Phenytoin loading doses in adult critical care patients: does current practice achieve adequate drug levels?

Evaluation of Goal Phenytoin Levels After an Initial Intravenous Loading Dose at an Academic Medical Center

BACKGROUND

Phenytoin intravenous loading doses are administered in status epilepticus to rapidly achieve therapeutic levels. Accurately assessing phenytoin levels after the initial load can be challenging because of its complex pharmacokinetic profile and nonstandardized weight-based loading doses.

OBJECTIVES

The objectives of this analysis were to determine the incidence of patients achieving goal phenytoin levels after the initial loading dose and characterize factors that contribute to achieving the goal level.

METHODS

This single-center, retrospective cohort analysis was approved by our institutional review board and included adult patients who received a phenytoin load from May 2016 to March 2021. Patients were excluded if no total phenytoin level was drawn within 24 hours of the load, if the maintenance dose was given before the first level was drawn, or if the patient was on phenytoin before the load. The major endpoint was the percentage of patients achieving a corrected goal phenytoin level of ≥10 mcg/mL after the initial load. Multivariate regression was used to determine predictors of achieving the goal phenytoin level.

RESULTS

Of the 152 patients included, 139 patients (91.4%) achieved a corrected goal level after the first load. Patients at goal received a significantly higher median weight-based loading dose (19.1 mg/kg [15.0-20.0] vs 12.6 mg/kg [10.1-15.0], P < 0.01). The multivariate analysis identified weight-based dosing as a statistically significant predictor of achieving the corrected goal level (odds ratio, 1.30; 95% CI, 1.12-1.53; P < 0.01).

CONCLUSION AND RELEVANCE

Most patients achieved a corrected goal phenytoin level after the initial load. A higher median weight-based loading dose was shown to be a predictor of achieving the goal level and should be encouraged for rapid seizure termination. Future studies are warranted to confirm patient-specific factors that affect rapid achievement of the goal phenytoin level.

The Effect of Plasma Protein Binding on the Therapeutic Monitoring of Antiseizure Medications

Epilepsy is a widely diffused neurological disorder including a heterogeneous range of syndromes with different aetiology, severity and prognosis. Pharmacological treatments are based on the use, either in mono- or in polytherapy, of antiseizure medications (ASMs), which act at different synaptic levels, generally modifying the excitatory and/or inhibitory response through different action mechanisms. To reduce the risk of adverse effects and drug interactions, ASMs levels should be closely evaluated in biological fluids performing an appropriate Therapeutic Drug Monitoring (TDM). However, many decisions in TDM are based on the determination of the total drug concentration although measurement of the free fraction, which is not bound to plasma proteins, is becoming of ever-increasing importance since it correlates better with pharmacological and toxicological effects. Aim of this work has been to review methodological aspects concerning the evaluation of the free plasmatic fraction of some ASMs, focusing on the effect and the clinical significance that drug-protein binding has in the case of widely used drugs such as valproic acid, phenytoin, perampanel and carbamazepine. Although several validated methodologies are currently available which are effective in separating and quantifying the different forms of a drug, prospective validation studies are undoubtedly needed to better correlate, in real-world clinical contexts, pharmacokinetic monitoring to clinical outcomes.

Anti-seizure prophylaxis in critically ill patients with traumatic brain injury in an intensive care unit

The objectives of this prospective observational study were to determine the proportion of patients with traumatic brain injury who received effective anti-seizure prophylaxis. The study was conducted in a tertiary level ICU of a major trauma referral centre between February 2012 and August 2013. A total of 2361 patients were admitted to the ICU in this study period, of whom125 patients (index) with traumatic head injury were included in this study. The patients had a mean age of 45 years (SD=19), a mean score on the Glasgow Coma Scale of 9 (SD=4), a mean injury severity score of 27 (SD=13) and a mean APACHE III score of 55 (SD=27). Only 13.6 % (17 of 125) of patients were given anti-seizure prophylaxis and phenytoin levels were measured in 9.6% (12 of 125). Although all 12 patients achieved an effective concentration for phenytoin therapy (>40 µmol/l) after the loading dose, no patient had their target concentration consistently maintained in the recommended therapeutic range (40 to 80 µmol/l) throughout the seven-day monitoring period. There was wide fluctuation in phenytoin levels in the patients in this study. Twenty-two (18%) of the index patients had post-traumatic seizures, indicating a high prevalence for this study. Poor compliance with guidelines could possibly explain this phenomenon. Future studies are needed to look at the dosing and monitoring of phenytoin and/or alternative anti-seizure prophylaxis in patients with traumatic brain injury.