Is there a difference between strong ion gap in healthy volunteers and intensive care unit patients?

Abnormalities of strong ion gap (SIG) are common in critically ill intensive care unit (ICU) patients in conjunction with a high incidence of acid-base abnormalities. However, it is unknown whether abnormalities in SIG are also seen in ICU patients without active acid-base abnormalities. Thus, we conducted this pilot study to examine differences in quantitative acid-base variables between healthy adult volunteers and stable ICU patients with no suspected acid-base abnormalities.

The study used a prospective observational study of 2 cohorts, 15 healthy adult volunteers and 15 stable adult patients just before ICU discharge who were not receiving renal replacement therapy and had no known active acid-base derangements. We analyzed venous blood for acid-base variables (potential hydrogen in central venous blood [pH(CV)], partial tension of carbon dioxide in central venous blood [p(CV)co(2)], standard base excess [SBE], lactate, Na(+), K(+), Cl(-), Mg(2+), Ca(2+), phosphate, and albumin). From these, we calculated strong ion difference (SID) and SIG for both cohorts.

Although mean values for pH(CV), p(CV)CO(2), and SBE were within the normal range in both cohorts, 10 (66.7%) of 15 of ICU patients were found to have occult acid-base disorders. The ICU patients also had reduced albumin and SID measurements and significantly greater mean SIG (5.1 ± 2.9 mEq/L) compared to healthy controls (1.4 ± 1.8 mEq/L) (P = .0002). None of the healthy controls had a SIG higher than 5.0 mEq/L, whereas 7 (46.7%) of 15 of ICU patients had a SIG higher than 5.0 mEq/L. Even among ICU patients with no abnormalities of pH(CV), p(CV)CO(2), and SBE, mean SIG was 6.4 (± 3.3 mEq/L).

Stable ICU patients have much higher levels of unexplained anions when comparing with healthy controls. Whether this finding represents occult acid-base disorders or simply metabolic differences among the critically ill is uncertain. Further study in larger populations is warranted to establish the significance of high SIG in otherwise stable ICU patients.