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As outlined in Chapter 12, sleep occurs in distinct states classified broadly as non-rapid eye movement (NREM) sleep and rapid eye movement (REM) sleep. These occur in a temporally organized fashion across the sleep period, with NREM dominating during the early hours of sleep and REM during the final hours of sleep. Myocardial infarction and ischemic stroke occur at a higher frequency during early morning hours,13 when REM sleep predominates. This temporal distribution of events may be explained at least in part by specific alterations in autonomic regulation during sleep. This chapter presents additional information on alterations in cardiopulmonary function during sleep.

Changes in Cardiovascular Control During Sleep

During NREM sleep, heart rate slows by 5% to 10%,4 and blood pressure drops by about 10%.4 These changes are most prominent in the deepest stage of NREM sleep,4 that is, stage 4 (now known as N35 or slow-wave sleep4). The mechanism for this decline was elucidated by the seminal studies of Somers et al.4 using microneurography to record sympathetic bursts in the peroneal nerve in healthy humans during wake and in the different stages of sleep (Fig. 101-1). These studies confirmed a fall in sympathetic activity during NREM sleep, which is responsible for the drop in blood pressure, cardiac output, and systemic vascular resistance.4 This fall is accompanied by an increase in parasympathetic (vagal nerve) activity that is believed to be responsible for the bradycardia of NREM sleep.6 Thus, the balance of parasympathetic/sympathetic activity is altered during NREM sleep, with the parasympathetic being dominant. This also results in alteration in heart rate variability during sleep. The high-frequency component of this heart rate variability is said to reflect parasympathetic activity, while the low-frequency component is related to sympathetic activity.4 Thus, during NREM sleep there is an increase in the high-frequency component compared with wakefulness and a quite marked reduction in the low-frequency component of heart rate variability.4

Figure 101-1

Alterations in recorded bursts of sympathetic nerve activity (SNA) and blood pressure in wakefulness, different stages of non-rapid eye movement sleep (NREM) (stages 2–4), and rapid eye movement (REM). With deepening of NREM sleep, there is progressive loss of sympathetic activity, which is virtually absent in slow-wave sleep (stage 4). Sympathetic activity returns in REM but is highly variable. (Reproduced with permission from Somers VK, Dyken ME, Mark AL, et al. Sympathetic-nerve activity during sleep in normal subjects. N Engl J Med. 1993;328(5):303–307.)

Changes in REM sleep differ from those that occur during NREM sleep. During REM sleep, which occurs at approximately 90-minute intervals, there is a return of sympathetic activity such that heart rate and blood pressure return to wakefulness levels.4 This has been shown directly by recording sympathetic nerve activity ...

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