Low-Molecular-Weight-Heparin as Thromboprophylaxis : a Dosage Problem in Obese Patients

Background: Prophylaxis with fixed doses of low-molecularweight heparin (LMWH) is standard procedure to reduce the risk of venous thromboembolism after surgery. Patient studies have rarely been stratified for body weight. There is evidence to suggest that lack of sufficient levels of anti-factor Xa-activity (anti-Xa) in morbidly obese patients on LMWH. Anti-Xa is used as a measurement of antithrombotic activity. The objective of the present study was to see, for obese patients, whether body weight-adjusted dosing of enoxaparin results in anti-Xa levels comparable to those of a standard fixed dose of normal weight patients. Material and Methods: Subcutaneous injections of enoxaparin 0.3 mg/kg and 0.6 mg/kg were administered to 10 morbidly obese volunteers (median body weight 127 kg). Plasma anti-Xa was measured at defined intervals for 10 hours after injection. Reference values for anti-Xa were obtained from a former study where a fixed dose of 40 mg enoxaparin was given to subjects with different body weights. Results: Body weight dosing with 0.6 mg/kg enoxaparin yields levels of anti-Xa in the same range as in normal weight patients who receive the recommended fixed dose of 40 mg. Conclusion: Body weightadjusted dosing may be considered in perioperative thromboprophylaxis with LMWH in obese patients.


Introduction
Venous thromboembolism is one of the major causes of mortality after surgery and one of the strongest patient-specific risk factors is obesity, a factor rapidly becoming more _______________________________________________________________________ ______________ Sven G. Frederiksen, Mikael Ekelund, Rickard Rothpfeffer, Ralph Peterli and Jan L. Hedenbro (2014), Journal of Research in Obesity, DOI: 10.5171/2014.282504.(Cheymol, 2000).Thus, proper dosing regimens need to be established.

Pharmacological
thromboprophylaxis is widely used, and recommendations for peak and through levels of the substitute end-point "anti-factor Xa-activity" (anti-Xa) have been given (Levine et al., 1989).Renal function and thus elimination has been taken into account in several studies, but rarely body weight/obesity.The recommendations of the Medical Products Agency in Sweden, give risk classification of the patient, renal function and patient age as the variables that indicate dose adjustments (FASS, 2013).Only recently body weight regarding standard thromboprophylaxis for the morbidly obese has been added in Swedish and American PDR (FASS, 2013, PDR, 2013).
In a previous study, we measured anti-Xa after fixed doses in patients with varying body weights, and found a negative linear correlation with body weight.Neither the peak activity nor the area-under-curve observed in the normal-weight subjects were reached in the obese, and we concluded that the recommended doses were inadequate in heavy patients (Frederiksen et al., 2003).
The objective of the present study was to test the hypothesis that, by using a weightdependent dose it would be possible to get the same anti-Xa activity in obese patients as in normal-weight subjects.

Patients and Methods
Ten patients scheduled for gastric bypass surgery were recruited after informed consent.Six were women, four were men.All invited participants concluded the study.Body weights ranged between 108 and 147 kg (median 127 kg); Body mass indices ranged between 35 and 58 kg/m 2 .No patient was planned for surgery within the next twoweek period.Each of our subjects was tested twice, the minimum interval between injections was set at five days and the order of injections was randomised.At each test a subcutaneous dose of enoxaparin (Klexane ® , Roerig, Sweden) was given.As estimated from data from our former study, (Frederiksen et al., 2003) the tested lower dose (0.3 mg/kg) corresponds to the standard dose recommendation for lowrisk patients of 20 mg, while the higher dose (0.6 mg/kg) corresponds to the 40 mg recommended standard dose for high risk patients.
Blood samples for anti-Xa were drawn before enoxaparin-injection and at one-hour intervals up to six hours post-injection, then at eight and ten hours.Samples were collected in tubes with 0.5 ml 0.129 mmol/l sodium citrate solution.Tubes were immediately centrifuged at 2000g for 20 minutes; plasma was separated and frozen at -80° C.After all subjects had been tested twice, samples were analysed in a batch using the Behring Coagulation System (Behring Diagnostica AB, Stockholm Sweden) and anti-Xa was measured using Coamatic® Heparin (Chromogenix Instrumentation Laboratory S.p.A., Milano, Italy).Some individuals had a (low) spontaneous anti-Xa at baseline, as previously described (Bendz et al., 1999).We subtracted this activity from all further samples from these individuals; incremental values were used in the further analyses.
Data were stored in Microsoft Excel ® and analysed using Graph Pad Prism 5.The study was approved by the local Ethics Committee in Lund.

