Heparin pretreatment does not alter heparin requirements during cardiopulmonary bypass

Heparin infusion may cause heparin resistance and may affect monitoring by measurement of the activated coagulation time (ACT), making the assessment of anticoagulation difficult, with the risk of over- or undertreatment, esp ecially during cardiac surgery. We studied two groups of patients undergoin g cardiopulmonary bypass (CPB): patients on heparin infusions (group H) and heparin-naive controls (group C). All patients received heparin 300 IU kg( -1) before CPB and a further dose of 5000 IU if the ACT 5 min after commenc ing bypass was less than 400 s. Measurements of ACT, heparin concentration, antithrombin-3, thrombin-antithrombin complex, prothrombin fragment F1+2 a nd D-dimers were made before and 5 and 20 min after start of CPB. A second dose of heparin was given to eight out of 18 patients in group C and 10 out of 24 in group H. Antithrombin-3 in group H was significantly less than in group C at 5 min [59 (14) vs 52 (9)%, P<0.05]. ACT was significantly lower in group H than group C at 20 min [387 (64) vs 431 (67) s, P<0.05]. Despit e ACTs of less than 400 s in both groups, no coagulation was seen, suggesti ng that 300 IU kg(-1) heparin is a safe dose for anticoagulation in CPB eve n after heparin therapy.