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Ann Thorac Surg 2007;84:108-109
© 2007 The Society of Thoracic Surgeons
Division of Cardiac Surgery, Department of Surgery, University of Toronto, 200 Elizabeth St, Toronto, Ontario, Canada M5G 2C4
(Email: stephanie.brister{at}uhn.on.ca).
Whitlock and colleagues [1] give us concrete evidence to back up clinical impressions of bleeding in cardiac surgical patients. The authors report a randomized, controlled trial of perioperative warfarin management in patients undergoing cardiopulmonary bypass (CPB). The aim of the study was threefold: (1) to describe the levels of vitamin K-dependent coagulation factor (VKDCF) in patients electively discontinuing warfarin who did or did not receive vitamin K, (2) to monitor the levels of VKDCF during and after CPB, and (3) to determine the residual anti-Xa activity after preoperative administration of a low molecular weight heparin (enoxaparin).
Decreased functional levels of factors II, VII, IX, and V were found at baseline. Warfarin was discontinued 5 to 6 days before surgery, and by the day of surgery these levels of factors were approaching normal with or without administration of vitamin K. Crowther and colleagues [2] had previously reported the normalization of the international normalized ratio within 24 to 48 hours after the administration of low-dose vitamin K. Therefore the findings in this study are not unexpected. Unfortunately in this study the authors do not explore the time course for recovery of the factors between preoperative day 6 and the day of surgery. This information would be particularly helpful in determining whether vitamin K administration would be of use if warfarin cessation were delayed for 2 to 3 days.
The authors go on to report that VKDCFs declined through the period of CPB, and in fact are only approaching normal by postoperative day 5. The 50% decrease in factor levels is more than would be expected secondary to hemodilution alone. Brister and colleagues [3] have reported ongoing thrombin generation through cardiac surgery. With increasing duration of surgery, thrombin generation increased. The additional decrease of VKDCFs seen in this study may be related to consumption during surgery. Regardless of the cause, the decrease in factor levels may contribute to postoperative bleeding.
Other authors have reported an increased incidence of postoperative bleeding associated with the preoperative administration of low molecular weight heparin. Suggested times for discontinuation range from 12 to 24 hours preoperatively [4, 5]. Whitlock and colleagues [1] report that when enoxaparin was discontinued the night before surgery, residual anti-X activity was 0.6 ± 0.3 µ/mL. Similar results have been reported by O’Donnel and colleagues [6]. Therapeutic levels of anti-X activity for treatment of deep venous thrombosis are in the range of 0.4 to 0.69 IU/mL. Anti-Xa levels greater than 1 IU/mL are associated with increased bleeding in nonsurgical patients [7]. In all probability an anti-x activity level of 0.6 µ/mL would increase the risk of bleeding in a cardiac surgical patient.
In conclusion, bleeding remains a significant problem in cardiac surgery. More studies similar to that of Whitlock and colleagues [1] are required. These studies will provide the facts for a long overdue "evidence-based" approach to blood management in cardiac surgery.
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