Heart rate variability: Measurement and meaning

The heart beat is not absolutely regular; under physiologic conditions, there are continuous small variations in the intervals between beats. Much of the recent interest in heart rate variability (HRV) is because of the fundamental observation that loss of this variability is a surprisingly powerful predictor of poor outcome in a variety of conditions, including ischemic cardiac disease. Assessment of HRV also provides a noninvasive window into the autonomic control of the heart. This review will discuss theoretical and practical aspects of the measurements of HRV and summarize studies relevant to its prognostic import. In doing so, it will provide some illustration of the use of HRV for autonomic assessment. The literature on HRV has grown exponentially, and we have focused this review on subjects most relevant to cardiology and cardiologists: HRV in patients with previous myocardial infartion or ventricular arrhythmias. Cardiologists also are most likely to be presented with heart rate variability information on such patients, obtained from analysis of ambulatory electrocardiographic recordings requested for other indications.