Monitoring of pentobarbital plasma levels in critical care patients suffering from increased intracranial pressure

Principles of Pharmacotherapy of Seizures and Status Epilepticus

Status epilepticus is a neurological emergency with an outcome that is highly associated with the initial pharmacotherapy management that must be administered in a timely fashion. Beyond first-line therapy of status epilepticus, treatment is not guided by robust evidence. Optimal pharmacotherapy selection for individual patients is essential in the management of seizures and status epilepticus with careful evaluation of pharmacokinetic and pharmacodynamic factors. With the addition of newer antiseizure agents to the market, understanding their role in the management of status epilepticus is critical. Etiology-guided therapy should be considered in certain patients with drug-induced seizures, alcohol withdrawal, or autoimmune encephalitis. Some patient populations warrant special consideration, such as pediatric, pregnant, elderly, and the critically ill. Seizure prophylaxis is indicated in select patients with acute neurological injury and should be limited to the acute postinjury period.

Deliberate Self-poisoning with a Lethal Dose of Pentobarbital with Confirmatory Serum Drug Concentrations: Survival After Cardiac Arrest with Supportive Care

INTRODUCTION

Pentobarbital (PB) is a euthanasia drug in doses of 2 to 10 grams, causing death within 15-30 minutes. We report a case of recovery from lethal pentobarbital deliberate self-poisoning with confirmatory serum drug concentrations.

CASE REPORT

A 45-year-old male purchased 20 grams of PB powder over the Internet. He ingested this powder and then alerted his mother 10 minutes later. She found him unresponsive and commenced cardiopulmonary resuscitation (CPR). Within 20 minutes of ingestion, emergency medical services arrived and initiated advanced life support. On arrival to the emergency department, heart rate was 116 bpm, BP 117/62 mmHg, on an epinephrine infusion. He was hypotonic and hypothermic, with absent brainstem reflexes. ECG and CT brain were normal. Activated charcoal was administered and he was admitted to ICU. He remained comatose with absent brainstem reflexes until day 5. Cerebral angiogram on day 3 was normal. Qualitative urine testing detected pentobarbital suggesting ongoing drug effects as the cause of coma. He was extubated on day 10, eventually making a full recovery. At 2.5 hours post-ingestion, PB concentration was 112 mg/L; PB peaked at 116 mg/L at 29 hours; PB was 2 mg/L at 190 hours and undetectable over 200 hours post-ingestion.

DISCUSSION

Average PB concentration in fatalities is reported around 30 mg/L. This patient survived higher serum concentrations with early CPR and prolonged cardiorespiratory support in the ICU. Assessment of brainstem death should be deferred until PB has been adequately eliminated.

Pentobarbital Removal During Continuous Venovenous Hemofiltration: Case Report and Review of the Literature

BACKGROUND:

Renal replacement therapy may enhance the elimination of barbiturates. Pentobarbital clearance during continuous venovenous hemofiltration (CVVH) has not been described previously. We report a patient case involving the measurement of serial pentobarbital levels during CVVH and review relevant literature characterizing extracorporeal pentobarbital elimination.

METHODS:

The following is a retrospective report of a previously healthy 26-year-old woman who sustained a severe traumatic brain injury (TBI) and required administration of pentobarbital on hospital day 0 for intracranial pressure (ICP) control. Given concern for interference with the patient's ongoing neurologic assessments, pentobarbital was discontinued on hospital day 4. The patient's hospital course was complicated by acute kidney injury (AKI), requiring initiation of CVVH on hospital day 5. Daily serum pentobarbital levels were obtained during CVVH.

RESULTS:

While on CVVH, the patient's estimated pentobarbital clearance ranged from 6 to 44 mL/min and the elimination half-life ranged from 17.7 to 65.9 hours. Based on reductions in pentobarbital clearance during CVVH interruption, the elimination of drug was dependent upon extracorporeal removal in this patient. CVVH facilitated pentobarbital elimination in a manner approaching endogenous clearance in healthy individuals.

CONCLUSION:

We report clinically significant pentobarbital removal by CVVH in a patient with severe TBI. Application of CVVH may expedite reliable neurologic assessments and facilitate the application of clinical brain death examination following pentobarbital exposure.

