Absorption and Elimination of Mannitol Solution when used as an Isotonic Irrigating Agent in Connection with Transurethral Resection of the Prostate

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Abstract
12 patients were studied in connection with transurethral resection of the prostate using the intermittent technique and isotonic 5% mannitol solution as an irrigating fluid. No diuretics were given postoperatively. The serum creatinine concentrations were normal in 10 patients and slightly elevated in 2. The plasma mannitol levels were followed for four hours postoperatively. The highest concentration observed in the series was 6275 mg/l (34.9 mmol/l) immediately postoperatively. The maximum level was observed immediately postoperatively in all patients (mean 2140 mg/l, 11.9 mmol/l). The mean half-life of mannitol in plasma was 163 min (10 patients). The patient with the highest serum creatinine concentration preoperatively (138 μmol/l) showed a marked prolongation of the half-life (692 min). The mean intravenous fluid absorption calculated from the immediate postoperative mannitol concentrations was 0.681 (range 0.05–1.781). A decrease in the serum sodium concentration was observed immediately postoperatively (mean 5.4 mmol/l, range 0–19 mmol/l). There was a correlation between the decrease in the serum sodium concentration and the simultaneous plasma mannitol concentration. There was no significant change in plasma osmolality. Mannitol elimination in urine was followed for 24 hours postoperatively in 7 patients. The mean absorbed volume of irrigating fluid was calculated from the elimination data and was found to be 0.481, which should be compared with the figure obtained from the calculation based on the immediate postoperative plasma mannitol concentration, which gave 0.491 (mean) in the same 7 patients. This shows that, in spite of the theoretical assumptions made in the calculation of absorbed fluid volume from the plasma mannitol concentration, this method of calculation is valid. The correlation between the plasma concentration immediately postoperatively and the decrease in the serum sodium level from the preoperative to the immediate postoperative status makes it possible and more practical to estimate absorbed fluid volume from the serum sodium decrease.