HRV

HRV can easily be determined from ECG recordings, producing time series of RR-intervals that are usually analysed in time and frequency domains. ^[6] Metrics include time-domain indices, frequency-domain values that calculate signal energy within component bands, and non-linear measurements that quantify the unpredictability and complexity of a series of interbeat intervals. ^[1] The ease of HRV collection should not obscure the difficulty of interpretation of HRV findings, which can be easily misconstrued but can be controlled through correct methodological processes. ^[4]

Lower HRV parameter values were significant predictors of higher mortality across different ages, sex, continents, populations and recording lengths in a meta-analysis of healthy and patient populations. ^[7] In that meta-analysis, studies comparing the lowest quartile of 5-minute root mean square of successive differences (RMSSD) versus the other quartiles yielded a combined hazard ratio of 1.56 (95% CI 1.32-1.85). ^[7] Despite many efforts there are no generally accepted HRV reference values yet, though some large studies provide values that may be used for orientation purposes.