OSA is characterized by repetitive episodes of complete (apnea) or partial (hypopnea) upper airway obstruction during sleep that often result in oxygen desaturation and terminate with brief arousals. By definition, apnea and hypopnea events last for 10 seconds or longer and are accompanied by continued efforts to breathe. Obstructive hypopneas are typically defined by a decrease in airflow of 30% or more with desaturation of 4% or more (Strollo and Rogers 1996; Figure 22–7). Because the neurocognitive and cardiovascular outcomes are similar for apneas and hypopneas, these events are typically counted together in providing an overall index of severity, the apnea–hypopnea index (AHI; number of apneas and hypopneas per hour of sleep) (Gottlieb et al. 1999). Mild OSA is defined as an AHI between 5 and 15, moderate OSA as an AHI of 15 to 30, and severe OSA as an AHI of greater than 30. In addition to apneas and hypopneas, the OSA syndrome includes a complaint of daytime sleepiness or insomnia, loud snoring, and/or episodes of breath holding, gasping, or choking during sleep. Other findings include complaints of fatigue, memory and cognitive difficulty, obesity, and hypertension or other cardiovascular disease.