Types of Acute Respiratory Failure
Acute respiratory failure is classified into two main types: Type 1 (hypoxemic) respiratory failure, defined by PaO₂ <8 kPa (60 mmHg) with normal or low PaCO₂, and Type 2 (hypercapnic) respiratory failure, defined by PaO₂ <8 kPa (60 mmHg) AND PaCO₂ >6 kPa (45 mmHg). 1, 2
Type 1 Respiratory Failure (Hypoxemic)
Definition and Gas Exchange Abnormalities:
- Characterized by failure to maintain adequate oxygenation with PaO₂ <8 kPa (60 mmHg) while PaCO₂ remains normal or low 1, 2
- The hypoxemia occurs despite normal or increased ventilatory effort 3
Pathophysiological Mechanisms:
- Primarily caused by ventilation-perfusion (V/Q) mismatch, which is the most common mechanism 3, 2
- Additional mechanisms include right-to-left intrapulmonary shunts, diffusion impairment, and alveolar hypoventilation 3
Common Clinical Scenarios:
- Acute respiratory distress syndrome (ARDS), which can be further classified by severity: mild (PaO₂/FiO₂ 200-300 mmHg), moderate (100-200 mmHg), or severe (≤100 mmHg) 3
- Pneumonia and cardiogenic pulmonary edema 3
Management Approach:
- Typically responds to oxygen therapy, which is the first-line treatment 3
- High-flow nasal oxygen (HFNO) may reduce intubation rates compared to conventional oxygen therapy 3
Type 2 Respiratory Failure (Hypercapnic)
Definition and Gas Exchange Abnormalities:
- Defined by elevated PaCO₂ >6 kPa (45 mmHg) with concurrent hypoxemia (PaO₂ <8 kPa) 1, 2
- Represents failure of the ventilatory pump function rather than just oxygenation failure 3
- Often associated with respiratory acidosis (pH <7.35) 1, 3
Pathophysiological Mechanisms:
- Alveolar hypoventilation is the primary mechanism, where minute ventilation is insufficient relative to CO₂ production 3, 2
- Increased airway resistance, dynamic hyperinflation with intrinsic PEEP (PEEPi), and inspiratory muscle dysfunction contribute significantly 3
- V/Q abnormalities worsen during acute exacerbations 3
Common Clinical Scenarios:
- COPD exacerbations are the most common cause 1, 4
- Chest wall deformities (scoliosis, thoracoplasty) and neuromuscular diseases 1
- Obesity hypoventilation syndrome 4
Management Approach:
- Non-invasive ventilation (NIV) is indicated when pH <7.35 and PaCO₂ >6 kPa, particularly in COPD with respiratory acidosis (pH 7.25-7.35) 1, 3
- Controlled oxygen therapy with target saturation of 88-92% to avoid worsening hypercapnia 3, 2
- NIV is contraindicated in patients with impaired consciousness, severe hypoxemia, or copious respiratory secretions 1, 3
Critical Diagnostic Considerations
Arterial Blood Gas Analysis:
- Knowledge of arterial blood gas tensions is critical for proper classification and management 1, 2
- Arterial blood gases should be measured in most patients with acute breathlessness 1
- A repeat sample should be taken after initial medical treatment to determine if NIV is still indicated 1
Common Pitfall to Avoid:
- Administering high-flow oxygen without monitoring CO₂ can precipitate CO₂ narcosis and respiratory arrest in Type 2 respiratory failure 3
- There should be a low threshold for measuring arterial blood gases in patients with neuromuscular diseases, chest wall deformity, or obesity who may be in respiratory failure without significant breathlessness 1
Additional Classifications
Temporal Classification:
- Acute respiratory failure is characterized by sudden onset with rapid deterioration of arterial blood gases 3
- Chronic respiratory failure develops gradually with compensatory mechanisms (e.g., renal bicarbonate retention) 3
- Acute-on-chronic respiratory failure presents unique challenges due to altered baseline physiology 3