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When we use a CPAP, our upper airway is part of a "semi-closed pressurized system" comprising the blower, the tube, the mask, and our upper airway. The system is "semi-closed" because of the intentional leakage built into the mask to prevent re-breathing. A full efficacy data CPAP measures the back pressure at blower end of "system" and uses that data to calculate the air flowing into and out of our lungs all night long. The resulting data is called the Flow Rate data, and the Flow Rate graph provides a trace of very breath you took all night long.

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Each event that is scored by your machine shows up as a tick mark in the Flow Rate graph. Zooming in on the Flow Rate curve often shows exactly why the event was scored; but sometimes it is not clear why an event was (or was not) scored. And since our CPAPs do not have EEG data, they cannot tell when we are awake and when we are asleep. Hence your CPAP will score "events" that meet the definitions in their scoring algorithm even if you are wide awake.

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On an in-lab sleep test (PSG), each apnea will be classified as either an ''obstructive apnea'' or ''central apnea''. The data from the belts around the chest and abdomen are used to distinguish between the two types of apnea on a PSG. If the belts indicate that the patient is trying to breathe, but no air is getting into the lungs, then it's assumed that the problem is that the upper airway has collapsed and the apnea is scored as an ''obstructive apnea''. But if the belts indicate that the patient is making ''no effort'' to breathe, then it's assumed that the problem is that the brain has "forgotten" to send the signal to the diaphragm and lungs to inhale. The patency of the airway is irrelevant when scoring a central apnea on a PSG because the main problem is with the brain's forgetting to tell the diaphragm and lungs to breath. And it's possible for the airway to collapse during a central apnea.

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In addition to scoring apneas and hypopneas, many CPAP machines score ''snoring and/or flow limitations (FL)''. The way machines score this things varies quite a bit, and they are discussed at length in [http://www.apneaboard.com/wiki/index.php?title=Beginner%27s_Guide_to_SleepyHead#Snoring_Data 10. Snoring Data] and [http://www.apneaboard.com/wiki/index.php?title=Beginner%27s_Guide_to_SleepyHead#Flow_Limitations 11. Flow Limitations].

In addition to scoring OAs, CAs, Hs, snoring, and FLs, the '''Philips Respironics System One machines''' also score ''respiratory effort related arousals (RERAs)'' and '''periodic breathing''' (PB) in the events table with flags on the Flow Rate curve.

On an overnight PSG, a ''RERA'' is scored when there is evidence of increasing respiratory effort that ends with an EEG arousal. Not all labs score RERAs, by the way. The line between RERA and an AASM Rule 4B "hypopnea with arousal" is a bit arbitrary: RERAs don't have have last 10 seconds, but 4B hypopneas do; RERAs do not require a 50% reduction in airflow, but 4B hypopneas do. And a RERA requires an EEG arousal, whereas a Rule 4B hypopnea requires either an arousal OR an O2 desat of 3%. Finally a RERA may nor may not have an O2 desat. When a lab chooses to score RERAs, the lab will usually compute both an AHI and an RDI:

Nonetheless, if your sleep study showed a large number of RERAs the Philips Respironics RERA scoring algorithm may be useful for partially monitoring how well the CPAP is doing in terms of preventing RERAs from occurring.

Philips Respironics machines flag breathing that meets their definition of ''periodic breathing'' (PB). On the Philips Respironics web pages PB is defined as, "Periodic breathing is defined as alternating periods of hyperventilation with waxing/waning tidal volume..." Typically the waxing and waning pattern must be quite regular in visual appearance and it must present for at least a minute or two for PB to be scored. PB is flagged with a green back ground on the flow rate data. Often, but not always, there will be CAs (or Hs or sometimes OAs) scored at the nadir of the cycle. Sometimes the System One will only flag the most obvious part of the cycle:

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Because the scoring of an apnea or a hypopnea depends on the moving baseline airflow, the actual airflow recorded during an apnea or a hypopnea can vary. This becomes important during periods when you are not soundly asleep. Wake breathing is controlled by the voluntary nervous system and it is much more variable than sleep breathing. Because of this variability it is not uncommon for OAs, CAs, and Hs to be scored during periods when you are awake: A few minutes of conscious deep yoga style breaths followed by a return to normal wake respiration may meet the criteria to be scored as an H. When we're tossing and turning in bed, it's not uncommon to hold our breath for a few seconds while concentrating on turning over or fixing the bed pillows and covers; if the pause in our breathing is close to 10 seconds, it can be mis-scored as a hypopnea. And some people's wake breathing patterns are more susceptible to being misinterpreted as sleep disordered breathing than others. These sleep/wake/junk dozing periods are most likely to happen at the ''beginning'' or ''end'' of the night, particularly if you are in the habit of lying in bed without being sound asleep.

Sleep transition breathing also poses problems for our machines. The control of respiration has to be handed off from the voluntary nervous system to the autonomous nervous system. it is not uncommon that during the transition process, the airflow into the lungs drops (and a breath or two is skipped) as the body resets the CO2 trigger for inhalation. On a PSG, these "sleep transitional central apneas" are not scored because they are not part of sleep disordered breathing. But our machines cannot tell that we're in the process of transitioning to sleep, and it's not uncommon for "false" events to be scored just as we're drifting off to sleep.

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SleepyHead has a neat tool that both lets you zoom in on a particular event and (estimate) how long it lasted. Between the Calendar and the AHI table in the standard view of the Daily Data table there is a navigation menu that looks like this:

This particular hypopnea's flag is more or less in the middle of the hypopnea. Most of the time the flag is closer to the end of the event, but there is some variability. Another thing that's worth pointing out is that the patient's BiPAP is in the middle of decreasing the IPAP as part of the Philips Respironics "search" algorithm; the isolated H does not cause the machine to increase the IPAP and that's a bit counter intuitive to some CPAP users. But it is how the Philips Respironics Auto algorithm works: It ignores isolated Hs and OAs; pressure is increased for events only if two or more events occur very close to each other.

One common concern CPAP users have about their data are clusters of events. It's important to understand that your CPAP machine is not going to prevent every apnea. If you have a bad night now and then with some clusters of events, it's not something to worry too much about. But if you tend to see dense clusters of events on most nights, your CPAP therapy may not be optimized.

If your diagnostic sleep study indicated that your OSA is much worse in REM and the clusters are roughly 90 minutes apart with longer clusters towards the end of the night, they may very well be REM related. If your diagnostic sleep study indicated that your OSA is much worse in supine sleep and you find yourself waking up on your back at times, then the clusters may be supine-sleep related. But there really is no way to tell for sure that clusters of events in your data are REM or supine sleep related. Sometimes its worth making the reasonable assumption that the clusters are REM or supine sleep related in these situations, and if the clusters are particularly persistent (as in they occur on most nights) and nasty (as in they involve a lot of events), it's worth checking with your sleep doctor since a (small) increase in pressure may help break up the clusters. But if the clusters continue after one or more pressure increases, more formal investigation into their cause may be needed.<br />

New CPAP users are sometimes surprised by the fact that their CPAP did nothing when an apnea was in progress. There's an assumption that the positive air pressure provided by the CPAP is supposed to "blast" through the obstruction and restart the breathing. But pressure is not used to try to "end" an on-going event. It's not even clear that 20cmH20, the maximum pressure delivered by a CPAP, would even be effective at trying to "blow" a collapsed airway open: 20cmH20 is not enough pressure to effectively blow up an ordinary balloon. In fact, 20 cmH20 is about the difference in atmospheric pressure between a very stormy, low pressure day, and a bright sunny high pressure day.