Pharmacokinetics of Bolus Versus Continuous Cefazolin Infusion in Patients Undergoing Major Surgery

The current incidence of post-operative surgical infections varies between 6% for patients undergoing non-cardiac surgery to greater than 30% in high-risk surgery. 1 The administration of an antibiotic preoperatively is an important intervention that helps to reduce the risk of post-operative health-care associated infections (HCAI). These include urinary tract, surgical site and blood stream infection. Based on the current Surgical Infection Prevention Project, (SCIP) recommendations, the choice of the antibiotic is based on the nature and site of the surgery and the presence of a β-lactam allergy (Table 2). The antibiotic of choice for perioperative prophylaxis remains first and second generation cephalosporin. These drugs have a broad range of antimicrobial activity against common skin pathogens and have an excellent safety profile.

Based on current recommendations, the cephalosporin is administered 1 hour prior to incision.1 Timing is critical, as both early and late administration of the antibiotic is associated with an increase risk of HCAI. Additional doses of the cephalosporin are administered at pre-set intervals based on the duration of the surgery in an attempt to maintain antibiotic concentrations at a desired level.

Antibiotics can be classified into two broad classes, based on their bacterial killing characteristics. Antibiotics with concentration-dependent killing characteristics require a high ratio between the peak concentration and the minimum inhibitory concentration (MIC) of the pathogen, during the dosing interval. Therefore large infrequent doses of these antibiotics will result in optimal antibacterial activity.

In contrast β-lactam antibiotics, like cephalosporins, demonstrate time-dependent pharmacokinetics. This means that effective microbiological activity only occurs if the unbound (or free) plasma concentration of the drug is above the MIC for a specified time period. In infection models, bacteriostatic and bacteriocidical activity occurs when antibiotic concentrations are maintained above the MIC for 35-40% and 60-70% of the dosing interval respectively. However, in surgical prophylaxis, it is preferred that antibiotic concentrations are maintained above the MIC of the likely pathogen/s for the entire duration of the procedure. Therefore more frequent dosing or use of extended or continuous infusion of these drugs will better maintain the pharmacokinetic goals. Recent studies have shown that plasma levels of β-lactam antibiotics fall below the MIC 1-2 hours after the initial bolus dose in both normal and obese patients.2, 3 Even with appropriate re-dosing of the β-lactam antibiotic, there are times when the plasma concentration of the drug is below the MIC. The combination of sub-MIC plasma drug level and high bacteremic load can potentially increase the risk of developing a post-operative HCAI. It follows that to effectively prevent microbiological activity in the perioperative period the β-lactam antibiotic must remain above the MIC during the entire operative period. This is of particular importance in patients undergoing major abdominal surgery where the risk of perioperative surgical site infection (including superficial, deep and organ specific infection) is 13.1%. The aim of this study is to compare the pharmacokinetics of bolus and continuous cefazolin infusion during major abdominal surgery.

Hypothesis to be Tested Hypothesis A continuous infusion of cefazolin maintains the unbound plasma concentration above Minimum Inhibitory Concentration (MIC) for the duration of the operative procedure better than the traditional bolus administration at every four hours.