Age-associated alteration of sympatho-vagal balance in a female population assessed through the tone–entropy analysis

Through our recent studies on heart rate variability (Oida et al. J Appl Physiol 82:1794–1801, 1997; J Gerontol 54A:M219–M224, 1999a; Acta Physiol Scand 165:129–134, 1999b; Acta Physiol Scand 165:421–422, 1999c), we discover that autonomic functions could be assessed quantitatively in time domain by the tone–entropy (T–E) methodology, where the tone represents sympatho-vagal balance, and the entropy, autonomic regulatory activity. The purpose of this study was then to elucidate an age-associated alteration of sympatho-vagal balance in a female population through this T–E method. ECG R–R time intervals at rest were acquired on 10 min for 73 female subjects. Ageing influence was examined by comparisons between two groups: middle-aged group (40–50), (51.5±0.7 year, n=28) and old-aged (60–70), (69.5±0.8 year, n=45)]. Evaluated tone: [−0.058±0.011 (40–50), and 0.027±0.003 (60–70) (P<0.01)], and entropy: [3.46±0.11 (40–50), and 3.06±0.08 bit (60–70) (P<0.01)]. The result showed that the tone was high and the entropy was low in the old-aged compared with the middle-aged group. When the result was plotted in two-dimensional T–E space, it revealed a curvi–linear relation between the tone and the entropy, consistent with our previous studies on pharmacological blockades, on heart recovery after dynamic exercise and on a male ageing. In conclusion, the result suggested that the sympatho-vagal balance altered or the vagal predominance was impaired with age significantly in this female population. Interestingly, comparing with corresponding male, the female had better autonomic functions.