Effect of continuous low-dose intravenous heparin administered during operation on postoperative measurements of antithrombin III and antifactor Xa.

A preliminary subcutaneous dose of heparin given with premedication, followed by a continuous infusion of low-dose heparin up to 2 hours after operation, was given to 20 of 40 patients undergoing major abdominal operation randomly allocated to either act as controls or receive prophylaxis. The anticoagulant effect of heparin was determined by immunologic measurements of antithrombin III (AT III) and by assay of AT III and antifactor Xa activity on postoperative days 1 and 3. Despite lowered plasma AT III levels on postoperative day 3, the patients who had received low-dose heparin had significantly increased plasma antifactor Xa activity when compared with control patients (P less than 0.05). The incidences of thrombosis within 48 hours (early deep venous thrombosis) after operation were 9 of 20 control patients and 1 of 20 patients who received prophylaxis (P less than 0.01 by Fisher's exact test). Preoperative blood samples from another group of patients undergoing major abdominal operation were examined by the thromboelastograph saline dilution test, which has previously been shown to be a predictor of the risk of early deep vein thrombosis. The patients who were considered to be at high risk were treated with low-dose heparin as before, and the low-risk patients received no prophylaxis. The high-risk patients had lower levels of AT III before operation and before heparin administration than the low-risk patients and significantly lower levels on day 3 (P less than 0.02), whereas on day 3 the high-risk patients had significantly raised levels of plasma antifactor Xa. No patient in either group developed deep vein thrombosis within 48 hours of operation.