Heart rate variability: why do spectral analysis?

Measurement of heart rate variability (HRV) by spectral analysis has become a hot issue in cardiology in recent years. Originally this technique was promoted by well-known clinicians and physiologists who reported that HRVgave insights into sympatho-vagal balance in autonomic outflow.'-3 This led to it being applied to the assessment of mental4 or physical stress5 on the one hand and the diagnosis of autonomic neuropathy6 on the other. Later it became apparent that HRV analysis might help in risk assessment of patients, for instance after myocardial infarction.7 It was found that decreased HRV points to a poor prognosis, probably related to the decreased cardioprotective activity of the parasympathetic system. Despite difficulties with the interpretation and in application of the techniques,8 many are using HRV analysis in clinical research. Heart rate variability is mainly a reflection of the influence of the autonomic nervous system on the sinus node of the heart. The heart rate alters with many of the changes in demand on the cardiovascular system that are related to changes in respiration, posture, and physical or mental activity. These changes are invoked by the control mechanisms that coordinate the total pattern of activity in the individual. We need information on this total activity to interpret HRV correctly. For instance, unless we were aware of the highly emotional conversation that took place, a sudden increase in blood pressure and heart rate in someone apparently sitting quietly at his desk would be incomprehensible. Data on blood pressure changes are essential for the understanding of short-term HRV because much of it is under baroreflex control.9 In the blood pressure to heart rate reflex loop the vagus nerve induces fast, brisk changes in the activity of the sinus node.'0 This to a large extent explains why a decrease in vagal outflow as observed after myocardial infarction induces lower HRV and lower baroreflex sensitivity scores.71'