What Does a PSG Entail?
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What Signals are Recorded?
PSGs in our lab have a standard montage of 6 EEG leads (bilateral frontal, central, and occipital leads), 2 EOG leads to measure eye movements, chin EMG for scoring atonia of REM sleep, single-lead ECG, pulse oxymetry, bilateral anterior tibialis EMG leads for leg movements, abdomen and chest belts to measure breathing effort, thermal and airflow measures of breathing near the mouth and nose, snoring sensor, and intercostal EMG. We have the capacity to increase the EEG leads to 12 (mainly to include temporal coverage). We also record audio and video through the night.
How is the record scored?
Our registered technicians manually review each patient’s PSG data, epoch by epoch, to score the sleep stages, breathing events, EEG arousals, leg movements, and other findings according to the standards set by the American Academy of Sleep Medicine. We regularly perform quality assurance tests to compare inter-rater reliability among our scoring technicians.
How is the record scored?
Our registered technicians manually review each patient’s PSG data, epoch by epoch, to score the sleep stages, breathing events, EEG arousals, leg movements, and other findings according to the standards set by the American Academy of Sleep Medicine. We regularly perform quality assurance tests to compare inter-rater reliability among our scoring technicians.
Interpreting the PSG
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Breathing Metrics
The most common metric for diagnosing sleep apnea and quantifying the severity is the frequency of apneas and hypopneas per hour of sleep, or the AHI. 0-5/hr is considered normal, 5-15 is considered mild, 15-30 is considered moderate, and >30 is considered severe. However, it may be useful to consider the degree of oxygen drop in estimating the clinical relevance of the sleep apnea findings, as some patients only have events during REM sleep (25% of the night), and their overall index may be mild or moderate range, but the oxygen loss could be profound. The RDI metric is a combination of apnea, hypopnea, and non-hypoxic partial obstructions called Respiratory Event Related Arousals, or RERAs. In some cases, the oxygen levels may hover below 90% even when no obvious obstructions are seen. This may occur in primary lung disease, or in obesity.
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Glossary of Terms Apnea-Hypopnea Index (AHI)the number of apneas + hypopneas per hour of sleep
Apnea, Central an apnea associated with absent effort on chest and abdomen belts
Apnea, Mixed an apnea with a combination of central and obstructive features
Apnea, Obstructive >90% decrease in airflow, lasting >10 seconds, with persistent effort based on chest and abdomen belts
Arousal “micro-awakenings” – these are 3-15 second long with episodes of increased EEG frequency
Awakening at least 15 seconds of waking EEG is required to score an epoch as wake
Complex Apnea typically refers to the emergence or worsening of central apnea with the administration of PAP
Epoch 30 second window, the standard scoring window for clinical human sleep studies
Hypnogram the graph of sleep-wake stages over the course of the night studies
Hypopnea 30-90% decrease in airflow, lasting >10 seconds, with >4% desaturation
Obstructive Sleep Apnea the typical form of sleep apnea;requires at least 5 pauses in breathing per hour of sleep. Many authorities use the AHI as the definition of events, but some use the RDI.
Respiratory Disturbance Index (RDI) the number of apneas + hypopneas + RERAs per hour of sleep
Respiratory Event Related Arousal (RERA) a non-hypoxic partial reduction in airflow associated with EEG arousal
Sleep Efficiency the amount of time spent asleep divided by the amount of time in bed
Sleep Latency various definitions are used, such as time from lights out to first 1-3 epochs of sleep
Wake After Sleep Onset (WASO) the amount of scored wake that occurs after sleep onset
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Sleep Stage Metrics
Many patients are interested in how much REM sleep or deep sleep they obtained, perhaps thinking that this is linked to their sleep symptoms. The main utility of measuring sleep stages during a PSG is to evaluate sleep apnea in REM sleep - specifically in supine-REM conditions. If supine-REM is not seen, we cannot confidently rule out sleep apnea, because obstructions may only be evident in this setting. The portion of stage N1, the lightest stage of sleep, can give an indication of how light/fragmented sleep was, as can sleep efficiency (how much sleep is obtained as a percentage of time in bed). However, on a single night in the lab, there is limited actionable data regarding the percentages. Many medications suppress REM sleep for example, and some data indicates that patients sleeping in the lab tend to have more fragmentation and less REM and slow wave, called the "first-night effect". Again, the main clinical consequence is that certain pathologies such as OSA or RBD require sufficient REM sleep to be obtained.
Restless Leg versus Periodic Limb Movements
RLS is a purely clinical diagnosis, based on the presence of uncomfortable sensations or desire to move, worse at night or at rest, and relieved by movement. Although most patients with RLS also have PLMS, the measurement of leg movements in a sleep study is not part of the diagnosis. PLMS, by comparison, are objective PSG findings. Most patients with PLMS do not have RLS. The evaluation and treatment pathways are similar however, and the pathophysiology may be shared. For patients with RLS, if there is concurrent OSA, it is important to treat the OSA, which may relieve some of the RLS symptoms.