Results
All ten subjects completed the study.Anti-Xa was plotted for the two different doses as a function of time for individual patients as well as for the group (summarized in figure 1).Anti-Xa peaked after approximately four hours.Peak values for 0.6 mg/kg were twice those for 0.3 mg/kg and the difference was statistically significant at all time-points.LMWHs are normally given in either of two standard doses, the choice of which is determined by assessment of patients being at normal or at high risk of thromboembolic disease.Dosing intervals have been discussed in ICU patients, but standard practice in elective abdominal surgery, is once every 24h.
The recommended doses of LMWH stem from large, well-designed studies not commenting the effect on morbidly obese patients being performed in a bariatric surgery setting (Kakkar et al., 1993, Group 1997).The proper dosage in heavy-weight patients is still not scientifically determined, although different fixed doses have been compared in the obese setting (Simone et al., 2008, Rowan et al., 2008).
Given the relative rarity of pulmonary embolism, a study comparing fixed to weightrelated dosing of LMWH, needs a substitute endpoint.Anti-Xa has been accepted as that endpoint, although questioned on correlation with thromboembolic complications in a paper on ICU-patients (Rutherford et al., 2005).Steady-state enoxaparin activity levels are well predicted by single-dose pharmacokinetics and peak and through values for desired activity are known.We have established plasma anti-Xa values for our laboratory in normal weight patients given recommended doses (Frederiksen et al., 2003).The values so obtained are taken to be the desired range.In the present set of experiments we compared the effect on plasma anti-Xa of two weightadjusted dosing regimens.In order to strengthen statistics, the study was designed to enable each patient to serve as its own control for the weight-adjusted two doses.Laboratory accuracy was enhanced by performing a batch analysis of deep frozen samples.Although the number of patients is small, differences in anti-Xa between doses are large.For all patients the 0.6 mg/kg AUCvalues fell within the reference range established for our laboratory when giving 40 mg enoxaparin once daily to normal-weight patients (fig 5).The dose of 0.3 mg/kg bodyweight for obese patients did not reach the reference values, neither for peak activity (fig 1 and 3), nor for AUC values (fig 2 and 5).
Since regimens with fixed doses every 24h are standard practice in abdominal surgery, our study could suggest that new recommendations for dosing LMWH may be indicated in obese patients.Our findings are clearly supported by a recent study on morbidly obese, medically ill patients (Rondina et al., 2010).The present data were recruited in obese patients, who were heavy for body height but also heavy in absolute terms.Further studies are indicated to see whether the implications of this study can be transferred to heavy, non-obese patients.
In summary obese patients do not reach the same level of thromboprophylaxis as normal weight subjects, if standard fixed doses of LMWHs are given, at least not as measured

Figure 1 .Figure 2 .
Figure 1.Panel A: Incremental plasma levels of anti of enoxaparin 0.6 mg/kg.after injection of enoxaparin 0.3 mg/kg.Panel C: Mean and 95% CI for values in pane and B. p> 0.001 at all time

Figure 3 .
Figure 3. Incremental plasma levels of anti of 40 mg of enoxaparin.(Non Individual values in upper diagrams, mean and 98% CI plotted in lower diagrams.