Pentobarbital pharmacokinetics in patients with severe head injury

Super-Refractory Status Epilepticus: Prognosis and Recent Advances in Management

Super-refractory status epilepticus (SRSE) is a life-threatening neurological emergency with high morbidity and mortality. It is defined as “status epilepticus (SE) that continues or recurs 24 hours or more after the onset of anesthesia, including those cases in which SE recurs on the reduction or withdrawal of anesthesia.” This condition is resistant to normal protocols used in the treatment of status epilepticus and exposes patients to increased risks of neuronal death, neuronal injury, and disruption of neuronal networks if not treated in a timely manner. It is mainly seen in patients with severe acute onset brain injury or presentation of new-onset refractory status epilepticus (NORSE). The mortality, neurological deficits, and functional impairments are significant depending on the duration of status epilepticus and the resultant brain damage. Research is underway to find the cure for this devastating neurological condition. In this review, we will discuss the wide range of therapies used in the management of SRSE, provide suggestions regarding its treatment, and comment on future directions. The therapies evaluated include traditional and alternative anesthetic agents with antiepileptic agents. The other emerging therapies include hypothermia, steroids, immunosuppressive agents, electrical and magnetic stimulation therapies, emergent respective epilepsy surgery, the ketogenic diet, pyridoxine infusion, cerebrospinal fluid drainage, and magnesium infusion. To date, there is a lack of robust published data regarding the safety and effectiveness of various therapies, and there continues to be a need for large randomized multicenter trials comparing newer therapies to treat this refractory condition.

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.

Antiepileptic drugs in critically ill patients

BACKGROUND

The incidence of seizures in intensive care units ranges from 3.3% to 34%. It is therefore often necessary to initiate or continue anticonvulsant drugs in this setting. When a new anticonvulsant is initiated, drug factors, such as onset of action and side effects, and patient factors, such as age, renal, and hepatic function, should be taken into account. It is important to note that the altered physiology of critically ill patients as well as pharmacological and nonpharmacological interventions such as renal replacement therapy, extracorporeal membrane oxygenation, and target temperature management may lead to therapeutic failure or toxicity. This may be even more challenging with the availability of newer antiepileptics where the evidence for their use in critically ill patients is limited.

MAIN BODY

This article reviews the pharmacokinetics and pharmacodynamics of antiepileptics as well as application of these principles when dosing antiepileptics and monitoring serum levels in critically ill patients. The selection of the most appropriate anticonvulsant to treat seizure and status epileptics as well as the prophylactic use of these agents in this setting are also discussed. Drug-drug interactions and the effect of nonpharmacological interventions such as renal replacement therapy, plasma exchange, and extracorporeal membrane oxygenation on anticonvulsant removal are also included.

CONCLUSION

Optimal management of antiepileptic drugs in the intensive care unit is challenging given altered physiology, polypharmacy, and nonpharmacological interventions, and requires a multidisciplinary approach where appropriate and timely assessment, diagnosis, treatment, and monitoring plans are in place.

Pharmacotherapy Overview of Seizure Management in the Adult Emergency Department

Seizures are a common cause of emergency department visits, and approximately 28% of epilepsy patients present to an emergency department annually for treatment. This article will provide an overview of the pharmacotherapeutic management of seizures and anticonvulsant therapy for patients who present to the adult emergency department, including practical information for pharmacists covering or cross-covering this practice area. The benzodiazepines are reviewed as a class, including dosing strategies, pharmacodynamic considerations, and advantages and disadvantages of lorazepam, diazepam, and midazolam. Indications for the use of phenytoin and fosphenytoin will be reviewed, as well as dosing, adverse effects, and cost-effectiveness data. In addition, dosing, administration, pharmacokinetics, and adverse effects of phenobarbital, carbamazepine, and valproate will be discussed. Clinical indications for serum anticonvulsant concentration monitoring and subsequent calculation of loading doses from serum concentrations are reviewed. Since status epilepticus is a life-threatening emergency, its therapeutic management is reviewed, including the use of continuous infusion midazolam, pentobarbital, and propofol. There are many opportunities for clinical pharmacists to collaborate with other members of the health care team to optimize efficacy and minimize adverse effects of anticonvulsant agents in the emergency department setting.

Population pharmacokinetics of pentobarbital in neonates, infants, and children after open heart surgery

OBJECTIVES

To determine the pharmacokinetics of pentobarbital in neonates, infants, and young children with congenital heart disease after open-heart surgery.

STUDY DESIGN

Thirty-five subjects (3.0 days-4.4 years) after open-heart surgery who received pentobarbital as standard of care were enrolled. Serial pharmacokinetic blood samples were obtained. A population-based, nonlinear mixed-effects modeling approach was used to characterize pentobarbital pharmacokinetics.

RESULTS

A two-compartment model with weight as a co-variate allometrically expressed on clearance (CL), inter-compartmental clearance, central (V1) and peripheral volume of distributions, bypass grafting time as a co-variate on CL and V1, and age and ventricular physiology as co-variates on CL best described the pharmacokinetics. A typical infant (two-ventricle physiology, 6.9 kg, 5.2 months, and bypass grafting time of 60 minutes) had a CL of 0.12 L/hr/kg, V1 of 0.45 L/kg, and peripheral volume of distributions of 0.98 L/kg. The bypass grafting effect was poorly estimated. For subjects <12 months age, an age effect on CL remained after accounting for weight and was precisely estimated.

