Original Article

Factors Affecting the Activated Clotting Time

Extracorporeal Circulation Technology Program and the Division of Cardiothoracic Surgery Medical University of South Carolina Charleston, South Carolina

^* Direct communications to: Phyllis B. Kase, 171 Ashley Avenue, Charleston, SC 29425

Abstract

The activated clotting time (ACT) has become an important tool for monitoring the status of coagulation potential in patients undergoing cardiopulmonary bypass (CPB). Variability in results leads to confusion in interpretation of results. This study examined several CPB variables that may affect the ACT.

Samples from a controlled patient population were analyzed to evaluate relationships between blood gas analysis, hematocrit, total protein concentration and temperature at which ACT was performed and the ACT.

Duplicate samples were drawn for ACT determination (one in a 37 °c. heat block and one at room temperature). Concurrent blood gas analysis, hematocrit and total protein measurements were made.

There were significant differences between room temperature and heat block ACT during all stages of CPB but, at most times, a strong correlation existed (r ranged between .21 and .89). A positive correlation between post heparin ACT and hematocrit was shown, r = .48, while a negative correlation between post protamine ACT and hematocrit occurred, r=-.23.

Conditions under which the ACT is measured are very important for accuracy and it may be desirable to adjust a patient’s dose response based on the hematocrit.