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The strong ion gap predicts mortality in children following cardiopulmonary bypass surgery
Andrew Durward1, Shane M Tibby, Sophie Skellett
1Department of Pediatric Intensive Care, Guy's Hospital, London, United Kingdom.
Insights
The strong ion gap, a measure of unmeasured tissue acids, is commonly elevated after heart surgery. This gap is a better predictor of intensive care unit mortality than blood lactate levels in children.
Area of Science:
- Biochemistry
- Critical Care Medicine
- Pediatric Surgery
Background:
- Stewart's strong ion theory quantifies unmeasured tissue acids from hypoxia or hypoperfusion.
- The strong ion gap (SIG) is a calculated variable reflecting these unmeasured acids.
Purpose of the Study:
- To determine the 24-hour profile of the SIG after cardiopulmonary bypass (CPB) surgery.
- To compare the prognostic value of SIG versus blood lactate for predicting intensive care unit (ICU) mortality.
Main Methods:
- Prospective, observational study in a tertiary pediatric ICU.
- Eighty-five children undergoing congenital heart disease surgery.
- Arterial blood samples collected at ICU admission and 24 hours for SIG and lactate measurement.
Main Results:
- Elevated SIG (>3 mEq/L) was observed in 41.1% (admission) and 51.7% (24 hours), present at both times in 30.5%.
- SIG and lactate increased with surgical complexity but did not correlate with CPB or aortic cross-clamp times.
- Admission SIG (cutoff >3.2 mEq/L) showed superior mortality prediction (AUC 0.85) compared to lactate (cutoff >3.0 mmol/L, AUC 0.71).
Conclusions:
- An elevated SIG is common after CPB surgery.
- SIG appears to be a more effective predictor of ICU mortality than lactate in this pediatric population.
Objective:
Stewart's strong ion theory quantifies unmeasured tissue acids produced following hypoxia or hypoperfusion, by calculation of the strong ion gap. Our study objectives were as follows: a) to determine the 24-hr profile of the strong ion gap following cardiopulmonary bypass surgery; and b) to compare the prognostic value in terms of intensive care unit mortality of this variable with blood lactate.
Design:
Prospective, observational study.
Setting:
Tertiary pediatric intensive care unit.
Patients:
Eighty-five children following surgery for congenital heart disease.
Interventions:
None.
Measurements And Main Results:
Arterial blood samples for lactate and strong ion gap calculation were obtained at intensive care unit admission and at 24 hrs. A raised strong ion gap (>3 mEq/L) was present in 41.1% and 51.7% of admission and 24-hr samples, respectively, being elevated at both time points in 30.5%. Both the strong ion gap and lactate increased with surgical complexity, but neither was correlated with length of bypass (r = .13 and -.02) or aortic cross-clamp (r = .13 and .10). The crude mortality was 5.8% (5/85). Four of the five deaths were associated with a persistently elevated strong ion gap, in contrast to two with ongoing hyperlactatemia (>2 mmol/L). The admission strong ion gap (cutoff, >3.2 mEq/L) was superior to lactate (cutoff, >3.0 mmol/L) as a mortality predictor (area under receiver operating characteristic curve of 0.85 [95% confidence interval, 0.74-0.95] vs. 0.71 [95% confidence interval, 0.44-0.98], respectively).
Conclusions:
An elevated strong ion gap occurs commonly following bypass surgery and appears to be superior to lactate as a mortality predictor.
