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The calculated ion gap: a novel predictor of mortality in the critically ill surgical patient

Introduction

Early identification of critically ill surgical patients who are not fully resuscitated improves outcome. Current markers of clinically occult hypoperfusion, such as lactate, have serious limitations. Increased oxidative stress as a consequence of inadequate cellular respiration results in elevated levels of unmeasured anions. We evaluated these anions as a novel marker of outcome.

Methods

We prospectively evaluated 109 consecutive patients admitted to a surgical high-dependency unit (HDU). Regional Ethics Committee approval was obtained. Serum electrolytes, albumin, phosphate and lactate were measured on admission and days 1 and 2. We derived the calculated ion gap (CIG) using our simplified modification of the Stewart–Figge equations.

Results

The CIG on day 1 predicted mortality (P = 0.001, analysis of variance). A CIG > 10 mmol/l correlated very strongly with mortality. The mortality in patients with a CIG < 10 mmol/l (n = 86) was 4.7%. The mortality in patients with a CIG > 10 mmol/l (n = 23) was 26.1% (P = 0.006, chi-square test). There were no differences in CIG with respect to mortality on admission or day 2 (P = 0.273 and 0.104, respectively). The mean hospital stay was significantly longer in patients with a CIG > 10 mmol/l (46.6 vs 18.7 days, P = 0.015, t test) (Table 1).

Table 1 (abstract P451)

Conclusion

We describe the CIG for the first time in the critically ill surgical patient, and quantify it using simple bedside calculations derived from routine blood investigations. Failure to normalise the CIG by day 1 after admission to the HDU is an excellent marker for mortality and length of hospital stay, and should be used to guide resuscitation.

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Leitch, F., Dickson, E., McBain, A. et al. The calculated ion gap: a novel predictor of mortality in the critically ill surgical patient. Crit Care 11, P451 (2007). https://0-doi-org.brum.beds.ac.uk/10.1186/cc5611

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Keywords

  • Lactate
  • Hospital Stay
  • Routine Blood
  • Ethic Committee Approval
  • Serum Electrolyte