Treatment with an
aminoglycoside plus
flucloxacillin is commonly used to prevent
wound infection and
endocarditis after cardiac surgery.
Cardiopulmonary bypass,
blood transfusion and the lack of a steady state greatly affect handling of
gentamicin. Urinary
gentamicin excretion is not predictable in terms of preoperative risk factors possibly because there is no clear relationship between serum levels and
gentamicin clearance. A study was performed to determine whether the existing prophylactic regimen gave adequate serum levels during surgery and to compare renal excretion of
gentamicin and the trough serum levels. Ten patients received
gentamicin (1.5 mg/kg at the start of surgery followed by 80 mg
tds for 2 days) and
flucloxacillin 500 mg qds for 2 days. Serum and urinary concentrations of
gentamicin were assayed during surgery and in the early postoperative period. The median apparent serum half-life during the first 8 h was 2.5 h (95%CI 1.7-3.2 h). The median
gentamicin clearance was 37 mL/min(95%CI 23-64 mL/min) and the
creatinine clearance 85 mL/min (95%CI 72-210 mL/min). Serum levels remained above 1 mg/L during surgery but urinary concentrations varied between 0.4 and 364 mg/L (median 70 mg/L). At 24 h (but not 8 or 16 h), trough serum levels appeared to be related to the amount of
gentamicin excreted but the relationship was not quite statistically significant (P = 0.057). Despite the effects of
cardiopulmonary bypass, therapeutic serum
gentamicin levels were maintained during surgery and reduced renal excretion in the postoperative period was associated with raised levels.