Surgical Wound Prophylaxis

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Introduction

Postoperative wound infection is common, expensive and disabling. The basic idea behind surgical wound prophylaxis is that antibiotic should already be in the tissue at the time the wound is inflicted. This has been shown both experimentally and clinically to dramatically decrease the infection rate. It has long been realised that some patients are at far greater risk of developing wound infection than are others. Administration of antibiotics is not innocuous - apart from the cost, there is a substantial risk of allergy, up to and including death from unexpected anaphylaxis, as well as the fear that we might promote the development of resistance by our profligate use of antibiotics. We therefore need to identify those patients at particular risk, and target them with our prophylactic antibiotics. What determines the risk of wound infection? Ten or so years ago, the answer was easy. It had been shown that if one classified the wound according to how "dirty" it was, one could get a good idea of the likelihood of infection, as follows:

Classification of Surgical Procedures
Procedure	Definition	Wound Infection rate
Clean	Atraumatic, gastrointestinal/ genitourinary / respiratory tracts not entered	1-2%
Clean-contaminated	GIT / respiratory tract entered BUT no spillage, or oropharynx, sterile biliary or sterile GUT entered, or minor break in technique	2-4%
Contaminated	Acute inflammation, infected bile/urine, or gross GIT spillage	7-10%
Dirty	Established infection	10-40%

We now know that the above is a gross oversimplification. Young fit patients with contaminated or dirty wounds may shrug off the infection far more often than predicted above. Older patients with clean surgery and sick organs might have a far higher infection rate. The host of factors that determine wound infection (and thus the need for prophylaxis) include:

• Patient factors: age, nutritional status, immune compromise, and the presence of underlying neoplasia; as well as obesity and diabetes. We know that pre-operative malnutrition is a strong predictor of increased hospital stay, and this is largely related to increased infection. Organ dysfunction is thought to be a particularly adverse factor.
• Surgical technique. This is probably the single most important determinant. Leaving vast volumes of dead space, failing to achieve adequate haemostasis with consequent haematoma formation, and not handling the tissues gently extend the welcome mat to any passing microbe. Diathermy may be associated with a substantial increase in the rate of wound infection.
• Duration of surgery is vital: long operations result in infected wounds.
• Wound contamination, as mentioned above.
• Bacterial virulence is important.
• Wound preparation is also a determinant. The barbaric traditional process of shaving the wound the evening before vastly increases infection rates owing to the micro-trauma inflicted on the skin. (Clipping the hair is fine. Newer wound preparation techniques may also show promise).
• In special circumstances such as insertion of orthopaedic prostheses, wound infection may also be decreased by use of fancy air-conditioning, laminar flow, and even uncomfortable "space suits". These are not for general use.

Procedures where there is a particular risk of infection, or where infection has particularly severe consequences include:

• Urological procedures, especially on the prostate;
• Gynaecological procedures;
• Upper gastrointestinal surgery where there has been gastric stasis;
• Lower GIT surgery;
• Insertion of prostheses (esp. orthopaedic ones);

The Infecting Organism

This depends on the flora colonising the skin. We know that forty-eight hours after admission to hospital, most patients are colonised by the local nasties peculiar to that institution. A variety of organisms can cause wound infection, including:

• Gram Positive Cocci:
  • Staphylococcus aureus
  • Coagulase negative Staphylococci
  • Enterococci
• Gram negative bacilli:
  • Enterobacteriaceae such as E. coli
  • Pseudomonas
  • Acinetobacter, and so on.
• and even Anaerobes such as Bacteroides fragilis.

How to give prophylactic antibiotics

1. Timing Because the antibiotic must be present in the tissue at the time the infecting organism gets there, timing is crucial. The initial dose should be given IMMEDIATELY BEFORE SURGERY. A second dose may be necessary with prolonged surgery - if the operation lasts longer than twice the half-life of the drug (see below) then give a second dose. There is no substantial evidence that continuing the antibiotic after the cessation of surgery is of any value.
2. Route This should generally be intravenous, (although some claim that oral administration prior to colorectal surgery is effective).
3. Which antibiotic? The antibiotic should cover the likely organism. This obviously changes with time, and depending on the institution. It is also probably not a good idea to use the same drug for prophylaxis as you would use for therapy. A wide variety of recommendations exists, but generally "simple is best".

The following simple table gives some idea of possible prophylaxis by site:

Recommended Antibiotic Prophylaxis
Procedure	Common micro-organisms	Rx
Gynaecologic	enteric G-ves, Strep Group B, Enterococci	Cefazolin 1g IV OR cefuroxime; some Add metronidazole
Caesarean section	As for gynaecologic, above	NO antibiotic if c/s is elective, otherwise as for gynaecologic, above. Some only give antibiotic after the cord is clamped! - is this prophylaxis?
Orthopaedic	S. aureus, S epidermidis	Cefazolin 1g IV, or Cefuroxime 1.5g IV
Gastric resection, percutaneous gastrostomy	Enteric G-ves, Enterococci, sometimes Bacteroides spp.	Cefazolin 1g IV
Biliary surgery, if patient is elderly, has obstructive jaundice, acute cholecystitis, prior biliary surgery, or biliary calculi	Enteric G-ves, Enterococci, Clostridium spp.	Cefoxitin 2g IV, or consider Amoxycillin+clavulanate
Colonic surgery	Enteric G-ves, Anaerobes especially B. fragilis	Cefoxitin 2g IV, OR combination of Penicillin G + metronidazole + aminoglycoside, or even Amoxycillin+clavulanate !
Head & Neck surgery	S viridans, S aureus, Enteric G-ves, oral anaerobes such as peptostreptococci, fusobacteria	Cefuroxime 1.5g IV, consider adding metronidazole especially if prior radiotherapy. Alternative: amoxycillin+clavulanate.
Vascular Surgery	S. aureus, S. epidermidis, Streptococci, Enteric G-ves	Cefazolin 1g IV
Cardiac / Thoracic surgery (eg. CABG, valve surgery, lung resection)	Staphylococci, S. pneumoniae, Enteric G-ves, oral anaerobes	Cefazolin 1g IV
Urologic surgery: prostate	E. coli, Klebsiella spp, Enterococci, Pseudomonas spp.	Cefazolin 1g IV.
In many specific circumstances such as neurosurgery, prophylaxis should depend on local hospital complication rates.

As can be seen, you are generally on pretty safe ground if you just give cefazolin 1g IV. Exceptions include those sites where anaerobes prevail (such as the colon).

References

Much of the above is loosely based on a lecture given by Dr Adrian Duse on 15 October 1997. Also see:

1. Can Med Assoc J 1994 151(7) 925-31pp
   Committee on Antimicrobial Agents
   This article contains very reasonable recommendations. Note that it has however been criticised [Can Med Assoc J 1995 152(9) 1381] for its 'simplistic' classification of wound infection. My table of recommendations (above) is partially based on their Table 2.