Heparin therapy for venous thromboembolism: are morbidly obese patients treated differently?

Purpose: Heparin dosing for obese patients with venous thromboembolism (VTE) poses challenges because of concern for bleeding if doses are based on body weight. The challenges and concerns are amplified in morbid obesity (body mass index (BMI) >40kg/m²)   We aimed to determine if: (1) obese and morbidly obese patients received appropriate heparin doses; (2)  the given doses were associated with delayed anticoagulation; (3) adverse events were more frequent in obese patients.

Methods: Our retrospective chart-review compared 155 non-obese (BMI<30kg/m²) patients with 91 obese (BMI=30.0-39.9 kg/m²) and 31 morbidly obese patients who presented to the emergency department with confirmed VTE.  We determined the proportion of patients who: (1) received the appropriate initial bolus (80 units/kg) and infusion (18 units/kg/hour) dose according to our protocol (for obese patients, we used an adjusted body weight-based nomogram), (2) achieved aPTT >48s within 12 hours of heparin treatment, and (3) experienced in-hospital bleeding or had VTE recurrence within 90 days.

Results: Underdosing (defined as >10% below target) for infusions occurred more frequently for non-obese patients (31%) than either obese group (obese=14%; morbidly obese=19%; P<0.01). Bolus underdosing was similarly high for non-obese (28%); but, did not differ between groups (obese=19%; morbidly obese=32%; P=0.16). In contrast, failure to achieve target aPTT was highest with morbid obesity (39%; vs. obese=10%; non-obese=19%; P<0.002). Nevertheless, bleeding (morbidly obese=3.2%; obese=1.1%; non-obese=0.6%) and VTE recurrence (3.2% vs. 3.3% vs. 2.6%) was similar (P=NS).

Conclusions: Morbidly obese patients were not treated differently; the proportion underdosed according to the adjusted body weight-based nomogram did not differ from that of obese patients.  Despite this equivalence, morbidly obese patients responded differently: exhibiting greater failure to achieve anticoagulation. Nonetheless, such delay was not associated with increased VTE recurrence.  The compromised response to heparin therapy in morbid obesity warrants further investigation to determine its clinical and practical significance.