Postoperative Hyponatremia

Mitchell Halperin; Manjula Gowrishankar; C. David Mazer

doi:10.7326/0003-4819-126-12-199706150-00031

Letters |15 June 1997

Postoperative Hyponatremia

Mitchell Halperin, MD, FRCP(C); Manjula Gowrishankar, MD, FRCP(C); C. David Mazer, MD, FRCP(C)

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IN RESPONSE:

Ayus and Arieff's letter deserves clarification on three points.

First, can the plasma sodium level decrease to 120 mmol/L if only isotonic saline was infused? It is universally agreed that severe hyponatremia must be avoided immediately after surgery. Anything that adds electrolyte-free water (for example, hypotonic infusions or desalination of isotonic saline) when antidiuretic hormone acts contributes to the degree of hyponatremia and should be minimized. Moreover, the amount given relative to body size is important. A calculation might help here. Almost 15% of total body water must be retained as electrolyte-free water to cause the plasma sodium level to decrease to 120 mmol/L. If isotonic saline were the sole fluid given to a patient with 25 L of total body water (50 kg, 50% water), one must “desalinate” 8 L (the volume infused in our patients was as high as 10 L); much less is needed with a smaller volume of total body water. Finally, a wrong impression is gleaned from an analysis of only mean values: Two patients had a plasma sodium level of 131 mmol/L, but no one had expected this decrease because no electrolyte-free water had been given.

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Letters |15 June 1997

Postoperative Hyponatremia

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Letters |15 June 1997

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