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Calculations (1). Scoring Records. {8/06 shl}. Once a record is recorded, it must be scored to be useful. Scoring actually consists of four separate but not exclusive tasks: sleep staging, event scoring, summation, and report generation.
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Calculations (1) Scoring Records {8/06 shl}
Once a record is recorded, it must be scored to be useful. Scoring actually consists of four separate but not exclusive tasks: sleep staging, event scoring, summation, and report generation.
Most sleep centers follow the AASM recommendations and score according to the guidelines established by the AASM Manual for the Scoring of Sleep and Associated Events published in 2007.
Unfortunately, the majority of patients seen by a technologist of sleep medicine are anything but normal, and it is sometimes difficult to apply the rules precisely. In this case, a modicum of judgment must be exercised on the part of the scorer to apply the rules in a clinical manner.
Sleep stages will be scored in 30 second “epochs”, or a page at a time when run at 10 mm/sec.
The score assigned will reflect the “majority” of the epoch, except in specific circumstances. • With the exception of REM, scoring of all stages is determined by the EEG. • REM is determined by specific criteria occurring in the EEG, EOG and chin EMG channels.
Stage W – The EEG contains alpha activity and/or low voltage, mixed frequency activity. • Stage N1 – A relatively low voltage, mixed frequency EEG without rapid eye movements. • Stage N2 – Sleep spindles and K-complexes on a background of relatively low voltage, mixed frequency EEG activity.
Stage N3 – 20% or higher of high amplitude slow waves in the EEG. • Stage REM – A relatively low voltage, mixed frequency EEG in conjunction with rapid eye movements and low amplitude EMG.
Sleep Architecture Total Recorded Time Time in Bed (TIB) REM Latency REM Sleep Latency Sleep Period Time (SPT) Sleep Onset Sleep Offset Wake After Sleep Onset (WASO) Wake Lights Out Lights On
Once each epoch is staged, the next step is to summarize the staging. Add up all epochs of each stage and tabulate the totals. For example: Since each epoch is 30 seconds, there are two epochs per minute. Therefore, divide epochs by 2 to obtain minutes.
Now that we have totals, we can easily obtain relative amounts by calculating percents. Divide the number of minutes of each stage by the total number of minutes of testing. Multiply by 100 and you have percent:
The question now is, “what are these percents of?” • Looking back at our architecture, we can see that there are really three components to the night’s sleep. First, there is the period from getting in bed (“lights out”) until sleep ensues. This is called “sleep latency (SL)”. Then there is the period of time in which the subject had to sleep. This is called “sleep period time (SPT)”. Finally, there was any wake time left after the subject awoke. As we already pointed out, this is called “final wake time (FWT)”. The entire test time can be called either “total recording time (TRT)” or “time in bed (TIB)”. The percents we have calculated are percents of TIB.
If we want to calculate the percents of SPT, we can do so by removing the time spent awake before and after the sleep period time.
One more set of values may be helpful. We can ignore all wakefulness, regardless of when it occurred, and look only at the sleep. In this case, we set wake to 0 and calculate the total sleep time (TST):
With the data above, we can calculate other useful statistics. We can calculate the % of time in bed that the patient was asleep. This is called “sleep efficiency (SE%)”. The formula is: SE% = TST/TIB X 100 Using our example above, SE% = 315.5/324.0 X 100 = 97.4%
We can also calculate how well the patient slept once he or she fell asleep. This is called “sleep maintenance (SM%)”. The formula is: SM% = TST/SPT X 100 In our example, SM% = 315.5/317.0 X100 = 99.5% SM% can never be less than SE%.
We would also report our sleep latency, which was the time it took to fall asleep (in minutes), and one more value: the REM latency (RL). • The REM latency is the amount of time elapsed between sleep onset and REM onset. Don’t confuse this by counting from the start of the test as you did with sleep latency.
Sleep Latency SL = Lights out -> Sleep Onset Sleep Latency REM LATENCY start sleep REM RL = Sleep Onset -> REM Onset
This brings up a good question. What exactly is sleep onset? • AASM Scoring Manual defines sleep onset as: • The start of the first epoch scored as any stage other than stage W
Example 1 TIB = Lights out -> Lights On TIB = 2230 -> 0700 = 510 minutes (90 minutes before midnight + 420 minutes after midnight) SPT = Sleep onset -> Sleep Offset SPT = 2300 -> 0645 = 465 minutes 60 minutes before midnight + 405 minutes after midnight
TIB = 510 min SPT = 465 min SL = Lights out -> Sleep onset SL = 2230 -> 2300 = 30 minutes RL = Sleep Onset -> REM Onset RL = 2300 -> 0030 = 90 minutes
TIB = 510 min SPT = 465 min SL = 30 min RL = 90 min TST = SPT - WASO TST = 465 – 30 = 435 min
TIB = 510 min SPT = 465 min SL = 30 min RL = 90 min TST = 435 min SE% = TST/TIB (x100) SE% = 435/510 x 100 = 85.3% SM% = TST/SPT (x100) SM% = 435/465 x 100 = 93.5%
Example 2 TIB = Lights out -> Lights On TIB = 2100 -> 0630 = 570 minutes (180 minutes before midnight + 390 minutes after midnight) SPT = Sleep onset -> Sleep Offset SPT = 2345-> 0630 = 405 minutes 15 minutes before midnight + 390 minutes after midnight
TIB = 570 min SPT = 405 min SL = Lights out -> Sleep onset SL = 2100 -> 2345 = 165 minutes RL = Sleep Onset -> REM Onset RL = 2345 -> 0005 = 15 minutes
TIB = 570 min SPT = 405 min SL = 165 min RL = 15 min TST = SPT - WASO TST = 405 – 10 = 395 min
TIB = 570 min SPT = 405 min SL = 195 min RL = 15 min TST = 395 min SE% = TST/TIB (x100) SE% = 395/570 x 100 = 69.3% SM% = TST/SPT (x100) SM% = 395/405 x 100 = 97.5%
