Heart rate variability measures: a fresh look at reliability
Résumé
Despite its extensive use in physiological and clinical research, the analysis of heart rate variability (HRV) is still poorly supported by sound reliability studies. The main aim of this study was to make an in-depth assessment of absolute and relative reliability of standard indexes of HRV from short-term laboratory recordings. In 39 healthy subjects (mean age (min-max): 38 (26-56) years, 18 men and 21 women) we recorded 5 + 5 min of supine ECG during spontaneous and paced (15 breaths/min) breathing. The test was repeated on the next day in the same conditions. From RR intervals we computed standard indexes of HRV: SDNN, RMSSD, LF and HF power (absolute and normalized units) and LF/HF. Absolute reliability was assessed by the 95% limits of random variation (RV); relative reliability was assessed by the intraclass correlation coefficient (ICC). The sample size needed to detect a mean difference ≥30% of the between-subject SD was also estimated. Although there was no significant mean change between the two tests, we found that in individual subjects the second measurement can be as high/low as 1.9/0.5 times (SDNN, best case) and 3.5/0.3 times (LF/HF, worst case) the first measurement, due to pure random variation. For most parameters the ICC was > 0.8 (range: 0.65-0.88). The estimated sample size ranged from 24 to 98 subjects. Reliability indexes tended to improve during paced breathing. We conclude that short-term HRV parameters are subject to large day-to-day random variations. Random error, however, represents a limited portion of between-subject variability; therefore observed differences between individuals reflect mostly differences in the subjects' error-free value rather than random error. Overall, paced breathing improves reliability.
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