Factors affecting warfarin therapy following cardiac valve surgery

OBJECTIVE: To determine the factors that affect the initial response to warfarin therapy in Korean patients after cardiac valve surgery. METHODS: A retrospective analysis of 127 patients who had undergone cardiac valve surgery at Seoul National University Hospital was performed. On the first day, most patients received warfarin 5 mg, while some received an individualized warfarin dose according to their physician's decision. Doses to be given on the following days were determined based on daily international normalized ratio (INR) and the previous doses. To measure warfarin sensitivity, the warfarin dose index (WDI), defined as the INR divided by the mean warfarin dose administered during the preceding 3 days, was introduced. The effects of age, gender, weight, serum albumin concentration, baseline INR, cardiopulmonary bypass time, and concurrent administration of amiodarone were evaluated. RESULTS: The patients' weight, initial serum albumin concentration, and baseline INR value influenced their initial response to warfarin. The initial WDI correlated negatively with the initial serum albumin concentration (p < 0.001) and body weight (p < 0.05) and positively with the baseline INR (p < 0.01). The initial WDI of the patients taking amiodarone was significantly higher (mean SD 0.74 +/- 0.34) than that of patients without amiodarone (0.46 +/- 0.22) (p < 0.001). Maintenance doses correlated negatively with the initial warfarin response (p < 0.001) and positively with body weight (p = 0,053). CONCLUSIONS: The factors associated with an increased initial warfarin response in patients after cardiac valve surgery were high baseline INR, low postoperative serum albumin concentration, and concurrent administration of amiodarone. Thus, patients with any of these factors should receive a smaller initial warfarin dose. Also, to predict the warfarin maintenance dose from the initial response, the effect of transient changes in the sensitivity to warfarin during the initial period should be considered.