Mechanical durability of pulmonary allograft conduits at systemic pressure. Angiographic and histologic study in lambs.

We examined the mechanical durability of cryopreserved pulmonary allograft conduits at systemic pressure in lambs. Composite valveless tube grafts made of cryopreserved pulmonary allograft and a length of Dacron tube were implanted in the thoracic aortic position in 10 lambs (aged 1 month, mean weight 11.4 kg). The pulmonary allografts were harvested from similar-sized lambs (aged 1 month, mean weight 10.5 kg) at 0 to 48 hours postmortem and were cryopreserved. Recipient sheep were catheterized at 1 week (baseline) and at 1, 4, 6, 9, and 12 months postoperatively to allow changes in the diameters of the grafts to be followed up by angiography. Samples of the grafts were examined histologically at the time of harvest, at implantation after cryopreservation and thawing, and at the end of the study. At the postoperative baseline study, the pulmonary allografts had almost doubled (mean 21.1 mm) their implantation diameters (unpressurized mean 12.6 mm). During the 12-month study period, the allografts further increased their angiographic diameter by 36.2% compared with baseline (p < 0.01). One animal died at 3 1/2 months postoperatively and was found to have an aneurysm of the allograft. Five of the remaining eight lambs showed aneurysmal dilatation (more than 40% increase in diameter relative to pressurized baseline) at 12 months. Postmortem study revealed disruption at the suture line between allograft and Dacron in these five recipients. Histologic study showed much less calcification than was seen in our previous study of aortic allografts. There was loss of most of the smooth muscle, intimal and adventitial fibrous proliferation, and occasional dense lymphocytic infiltrates associated with foci of persistent smooth muscle cells. Time of harvest after donor death did not affect the histologic appearance after 12 months' implantation. Pulmonary allografts dilate substantially at systemic pressure. This characteristic has the benefit of allowing progressive enlargement with time as a recipient grows but also carries the risk of aneurysm formation, particularly pseudoaneurysm at the suture line when anastomosed with a noncompliant Dacron prosthesis.