Guide — Sleep Medicine
Polysomnography & Sleep Study Essentials
Polysomnography is the overnight, multichannel recording of physiologic signals during sleep, scored in 30-second epochs. This guide walks through every channel, the AASM scoring rules for apneas and hypopneas, sleep stage architecture, study types, and the indices RTs are expected to know — AHI, RDI, ODI, and REI.
10 min read · Sleep Medicine
Written by Apex Respiratory Editorial Team
Educational use only. This material supports respiratory therapy education and exam review. It is not medical advice and is not a substitute for clinical judgment, institutional protocols, or physician orders. Always follow facility policies and current provider orders, and verify calculations independently before clinical use.
Overview
A polysomnogram (PSG) is the diagnostic gold standard for sleep-disordered breathing. The recording captures multiple physiologic signals simultaneously and is divided into 30-second epochs that are scored individually for sleep stage and respiratory events. From those epochs, technologists and physicians extract the indices that characterize a patient’s sleep architecture, breathing, and oxygenation overnight.
The respiratory therapist’s role spans both the diagnostic and therapeutic sides of the sleep lab: applying and verifying sensors before lights-out, monitoring signal quality and recognizing events in real time throughout the night, and performing positive airway pressure (PAP) titration during split-night or dedicated titration studies.
Key Concepts — Channels & Signals
A full in-lab PSG uses a standard montage of sensors. Each channel serves a specific diagnostic purpose — knowing why each one is there is as important as knowing how to apply it.
| Channel | Full Name | Diagnostic Role |
|---|---|---|
| EEG | Electroencephalogram | Stages sleep (Wake, N1, N2, N3, REM) |
| EOG | Electrooculogram | Records eye movements; detects REM-characteristic rapid eye movements |
| Chin EMG | Electromyogram (chin) | Measures muscle tone; atonia in REM confirms the sleep stage |
| Leg EMG | Electromyogram (legs) | Detects periodic limb movements |
| ECG | Electrocardiogram | Monitors heart rhythm throughout the night |
| Thermistor | Oronasal thermistor (airflow, qualitative) | Used to score apneas (≥90% drop in airflow) |
| Nasal pressure | Nasal pressure transducer (airflow, quantitative) | Used to score hypopneas (≥30% drop in airflow) |
| RIP belts | Respiratory inductance plethysmography | Thoracic + abdominal effort — distinguishes obstructive (effort present) from central (no effort) events |
| SpO₂ | Pulse oximetry | Tracks oxygen desaturations throughout the night |
| Position / snore | Body position + snore sensors | Identifies positional OSA and snoring episodes |
AASM Scoring Rules — Events
- Apnea— ≥90% drop in airflow (thermistor) lasting ≥10 seconds. Classified as obstructive (effort continues on RIP belts), central (no effort), or mixed (central onset, obstructive end).
- Hypopnea— ≥30% drop in airflow (nasal pressure transducer) lasting ≥10 seconds WITH either a ≥3% oxygen desaturation OR an arousal (AASM recommended rule). Some payers and older protocols require a ≥4% desaturation instead.
- RERA(respiratory effort-related arousal) — flow limitation or increasing respiratory effort causing an EEG arousal that does not meet full apnea or hypopnea criteria.
Key Indices
- AHI— apnea-hypopnea index: (apneas + hypopneas) per hour of sleep time.
- RDI— respiratory disturbance index: (apneas + hypopneas + RERAs) per hour. Equal to or greater than the AHI.
- ODI— oxygen desaturation index: number of desaturations per hour of sleep.
- REI— respiratory event index (HSAT): events per hour of recording time (not sleep time). Because recording time includes wakefulness, the REI can underestimate the AHI.
Assessment & Findings — Sleep Architecture & Study Types
The AASM staging system recognizes five states: Wake, N1, N2, N3, and REM. Normal adults cycle through NREM stages and REM approximately every 90 minutes, with REM periods becoming longer and more frequent toward morning.
| Stage | % of Night | Key Features |
|---|---|---|
| Wake | Variable | Alpha and beta EEG waves; voluntary eye movements; high muscle tone |
| N1 | ~5% | Theta EEG; slow rolling eye movements; light, transitional sleep |
| N2 | ~45-55% | Sleep spindles and K-complexes; true sleep; most time is spent here |
| N3 | ~15-20% | Slow-wave (delta) EEG; deepest NREM; restorative; hardest to arouse |
| REM | ~20-25% | Low-amplitude mixed EEG; muscle atonia; dreaming; REMs on EOG; predominates in latter-night cycles |
Beyond sleep staging, the PSG report includes sleep efficiency (time asleep / time in bed), sleep latency (time from lights out to first epoch of sleep), REM latency (time from sleep onset to first REM epoch), and total sleep time — all of which inform the clinical picture.
Study Types
Type 1 — In-lab PSG
Attended overnight polysomnogram with full channel montage. Gold standard for diagnosing all sleep disorders.
Mandatory for suspected narcolepsy, parasomnias, or when HSAT is negative in a symptomatic patient.
Split-night PSG
Diagnostic first half; CPAP titration begins in the second half if enough events are recorded (commonly AHI ≥20-40 events/hour).
Saves the patient a second night in the lab when OSA severity is confirmed early.
HSAT (Type 3/4)
Home sleep apnea testing with fewer channels (airflow, effort, oximetry). Reports REI (events/recording time), not AHI.
Appropriate for uncomplicated adults with high pretest probability of moderate-severe OSA. Underestimates severity; negative result needs follow-up in-lab.
