Overview
This lecture reviews the three major approaches to acid-base analysis—traditional (Boston), base excess (Copenhagen), and Stewart—covering their principles, differences, and practical relevance in clinical medicine.
Historical Context & Evolution
- Acid-base chemistry evolved from Arrhenius' definition (1880s) to Bronsted-Lowry (1923) to more complex clinical models.
- Early focus was on pH, PCO₂, and bicarbonate; the Henderson-Hasselbalch equation became central in clinical use.
- Two major 20th-century camps: Boston (Schwarz, Relman) and Copenhagen (Astrup, Siggaard-Andersen).
Traditional (Boston) Approach
- Based on Bronsted-Lowry acid/base theory and bicarbonate buffer system.
- Key variables: pH, PCO₂, bicarbonate, anion gap.
- Defines 6-7 acid-base disorders (metabolic and respiratory types, with compensation).
- Compensation rules empirically derived for each disorder.
- Criticized for not including all buffers and for interdependence of bicarbonate and respiratory status.
Base Excess (Copenhagen) Approach
- Also uses Bronsted-Lowry theory and bicarbonate buffer system.
- Focuses on "base excess": the amount of acid needed to normalize blood pH at set PCO₂.
- Base excess is calculated by ABG analyzers; negative = metabolic acidosis, positive = metabolic alkalosis.
- Standard base excess adjusts for a set hemoglobin level to better reflect in vivo physiology.
- Overall, approach and diagnosis closely parallel the traditional method.
Stewart (Strong Ion/Physicochemical) Approach
- Uses Arrhenius and Nin theory, focusing on ion concentrations' effects on water dissociation.
- Bicarbonate is a dependent variable, not a central one.
- Key variables: PCO₂, strong ion difference (SID), and total weak acid concentration (aₜₒₜ).
- Classifies six acid-base disorders, including two unique to the Stewart approach (nonvolatile buffer acidosis/alkalosis).
- Analysis is mathematically complex, with no standard bedside algorithm and limited clinical practicality.
Comparing Approaches
- Traditional and base excess methods lead to similar diagnoses when the anion gap is properly corrected.
- Both focus on pH, PCO₂, and (often corrected) anion gap.
- Stewart’s model is more complex, less practical, and offers no demonstrated clinical advantage.
- Despite controversy, the Stewart approach has limited influence on real-world clinical decision-making.
Key Terms & Definitions
- pH — Measure of hydrogen ion concentration; reflects acidity.
- PCO₂ — Partial pressure of carbon dioxide; respiratory component in acid-base balance.
- Bicarbonate (HCO₃⁻) — Major blood buffer in traditional models.
- Anion Gap (AG) — Difference between major measured cations and anions; helps identify metabolic acidosis types.
- Base Excess (BE) — Amount of acid or base needed to return blood pH to normal; indicator of metabolic disturbance.
- Strong Ion Difference (SID) — Difference between sums of fully dissociated cations and anions in plasma (e.g., Na⁺ + K⁺ – Cl⁻).
- Aₜₒₜ — Total concentration of weak, nonvolatile acids (mainly albumin, phosphate).
- Strong Ion Gap (SIG) — Difference between SIDa and SIDe; indicates unmeasured anions.
Action Items / Next Steps
- Review previous lectures for details on compensation rules and anion gap calculation.
- Prepare for upcoming lectures on oxygenation in ABG analysis.