Intravenous phenytoin: a loading scheme for desired concentrations

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

Phenytoin is regularly employed in the critically ill for prophylaxis against or treatment of seizure disorders. No prior studies have examined current dosing practices in an Australasian intensive care unit (ICU) setting. The aims of this study were to: a) describe the adequacy of contemporary dosing in respect to free and total serum phenytoin concentrations; b) identify factors associated with therapeutic drug concentrations; and c) examine the accuracy of predictive equations that estimate free concentrations in this setting. All patients receiving a loading dose of phenytoin in a tertiary-level ICU were eligible for enrolment; 53 patients were enrolled in the study. Serum samples to determine free and total phenytoin concentrations (measured by high performance liquid chromatography) were then drawn prior to the following dose. Free concentrations below the recommended target (<1 mg/l) were considered as suboptimal. The most common indication for phenytoin loading was traumatic brain injury (49%) and the mean administered dose was 14.5 (3.66) mg/kg. Twenty-six patients (49%) had suboptimal trough free concentrations, although this subgroup was significantly heavier and therefore received a lower per kilogram dose (12.8 [3.1] vs 16.3 [3.4] mg/kg, P=0.001). In multivariate analysis, larger weight adjusted doses (P=0.018), higher albumin concentration (P=0.034) and receiving phenytoin for an indication other than seizure (P=0.035), were associated with a greater likelihood of adequate concentrations. In conclusion, phenytoin dosing remains complex in critically ill patients, although lower per kilogram loading doses are strongly associated with free concentrations below the desired target.

Rapid i.v. loading with phenytoin with subsequent dose adaptation using non-steady-state serum levels and a Bayesian forecasting computer program to predict maintenance doses

OBJECTIVE

To evaluate the suitability of a phenytoin loading dose regimen; to assess whether dose-individualization was necessary and to investigate the reliability of a Bayesian forecasting method for phenytoin dose adaptation using non-steady-state levels in hospital-admitted patients.

METHOD

An initial loading dose (15 mg phenytoin acid/kg BW) was given i.v. over 4 h, followed by standardized maintenance doses given i.v. in 12-h intervals from days 1 to 5 (175 mg </= 70 kg; 202 mg > 70 kg BW). The evening dose of day 5 was individualized based on three serum trough levels: L1 (after 16 h), L2 (morning day 4) and L3 (morning day 5).

RESULTS

Ninety of 136 consecutive patients were evaluable in a prospective study for the standardized phase; 50 of them had additional serum levels in the individualized phase. There was no exclusion of patients with interacting co-medication. Seventy-seven per cent (L1) and 68% (L3) of patients showed therapeutic values (10-20 mg/L). The prediction error of the forecasting was 3.95 mg/L, the root mean squared error 6.27 mg/L (target trough level 11 mg/L). Seventy per cent of the levels (n=50) were within the 68% confidence interval.

CONCLUSION

The effectiveness and safety of the regimen with rapid i.v. loading and the necessity to individualize phenytoin dosing after day 5 were demonstrated.

Phenytoin reduces suxamethonium-induced myalgia

A prospective, randomised trial was undertaken in 60 healthy adults to determine the efficacy of intravenously administered phenytoin in doses of 5 mg.kg-1 for the prevention of suxamethonium-induced fasciculations, a rise in serum K+ and myalgia. This was compared with tubocurarine pretreatment and no pretreatment (control group). Phenytoin pretreatment significantly reduced myalgia from 45% (nine patients) in the control group to 10% (two patients) (p less than 0.05). It also decreased the duration and mean intensity of fasciculations. Incidentally, phenytoin was also found to decrease significantly mean serum Na+ levels (p less than 0.001) both at 5 and 20 min after administration. Tubocurarine pretreatment (3 mg) resulted in a significant decrease in fasciculations, but myalgia, which occurred in five patients, remained the same. No significant correlation was found between muscle fasciculations, postoperative myalgia and K+ changes, but patients with myalgia had a significant decrease in mean serum Na+ levels at 5 and 20 min after suxamethonium (p less than 0.01).

Evaluation of population (NONMEM) pharmacokinetic parameter estimates

The application of population pharmacokinetic analysis has received increasing attention in the last few years. The main goal of this report is to make investigators aware of the necessity of independent evaluation of the results obtained from a population analysis based on observational studies. We also describe with the help of a specific example (a new synthetic opiate Alfentanil) how such evaluation can be performed for parameter estimates obtained with the software system NONMEM. The method differs depending on the type of serum concentration data that are used for the evaluation. A general method is described, based on the regression model used in NONMEM, that can test for bias in the estimates of fixed and random effects independent of the number of observations per patient and dosing. Since the procedure for testing for statistically significant bias in the prediction of the average concentration and its variability can be relatively complex, we propose that generally available program packages performing estimation of the pharmacokinetic parameters from observational data should contain the necessary software to evaluate the reliability of the parameter estimates on a second data set.