The Effect of Hypothermia on Heparin Anticoagulation as Measured by Activated Clotting Time and Factor Xa Inhibition

The effect of hypothermia on heparin anticoagulation was studied using (siliceous earth) activated clotting time (ACT) measured by the modified Hattersley technique and anti-factor Xa assay measured by the modified Teien technique during cardiopulmonary bypass (CPB). Seventeen adult patients with normal blood coagulation function undergoing their first time coronary artery bypass grafting surgery or valve replacement surgery were randomly selected. Blood samples were collected 3-5 minutes after CPB initiation, before cooling at a mean temperature of 32.9°C, and then again after cooling to a mean temperature of 28.4°C. No additional heparin, citrate or packed red blood cells were added to the CPB circuit between sampling points. The mean heparin concentration measured by the anti-factor Xa assay was 4.1 ± 0.5 units/ml before cooling and 4.1 ± 0.4 units/ml after cooling. The mean ACT was 465 ± 75 seconds prior to cooling and 499 ± 69 seconds at the cold temperature (p<0.05). This suggests that the cooling process itself must induce changes in whole blood coagulation resulting in a prolongation in the ACT. The change in ACT with cooling is not due to a change in heparin concentration. The presented data supports previous research on the inadequacy of ACT to trend heparin concentration when steady state conditions are altered.