(12-13-2013, 12:03 PM)Brian Rule Wrote: I sleep fine for about 4 hours but then need a bathroom break. When I get up for this the S9 is showing good results with pressure of about 12 to 13 and AHI of about 4. So far so good. After the break the machine starts fine but as I don't go back to sleep easily it seems to get fed up and ups the pressure gradually and intermittently even though I am still wide awake. My main question then is why is it trying to treat an event when I am wide awake and obviously not having one. Common sense tells me that the machine should run at minimum pressure until I have an apnea.
The machine has no EEG data and, unlike Santa Claus, it does not know when you are awake and when you are asleep. The presumption by the engineers who designed the machine is that if you're using the machine for any length of time, there should be a high probability that you are asleep. Hence the Auto algorithm is based on what the characteristics of what normal sleep breathing
should look like and the machine responds by increasing pressure when the breathing pattern does not resemble normal sleep breathing
But normal sleep breathing
and normal wake breathing
are two very, very different things. Indeed, sleep breathing
and wake breathing
are even controlled by different parts of our nervous system. Wake breathing
is controlled by the conscious part of our nervous system---the part that we use to control our arms and legs; it's the part that allows us to walk (without thinking too much about walking). Sleep breathing
, however, is controlled by the autonomic part of our nervous system---the part that controls the beating of our heart.
And the Auto algorithm is based on what normal sleep breathing
is supposed to look like. Normal sleep breathing
is much more regular than normal wake breathing
is in terms of both respiratory rate and in the depths of the inhalations and exhalations. When we're awake, it's the conscious part of our nervous system that controls our wake breathing
. And we do all kinds of things that affect our wake breathing
without really thinking about them, but it's still controlled by the conscious part of the nervous system when we're awake. We take an extra deep breath now and then that may have a prolonged exhalation. We sigh. We momentarily hold our breath when we're concentrating on something (and we may not even be aware of this.) We clear our throats. We "sniff" more vigorously through a slightly stuffed up nostril. We take a series of deep, full breaths (think relaxing yoga breaths) and then naturally return to a more shallow breathing pattern.
All this normal amount of variation in wake breathing
is capable of fooling an APAP which is programmed to expect the wave flow pattern to be the normal sleep breathing
pattern---which is exceptionally regular in terms of the size of the inhalations and the lengths of the breaths. And the auto algorithm kicks in when the detected breathing starts to look "ragged" because it's not super regular and there's too much variation in the size of the breaths. So if you're using the APAP for long periods of time when you are very restless, but awake in bed, your wake breathing
can fool the machine into thinking its seeing signs of abnormal sleep breathing
In other words, if you're lying in bed fighting to get to sleep, your wake breathing
can appear very ragged to the machine: If you hold your breath just a bit too long, the machine thinks you might be having an apnea and starts the CA/OA detection algorithm, which on the Resmed involves sending a series of oscillating small changes in pressure. If the machine thinks the air flow has dropped by something like 40-50% from baseline for at least 10 seconds, it will score a hypopnea. And when you're awake a drop of 40-50% in airflow can just me that you were breathing very deeply for a while (maybe as part of conscious relaxation exercises) and you let your breathing return to a more normal depth for your resting, but wakeful breathing. Or as you transition to sleep, the depths of your inhalations naturally become shallower, but relative to the last several minutes of wake breathing, the new "sleep breathing" inhalations meet the 40-50% reduction needed to flag a hypopnea. And if the machine scores two apneas or hyponeas with in a minute or two of each other, it's going to increase the pressure. And the pressure increase startles you back to full wake status and the cycle starts over ...
Also consider this: As part of its Auto algorithm the machine is measuring the shape
of each and every inhalation in the wave flow data and comparing it to an expected shape for an unobstructed open airway during sleep breathing
. Certain changes in the shape of the inhalation part of wave flow for sleep breathing
are thought to indicate that the airway is beginning to collapse; these changes are called flow limitations
and your S9 Autoset responds to flow limitations by increasing the pressure. And here again, the things that we do during wake breathing
can fool the Auto algorithm into thinking that it is seeing flow limitations, and so it starts to increase the pressure because the machine has no way of knowing that you are awake.
Finally, the S9 Autoset will increase the pressure when it thinks it is detecting snoring. Since there is no microphone to pick up the audible noise of snoring, the machine is using certain characteristics in the wave flow that are strongly associated with snoring during sleep breathing
. But a few people will occasionally have a relaxed wake breathing
pattern that can fool the machine into thinking it is picking up snoring, and so the machine increases the pressure ....
So the net result of all this is: The Auto algorithm can misinterpret normal wake breathing patterns as evidence of problematic sleep breathing, and hence it increases the pressure even though you're wide awake.
Quote:I have asked the hospital and ResMed about this and they are completely fazed by the question which is simple enough in my mind. Not only this but after a few hours of wrestling with the high pressures then going to sleep (or not) the AHI reading is magically increased to some stunning figure much higher than the one when I got out of bed for the bathroom break ( I have had 18.7 on one occasion!!). On no occasion has the machine shown a leakage level greater than 10 and it is usually much lower than that.
Excessive restlessness while using the machine often leads to it scoring a whole lot of "false" events.
As to what to do: It sounds like what's happening is that after that bathroom break you get trapped in a nasty, positive feedback loop that looks like this:
- ... ragged but normal wake breathing is misinterpreted by the machine as abnormal sleep breathing which leads to a pressure increase which leads to more restlessness which leads to ragged but normal wake breathing which is misinterpreted by the machine as abnormal sleep breathing which leads to an increase in pressure which leads to more restlessness which leads to more ragged, but normal wake breathing which is misinterpreted by the machine as abnormal sleep breathing which leads to an increase in pressure which leads to ...
You get the picture. The obvious thing is that you need to decrease the restlessness that occurs after that bathroom break. If you were getting back to sleep as quickly as you get to sleep at the beginning of the night, then this problem would disappear. So the question becomes, what kind of things can you do to decrease the restlessness? And what's triggering the restlessness in the first place?
So some questions:
1) Pre-CPAP did you have one or more bathroom breaks during the night? If you did, did you have the same trouble getting back to sleep pre-CPAP that you are currently having now?
2) Can you identify what is causing the restlessness? Does the problem start as soon as you mask up? Or does it start after
you notice the pressure increases?
3) Are you using the ramp? If you are using the ramp, it may be that the linear pressure increase during the ramp period starts making you get more restless and increases your wakefulness, but the wakefulness, that's the start of the negative feedback loop. If you think this might be the problem, then turning off the ramp might go a long way in fixing your problem.
Finally a suggestion: If you haven't gotten back to sleep in about 20 minutes or so after that bathroom break, you may do better to just get out of bed and go into a different room and do something relaxing (not work related) for a while until you start to get sleepy enough where you won't be so restless when you mask back up. Because the sooner you actually fall back asleep with the mask on, the less likely you are to get trapped in the feedback loop of "restlessness triggers the machine scoring false events which triggers pressure increases which triggers more restlessness ...."