Effect of extracorporeal membrane oxygenation on tobramycin pharmacokinetics in sheep

Overview of Pharmacological Considerations in Extracorporeal Membrane Oxygenation

Extracorporeal membrane oxygenation has become more widely used in recent years. Although this technology has proven to be lifesaving, it is not devoid of complications contributing to significant morbidity and mortality. Nurses who care for patients receiving extracorporeal membrane oxygenation should further their understanding of changes in medication profiles due to complex interactions with the extracorporeal membrane oxygenation circuitry. The aim of this comprehensive review is to give nurses a better understanding of analgesic, sedative, anti-infective, and anticoagulation medications that are frequently used to treat patients receiving extracorporeal membrane oxygenation.

Phenobarbital use in an infant requiring extracorporeal membrane life support

Over the past two decades, there has been an increased use of extracorporeal membrane life support (ECLS) for critically ill neonates and infants. Approximately 20% of these children will experience seizures as a complication of ECLS or the comorbid condition which necessitated extracorporeal support. While phenobarbital is one of the most common drugs used to treat seizures in children, little is known about its dosing while on ECLS. We present a 3-month-old girl who required ECLS after cardiac arrest in the postoperative period following surgery for complex congenital heart disease. The patient subsequently developed seizure activity, which was treated with phenobarbital. Following an initial loading dose of 30 mg/kg, the serum concentration was 47.9 mcg/ml. A supplementary loading dose of 10 mg/kg was administered 8 h later with an increase of the maintenance dose to 8 mg/kg/day. The phenobarbital serum concentrations were 65.9 and 72.8 mcg/ml on the subsequent days. Despite therapeutic levels of phenobarbital, the patient continued to exhibit clinical and electroencephalographic evidence of seizure activity and a midazolam infusion was started at 0.3 mg/kg/h. Because of continued seizure activity, the patient ultimately required titration of midazolam to 1.2 mg/kg/h by day 7 of ECLS to control seizure activity. Due to severe intracerebral bleeding on day 9, ECLS was withdrawn and the patient expired. Our experience demonstrates some of the challenges of medication titration during ECLS. Previous reports of phenobarbital dosing during ECLS are reviewed and considerations for the dosing of anticonvulsant medications during extracorporeal support are discussed.

Pharmacokinetic issues for antibiotics in the critically ill patient

OBJECTIVE

To discuss the altered pharmacokinetic properties of selected antibiotics in critically ill patients and to develop basic dose adjustment principles for this patient population.

DATA SOURCES

PubMed, EMBASE, and the Cochrane-Controlled Trial Register.

STUDY SELECTION

Relevant papers that reported pharmacokinetics of selected antibiotic classes in critically ill patients and antibiotic pharmacodynamic properties were reviewed. Antibiotics and/or antibiotic classes reviewed included aminoglycosides, beta-lactams (including carbapenems), glycopeptides, fluoroquinolones, tigecycline, linezolid, lincosamides, and colistin.

DATA SYNTHESIS

Antibiotics can be broadly categorized according to their solubility characteristics which can, in turn, help describe possible altered pharmacokinetics that can be caused by the pathophysiological changes common to critical illness. Hydrophilic antibiotics (e.g., aminoglycosides, beta-lactams, glycopeptides, and colistin) are mostly affected with the pathphysiological changes observed in critically ill patients with increased volumes of distribution and altered drug clearance (related to changes in creatinine clearance). Lipophilic antibiotics (e.g., fluoroquinolones, macrolides, tigecycline, and lincosamides) have lesser volume of distribution alterations, but may develop altered drug clearances. Using antibiotic pharmacodynamic bacterial kill characteristics, altered dosing regimens can be devised that also account for such pharmacokinetic changes.

CONCLUSIONS

Knowledge of antibiotic pharmacodynamic properties and the potential altered antibiotic pharmacokinetics in critically ill patients can allow the intensivist to develop individualized dosing regimens. Specifically, for renally cleared drugs, measured creatinine clearance can be used to drive many dose adjustments. Maximizing clinical outcomes and minimizing antibiotic resistance using individualized doses may be best achieved with therapeutic drug monitoring.

