Effect of vasoactive drugs on carotid diameter in humans

We studied whether vasoactive drugs used to determine baroreflex sensitivity influence baroreceptor firing by affecting carotid sinus smooth muscle or simply by stretching the sinus wall through changes in pressure. In six young healthy subjects, the diameter of the carotid artery and its change with arterial pulse were measured with ultrasonography. Blood pressure was measured by Finapres. Phenylephrine and nitroglycerin doses were injected intravenously to raise and lower pressure by approximately 15-25 mmHg. Carotid dimensions increased in all subjects during the phenylephrine-induced rise and decreased during the nitroglycerin-induced fall in pressure. Diastolic diameter changed more than systolic diameter; changes were significantly different from the control value (assessed by single-factor analysis of variance and Scheffé's post hoc test). The systolic pressure-diameter relationship appeared to be nonlinear, with a steeper slope above than below baseline, and contributed significantly to the nonlinearity of the R-R interval-systolic pressure relationship. It is concluded that during drug-induced changes in blood pressure, baroreceptor activity in humans is influenced more by passive stretch than by local smooth muscle contraction.