2006, Number 4
Rev Mex Anest 2006; 29 (4)
Carrillo-Esper R, Visoso-Palacios P
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ABSTRACTAcid-base disturbances are frequent in the perioperative period. When severe and rapidly-evolving, they can cause organ dysfunction, with increased morbidity and mortality. The Henderson-Hasselbalch model cannot explain all acid-base disturbances in surgical patients. Alternately, the Stewart model explains that the source of hydrogen ions, and hence of pH, is the dissociation of water due to an increase in strong ion difference (SID), pCO2, and the total concentration of dissociated weak nonvolatile acids (ATOT). Three models are used to explain acid-base disturbances. The first one is the Henderson-Hasselbalch model, based on the law of mass action and using HCO3- and pCO2 as independent variables. The «rule of fives», based on this model, makes it possible to systematically diagnose simple, double and triple acid-base disturbances. The second model involves the concept of titratable hydrogen ion concentration in extended extracellular fluid (ctH+ Ecf), defined as the amount of hydrogen ions added or removed in relation to a reference pH of 7.40. This model uses the Van Slyke equation and the Siggaard-Andersen chart, respectively, to calculate and plot acid-base disturbance. The third approach is the Stewart model, based on the laws of mass conservation and electroneutrality, with pCO2, SID and ATOT as independent variables causing water dissociation and hydrogen ion formation. This approach leads to such diagnoses as hyperchloremic metabolic acidosis and metabolic acidosis due to unmeasured anions, the former due to resuscitation with non-balanced solutions, and the latter, due to sepsis.