Tuesday, 26 May 2015

Administration of parenteral prophylactic beta lactam antibiotics in 2014: a review. Anesthesia & Analgesia April 2015.

Perhaps this is a good time to review our antibiotic dosing given the recent Antibiotic Audit. Key points:

1. Dosing of antibiotics
Prophylactic antibiotics are given to minimise incidence of surgical site infections. The time from skin incision to a few hours after skin closure is a “decisive period” during which infection is established. Prophylactic antibiotics are most effective when given during this period, however they need to be given at a sufficient dose and frequency so that maximum tissue concentration occurs at the time of incision and tissue concentration exceeds the MIC (minimum inhibitory concentration) of potential microbial pathogens for the duration of this period.  Beta lactams (as well as clindamycin) are time dependent antibiotics, i.e. the fraction of time the drug concentration exceeds MIC is the primary determinant of effectiveness and it does not have any post antibiotic effect. Gentamicin is concentration dependent and has significant post antibiotic effect.

2. Timing
Antimicrobials need to reach the operative site before contamination occurs, and the tissue concentration exceeds MIC for the probable pathogen. Consensus seems to be 30-45min prior to skin incision, especially for obese patients (there is more tissue for the drug to travel).

3. Redosing
Cefazolin has plasma half life of 1.2-2.2h and should be redosed every 2 half lives thus every 4 hours; ampicillin redosed every 2h. Many suggest a continuous infusion after the initial prophylactic bolus injection is more effective in preventing plasma concentration falling below MIC.

4. Dose amount
Cephazolin: 1g <70kg; 2g >70kg and 3g if >120kg; the article does have a more complex dosing calculator.

5. Use of a tourniquet
First dose 15-45min prior to tourniquet application and then another dose after tourniquet release for joint replacements; continuous infusion is not logical.

6. In patients with penicillin allergy, the article suggests that the patient needs to have had a serious adverse reaction to dismiss cephazolin.

The April issue of Anesthesia and Analgesia is all about anaesthetists making a difference in hospital acquired infections. In fact bacterial transmission within the anaesthesia work area as the root cause of 30day postoperative infections occuring in up to 16% of patients undergoing surgery! A multi-modal approach is required to reduce the risk of “within patient” and “between patient” pathogen transmission and post-operative infection. This multimodal approach targets decontaminating the patients’ bacterial reservoirs and the anaesthetic working space, better IV access handling and stricter hand hygiene. Double gloving during intubation significantly reduces workspace contamination, analogous to surgeons double gloving during surgical draping. Speaking of IV access ports, a small studys showed that more bacteria was grown from stop cocks in patient who had propofol as part of their anaesthesia than those who didn’t. By 48h, the stop cocks with propofol had >100 times CFU! If propofol is used and the IV line is to remain in place, should we change the infusion sets or at least the stopcocks before the patients leave PACU? Knowing that we recently changed our practice to not change IV extension tubing in PACU prior to going to the ward…


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