Heart rate variability

Heart rate variability (HRV) is the physiological phenomenon of variation in the time interval between heartbeats. It is measured by the variation in the beat-to-beat interval. Other terms used include cycle length variability, R–R variability (where R is a point corresponding to the peak of the QRS complex of the ECG wave; and R–R is the interval between successive Rs), and heart period variability. Measurement of the R–R interval (often termed normal-to-normal or N–N interval when additional filtering is used) is used to derive heart rate variability. Methods used to detect beats include ECG, blood pressure, ballistocardiograms, and the pulse wave signal derived from a photoplethysmograph (PPG). ECG is considered the gold standard for HRV measurement because it provides a direct reflection of cardiac electric activity.

Variability Variability (or Variation) in the beat-to-beat interval is a physiological phenomenon. Power spectral analysis of the beat-to-beat variations of heart rate or the heart period (R–R interval) partitions the total variance (the "power") of a continuous series of beats into its frequency components, typically identifying two or three main peaks: Very Low Frequency (VLF) <0.04 Hz, Low Frequency (LF), 0.04–0.15 Hz, and High Frequency (HF) 0.15–0.4 Hz. The HF peak is widely believed to reflect cardiac parasympathetic nerve (PSNS) activity while the LF, although more complex, is often assumed to have a dominant sympathetic (SNS) component. Though LF was previously thought to reflect primarily SNS activity, it is now widely accepted that it reflects a mixture of both the SNS and PSNS. Decreased PSNS activity or increased SNS activity will result in reduced HRV. High frequency (HF) activity (0.15 to 0.40 Hz), especially, has been linked to PSNS activity. Activity in this range is associated with the respiratory sinus arrhythmia (RSA), a vagally mediated modulation of heart rate (which increases during inspiration and decreases during expiration). Less is known about the physiological inputs of the low frequency (LF) activity (0.04 to 0.15 Hz). The SA node receives several different inputs and the instantaneous heart rate or RR interval and its variation are the results of such inputs. The main inputs are the sympathetic and the parasympathetic nervous system (PSNS) and humoral factors. Respiration gives rise to waves in heart rate mediated primarily via the PSNS, and it is thought that the lag in the baroreceptor feedback loop may give rise to 10 second waves in heart rate (associated with Mayer waves of blood pressure), but this remains controversial. Factors that affect the input are the baroreflex, thermoregulation, hormones, sleep–wake cycle, meals, physical activity, and stress.

Clinical significance Reduced HRV has been shown to be a predictor of mortality after myocardial infarction although others have shown that the information in HRV relevant to acute myocardial infarction survival is fully contained in the mean heart rate. A range of other outcomes and conditions may also be associated with modified (usually lower) HRV, including congestive heart failure, diabetic neuropathy, post–cardiac-transplant depression, susceptibility to SIDS and poor survival in premature babies, and fatigue severity in chronic fatigue syndrome. On the other hand, for patients having high blood pressure (hypertension), higher HRV is a risk factor for atrial fibrillation.

Psychological and social aspects

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