The use of nitroprusside to characterize aortic pressure-diameter relationships

The viscoelastic properties of the wall of the ascending aorta can be determined by calculating the relationship between pressure and diameter of the vessel. Because of potential nonlinearities in aortic pressure-diameter relationships, however, pressure-diameter curves are more accurate than compliance expressed as a single value in measuring aortic viscoelastic properties. To determine whether nitroprusside could be used to obtain aortic pressure-diameter curves over a wide range, we measured simultaneous aortic pressure and diameter in anesthetized dogs. The inferior vena cava (IVC) of each animal was briefly occluded to generate a baseline series of pressure-diameter points over a wide range of pressure and diameter. We found that moderate lowering of systolic arterial pressure (30 mmHg) with nitroprusside did not significantly affect the aortic pressure-diameter relationship in comparison with control measurements during brief IVC occlusions at similar pressures and diameters. Prolonged inferior vena caval occlusion and a more profound lowering of arterial pressure with nitroprusside or IVC occlusion resulted in a leftward and upward shift of the aortic pressure-diameter relationship, with higher pressures at comparable diameters and lower diameters at comparable pressures. However, with more profound changes in arterial pressure, possible reflex-mediated mechanisms that alter the baseline aortic pressure-diameter relationship may be activated. We conclude that nitroprusside can be used to obtain aortic pressure-diameter data over a wider range than that possible from a single cardiac cycle.