High-dose pentobarbital pharmacokinetics in hypothermic brain-injured children

A Population Pharmacokinetic Model of Pentobarbital for Children with Status Epilepticus and Severe Traumatic Brain Injury

BACKGROUND

Pentobarbital pharmacokinetics (PK) remain elusive and the therapeutic windows narrow. Administration is frequent in critically ill children with refractory status epilepticus (SE) and severe traumatic brain injury (sTBI).

OBJECTIVES

To investigate pentobarbital PK in SE and sTBI patients admitted to the paediatric intensive care unit (PICU) with population-based PK (PopPK) modelling and dosing simulations.

METHODS

Develop a PopPK model with non-linear mixed-effects modelling (NONMEM®) with retrospective data (n = 36; median age 1.3 years; median weight 10 kg; 178 blood samples) treated with continuous intravenous pentobarbital. An independent dataset was used for external validation (n = 9). Dosing simulations with the validated model evaluated dosing regimens.

RESULTS

A one-compartment PK model with allometrically scaled weight on clearance (CL; 0.75) and volume of distribution (Vd; 1) captured data well. Typical CL and Vd values were 3.59 L/70 kg/h and 142 L/70 kg, respectively. Elevated creatinine and C-reactive protein (CRP) levels significantly correlated to decreased CL, explaining 84% of inter-patient variability, and were incorporated in the final model. External validation using stratified visual predictive checks showed good results. Simulations demonstrated patients with elevated serum creatinine and CRP failed to achieve steady state yet progressed to toxic levels with current dosing regimens.

CONCLUSIONS

The one-compartment PK model of intravenous pentobarbital described data well whereby serum creatinine and CRP significantly correlated with pentobarbital CL. Dosing simulations formulated adjusted dosing advice in patients with elevated creatinine and/or CRP. Prospective PK studies with pharmacodynamic endpoints, are imperative to optimise pentobarbital dosing in terms of safety and clinical efficacy in critically ill children.

Role of barbiturate coma in the management of focally induced, severe cerebral edema in children

Barbiturates are widely used in the management of high intracranial pressure (ICP) caused by diffuse brain swelling. The cardiovascular, renal, and immunological side effects of these drugs limit them to last-line therapy. There are few published data regarding the role of barbiturates in focal brain lesions causing refractory elevated ICP and intraoperative brain swelling in the pediatric population. The authors here present 3 cases of nontraumatic, focally induced, refractory intracranial hypertension due to 2 tumors and 1 arteriovenous malformation, in which barbiturate therapy was used successfully to control elevated ICP. They focus on cardiovascular, renal, and immune function during the course of pentobarbital therapy. They also discuss the role of pentobarbital-induced hypothermia. From this short case series, they demonstrate that barbiturates in conjunction with standard medical therapy can be used to safely reduce postoperative refractory intracranial hypertension and intraoperative brain swelling in children with focal brain lesions.

The impact of extracorporeal life support and hypothermia on drug disposition in critically ill infants and children

Extracorporeal membrane oxygenation (ECMO) support is an established lifesaving therapy for potentially reversible respiratory or cardiac failure. In 10% of all pediatric patients receiving ECMO, ECMO therapy is initiated during or after cardiopulmonary resuscitation. Therapeutic hypothermia is frequently used in children after cardiac arrest, despite the lack of randomized controlled trials that show its efficacy. Hypothermia is frequently used in children and neonates during cardiopulmonary bypass (CPB). By combining data from pharmacokinetic studies in children on ECMO and CPB and during hypothermia, this review elucidates the possible effects of hypothermia during ECMO on drug disposition.

Clinical implications of pharmacokinetics and pharmacodynamics of procedural sedation agents in children

PURPOSE OF REVIEW

Procedural sedation has become the standard of care for managing pain and anxiety in children in the emergency department.

RECENT FINDINGS

Numerous articles have been published on pediatric procedural sedation with, however, little in-depth discussion of the pharmacodynamics and pharmacokinetics of the sedation agents utilized.

SUMMARY

We review the pharmacokinetics and pharmacodynamics of the pediatric procedural sedation pharmacopeia from a clinical perspective with emphasis on the practical implications for drug titration and dosing.

