Effect of mannitol on diuresis and acid-base equilibrium in critically ill patients.

Background: Mannitol is the most commonly used osmotic diuretic, but its effect on plasma and urine electrolytes and on acid-base equilibrium have not been well investigated. The aim of this study was to evaluate the short-term effects of mannitol on diuresis and plasma and urine acid-base equilibrium in a group of critically ill patients according to the Stewart approach.

Results: Prospective observational study enrolling all consecutive sedated and mechanically ventilated patients requiring mannitol infusion for clinical purpose. Plasma and urine acid-base variables and electrolytes were measured before mannitol infusion and every 60 and 30 min, respectively, following the infusion of 1 g/kg of ideal body weight of mannitol. Forty-two patients were enrolled. Diuresis increased significantly 30 min after the mannitol infusion was completed and remained significantly higher as compared with T₀. Plasma sodium and chloride concentrations and plasma SID significantly decreased after mannitol infusion ended; urine sodium and chloride concentration remained unchanged, while urine ammonium increased increasing urine SID.

Conclusions: Since the end of the infusion, mannitol promoted a significant increase in diuresis, with a reduction in plasma electrolytes due to volume expansion, and a slight decrease in arterial pH due to dilutional acidosis. Kidney relative excretion pattern was unmodified during the study.