The effect of extraanatomic bypass on aortic input impedance studied in open chest dogs. Should the vascular prosthesis be compliant to unload the left ventricle?

Left ventricular hypertrophy has been reported after ascending aorta-abdominal aorta bypass, despite seemingly insignificant changes in cardiac output and mean arterial pressure. Such a bypass procedure may be used for the treatment of complex coarctation of the aorta, hypoplastic aortic arch, or thoracoabdominal aortic aneurysm. To investigate the effect of the bypass procedure on left ventricular afterload, we measured aortic input impedance in six open chest dogs by placing a knitted Dacron graft from the ascending aorta to the abdominal aorta and occluding the aortic arch. Cardiac output and mean arterial pressure remained unaltered throughout the experiment, consistent with clinical reports. Systolic pressure increased by 25% of control, and the ratio of diastolic pressure-time index to tension-time index decreased by 27%. The measured input impedance was then approximated with the three-element windkessel model, which consists of resistance, compliance, and characteristic impedance (average of impedance modulus between 5 and 15 Hz). There was no change in resistance and compliance; characteristic impedance increased to 255% of control. Connecting an air chamber to the vascular prosthesis doubled the compliance and decreased the characteristic impedance nearly to the control value without altering resistance. It also reduced the systolic pressure by 14% of the bypass protocol and increased the ratio of diastolic pressure-time index to tension-time index (by 32% of control value and 82% of bypass value). Arterial systolic pressure and pulse pressure were both linearly correlated with the characteristic impedance. Thus we conclude that although ascending aorta-abdominal aorta bypass does not affect cardiac output, mean arterial pressure, resistance, or compliance, it does increase characteristic impedance. Left ventricular systolic load is directly correlated with characteristic impedance. Increased systolic wall stress might be the cause of left ventricular hypertrophy of the previously reported cases. Because decrease in the distensibility of the proximal aorta is one of the factors causing the increase in characteristic impedance, using a compliant graft might help to unload the heart.