Alterations in urinary strong ion difference in critically ill patients with metabolic acidosis: a prospective observational study.

BACKGROUND AND OBJECTIVE

The correct renal response to metabolic acidosis should be a negative shift in the urinary strong ion difference (SID = Na(+) + K(+) - Cl(-)). Our hypothesis was that the failure to decrease the SID is frequently present and leads to a more severe metabolic acidosis.

DESIGN, SETTING AND PARTICIPANTS: Prospective observational study conducted in the medical/surgical intensive care unit of a teaching hospital between 1 January 2006 and 30 April 2007. Participants were 98 patients with metabolic acidosis on ICU admission and 10 healthy volunteers.

INTERVENTIONS

None.

MAIN OUTCOME MEASURES

Severity of metabolic acidosis; behaviour of acid-base variables according to positive or negative SID.

RESULTS

Twelve patients (12%) had negative SID and 86 (88%) had positive SID. Compared with patients with positive SID, those with negative SID had higher [HCO(3) (-)] (20 ±2 v 18 ±3 mmol/L), base excess ([BE]) (-5 ±2 v -7 ±2 mmol/L), anion gap ([AG]) (21 ±5 v 17 ±4 mmol/L), Δ[AG] - Δ[HCO(3)(-)] (1 ±5 v -3 ±3 mmol/L) and lower [Cl(-)] (105 ±5 v 111 ±3 mmol/L).

CONCLUSIONS

Most of the critically ill patients with metabolic acidosis showed inappropriate renal compensation, as evidenced by positive SID and higher plasma [Cl(-)]. These patients had more severe metabolic acidosis. On the other hand, patients with adequate renal response and negative SID had positive Δ[AG] - Δ[HCO(3)(-)]. These findings, usually considered as a diagnosis of associated metabolic alkalosis, might be interpreted as the proper renal response to metabolic acidosis.