Phenytoin dosing in obese patients: two case reports

IV fosphenytoin in obese patients: Dosing strategies, safety, and efficacy

Background

Previous studies evaluated the disposition of IV phenytoin loading doses and found that obese patients had increased drug distribution into excess body weight, larger volumes of distribution, and longer half-lives when compared to their nonobese counterparts. We assess the safety and efficacy of fosphenytoin loading doses in patients with different body mass indices (BMIs).

Methods

A retrospective chart review was conducted in 410 patients who received fosphenytoin. Patients were divided into 2 groups: BMI <30 (nonobese) and BMI ≥30 (obese). Patient demographics, fosphenytoin dose administered in mg/kg body weight, renal and liver function tests, fosphenytoin drug levels, and pre- and post-fosphenytoin administration vital signs were collected to assess for adverse events. Necessity of additional antiepileptic loading doses was used as a surrogate for clinical efficacy.

Results

The median dose of fosphenytoin administered was 19 mg/kg (interquartile range 15-20). The most frequently encountered adverse event was hypotension, which occurred in 39% of the cohort. Using a Bonferroni adjustment for multiple comparisons, there were no differences in adverse events between the 2 groups. The need for additional antiepileptic loading doses was not different between the 2 groups (p = 0.07).

Conclusions

The incidence of adverse events and the need for repeat loading antiepileptic medications was similar between the 2 groups. From our findings, the patients in our study did not receive empiric loading dose adjustments and the current method of loading fosphenytoin achieves similar outcomes, regardless of the patient's BMI.

Resuscitation of the morbidly obese patient

Obesity is a major health care problem in the United States. The body mass index (BMI) is the standard measure of obesity. A BMI >25 kg/m2 is defined as overweight and obesity as a BMI > 30 kg/m2. Recent surveys indicate that 54% of adults, or roughly 97 million people, are overweight. Given the incidence of obesity in the general population, it is likely that EM physicians will be involved in the emergency care of critically ill or injured obese patients. The objective of this article is to present the clinical problems associated with the resuscitation of the critically ill or injured obese patient and their potential solutions.

Phenobarbital pharmacokinetics in obesity. A case report

A morbidly obese woman [190 kg total bodyweight (TBW)] was admitted to hospital with a rapidly progressing wound infection. Over the next 2 weeks the patient developed congestive heart failure, acute renal failure, septic shock and multiple seizure episodes. Intravenous phenobarbital was added to phenytoin therapy to achieve seizure control. A total loading dose of phenobarbital 3700 mg (19.5 mg/kg TBW) was administered in 3 divided doses. The initial dose of 1100 mg resulted in a serum phenobarbital concentration of 6.3 mg/L 5h postinfusion, a second 1100 mg dose increased the concentration to 13.1 mg/L 1h postinfusion and a final dose of 1500 mg resulted in a 22.5 mg/L concentration at the end of the infusion. A phenobarbital maintenance regimen of 120 mg every 12h was then started. Peak serum concentrations of 19.8 and 17.8 mg/L were measured. All of the available serum phenobarbital concentrations and dosage amounts were fitted with least-squares nonlinear regression analysis to a 1-compartment model. An apparent volume of distribution (Vd) of 154.9L (0.82 L/kg TBW), total body clearance (CL) of 29 ml/min (1.74 L/h) and elimination half-life of 61h were determined. Our case report suggests that the dose of intravenous phenobarbital should be calculated using TBW. Additional studies are needed to precisely define the appropriate dosage weight, serum concentrations and clinical efficacy associated with intravenous phenobarbital in morbidly obese patients.