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Anticoagulation with unfractionated heparin (UFH) after pediatric cardiac surgery can be monitored using either activated partial thromboplastin time (aPTT) or anti-factor Xa activity (anti-Xa). However, correlation of bleeding with either of these laboratory values has not been established. We sought to determine the correlation between bleeding events and aPTT and anti-Xa in patients who undergo anticoagulation after congenital heart surgery.We prospectively studied pediatric patients treated with UFH after cardiac surgery over an 11-month period. Bleeding events were prospectively assessed and adjudicated. The highest aPTT and corresponding anti-Xa for the 24 hours before bleeding events were collected to assess for association with bleeding. Statistical analysis was performed using generalized additive logistic regression.A total of 202 patients received UFH over 1488 patient-days. The median age at surgery was 0.4 years (interquartile range, 0.1-2.2). A total of 45 major or clinically relevant bleeding events were observed. The correlation between aPTT and anti-Xa was of moderate strength (R = 0.58; P < .001). The odds of bleeding increased significantly when aPTT exceeded 150 (odds ratio, 1.71 per 10-second increase in aPTT, 95% confidence interval, 1.21-2.42; P = .003). Anti-Xa was not associated with bleeding (odds ratio, 1.11 per 0.1 IU/mL increase, 95% confidence interval, 0.89-1.29; P = .34).In heparinized pediatric patients after cardiac surgery, increased risk of bleeding is more closely associated with elevated aPTT levels than elevated anti-Xa levels. In addition to anti-Xa, monitoring of aPTT levels should be considered during titration of UFH in pediatric patients after cardiac surgery.