Rebecca J. Cusack

The strong ion gap does not have prognostic value in critically ill patients in a mixed medical/surgical adult ICU

AbstractObjective. To examine whether the strong ion gap (SIG) or standard base excess corrected for abnormalities of serum chloride and albumin (BEUA) can predict outcome and to compare the prognostic abilities of these variables with standard base excess (SBE), anion gap (AG), pH, and lactate, the more traditional markers of acid-base disturbance. Design. Prospective, observational study. Setting. University teaching hospital, general adult ICU. Patients. One hundred consecutive patients on admission to the ICU. Measurements and results. The anion gap (AG) was calculated and corrected for abnormal serum albumin (AGcorrected). Serum lactate was measured and SBE, BEUA, SIG, and APACHE II scores calculated for each patient. 28-day survival was recorded. There was a significant difference between the mean APACHE II (P<0.001), SBE (P<0.001), lactate (P=0.008), AG (P=0.007), pH (P<0.001), and BEUA (P=0.009) of survivors and non-survivors. There was no significant difference between the mean SIG (P=0.088), SIDeff (P=0.025), and SID app (P=0.254) between survivors and non-survivors. The pH and SBE demonstrated the best ability of the acid-base variables to predict outcome (AUROC curves 0.72 and 0.71, respectively). Neither of these were as good as the APACHE II score (AUROC 0.76) Conclusion. Traditional indices of SBE, BEUA, lactate, pH, AG, and APACHE II all discriminated well between survivors and non-survivors. In this group of patients the SIG, SIDeff, and SIGapp appear to offer no advantage in prediction of outcome and their use as prognostic markers can therefore not be advocated.

Arterial blood gas analysis and lactate

It has long been recognized that a disturbance of the homeostatic mechanisms controlling the body’s acid-base balance is associated with an increased risk of organ dysfunction and death in the critically ill patient. Despite acid-base imbalance being an integral aspect of ongoing pathologic processes in a large number critically ill patients, the importance of understanding the fundamental principals behind the physiology has been largely ignored. Measurement of the standard bicarbonate and base excess have been used for decades as indicators of metabolic acid-base disturbances. These techniques, however, are dependent upon normal electrolyte, water, and albumin content in the patients. Needless to say, these conditions are rarely met in the critically ill patient, and estimations of the degree of metabolic disturbance may be, therefore, inaccurate. The anion gap has been proposed to give a better indication as to the origin of the metabolic upset. However, this measurement is still reliant upon normal albumin and PCO2 levels. Serum lactate levels have been used as a prognostic marker in the critically ill with some degree of success, but the metabolism of lactate is complex and incompletely understood. The belief that lactates can be accurately used as a stand-alone marker of outcome and mortality is thus naive. More recently, quantitative studies on acid-base imbalance have shown that three independent variables determine the hydrogen and bicarbonate ion concentrations in complex solutions such as plasma. These principles have been developed such that estimates of metabolic derangement can be made in the critically ill without making incorrect assumptions about albumin and free water concentrations. These newer techniques may give improved insights as to the origins to the acid-base disturbances seen in the critically ill and thus aid therapy.