CONCLUSIONS

Pentobarbital pharmacokinetics is influenced by age and weight. Subjects with single-ventricle physiology demonstrated a 15% decrease in clearance when compared with subjects with two-ventricle physiology.

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.

Contemporary pharmacologic issues in the management of traumatic brain injury

Traumatic brain injury (TBI) is a major cause of death and disability in the United States. While there are no pharmacotherapeutic options currently available for attenuating the neurologic injury cascade after TBI, numerous pharmacologic issues are encountered in these critically ill patients. Adequate fluid resuscitation, reversal of coagulopathy, maintenance of cerebral perfusion, and treatment of intracranial hypertension are common interventions early in the treatment of TBI. Other deleterious complications such as venous thromboembolism, extremes in glucose concentrations, and stress-related mucosal disease should be anticipated and avoided. Early provision of nutrition and prevention of drug or alcohol withdrawal are also cornerstones of routine care in TBI patients. Prevention of infections and seizures may also be helpful. Clinicians caring for TBI patients should be familiar with the pharmacologic issues typical of this vulnerable population in order to develop optimal strategies of care to anticipate and prevent common complications.

Critical care sedation for neuroscience patients

In 2000, the Joint Commission on Accreditation of Healthcare Organizations (JCAHO) introduced the mandate for the implementation of standards for both pain assessment and need for therapy in hospitalized patients. The need for the appropriate titration of sedation and analgesia is particularly poignant in an intensive care unit (ICU) setting where iatrogenic discomfort often complicates patient management. Neurologically ill patients in ICUs present particularly complex sedation issues, owing to the need to monitor these patients with serial neurological exams. Hence, maximal comfort without diminishing neurological responsiveness is desirable. Here, we review the frequently applied methods of evaluating levels of pain and agitation in critically ill patients as well as discuss the appropriate classes of pharmaceutical agents common to this population, with particular emphasis on the potential neurophysiological impact of such therapy.

Pharmacokinetics and interactions of headache medications, part I: introduction, pharmacokinetics, metabolism and acute treatments

Recent progress in the treatment of primary headaches has made available specific, effective and safe medications for these disorders, which are widely spread among the general population. One of the negative consequences of this undoubtedly positive progress is the risk of drug-drug interactions. This review is the first in a two-part series on pharmacokinetic drug-drug interactions of headache medications. Part I addresses acute treatments. Part II focuses on prophylactic treatments. The overall aim of this series is to increase the awareness of physicians, either primary care providers or specialists, regarding this topic. Pharmacokinetic drug-drug interactions of major severity involving acute medications are a minority among those reported in literature. The main drug combinations to avoid are: i) NSAIDs plus drugs with a narrow therapeutic range (i.e., digoxin, methotrexate, etc.); ii) sumatriptan, rizatriptan or zolmitriptan plus monoamine oxidase inhibitors; iii) substrates and inhibitors of CYP2D6 (i.e., chlorpromazine, metoclopramide, etc.) and -3A4 (i.e., ergot derivatives, eletriptan, etc.), as well as other substrates or inhibitors of the same CYP isoenzymes. The risk of having clinically significant pharmacokinetic drug-drug interactions seems to be limited in patients with low frequency headaches, but could be higher in chronic headache sufferers with medication overuse.

Kinetic analysis of the agonistic and blocking properties of pentobarbital on recombinant rat alpha(1)beta(2)gamma(2S) GABA(A) receptor channels

Barbiturates have three different effects on the GABA(A) receptor channels: coactivation, direct activation, and blockage. We investigated the activation and blockage of the GABA(A) receptor channels by pentobarbital using the alpha(1)beta(2)gamma(2S) GABA(A) receptor channels transiently expressed in HEK293 cells in combination with the ultrafast application of agonists. The peak current amplitude of the pentobarbital activated ionic current proportionally increased to the first power of the pentobarbital concentration (Hill coefficient approximately 0.7), indicating that one binding step of pentobarbital at alpha(1)beta(2)gamma(2S) GABA(A) receptor channels can describe the experimental dose-response relation. The maximum peak current amplitude occurred at 1 mM pentobarbital and decreased at higher concentrations due to an open channel block. After the end of the pentobarbital pulses, rebound currents due to transition from the open-blocked to the open state of the receptor were observed. A kinetic scheme was constructed allowing the quantitative analysis of the pentobarbital activated ionic currents through GABA(A) receptor channels.