Therapeutic Drug Monitoring of Pentobarbital: Experience at an Academic Medical Center

BACKGROUND

Pentobarbital is used for management of intractable seizures and for reducing elevated intracranial pressure. Dosing of pentobarbital can be aided by therapeutic drug monitoring (TDM). There is no commercially available automated assay for measurement of pentobarbital serum/plasma concentrations; consequently, chromatography-based assays are often used.

METHODS

Pentobarbital TDM was studied over a 14-year period at an academic medical center. 154 patients (94 adult, 60 pediatric) were identified who had pentobarbital levels ordered at least once during a hospital encounter. Chart review included patient diagnosis, indication for pentobarbital therapy, recent or concomitant medication with other barbiturates, patient disposition, organ donation, pentobarbital dosing changes, and neurosurgical procedures. Pentobarbital serum/plasma concentrations were determined on an automated clinical chemistry platform with a laboratory-developed test adapted from a urine barbiturates immunoassay.

RESULTS

Chart review showed therapeutic use of pentobarbital generally consistent with previously published literature. The most common errors observed involved confusion in barbiturate names (eg, mix-up of pentobarbital and phenobarbital in test ordering or in provider notes) that seemed to have minimal impact on TDM effectiveness, with pentobarbital serum/plasma concentrations generally within target ranges. The laboratory-developed pentobarbital immunoassay showed cross-reactivity with phenobarbital and butalbital that was eliminated by alkaline and heat pretreatment. The immunoassay was linear to 20 mcg/mL and correlated closely with gas chromatography-mass spectrometry measurements at a reference laboratory.

CONCLUSIONS

Pentobarbital TDM can be performed by immunoassay on an automated clinical chemistry platform, providing an alternative to chromatography-based methods. Confusion in barbiturate names is common, especially pentobarbital and phenobarbital.

Enhanced elimination in acute barbiturate poisoning - a systematic review

CONTEXT

Despite a worldwide decline in barbiturate use, cases of acute poisoning with severe toxicity are still noted, particularly in developing countries. Severe poisonings often require prolonged admission to an intensive care unit, so enhanced elimination might be useful to hasten recovery. Information regarding the efficacy of these techniques for individual barbiturates is not available in standard textbooks.

OBJECTIVE

To determine the evidence supporting the effect of enhanced elimination and its role in the management of acute barbiturate poisoning.

METHODS

A systematic review was conducted using broad search criteria in three databases. All potentially relevant articles were obtained, and reference lists were manually reviewed. Ninety-four publications fulfilling inclusion criteria were located. Studies were classified as controlled or uncontrolled, and clinical and pharmacokinetic end points were manually extracted. If not directly stated, standard pharmacokinetic methods were used to calculate the clearance and efficiency of enhanced elimination techniques for each barbiturate and tabulated for direct comparison. PROSPECTIVE CONTROLLED CLINICAL TRIALS: Two of the 94 publications were prospective controlled studies (only one stated that allocation was via blinded randomisation), and both assessed the effect of multiple-dose activated charcoal for acute phenobarbital poisoning. These studies demonstrated enhanced elimination with a decrease in elimination of half-life from approximately 80 to 40?h, but only one study reported clinical benefits. UNCONTROLLED SERIES AND SINGLE CASE REPORTS: Sufficient data to determine the clearance due to enhanced elimination were available in only 52 of these papers. Barbiturate clearances by enhanced elimination varied markedly among studies. While extracorporeal modalities appeared to increase the direct clearance of many barbiturates, there was insufficient information to confirm a clinical benefit.

CONCLUSIONS

There is limited evidence to support the use of enhanced elimination in the treatment of poisoning with most barbiturates. There is no role for urine alkalinisation, while multiple-dose activated charcoal may be useful for most phenobarbital and possibly primidone poisonings. Extracorporeal techniques appear to enhance elimination, but the clinical benefits, relative to the potential complications and cost, are poorly defined. Extracorporeal techniques such as haemodialysis and haemoperfusion can be considered for patients with life-threatening barbiturate toxicity such as refractory hypotension.

