A Novel Correction Equation Avoids High-Magnitude Errors in Interpreting Therapeutic Drug Monitoring of Phenytoin Among Critically Ill Patients

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.

Impact of an enteral nutrition holding guideline on daily nutrition goals in patients taking phenytoin

BACKGROUND

Concomitant administration of enteral nutrition (EN) and phenytoin decreases phenytoin absorption. Concerns over impaired nutrition, however, may prevent EN from being held surrounding phenytoin administration. This study aimed to evaluate whether EN holding guidelines impacted nutrition goal achievement in patients taking phenytoin.

METHODS

Adult patients administered enteral phenytoin for acute or chronic seizures while receiving EN during a neurocritical care admission 6 months before and after EN holding guideline implementation were eligible. Patients without phenytoin concentrations or a clinical registered dietitian assessment were excluded. The primary outcome was the percentage of nutrition daily goals attained before and after implementation. Secondary end points included the incidence of hypoglycemia, differences in measured phenytoin concentrations, and rates of therapeutic (10-20 mcg/ml) and high-therapeutic (15-20 mcg/ml) concentration attainment. Concentrations were adjusted for hypoalbuminemia using the Winter-Tozer equation.

RESULTS

Fifty-five patients representing 412 patient days and 1110 phenytoin administrations were included with 29 preimplementation and 26 postimplementation patients. Median percent attainment of daily EN goals was consistent preimplementation and postimplementation (86% vs 83%, P = 0.48). No significant change in rates of days with hypoglycemia was observed. Adjusted phenytoin concentrations were similar before and after implementation (14.1 vs 15.2 mcg/ml, P = 0.45), but the preimplementation cohort had a lower proportion of high-therapeutic concentrations (23% vs 36%, P = 0.018).

CONCLUSION

Holding EN for phenytoin did not impact attainment of daily nutrition goals and was not associated with increased rates of hypoglycemia. This is the first study to evaluate the effect of EN holding on nutrition goals in patients receiving phenytoin.

Therapeutic drug monitoring of phenytoin and valproic acid in critically ill patients at Windhoek Central Hospital, Namibia

Background

Phenytoin and valproic acid, anticonvulsants, have a low therapeutic index and are highly plasma protein bound, mainly to albumin. Hypoalbuminaemia is common in critically ill patients and increases the unbound drug concentration. Thus, monitoring unbound rather than total plasma drug concentrations is recommended to optimise the dosing of these drugs.

Objective

This retrospective study determined unbound plasma concentrations of phenytoin and valproic as a more accurate value of drug levels than total plasma drug concentrations.

Methods

Total plasma concentrations were retrieved for 56 Intensive Care Unit patients for phenytoin and 93 for valproic acid. Total drug concentrations were converted to unbound concentrations using a serum albumin-based normalising equation.

Results

Total phenytoin plasma concentration was below (41.1% of patients), within (46.4%) or above (12.5%) the therapeutic range (10 μg/mL – 20 μg/mL). However, the predicted unbound plasma concentration of phenytoin was above the therapeutic range (1 μg/mL – 2 μg/mL) in the majority of patients (57.1%). For valproic acid, the total plasma concentration of most patients (87.1%) was below the therapeutic range (50 μg/mL – 100 μg/mL); among remaining patients (12.9%), it was within the therapeutic range. In the majority of patients (91.4%), the predicted unbound plasma concentration of valproic acid was between 2.5 μg/mL and 20 μg/mL.

Conclusion

The usefulness of monitoring the total phenytoin or valproic acid levels for dose optimisation is limited as it is an inaccurate indicator of a patient’s drug therapeutic state. Thus, the unbound plasma drug concentrations should be quantified experimentally or predicted in resource-limited settings.

Estimation of Free Phenytoin Concentration in Critically Ill Patients with Hypoalbuminemia: Direct Measurement vs Traditional Equations

Background

In critically ill patients with low albumin, dose individualization of phenytoin is a challenge. The currently used Sheiner–Tozer equation does not accurately predict the free phenytoin concentration in serum and can result in incorrect dose modifications. The best measure to advocate in these patients is the direct-measurement of free phenytoin concentration.

