Heart Rate Variability (HRV) from PPG Signals
Heart Rate Variability (HRV) quantifies the variation in time between successive heartbeats. PPG-derived HRV uses interbeat intervals (IBI) extracted from pulse peaks as a surrogate for ECG R-R intervals. HRV reflects autonomic nervous system balance and is used as a biomarker for stress, recovery, and cardiovascular health.
HRV analysis methods fall into three domains. Time-domain metrics include SDNN (standard deviation of all IBI), RMSSD (root mean square of successive differences — a vagal tone proxy), and pNN50 (percentage of successive IBIs differing by >50 ms). Frequency-domain metrics decompose IBI variability into very low frequency (VLF: 0.003–0.04 Hz), low frequency (LF: 0.04–0.15 Hz), and high frequency (HF: 0.15–0.4 Hz) bands. The LF/HF ratio is widely used as a sympathovagal balance index, though its interpretation remains debated. Nonlinear metrics including Poincaré plots (SD1, SD2) and detrended fluctuation analysis (DFA α1) capture complexity dimensions not captured by linear methods.
PPG-derived HRV is highly concordant with ECG-HRV for time-domain metrics like RMSSD when signal quality is high (Pearson r > 0.95 in controlled settings). Frequency-domain metrics are more sensitive to IBI interpolation artifacts. The TRIPOD guidelines for HRV analysis recommend flagging any 5-minute recording segment where >5% of IBIs are interpolated or corrected.
Consumer wearables now incorporate optical HRV monitoring as a recovery and stress index. Garmin's Body Battery, WHOOP's Recovery Score, and Oura's Readiness Score all incorporate HRV as a primary input. Validation studies show wearable HRV RMSSD correlates well with ECG-RMSSD (r = 0.88–0.97) at rest but degrades significantly during sleep movement and daytime activity.
Frequently Asked Questions
How long should a HRV recording be for valid analysis?
Standard guidelines recommend 5-minute short-term recordings for frequency-domain HRV and 24-hour Holter recordings for long-term metrics. Ultra-short (1-minute) recordings can estimate RMSSD with reasonable accuracy.
What is a good HRV score?
HRV is highly individual and age-dependent. Rather than absolute values, tracking trends relative to your baseline is more clinically meaningful. RMSSD typically ranges from 20–100 ms in healthy adults.
Does PPG-based HRV work during sleep?
PPG HRV during sleep is reasonably accurate during light and REM sleep when movement is minimal. Deep sleep measurements are most reliable due to low motion artifact.