Search

Quantification of variations in instantaneous heart rate (HR) can be used to evaluate cardiac autonomic function. A 24-hour standard deviation of all normal RR intervals 6%--an index of parasympathetic activity. The 24-hour standard deviation varied between 68 and 261 ms (median 139). Range for index of parasymphatetic activity was 0.1 to 29.6% (median 4.4). Twenty percent of the interindividual variation in HR variability was explained by impact of risk factors. Standard deviation was uninfluenced by age, whereas parasympathetic activity decreased by increasing age. High physical training level was independently associated with significantly higher standard deviation (and parasymphatetic activity) values during both day and night. Hourly figures of standard deviation decreased during the night, whereas parasympathetic activity increased and peaked early morning. Standard deviation values as low as those reported in high-risk patients were not observed, but comparable low values for, and lack of diurnal variation in, parasympathetic activity were seen in healthy subjects also. In conclusion, risk factors and, in particular, the physical training level have impact on 24-hour HR variability in healthy subjects. This may prove valuable for modification of cardiac autonomic activity in patients., The rate at which the heart beats at any given instant is largely determined by the opposing actions of the two branches of the peripheral (autonomic) nervous system, the sympathetic and parasympathetic. The beat-to-beat changes in heart rate effected by the autonomic nervous system are reflected in the degree of variability in heart rate over relatively short periods of time (minutes or hours). Low 24-hour heart rate variability has been reported to be associated with high mortality in heart attack survivors, and has been observed in victims of sudden cardiac death. To determine normal levels of heart rate variability and to investigate the effects of risk factors for cardiovascular disease on this index of heart function, a study was carried out with 140 healthy subjects between the ages of 40 and 77 years. Analysis of heart rate data collected by 24-hour monitoring allowed estimation of the heart rate variability as well as the amount of parasympathetic autonomic activity. Normative data were collected and stratified by subject age and risk factors. Heart rate variability was not influenced by age, whereas parasympathetic activity decreased with increasing age. Twenty percent of the between-subject variation in heart rate was attributable to the influence of risk factors. High levels of physical fitness were associated with increased heart rate variability and increased parasympathetic activity. Levels of heart rate variability as low as those reported in high-risk individuals were not observed in this healthy population, but levels of parasympathetic activity comparable to those reported in high-risk patients were seen in this group. (Consumer Summary produced by Reliance Medical Information, Inc.)