Last Word on Viewpoint: Stewart's approach to quantitative acid-base physiology should replace traditional bicarbonate-centered models

Strong ions and charge-balance

It has been shown that the ability to predict the pH in any chemically characterized fluid, together with its buffer-capacity and acid content can be based on the requirement of electroneutrality, conservation of mass, and rules of dissociation as provided by physical chemistry. More is not required, and less is not enough. The charge in most biological fluids is dominated by the constant charge on the completely dissociated strong ions but, nonetheless, a persistent narrative in physiology has problematized the notion that these have any role at all in acid-base homeostasis. While skepticism is always to be welcomed, some common arguments against the importance of strong ions are examined and refuted here. We find that the rejection of the importance of strong ions comes with the prize that even very simple systems such as fluids containing nothing else, or solutions of sodium bicarbonate in equilibrium with known tensions of CO₂ become incomprehensible. Importantly, there is nothing fundamentally wrong with the Henderson-Hasselbalch equation but the idea that it is sufficient to understand even simple systems is unfounded. What it lacks for a complete description is a statement of charge-balance including strong ions, total buffer concentrations, and water dissociation.

Clinical Approach to Assessing Acid-Base Status: Physiological vs Stewart

Evaluation of acid-base status depends on accurate measurement of acid-base variables and their appropriate assessment. Currently, 3 approaches are utilized for assessing acid-base variables. The physiological or traditional approach, pioneered by Henderson and Van Slyke in the early 1900s, considers acids as H⁺ donors and bases as H⁺ acceptors. The acid-base status is conceived as resulting from the interaction of net H⁺ balance with body buffers and relies on the H₂CO₃/HCO₃^- buffer pair for its assessment. A second approach, developed by Astrup and Siggaard-Andersen in the late 1950s, is known as the base excess approach. Base excess was introduced as a measure of the metabolic component replacing plasma [HCO₃^-]. In the late 1970s, Stewart proposed a third approach that bears his name and is also referred to as the physicochemical approach. It postulates that the [H⁺] of body fluids reflects changes in the dissociation of water induced by the interplay of 3 independent variables-strong ion difference, total concentration of weak acids, and PCO₂. Here we focus on the physiological approach and Stewart's approach examining their conceptual framework, practical application, as well as attributes and drawbacks. We conclude with our view about the optimal approach to assessing acid-base status.