Haemodynamic consequences of arterial replacement with a synthetic graft

Because of structural differences between a synthetic graft and the natural artery, a graft would be expected to alter the haemodynamics of the vessel which is bypassed. To examine this, a segment of the lower descending thoracic aorta in six calves was removed and replaced with a Dacron graft. The gradients in pressure and flow across this segment were calculated from simultaneous measurements of pressure and flow made at both ends. From these, the longitudinal (Zl) and transverse (Zt) impedance of the segment was computed. In three animals pressure and flow were measured in the ascending aorta so that the systemic input impedance (Zi) could also be computed. Neither the magnitude nor phase of Zl was affected by the graft. However, the magnitude of Zt was approximately five times higher at all frequencies above DC following insertion of the graft. The characteristic impedance (Zo) was computed from Zl and Zt. Zo for the aortic segment remained approximately constant between 20 and 40 Pa X s X cm-3 for the first 10 harmonics. In contrast, Zo for the graft was strongly frequency dependent, and progressively increased in magnitude up through the tenth harmonic. The input impedance at the entrance to the segment (Zx) was strongly influenced by Zo. As a result, the higher frequency components of Zx were increased following insertion of the graft. In spite of these local changes in impedance, Zi was not affected by the graft. This was probably due to effective filtering by the healthy, compliant aorta left intact between the graft and the left ventricle.