Potential drug sequestration during extracorporeal membrane oxygenation: results from an ex vivo experiment

OBJECTIVE

Using an ex vivo simulation model we set out to estimate the amount of drug lost due to sequestration within the extracorporeal circuit over time.

DESIGN

Simulated closed-loop extracorporeal membrane oxygenation (ECMO) circuits were prepared using a 1.5-m2 silicone membrane oxygenator. Group A consisted of heparin, dopamine, ampicillin, vancomycin, phenobarbital and fentanyl. Group B consisted of epinephrine, cefazolin, hydrocortisone, fosphenytoin and morphine. Drugs were tested in crystalloid and blood-primed circuits. After administration of a one-time dose of drugs in the priming fluid, baseline drug concentrations were obtained (P0). A simultaneous specimen was stored for stability testing at 24 h (P4). Serial post-membrane drug concentrations were then obtained at 30 min (P1), 3 h (P2) and 24 h (P3) from circuit fluid.

MEASUREMENTS AND RESULTS

One hundred and one samples were analyzed. At the end of 24 h in crystalloid-primed circuits, 71.8% of ampicillin, 96.7% of epinephrine, 17.6% of fosphenytoin, 33.3% of heparin, 17.5% of morphine and 87% of fentanyl was lost. At the end of 24 h in blood-primed extracorporeal circuits, 15.4% of ampicillin, 21% of cefazolin, 71% of voriconazole, 31.4% of fosphenytoin, 53.3% of heparin and 100% of fentanyl was lost. There was a significant decrease in overall drug concentrations from 30 min to 24 h for both crystalloid-primed circuits (p = 0.023) and blood-primed circuits (p = 0.04).

CONCLUSIONS

Our ex vivo study demonstrates serial losses of several drugs commonly used during ECMO therapy. Therapeutic concentrations of fentanyl, voriconazole, antimicrobials and heparin cannot be guaranteed in patients on ECMO.

Pharmacokinetic changes during extracorporeal membrane oxygenation: implications for drug therapy of neonates

Extracorporeal membrane oxygenation (ECMO) is a prolonged form of cardiopulmonary bypass used to support patients with life-threatening respiratory or cardiac failure. In neonates, ECMO is used for a variety of indications, including sepsis and pulmonary diseases such as meconium aspiration syndrome, persistent pulmonary hypertension or congenital diaphragmatic hernia. In recent years, ECMO has been increasingly used after surgery to correct congenital cardiac defects. Despite the need for numerous drugs to maintain the ECMO circuit and treat the patient's underlying illness, relatively little is known of the disposition of drugs in this patient population. To date, the largest number of pharmacokinetic studies have been conducted with gentamicin and vancomycin. Both drugs have been found to have an increased volume of distribution, probably as a result of the addition of a large exogenous blood volume for circuit priming. Elimination half-lives for both drugs are prolonged during ECMO, with several studies demonstrating a return to expected values after decannulation. The reason for this prolonged elimination is probably multifactorial, with a reduction in renal function as the primary determinant. This same pattern of an increased volume of distribution and prolonged elimination has been found for several other drugs, including tobramycin, bumetanide and ranitidine. Other factors that affect drug disposition during ECMO include loss of the drug from adhesion to the circuit components and loss in the circulating blood volume during changes in the equipment. The benzodiazepines and propofol are largely sequestered within the circuit. Serum concentrations of heparin, morphine, fentanyl, furosemide, phenytoin and phenobarbital are also reduced by these mechanisms. The addition of haemofiltration or dialysis in up to a quarter of ECMO patients further complicates the determination of population pharmacokinetic parameters. The literature published to date on the pharmacokinetic changes associated with ECMO provide preliminary support for dosage adjustment; however, more research is needed to identify optimal administration strategies for this patient population.

