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Ann Thorac Surg 2006;82:1573-1574
© 2006 The Society of Thoracic Surgeons
Division of Cardiac Surgery, University of Ottawa Heart Institute, 40 Ruskin Street, Suite H3204, Ottawa, Ontario, K1Y 4W7 Canada
(Email: bklam{at}ottawaheart.ca).
We read with interest the report by Jones and colleagues [1] evaluating the hemorrhagic risk of postoperative unfractionated heparin and enoxaparin after cardiac surgery. Using data collected on 2,977 consecutive cardiac surgery patients treated in Salt Lake City, Utah, the authors reported that early postoperative use of enoxaparin and unfractionated heparin was associated with a significant increase in reexploration for postoperative bleeding.
We recently reviewed the literature regarding the role of early anticoagulation after mechanical valve replacement [2], and we were enthusiastic to learn of the Salt Lake City experience. Yet given the paucity of methodological definitions, we found it difficult to interpret the study's results. The authors suggested that early postoperative anticoagulation increased the risk of bleeding, but gave no time reference to the term early. Institutionally biased, the timing of anticoagulation initiation can vary from 6 to 48 hours after mechanical valve implantation. The majority of patients in Jones and colleagues' [1] study were reexplored in the initial 3 days. However, without knowledge of the time interval between anticoagulation and reopening, it was difficult to discern whether the rate of reexploration was attributable to the anticoagulation strategy, or whether it was attributable to patient-related factors or surgery-related factors, or both. Furthermore, the dose and route of unfractionated heparin administration were not defined in the heparin group, and the specific indications for anticoagulation were not discussed.
Bleeding and cardiac tamponade are serious complications after cardiac surgery, and both have been demonstrated to occur at higher rates in patients undergoing valve surgery compared with isolated coronary bypass grafting [3–5]. Table 1 in Jones and colleagues' [1] article clearly illustrated that the heparin and enoxaparin groups underwent more complex and longer operations than the control group [1]. The proportion of redo operations among this cohort was not stated. The authors performed a Cox regression analysis to evaluate the effect of postoperative anticoagulation while controlling for the covariates of age, gender, type of preoperative and perioperative anticoagulant, warfarin, and antiplatelet therapy. However, the type and length of surgical procedure were not factored into the analysis. Although an attempt was made to factor warfarin into the analysis, we do not believe that Cox regression analysis can eliminate the confounding created by differing rates of postoperative warfarin therapy in the groups (enoxaparin patients, 25%; control patients, 12%). Given the number of differences between the groups, some controlled for and others not, we find it difficult to agree with the conclusion that the higher reexploration rate was solely attributable to enoxaparin.
Unlike Jones and colleagues [1], in our systematic review we found that the bleeding risk associated with low molecular weight heparin was less than that associated with intravenous heparin (hemorrhage risk with oral anticoagulation alone, 3.3%; oral anticoagulation with intravenous unfractionated heparin, 7.2%; oral anticoagulation with low molecular weight heparin, 4.8%) [2]. Although we may argue this study's methodology and results, we wholeheartedly agree with the authors that prospective randomized studies are necessary to fully ascertain the relative risks and benefits of different anticoagulation regimens with a variety of clinical circumstances after cardiac surgery.
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