Definition and History:

A working definition of antimicrobial prophylaxis in surgery is the administration of an antimicrobial drug to a patient, in the absence of infection, prior to surgery. The history of the use of these agents during surgery is interesting and reveals many of the problems which occur with their use. When antimicrobial agents became available to surgeons, they did not provide the panacea for prevention of all surgical infections. In fact, a twenty-year analysis done in the mid-seventies indicated that no significant alteration of infection rates had occurred since the advent of prophylactic antimicrobial usage in human surgery. The study went on to identify the following misuses:
     1. Excessive use in clean surgical procedures
     2. Faulty timing of administration of the antimicrobial agent
     3. Continued use beyond the time necessary for benefit.

Today, unfortunately, some of these misuses are still occurring in veterinary surgery.

Wound Infections:

In the evolution of wound infections, there are three main components. These are bacterial inoculum, bacterial nutrition, and impaired host resistance. The mere presence of bacteria is less important than the level of bacterial growth. Therefore, the goal of the surgeon is to maintain a favorable balance between patient and bacteria. It is important to remember that proper surgical technique and proper patient preparation, strict adherence to aseptic technique and application of atraumatic surgical technique are far more important in the prevention of infection than the use of antibiotics.

Patient Profile for Antimicrobial Prophylaxis:

The next question to ask is "in which patients should I use antimicrobial prophylaxis?" There are no hard and fast rules to follow but the following examples can be used for some general guidelines. Most orthopedic procedures are defined as clean surgical wounds, and, in general, the use of prophylaxis is not recommended. Important factors to consider when giving antibiotics prior to surgery include: anticipated duration of the operation (degree of contamination), local wound factors favoring infection (e.g., extensive tissue trauma, placement of large implants) and systemic factors favoring infection (e.g., concurrent infections, diseases suppressing immunity).

Procedures in which it is difficult to justify giving antibiotics include:
     1. Cranial cruciate repair
     2. Patellar repair
     3. Arthrotomies including removal of endochondral ossification defects or open joint reductions
     4. Many closed fracture repairs

Procedures which can be more easily justified for the use of prophylaxis are:
     1. Total hip replacement
     2. Complex multiple fractures
     3. Open fractures
     4. Systemically comprised patients

Timing of administration:

Maximal therapeutic concentrations of the antibiotic must be present in the tissue at the time of contamination (i.e. beginning of surgery)!!! Experimental work has demonstrated a short, early period in which "decisive biochemical interactions" between the microorganisms and the host tissue occur. During this time the development of the primary bacterial lesion is susceptible to the action of parenterally administered antibiotics. The major effect is in the first minutes of the contamination and no effect is seen if antibiotics are given 3 hours after contamination has begun. Thus, if given intramuscularly, administer 30 minutes prior to your incision. If given intravenously, administer 15 minutes prior to the incision. Repetitive dosing during surgery should occur depending on the antimicrobial given. As an example with a first generation cephalosporin (cefazolin) every 2 to 2.5 hours is adequate according to published data. Serum half-life has been used as a guideline for this dosing, but it is not consistent with concentrations in the tissue (i.e., the drug is given at every half-life).