Safety and Efficacy of Lean Body Weight-based IV Heparin Dosing in Obese/Morbidly Obese Patients

Overview

Standard weight-based IV heparin for normal weight patients is based on actual body weight (ABW). However, no well-defined guidelines have been established for patients considered to be obese or morbidly obese. In current practice, the calculated ABW based heparin initial bolus dose and infusion rates are quite high, and therefore often not used for obese/morbidly obese patients for fear of bleeding. Heparin is distributed in the body approximately the same as blood and does not get distributed to adipose tissue. There are some studies suggesting that lean body weight (LBW) might be a better basis for dosing heparin. LBW is a calculated weight that excludes the weight of fat. The investigators hypothesize that intravenous heparin dosing based on the Lean body weight of obese/morbidly obese patients would be safe and effective in achieving a therapeutic level of heparin in 24 hours compared to the usual practice in this patient population.

Unfractionated Heparin has been used as an anticoagulant in various disease states and conditions, such as atrial fibrillation, deep vein thrombosis (DVT), pulmonary embolism (PE), myocardial infarction, acute coronary syndrome, and peripheral vascular disease. Prior studies have demonstrated that a weight-based heparin (WBH) dosing regimen is superior to a fixed dosing regimen (e.g. 5000 units bolus and 1000 units/hour) in achieving a therapeutic activated partial thromboplastin time (aPTT) more quickly and with fewer complications (1,2). The importance of achieving anticoagulation to a targeted aPTT range within 24 hours after starting intravenous heparin infusion has demonstrated a therapeutic benefit (1,3,30-31). WBH dosing is now the standard of care for intravenous heparin therapy. Prior studies have assessed the benefit of WBH utilizing the patients' actual body weight (ABW), but have not significantly considered the issue of dosing and titrating WBH in the obese and morbidly obese (O/MO) patient population (1,16-17, 32). There are few studies that have examined the effect of obesity in heparin dosing, and there are wide variations in how obesity is defined and how (if at all) WBH nomograms based on ABW are adjusted in this patient population (4,12,16-18, 23, 25). In addition, there are but a few case reports addressing WBH-dosing in the O/MO patient population according to a more modern definition of obesity. These limitations make it difficult to apply in the clinical setting (16-18, 21-28). In current practice, ABW-based heparin bolus doses and infusion rates are quite high and therefore often not used for O/MO patients due to fears of over-anticoagulation and of serious bleeding. Studies have in fact reported a higher potential of supratherapeutic aPTT in the bariatrically-challenged population (19, 23-24, 26-27). Therefore, it remains uncertain as to how to optimize intravenous heparin dosing in these patients. The volume of distribution (Vd) of heparin is similar to blood volume and adipose tissue has little vasculature to account for heparin's distribution (5-11). LBW has recently been proposed to be a better metric for dosing heparin in obese/morbidly obese patients (14-15,20,29,34). The investigators hypothesize that intravenous heparin (IVH) dosing based on LBW of O/MO patients would be safe and effective in achieving aPTT within 24 hours compared to the usual practice in this patient population. In addition, the investigators postulate that it would be cost-effective since it would reduce the number of blood draws and laboratory tests (13). This prospective study will involve the departments of emergency medicine, internal medicine (sections of cardiology and critical care), laboratory, nursing, and pharmacy. Those patients who are O/MO according to 2004 World Health Organization (WHO) definition (BMI \> 30) (33) and have an IVH order will be enrolled to receive heparin via a LBW-based nomogram. As a comparison, the investigators will also have the calculated initial bolus and infusion doses that would have been given in a traditional ABW-based treatment algorithm. A signed consent form from the patients will be obtained in order to use the LBW-based heparin nomogram. In accordance with the standard weight-based heparin protocol and 2008 CHEST guidelines, the modified version of current Mercy Nazareth Hospital (MNH) weight-based heparin protocol will be followed in all other aspects beyond the dosing nomogram. An aPTT tracking form and data collection tool will be utilized to monitor and gather results. References on File - provided upon request.