Mechanics of the Aorta In Vivo

The hydraulic properties of the unexposed thoracic aorta were studied in five cats by angiography. Repeated measurements of pressure and diameter were obtained at various distances from the aortic root, and it became obvious that the unexposed aorta in cats was more than twice as distensible as the exposed aorta. Diameter of the aortic root, for example, varied by about 20% over a physiologic pulse pressure (40 mm Hg); even in the lower thoracic aorta variations exceeded 15%. When plotted as a function of distance from the root, aortic diameter and distensibility both decreased, and the calculated impedance increased. Despite these changes, however, the calculated reflection coefficients were quite small. It was concluded that geometric and elastic inhomogeneities in the thoracic aorta tend to balance one another, keeping the reflection coefficient quite low and helping to "decouple" the oscillatory left ventricular pump from the static peripheral vasculature. Since the aortic pulse contour is reflection-free during the first 50 msec following the onset of ventricular ejection, it may reliably indicate left ventricular properties and their changes with stress. In several animals, norepinephrine infusion was used to elevate the aortic pressure to hypertensive levels. When compared to hypertension induced by carotid reflex stimulation or mechanical occlusion of the aorta, however, aortic wall properties were essentially the same, suggesting that this agent has no important direct effect on the aortic wall.