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.
[EDIT: There sometimes is a difference between cumulative "Therapy Time" and "Blower Time" with both Respironics and ResMed machines, and this difference can go in either direction. "Blower Time" would ordinarily be expected to be larger, since there can be no "therapy" when the blower is off, and "Blower Time" is "hard-wired" to a dedicated non-resettable clock. "Blower Time" may be larger for either machine when the unit is used for drying out tubing or other purposes during which the machine does not sense "breathing" patterns, and therefore does not credit any time to "therapy". When "Blower Time" is larger for apparently inexplicable reasons (which seems to predominate for Respironics), this may occur because "Therapy Time" may be linked to additional "soft" considerations, such as a slight delay in initiation of the "Therapy Time" clock while the machine moves toward its steady-state conditions, and/or while the sensors determine that an airflow pattern consistent with breathing is actually present. When "Therapy Time" is larger (which seems to predominate with ResMed), this seems to occur because the machine "credits" as therapy time short breaks when the blower is in fact off. --Velbor 15:27, 14 December 2009 (UTC) ]
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 is not always clear what "usage time" means. As an example, a ResMed S8 Auto machine reports "usage" as 8:04 on a night with two arisings. Review of the 60-second-reporting-window data shows that (presumably median) pressure was >0 for only 8:01. Similarly, pressure was >=10.0 cm (the minimum set pressure) for only 7:57. Which number is "correct" to use in evaluating respiratory disturbance indices?
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)