Patients with Cheyne-Stokes respiration may mimic OSA but with a combination of airflow, respiratory movement, and saturation measurements; this should be apparent on a portable monitor. Patients with chronic obstructive pulmonary disease (COPD) may have periods of desaturation that typically occur during REM sleep (67). Since the portable monitor does not measure REM sleep, studies in patients with severe COPD should be avoided if attempting to diagnose OSA.
As mentioned, daytime sleepiness can occur in sleep disorders other than OSA (2). The typical Level III portable monitor is of little use in these cases and patients with daytime sleepiness and a negative portable monitor study should have the cause of the daytime sleepiness characterized. This will often require a PSG and possibly a multiple sleep latency test, which requires measurement of sleep staging (2,68).
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