Cardiac arrest and therapeutic hypothermia decrease isoform-specific cytochrome P450 drug metabolism

Mild therapeutic hypothermia is emerging clinically as a neuroprotection therapy for individuals experiencing cardiac arrest (CA); however, its effects combined with disease pathogenesis on drug disposition and response have not been fully elucidated. We determined the activities of four major hepatic-metabolizing enzymes (CYP3A, CYP2C, CYP2D, and CYP2E) during hypothermia after experimental CA in rats by evaluating the pharmacokinetics of their probe drugs as a function of altered body temperature. Animals were randomized into sham normothermia (37.5-38°C), CA normothermia, sham hypothermia (32.5-33°C), and CA hypothermia groups. Probe drugs (midazolam, diclofenac, dextromethorphan, and chlorzoxazone) were given simultaneously by intravenous bolus after temperature stabilization. Multiple blood samples were collected between 0 and 8 h after drug administration. Pharmacokinetic (PK) analysis was conducted using a noncompartmental approach and population PK modeling. Noncompartmental analysis showed that the clearance of midazolam (CYP3A) in CA hypothermia was reduced from sham normothermia rats (681.6 ± 190.0 versus 1268.8 ± 348.9 ml · h(-1) · kg(-1), p < 0.05). The clearance of chlorzoxazone (CYP2E) in CA hypothermia was also reduced from sham normothermia rats (229.6 ± 75.6 versus 561.89 ± 215.9 ml · h(-1) · kg(-1), p < 0.05). Population PK analysis further demonstrated the decreased clearance of midazolam (CYP3A) was associated with CA injury (p < 0.05). The decreased clearance of chlorzoxazone (CYP2E1) was also associated with CA injury (p < 0.01). Hypothermia was found to be associated with the decreased volume of distribution of midazolam (V(1)), dextromethorphan (V(1)), and peripheral compartment for chlorzoxazone (V(2)) (p < 0.05, p < 0.05, and p < 0.01, respectively). Our data indicate that hypothermia, CA, and their interaction alter cytochrome P450-isoform specific activities in an isoform-specific manner.

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.

The effect of therapeutic hypothermia on drug metabolism and response: cellular mechanisms to organ function

INTRODUCTION

Therapeutic hypothermia is being employed clinically due to its neuro-protective benefits. Both critical illness and therapeutic hypothermia significantly affect drug disposition, potentially contributing to drug-therapy and drug-disease interactions. Currently, there is limited information on the known alterations in drug concentration and response during mild hypothermia treatment, and there is a limited understanding of the specific mechanisms that underlie alterations in drug concentrations and the potential clinical importance of these changes.

AREAS COVERED

A systemic review of the effect of therapeutic hypothermia on drug metabolism, disposition and response is provided. Specifically, the clinical and preclinical evidence of the effects of therapeutic hypothermia on blood flow, specific hepatic metabolism pathways, transporter function, renal excretion, pharmacodynamics and the effects during rewarming are reviewed.

EXPERT OPINION

Available evidence demonstrates that mild hypothermia decreases the clearance of a variety of drugs with apparently little change in drug-protein binding. Recent evidence suggests that the magnitude of the change is elimination route specific. Further research is needed to determine the impact of these alterations on both drug concentration and response in order to optimize the therapeutic hypothermia in this vulnerable patient population.

Therapeutic hypothermia and controlled normothermia in the intensive care unit: practical considerations, side effects, and cooling methods

BACKGROUND

Hypothermia is being used with increasing frequency to prevent or mitigate various types of neurologic injury. In addition, symptomatic fever control is becoming an increasingly accepted goal of therapy in patients with neurocritical illness. However, effectively controlling fever and inducing hypothermia poses special challenges to the intensive care unit team and others involved in the care of critically ill patients.

OBJECTIVE

To discuss practical aspects and pitfalls of therapeutic temperature management in critically ill patients, and to review the currently available cooling methods.

DESIGN

Review article.

INTERVENTIONS

None.

