To determine whether heparin resistance (HR) occurring during cardiopulmonary bypass (CPB) is associated with antithrombin (antithrombin III) activity.

Retrospective review of data collected prospectively in a single center.

A tertiary referral hospital.

A total of 605 patients who underwent cardiovascular surgery with CPB, excluding emergency aortic dissection cases.

Cardiac valve and aortic surgeries were performed.

HR was defined as an activated clotting time of less than 450 seconds measured after initial heparin administration (3 mg/kg) prior to CPB. Among the 605 patients, 164 were categorized into the HR group. Multivariable analysis revealed that patients with an active infection who underwent surgery had a significantly higher risk of developing HR (odds ratio [OR]: 2.30, 95% confidence interval [CI]: 1.25-4.21, p = 0.007). Compared with isolated valve surgery, isolated aortic surgery was associated with a higher risk of HR (OR: 1.74, 95% CI: 1.12-2.71, p = 0.014). Preoperative antithrombin activity did not show a significant association with HR (OR: 1.00, 95% CI: 0.98-1.01, p = 0.786). The incidence of major composite complications was significantly higher in the heparin-resistant group (33/164, 20.1%) compared with the non–heparin-resistant group (55/441, 12.5%, p = 0.025). However, no clear differences were observed in the incidence of individual complications.

An active infection and aortic surgery were identified as risk factors for HR. Antithrombin deficiency was not a significant risk factor for HR in cardiovascular surgery; thus, preoperative antithrombin supplementation for the prevention of HR may not be necessary.