Calculations (1)

Scoring Records

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• 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.

1. Sleep stages will be scored in 30 second “epochs”, or a page at a

time when run at 10 mm/sec.

2. Each epoch will receive one of the following 7 “scores”:

STAGE SYMBOL

wake W or 0

stage N1 1

stage N2 2

stage N3 3

stage REM R or 5

3. The score assigned will reflect the “majority” of the epoch, except in specific circumstances.

4. With the exception of REM, scoring of all stages is determined by the EEG.

5. 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

Lights Out Lights On

Total Recorded TimeTime in Bed (TIB)

Sleep Onset Sleep Offset

Sleep Period Time (SPT)

Sleep Latency REMREM Latency

WakeWake After Sleep Onset (WASO)

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:

stage epochs minutes

W 19 9.5

N1 22 11.0

N2 368 184.0

N3 115 57.5

R 124 62.0

TOTAL 648 324.0

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:

stage minutes %

W 9.5 2.9

N1 11.0 3.4

N2 184.0 56.8

N3 57.5 17.7

R 62.0 19.1

TOTAL 324.0 100.0

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. stage minutes %SPT

WASO 3.5 1.1

N1 11.0 3.5

N2 184.0 58.0

N3 57.5 18.2

R 62.0 19.6

TOTAL 317.0 100.0

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):

stage minutes %TST

W ----- -----

N1 11.0 3.5

N2 184.0 58.3

N3 57.5 18.2

R 62.0 19.7

TOTAL 315.5 100.0

Comparing %TIB, %SPT & %TST

stage minutes %TIB %SPT %TST

W9.5

(1.5 WASO)2.9 0.8 -----

N1 11.0 3.4 3.5 3.5

N2 184.0 56.8 58.0 58.3

N3 57.5 17.7 18.2 18.2

R 62.0 19.1 19.6 19.7

TOTAL 324 100.0 100.0 100.0

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

RL = Sleep Onset -> REM Onset

start sleep REM

Sleep LatencyREM

LATENCY

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 1Lights

out2230

Sleep Onset

2300REM Onset

0030

Lights on

0700Sleep Offset

0645 WASO 30 min

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 minutes60 minutes before midnight + 405 minutes after midnight

Lights out

2230Sleep Onset

2300REM Onset

0030

Lights on

0700Sleep Offset

0645 WASO 30 min

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

Lights out

2230Sleep Onset

2300REM Onset

0030

Lights on

0700Sleep Offset

0645 WASO 30 min

TIB = 510 min SPT = 465 min

SL = 30 min

TST = SPT - WASO

RL = 90 min

TST = 465 – 30 = 435 min

Lights out

2230Sleep Onset

2300REM Onset

0030

Lights on

0700Sleep Offset

0645 WASO 30 min

TIB = 510 min SPT = 465 min

SL = 30 min

SE% = TST/TIB (x100)

RL = 90 min

SE% = 435/510 x 100 = 85.3%

TST = 435 min

SM% = TST/SPT (x100)

SM% = 435/465 x 100 = 93.5%

Example 2Lights

out2100

Sleep Onset

2345REM Onset

0005

Lights on

0630Sleep Offset

0630 WASO 10 min

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 minutes15 minutes before midnight + 390 minutes after midnight

Lights out

2100Sleep Onset

2345REM Onset

0005

Lights on

0630Sleep Offset

0630 WASO 10 min

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

Lights out

2100Sleep Onset

2345REM Onset

0005

Lights on

0630Sleep Offset

0630 WASO 10 min

TIB = 570 min SPT = 405 min

SL = 165 min

TST = SPT - WASO

RL = 15 min

TST = 405 – 10 = 395 min

Lights out

2100Sleep Onset

2345REM Onset

0005

Lights on

0630Sleep Offset

0630 WASO 10 min

TIB = 570 min SPT = 405 min

SL = 195 min

SE% = TST/TIB (x100)

RL = 15 min

SE% = 395/570 x 100 = 69.3%

TST = 395 min

SM% = TST/SPT (x100)

SM% = 395/405 x 100 = 97.5%