MAIN RESULTS

Cooling can be divided into three distinct phases: induction, maintenance, and rewarming. Each has its own risks and management problems. A number of cooling devices that have reached the market in recent years enable reliable maintenance and slow and controlled rewarming. In the induction phase, rapid cooling rates can be achieved by combining cold fluid infusion (1500-3000 mL 4 degrees C saline or Ringer's lactate) with an invasive or surface cooling device. Rapid induction decreases the risks and consequences of short-term side effects, such as shivering and metabolic disorders. Cardiovascular effects include bradycardia and a rise in blood pressure. Hypothermia's effect on myocardial contractility is variable (depending on heart rate and filling pressure); in most patients myocardial contractility will increase, although mild diastolic dysfunction can develop in some patients. A risk of clinically significant arrhythmias occurs only if core temperature decreases below 30 degrees C. The most important long-term side effects of hypothermia are infections (usually of the respiratory tract or wounds) and bedsores.

CONCLUSIONS

Temperature management and hypothermia induction are gaining importance in critical care medicine. Intensive care unit physicians, critical care nurses, and others (emergency physicians, neurologists, and cardiologists) should be familiar with the physiologic effects, current indications, techniques, complications and practical issues of temperature management, and induced hypothermia. In experienced hands the technique is safe and highly effective.

Therapeutic hypothermia changes the prognostic value of clinical evaluation of neonatal encephalopathy

OBJECTIVE

To evaluate whether therapeutic hypothermia alters the prognostic value of clinical grading of neonatal encephalopathy.

STUDY DESIGN

This study was a secondary analysis of a multicenter study of 234 term infants with neonatal encephalopathy randomized to head cooling for 72 hours starting within 6 hours of birth, with rectal temperature maintained at 34.5 degrees C +/- 0.5 degrees C, followed by re-warming for 4 hours, or standard care at 37.0 degrees C +/- 0.5 degrees C. Severity of encephalopathy was measured pre-randomization and on day 4, after re-warming, in 177 infants; 31 infants died before day 4, and data were missing for 10 infants. The primary outcome was death or severe disability at 18 months of age.

RESULTS

Milder pre-randomization encephalopathy, greater improvement in encephalopathy from randomization to day 4, and cooling were associated with favorable outcome in multivariate binary logistic regression. Hypothermia did not affect severity of encephalopathy at day 4, however, in infants with moderate encephalopathy at day 4, those treated with hypothermia had a significantly higher rate of favorable outcome (31/45 infants, 69%, P = .006) compared with standard care (12/33, 36%).

CONCLUSION

Infants with moderate encephalopathy on day 4 may have a more favorable prognosis after hypothermia treatment than expected after standard care.

Adverse effects of sedatives in children

The application of sedation/analgesia in paediatric patients is rapidly expanding as less invasive, non-operative techniques of diagnosis and treatment are applied to the paediatric population. Medical providers who are asked to provide sedation may include radiologists, paediatricians, nurses and emergency physicians, as well as anaesthesiologists and intensive care physicians. At the same time, the range of drugs used in these settings has expanded considerably. As there is no single drug fulfilling the criteria for the ideal sedative (rapid-onset, rapid recovery, no adverse effects, immobility appropriate to procedure being performed), multiple drugs may be used in combination. It is imperative that practitioners using drugs for sedation/analgesia in children be aware of the adverse effect profile(s) of these drugs, both individually and in combination. The purpose of this review is to describe the adverse effects of sedative and reversal agents currently used in paediatric sedation/analgesia.

Adverse sedation events in pediatrics: analysis of medications used for sedation

OBJECTIVES

To perform a systematic investigation of medications associated with adverse sedation events in pediatric patients using critical incident analysis of case reports.

METHODS

One hundred eighteen case reports from the adverse drug reporting system of the Food and Drug Administration, the US Pharmacopoeia, and the results of a survey of pediatric specialists were used. Outcome measures were death, permanent neurologic injury, prolonged hospitalization without injury, and no harm. The overall results of the critical incident analysis are reported elsewhere. The current investigation specifically examined the relationship between outcome and medications: individual and classes of drugs, routes of administration, drug combinations and interactions, medication errors and overdoses, patterns of drug use, practitioners, and venues of sedation.

