Stewart Acid–Base

Abnormalities in ABG

ABG from question not available

Stewart approach to acid-base interpretation
(Physico-chemical approach)

• Developed to address the criticism that traditional approach is merely a mathematical description of pH and fails to provide any mechanistic insight into rising and falling [H+]
• Peter Stewart (1978) modeled a solution that contained a complex mixture of ions of constant charge over the physiological pH range (strong ions), non-volatile proton donor/acceptors which transfer H⁺ within the physiological pH range (weak acid/base), and the volatile bicarbonate–CO₂ buffer system.
• Key aspect of Stewart’s concept was the classification of each variable as dependent or independent in determining the H⁺ concentration of the solution.
• Three independent variables that independently determine the dissociation of water, and consequently the [H⁺] and [HCO₃^–] to maintain electrical neutrality:
  • partial pCO₂ of the solution
  • Total concentration of weak acids (A_TOT)
  • Strong ion difference (SID)
• Thus, in the Stewart’s approach, metabolic disorders are the results of changes in SID or A_TOT.

PaCO2

• ↑ = respiratory acidosis
• ↓ = respiratory alkalosis

Apparent SID and effective SID

Apparent SID (SIDa): represents the difference between measured strong cations and strong anions

• Normal plasma SID is 42 mEq/L
• ↑SID = Metabolic Alkalosis
• ↓SID = Metabolic Acidosis

SID can be changed by:

• Concentration change
  • ↑ H₂O: concentrates alkalinity and ↑ SID
  • ↓ H₂O: dilutes alkalinity and ↓ SID
• Strong ion change
  • ↓ Na+: ↓ SID and acidosis
  • ↑ Na+: ↑ SID and alkalosis
  • ↑ Cl-: ↓ SID and acidosis (~NAGMA, for e.g with Normal saline)
  • ↑ organic acids with pKa <4 (lactate, formate, ketoacids): ↓SID and acidosis (~HAGMA)
  • ↑ ↓

Effective SID (SIDe): calculated to account for electrical neutrality. Sum of bicarbonate and weak acids (Albumin and phosphate).

SIG

• Gap between SIDa and SIDe due to failure to measure the concentration of all strong and weak ions in plasma

• quantifies [unmeasured anions] – [unmeasured cations] of both strong and weak ions
• theoretical advantage over AG due to pure representation of unmeasured ions
  • Unmeasured anions in AG: Mg,Ca,Alb,phosphate,lactate and other ions
  • Unmeasured anions in SIG: just other ions
• normal AG 8 to 12, whereas SIG closer to zero in normal situations
• ↑SIG = ↑ unmeasured anions
• ↓SIG = ↑ unmeasured cations

A_TOT

• represents all non-bicarbonate buffers
• is made up of mainly serum albumin and other minor charges such as phosphate and globulins
• ↑A_TOT = Metabolic acidosis
• ↓A_TOT = Metabolic alkalosis

Advantages and Disadvantages

• Advantages
  • provides physico-chemical basis and mechanistic insight into rising and falling [H+]
  • Pure representation if unmeasured ions
  • Diminishes importance of [HCO3-] which is just a dependent variable
  • Better explanation of some phenomenon such as acidosis from Normal Saline
• Disadvantages
  • Complex calculation, cannot be used bedside quickly
  • Substantially different from well-validated classical approach
  • Only reflects plasma (base excess reflects whole body and influence of Hb)
  • Unclear clinical correlation, not extensively validated
  • Higher chance of error due to numerous variables