Age and Blood Pressure Determine Vasodepressor Response to Sodium Nitroprusside

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Abstract
To examine the determinants of the vasopresser response, 22 patients, aged 33-72 yr, with uncomplicated essential hypertension were given sequential incremental i.v. sodium nitroprusside infusions of 10 min duration. Changes in mean arterial pressure (.DELTA.MAP), heart rate (.DELTA.HR), and plasma norepinephrine (.DELTA.nc) were determined at the end of each infusion period. The slopes of the dose-response curves obtained were directly proportional to predrug blood pressure (P < 0.0001) and inversely proportional to baroreflex sensitivity as measured by the slope of the .DELTA.HR vs. .DELTA.MAP relationship (P = 0.0007). Baroreflex sensitivity was inversely proportional to and approximately equally dependent on predrug blood pressure and age (P = 0.0116). The slopes of the dose-response curves were determined by predrug blood pressure and patient age. The predrug blood pressure accounted for .apprx. 75% of the variability in the slope of the dose-response curve, and age for only 25%. The age-related reduction in baroreflex sensitivity evidently is mechanistically different from the hypertension-related reduction and is of less importance in modifying homeostatic responses to vasodilation.