Such measures may decrease the incidence of late prosthetic valve endocarditis caused by viridans group streptococci. Patients who undergo surgery for placement of prosthetic heart valves or prosthetic intravascular or intracardiac mate- rials are at risk for the development of infection.152 Because the morbidity and mortality of infection in these patients are high, perioperative prophylactic antibiotics are recommended (Class I, LOE B). Early-onset prosthetic valve endocarditis is most often caused by S aureus, coagulase-negative staphylo- cocci, or diphtheroids. No single antibiotic regimen is effec- tive against all these microorganisms. Prophylaxis at the time of cardiac surgery should be directed primarily against staphylococci and should be of short duration. A first- generation cephalosporin is most often used, but the choice of an antibiotic should be influenced by the antibiotic suscepti- bility patterns at each hospital. For example, a high preva- lence of infection by methicillin-resistant S aureus should prompt the consideration of the use of vancomycin for perioperative prophylaxis. The majority of nosocomial coagulase-negative staphylococci are methicillin-resistant. Nonetheless, surgical prophylaxis with a first-generation cephalosporin may be recommended for these patients (Class I, LOE A).107 In hospitals with a high prevalence of methicillin-resistant strains of S epidermidis, surgical prophylaxis with vancomycin may be reasonable but has not been shown to be superior to prophylaxis with a cephalosporin (Class IIb, LOE C). Prophylaxis should be initiated immediately before the operative procedure, repeated during prolonged procedures to maintain serum concentrations intraoperatively, and continued for no more than 48 hours postoperatively to minimize emergence of resistant micro- organisms (Class IIa, LOE B). The effects of cardiopulmonary bypass and compromised renal function on antibiotic concentra- tions in serum should be considered and dosages adjusted as necessary before and during the procedure. 