Clinical review: the meaning of acid-base abnormalities in the intensive care unit part I - epidemiology

Abstract

Acid-base abnormalities are common in critically ill patients. Our ability to describe acid-base disorders must be precise. Small differences in corrections for anion gap, different types of analytical processes, and the basic approach used to diagnose acid-base aberrations can lead to markedly different interpretations and treatment strategies for the same disorder. By applying a quantitive acid-base approach, clinicians are able to account for small changes in ion distribution that may have gone unrecognized with traditional techniques of acid-base analysis. Outcome prediction based on the quantitative approach remains controversial. This is in part due to use of various technologies to measure acid-base variables, administration of fluid or medication that can alter acid-base results, and lack of standardized nomenclature. Without controlling for these factors it is difficult to appreciate the full effect that acid-base disorders have on patient outcomes, ultimately making results of outcome studies hard to compare.

Figure 1

Distribution of patients and contributing ion responsible for majority of metabolic acidosis present. Shown is the distribution of patients within different types of intensive care unit (ICU) locations and their respective hospital mortality associated with the major ion contributing to the metabolic acidosis. These results were obtained from a large teaching institution comprised of two hospitals and seven ICUs over a 1 year period and included patients with a suspected lactic acidosis. No metabolic acidosis is defined as a standard base excess of -2 mEq/l or higher. CCU, cardiac (nonsurgical) ICU; CTICU, cardiothoracic ICU; LTICU, liver transplant ICU; Med, medical ICU; Neuro, neurosurgical and neurological ICU; Surg, general surgical ICU; Trauma, trauma ICU.

Figure 2

Proposed metabolic acidosis classification flow diagram based on the contributing anion group. This flow diagram is one proposed way to classify metabolic acidosis based on the major contributing anion group. The definition of metabolic acidosis component is a standard base excess (SBE) below -2 mEq/l. It is not based on pH because of the possibility of respiratory compensation. SIDa, apparent strong ion difference; SIDe, effective strong ion difference; SIG, strong ion gap.