Introduction to an alternate view of acid/base balance: the strong ion difference or Stewart approach

The carbonic acid/bicarbonate system, as defined by the Henderson-Hasselbach (H-H) equation, has traditionally formed the centrepiece of the presentation of acid/base physiology in nursing education. However, an alternative approach to describe acid/base physiology was proposed by Peter Stewart in 1983. Stewart determined, using the physiochemical principles of dissociation equilibrium, electroneutrality and conservation of mass, that hydrogen ion concentration [H+] was dependent upon the difference between the concentrations of strong cations and strong anions in a solution (the strong ion difference or SID), concentration of weak acid anions, and the partial pressure of carbon dioxide in plasma. Therefore, a change in pH (the [H+] expressed as its negative log) indicates that there must be a change in one of these independent variables, and not simply explained by movement of hydrogen ions or bicarbonate into or out of the body fluids. An analysis of the complex acid/base derangements commonly seen in the critically ill can be achieved using this approach. The acid/base consequences of vomiting, gastric aspiration, diarrhoea, diuretic therapy, the infusion of large volumes of normal saline, the contribution of lactate, and the effects of methanol and ethylene glycol poisoning can all be more readily understood considering Stewart's explanation of acid/base balance. This paper outlines this alternative approach and provides some examples for the intensive care setting.