Induction of anticoagulation using warfarin requires regular monitoring because patient responses vary widely. A history of heart valve replacement (HVR) presents a particular problem because of the presence of pacing wires that are routinely left in place for approximately five days postoperatively. Most patients start oral anticoagulant treatment 24 to 48 hours after surgery but, until the wires are removed, anticoagulation should be maintained at low levels, with an International Normalization Ratio (INR) of less than 2.6. Ageno and Turpie conducted a retrospective review of clinical records of patients receiving early anticoagulation treatment after HVR and a control group of nonsurgical patients. Factors that might interfere with the induction phase of anticoagulation were also reviewed.
Basic data about the course of oral anticoagulation treatment with warfarin were collected, along with initial serum albumin levels. The mean daily dose was collected for the first five days of treatment, as was mean daily prothrombin time expressed as INR. All surgical patients received low-dose heparin for prophylaxis of venous thromboembolism. Daily doses of warfarin were compared with initial serum albumin levels, patient age and body weight.
A total of 84 surgical patients and 32 non-surgical patients was included in the study. During the first five days of the study, the average daily dose of warfarin was significantly lower in the HVR group than in the nonsurgical group. The mean INR was initially higher in the HVR group (2.08) than in the nonsurgical group (1.60). Patients in the HVR group achieved the therapeutic range (1.5 to 2.6 in the HVR group and 2.0 to 3.0 in the control group) in approximately 36 hours, compared with 53 hours in the nonsurgical group. The incidence of bleeding was similar between groups, and the INR did not appear to be related to bleeding episodes. In addition, patients with lower-than-normal serum albumin levels (less than 3.5 g per dL [35 g per L]) also received significantly less warfarin than patients with normal levels.
The authors conclude that patients after HVR tend to be more sensitive to warfarin treatment than nonsurgical patients during the initial phase of anticoagulation. The tendency to need the lower dosage appears to be independent from the lower target INR and, according to other studies, is not maintained during long-term treatment. Concomitant drug use, specifically antibiotics, does not seem to be related to this phenomenon. The decrease in serum albumin levels following surgery may reduce the binding capacity of warfarin, thereby altering response. Therefore, obtaining serum albumin levels may be useful in determining when to initiate anticoagulation treatment.