ABG Examples: Respiratory vs Metabolic Acidosis
Respiratory acidosis shows pH <7.35 with elevated PaCO₂ (>45 mmHg and typically ≥6.5 kPa), while metabolic acidosis shows pH <7.35 with low bicarbonate (<18 mmol/L) and compensatory low PaCO₂ (<35 mmHg).
Respiratory Acidosis - Typical ABG Pattern
Primary problem: CO₂ retention from hypoventilation
- pH: <7.35 (acidemic)
- PaCO₂: >45 mmHg (>6.0-6.5 kPa) - this is the PRIMARY abnormality 1, 2
- HCO₃⁻: Normal acutely, or elevated (>26 mmol/L) if chronic (metabolic compensation)
- Respiratory rate: Often >23 breaths/min despite inadequate ventilation 1
Clinical Context
The guidelines specifically define acute hypercapnic respiratory failure as pH <7.35 with PaCO₂ ≥6.5 kPa (approximately 49 mmHg), particularly in COPD exacerbations where respiratory muscle failure leads to inadequate alveolar ventilation despite increased work of breathing 1, 2. The elevated CO₂ is the driving force behind the acidosis.
Example ABG:
- pH: 7.28
- PaCO₂: 65 mmHg (8.7 kPa)
- HCO₃⁻: 28 mmol/L (slight elevation suggesting early compensation)
- PaO₂: Variable depending on oxygen therapy
Metabolic Acidosis - Typical ABG Pattern
Primary problem: Excess acid production or bicarbonate loss
- pH: <7.35 (acidemic) 3
- HCO₃⁻: <18 mmol/L - this is the PRIMARY abnormality 3
- PaCO₂: <35 mmHg (respiratory compensation through hyperventilation) 3, 4
- Base excess: ≤ -7 mmol/L 5
Clinical Context
The body compensates for metabolic acidosis through hyperventilation, which lowers PaCO₂ 3, 4. This hyperventilation is usually well-sustained even in severe acidosis (pH <7.10), except when acute tissue hypoxia or circulatory failure is present 4. The low CO₂ distinguishes this from respiratory acidosis.
Example ABG:
- pH: 7.22
- PaCO₂: 28 mmHg (compensatory hyperventilation)
- HCO₃⁻: 12 mmol/L
- Base excess: -12 mmol/L
- Calculate anion gap to determine if high anion gap (lactic acidosis, ketoacidosis) or normal anion gap (GI/renal bicarbonate loss) 3, 6
Key Distinguishing Feature: The CO₂ Direction
The critical difference is the PaCO₂ direction relative to pH:
- Respiratory acidosis: pH ↓ and PaCO₂ ↑ (move in OPPOSITE directions - "Respiratory opposite") 7
- Metabolic acidosis: pH ↓ and PaCO₂ ↓ (move in SAME direction - "Metabolic equal") 7
Common Pitfall: Mixed Disorders
Beware that approximately 20% of acute hypercapnic respiratory failure cases have a mixed metabolic and respiratory acidosis 1. In these cases:
- pH <7.35
- PaCO₂ elevated (>45 mmHg)
- HCO₃⁻ also low (<18 mmol/L)
- The acidosis is MORE severe than either disorder alone would predict
This mixed picture is particularly common in critically ill patients and requires treating both the underlying respiratory failure and the metabolic component 1, 5. For example, severe asthma patients may develop metabolic acidosis from beta-agonist therapy (causing lactic acidosis) superimposed on their primary respiratory condition 5.