reported AHI versus the real AHI:
1) You've got the possibility of a fair number of "false" events---events scored when the user was actually awake. This increases the number of events (the numerator of the AHI), but (obviously) does not change the run time (the denominator). And so a large number of these false events might make the reported AHI higher than it should be.
2) You've definitely got the denominator of the AHI ratio wrong. Since the machine AHI uses "time machine was on and delivering therapy" as a stand in for the correct denominator "time the person was asleep", if there is a LARGE discrepancy between
run time and
actual sleep time, the reported AHI may be
smaller (potentially much smaller) than the real AHI.
The common assumption in manufacturers seems to be that problems 1 and 2 will probably cancel each other out when it comes to a patient who is spending a lot of time in very restless dozing---going from WAKE to (light) SLEEP for long periods of time. And that's not an unreasonable assumption to make about aggregate data across large numbers of PAPers. But whether that happens for a particular individual really depends on the individual.