MSLT / MWT
Multiple Sleep Latency Test and Maintenance of Wakefulness Test — daytime nap studies, not overnight.
MSLT evaluates hypersomnia and narcolepsy; MWT measures ability to stay awake.
RT Priorities & Interventions
- Sensor application and verification. Before lights-out, confirm impedance on EEG electrodes, verify both airflow sensors (thermistor and nasal pressure) are producing signal, and check that RIP belt excursions are visible and in phase. A poor-quality montage invalidates scoring.
- Real-time event recognition. Monitor the signal in the control room throughout the night. Recognize the difference between obstructive and central apneas on the effort channels before events are formally scored — this matters most during a split-night study when the threshold to switch to PAP must be judged in real time.
- PAP titration. During split-night or dedicated titration studies, the RT remotely adjusts CPAP (or bilevel) pressure to eliminate apneas, hypopneas, snoring, and flow limitation while optimizing SpO₂ and sleep continuity. Document the optimal pressure and the patient’s response.
- Patient education. Many patients are anxious about sleeping in a lab. Explain the sensor application process, reassure them that leads are non-invasive, and review what to expect during the night. A relaxed patient produces better data.
- Pediatric and hypoventilation studies. Selected studies — particularly pediatric PSGs and studies for suspected hypoventilation syndromes — also include end-tidal or transcutaneous CO₂ monitoring. Ensure the capnography waveform is stable before sleep onset.
Common Pitfalls
- Swapping the airflow sensors. The oronasal thermistor scores apneas; the nasal pressure transducer scores hypopneas. Using the wrong channel for the wrong event type will produce incorrectly classified data. Both channels must be active and correctly labeled from the start.
- Forgetting the effort belts distinguish event type. The RIP belts are the only channel that separates obstructive from central apneas. If belt signals are lost or noisy, central vs. obstructive classification is not possible — always troubleshoot belt signal before lights-out.
- Treating HSAT REI as equivalent to PSG AHI. Because HSAT divides events by total recording time rather than sleep time, the REI underestimates the true AHI. A borderline HSAT result in a symptomatic patient warrants in-lab PSG, not reassurance.
- Ignoring the first-night effect. Sleeping in a laboratory setting commonly reduces total sleep time, suppresses REM, and may alter the distribution of events compared to the patient’s habitual sleep. This is a recognized limitation of single-night in-lab studies.
Board Exam Pearls
- Apnea = ≥90% airflow drop (thermistor) ≥10 seconds. Hypopnea = ≥30% airflow drop (nasal pressure) ≥10 seconds with ≥3% desaturation or arousal (AASM recommended rule).
- The effort channel (RIP belts) is the key to distinguishing central (no effort) from obstructive (effort present) apneas. The airflow channels alone cannot make that distinction.
- RDI = AHI + RERAs/hour. RDI is always ≥ AHI. If a question gives both, the RDI will be the higher number.
- Split-night criteria: AHI is typically ≥20–40 events/hour in the first portion. The exact threshold varies by lab protocol, but the concept — enough events to confirm diagnosis before switching to CPAP — is consistent.
- HSAT reports REI, not AHI. REI divides by recording time (includes wake), so it underestimates. Choose in-lab PSG when HSAT is negative in a symptomatic patient.
- REM predominates in the latter half of the night. OSA tends to be most severe during REM (muscle atonia worsens upper airway collapse). Supine position also worsens events.
FAQ
What does a sleep study measure?
A full polysomnogram records EEG, eye movements, chin and leg EMG, ECG, airflow (both oronasal thermistor and nasal pressure transducer), respiratory effort via RIP belts, oxygen saturation, body position, and snoring — combined to stage sleep and score breathing events across the night.
What is the difference between AHI and RDI?
AHI (apnea-hypopnea index) counts apneas and hypopneas per hour of sleep. RDI (respiratory disturbance index) adds respiratory effort-related arousals (RERAs), so the RDI is always equal to or higher than the AHI for the same study.
What is a split-night study?
A split-night study is a single overnight PSG that serves two purposes: the first portion is diagnostic, and if enough events are recorded (often an AHI of 20-40 or more), the study is paused and CPAP titration begins in the second half of the night.
Is a home sleep test as good as an in-lab study?
For uncomplicated patients with a high pretest probability of moderate-to-severe OSA, home sleep apnea testing (HSAT) is appropriate and convenient. However, it monitors fewer channels and reports a respiratory event index (REI) based on total recording time rather than sleep time, which can underestimate severity. A negative or inconclusive HSAT in a symptomatic patient should be followed by in-lab PSG.
Practice with real values
Sleep-disordered breathing causes hypoxemia and acid-base changes. Use the ABG interpreter to work through the arterial blood gas patterns you will encounter in patients with OSA and hypoventilation syndromes.
Open the ABG Interpreter →Related Resources
Sources
- Kacmarek RM, Stoller JK, Heuer AJ. Egan's Fundamentals of Respiratory Care. 12th ed. Elsevier; 2021.
- Berry RB, Quan SF, Abreu AR, et al. The AASM Manual for the Scoring of Sleep and Associated Events: Rules, Terminology and Technical Specifications. Darien, IL: American Academy of Sleep Medicine; 2020.
- Kapur VK, Auckley DH, Chowdhuri S, et al. Clinical practice guideline for diagnostic testing for adult obstructive sleep apnea: an American Academy of Sleep Medicine clinical practice guideline. J Clin Sleep Med. 2017;13(3):479-504.