Heparin Pharmacokinetics in Response to Temperature Variation during Cardiopulmonary Bypass

Most patients undergoing cardiac surgery will require a heart-lung machine, or cardiopulmonary bypass (CPB). This is to allow the heart to be temporarily stopped during surgery to allow the surgeon to operate on a still, bloodless field. The heart-lung machine allows blood to circulate in the body during this time, keep the body supplied with oxygen and for carbon dioxide to be removed. During CPB, patients are often cooled to protect the organs. The 'target temperature' for cooling ranges between 28 and 34°C depending on the type of surgery and the surgeon preference. The duration of cooling and the duration of CPB are determined by the operation, not the study protocol. Patients will spend the same amount of time on CPB and cooled during their surgery regardless of whether they participate in the study. All patients receive a dose of heparin to stop blood clotting in the heart-lung machine during surgery. For this study, patients will receive 400 IU/kg based on ideal body weight. They will then be cooled to the target temperature during CPB. Once the target temperature is reached as measured by a catheter in the bladder, a further dose of 100 IU/kg based on ideal body weight will be administered, and a series of blood samples (2 mL every 6 minutes for 30 minutes) will be taken to measure heparin levels. The dose of heparin will not be altered by changes in clearance, although if the measured activated clotting time (ACT) falls below 450 seconds, a further dose of 100 IU/kg of heparin can be administered.