Increased sensitivity to warfarin for valve patients

By: Alere Staff
Publication Date: Tue, 11/01/2011

Previous studies have shown that patients emerging from surgery have an increased sensitivity to warfarin. Mechanical heart valve patients have shown to benefit from lower initial doses of warfarin therapy. A February 2006 published study by Rahman et al. found that initiating patients at 5mg for the first two days reduced the risk of bleeding.1

The initial Rahman study evaluated 111 mechanical heart valve patients on no other known competitive medications (amiodarone, antibiotics). The average length of hospital stay for patients was 9 days (range: 7-15). The goal of the study was to rapidly and safely reach therapeutic levels of protection.

An ongoing study by the Royal Hobart Hospital (Tasmania, Australia) is taking the Rahman study protocol one step further initiating patients with 3 milligrams of warfarin for the first two days following mechanical heart valve surgery. The Australian study evaluated 127 patients. Despite 30% of the study patients on amiodarone (notorious for prolonging International Normalized Ratio [INR] test results) and 44% of patients on antibiotics, patients remained in target range of 100% of the time (INR < 3.5). It remains to be seen what affect amiodarone and or antibiotics played on assisting the lower initiation dose of warfarin.

The two differences between the studies were concomitant interacting medications used and initiating doses. The 5mg initiating protocol resulted in 25% of patients with an INR result higher than 4.0. Rahman’s study removed competing medications to isolate warfarin and to isolate patient’s sensitivity exclusively to warfarin.

Despite the improved time in range for the 3mg initiation group, the average length of stay was 9 days for both groups. The use of competing, concomitant medications in the 3mg cohort found only 5% of their patients with INR test results over 4.0 (INR).

  1. Rahman, M, BinEsmael, TM, Payne, N, Butchart, EG. Increased sensitivity to warfarin after heart valve replacement. Ann Pharmacother. 2006 Mar;40(3):397-401. Epub 2006 Feb 28.