Overview
This lecture covers the mechanisms and formulas for physiologic compensation in acid-base disorders and using compensation assessments to diagnose mixed acid-base disturbances.
Mechanisms of Compensation
- The body compensates for acid-base disorders to minimize abnormal arterial pH.
- Primary metabolic disorders trigger respiratory compensation; primary respiratory disorders trigger metabolic compensation.
- Compensation cannot fully normalize pH and does not result in overcompensation.
- In metabolic acidosis, increased ventilation lowers pCO₂ to raise pH.
- In metabolic alkalosis, decreased ventilation raises pCO₂ to lower pH, but this compensation is limited by hypoxia.
- In respiratory acidosis, kidneys increase bicarbonate (HCO₃⁻) reabsorption and hydrogen ion excretion to raise pH.
- In respiratory alkalosis, kidneys reduce HCO₃⁻ reabsorption and excretion of hydrogen ions to lower pH.
Timing and Efficacy of Compensation
- Respiratory compensation for metabolic disorders begins within minutes, maximizes by 24 hours.
- Metabolic compensation for respiratory disorders takes 12 hours to 5 days to reach full effect.
- Compensation for metabolic alkalosis is weak, often limited by hypoxia.
Identifying Mixed Acid-Base Disorders
- Predicting normal compensation helps identify additional acid-base disorders.
- If actual compensation deviates from predictions, a second or third disorder is present.
Compensation Formulas
- Metabolic acidosis: pCO₂ ≈ 1.5 × [HCO₃⁻] + 8 ("Winter's formula"; ±5 mmHg)
- Metabolic alkalosis: pCO₂ ≈ 40 + 0.7 × ([HCO₃⁻] – 24) (±5 mmHg)
- Acute respiratory acidosis: [HCO₃⁻] ≈ 24 + 1 × (pCO₂ – 40)/10 (±3 mEq/L)
- Chronic respiratory acidosis: [HCO₃⁻] ≈ 24 + 4 × (pCO₂ – 40)/10 (±3 mEq/L)
- Acute respiratory alkalosis: [HCO₃⁻] ≈ 24 – 2 × (40 – pCO₂)/10 (±3 mEq/L)
- Chronic respiratory alkalosis: [HCO₃⁻] ≈ 24 – 5 × (40 – pCO₂)/10 (±3 mEq/L)
Alternative Shortcuts
- For metabolic disorders, pCO₂ ≈ first two digits after the decimal in pH (e.g., pH 7.27 → pCO₂ ≈ 27).
- For respiratory disorders:
- Acute acidosis: HCO₃⁻ increases 1 mEq/L per 10 mmHg pCO₂ above 40.
- Chronic acidosis: HCO₃⁻ increases 4 mEq/L per 10 mmHg pCO₂ above 40.
- Acute alkalosis: HCO₃⁻ decreases 2 mEq/L per 10 mmHg pCO₂ below 40.
- Chronic alkalosis: HCO₃⁻ decreases 5 mEq/L per 10 mmHg pCO₂ below 40.
Examples
- Correct compensation confirms a single disorder; deviation indicates a mixed disorder.
- Compare measured values to expected compensation to identify second disorders.
Key Terms & Definitions
- Compensation — Physiologic response to minimize changes in pH after primary acid-base disturbance.
- Metabolic acidosis/alkalosis — Disorders due to changes in bicarbonate; compensated by changes in ventilation.
- Respiratory acidosis/alkalosis — Disorders due to changes in CO₂; compensated by renal adjustment of bicarbonate.
- Winter's formula — Formula to predict appropriate pCO₂ in metabolic acidosis.
Action Items / Next Steps
- Review compensation formulas and shortcut methods.
- Practice using compensation assessment with ABG examples.
- Prepare for the next lecture on the anion gap.