BAROREFLEXES AND CONGESTIVE-HEART-FAILURE

Abnormal responses are found in the early stages of heart failure with increased sympathetic and decreased parasympathetic activity, causing peripheral arteriolar vasoconstriction and tachycardia respectively. The cardiopulmonary baroreflex may be studied by decreasing venous return (''low body negative pressure'') and by measuring vascular resistance forearm. The arterial baroreflex may be studied by changing aortic pressures (by intravenous phenylephrine or nitroglycerin). Orthostatism and the tilt test deactivate the cardiopulmonary and arterial baroreflexes simultaneously. These baroreflexes are impaired in patients with heart failure. Their activation does not cause the usual sympatho-inhibition so contributing to increased sympathetic tone. This dysfunction may result from a change at any point on the reflex pathway : the baroreceptors themselves, the afferent, central and efferent pathways. It is selective as during the cold presser test, the vasoconstrictor response remains intact. One of the possible mechanisms of baroreflex dysfunction in heart failure is loss of sensitivities of the baroreceptors. This may be multifactorial : structural abnormalities, changes in compliance or functional abnormality. Even if the loss of sensitivity is partially related to a change in compliance, other factors play a role. It is more functional than structural abnormalities because, after cardiac transplantation, the baroreceptors regain their sensitivity within 2 to 3 weeks. Excessive Na-K dependant ATPase activation of the smooth muscle cells of the carotid sinus could lead to hyperpolarisation of the cell membrane, so reducing the excitability of the receptor. Aldosterone is one of the factors which could activate the Na-K ATPase, as this hormone directly increases pump activity and favorises the synthesis of new pumps in the vascular smooth muscle cells. Digitalis may resensitise the baroreceptors in heart failure and restore the baroreflex sympathetic inhibitory effect. This has a favourable effect on the neurohormonal profile in heart failure.