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Objective: The traditional approach for acid base interpretation is based on Handerson-Hasselbalch formula and includes Base Excess (BE), bicarbonate (HCO3), albumin corrected anion gap. The Physicochemical approach is centered on the Carbon Dioxide tension (PCO2), the strong ion difference (SID), strong ion gap (SIG) = SID apparent-SID effective and totally weak acids (Atot). The study aims to compare between the traditional approach and the physicochemical approach in acid base disorder interpretation. Design: Prospective observational study in an adult Intensive Care Unit (ICU) recruiting six hundred and sixty one patients. Methods: Arterial blood samples were analyzed to measure pH, PaCO2 sodium, potassium, chloride and lactate. Venous blood samples were analyzed to measure ionized calcium, magnesium, phosphorous and albumin. These samples were interpreted by both techniques. Results: Normal HCO3 and BE were detected by traditional approach in 49 cases of which SIG acidosis was detected in 22 cases (46%) and Hyperchloremic acidosis was detected in 29 cases (60%) by physicochemical method. SIG was elevated in 72 cases (58%) of 124 cases with high anion gap acidosis. SIDeff and BE were strongly correlated, r = 0.8, p 0.0001, while SIG and Albumin corrected Anion Gap (ALAG) were moderately correlated r = 0.56, p Conclusion: Both approaches are important for interpretation of the acid base status. Traditional approach identifies the diagnostic description without many calculations and detects body compensatory response to acid base disorders. Physicochemical approach is essential to identify the exact causation and the severity of the acid base disorders.

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 (ATOT), 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.

As has been theoretically and empirically demonstrated by Morgan et al.2,3 a truely balanced intravenous fluid has a concentration of 24 mEq/l bicarbonate or metabolisable anions. In plasmalyte, though, the combined charge of these (acetate and gluconate) is 50 mEq/l. This would not qualify as balanced, in our view. It leads to an increased strong ion difference, ultimately introducing a iatrogenic metabolic alkalosis. . Aksu et al. find that the amount of fluid needed to resuscitate the animals in haemorrhagic shock to the target blood pressure is much less with Plasmalyte than with isotonic saline (30 vs. 57 ml). This would seem clinically relevant. Do the authors have any explanation for this?

These formulae are erroneous and lead to predictably grossly wrong results. The authors find SIG values of mean -14 mEq/l in the non-survivors' and -10 mEq/l in the survivors' group. ' According to our data the SIG approach does not seem to add further information to usual parameters in acid-base evaluation or early risk stratification in cardiogenic shocks patients.'

The Stewart approach to acid–base management has evolved to be the method of choice [1]. While the quantitative approach and the traditional Henderson–Hasselbalch method do not mathematically exclude one another, the Stewart approach may provide better mechanistic insight in complex acid–base disturbances. However, the approach is perceived as complex, requiring many calculations. This is why we developed an online tool that may facilitate its use.