In 1981, Stewart proposed what was described as a non-empirical method based on the physicochemical fundamentals of acid-base physiology. Stewart’s pH determinants included the following as shown in the flowchart below:

Where, pCO2: partial pressure of carbon dioxide, Na+ : Sodium, K+ : Potassium, Ca++ : Calcium, Mg++ : Magnesium, Cl- : Chloride Gamblegrams, named after physician James L. Gamble, are bar diagrams displaying the relative abundance of extracellular anions and cations and are commonly used to analyze acid-base disorders.

Wrong Answer:1. A: Bicarbonate (HCO3-)
B: Strong Ion Difference (SID)
C: Weak acids (ATOT)
D: Unmeasured anions (UMA)

Right Answer:2. A: Strong Ion Difference (SID)
B: Unmeasured anions (UMA)
C: Bicarbonate (HCO3-)
D: Weak acids (ATOT)

Explaination:

SID = Strong Cations– Strong Anions
SID = ([Na+] + [K+] + [Mg++] + [Ca++]) – ([Cl-] + [lactate])

Solving the complex polynomial equations involved in it at the bedside was difficult and resulted in poor adoption of the method. In 1993, based on the preliminary work of Fencl, Gilfix and other simplified Stewart’s method to make it useful at the bedside. Stewart's method, also known as the Stewart approach or physicochemical approach, focuses on the physical and chemical principles governing the behavior of electrolytes and their impact on acid-base balance.
They proposed simple equations to measure UA and also to correct the Standard Base Excess (SBE) for the changes in sodium, chloride and albumin.
Stewart's method identifies three independent variables that determine acid-base balance:
● Strong Ion Difference (SID): The difference between the concentrations of strong cations (e.g., Na⁺, K⁺) and strong anions (e.g., Cl⁻, lactate). SID influences the dissociation of water and the concentration of hydrogen ions (H⁺).
● Partial Pressure of Carbon Dioxide (PaCO₂): Reflects the respiratory component of acid-base balance. CO₂ is a volatile acid that affects pH.
● Total Concentration of Weak Acids (Aₜₒₜ): Primarily includes plasma proteins (e.g., albumin) and phosphate, which act as buffers.

* pH, bicarbonate (HCO₃⁻), and other traditional acid-base parameters are considered dependent variables. They are determined by the interplay of the three independent variables.

Wrong Answer:3. A: Weak acids (ATOT)
B: Bicarbonate (HCO3-)
C: Unmeasured anions (UMA)
D: Strong Ion Difference (SID)

Wrong Answer:4. A: Unmeasured anions (UMA)
B: Weak acids (ATOT)
C: Strong Ion Difference (SID)
D: Bicarbonate (HCO3-)