(05-15-2012 11:51 AM)PaulaO2 Wrote: And see, if this fact is to be so important--and I agree it is--why should/would the machines record it differently? Users would not tolerate a glucose meter that is off. That company would be out of business. Why do we tolerate machines that record such an allegedly important number as the AHI differently? If one machine says my AHI is below 5 and another says it is above 5, which is right? Both? Neither?
Actually, glucose meters aren't all that accurate either, even when you use the same strips and meter, let alone when you switch between brands. As far as official standards, meter readings can be off +/- 20% and still be acceptable. From this
Quote:Blood glucose meters must meet accuracy standards set by the International Organization for Standardization (ISO). According to ISO 15197 Blood glucose meters must provide results that are within 20% of a laboratory standard 95% of the time (for concentrations about 75 mg/dL, absolute levels are used for lower concentrations).
That's a huge difference. That means that my meter can record my blood sugars anywhere between 80-120, roughly and it's just fine. I've tested my sugar levels using nearly the same spot on my finger or thumb using the same strips and the same meter, 2-3 times, and each time, I can get a substantially different reading: something like 120, 134 and 105, all tested at roughly the same point on my hand, using the same strips and meter within 3 minutes of each other.
For Sleep Apnea and trend analysis with the AHI, you don't need to have a completely and perfectly calibrated system of measure. All you really need is consistency within the algorithm of the CPAP you're using. That's good enough to spot trends. This is why we often tell patients not to go by the readings of a single night's AHI, or even two nights. We really need to come up with a larger statistical sample (time-wise) to get a more accurate trend. I won't make a pressure change unless I've been monitoring my AHI at a specific pressure for at least a week.
I actually think that there's not a huge difference in comparable readings of AHI amongst the different data-reporting CPAP machines. Maybe a point or two overall - but that doesn't really matter. Patients utilize the AHI readings of their one machine, generally, and when testing pressure and other settings, don't generally switch to another brand of CPAP. Thus, over time, using the same machine, they can actually get a pretty accurate representation of their own AHI trending data... at least enough to make a reasonable decision regarding whether or not a specific CPAP setting change has helped or hindered their therapy.
Quote:If one machine says my AHI is below 5 and another says it is above 5, which is right? Both? Neither?
It doesn't really matter. It's not an issue of "right" or "wrong"-- There is no sure-fire method for determining whether or not AHI is being reported correctly or not in a CPAP machine. Even if you were to compare (if possible) 2 overnight studies in a sleep lab with a full PSG (sleep test) in a fully-equipped clinical sleep lab, you'd get differences in the AHI numbers for the same patient. Labs don't have identical equipment all the time, so there's going to be variations.
The important thing for using AHI as a trend indicator is that the machine's algorithm and associated firmware be relatively consistent from night to night, which it generally is. It doesn't
have to be calibrated exactly to the sleep lab's standards or even to other CPAP machines.
I'm over-simplifying in this example, I realize, but let's say, for instance, my actual treated
AHI levels (on CPAP at 12 cmH2O), according to a fully functional sleep lab was 4.0. Now, I go home and put on my CPAP machine and it records my AHI as 3.5. Which is right? Doesn't matter. My home CPAP machine has now become my baseline standard.
I know that at 12 cmH2O of pressure my CPAP machine is off a little bit in reporting my AHI (by 0.5 AHI, roughly). I don't really care, because I'm going to take that 3.5 level and mess around with my pressure settings to try to lower that 3.5 AHI that my CPAP is reporting. I couldn't care less about the slight difference between my machine's AHI of 3.5 and the sleep lab's at 4.0. What I'm going to do is look at my machine's trends from now on. The sleep lab's data is meaningless to me for trend analysis, because I can't go in for a sleep test every week to check AHI levels. I use my home CPAP for that now. If increasing pressure to 13 gets my machine's AHI down to 2.5 AHI on average over a week, my machine's readings have proven effective at giving me enough trend data to make a logical pressure change. Doesn't matter that the actual AHI numbers disagree by a small margin with the sleep lab's numbers.
Hope that made some sort of sense. I get long-winded, I know.