The pharmacokinetics of anesthetic drugs and adjuvants during cardiopulmonary bypass

The institution of cardiopulmonary bypass during cardiac surgery has profound effects on the plasma concentration of drugs and thus their therapeutic effectiveness. These changes occur through acute hemodilution, altered plasma protein binding, hypotension, as well as the use of hypothermia and heparin administration. Isolation of the lungs from the circulation and the possible sequestration of drugs in the bypass circuit also affect drug plasma concentrations on bypass. The individual characteristics of the drug in question are also important in determining the final plasma concentration: Lipid soluble drugs with a high volume of distribution may be more readily taken up by bypass equipment, but the initial fall in concentration at the start of cardiopulmonary bypass may be more readily counteracted by back diffusion into plasma, if large tissue stores have accumulated. The extent of the drug's plasma protein binding is of importance as the effective free fraction in plasma for highly bound drugs will be sensitive to changes in plasma protein binding brought on by factors such as hemodilution, heparin administration as well as alpha, acid-glycoprotein binding. Clearly the fate of drugs administered before or on bypass is complex and can only be accurately determined by specific studies evaluating drug plasma concentrations. This review updates the available data on anesthetics and drugs used during cardiac surgery in order that anesthetists may predict better the likely effect of drugs administered before or during cardiopulmonary bypass.

Pharmacokinetics of drugs used in critically ill adults

Critically ill patients exhibit a range of organ dysfunctions and often require treatment with a variety of drugs including sedatives, analgesics, neuromuscular blockers, antimicrobials, inotropes and gastric acid suppressants. Understanding how organ dysfunction can alter the pharmacokinetics of drugs is a vital aspect of therapy in this patient group. Many drugs will need to be given intravenously because of gastrointestinal failure. For those occasions on which the oral route is possible, bioavailability may be altered by hypomotility, changes in gastrointestinal pH and enteral feeding. Hepatic and renal dysfunction are the primary determinants of drug clearance, and hence of steady-state drug concentrations, and of efficacy and toxicity in the individual patient. Oxidative metabolism is the main clearance mechanism for many drugs and there is increasing recognition of the importance of decreased activity of the hepatic cytochrome P450 system in critically ill patients. Renal failure is equally important with both filtration and secretion clearance mechanisms being required for the removal of parent drugs and their active metabolites. Changes in the steady-state volume of distribution are often secondary to renal failure and may lower the effective drug concentrations in the body. Failure of the central nervous system, muscle, the endothelial system and endocrine system may also affect the pharmacokinetics of specific drugs. Time-dependency of alterations in pharmacokinetic parameters is well documented for some drugs. Understanding the underlying pathophysiology in the critically ill and applying pharmacokinetic principles in selection of drug and dose regimen is, therefore, crucial to optimising the pharmacodynamic response and outcome.

Morphine pharmacokinetics during continuous infusion of morphine sulfate for infants receiving extracorporeal membrane oxygenation

OBJECTIVES

To determine a) if serum morphine concentration changes during the first 3 hrs of extracorporeal membrane oxygenation (ECMO); and b) if absorption of morphine onto the membrane oxygenator is responsible for these changes. Also, morphine clearance during the first 5 days of ECMO was studied.

DESIGN

Prospective, open-label study with consecutive patient enrollment.

SETTING

Neonatal intensive care unit at a university-affiliated, children's hospital.

SUBJECTS

Eleven neonates with severe persistent pulmonary hypertension of the newborn receiving continuous intravenous infusions of morphine sulfate and requiring ECMO.

INTERVENTIONS

Blood samples were obtained from the subjects and ECMO circuits at predetermined time intervals.

MEASUREMENTS AND MAIN RESULTS

Serum morphine concentration was determined using high-performance liquid chromatography. Morphine concentrations were no different from baseline at 5 mins, 1 hr, or 3 hrs after beginning ECMO. There was no significant difference in morphine concentration from samples taken immediately proximal and distal to the membrane oxygenator at 5 mins, 1 hr, and 3 hrs after the start of ECMO. Morphine clearance was calculated on days 1, 3, and 5 of ECMO. The mean value for morphine clearance was 11.7 +/- 9.3 (SD) ml/min/kg (range 2.6 to 34.5).

CONCLUSIONS

The initiation of ECMO does not lead to a significant decrease in serum morphine concentration and there is no uptake of morphine onto the membrane oxygenator of the ECMO circuit. Morphine clearance for infants receiving ECMO is variable.