Identifying Optimal Initial Infusion Rates for Unfractionated Heparin in Morbidly Obese Patients

Background: Most literature available for unfractionated heparin (UFH) supports the use of actual body weight for dosing all patients, yet a small proportion of the patients in these studies were morbidly obese. The most appropriate dosing strategy for therapeutic UFH in this patient population is not clearly defined. Objective: To better define appropriate UFH dosing strategies in morbidly obese patients and to evaluate the safety of a weight-based heparin nomogram in this patient population. Methods: Patients with class III (morbid) obesity receiving therapeutic doses of a UFH infusion for greater than 24 hours were evaluated. Two comparator groups of overweight/class I–II obesity and normal/underweight patients were created by matching patients to the class III obesity group. Doses and times to therapeutic activated partial thromboplastin time (aPTT), bleeding rates, and mortality were assessed. Results: The mean infusion rate required to obtain a first therapeutic aPTT was 11.5 units/kg/h in the class III obesity group (n = 94) versus 12.5 units/kgm and 13.5 units/kg/h for the overweight/class I–II obesity (n = 92) and normal/underweight (n = 87) groups, respectively (p = 0.001). The mean times to a first therapeutic aPTT were 21.3, 22.1, and 29.9 hours, respectively (p = 0.421). There was a statistically significant difference in the infusion rate required to obtain 2 consecutive therapeutic aPTTs between groups (p = 0.016), with higher weight groups requiring smaller (per kilogram actual body weight) infusion rates, but there was no significant difference in the time to reach 2 consecutive therapeutic aPTTs (p = 0.776). There was no significant difference in bleeding (p = 0.517) or mortality (p = 0.475) among groups. Conclusions: Morbidly obese patients require smaller UFH infusion rates per kilogram actual body weight compared to patients with lower body mass indices. UFH dosing recommendations should be modified to reflect body mass index classification.