Warfarin is a drug used to stop blood clotting as quickly as normal. It is used to prevent blood clots after certain types of cardiac surgery, such as mechanical valve replacement, mitral valve repair and tissue mitral valve replacement. It is also used to reduce the risk of stroke in patients suffering from atrial fibrillation, a condition where the heart beats irregularly. Patients who take warfarin need regular blood tests to see how quickly their blood clots; this test measures their International Normalised Ratio (INR). Dosing is then adjusted to keep the INR within a safe, but effective, target range. The initial dose of warfarin is based on patient age, weight and risk factors for sensitivity.
Patients who start warfarin after heart surgery appear to be more sensitive to warfarin than to those who have not had heart surgery. We do not know why this is. Despite using lower doses of warfarin and more frequent monitoring, the increase in INR is still very unpredictable in these patients.
When the INR exceeds 4.5, the risk of major bleeding increases considerably, and discharge may be delayed if the INR is unstable. This study is looking to find out the reasons why this patient group are more sensitive to warfarin by examining the relationship between warfarin sensitivity and albumin levels, an inflammatory protein called C-Reactive Protein (CRP), heart function (ejection fraction and cumulative inotrope dose) and interacting drugs. This will help us to improve the way we use warfarin by highlighting those patients who may be more sensitive to the effects of warfarin.