Short-term effects of small volume saline infusion on acid base equilibrium in critically ill patients.

BACKGROUND:Short-term acid-base effects of 0.9% saline solution infusion are not well described. Aim of this study was to assess the effects of a fluid challenge with 0.9% saline in critically ill patients according to the Stewart's approach, which allows a precise determination of acid base equilibrium. METHODS:In 40 mechanically ventilated critically ill patients, acid-base variables according to Stewart's approach were measured before and after 30 minutes from the infusion of 0.5L of 0.9% saline. Patients were divided in saline responder (fractional sodium excretion increase <0.5) and non-responders, and in patients with (estimated glomerular filtration rate >63 mL/min) and without renal impairment. RESULTS:After saline infusion, plasma sodium concentration did not change (138 [135-141] vs. 138 [135-140] mEq/L, P=0.646), while chloride concentration significantly increased (102 [100-106] vs. 104 [191-106] mEq/L, P=0.003), reducing strong ion difference (37.0 [34.9-38.0] vs. 35.4 [32.7-37.5] mEq/L, P=0.004) without any impact on pH, due to the concomitant albumin dilution. In saline non-responders, the increase of plasma chloride concentration caused a reduction in strong ion difference, while in saline responders both plasma chloride concentration and strong ion difference remained similar. Patients with and without renal impairment presented a similar acid-base response. CONCLUSIONS:The infusion of 0.9% saline reduced strong ion difference by increasing plasma chloride concentration, with no effect on pH due to concomitant albumin dilution. Saline non-responders, characterized by the ability to excrete the sodium excess, were more likely to suffer the acidifying effects of saline infusion, while renal function did not affect the acid-base response to saline infusion.