Extracorporeal circulation in sheep with normal bleeding time using a surface heparinized circuit

Bleeding due to systemic heparinization represents the major side effect of extracorporeal respiratory support. In the present animal study, a surface heparinized system (Carmeda Biological Active Surface) was applied to assess the feasibility of prolonged perfusion at low circulating heparin levels. Eight sheep divided into two groups: group A (5 animals) and group B (3 animals) underwent venovenous bypass using a heparin coated surface circuit. The following protocol was used: a) 24 hours at high heparin dose (30 to 100 U/kg/hr with an ACT [activated coagulation time] three to four times normal); b) 24 hours at low heparin dose (3 to 8 U/kg/hr with an ACT within the normal range); c) 24 hours at high heparin dose. Group B animals also received fresh frozen sheep plasma (14 ml/kg/day). During Period b, the clotting times were within baseline range. The bleeding time showed a dramatic decrease after change from a to b (27.9 +/- 3 minutes vs. 10.2 +/- 5.6 minutes). There was a negative relationship between antithrombin III (AT III) and thrombin coagulase time (TC); the latter is considered to be an aspecific indicator of circulating fibrin(ogen) degradation products. Maintaining AT III over 70%, TC changes were only minor. The use of the bioactive heparin surface allowed the performance of a 24 hour bypass, with normal coagulation times, at low circulating heparin levels.