HF Power: Parasympathetic HRV from PPG
High-frequency (HF) power in HRV spectral analysis quantifies the variance of interbeat intervals in the 0.15–0.4 Hz band, corresponding to respiratory sinus arrhythmia (RSA). HF power is the primary frequency-domain marker of cardiac parasympathetic (vagal) modulation.
HF power is computed by integrating the IBI power spectral density over the 0.15–0.4 Hz band, reported in ms² (absolute power) or normalized units (nu, as percentage of total LF+HF power). Absolute HF power in healthy adults at rest typically ranges from 100–3000 ms², with high individual variability. HF power in normalized units (HFnu) provides better between-subject comparability by removing total power variability.
Respiratory sinus arrhythmia — the physiological phenomenon where heart rate increases during inspiration and decreases during expiration — is the primary source of HF power. This RSA is mediated by vagal efferent activity modulated by central respiratory drive and pulmonary stretch receptor afferents. HF power magnitude directly correlates with cardiac vagal tone as confirmed by atropine blockade studies (complete vagal blockade eliminates >90% of HF power).
For PPG-derived HF power, IBI accuracy in the HF band requires sampling rates ≥25 Hz and careful interpolation of unevenly sampled IBI series. Cubic spline interpolation with 4 Hz resampling before FFT is the standard approach. PPG HF power shows ICC = 0.80–0.90 vs. ECG HF power at rest — lower concordance than time-domain metrics due to spectral estimation sensitivity to IBI timing jitter inherent in PPG pulse detection.
Frequently Asked Questions
Why does HF power decrease with age?
Cardiac vagal tone declines with aging due to parasympathetic neural degeneration and reduced baroreflex sensitivity. HF power decreases approximately 10% per decade, with accelerated decline after age 50.
Should HF power be reported in absolute or normalized units?
Absolute units (ms²) preserve physiological magnitude. Normalized units (nu) facilitate comparison across individuals with different total power. Report both when possible. For longitudinal within-subject tracking, absolute units are preferred.
Can HF power be measured from ultra-short PPG recordings?
Reliable HF power estimation requires at least 1 minute of data (minimum 10 respiratory cycles for the 0.15–0.4 Hz band). 2-minute recordings provide acceptable accuracy (r > 0.85 vs. 5-minute standard). Below 1 minute, spectral resolution is insufficient.