Effects of mental load on the spectral components of heart period variability in twins.
The contribution of genetic and environmental control to stress-related cardiovascular reactions was investigated in 10 monozygotic and 10 dizygotic twin pairs during mental arithmetics. Non-invasive indices reflecting vagal and sympathetic activity were used, namely: indices of myocardial contractility based on impedance cardiogram, and spectral components of heart period variance. Autoregressive algorithms were developed for heart period power spectral density estimation providing automatic decomposition of heart period spectra into individual spectral components. During the mental task spectral energy of the mid-frequency (central frequency approximately 0.1 Hz) and high frequency (around respiratory frequency) components of heart period variance significantly decreased indicating vagal withdrawal. A task-related increase of the mid-frequency component relative to the high-frequency component was obtained. This change in the ratio of the two components as well as the considerable shortening of the contractility indices are pointing to sympathetic activation. When comparing intraclass correlations computed separately for monozygotic and dizygotic twins highly significant correlations were found for the mid-frequency component in monozygotic but not in dizygotic twin pairs in resting condition indicating a substantial genetic contribution to the control mechanisms involved in the baroreflex. Contribution of genetic factors to the control of stress-related interplay of autonomic outflows has been shown.