Effect of postural stimulation on systemic hemodynamics and sympathetic nervous activity in systemic hypertension.

The contributions of the carotid sinus and cardiopulmonary baroreflexes to the interindividual variation in sympathetic nervous system activation caused by postural adaptation were indirectly assessed in 68 mild hypertensive subjects. Supine and upright plasma norepinephrine (NE), blood pressure (cuff) and cardiac output (acetylene rebreathing) were measured. Mean arterial pressure (MAP), carotid sinus pressure, stroke volume and systemic vascular resistance were calculated. Stroke volume was assumed to be proportional to the degree of stretch of cardiac mechanoreceptors, carotid sinus MAP was assumed to be proportional to carotid sinus stretch and plasma NE to reflect sympathetic nervous activity. Plasma NE correlated inversely with stroke volume (r = -0.62, p less than 10(-14] and estimated carotid sinus MAP (r = -0.33, p less than 0.0002) and positively with systemic vascular resistance (r = 0.59, p less than 10(-10]. Holding systemic vascular resistance constant by partial regression, the inverse relation between plasma NE and stroke volume remained (partial r = -0.36, p less than 0.02). Multiple linear regression yielded the equation: plasma NE (pg/ml) = 720 + 4.3 age - 5.1 stroke volume (ml) - 1.0 carotid sinus MAP (mm Hg). Substituting mean supine and upright values for stroke volume and carotid sinus MAP in this equation, it can be roughly estimated that changes in stroke volume account for as much as 60% of the postural variation in plasma NE in hypertensives, whereas only 15% of this variation is caused by changes in carotid sinus pressure. These findings suggest that cardiopulmonary baroreflexes are primary activators of the sympathetic nervous system during postural adaptation.(ABSTRACT TRUNCATED AT 250 WORDS)