What are the types of respiratory failure?

Types of Respiratory Failure

Respiratory failure is classified into two main types: Type 1 (hypoxemic) and Type 2 (hypercapnic), with additional classifications based on onset (acute vs. chronic) and underlying pathophysiology. 1

Primary Classification

Type 1 Respiratory Failure (Hypoxemic)

• Defined by low oxygen levels (PaO₂ < 60 mmHg or SaO₂ < 88%) with normal or low carbon dioxide levels 1, 2
• Results from failure to maintain adequate oxygenation despite normal or increased ventilatory effort 1
• Primary pathophysiological mechanisms include:
  • Ventilation-perfusion mismatch 1, 2
  • Right-to-left shunts (intrapulmonary shunting) 1, 2
  • Diffusion impairment 1, 2
  • Alveolar hypoventilation 1
• Common clinical scenarios include acute respiratory distress syndrome (ARDS), pneumonia, and pulmonary edema 1
• Typically responds to oxygen therapy as recommended by the American Thoracic Society 1

Type 2 Respiratory Failure (Hypercapnic)

• Defined by high carbon dioxide levels (PaCO₂ ≥ 45 mmHg) with pH < 7.35, often with concurrent hypoxemia 1, 2
• Represents failure of ventilatory pump function 1, 3
• Normal range for carbon dioxide levels is 4.6-6.1 kPa (34-46 mmHg) 1
• Common causes include:
  • COPD exacerbations 1, 4
  • Neuromuscular disorders 1
  • Chest wall deformities (scoliosis, thoracoplasty) 1
• Requires cautious oxygen therapy with a target oxygen saturation of 88-92% as recommended by the European Respiratory Society 1
• Non-invasive ventilation is recommended as first-line treatment when pH < 7.35 1, 4

Temporal Classification

Acute Respiratory Failure

• Characterized by sudden onset with rapid deterioration of arterial blood gases 1, 5
• During acute exacerbations, ventilation-perfusion abnormalities worsen significantly 5
• Airway resistance, end-expiratory lung volume, and intrinsic PEEP increase substantially 5
• Breathing pattern becomes abnormal with decreased tidal volume and increased respiratory frequency 5
• May lead to respiratory muscle fatigue due to high inspiratory muscle activity 5

Chronic Respiratory Failure

• Develops gradually over time 1
• Often involves compensatory mechanisms (e.g., renal bicarbonate retention) 1
• In chronic hypercapnic failure, the "theory of natural wisdom" suggests patients adapt to respiratory loads by reducing minute ventilation to avoid fatigue and exhaustion, at the expense of alveolar ventilation 3

Acute-on-Chronic Respiratory Failure

• Presents unique challenges due to altered baseline physiology 1
• Often seen in COPD patients with acute exacerbations 5, 4

Pathophysiological Mechanisms

Mechanisms of Hypercapnic Respiratory Failure

• Results from reduced alveolar ventilation for a given carbon dioxide production 1, 3
• May be caused by:
  • Mechanical defects of the respiratory system 3
  • Central nervous system depression 3
  • Imbalance between energy demands and supplies 3
  • Adaptation of central respiratory controllers 3
  • Increased work of breathing (e.g., in COPD with intrinsic PEEP) 1, 5

Mechanisms of Hypoxemic Respiratory Failure

• Can be further classified based on severity (e.g., mild, moderate, severe ARDS) 1
• Primary mechanisms include:
  • Ventilation-perfusion mismatch 1
  • Intrapulmonary shunting 1
  • Diffusion limitation 1
  • Alveolar hypoventilation 1

Clinical Considerations

Monitoring Requirements

• Regular assessment of arterial blood gases 1
• Continuous pulse oximetry 1
• Use of early warning scores 1

Ventilation Strategies

• Ventilator settings must accommodate the underlying pathophysiology 1
• For COPD exacerbations, settings should include longer expiration and shorter inspiration time to avoid further hyperinflation and increased intrinsic PEEP 5, 4
• In severe cases where mechanical ventilation is insufficient, extracorporeal CO₂ removal may be considered 4

Common Pitfalls

• Failure to recognize the predominant pathophysiological component when selecting ventilation strategies 4
• Delayed recognition of NIV failure in patients with severe respiratory acidosis 4
• Inappropriate oxygen therapy in Type 2 respiratory failure, which can worsen ventilation-perfusion balance and increase PaCO₂ 5
• Overlooking the possibility of respiratory muscle fatigue in patients with high work of breathing 5