AHI and being awake

So, I was thinking.....the machine will calculate my AHI but doesn't know if I'm awake or asleep. So, if I lie awake for 2 hours with the mask on, obviously I won't have any events during that time, which will make my AHI look better than it really is, right? I'm assuming the AHI is calculated based on number of events divided by hours recorded. Please correct me if I'm wrong about that. So, then I'm wondering, is the AHI from the sleep study based on total hours or just hours asleep?
Nothing urgent in my question, just a point of curiosity.

AHI from the sleep study is from actual time asleep since they know exactly when/if you are asleep.

AHI from the machine at home is from the time you turn it on until you turn it off. It doesn't know if you are asleep or not.
If you use the machine for 6 hours but actually only sleep 3 hours...the AHI will be skewed by the 3 hours the machine was on and you were asleep. Assuming nothing was flagged then it is skewed lower but actually most of the time the opposite happens because our awake breathing is irregular and often confuses the machine and we get a lot more events while awake and in that case the AHI gets skewed high.

Thanks for the reply. Interesting that it might register events while you're awake. I wouldn't have thought that would happen!

If you are using Sleepyhead to monitor your treatment, it is possible to highlight just a portion of the graphs, i.e. just the period you were actually asleep, and the AHI for that period will show up in the small text line at the top of the graph. In this way, you can still see what your "real" AHI was for that night. Usually I'm aware of the times I am awake with the machine running, but I also check those times against the Respiration Rate graph: when I'm asleep, the respiration rate goes up and stays up; when I'm awake or only on the verge of sleep, the rate is low or mixed high/low.

These machines just measure air flow or the lack or it or the irregularity of the air flow.
They have no way at all to know if you are asleep or not.
Awake breathing is much more irregular than asleep breathing and sometimes the machines mistakes awake breathing irregularity for some sort of apnea event.

With time and experience we can usually spot the changes in the air flow and know that it wasn't a real apnea event getting a flag.
See this thread.
viewtopic.php?f=1&t=114600&p=1110933#p1110933

For newbies whose eyes glaze over when I talk about this sort of stuff I usually just say to watch where the flagging is happening and if it is close to a known awake time then there's a good chance it is SWJ flags.
Don't worry about it for right now and just concentrate on getting comfortable with the mask and machine and getting to sleep first.

I once helped a guy whose AHI was 15 to 20 and there were a lot of all sorts of events being flagged. Very sporadic on the graphs...some times with nothing and some times with a big mess of stuff. I was really scratching my head over the reports until I asked him if he spent much time wake with the mask and machine on and his response was LOTS of time because he was having insomnia issues. He was fighting the mask and machine a lot and doing a lot of tossing and turning.
So I told him to quit laying there for hours worrying about falling asleep and if he couldn't fall asleep or stay asleep turn the machine off and get up and do something else...pretty much standard insomnia advice.
He worked on the not worrying part and within a couple of weeks was sleeping much more soundly and his AHI went from 20 with a truck load of all sorts of events to less than 2.0 and never changed a setting on his machine. Only thing different was he quit laying there for hours wide awake. This is why you see me so often ask a person whose AHI is a bit high if they are sleeping well or having a lot of wake ups with a lot of time spent with the mask and machine on. I am first trying to figure out if the events flagged are real or they are SWJ flags.

It would be nice if the machine could do that for us but so far they can't do that.

Thanks for sharing that story. I seem to be tossing and turning a lot too. Will try to worry less.

>So I told him to quit laying there for hours worrying about falling asleep and if he couldn't fall asleep or stay asleep turn the machine off and get up and do something else...pretty much standard insomnia advice.<

Great advice on getting up to do something else if you have insomnia. I used to travel full time for work, so always a different hotel, bedding to get used to. I tried deep abdominal breathing and slow stretching to elongate the body. After long existing insomnia on the road, it got to be a routine to breathe and stretch out long. The time for this became very relaxing and soon shortened the time to sleepiness. Now it has become an actual queue to my body to fall asleep almost immediately. <2 mins. I actually use this on planes too. Stretch legs just one minute before boarding. Then just before I take my seat, I stretch up high to store my bag with both arms extended. I can now sleep on most flights, just by implementing these body queues that have become a familiar practiced activity with outcome of sweet sleep.

Hope this helps.
PawsnRefreshed