Determinants and interindividual variation of R-R interval dynamics in healthy middle-aged subjects

Determinants and intersubject variations of fractal and complexity measures of R-R interval variability were studied in a random population of 200 hea lthy middle-aged women (age 51 +/- 6 yr) and 189 men (age 50 +/- 6 yr) duri ng controlled conditions in the supine and sitting positions. The short-ter m fractal exponent (alpha (1)) was lower in women than men in both the supi ne (1.18 +/- 0.20 vs. 1.12 +/- 0.17, P< 0.01) and sitting position (P< 0.00 1). Approximate entropy (ApEn), a measure of complexity, was higher in wome n in the sitting position (1.16 +/-17 vs. 1.07 +/-19, P<0.001), but no gend er-related differences were observed in ApEn in the supine position. Fracta l and complexity measures were not related to any other demographic, labora tory, or lifestyle factors. Intersubject variations in a fractal measure, < alpha>(1) (e.g., 1.15 +/- 0.20 in the supine position, z value 1.24, not si gnificant), and in a complexity measure, ApEn (e.g., 1.14 +/- 0.18 in the s upine position, z value 1.44, not significant), were generally smaller and more normally distributed than the variations in the traditional measures o f heart rate variability (e.g., standard deviation of R-R intervals 49 +/- 21 ms in the supine position, z value 2.53, P< 0.001). These results in a l arge random population sample show that healthy subjects express relatively little interindividual variation in the fractal and complexity measures of heart rate behavior and, unlike the traditional measures of heart rate var iability, they are not related to lifestyle, metabolic, or demographic vari ables. However, subtle gender-related differences are also present in fract al and complexity measures of heart rate behavior.