Abnormalities of autonomic nervous control in human hypertension

Abstract

The pathophysiology of various stages of hypertension is different. In early hyperkinetic borderline hypertension, the sympathetic drive to the heart and blood vessels is increased while the parasympathetic cardiac inhibition is decreased. The elevated cardiac output, vascular resistance, and blood pressure at that stage can be fully normalized by autonomic blockade. As hypertension advances, a hyperkinetic circulation is less evident, since beta-adrenergic responsiveness and cardiac compliance tend to decrease. Simultaneously hypertrophy of the resistance vessels increases the baseline vascular resistance and the vessels' responsiveness to constrictive stimuli. Eventually a picture of a normal cardiac output/high vascular resistance typical for established essential hypertension emerges. As the blood vessels become hyperreactive, the same degree of vasoconstriction/blood pressure elevation can be achieved with less sympathetic tone. In that phase the sympathetic overactivity is less evident, as the brain resets itself to maintain the same blood pressure elevation with a small amount of sympathetic discharge. While sympathetic overactivity may be less evident in established hypertension, it remains an important pathophysiologic factor, not only for the maintenance of blood pressure, but also for a number of other abnormalities in hypertension. Hypertension is intimately associated with higher levels of pressure-unrelated risk for development of atherosclerosis: dyslipidemia, overweight, and hyperinsulinemia. Furthermore, a number of factors in hypertension favor a poorer outcome from coronary heart disease. These pressure-independent factors increase the risk of coronary thrombosis, arrhythmic deaths, and coronary spasms. Sympathetic overreactivity appears to be crucially implicated in the evolution of this added coronary risk in hypertension. Understanding the pathophysiology of coronary risk and its relationship to sympathetic overreactivity in hypertension is helpful in seeking further improvements in clinical practice. At present antihypertensive treatment is less efficacious in reducing coronary events in hypertension than would be expected. Judicious use of appropriate drugs promises to further improve the efficacy of antihypertensive treatment in those patients who, in addition to high blood pressure, also have other associated risk factors.