HDCN Article Review/Hyperlink

Steele A, Gowrishankar M, Abrahamson S, Mazer D, Feldman RD, Halperin ML

Postoperative hyponatremia despite near isotonic saline infusion: a phenomenom of desalination

Ann Int Med (Jan) 126:20-25 1997

Acute post-operative hyponatremia leading to brain swelling and death has been described, especially in young women. The pathogenesis has been assumed to be administration of large amounts of hypotonic fluid which cannot be excreted because of elevated ADH secretion (due to pain, drugs, stress, nausea). However, the authors reviewed five such cases and found that the amount of hypotonic fluid administered did not explain the degree of hyponatremia that occurred. Therefore, they prospectively studied 22 women (42 +/- 1 yrs) undergoing gynecologic surgery with infusion of only near-isotonic solutions (sodium chloride 154 mmol/L plus Ringer's lactate sodium 130 mmol/L and potassium 4 mmol/L). Electrolyte and water balance were determined during the first 24h post-operatively.

The pre-operative plasma sodium concentration was 140 +/- 1 mmol/L; 24h later it decreased in 21/22 patients (mean decrease 4.2 +/- 0.4 mmol/L, p < 0.001; nadir 131 mmol/L in 2 patients). Of note, the urine remained hypertonic (urine sodium plus potassium levels > 150 mmol/L) in all patients for at least the first 16h post-operatively. The average volume of isotonic saline (2.6 +/- 0.3L) and Ringer lactate (2.8 +/- 0.3L) that was infused for 24 h was 5.3 +/- 0.2L. The average 24h urine volume was 2.5 +/- 0.3L, resulting in a net water gain of 2.9 +/- 0.3L. Sodium balance was +367 mmol/24h and potassium balance was -90 mmol/24h; therefore there was a gain of 277 mmol of cations; however, since water balance was +2.9L, this indicated a net gain of 1.8L of isotonic saline and 1.1L of electrolyte-free water. Retention of this amount of electrolyte-free water (approx 3% of TBW) accounts for the decline in serum sodium level.

The authors conclude that administration of large volume of near-isotonic fluid (and presumably isotonic fluid as well) to post-operative patients can result in hyponatremia because of retention of electrolyte-free water in the body. This complication can be prevented by administering smaller volumes of isotonic fluid; if it occurs, a reduction in iv fluid rate with or without administration of a loop diuretic (to increase water loss) is indicated.

Comment: This is a well-designed study that demonstrates nicely that patients are not closed boxes when it comes to predicting the effect of iv fluid on serum sodium levels. Hyponatremia occurred because the retention of administered water exceeded the retention of administered electrolytes, or in the author's words, there was relative "desalination". The crucial point here is that the concentration of urinary cations exceeded the concentration of plasma cations. The authors did not calculate electrolyte-free water clearance:

CeH20 = V - (UNa+K x V / PNa+K) = V (1 - UNa+K / PNa+K)

However, it can easily be seen that electrolyte-free water clearance would be negative; i.e., there was net electrolyte-free water reabsorption by the kidney. (David J. Leehey, M.D., Loyola University at Chicago)

The full text of this paper is available from the ACP site.