Systemic availability of prophylactic cefuroxime in patients submitted to coronary artery bypass grafting with cardiopulmonary bypass

Pharmacokinetic Model for Cefuroxime Dosing during Cardiac Surgery under Cardiopulmonary Bypass

Cefuroxime (CXM) is an antibiotic recommended for surgical site infection prevention in cardiac surgery. However, the dosing regimens commonly used do not sustain therapeutic concentrations throughout surgery. The aim of this study was to conduct a population analysis of CXM pharmacokinetics (PK), and to propose an optimized dosing regimen. Adult patients undergoing cardiac surgery under cardiopulmonary bypass (CPB) received a 1,500 mg CXM intravenous bolus followed by a 750 mg bolus at CPB priming, then every 2 h thereafter. Model-based PK simulations were used to develop an optimized dosing regimen and evaluate its efficacy in attaining various concentration thresholds, including those recommended in US and European guidelines. In total, 447 CXM measurements were acquired in 50 patients. A two-compartment model best fit the data, with total body weight and creatinine clearance determining interpatient variability in the central and peripheral volumes of distribution, and in elimination clearance, respectively. Using our optimized dosing regimen, different dosing schemes adapted to body weight and renal function were calculated to attain total concentration thresholds ranging from 12 to 96 mg/liter. Our simulations showed that the dosing regimens recommended in US and European guidelines failed to maintain concentrations above 48 mg/liter. Our individualized dosing strategy was capable of ensuring therapeutic CXM concentrations conforming to each target threshold. Our model yielded an optimized CXM dosing regimen adapted to body weight and renal function, and sustaining therapeutic concentrations consistent with each desired threshold. The optimal target concentration and necessary duration of its maintenance in cardiac surgery still remain unclear.

Population Pharmacokinetic Model-Based Evaluation of Standard Dosing Regimens for Cefuroxime Used in Coronary Artery Bypass Graft Surgery with Cardiopulmonary Bypass

The purpose of this study was to investigate the population pharmacokinetics (PK) of cefuroxime in patients undergoing coronary artery bypass graft (CABG) surgery. In this observational pharmacokinetic study, multiple blood samples were collected over a 48-h interval of intravenous cefuroxime administration. The samples were analyzed by using a validated high-performance liquid chromatography (HPLC) method. Population pharmacokinetic models were developed using Monolix (version 4.4) software. Pharmacokinetic-pharmacodynamic (PD) simulations were performed to explore the ability of different dosage regimens to achieve the pharmacodynamic targets. A total of 468 blood samples from 78 patients were analyzed. The PK for cefuroxime were best described by a two-compartment model with between-subject variability on clearance, the volume of distribution of the central compartment, and the volume of distribution of the peripheral compartment. The clearance of cefuroxime was related to creatinine clearance (CL_CR). Dosing simulations showed that standard dosing regimens of 1.5 g could achieve the PK-PD target of the percentage of the time that the free concentration is maintained above the MIC during a dosing interval (fT_MIC) of 65% for an MIC of 8 mg/liter in patients with a CL_CR of 30, 60, or 90 ml/min, whereas this dosing regimen failed to achieve the PK-PD target in patients with a CL_CR of ≥125 ml/min. In conclusion, administration of standard doses of 1.5 g three times daily provided adequate antibiotic prophylaxis in patients undergoing CABG surgery. Lower doses failed to achieve the PK-PD target. Patients with high CL_CR values required either higher doses or shorter intervals of cefuroxime dosing. On the other hand, lower doses (1 g three times daily) produced adequate target attainment for patients with low CL_CR values (≤30 ml/min).

Dosing antibiotic prophylaxis during cardiopulmonary bypass-a higher level of complexity? A structured review

In highly invasive procedures such as open heart surgery, the risk of post-operative infection is particularly high due to exposure of the surgical field to multiple foreign devices. Adequate antibiotic prophylaxis is an essential intervention to minimise post-operative morbidity and mortality. However, there is a lack of clear understanding on the adequacy of traditional prophylactic dosing regimens, which are rarely supported by data. The aim of this structured review is to describe the relevant pharmacokinetic/pharmacodynamic (PK/PD) considerations for optimal antibiotic prophylaxis for major cardiac surgery including cardiopulmonary bypass (CPB). A structured review of the relevant published literature was performed and 45 relevant studies describing antibiotic pharmacokinetics in patients receiving extracorporeal CPB as part of major cardiac surgery were identified. Some of the studies suggested marked PK alterations in the peri-operative period with increases in volume of distribution (V_d) by up to 58% and altered drug clearances of up to 20%. Mechanisms proposed as causing the PK changes included haemodilution, hypothermia, retention of the antibiotic within the extracorporeal circuit, altered physiology related to a systemic inflammatory response, and maldistribution of blood flow. Of note, some studies reported no or minimal impact of the CPB procedure on antibiotic pharmacokinetics. Given the inconsistent data, ongoing research should focus on clarifying the influence of CPB procedure and related clinical covariates on the pharmacokinetics of different antibiotics during cardiac surgery. Traditional prophylactic dosing regimens may need to be re-assessed to ensure sufficient drug exposures that will minimise the risk of surgical site infections.

