Heart Rate Variability:

Heart rate and blood pressure represent the integrated response to ongoing and dynamic changes in autonomic, cortical, environmental, and other inputs. The relatively “coarse” quantification of heart rate and blood pressure nevertheless is invaluable in predicting and treating cardiovascular disease and in understanding disease mechanisms. As part of an approach to better understanding cardiovascular physiology and possibly provide even earlier predictions of risk and progression of cardiovascular disease, a myriad of approaches have been taken to dissect out the finer points of blood pressure and heart rate regulation, and to understand the mechanisms and implications of the shortand long-term dynamics that regulate their change. Unprecedented software sophistication and computer processing power, and the availability of relatively simple measures for evaluating RR interval, have made analyses of changes in heart rate, or heart rate variability, widely accessible to physiologists and clinicians. For practical purposes, heart rate variability may be understood as the magnitude of “sway” or flexibility that is spontaneously inherent in cardiac rate control. Much as the sway inherent in a bridge or skyscraper will impart resilience to external stress, the magnitude of variability in the beat-to-beat changes of RR interval are generally a sign of sound cardiovascular health. This is demonstrated by visual inspection of the ECG of a young fit individual compared to that of someone with severe heart failure. The slower heart rate and marked sinus arrhythmia in a young athlete contrast markedly with the tachycardia and relatively fixed RR interval in a patient with heart failure in sinus rhythm. Approaches to study and quantify the variability of heart rate include measurements in the time and frequency domains, and application of these methods to short-term (several minutes) and long-term (24-hour Holter) measurements.1-7 Frequency-domain analysis seeks to quantify the magnitude of RR fluctuations at specific frequencies, whereas time-domain analysis quantifies the absolute variability of RR interval, measured, for example, as standard deviation, over a given period of time, varying from 5 minutes or up to 24 hours or longer. Other approaches include variations on these themes, such as wavelet transform analysis8 and fractal scaling.9,10 In patients with overt cardiovascular disease, abnormalities in heart rate variability, perhaps reflecting autonomic dysregulation or baroreflex impairment, may be linked to