Reference — Sleep Medicine
OSA Severity & Sleep Study Metrics
AHI severity thresholds, AASM respiratory event definitions, key polysomnography indices, normal sleep architecture percentages, and the screening tools used to classify obstructive sleep apnea — the reference every RT working in sleep needs at hand.
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
Obstructive sleep apnea (OSA) is characterized by recurrent upper airway obstruction during sleep, producing apneas, hypopneas, and oxygen desaturations. Severity is graded by the apnea-hypopnea index (AHI) derived from in-lab polysomnography (PSG) or, when PSG is unavailable, estimated from a home sleep apnea test (HSAT). The American Academy of Sleep Medicine (AASM) defines the scoring rules used for all indices below; applying them consistently is essential before comparing results across studies or titrating positive airway pressure therapy.
AHI Severity Classification (Adult)
| Severity | AHI (events/hour) |
|---|---|
| Normal | < 5 |
| Mild | 5 to < 15 |
| Moderate | 15 to < 30 |
| Severe | ≥ 30 |
Adult criteria. Pediatric OSA uses lower thresholds — an obstructive AHI ≥ 1 event/hour is considered abnormal in children.
Respiratory Event Definitions (AASM)
| Term | Definition |
|---|---|
| Apnea | ≥ 90% drop in airflow lasting ≥ 10 seconds |
| Obstructive apnea | Apnea WITH continued respiratory effort |
| Central apnea | Apnea WITHOUT respiratory effort |
| Mixed apnea | Central onset, obstructive end |
| Hypopnea | ≥ 30% drop in airflow ≥ 10 s WITH a ≥ 3% desaturation OR an arousal |
| RERA | Respiratory effort-related arousal not meeting apnea/hypopnea criteria |
Key Sleep Study Indices
| Index | Meaning |
|---|---|
| AHI | (Apneas + hypopneas) per hour of SLEEP |
| RDI | (Apneas + hypopneas + RERAs) per hour |
| REI | Respiratory event index (home testing; per hour of RECORDING time — tends to underestimate AHI) |
| ODI | Oxygen desaturation index — desaturations (≥ 3% or ≥ 4%) per hour |
| Arousal index | Arousals per hour of sleep |
Normal Adult Sleep Architecture
| Stage | % of Total Sleep |
|---|---|
| N1 | ~ 2 to 5% |
| N2 | ~ 45 to 55% |
| N3 (slow-wave) | ~ 13 to 23% |
| REM | ~ 20 to 25% |
Sleep efficiency is normally > 85%.
Diagnostic Thresholds & Screening
OSA Diagnostic Thresholds
OSA is diagnosed at AHI ≥ 5 with symptoms or comorbidities, or AHI ≥ 15 regardless of symptoms.
STOP-BANG Screening (each item = 1 point)
Snoring (loud) · Tiredness (daytime) · Observed apnea · Pressure (hypertension) · BMI > 35 kg/m² · Age > 50 · Neck circumference > 40 cm · Gender male.
Risk: 0–2 low · 3–4 intermediate · ≥ 5 high risk for moderate–severe OSA.
Epworth Sleepiness Scale
Scored 0–24; a score > 10 suggests excessive daytime sleepiness.
Nocturnal Oxygenation Reporting
Report nadir SpO₂, total time with SpO₂ < 88–90%, and the ODI. Significant nocturnal desaturation is often defined as ≥ 5 minutes with SpO₂ < 88%.
Clinical Notes
- The hypopnea desaturation rule changes the measured AHI. The AASM allows either a 3% or 4% oxygen desaturation threshold when scoring hypopneas. The 3% rule yields a higher AHI than the 4% rule for the same recording. Always note which scoring rule was applied when comparing studies or adjusting therapy.
- Home testing reports REI, not AHI. HSATs record respiratory events per hour of recording time (REI), not per hour of actual sleep time (AHI). Because recording time includes wake time, the REI tends to underestimate severity relative to in-lab PSG, particularly in patients with poor sleep efficiency.
- Pediatric thresholds differ from adult thresholds. An obstructive AHI ≥ 1 event/hour is abnormal in children. Pediatric OSA is also more likely to present with hyperactivity and behavioral changes than with the classic daytime sleepiness seen in adults.
Related Resources
Sources
- Kacmarek RM, Stoller JK, Heuer AJ. Egan's Fundamentals of Respiratory Care. 12th ed. Elsevier; 2021.
- 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.
- 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.