The effect of high-dose mannitol on serum and urine electrolytes and osmolality in neurosurgical patients.

The effect of mannitol on serum and urine electrolytes and osmolality was investigated intraoperatively in neurosurgical patients. Patients in Group A (n = 7) received 1 gm . kg-1 of 20 per cent mannitol ("low"-dose) and in Group B, (n = 7) 2 gm . kg-1 ("high"-dose). There was a significant decrease in serum sodium and bicarbonate, and a significant increase in serum osmolality in both groups after mannitol administration. The decrease in serum sodium and the increase in serum osmolality were significantly greater in patients receiving the larger dose of mannitol. The infusion of low-dose mannitol resulted in a slight decrease in serum potassium. In contrast, after high-dose mannitol there was a significant rise in serum potassium reaching a maximum mean increase of 1.5 mmol . l-1. Urine electrolyte concentration and osmolality showed a similar decrease in both groups. The significant changes that occurred with the administration of mannitol were of short duration in these patients with normal cardiac and renal function. The clinically most important change is the increase in serum potassium with high-dose mannitol. The exact mechanism of this increase remains unclear.