Pharmacokinetic changes in critical illness

Physiologic alterations in critically ill patients can significantly affect the pharmacokinetics of drugs used in the critically ill patient population. Understanding these pharmacokinetic changes is essential relative to optimizing drug therapy. This article outlines the major differences seen in the absorption, distribution, metabolism, and excretion of drugs in critically ill patients. Important strategies for drug therapy dosing and monitoring in these patients are also addressed.

Midazolam Therapeutic Drug Monitoring in Intensive Care Sedation

During a 5-year period, 1997 to 2002, therapeutic drug monitoring of midazolam plasma concentrations in combination with the level of sedation as assessed by the Ramsay sedation scale was performed in 648 critically ill patients requiring artificial ventilation. In a subgroup of 189 patients sepsis-related organ failure assessment procedure was additionally performed. A total number of 3354 samples were analyzed. Significantly reduced clearance of midazolam was observed within the first 4 days of midazolam treatment of critically ill patients. As a result, accumulation of midazolam and its metabolites occurred within the first week of treatment. In contrast, parameters such as serum bilirubin or creatinine, which are commonly used to adapt drug therapy to organ dysfunction, showed significant changes with a delay of more than 10 days as compared with the findings of midazolam monitoring. Midazolam plasma concentrations showed a good correlation with the sedative capacity of the drug (r2 = 0.906). However, a great variability of the drug effect between patients could be demonstrated, which, as a consequence, may complicate the development of dosing strategies based on midazolam plasma concentrations to better control sedation in critically ill patients. Furthermore, patient age seems to be an important factor for the considerable variability of the sedative effect of midazolam. To achieve a certain levels of sedation, significantly lower midazolam infusion rates as well as plasma concentrations were required as the patients age increased. No significant sex-related differences could be observed for any pharmacologic parameter obtained in this study. Our findings suggest that midazolam therapeutic drug monitoring might be a useful tool to individualize midazolam therapy, especially in critically ill patients developing organ dysfunction and requiring long-term sedation to minimize the risk of drug accumulation and excessive sedation.

Differential effects of traumatic brain injury on the cytochrome p450 system: a perspective into hepatic and renal drug metabolism

Traumatic brain injury is known to cause several secondary effects, one of which is altered drug clearance. Given the fact that patients who sustain TBI are subsequently treated with a variety of pharmacological agents for the purpose of either neuroprotection or physiological support, it is imperative to clarify changes in expression and/or activities of enzymes involved in clearing drugs. The mixed function oxidase system, which consists of cytochrome P450 and cytochrome P450 reductase, plays a vital role in phase I drug metabolism. This paper addresses the issue as to what extent TBI affects the levels and activity of various rat CYP450 subfamilies. Our results show that TBI induces tissue-specific and time-dependent alterations. Total hepatic CYP450 content showed a biphasic response with a decrease seen at 24 h followed by an increase at 2 weeks. CYP450 reductase, in contrast, showed an opposite temporal profile. Immunoblot analyses and marker substrate metabolism demonstrated a clear decrease in hepatic CYP1A levels while a significant increase in kidney was seen at both 24 h and 2 weeks. A dramatic induction of CYP3A was evident at 2 weeks in liver, while no changes were noticed in CYP2B or CYP2D subfamilies. CYP4F subfamily showed induction in kidney only. Collectively, the data reveal the differential effects of TBI on hepatic and renal drug metabolism.

Pharmacokinetic alterations after severe head injury. Clinical relevance

Pharmacological therapy, present and future, will undoubtedly continue to play a large role within the overall management of patients with severe head injury. Nevertheless, limited clinical data are available to evaluate the effect of severe head injury on pharmacokinetics. The disruption of the blood-brain barrier secondary to trauma and/or subsequent hyperosmolar therapy can be expected to result in higher than expected brain drug concentrations. Aggressive dietary protein supplementation may result in increased oxidative drug metabolism. These effects may counterbalance inhibitory influences on drug metabolism secondary to cytokine release during the acute phase response. Alterations in protein binding can also be anticipated with the hypoalbuminaemia and increases in alpha 1-acid glycoprotein typically observed in these patients. Based on studies in other patient populations, moderate hypothermia, a treatment strategy in patients with head injury, can decrease drug metabolism. The pharmacokinetics of the following drugs in patients with severe head injury have been studied: phenytoin, pentobarbital (pentobarbitone), thiopental (thiopentone), tirilazad, and the agents used as marker substrates, antipyrine, lorazepam and indocynanine green (ICG). Several studies have documented increase in metabolism over time with phenytoin, pentobarbital, thiopental, antipyrine and lorazepam. Increases in tirilazad clearance were also observed but attributed to concurrent phenytoin therapy. No changes in the pharmacokinetics of ICG were apparent following head injury. With the frequent use of potent inhibitors of drug metabolism (e.g., cimetidine, ciprofloxacin) the potential for drug interaction is high in patients with severe head injury. Additional pharmacokinetic investigations are recommended to optimise pharmacological outcomes in patients with severe head injury.

