The recently recommended activated clotting time (ACT) to be maintained at the initiation of and during cardiopulmonary bypass (CPB) is ≥480 s. However, the post-unfractionated heparin (UFH) administration ACT occasionally does not exceed 480 s. Therefore, in this study, we retrospectively evaluated the factors influencing post-heparin administration ACT before initiating CPB.

In this retrospective study, patients aged <7 years who had undergone open-heart surgery with CPB between August 2021 and June 2023 were investigated. Those who lacked preoperative data or received antithrombin or fresh frozen plasma preparations prior to undergoing CPB were excluded. Multiple regression analysis was performed using the initial ACT as the dependent variable and preoperative covariates as independent variables.

This retrospective study included 91 patients. The median age of the patients was 265 (interquartile range [IQR]: 127–750) days. The median initial ACT was 589 (IQR: 506–713) s. In 17 (19%) patients, the initial ACT was <480 s. Multiple regression analysis revealed a statistically significant association between the platelet count and initial ACT, with a regression coefficient of −5.26 (95% confidence interval [−8.56 to −1.95]) and standard regression coefficient of −0.39 (p = .002).

A high preoperative platelet count was associated with a reduced heparin response. Nevertheless, the platelet count cannot solely elucidate the heparin response, and further investigations are required to determine the predictive factors affecting this response.