LFHF

LFHF is a shorthand used in heart rate variability (HRV) analysis referring to the low-frequency to high-frequency power ratio, commonly written as LF/HF. In many studies, LFHF denotes the ratio of spectral power in the low-frequency band (LF, about 0.04–0.15 Hz) to that in the high-frequency band (HF, about 0.15–0.40 Hz). Some literature also uses LFHF to describe the two-band powers themselves, but the ratio interpretation is the more frequent context.

Measurement and calculation involve recording heartbeats, extracting the RR interval time series from an electrocardiogram or similar device, and applying spectral analysis (such as fast Fourier transform or autoregressive modeling) to estimate the power in the LF and HF bands. The LF/HF ratio is then computed by dividing the LF power by the HF power. Recordings are typically short-term (around 5 minutes) for LF/HF, though longer recordings are also used in research.

Interpretation of LFHF has evolved. Traditionally, LF was thought to reflect both sympathetic and parasympathetic activity with a contribution from baroreflexes, while HF mainly reflected parasympathetic (vagal) activity. The ratio was presented as an index of sympathovagal balance. However, many researchers caution that LF is not a pure sympathetic marker and HF is not exclusively vagal; factors such as respiration rate, breathing pattern, posture, medications, and health status can influence both bands. Consequently, LFHF should be interpreted in the context of other measures and experimental conditions rather than as a direct measure of autonomic tone.

Applications include research on stress, sleep, exercise recovery, and certain clinical conditions, where LFHF is used alongside time-domain and other HRV metrics to assess autonomic regulation.