Spectral analysis of the heart rate variability : a methodological study

Sammanfattning: Heart rate and other cardiovascular variables are not steady but fluctuate over time. The evolution of computer technology the last 2 decades has made detailed analysis of the fluctuations possible and has focused the attention to the information content of the variability.Autonomic nervous influence on the heart can be assessed by measuring the heart rate variability from R-R intervals in the electrocardiogram. Applications have been developed in many fields where the pathophysiological features of the patients include autonomic dysfunction.This thesis is based on clinical studies and model studies compared with results from healthy volunteers. Two patient groups, with familial amyloidosis and with myotonic dystrophy, were evaluated using simple indexes of heart rate variability. Signs of autonomic dysfunction which is a dominant clinical feature in familial amyloidosis with polyneuropathy was confirmed by abnormal heart rate variability indexes. In myotonic dystrophy heart rate variability was normal but recumbent and standing plasma catecholamines were elevated. Reassessment of the heart rate variability with spectral analysis confirmed the results.The integral pulse frequency modulation (IPFM) model can be used as a simple analogy of the interface between autonomic activity and the cardiac pacemaker. Computer simulations using the IPFM model revealed marked differences in the estimated modulating variability depending on whether the intervals between simulated beats or the frequency of beats was analyzed. Comparison with data from healthy volunteers indicated that the IPFM model was valid and that heart rate should be preferred instead of R-R intervals for variability measurements.Beat-by-beat heart rate can be viewed as an irregular sample of the underlying autonomic activity. Spectral analysis of simulated heart rate variability data using the IPFM model showed attenuation of high frequency variability with decreasing mean heart rate and with increasing variability mean frequency. Application of these results on data from healthy volunteers suggest that the effects can be significant in actual variability measurements.Cholinergic blockade resulted in reduced heart rate variability at all spectral frequencies. Adrenergic beta-blockade reduced high-frequency variability in standing position indicating that high-frequency variability might partly be sympathetically mediated.