[Fluid therapy in surgical patients: composition and influences on the internal milieu]

Intravenous fluids administration is the usual practice in the postoperative period. Nonetheless, consensus about the more appropriate fluid reposition recipe is still insufficient.

To study the type of intravenous reposition used in Surgical Units and its impact on the internal milieu.

prospective study of 112 patients with scheduled surgery, receiving only intravenous fluids.

biochemical study on postoperative fluids management in uncomplicated surgery. Principal variables: 1. Water and electrolytes administrated. 2. Differences in sodium/water balances before surgery vs first day after surgery. 3. Symptoms related to hyponatremia.

Median P[Na] before and after surgery was 139.9+/-2.9 and 137.7+/-3.7, respectively (p<0.01). Fourteen patients (12.5%) had P[Na]<135, and 12 of them had a reduction of more than 6 mmol/L; accordingly, twenty-six patients (23.2%) had an increased free-water retention (p<0.05). Relevantly, they did not receive a higher amount of free-water and the proportion of isotonic saline/free water varied from <1 to >3. As possible mechanism of free-water accumulation: the postoperative P[Na] was lower in the patients who had a negative free-water clearance >or= -1 L (136.7+/-4.1 vs 138.5+/-3.2 mmol/L, p 0.015).

The present study provides new information about the intravenous fluids prescribed in postoperative patients, ie, different proportions saline/water are basically equivalent with respect to inducing symptomatic hyponatremia. The mean value of the relation saline/water is 2:1. Hypotonic fluids input is not clearly related to more intense hyponatremia; the latter appears to depend more on a reduced capacity of the kidney to generate sufficient free water output.