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.
The reason that misuses still occur in our profession is partly due to the limited amount of data based on clinical studies
in veterinary medicine. Most of the studies available do not justify the use of prophylactic antibiotics in the study populations
examined. Despite this fact, it is safe to say that a majority of surgeries done in veterinary practices are performed with
antimicrobials given to the patient.
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).