inkdesigner wrote:
I really don't understand what the AHI & AI numbers mean.
The screen shot of the ResMed report you included in your post indicates that your machine is an ResMed S9 Auto, which is the same as my machine. I use the Swift FX nasal pillows mask, but you didn't specify which nasal pillows mask you're using. So my answers are based on what I know about my machine and my nasal pillows.
The S9's software detects four types of respiratory events: Obstructive apneas, central apneas, unknown apneas, and hypopneas. The unknown apneas are events that the S9's algorithm detects as apneas, but cannot tell whether the apnea is obstructive or central with enough certainty to classify them.
The AHI number is the Apnea/Hypopnea Index for the reporting period, which on the S9 LCD is typically the overnight number. On the ResScan screen shot you included, the reporting period appears to be five days. For overnight (one day) data, the AHI is computed by the S9 as follows: It takes the number of all detectable events (obstructive apneas, central apneas, unknown apneas, and hypopneas detected) and divides that number by the time the machine was running. In other words, the AHI is the
average number of apneas and hypopneas that occurred each hour the machine was on. The LCD display number on the S9 is simply truncated at one decimal place
without any rounding as near as I can tell. I believe that for longer reporting periods (such as five days), ResScan simply averages the nightly AHI numbers to compute the one in the report. But I don't know for sure.
The AI number is the Apnea Index for the reporting period. It only counts the obstructive, central, and unknown apneas. In other words, the S9 computes this number by dividing the number of detected apneas (includes obstructive, central, and unknown apneas) by the time the machine was running. In other words, the AI is the
average number of apneas that occurred each hour the machine was on. Again, the S9's LCD display number seems to simply be truncated at one decimal place instead of being rounded to one decimal place. In ResScan, if the reporting period is more than one day, I think the number is just the average of the daily AI numbers.
An example might help. Suppose that on a given night the S9 detects 15 total events---lets say there are 9 obstructive apneas, 2 central apneas, and 4 hypopneas---and that the machine was run for a total of 6.6 hours.
The S9's LCD's overnight AHI, AI, and CAI numbers are computed as follows:
AHI = 15/6.6 = 2.272727 = 2.2 (due to truncating the decimal)
AI = 11/6.6 = 1.666667 = 1.6 (due to truncating the decimal)
CAI = 2/6.6 = 0.303030 = 0.3 (due to truncating the decimal)
Note that you can use the LCD's data each morning to estimate the
number of apneas, central apneas, obstructive apneas, and hypopneas by doing some quick multiplication of the LCD's overnight indices by the usage number. Here's a typical example:
Suppose you look at the detailed Sleep Quality Data and this is what you see:
Usage: 7.3
Pressure: 11.0
Leak: 9.6
AHI: 1.8
AI: 1.4
CAI: 0.3
The first number indicates you used the machine 7.3 hours overnight. The second is the 95% pressure level. Since your machine is running in straight CPAP mode, this should stay at 11 cm since your pressure shouldn't be changing. The 95% leak rate is 9.6 L/minute. That means 95% of the time your leak rate was LESS THAN or EQUAL to 9.6 L/min. Not fantastic, but not bad. SInce you're still getting use to the machine, I'd say this 95% leak rate is not something to be too worried about. More on this later.
Since your usage was about 7.3 hours, we can estimate the number of events you had and what type they were from the AHI, AI and CAI as follows:
Total number of apneas and hypopneas:
AHI x Usage = (1.8 events per hour) x (7.3 hours) = approx. 13.14 events = 14 events (round UP since there's a fraction)
Total number of apneas (all types):
AI x Usage = (1.4 apneas per hour) x (7.3 hours) = approx. 10.22 apneas = 11 apneas (round UP since there's a fraction)
Total number of central apneas:
CAI x Usage = (0.3 CAs per hour) x (7.3 hours) = aprox. 2.19 central apneas = 3 central apneas (round UP since there's a fraction)
Estimated Total number of obstructive apneas:
First, an estimate of the OAI is simply AI-CAI. Note that if your S9 detects any Apneas of Unknown type, that will throw this calculation off, but it's still a good estimate. So here we go:
OAI = AI - CAI = 1.4 - 0.3 = 1.1 and
OAI x Usage = (1.1 OAs per hour) x (7.3 hours) = 8.03 obstructive apneas = 8 apneas (since we truncate to one decimal and because 8 + 3 = 11 apneas)
Total number of hypopneas:
First, an estimate of the HI is simply AHI-AI. So here we go:
HI = AHI - AI = 1.8 - 1.4 = 0.4
HI x Usage = (0.4 Hypopneas per hour) x (7.3 hours) = 2.93 hypopneas = 3 hypopneas (which make since because 11 apneas + 3 hypopneas = 14 events.
Of course, to get more detailed information about
when the events happened and
how long each of the apneas lasted, you will need to download the data into ResScan and look at the graphs for the night.
And if you don't know how to get to the "detailed" LCD information about AHI, AI, and CI, here's how to do it:
Step 1) Press the info button on the S9 to get into the short Sleep Quality menu.
Step 2) Hold both the info and the double check buttons down at the same time for three seconds or so. You may notice a brief flash.
Step 3) Use the large round knob to scroll down ONE icon on the left hand list of icons. You should now see the top of the detailed Sleep Quality menu. And then CLICK the round button to select the detailed Sleep Quality menu. This will now let you scroll all the way through the detailed Sleep Quality menu to see the AHI, AI, and CAI, which are at the bottom of this menu. It will also allow you to change the data period from 1 Day to 1 Week to 1 Month to 3 Months to get an idea of the averages of the data over longer time frames than over night.
Also, should I be concerned at all about the leak numbers. Whenever I use the mask check feature on the S9, it always reports good and I really don't think I hear anything leaking. Could the 95% and maximum numbers be so high because I take my mask off before turning the machine off? Should I turn the machine off first prior to removing my mask?
A bit more information on the five day leak data you provided: Your five day average median leak rate is 0.0 L/min. So 50% of the time the machine was running each night, the S9 could detect no mask leaks above and beyond what it expects from your mask provided your mask setting is correct. And that's pretty nice data to have.
Your average maximum is 23.4 L/min. Now if you know that you've taken the mask off (on most nights) without turning the machine off, that more than likely explains the maximum leak rate being relatively high. Since your average 95% leak rate is 9.6, that indicates that any really bad leaks did not last very long---less than 5% of the time the machine was running each night.
Hope the math lesson is useful instead of intimidating.
robysue
