Accentuated antagonism in the control of human heart rate.

Invasive animal models indicate that the accelerative effects of the sympathetic nervous system on heart rate are highly dependent on the background level of vagal activity. A noninvasive, parasympathetic chronotropic index (respiratory sinus arrhythmia) and a sympathetic chronotropic index (left ventricular ejection time) were used to evaluate autonomic control of human heart rate. A strong interaction, previously called accentuated antagonism, was found. Sympathetic heart rate effects were substantially smaller with high levels of vagal tone than with low vagal background activity. Furthermore, vagal effects became progressively stronger with increasing sympathetic background activity, demonstrating the predominance of parasympathetic control of human heart rate. This finding implies that changes in cardiac activity resulting from changes in sympathetic control cannot be interpreted accurately unless concurrent vagal activity is taken into account, as well.