Mannitol concentrations in blood plasma in connection with transurethral resection of the prostate using mannitol...

Thirteen patients undergoing transurethral resections of the prostate (TURP) using iso-osmolar 5% mannitol as an irrigating fluid were studied. Mannitol was determined in serum (plasma), as were sodium, prostatic acid phosphatase protein (PAP) and osmolality as probable indicators of absorption of irrigating fluid. The plasma level of mannitol (mean 2.7 g/l = 15 mmol/l) immediately postoperatively, the increase in serum PAP (mean 93 micrograms/l) and the decrease in serum sodium (mean 8.7 mmol/l) all reflect the amount of irrigating fluid absorbed during TURP. The three variables are intercorrelated. The plasma osmolality was unchanged (mean -1 mosmol/kg). A small but constant fraction of mannitol was found in the erythrocytes 2 hours after the operation, amounting to about 3% of the simultaneous plasma concentration. The mean plasma half-life of mannitol was 127 min in the absence of uraemia. In two cases showing a slight increase in serum creatinine the half-lives were prolonged. An estimate of the volume of fluid absorbed was made from the observed plasma mannitol levels. A fluid absorption of up to 3 litres (mean 1.1 l) was found. A strong diuretic effect was observed in some cases when irrigation with mannitol was combined with i.v. furosemide. We conclude that the i.v. diuretic should be withheld until the extent of fluid absorption has been estimated. If the sodium concentration in the serum is largely unchanged immediately postoperatively, diuresis can be induced by an intravenous diuretic.