Aims and objectives

Phenytoin exhibits complex pharmacokinetics, requiring careful therapeutic drug monitoring. This study aimed to compare the accuracy of the established Sheiner–Tozer calculation method against the direct-measurement of free phenytoin concentration in serum by high performance liquid chromatography in critically ill patients with low albumin.

Materials and methods

Blood specimens for direct-measurement of both total and free phenytoin concentration were obtained from 57 patients with hypoalbuminemia monitored in the intensive care unit.

Results

The median [inter-quartile range (IQR)] for Sheiner–Tozer equation calculated total phenytoin concentration and direct-measured total was 17.14 (10.63–24.53) and 9.82 (6.02–13.85) μg mL⁻¹, respectively. Approximately 53 and 5% of patients were found to be subtherapeutic and supratherapeutic for direct-measured total phenytoin concentrations, respectively. In contrast, on applying the Sheiner–Tozer calculation, 23 and 40% had subtherapeutic and supratherapeutic concentrations, respectively, for total phenytoin concentration. The median (IQR) for direct-measured, routine and Sheiner–Tozer equation calculated free phenytoin concentration were 1.92 (1.06–2.76), 0.98 (0.60–1.39), and 1.71 (1.06–2.45) μg mL⁻¹, respectively. Only 45.7% of patients were in agreement with respect to the therapeutic category when direct-measured free was compared against routine calculation free.

Conclusion

In patients with low albumin, free phenytoin concentration based on the Sheiner–Tozer corrected equation accurately classified patients based on their therapeutic category of free phenytoin in 73.7% of patients. Hence, for individualization of phenytoin dosage in critically ill patients with low albumin, we recommend direct-measurement of free phenytoin concentration.

How to cite this article

Wilfred PM, Mathew S, Chacko B, Prabha R, Mathew BS. Estimation of Free Phenytoin Concentration in Critically Ill Patients with Hypoalbuminemia: Direct-measurement vs Traditional Equations. Indian J Crit Care Med 2022;26(6):682–687.

Seizures, Status Epilepticus, and Continuous EEG in the Intensive Care Unit

PURPOSE OF REVIEW

This article discusses the evolving definitions of seizures and status epilepticus in the critical care environment and the role of critical care EEG in both diagnosing seizure activity and serving as a predictive biomarker of clinical trajectory.

RECENT FINDINGS

Initial screening EEG has been validated as a tool to predict which patients are at risk of future seizures. However, accepted definitions of seizures and nonconvulsive status epilepticus encourage a treatment trial when the diagnosis on EEG is indeterminate because of periodic or rhythmic patterns or uncertain clinical correlation. Similarly, recent data have demonstrated the diagnostic utility of intracranial EEG in increasing the yield of seizure detection. EEG has additionally been validated as a diagnostic biomarker of covert consciousness, a predictive biomarker of cerebral ischemia and impending neurologic deterioration, and a prognostic biomarker of coma recovery and status epilepticus resolution. A recent randomized trial concluded that patients allocated to continuous EEG had no difference in mortality than those undergoing intermittent EEG but could not demonstrate whether this lack of difference was because of studying heterogeneous conditions, examining a monitoring tool rather than a therapeutic approach, or examining an outcome measure (mortality) perhaps more strongly associated with early withdrawal of life-sustaining therapy than to a sustained response to pharmacotherapy.

SUMMARY

Seizures and status epilepticus are events of synchronous hypermetabolic activity that are either discrete and intermittent or, alternatively, continuous. Seizures and status epilepticus represent the far end of a continuum of ictal-interictal patterns that include lateralized rhythmic delta activity and periodic discharges, which not only predict future seizures but may be further classified as status epilepticus on the basis of intracranial EEG monitoring or a diagnostic trial of antiseizure medication therapy. In particularly challenging cases, neuroimaging or multimodality neuromonitoring may be a useful adjunct documenting metabolic crisis. Specialized uses of EEG as a prognostic biomarker have emerged in traumatic brain injury for predicting language function and covert consciousness, cardiac arrest for predicting coma recovery, and subarachnoid hemorrhage for predicting neurologic deterioration due to delayed cerebral ischemia.

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.