Acid-Base Balance II: Physicochemical Approach (Stewart)

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The Three Independent Variables of the Stewart Model

1. Strong Ion Difference (SID): Represents the net difference in electrical charge between completely dissociated cations and completely dissociated anions in the plasma. Its normal value in plasma ranges between 40 and 42 mEq/L and is calculated according to the formula: SID = ( [Na⁺] + [K⁺] + [Ca²⁺] + [Mg²⁺] ) - ( [Cl^-] + [Lactate^-] )
2. Partial Pressure of Carbon Dioxide (pCO₂): Dynamically regulated exclusively by pulmonary alveolar ventilation.
3. Total Mass of Weak Non-Volatile Acids (A_tot): Represents the content of plasmatic organic chemical buffers that can donate or accept hydrogenions, made up almost entirely of plasma albumin and circulating inorganic phosphates.

Mechanism of Plasma pH Generation

According to Stewart's model, plasma water functions as an inexhaustible compartment of hydrogen ion (H⁺) and hydroxyl (OH^-), continuously dissociating according to its biological constant of the ionic product (K'_w = [H⁺] · [OH^-]). The final concentration of H⁺ (and consequently the pH) is instantly adjusted in a purely physical way to maintain the strict balance of electrical neutrality and mass conservation.

Under this approach, bicarbonate is simply a secondary variable and not the cause of acidosis or alkalosis. If the strong ion difference (SID) decreases (for example, due to hyperchloremia), electrical neutrality requires an increase in free hydrogen ions derived from the autodissociation of cellular water, inducing a metabolic acidosis instantaneously.

Apparent, Effective Strong Ion Difference and the Strong Ion Gap (SIG)

For advanced quantitative characterization in critically ill patients, two derived variables are calculated:

• Apparent Strong Ion Difference (SID_a): Evaluated using electrolytes measured in a traditional way: SID_a = [Na⁺] + [K⁺] + [Ca²⁺] + [Mg²⁺] - ([Cl^-] + [Lactate^-] )
• Effective Strong Ion Difference (SID_e): Incorporates the buffering effect of the negative charges of plasmatic non-volatile weak acids (albumin and phosphates) dependent on the ongoing pH: SID_e = 2.46 × 10^-8 × pCO₂10^-pH + [Albumin] × (0.123 × pH - 0.631) + [Phosphate] × (0.309 × pH - 0.469)
• The Strong Ion Gap (SIG): It represents the exact difference between the SID_a and the SID_e: SIG = SID_a - SID_e

The normal value of SIG should theoretically be zero. A significant positive GIS (> 2 mEq/L) mathematically indicates the unequivocal presence of circulating unmeasured anions (ketones, uremic anions or exogenous toxicants) with greater clinical precision and diagnostic sensitivity than the conventional measurement of the classic Anion Gap in critically ill patients in septic shock.