Introduction: Antithrombin is a natural anticoagulant that is consumed during cardiac operations with cardiopulmonary bypass. The aim of this observational study was to identify the factors determining antithrombin (AT) consumption during cardiac operations.
Method: 250 consecutive adults patients undergoing cardiac operations with cardiopulmonary bypass (CPB) were admitted to the study. Preoperative and intraoperative variables were tested with respect to their role in determining the AT activity at the end of the operation. No fresh frozen plasma nor purified AT were administered.
Results: With univariate analysis, 8 predictors of the AT activity at the end of the operation have been identified: preoperatively: AT activity; age; diabetes on medication; preoperative haematocrit value; preoperative dialysis; combined operation; CPB duration; lowest temperature on CPB. The multivariate model identified in our study includes five predictors of AT consumption after cardiac operation with CPB: preoperative AT activity; age; diabetes; CPB duration; combined operation. The highest significance was reached for the preoperative AT activity; age is the second independent factor determining the AT activity in the intensive care unit (ICU-AT) with a decrease of 1.5% for every decade, while, from our analysis, diabetes on medication introduces a predictable decrease of AT activity of about 6%. CPB duration is inversely correlated to ICU-AT with a predictable reduction of 3% for every hour on CPB. Combined operation carries a decrease of AT activity at the end of operation of 4% regardless of the CPB duration.
Discussion: At present, there is a great deal of information demonstrating that low levels of AT during CPB are responsible for heparin resistance and there are some reports of oxygenator failure in patients with very low AT activity levels. Conversely, despite several indications that at the end of cardiac operation with CPB many patients may experience extremely low level of AT activity there is not evidence-based information linking low levels of AT to a bad outcome. A “safe” AT activity threshold has not yet been defined during or after the operation. A postoperative target value of 50% appears empirically reasonable, and on this value we have settled the predictive tables for AT replacement. Nevertheless, we believe that it is time to design a large prospective trial aimed at determining if a low postoperative AT level is associated with a bad outcome, and if so, which complications are related to an AT deficiency and what is a “safe” postoperative AT activity level in cardiac surgery.
1 Koster A, Chew D, Kuebler W, et al. High antithrombin III levels attenuate hemostatic activation and leukocyte activation during cardiopulmonary bypass. J Thorac Cardiovasc Surg