EVALUATION OF THE DIFFERENT ANION GAPS AS A DIAGNOSTIC AND PROGNOSTIC TOOLS IN THE CRITICALLY ILL PATIENTS

Abstract

Over the past few decades, a number of scoring systems have been developed to provide standardized means of assessing illness severity in the critically ill patients<!). Most of these systems utilise specific physiologic parameters as; vital signs or laboratory values. With the wide spread utilization of blood gas analyzers; acid base measurements became one of the basic parameters in I.C.U planning and comparison of outcome of different groups ofiCU patients.(2)

The traditional approach used to clinically describe human acid base equilibrium focuses on H+as the determinant of pH with its relationship to PaC02 , HC03- concentration, to determine pH this relationship is expressed as Henderson-Hassel balch (H.H) equation.<3l

Patients in critical care units frequently manifest double or triple acid base defects these conditions are associated with either inadequate or excessive compensation for a primary defect or two primary disturbances affecting acid base metabolism. Patients who manifest double or triple defects often have a normal or near normal pH, but are suffering from complex acid base disturbances and may be critically ill, that could not be explained by the traditional approach. To help uncover these complex disturbances it should be clear the effects of double or triple defects on the internal environment as effects ofHC03- or carbon dioxide changes on pH, base disturbances or electrolytes.< 4 )

After H.H the improved ability to measure bicarbonate, pH and electrolytes led to further mathematical equations and conceptual definitions to understand acid base derangements especially nonrespiratory (metabolic) acid base disorders.<5l Some of the proposed concepts were whole blood buffer base, base excess, standard base excess that help partly to understand complex acid base disorders.