Respiratory disturbance index
The respiratory disturbance index (RDI) — or respiratory distress Index — is a formula used in reporting polysomnography (sleep study) findings. Like the apnea-hypopnea index (AHI), it reports on respiratory events during sleep, but unlike the AHI, it also includes respiratory-effort related arousals (RERAs). RERAs are arousals from sleep that do not technically meet the definitions of apneas or hypopneas, but do disrupt sleep. They are abrupt transitions from a deeper stage of sleep to a shallower.
A RERA is characterized by increasing respiratory effort (and thus decreasing esophageal pressures) for 10 seconds or more leading to an arousal from sleep, but one that does not fulfill the criteria for a hypopnea or apnea. The gold standard for measuring RERAs is esophageal manometry, as recommended by the American Academy of Sleep Medicine (AASM). However, esophageal manometry is uncomfortable for patients and impractical to use in most sleep centers.
Some research studies have found that a high RDI was significantly correlated with excessive daytime sleepiness, and that this correlation was stronger than that for the frequency of oxygen saturation decreases below 85%, but other studies have found only a weak correlation. More recent studies have found more impressive outcomes in treating patients with RERA and respiratory events that satisfy the definition of hypopneas in all aspects other than reduction of oxygen saturation levels.  
RDI = (RERAs + Hypopneas + apneas) X 60 / TST (in minutes). That is, RDI means the average number of episodes of apnea, hypopnea, and respiratory event-related arousal per hour of sleep. (TST is "total sleep time".)
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