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Postoperative Hyponatremia despite Near-Isotonic Saline Infusion: A Phenomenon of Desalination

Andrew Steele, MD; Manjula Gowrishankar, MD; Simon Abrahamson, MD, ChB; David Mazer, MD; Ross D. Feldman, MD; and Mitchell L. Halperin, MD
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From St. Michael's Hospital, University of Toronto, Toronto, Ontario, Canada. Requests for Reprints: Mitchell L. Halperin, MD, St. Michael's Hospital, Lab #1, Research Wing, 38 Shuter Street, Toronto, Ontario M5B 1A6, Canada. Current Author Addresses: Drs. Steele, Gowrishankar, Abrahamson, Mazer, and Halperin: St. Michael's Hospital, 38 Shuter Street, Toronto, Ontario M5B 1A6, Canada.

Copyright ©2004 by the American College of Physicians

Ann Intern Med. 1997;126(1):20-25. doi:10.7326/0003-4819-126-1-199701010-00003
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Background: It is widely presumed that the development of postoperative hyponatremia (which may be severe) results from administration of hypotonic fluids while antidiuretic hormone is acting.

Objective: To show that hyponatremia would occur in patients 24 hours after surgery if only near-isotonic solutions are given and to evaluate the mechanisms responsible for hyponatremia in this setting.

Design: Prospective cohort study.

Setting: University medical center.

Patients: 22 women who were having uncomplicated gynecologic surgery with infusion of near-isotonic solutions only (sodium chloride, 154 mmol/L, or Ringer lactate [sodium, 130 mmol/L, and potassium, 4 mmol/L]).

Measurements: Plasma electrolyte levels were measured at the time of induction of anesthesia and 24 hours later. Data on the balance of water and electrolytes were obtained for the same 24-hour period.

Results: At the time of induction of anesthesia, the plasma sodium concentration was 140 ± 1 mmol/L; 24 hours later, it decreased in 21 of 22 patients (mean decrease, 4.2 ± 0.4 mmol/L [P < 0.001]; lowest level, 131 mmol/L in 2 patients). The urine remained hypertonic (peak sodium plus potassium concentration in urine, 294 ± 9 mmol/L) in all patients for the first 16 hours after induction of anesthesia.

Conclusions: Postoperative hyponatremia occurred within 24 hours of induction of anesthesia when only near-isotonic fluids were infused. Hyponatremia was generally caused by generation of electrolyte-free water during excretion of hypertonic urine-a desalination process. This electrolyte-free water was retained in the body because of the actions of antidiuretic hormone. If the pathophysiology of this hyponatremic state is understood, recommendations for its prevention and treatment can be deduced.


Grahic Jump Location
Figure 1.
Urinary excretion of sodium and potassium after surgery.dotted line

A sodium plus potassium concentration greater than 150 mmol/L ( ) represents hypertonic urine and thus contributes to generation of hyponatremia. In contrast, values below this level represent excretion of hypotonic urine and thus a period when hyponatremia was being corrected. In samples obtained after 1000 minutes (from 16 to 24 hours), the urine was hypertonic in some patients and hypotonic in others.

Grahic Jump Location
Grahic Jump Location
Figure 2.
The desalination process.

Hatching represents isotonic saline in the extracellular fluid. The rectangle on the left represents the preoperative setting with a normal plasma sodium concentration (140 mmol/L). The rectangle on the right represents the postoperative state during which hypotonicity and edema have resulted from partial desalination of the infused near-isotonic saline. Expansion of extracellular fluid volume is caused by retention of isotonic saline. The desalination process involves administration of near-isotonic solutions and loss of hypertonic urine. Antidiuretic hormone causes retention of electrolyte-free water, which forms as a result of hypertonic urinary losses. Thus, two volumes are retained: The first volume (larger hatched rectangle) represents retained isotonic saline, which would not change the plasma sodium concentration. The second volume (white rectangle) represents retained electrolyte-free water (generated by excretion of hypertonic urine), which leads to hyponatremia and cell swelling.

Grahic Jump Location




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