Use of sedative and analgesic agents in neurotrauma patients: effects on cerebral physiology

Sedation and analgesia is used primarily in the intensive care unit (ICU) to limit the stress response to critical illness, provide anxiolysis, improve ventilatory support, and facilitate adequate ICU care. However, in the neurotrauma ICU there are many other reasons for the use of these agents. The primary aim is to prevent secondary cerebral damage by maintaining adequate cerebral perfusion pressures. This is accomplished in several different ways. Controlling intracranial pressure (ICP) and maintaining an adequate mean arterial pressure (MAP) is at the cornerstone of this management. Lowering the metabolic demands of the brain is also an important consideration as a treatment strategy. Analgesic and sedative agents are utilized to prevent undesirable increases in ICP and to lower cerebral metabolic demands. Concerns surrounding the use of these agents include time to awakening after discontinuation, effect on the cerebrovasculature, and the effect on patient outcome. There are many different pharmacological agents available, each with their distinct advantages and disadvantages. The purpose of this review is to evaluate the pharmacokinetic and pharmacological effects of each of these agents when used in neurotrauma patients.

Pentobarbital has curare-like effects on adult-type nicotinic acetylcholine receptor channel currents

Pentobarbital (PB) is widely used as a short-term sedative and anticonvulsive drug with a side-effect of relaxing muscle tone. We investigated block of nicotinic acetylcholine receptor (nAChR) channel currents by PB using the patch-clamp technique in combination with an ultrafast system for solution exchange. As a preparation, recombinant rat adult-type nAChR channels transiently expressed in HEK293 cells were used. Appli-cation of 1 mM acetylcholine to small cells or outside-out patches showed a transient current with fast activation and desensitization kinetics. Adding PB to the acetylcholine-containing solution resulted in a decrease of the time constant of current decay and of the peak current amplitude starting at concentrations >0.01 mM PB. Preincubation of nAChR channels with PB led to a decrease of the peak current amplitude without alteration of activation and desensitization kinetics caused by competitive block of nAChR channels. In conclusion, similar to the effect of d-Tubocurarine, block of nAChR channel currents by PB can be explained by a combination of open-channel and competitive block.

IMPLICATIONS

The interaction between adult-type nicotinic acetylcholine receptors, acetylcholine, and pentobarbital was biophysically investigated by using the patch-clamp technique in combination with tools for ultrafast solution exchange. PB elicited open-channel block and competitive block of nicotinic acetylcholine receptor channel currents, whereas the latter seems to be effective in clinically relevant concentrations.

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.

Effect of acute phase response on phenytoin metabolism in neurotrauma patients

The purpose of this prospective study was to correlate measures of the acute phase response, associated therapeutic interventions, and other clinical variables with the process of altered drug metabolism previously observed in patients with severe neurotrauma. Nine patients with severe head injury (Glasgow Coma Scale < or = 8) requiring intravenous phenytoin were included in the study. A loading dose of phenytoin was followed by daily maintenance doses. Serial blood samples were taken after the loading dose and every even-numbered study day for 10 to 14 days for measurement of total and unbound concentrations of phenytoin, interleukin-1 beta, interleukin-6 (IL-6), tumor necrosis factor alpha, alpha 1-acid-glycoprotein, C-reactive protein, and albumin. Time-invariant and time-variant Michaelis-Menten models were fit to the phenytoin concentration-time data. Protein intake was closely monitored. The mean (+/- SEM) unbound fraction of phenytoin increased from 0.17 +/- 0.02 on day 1 to 0.24 +/- 0.04 on day 10 (P < 0.05). The time-variant model was superior in describing the concentration-time data of unbound phenytoin in eight of nine patients. Mean (+/- SEM) pharmacokinetic parameter estimates for unbound phenytoin were: Vmax delta = 605 +/- 92 mg/day, VmaxB = 149 +/- 26.3 mg/day, K(ind) = 0.013 +/- 0.004 hr-1. Interleukin-6 was the only cytokine with significant concentration changes over time; it was inversely correlated with Vmax,t. Peak concentrations of interleukin-6 also proved to be inversely correlated with VmaxB. The daily amount of protein administered was significantly correlated with Vmax,t. Significant alterations in the metabolism of phenytoin occur after severe neurotrauma. The etiology of these changes is probably multifaceted. These results suggest that low initial phenytoin Vmax may be explained by the presence of interleukin-6. An increase in oxidative metabolism that correlated with nutritional protein administration was observed later in these patients.

Intensive management of severe head injury

Intensive management of patients with severe head injury offers the best hope of minimizing death and functional disability in a young, working population. Secondary neurologic insult can be decreased by cardiorespiratory support and ICP control from the outset. Rapid neurologic assessment, airway management, and support of circulation are the basis of emergency management for head injury. Patients with severe head injury require intensive care management for two major reasons: management of ICP and management of organ system dysfunction. Care should not be withheld because of initially grim (and inaccurate) prognostic assessment. Newer techniques for assessing the adequacy of cerebral circulation may allow refinement of management strategies in the future.

Pentobarbital suppresses human brain sodium channels

Ion channels, key components in neuronal signal transmission and processing, are likely to be important molecular sites of anesthetic action. Sodium channels from human brain tissue were incorporated into planar lipid bilayers in the presence of batrachotoxin and exposed to the anesthetic pentobarbital. This barbiturate, in a dose-dependent manner and at clinically relevant concentrations, reduced fractional channel open time independent of membrane potential, and interfered with the steady-state activation process.