What are respiratory events during sleep?
Respiratory events are apnea, hypopnea, and respiratory effort–related arousal (RERA). An apneic episode is an airflow decrease of at least 90% from baseline lasting at least 10 seconds (try holding your own breath for 10 seconds!). Hypopnea is defined as 10 seconds of at least a 30% decrease in airflow, resulting in ≥ 4% desaturation on pulse oximetry. RERA criteria should be sought if an observed event does not meet apnea or hypopnea criteria. RERA is defined as a sequence of breaths > 10 seconds in duration associated with increased respiratory effort and results in an arousal from sleep. The American Academy of Sleep Medicine (AASM) directs that apnea, hypopnea, and RERA, if present, be scored in the routine PSG interpretation. The average number of apnea and hypopnea episodes in 1 hour is referred to as the apnea–hypopnea index (AHI). However, if RERA is present, then the average number of apnea, hypopnea, and RERA should be calculated. This is the respiratory disturbance index (RDI). Note that the AHI does not equal the RDI, even though the terms are sometimes used interchangeably; such an interchange could create confusion.