Strong Ionic Difference

Abstract

Abstract Physicians and scientists have been using the Henderson-Hasselbach equation to decipher acid-base imbalances since the early 1900s. By using the concentration of arterial CO2 (partial pressure of CO2) and the concentration of bicarbonate [HCO3−], the Henderson-Hasselbach approach helps differentiate between respiratory and metabolic derangements. Subsequently, the concept of anion gap was created to help further differentiate metabolic derangements. However, these traditional models often struggled to explain why acid-base derangements occurred in certain patient populations, such as hypoalbuminemic patients. In 1981, Peter Stewart introduced a novel concept that redefined the independent and dependent variables that affect pH. This new concept, sometimes referred to as the physicochemical approach or the strong ion approach, posits that the three independent variables affecting pH are partial pressure of CO2, total concentration of nonvolatile weak acids, and the strong ion difference. Similar to the traditional approach, Stewart’s approach splits acid-base disturbances into respiratory and metabolic derangements; however, unlike the traditional approach, Stewart’s approach further splits metabolic derangements into strong ion and buffer ion derangements. Perhaps the most controversial aspect of Stewart’s approach is that the concentration of bicarbonate is considered a dependent variable; changes in bicarbonate concentration do not cause pH changes but rather occur in response to pH changes.