To prevent uncontrolled activation of the hemostatic system and thrombin formation that could be triggered by cardiopulmonary bypass (CPB), unfractionated heparin (hereinafter heparin) is administered to induce profound anticoagulation.^1-3 Effective heparin monitoring is essential to avoid both over- and under-anticoagulation,⁴ and activated clotting time (ACT) is routinely used at the point-of-care for this purpose.³ Despite its well-recognized limitations, ACT remains the most practical and guideline-recommended method for intraoperative heparin monitoring.

 

Recently, Russo and colleagues published their results from a prospective observational two-center study in the Journal of Cardiothoracic and Vascular Anesthesia. The authors compared two ACT devices—Hemochron Response and Hemochron Signature Elite—before and after administration of unfractionated heparin in 35 patients undergoing on-pump cardiac surgery. The devices demonstrated poor correlation with each other. Notably, the Hemochron Signature Elite produced below-target ACT values despite anti-Xa activity exceeding 4 IU/mL following the initial heparin bolus in 18 of 35 patients (51%). Hemochron Response consistently reached target ACT values in most patients (32/35), but these did not correlate with anti-Xa levels. In contrast, ACT values measured by Hemochron Signature Elite showed a moderate correlation with anti-Xa activity (r = 0.587, p < 0.001). Based on these findings, the authors advocated for device-specific ACT targets to assess adequate heparinization during CPB.⁵

Despite the longstanding use of ACT for heparin monitoring, evidence supporting an optimal heparin management based on ACT during CPB remains limited. The ACT test was first described by Hattersley in 1966⁶ and became the preferred point-of-care test to monitor heparin anticoagulation in patients on CPB after the two landmark papers by Bull and colleagues published in 1975.^7,8 In their initial report, Bull et al. described their technique as follows: “2 ml of blood was placed in a glass tube containing 12 mg of celite. The tube was inverted once a second for the first 30 seconds to mix the content thoroughly. It was then placed on a heat block, held over a 40-watt light bulb, and rocked slowly until clotting occurred. The stopwatch was started when blood entered the tube and was stopped when the first clearly defined clot was visible.”⁷ The authors found that blood in the CPB system never tends to form even small clots with an ACT of more than 300 seconds.⁷ However, they admitted that clotting might still occur when contacts of blood and abnormal surfaces were excessive enough.⁸ In their second report, the authors defined a “safe zone” of ACT values between 300 and 600 seconds, but the authors omitted any further explanation on how they chose their optimal ACT target of 8 minutes (i.e., 480 seconds).^7,9