Sympathetic Influences on the Human Heart: Measurement, Control and Role in Hypertension

Abstract

The pathological consequences of even mild increases in blood pressure warrant treatment for hypertension in its early stages. However, chronic drug treatment programmes are generally not advantageous during the early stages of hypertension. Augmented sympathetic outflow to the heart plays a role in the early stages of hypertension, and perhaps the development of hypertension. Environmental factors are often responsible for increases in sympathetic outflow to the heart. Therefore, an alternative hypertensive treatment involves behavioural control over increases in sympathetic activity. This treatment includes biofeedback training. The literature indicates that the R-wave to ear pulse wave interval (RPI) is the most appropriate index of sympathetic influences for biofeedback training. An experiment is reported in which unconstrained normotensive subjects were asked to produce changes in RPI with and without the aid of analog feedback. Five subjects learned to produce bidirectional changes in RPI. These subjects generally showed more RPI shortening than lengthening. The data indicate that moderately heavy levels of exercise were employed to shorten RPI. This is consistent with increased sympathetic activity. Some subjects were consistently able to lengthen RPI. However, this study produced converging evidence indicating that RPI lengthening was often a product of reduced left ventricular preload. Preload influences on RPI appear to have led subjects to adopt behavioural strategies which were inconsistent with reduced sympathetic activity during attempts to lengthen RPI. Therefore, caution must be employed when using RPI to index and teach control over sympathetic activity. It is suggested that incorporating information about left ventricular ejection time or cardiac interbeat interval will improve RPI as a measure of sympathetic influences on the human heart.