Enhancement of thrombus formation by vascular prostheses.

The stagnation point flow experiment has been used to identify the microscopic events that occur in thrombus formation on foreign surfaces. In this investigation it was used to study the role of vascular grafts in preactivating blood, as well as the role of pretreatment of these grafts to remove the gas nuclei. The experiment was carried out under two conditions: (a) in which whole blood was led from the carotid artery of a dog to the undersurface of a microscope coverslip (control experiment) and (b) in which blood flowed from the left carotid artery of the dog through a 10-cm vascular graft to the right carotid artery and finally to the flow chamber for visualization. The thrombus morphology, time to thrombus formation and thrombus adhesiveness to the foreign surfaces were compared using knitted Dacron, woven Dacron, expanded polytetrafluoroethylene (PTFE) and saphenous vein grafts. Blood led through the vascular grafts showed a marked predisposition to thrombus formation, with no change in clotting times. Knitted Dacron most highly preactivated the blood, while PTFE showed a lesser degree of preactivation. Treating expanded PTFE so that it may be completely wetted eliminated any activation of blood and caused blood to behave as if it had not contacted a foreign surface. It was the only graft material, including autogenous vein to demonstrate this property. Therefore, of the materials tested, hydrophobic Teflon with no entrapped gas appears blood compatible.