RESULTS

Ninety-five incidents fulfilled study criteria and all 4 reviewers agreed on causation; 60 resulted in death or permanent neurologic injury. Review of adverse sedation events indicated that there was no relationship between outcome and drug class (opioids; benzodiazepines; barbiturates; sedatives; antihistamines; and local, intravenous, or inhalation anesthetics) or route of administration (oral, rectal, nasal, intramuscular, intravenous, local infiltration, and inhalation). Negative outcomes (death and permanent neurologic injury) were often associated with drug overdose (n = 28). Some drug overdoses were attributable to prescription/transcription errors, although none of 39 overdoses in 34 patients seemed to be a decimal point error. Negative outcomes were also associated with drug combinations and interactions. The use of 3 or more sedating medications compared with 1 or 2 medications was strongly associated with adverse outcomes (18/20 vs 7/70). Nitrous oxide in combination with any other class of sedating medication was frequently associated with adverse outcomes (9/10). Dental specialists had the greatest frequency of negative outcomes associated with the use of 3 or more sedating medications. Adverse events occurred despite drugs being administered within acceptable dosing limits. Negative outcomes were also associated with drugs administered by nonmedically trained personnel and drugs administered at home. Some injuries occurred on the way to a facility after administration of sedatives at home; some took place in automobiles or at home after discharge from medical supervision. Deaths and injuries after discharge from medical supervision were associated with the use of medications with long half-lives (chloral hydrate, pentobarbital, promazine, promethazine, and chlorpromazine).

CONCLUSIONS

Adverse sedation events were frequently associated with drug overdoses and drug interactions, particularly when 3 or more drugs were used. Adverse outcome was associated with all routes of drug administration and all classes of medication, even those (such as chloral hydrate) thought to have minimal effect on respiration. Patients receiving medications with long plasma half-lives may benefit from a prolonged period of postsedation observation. Adverse events occurred when sedative medications were administered outside the safety net of medical supervision. Uniform monitoring and training standards should be instituted regardless of the subspecialty or venue of practice. Standards of care, scope of practice, resource management, and reimbursement for sedation should be based on the depth of sedation achieved (ie, the degree of vigilance and resuscitation skills required) rather than on the drug class, route of drug administration, practitioner, or venue.

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.

Head trauma in the child

Head injury, either alone or in combination with multiple injuries, is common in children. Its pattern is different in children compared to adults, with diffuse cerebral swelling rather than localized hematoma being most common. The pathophysiology of pediatric head trauma is not yet clearly elucidated, but may be closely related to changes in the regulation of cerebral blood flow. The initial management and subsequent care of the child with severe brain injury are discussed from a multisystem viewpoint. The prognosis for children with severe head injury seems brighter than for adults, but there are not yet enough data to allow prediction of outcome in any individual case. Efforts to prevent, rather than treat, head injury in childhood are more likely to be beneficial.

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.

Blood pentobarbital concentrations during thiopental therapy

The desulfuration of thiopental to pentobarbital has previously been shown to be a relatively minor pathway of thiopental metabolism. In two cases, we observed significant conversion, resulting in blood pentobarbital concentrations up to 50 percent of total blood barbiturate (thiopental and pentobarbital) concentrations. Both patients received continuous infusions of thiopental and had present a condition (hypothermia) or drug (cimetidine) known to inhibit hepatic microsomal enzyme activity. It is suggested that inhibition of hepatic microsomal enzyme activity may prevent thiopental's metabolism to its major metabolite, a carboxylic acid analogue, and increase the amount of thiopental desulfurated to pentobarbital. Inhibition of hepatic microsomal metabolism also decreases the metabolism of pentobarbital. Until further elucidation of the causes of altered thiopental metabolism is available to identify patients more likely to have elevated concentrations of pentobarbital, monitoring of blood drug concentrations in patients receiving thiopental should include determination of both thiopental and pentobarbital concentrations.

Intracranial pressure monitoring by subarachnoid bolt in comatose children

The safety of intracranial pressure (ICP) monitoring by the subarachnoid bolt was studied in 124 comatose children with global (103 patients) and focal (21 patients) cerebral lesions. None of the children developed cerebral hematoma, diffuse bleeding, epileptical focus, or local or systemic infection. Based on our experience, we advocate the use of the subarachnoid bolt for the purpose of ICP monitoring as a simple and safe modality.

Pharmacologic management of acute intracranial hypertension

Acute intracranial hypertension requires aggressive treatment with physiologic and pharmacologic measures guided by intracranial pressure monitoring devices. Therapy involves the use of diuretics, corticosteroids, and barbiturates in combination with hyperventilation, ventricular drainage, and general supportive measures. This review focuses on the pathophysiology of increased intracranial pressure and the pharmacologic agents used in its management.