Are children undergoing cardiac surgery receiving antibiotics at subtherapeutic levels?

OBJECTIVES

Perioperative antibiotics have decreased-but not eradicated-postoperative infections. In patients undergoing cardiac surgery with cardiopulmonary bypass, the dilutional effect of the priming and any additional volume given during the procedure may lead to subtherapeutic antibiotic levels. Our aim was to determine if children undergoing cardiac surgery with cardiopulmonary bypass receive perioperative antibiotics at subtherapeutic levels.

METHODS

Using published pharmacokinetic data on cefuroxime, we developed a computer simulation model to generate a nomogram predicting patients at risk for subtherapeutic cefuroxime levels based on time from initial dosing and additional volume given.

RESULTS

A computer-generated 1-compartment pharmacokinetic model was created to predict cefuroxime plasma levels over time for patients of all weights and additional volumes given for both a 25- and 50-mg/kg intravenous dose. For example, following a 25-mg/kg dose, a patient receiving an additional volume of 275 mL/kg is predicted to be subtherapeutic (<16 mg/L=4×minimum inhibitory concentration) at 4 hours. Our nomogram predicts all patients will be subtherapeutic at 8 hours, consistent with general pediatrics dosing schemes. Following a 50-mg/kg dose, levels are predicted to be subtherapeutic after an additional volume of 315 mL/kg at 5.5 hours.

CONCLUSIONS

Our model predicts which patients undergoing cardiac surgery with cardiopulmonary will have subtherapeutic cefuroxime levels. This nomogram enables providers to determine when to administer additional antibiotics in patients receiving large additional volumes during cardiac surgeries. This rational approach to perioperative antibiotic dosing may result in a reduction in postoperative infection in this vulnerable patient population.

Clinical relevance of pharmacokinetics and pharmacodynamics in cardiac critical care patients

Pharmacokinetics is a discipline aimed at predicting the best dosage and dosing regimen for each single drug in order to ensure and maintain therapeutically effective concentrations at the action sites. In cardiac critical care patients, various pathophysiological conditions may significantly alter the pharmacokinetic behaviour of drugs. Gastrointestinal drug absorption may be erratic and unpredictable in the early postoperative period, and so patients may be unresponsive to oral therapy; thus the intravenous route should be preferred for life-saving drugs whenever feasible. Variations in the extracellular fluid content as a response to the trauma of surgery and the fluid load or significant drug loss through thoracic drainages may significantly lower plasma concentrations of extracellularly distributed hydrophilic antimicrobials (beta-lactams, aminoglycosides and glycopeptides). Drug metabolism may be altered by the systemic inflammatory response and/or multiple organ failure and/or drug-drug pharmacokinetic interactions that can potentially occur during polytherapy, especially in immunosuppressed cardiac transplant patients. Instability of renal function may promote significant changes in body fluid concentrations of renally eliminated drugs, even in a brief period of hours. Finally, the application of extracorporeal circulation by means of cardiopulmonary bypass may significantly alter the disposition of several drugs during the operation because of acute haemodilution, hypoalbuminaemia, hypothermia and/or adsorption to the bypass equipment. Accordingly, to avoid either overexposure and the consequent increased risk of toxicity or underexposure and the consequent risk of therapeutic failure in critically ill cardiac patients, the dosing regimens of several drugs are expected to be significantly different from those suggested for clinically stable patients. Additionally, therapeutic drug monitoring may be helpful in the management of drug therapy and should be routinely used to guide individualized dose adjustments for (i) immunosuppressants whenever cytochrome P450 3A4 isoenzyme inhibitors (e.g. macrolide antibacterials, azole antifungals) or inducers (e.g. rifampicin [rifampin]) are added to or withdrawn from the regimen; and (ii) glycopeptide and aminoglycoside antibacterials whenever haemodynamically active agents (such as dopamine, dobutamine and furosemide [frusemide]) are added to or withdrawn from the regimen, and also whenever significant changes of haemodynamics and/or of renal function occur.