The principles governing use of antimicrobials in this context are as follows.
Chemoprophylaxis is justified:
— When the risk of infection is high because of the presence of large numbers of bacteria in the viscus which is being operated on, e.g. the large bowel
— when the risk of infection is low but the consequences of infection would be disastrous, e.g. infection of prosthetic joints or prosthetic heart valves, or of abnormal heart valves following the transient bacteraemia of dentistry
— when the risks of infection are low but randomised controlled trials in large numbers of patients have shown the benefits of prophylaxis to outweigh the risks, e.g. singledose antistaphylococcal prophylaxis for uncomplicated hernia and breast surgery. This indication remains controversial.
Antimicrobials should be selected with a knowledge of the likely pathogens at the sites of surgery and their prevailing antimicrobial susceptibility.
Antimicrobials should be given i.v., i.m. or occasionally rectally at the beginning of anaesthesia and for no more than 48 h. A single preoperative dose, given at the time of induction of anaesthesia, has been shown to give optimal cover for many different operations. Specific instances are:
1. Colorectal surgery, because there is a high risk of infection with Escherichia coli, Clostridium spp, streptococci and Bacteroides spp which inhabit the gut (a cephalosporin plus metronidazole, or benzylpenicillin plus gentamicin plus metronidazole are commonly used)
2. Gastroduodenal surgery, because colonisation of the stomach with gut organisms occurs especially when acid secretion is low, e.g. in gastric malignancy, following use of a histamine H2-receptor antagonist or following previous gastric surgery (usually a cephalosporin will be adequate)
3. Gynaecological surgery, because the vagina contains Bacteroides spp and other anaerobes, streptococci and coliforms (metronidazole and a cephalosporin are often used).
4. Leg amputation, because there is a risk of gas gangrene in an ischaemic limb and the mortality is high (benzylpenicillin, or metronidazole for the patient with allergy to penicillin)
5. Insertion of prosthetic joints. Chemoprophylaxis is justified because infection (Staphylococcus aureus, coagulase-negative staphylococci and coliforms are commonest) almost invariably means that the artificial joint, valve or vessel must be replaced (various regimens are used, with inclusion of vancomycin when the local MRSA prevalence is high). Single perioperative doses of appropriate antibiotics with plasma elimination half-lives of several hours (e.g. cefotaxime) are adequate, but if short half-life agents are used (e.g. flucloxacillin) several doses should be given during the first 24 hours.
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