Acute Effects of Furosemide and Mannitol on Renal Function in the Early Postoperative Period

The effects of furosemide and mannitol on renal function in the immediate postoperative period were studied in 16 patients, who had undergone upper abdominal surgery under neurolept anaesthesia. Renal function was studied preoperatively and during the first postoperative hour with a standard clearance technique. The patients were then randomly given furosemide, 1 mg/kg b.w., or mannitol, 0.5 g/kg b.w. Renal function was further investigated during the following 6 h in periods of varying duration. The injection of furosemide caused an initial increase of renal plasma flow (RPF), while glomerular filtration rate (GFR) decreased. Striking increases of urine flow, fractional sodium, fractional chloride and fractional osmolal excretion were found. These increases reached maximum levels during the second 20‐min clearance period after the injection. Fractional free water reabsorption decreased and during the first 20‐min clearance period it changed into free water clearance. These changes had reached or nearly reached the preoperative control levels after 6 h. The infusion of mannitol caused an initial rise in RPF, while GFR remained stable. Increases were found in urine flow, fractional sodium, fractional chloride, fractional osmolal excretions and fractional free water reabsorption. The changes reached maximum levels during the first 20‐min control period and then gradually returned to the preoperative control levels. Mannitol did not impair GFR and had a milder effect on renal function than furosemide. The losses of sodium and water were much smaller in the mannitol group than in the furosemide group. It is therefore concluded that mannitol should be used as the diuretic of choice in the treatment of postoperative oliguria in patients without known cardiovascular disease associated with increased preload and/or left ventricular insufficiency.