Short‐term heart rate variability in healthy adults

A decline in heart rate variability (HRV) is associated with increased cardiovascular morbidity and mortality 1 and may predict patient outcome in the peri-operative period 2. Heart rate variability analysis has been proposed as a diagnostic and prognostic tool for peri-operative risk stratification 1. Heart rate variability over a 5-minute period at rest in the supine position can be used to assess cardiovascular autonomic function 3, and its application in research and clinical practice remains a topic of discussion 4, 5. We attempted to determine values for short-term HRV in healthy adult subjects, as these are currently lacking 6. The standard HRV of 93 healthy subjects (18–50 years) was studied under standardised test conditions 3. R-R intervals were analysed by spectral Fast Fourier Transformation analysis. Analysis included evaluation of mean R-R intervals, SD of normal-to-normal (sinus rhythm) R-R intervals, and root mean square of successive differences between normal-to-normal R-R intervals. Very low, low, high frequency band and total spectral powers were determined from all sinus rhythm R-R intervals, together with low frequency to total low + high frequency powers. The study population had a mean (SD) age of 28 (8) years and body mass index of 22.6 (2.6) kg.m−2. Mean (SD) systolic and diastolic blood pressures were 108 (12) mmHg and 64 (8) mmHg, respectively, with a mean (SD) heart rate of 64 (10) beats.minute−1. The HRV parameters are shown in Table 1. Age was not correlated with any HRV parameters (R −0.21 to 0.27). Consistent with data in older subjects, we found significant differences between male and female subjects in the sympathetic and parasympathetic components of heart rate variability 1, 2. Larger sex differences in sympathetic parameters, such as the very low and low frequency power, have been previously reported 6, perhaps relating to greater sympathetic activity in men compared to women, in whom muscarinic regulation of HRV may be more important 7, although this hypothesis is not consistent with previous data in young and middle-aged adults 8. Our data tend to support van Hoogenhuyze's alternative hypothesis that sex differences are related to differences in the mean heart rate 9, which has a nonlinear inverse relation to the R–R interval. The significantly lower resting heart rates measured in male study subjects may therefore account for the higher power frequencies observed in males. Other possible explanations for the HRV differences measures include differences in hormone status, such as androgens or oestrogens levels, different resting levels of catecholamines or plasma cholesterol levels, but this was not within the scope of our investigation 7, 10. Interestingly, we found no age association with any of the HRV parameters. In contrast, other studies showed a decline in HRV with age 2, 6, although those studies were mainly carried out in older subjects, whilst our investigation included healthy young subjects. The present study provides further evidence of normal rasnges for short-term HRV analysis in younger adults. Future studies that focus on the practical implementation of HRV may elucidate causes of the differences between men and women.