What is the definition of acute respiratory failure?

Definition of Acute Respiratory Failure

Acute respiratory failure is defined as a severe impairment of pulmonary gas exchange characterized by hypoxemia (PaO2 <60 mmHg or SpO2 <90%) and/or hypercapnia (PaCO2 >45 mmHg with pH <7.35) that develops rapidly and requires immediate intervention.

Types of Acute Respiratory Failure

Type I (Hypoxemic) Respiratory Failure

• Characterized by:
  • PaO2 <60 mmHg or SpO2 <90%
  • Normal or low PaCO2 (hypocapnia or normocapnia)
• Primary pathophysiology: Oxygenation failure
• Common mechanisms:
  • Ventilation-perfusion (V/Q) mismatch
  • Intrapulmonary shunt
  • Diffusion impairment

Type II (Hypercapnic) Respiratory Failure

• Characterized by:
  • PaO2 <60 mmHg or SpO2 <90%
  • PaCO2 >45 mmHg with pH <7.35 (respiratory acidosis)
• Primary pathophysiology: Ventilatory failure (inadequate alveolar ventilation)
• Common mechanisms:
  • Respiratory pump dysfunction
  • Central respiratory drive depression
  • Increased airway resistance
  • Respiratory muscle fatigue

Pathophysiologic Mechanisms

1. Ventilation-Perfusion Mismatch:

   • Most common cause of hypoxemia
   • Areas of lung receive inadequate ventilation relative to perfusion

2. Intrapulmonary Shunt:

   • Blood passes through non-ventilated lung regions
   • Resistant to oxygen supplementation

3. Alveolar Hypoventilation:

   • Inadequate minute ventilation relative to CO2 production
   • Results in hypercapnia and hypoxemia

4. Diffusion Limitation:

   • Impaired gas transfer across alveolar-capillary membrane
   • Less common primary mechanism

5. Respiratory Pump Failure:

   • Neuromuscular disorders
   • Chest wall deformities
   • Respiratory muscle fatigue

Diagnostic Criteria

According to the BTS/ICS guidelines 1, acute respiratory failure is conventionally defined as:

• pH <7.35 and PaCO2 >6.5 kPa (>45 mmHg) for acute respiratory acidosis
• More severe acidosis (pH <7.25) is often used as a threshold for considering invasive mechanical ventilation

For hypoxemic respiratory failure, the European Society of Cardiology 1 defines criteria as:

• SpO2 <90% or PaO2 <60 mmHg (8.0 kPa)
• Respiratory distress (respiratory rate >25 breaths/min)

Clinical Assessment and Monitoring

Essential monitoring for patients with suspected acute respiratory failure includes:

• Continuous transcutaneous arterial oxygen saturation (SpO2) monitoring 1
• Measurement of blood pH and carbon dioxide tension 1
• Assessment of respiratory rate and work of breathing
• Evaluation for signs of respiratory distress:
  • Use of accessory muscles
  • Paradoxical breathing
  • Altered mental status

Management Considerations

The management approach depends on the type and severity of respiratory failure:

1. Oxygen Therapy:

   • Recommended for patients with SpO2 <90% or PaO2 <60 mmHg 1
   • Titrate to target SpO2 92-97% when PEEP <10 cmH2O and 88-92% when PEEP ≥10 cmH2O 2
   • Caution with excessive oxygen in COPD patients due to risk of worsening hypercapnia

2. Non-invasive Ventilation (NIV):

   • Consider in patients with respiratory distress (respiratory rate >25 breaths/min, SpO2 <90%)
   • Particularly effective in hypercapnic respiratory failure due to COPD exacerbations 1
   • Bi-level pressure support ventilators are recommended for acute NIV services 1

3. Intubation and Invasive Mechanical Ventilation:

   • Indicated when respiratory failure cannot be managed non-invasively
   • Criteria include: PaO2 <60 mmHg (8.0 kPa), PaCO2 >50 mmHg (6.65 kPa), and pH <7.35 1
   • Implement lung-protective ventilation strategies with tidal volumes of 4-8 ml/kg predicted body weight 2

Common Pitfalls and Caveats

1. Delayed Recognition:

   • Arterial blood gas analysis is critical for diagnosis and should be performed promptly in suspected cases 1
   • Clinical signs may be subtle, especially in neuromuscular diseases or obesity 1

2. Inappropriate Oxygen Therapy:

   • Excessive oxygen in COPD patients can worsen hypercapnia through suppression of hypoxic respiratory drive
   • Insufficient oxygen can lead to tissue hypoxia and organ dysfunction

3. Failure to Identify Underlying Cause:

   • Acute respiratory failure is a manifestation of an underlying condition that requires specific treatment
   • Common causes include pneumonia, pulmonary edema, COPD exacerbation, asthma, and neuromuscular disorders

4. Delayed Escalation of Care:

   • Failure to recognize when non-invasive support is inadequate can lead to worse outcomes
   • NIV should not substitute for intubation when the latter is clearly more appropriate 1

By understanding the definition, pathophysiology, and management principles of acute respiratory failure, clinicians can provide timely and appropriate interventions to improve patient outcomes.