Evidence in support of this demonstrates that suppression of PVCs by sleep predicts success of β blockade in PVC suppression. Night time reduction in sympathetic tone is reflected in the diminished incidence of premature ventricular contractions (PVCs) during sleep 2 w5 despite a relatively slow heart rate. Further monitoring may be appropriate to ensure that such arrhythmias are within normal limits, as described above. This may give rise to arrhythmias detected at night on Holter monitoring or inpatient cardiac monitoring even in the awake patient-a finding that does not necessarily imply the need for therapeutic intervention. This implies that parasympathetic mediated arrhythmias may persist in the awake patient overnight. w4 There was no difference in parasympathetic activity which was measured by the degree of sinus arrhythmia. w3 In a study that compared results from individuals after a night of sleep and then a subsequent night deprived of sleep, sympathetic activity, as measured by the systolic pre-ejection period, differed significantly between the two nights. Parasympathetic tone is controlled by circadian rhythms, and sympathetic tone by sleep stage and the changes in posture and activity accompanying sleep. Non-REM sleep is characterised by an overall increase in parasympathetic tone, and a decrease in sympathetic tone (table 2 2). 2ĪUTONOMIC INFLUENCE ON CARDIAC RHYTHM DURING SLEEP They are a reflection of changes in autonomic tone that occur during sleep and require no intervention unless accompanied by symptoms. w2 These arrhythmias are, for the most part, both asymptomatic and benign. 1 This is seen more frequently in athletes, w1 and less frequently in those over 80 years of age. 1 Sinus pauses up to two seconds in duration occur commonly in young people in association with sinus arrhythmia (fig 1 1 ). Studies of individuals free of cardiac disease show that sinus bradycardia, sinus pauses, and type 1 second degree atrioventricular (AV) block are common during sleep (table 1 1 ). REM sleep occurs at approximately 90 minute intervals, encompasses most dream activity, and is characterised by rapid eye movements and reduced voluntary muscle activity. Non-REM sleep is graded 1–4 according to electroencephalogram (EEG) characteristics and diminished arousability. The stages of sleep, conventionally divided into rapid eye movement (REM) and non-rapid eye movement (non-REM), are characterised by unique autonomic influences over cardiac rhythm and haemodynamics.
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