What is respiratory failure?

Definition of Respiratory Failure

Respiratory failure is characterized by significant deterioration of arterial blood gas tensions, specifically hypoxemia (PaO2 < 60 mmHg) and/or hypercapnia (PaCO2 > 45 mmHg), resulting from the inability of the respiratory system to maintain adequate gas exchange between ambient air and peripheral organs. 1

Types of Respiratory Failure

Respiratory failure is classified into two main types:

1. Type I (Hypoxemic) Respiratory Failure:

   • Characterized by hypoxemia (PaO2 < 60 mmHg) with normal or low PaCO2
   • Results primarily from ventilation-perfusion mismatch, intrapulmonary shunting, or diffusion impairment
   • Common causes include pneumonia, pulmonary edema, and acute respiratory distress syndrome (ARDS) 2

2. Type II (Hypercapnic) Respiratory Failure:

   • Characterized by hypercapnia (PaCO2 > 45 mmHg) and usually accompanied by hypoxemia
   • Results from alveolar hypoventilation or increased dead space ventilation
   • Common causes include COPD exacerbations, neuromuscular disorders, and chest wall deformities 3, 4

Pathophysiological Mechanisms

Several mechanisms contribute to respiratory failure:

• Ventilation-Perfusion (V'/Q') Mismatch: The most common mechanism, where areas of the lung receive ventilation but inadequate perfusion or vice versa 3

• Alveolar Hypoventilation: Reduced minute ventilation relative to CO2 production, leading to hypercapnia 4

• Increased Dead Space Ventilation: Ventilation of non-perfused or poorly perfused lung units, leading to ineffective CO2 elimination 4

• Intrapulmonary Shunting: Blood passes through the lungs without participating in gas exchange, causing hypoxemia resistant to oxygen therapy 3

• Diffusion Impairment: Thickening of the alveolar-capillary membrane, limiting oxygen transfer 2

Clinical Manifestations

The clinical presentation of respiratory failure includes:

• Tachypnea (respiratory rate > 35 breaths/min)
• Abnormal breathing pattern (decreased tidal volume, increased respiratory frequency)
• Use of accessory respiratory muscles
• Cyanosis (in severe hypoxemia)
• Altered mental status (confusion, somnolence)
• Tachycardia
• Hypotension (in severe cases) 3

Diagnostic Criteria

Respiratory failure is diagnosed based on:

• Arterial Blood Gas Analysis:

  • Type I: PaO2 < 60 mmHg with normal or low PaCO2
  • Type II: PaCO2 > 45 mmHg, usually with hypoxemia 3, 1

• Pulse Oximetry:

  • SpO2 < 90% suggests significant hypoxemia
  • However, clinical recognition of hypoxemia is not reliable, so continuous monitoring is recommended 3

• Chest Imaging:

  • May show bilateral infiltrates in ARDS
  • Can be normal or show specific patterns depending on the underlying cause 3

Management Principles

Management of respiratory failure focuses on:

1. Oxygen Therapy:

   • Target SpO2 92-96% for most patients 5
   • For patients with COPD or risk of hypercapnic failure, target 88-92% 5
   • Delivery devices should be selected based on severity:
     • Mild hypoxemia: Nasal cannula (1-2 L/min)
     • Moderate hypoxemia: Simple face mask (5-6 L/min)
     • Severe hypoxemia: Non-rebreather mask (15 L/min) 5

2. Ventilatory Support:

   • Non-invasive Ventilation (NIV):

     • First-line for hypercapnic respiratory failure, especially in COPD
     • Can be used in selected cases of hypoxemic failure 1

   • Invasive Mechanical Ventilation:

     • Indicated when NIV fails or is contraindicated
     • Lung-protective strategies should be employed (low tidal volumes, appropriate PEEP) 6

   • High-Flow Nasal Oxygen (HFNO):

     • May reduce intubation rates and improve patient comfort in certain types of respiratory failure 3

3. Treatment of Underlying Cause:

   • Antibiotics for pneumonia
   • Bronchodilators and corticosteroids for COPD exacerbations
   • Diuretics for cardiogenic pulmonary edema 4

Complications and Prognosis

• Respiratory failure is associated with significant morbidity and mortality
• Complications include ventilator-associated pneumonia, barotrauma, and multi-organ dysfunction
• Prognosis depends on the underlying cause, patient comorbidities, and response to treatment 7
• Death from refractory respiratory failure is uncommon; most deaths result from multiple organ failure or recurrent sepsis 3

Special Considerations

• Right Ventricular Function: Respiratory failure can lead to pulmonary hypertension and right heart dysfunction, which should be monitored and managed appropriately 3

• Mechanical Ventilation Strategy: Should aim to unload the right ventricle by avoiding lung derecruitment and overdistension 3

• Monitoring: Continuous monitoring of oxygen saturation, respiratory rate, and hemodynamic parameters is essential 3, 5