Rhythmic heart rate variability (sinus arrhythmia) related to stages of sleep

Evidence that the excursion of rhythmic heart rate variability (sinus arrhythmia) may be manipulated by altering the subject’s state of concentration, prompted a study of effects associated with different stages of sleep. There were 9 healthy subjects, ages 16–69, and one 55-year-old subject (C.P.) with a myocardial infarction history. Awake and during sleep the excursion of heart rate variability was greatest among the younger subjects, decreasing progressively with age. C.P. displayed rhythmic heart rate variability only when asleep. In light sleep (Stages 1 and 2) all subjects, with the exception of the 69-year-old, whose rate remained almost fixed throughout the night, displayed rhythmic heart rate variability, roughly synchronous with respiration. Neither the elderly subjeot nor C.P. entered into deep sleep (Stages 3 and 4). Among the other subjects, deep sleep was marked by a precise correlation between the frequency of respiration and heart rate variability. In REM sleep, however, in all subjects, there was often a total dissociation between respiration and rhythmic heart rate variability. There were periods of no heart rate variability with continuing regular respiration and of rhythmic heart rate variability with spontaneous breath holding. In the presence of heart rate variability during REM, C.P. displayed frequent ectopic beats, but ectopic beats were nearly absent in REM periods characterized by a steady, unvarying heart rate. The range of heart rate variability at rest and during sleep (sinus arrhythmia) reflects the manner in which autonomic activity is brought to bear in regulating the heart beat. The study of the behavior of sinus arrhythmia may offer a way to anticipate the occurrence of dangerous cardiac dysarrhythmias and sudden death, especially during sleep.