Time-dependent pharmacokinetics of high dose thiopental infusion in intensive care patients

PURPOSE

In patients with severe head injuries receiving long-term infusion for reducing intracranial pressure, a decline in concentrations was apparent following attainment of an initial steady state. This could be explained by an increased rate of elimination. An adequate modeling of the plasma disposition curves was used to demonstrate clearly the metabolic induction.

METHODS

The concentration-time data of 17 patients were fit by a one compartment pharmacokinetic model in which the decline of plasma concentration during infusion was due to an increase in the clearance rate of thiopental following a latency period. This time-dependent clearance model provided estimates of initial and final clearance rates.

RESULTS

This study demonstrated that large interindividual variations were observed during the course of the thiopental time-dependent pharmacokinetics. Depending on the patient, one or two steps of induction occurred. The mean initial and final clearance rates were 1.22 +/- 0.82 mL/min/kg and 10.5 +/- 23 mL/min/kg. The latency period for the first induction averaged 69 +/- 56 h. For 6 subjects, the rate of thiopental metabolism continued to change with time and there was a second step of induction.

CONCLUSIONS

Induction of thiopental metabolism occur within therapeutic ranges, but it was not established that attainment of individual limits in dosing rate, total dose, or treatment duration occur in the process. Thus, monitoring is needed for achievement of a target plasma concentration.

The kinetics of metamizol and its metabolites in critical-care patients with acute renal dysfunction

We have studied the clearance of monomethylaminoantipyrine (MMAAP), the pharmacologically active form of metamizol, in 46 patients in surgical intensive care with different degrees of renal dysfunction. In 23 patients without any renal impairment, mean clearance was 2.8 ml.min-1 x kg-1. Twenty-one patients with acute renal impairment had a significantly reduced clearance of MMAAP (0.83 ml.min-1 x kg-1). There was also reduced clearance in four patients with septic shock (1.0 ml.min-1 x kg-1). Kinetics of the metabolites of MMAAP (N-formylaminoantipyrine (FAAP), aminoantipyrine (AAP), and its secondary product N-acetylaminoantipyrine (AcAAP)) were calculated. FAAP and AcAAP showed delayed invasion, which can be explained by reduced hepatic metabolic activity. The product of N-demethylation, AAP, was not significantly altered. The delayed elimination of monomethylaminoantipyrine can be explained by reduced hepatic function in parallel with acute renal failure due to disturbed cardiovascular function caused by septic shock. This may also lead to disturbed hepatic macro- and microperfusion associated with altered oxygen supply and oxygen consumption.

Non linear disposition of thiopentone following long-term infusion

The pharmacokinetics of thiopentone administered at infusion rates ranging 4.5-11.5 mg.kg-1.h-1 for 50-130 h was studied in 6 patients with neurologic evidence of severe cerebral damage. Arterial plasma concentrations of thiopentone were measured during and after discontinuation of the infusion. The postinfusion plasma concentrations were fitted to the one compartment Michaelis-Menten model. At the end of the infusion, the level of saturation of the enzymatic systems ranged 67.0-95.7%. Vm was on average 0.93 +/- 0.57 mg.l-1.h-1. The mean plasma clearance was 2.0 +/- 1.4 ml.min-1.kg-1. The apparent half-life of the terminal phase was 5.5 +/- 3.9 h.

