Borderline hypertension, a condition in which the blood pressure oscillates between normal and high values, is a predictor of furture more severe hypertension. Pathophysiologically, borderline hypertension is different from established hypertension. A large proportion of such patients have elevated cardiac output and a normal vascular resistance. In established hypertension, the output is normal and resistance is elevated. The elevation of cardiac output in borderline hypertension is neurognic; it can be abolished by an autonomic blockade of the heart. In addition to an increased cardiac sympathetic drive, increased sympathetic tone to the kidney, arterioles, and viens has also been found. In parallel with the hypersympathetic state, patients with borderline hypertension also show decreased parasympathetic tone. The enhanced sympathetic tone leads to a decreased cardiac responsiveness, and eventually, the cardiac output returns to the normal range. High blood pressure causes vascular hypertrophy, and hypertrophic vessels are hyperresponsive to vasoconstriction. These secondary changes in the responsiveness of the heart and blood vessels are the basis of transition from a high cardiac output to high-resistance hypertension. These hemodynamic changes are associated with a downregulation of the sympathetic tone. A picture of an apparently nonneurogenic high-resistance hypertension emerges. Nevertheless, when assessed in regard to the enhanced pressor resposiveness, the sympathetic drive in such patients is still excessive. Despite the apparently normal tone, the sympathetic nervous system continues to play an important pathophysiological role in established hypertension. Borderline hypertension is associated with numerous metabolic abnormalities including obesity and insulin resistance. It is tempting to view all these abnormalities as a common expression of the increased sympathetic drive in hypertension. Explanation of the basis of the association of hypertension and metabolic abnoralities promises to bring new insights into the pathophysiology of two common diseases of civilization: hypertension and diabetes mellitus.
