Smooth muscle relaxation and local hydraulic impedance properties of the aorta

Smooth muscle relaxation is expected to yield beneficial effects on hydraul ic impedance properties of large vessels. We investigated the effects of in travenous diltiazem infusion on aortic wall stiffness and local hydraulic i mpedance properties. In seven anesthetized, closed-chest dogs, instantaneou s cross-sectional area and pressure of the descending thoracic aorta were m easured using transesophageal echocardiography combined with acoustic quant ification and a micromanometer, respectively. Data were acquired during a v ena caval balloon inflation, both at the control condition and with diltiaz em infusion. At the operating point, diltiazem reduced blood pressure in al l dogs but did not alter aortic dimensions or wall stiffness. Over the obse rved pressure range, aortic area-pressure relationships were linear. Wherea s diltiazem affected the slope of this relationship variably (no change in 3 dogs, increase in 1 dog, decrease in 3 dogs), the zero-pressure area inte rcept was significantly increased in every case such that higher area was o bserved at any given pressure. When comparisons were made at a common level of wall stress, wall stiffness was either increased or unchanged during di ltiazem infusion. In contrast, diltiazem decreased wall stiffness in every case when comparisons were made at a common level of aortic midwall radius. Aortic characteristic impedance and pulse wave velocity, components of lef t ventricular hydraulic load that are determined by aortic elastic and geom etric properties, were affected variably. A comparison of wall stiffness at matched wall stress appears inappropriate for assessing changes in smooth muscle tone. Because of the competing effects of changes in vessel diameter and wall stiffness, smooth muscle relaxation is not necessarily accompanie d by the expected beneficial changes in local aortic hydraulic impedance. T hese results can be reconciled by recognizing that components other than va scular smooth muscle (e.g., elastin, collagen) contribute to aortic wall st iffness.