Total Recording Time
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Total Recording Time (TRT) is the amount of time that is recorded during a Polysomnogram. This statistic can be used to compare how long it takes a patient to fall asleep, and Total Sleep Time (TST).
Although this terminology is not often directly employed for home CPAP use, the concept is transportable, and CPAP machines and software often display the "total time" they are in use. However, the paradigms for figuring "usage time" differ between machine manufacturers.
If you turn the machine on, for example, at 11:30pm, do not turn it off overnight, and arise at 7:30am, machines can be expected to show 8:00 as the usage time.
If however, you arose 3 times, and turned the machine off for 3 minutes at each time, different machines may show different results. Respironics machines show "usage time" as "blower on time" for the session. Changes in the status of the blower are recorded to the second, and "usage time" is the difference between the first "blower-on" time and the last "blower off" time for the session. Thus, in the scenario proposed, Respironics machines would show a "usage time" of 7:51. ResMed, on the other hand, utilizes the concept of "mask on time". This is also based on "blower on" status, but ResMed does not "penalize" the user for short (documentation the precise cutoff time has not been found, but seems to be under 10 minutes) arisings. Thus, in the scenario proposed, ResMed machines would show a "usage time" of 8:00, ignoring the brief arisings. However, if you arose the same 3 times, but turned the machine off for 10 minutes each time, the two machines would display the same results - a "usage time" of 7:30.
A consequence of this difference is that ResMed calculation of respiratory event indices, in the presence of multiple short arisings, may be reported slightly on the low side, since the time factor includes minutes during which the machine was not actually blowing. However, ten minutes "extra" during an 8 hour night would result in a 2% "error" which would not likely be noticeable.
It should be noted that while "usage time" can often be reset on a CPAP machine, total cumulative "blower on" time for a unit cannot be reset. It might also be noted here that ResMed machines cease recording pressure and leak data after 10 hours. This is apparently done to standardize the quantity of data which can be stored. Respiratory events, and usage time, continue to be recorded even after the 10 hour limit.
--Velbor 15:33, 24 November 2009 (UTC)