Acid–base

Stewart’s strong ion approach to acid–base disorders is increasingly popular, especially in critical care medicine. However, the Henderson–Hasselbalch-based bicarbonate-centered approach remains widely used. Basic concepts and clinical application of these methods are discussed in this chapter. According to the Stewart approach, only three independent parameters determine [H⁺] concentration or pH. These are the so-called strong ion difference (SID), the total amount of weak acids (A_TOT), and the partial pressure of carbon dioxide (PCO₂). A large contributor to SID is the difference between [Na⁺] and [Cl], whereas albumin is the most important contributor to A _TOT. If SID decreases, [H⁺] will increase. If A _TOT or PCO₂ decreases, [H⁺] will also decrease. Understanding how physiology and pathophysiology influences [H⁺] becomes easy when considering their effects on the three independent parameters. However, provided that hypoalbuminemia is appropriately corrected for, both the Stewart and bicarbonate-centered methods are mathematically compatible and may perform equally well in clinical practice, especially in simple acid–base disorders. However, if acid–base disorders become increasingly complex, which is the case in many critically ill patients, Stewart approach may be superior. In addition, by explicitly clarifying the relationship between electrolyte disorders and acid–base physiology, the Stewart approach helps to demystify the effects of resuscitation fluids on acid–base balance.