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Prevention of acute hyponatremia by mannitol: an unanticipated mechanism
M Gowrishankar1, C B Chen, S Cheema-Dhadli
1Division of Nephrology, St. Michael's Hospital, University of Toronto, Ontario, Canada.
Purpose:
To evaluate the effectiveness and mode of action of the osmotic diuretic mannitol to prevent the development of acute hyponatremia in a setting designed to mimic the acute hyponatremia observed on the day of surgery.
Results:
Hyponatremia (129+/-1 mM, fall of 10+/-1 mM, p <0.01) was induced by the intraperitoneal administration of half-isotonic saline plus DDAVP to rats (n = 8); hyponatremia was due to a positive balance of electrolyte-free water (EFW). Rats given mannitol (10 mmol/kg body wt) in addition to the hypotonic saline plus DDAVP had only a minor degree of hyponatremia (plasma [Na+] 136+/-1 mM, fall 3+/-2 mM, p >0.05). All the mannitol given was excreted over the 16 h of observation, but the urine volume was not higher in these rats. The higher rate of excretion of EFW was due to a marked reduction in the rate excretion of Na+ with mannitol. This antinatriuresis was also present when mannitol was given intravenously.
Conclusions:
Although mannitol increased the excretion of EFW, the mechanism required an enhanced renal reabsorption of Na+ rather than an increase in the urine flow rate.