REM rebound has been suggested to contribute to hemodynamic instability, myocardial ischemia and infarction, stroke, mental confusion, and wound breakdown.17–21 In REM sleep, the neural drive to the pharyngeal muscles is at a minimum, and the atonia of antigravity muscles predisposes the patient to airway instability causing episodic hypoxemias.22 In a small study23 of 25 patients undergoing minor limb surgery, the need for using positive pressures to maintain upper airway patency in patients with sleep-disordered breathing was highest during REM sleep. Episodic hypoxemias during REM sleep lead to brief arousals associated with profound sympathetic activation, which may cause hemodynamic instability and increased mean arterial pressure.,24–25 Postoperative respiratory obstructions are associated with large fluctuations in systolic and diastolic BPs in patients with OSAS26 (Fig 1
). Surgical stress, including postoperative pain and endocrine changes, increases the sympathetic activation further. As a result of chronic adrenergic arousal, patients with sleep apnea may have down-regulated α-receptors and β-receptors, and thus have an attenuated response to vasopressors.,21 REM sleep rebound and the link to sympathetic tone may be particularly dangerous, leading to myocardial ischemia, infarction, and even unexplained postoperative death. This hypothesis is supported by the finding that the majority of unexpected and unexplained postoperative deaths occur at night within 7 days of surgery.27It is interesting to note that nocturnal ST-segment changes consistent with myocardial ischemia are evident in patients with OSAS who are free of clinically significant coronary artery disease.28The Sleep Heart Health Study Research Group found29apnea-hypopnea index (AHI) scores to be modest, with scores of 1 to 10 associated with manifestations of cardiovascular disease. Hung and colleagues30 studied 101 male survivors of acute myocardial infarction (MI) and 63 age-matched control subjects. An apnea index of > 5 was found in 36% of MI patients compared with only 3.8% of the control patients. After adjustment for age, body mass index, hypertension, smoking, and serum cholesterol level, they found that an apnea index of > 5.3 was independently predictive of MI with an odds ratio of 23.3 (p < 0.001).