HRV

Heart rate variability (HRV) is the physiological phenomenon of variation in the time interval between consecutive heartbeats. It reflects autonomic nervous system regulation of the sinoatrial node, with parasympathetic activity (vagal tone) and sympathetic activity contributing to beat-to-beat fluctuations. Higher HRV generally indicates greater adaptability of the autonomic nervous system, whereas reduced HRV has been associated with stress, fatigue, aging, and various health conditions.

HRV can be assessed from electrocardiogram (ECG) or photoplethysmography (PPG) data. Data are analyzed in time domain, frequency domain, and non-linear methods. Common time-domain metrics derived from NN intervals (or RR intervals) include SDNN, RMSSD, and pNN50. Frequency-domain analysis estimates spectral power in low-frequency (LF, ~0.04–0.15 Hz) and high-frequency (HF, ~0.15–0.40 Hz) bands, sometimes with a total power measure; the LF/HF ratio is sometimes used but interpretation is contested. Non-linear measures such as Poincaré plot descriptors (SD1, SD2) and others provide additional information about autonomic dynamics.

Short-term HRV is typically measured for 5 minutes in a controlled posture (supine or seated) with minimal artifacts; ultra-short-term measures use shorter periods. Long-term HRV uses 24-hour recordings. Preprocessing includes artifact correction and ectopic beat removal; breathing rate and posture influence HRV and should be documented.

HRV is used in clinical risk stratification, monitoring autonomic function in cardiac patients, sleep studies, and athletic training. It is sensitive to a range of factors including age, gender, medications, caffeine, alcohol, temperature, and circadian rhythms. Limitations include sensitivity to artifacts, variable reporting standards, and the non-specific nature of HRV changes; HRV should be interpreted in context and not as a stand-alone diagnostic.