Arousal Index in Apnea-Predominant vs. Hypopnea-Predominant OSA
Apnea events cause more severe arousals than hypopnea events, but hypopneas are more likely to cause arousals overall, making the arousal index potentially higher in hypopnea-predominant OSA. 1
Arousal Characteristics in OSA Subtypes
- Apneas cause less frequent arousals than hypopneas in N1, N2, and N3 sleep stages across all OSA severity categories 1
- When arousals do occur following apneas, they tend to be longer in duration than those following hypopneas (p<0.001) 1
- Apnea-related desaturations are more severe than hypopnea-related desaturations (p<0.001) in N1, N2, and REM sleep, even after adjusting for respiratory event duration 1
Importance of Arousal-Based Scoring in OSA
- The American Academy of Sleep Medicine (AASM) recommends including arousal-based scoring when evaluating OSA patients 2
- Evidence demonstrates that arousals, rather than hypoxemia, better predict hypersomnia in OSA patients 2
- Not including arousal-based respiratory events may lead to misclassification of OSA severity or failure to diagnose OSA altogether 2
Relationship Between Arousals and Respiratory Events
- The AASM defines a hypopnea as a ≥30% drop in airflow for ≥10 seconds with either ≥3% oxygen desaturation OR an arousal 2
- Respiratory effort-related arousals (RERAs) are sequences of breaths lasting ≥10 seconds with increasing respiratory effort leading to arousal when not meeting criteria for apnea or hypopnea 2
- Recent research shows that when using the 3% desaturation or arousal criteria for hypopneas, fewer events are scored as RERAs compared to using the 4% desaturation-only criteria 3
Clinical Implications of Arousal Differences
- Sleep fragmentation from frequent arousals contributes significantly to daytime sleepiness in OSA patients 4
- Eliminating sleep fragmentation, even with persistent hypoxemia, improves sleepiness compared to baseline 2
- The arousal index has been found to correlate with risk of hypertension in some studies, whereas desaturation did not 2
- Increased arousals have also been associated with white matter disease in older adults 2
Measurement Considerations
- The arousal index is calculated as the number of arousals per hour of sleep 2
- Scoring reliability for arousals has been questioned, with some studies showing intraclass correlation coefficients of 0.54 2, though more recent large studies show higher reliability 2
- When using the 3% desaturation or arousal criteria for hypopneas, the apnea-hypopnea index (AHI) is significantly higher than when using the 4% desaturation-only criteria 3
Clinical Relevance
- Apneas and hypopneas are currently given equal diagnostic weight in the AHI despite differences in their associated arousals and desaturations 1
- The severity of arousals and desaturations should be considered when assessing OSA severity and related cardiovascular risk 1
- Prior sleep apnea increases the arousal threshold to upper airway occlusion on subsequent nights, potentially prolonging apneic events 5