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](urinary) = [Na(+)](urinary) + [K(+)](urinary) - [Cl(-)](urinary)). Our hypothesis was that the failure to decrease the [SID](urinary) 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](urinary).

RESULTS

Twelve patients (12%) had negative [SID](urinary) and 86 (88%) had positive [SID](urinary). Compared with patients with positive [SID](urinary), those with negative [SID](urinary) 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](urinary) and higher plasma [Cl(-)]. These patients had more severe metabolic acidosis. On the other hand, patients with adequate renal response and negative [SID](urinary) 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.