How accurate are CPAP machines for measuring AHI?

[joem18b]

If my sleep clinic number was 26 and I go home and use a cpap and get a 0.0 (not the first time, but after a few months), how trustworthy is that number, relatively speaking?

In the clinic, you're all wired up from head to foot and at home, the machine is just checking the pressure variations in your mask? Can it be that accurate?

[Gasper62]

It all comes down to actual breaths, that's exactly what the pap machine reports on. Did you wake up feeling like you had a 0.0 night's sleep ? That's the ultimate measure. (IMO)

[palerider]

If my sleep clinic number was 26 and I go home and use a cpap and get a 0.0 (not the first time, but after a few months), how trustworthy is that number, relatively speaking?

In the clinic, you're all wired up from head to foot and at home, the machine is just checking the pressure variations in your mask? Can it be that accurate?

it's *EASY* to see when you're breathing and not breathing. just look at the flow trace on a sleepyhead report.

[robysue]

Piggybacking on palerider's answer:

It is *easy* to spot the places where you stop breathing entirely while using the CPAP if you zoom in on the flow rate data to the point where you can see the individual breaths.

Visually spotting hypopneas, where you do not completely stop breathing, is a bit harder to do since a hypopnea is typically scored when the airflow into your lungs has dropped by at least 50% from the moving baseline airflow, and it's tough for us to see what that moving baseline is from the data.

In general there are two ways where the AHI data can theoretically become distorted:

1. Lots of "events" scored while you are awake. If you are lying in bed awake, or bouncing back and forth between WAKE and Stage 1 sleep, the machine can score "events" that would not be scored on a PSG because they occur when you are awake. Our wake breathing is much less regular than our sleep breathing and our machines cannot tell when we are awake and when we are asleep. And some people's normal wake breathing with the machine results in a lot of "fake" events being scored since the machine makes the assumption that if you are using the machine, you are probably asleep.

2. Lots of wake time with NO events being scored. Some people's wake breathing is just regular enough where they don't get a lot of fake events scored when they are lying in bed using the machine while they are awake.

In Case 1 the reported AHI may be a bit higher than it would be if the fake events were tossed out and the correct amount of sleep time was used to compute the AHI. In Case 2 the reported AHI may be a bit lower than it would be if the number of events was divided by the correct amount of sleep time. But in the grand scheme of things, the two problems typically "cancel" each other out if you look at long term data rather than data from each night. In other words, if the machine says that your long term AHI is around 1.5, it is very, very likely that your true, long term AHI is somewhere between 1 and 2 and that's good enough to say the machine is doing its job in treating the OSA.