Clinical pharmacokinetic considerations in the treatment of increased intracranial pressure

Life-threatening increased intracranial pressure can be reversed by a variety of drugs. These compounds all have some disadvantages, producing rebound effects, severe coma or cardiovascular depression and electrolyte imbalance. However, reduction of intracranial pressure is a prerequisite for recovery and the benefits of treatment outweigh the risks. Dexamethasone is rapidly eliminated, the short half-life (about 3 hours) indicating that dosage intervals should be kept small. As yet, however, its therapeutic efficacy has not been clearly demonstrated. Therefore, an association between pharmacokinetics and pharmacodynamics cannot be established. Osmotic diuretics are the most widely used agents for reduction of intracranial pressure. Pharmacokinetics show a very close relationship to changes in serum osmolality, but there are large variations in the clearance. For the use of osmotics, the blood-brain barrier must be intact. Osmotic diuretics may lead to intracerebral oedema or to acute renal failure as serum osmolality increases. Considering the pharmacokinetics of each drug, and the dynamics of intracerebral pressure and osmolality, an intermittent, individually titrated dosage should be administered, with simultaneous monitoring of intracranial pressure. Frusemide (furosemide) can be used as an adjunct, to enhance the effect of osmotic diuretics. Its pharmacokinetics are limited by renal function, depending on age as well as on the extent of renal impairment. Altered renal elimination of concomitantly administered drugs, and electrolyte imbalances should be anticipated when diuretics are used. Barbiturates are certain to decrease intracranial pressure in humans by an as yet unknown mechanism. Their administration is recommended for patients that do not respond to conventional therapy. As barbiturates can result in deep coma, knowledge of their pharmacokinetics is of great importance for recovery. Following single doses, pentobarbitone has a relatively long elimination half-life (about 22 hours). However, after repeated administration for several days, its elimination may be enhanced due to autoinduction. Thiopentone kinetics are characterised by distribution and redistribution into deep peripheral compartments. Administration of high and frequent doses leads to considerably delayed recovery. This is not true for methohexitone, which shows comparable pharmacokinetics after single and multiple dose administration. Elimination depends on liver blood flow. Thus, recovery from methohexitone-coma is rapid. Rapid elimination is also an important characteristic of etomidate and alphaxalone/alphadolone, two non-barbiturate hypnotics.(ABSTRACT TRUNCATED AT 400 WORDS)

Pentobarbital pharmacokinetics in patients with severe head injury

Intravenous administration of high-dose pentobarbital has been proposed as a treatment for elevated intracranial pressure refractory to other measures in brain-injured patients. The purpose of this clinical study was to examine the pharmacokinetics of high-dose continuous intravenous infusion of pentobarbital in this critical care setting. Six patients received a 25-34 mg/kg intravenous loading dose followed by a 1-3 mg/kg/h continuous infusion for 61-190 hours. Dosing rates were adjusted based on the patient's clinical status. The mean clearance was 0.72 ml/min/kg, with a volume of distribution (Vd) of 1.03 L/kg and a terminal half-life of 19.1 h. Considerable variation in individual patient parameters was observed. In addition, a change in clearance was suggested in patients requiring a longer infusion duration.

Kinetics of hexobarbital and dipyrone in critical care patients receiving high-dose pentobarbital

The effect of pentobarbital treatment in a mean dose of 30 mg/kg/day on the clearance of hexobarbital (Evipan) and dipyrone (Novalgin) has been evaluated in critical care patients receiving a large number of drugs as comedication. Eleven patients treated with pentobarbital showed a hexobarbital half-life of 2.79 h and a total plasma clearance of 9.80 ml X min-1 X kg-1 as compared to 10 patients without pentobarbital administration in whom there was a significantly longer half life (6.92 h) and lower clearance (2.97 ml X min-1 X kg-1). The kinetics of hexobarbital were correlated with the urinary excretion of D-glucaric acid, a non-invasive parameter of drug metabolising activity. In 10 patients on pentobarbital, the total plasma clearance of N-4-methyl-aminoantipyrine, the active form of dipyrone, did not differ from that in 8 patients not receiving pentobarbital. As drug kinetics show great variability in these patients, it is difficult to discriminate enzyme induction from other mechanisms, for example competitive inhibition or changes in volume of distribution. In the presence of pentobarbital, however, induction of drug metabolising enzymes should be considered as a possible reason for the higher clearance of hexobarbital.