Formulas for Acid-Base Disorders
The fundamental formulas for acid-base disorders are based on the relationship between carbon dioxide, bicarbonate, and pH, with distinct patterns for respiratory acidosis, respiratory alkalosis, metabolic acidosis, and metabolic alkalosis. 1
Respiratory Acidosis
Respiratory acidosis occurs when there is accumulation of carbon dioxide in the blood due to hypoventilation.
- Formula: CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻ 1
- Characterized by: ↑ PaCO₂, ↓ pH (< 7.35), and normal or ↑ HCO₃⁻ (compensatory) 1
- In acute respiratory acidosis: minimal compensation occurs, with primarily elevated PaCO₂ and decreased pH 1
- In chronic respiratory acidosis: renal compensation occurs with bicarbonate retention, resulting in high PaCO₂, high HCO₃⁻, and pH that may approach normal (compensated respiratory acidosis) 1
Respiratory Alkalosis
Respiratory alkalosis results from excessive elimination of carbon dioxide due to hyperventilation.
- Formula: ↓ CO₂ → ↓ H₂CO₃ → ↓ H⁺ + ↓ HCO₃⁻ 2, 3
- Characterized by: ↓ PaCO₂, ↑ pH (> 7.45), and normal or ↓ HCO₃⁻ (compensatory) 1, 2
- In acute respiratory alkalosis: minimal compensation occurs 2
- In chronic respiratory alkalosis: renal compensation with decreased bicarbonate reabsorption 2
Metabolic Acidosis
Metabolic acidosis occurs due to either increased acid production, decreased acid excretion, or bicarbonate loss.
- Formula: ↓ HCO₃⁻ → ↓ pH (primary defect is decreased bicarbonate) 1, 4
- Can be further categorized using the anion gap formula: Anion Gap = [Na⁺ + K⁺] - [Cl⁻ + HCO₃⁻] 4, 5
- High anion gap metabolic acidosis: accumulation of organic acids (e.g., lactic acid, ketoacids) 3
- Normal anion gap (hyperchloremic) metabolic acidosis: bicarbonate loss or chloride gain 3
- Respiratory compensation: ↑ minute ventilation leading to ↓ PaCO₂ 2
Metabolic Alkalosis
Metabolic alkalosis results from either excessive loss of hydrogen ions or gain of bicarbonate.
- Formula: ↑ HCO₃⁻ → ↑ pH (primary defect is increased bicarbonate) 2, 3
- Characterized by: ↑ HCO₃⁻, ↑ pH (> 7.45), and normal or ↑ PaCO₂ (compensatory) 1
- Respiratory compensation: hypoventilation leading to ↑ PaCO₂ 2
- Can be classified as chloride-responsive (urinary chloride < 10 mEq/L) or chloride-resistant (urinary chloride > 20 mEq/L) 2
Clinical Application and Compensation
- When assessing acid-base disorders, remember that normal pH range is 7.35-7.45, with acidosis defined as pH < 7.35 and alkalosis as pH > 7.45 1
- Normal PaCO₂ range is 4.6-6.1 kPa (34-46 mmHg) 1
- Compensation mechanisms attempt to normalize pH but rarely achieve complete normalization in simple disorders 3
- Mixed disorders can be identified when compensation is inappropriate for the primary disorder 5
Common Pitfalls to Avoid
- Failing to recognize mixed acid-base disorders (e.g., respiratory acidosis with metabolic alkalosis) 5, 6
- Overlooking the relationship between delta anion gap and delta bicarbonate in high anion gap metabolic acidosis 5
- Treating the acid-base disorder without addressing the underlying cause 4, 3
- Not considering the time course (acute vs. chronic) when evaluating compensation 1, 2