Autonomic nervous system function.

An age-related increase in autonomic nervous system activity, as reflected by increased plasma norepinephrine levels, has been reported by a number of investigators. This increase in plasma norepinephrine levels cannot be solely explained by increased obesity and decreased norepinephrine clearance with ageing. It has also been suggested that enhanced autonomic nervous system (sympathetic) activity may play a role in the pathogenesis of essential hypertension. However, recent studies from several laboratories have demonstrated that plasma norepinephrine levels increase with age in normotensives but not in patients with essential hypertension. A recent study from our laboratory has demonstrated no difference in basal or cardiopulmonary baroreflex-mediated increases in plasma norepinephrine levels in older hypertensives compared with middle-aged hypertensives. These observations suggest that elevated norepinephrine levels, per se, are not sufficient to explain the high prevalence of hypertension in the elderly, who are otherwise normal. In fact, it is likely that high norepinephrine levels in the elderly may be due to factors other than a primary hyperadrenergic state. Although arterial baroreceptor sensitivity decreases with ageing, studies from our laboratory indicate that cardiopulmonary baroreflex control of vascular resistance is relatively well preserved in older subjects with mild-to-moderate established essential hypertension. Thus, the relative influence of altered baroreflex sensitivity with ageing, or of a disturbed sympathetic outflow affecting autonomic nervous system activity and leading to variable blood pressure in the elderly, remains to be determined.