Increased sensitivity to warfarin after heart valve replacement.
BACKGROUND: The dosage requirement of warfarin to achieve a given international normalized ratio (INR) often varies considerably between the immediate postoperative period and long-term follow-up in patients with prosthetic heart valves, leading to INR instability. OBJECTIVE: To document the extent of warfarin sensitivity in a prospective study of patients receiving heart valve replacements. METHODS: Clinical and laboratory data regarding anticoagulation for 111 patients who received warfarin following heart valve replacement were collected during their hospital stay (induction period) and between 1 and 3 months after surgery (follow-up period). RESULTS: Mean patient age was 65.39 +/- 10.55 years (range 29-85), with 66 men. The mean INR value during the follow-up period was, on average, 0.21 higher than the induction period (2.81 +/- 0.5 vs 2.6 +/- 0.6; p = 0.007). The mean follow-up warfarin dose was 1.54 mg higher than the mean induction warfarin dose (5.09 +/- 2.03 vs 3.55 +/- 1.94 mg; p < 0.001). The warfarin dose index, which indicates relative sensitivity of warfarin, decreased from 1.16 to 0.65 (p < 0.001). Although the INR values during the induction and follow-up periods were similar, the dose requirement in the follow-up period was, on average, 43% higher than that of the induction period. CONCLUSIONS: Immediately after heart valve replacement, patients are more sensitive to warfarin and should receive a lower warfarin dose during the initial phase of oral anticoagulation treatment. This enhanced sensitivity decreases with time. Patients require frequent monitoring and are likely to need an increase in the warfarin dose to avoid insufficient anticoagulation during the early follow-up period.