What are the criteria for respiratory failure?

Criteria for Respiratory Failure

Respiratory failure is defined by arterial blood gas (ABG) criteria as PaO2 < 60 mmHg (8.0 kPa) or SpO2 < 88% for hypoxemic respiratory failure (Type 1), and PaCO2 ≥ 45 mmHg with pH < 7.35 for hypercapnic respiratory failure (Type 2). 1

Types of Respiratory Failure

Type 1 (Hypoxemic) Respiratory Failure

• Defined as PaO2 < 60 mmHg (8.0 kPa) or SpO2 < 88% with normal or low PaCO2 2
• Results from conditions causing hypoxemia such as:
  • V/Q mismatch
  • Shunt
  • Diffusion limitation
  • Low inspired oxygen tension 1
• Common causes include pneumonia, pulmonary edema, ARDS, and pulmonary embolism 2

Type 2 (Hypercapnic) Respiratory Failure

• Defined as PaCO2 ≥ 45 mmHg with arterial pH < 7.35 1
• Results from:
  • Alveolar hypoventilation
  • Increased dead space ventilation
  • Increased CO2 production 1
• Common causes include COPD exacerbation, neuromuscular disorders, and chest wall deformities 2

Diagnostic Criteria

Clinical Assessment

• Respiratory rate > 30 breaths/minute 2
• Use of accessory respiratory muscles 2
• Paradoxical breathing pattern 2
• Altered mental status (confusion, agitation) potentially indicating hypoxemia or hypercapnia 2
• Inability to protect airway or clear secretions 2

Laboratory Criteria

• Arterial Blood Gas (ABG) analysis is essential for definitive diagnosis 2
• Type 1 (Hypoxemic) Respiratory Failure:
  • PaO2 < 60 mmHg (8.0 kPa) or SpO2 < 88% 2
  • Normal or low PaCO2 2
• Type 2 (Hypercapnic) Respiratory Failure:
  • PaCO2 ≥ 45 mmHg 1
  • pH < 7.35 (indicating acute respiratory acidosis) 1

Severity Assessment for ARDS (a cause of respiratory failure)

• According to the Berlin Definition 2:
  • Mild: PaO2/FiO2 201-300 mmHg with PEEP/CPAP ≥ 5 cmH2O
  • Moderate: PaO2/FiO2 101-200 mmHg with PEEP ≥ 5 cmH2O
  • Severe: PaO2/FiO2 ≤ 100 mmHg with PEEP ≥ 10 cmH2O

Indications for Mechanical Ventilation

Non-invasive Ventilation Criteria

• Moderate to severe dyspnea with use of accessory muscles and paradoxical abdominal motion 2
• Acute respiratory acidosis (pH < 7.35 with PaCO2 > 45 mmHg) 2
• Respiratory rate > 25 breaths/minute 2
• SpO2 < 90% despite oxygen therapy 2

Invasive Ventilation Criteria

• Respiratory arrest 2
• Failure of non-invasive ventilation:
  • Worsening of ABGs and/or pH within 1-2 hours 2
  • Lack of improvement in ABGs and/or pH after 4 hours 2
• Severe acidosis (pH < 7.25) and hypercapnia (PaCO2 > 60 mmHg) 2
• Life-threatening hypoxemia (PaO2/FiO2 < 200 mmHg) 2
• Tachypnea > 35 breaths/minute 2
• Inability to protect airway (GCS < 8) 2
• Hemodynamic instability despite fluid resuscitation 2

Monitoring Parameters

• Continuous monitoring of SpO2 for at least 24 hours after initiating respiratory support 2
• Regular ABG measurements to assess response to therapy:
  • Within 1-2 hours after initiating non-invasive ventilation 2
  • Within 4-6 hours if earlier sample showed little improvement 2
• Clinical assessment including:
  • Respiratory rate and pattern
  • Use of accessory muscles
  • Patient comfort and conscious level
  • Coordination with ventilator 2

Important Considerations

• The PaO2/FiO2 ratio measured after 24 hours of standardized ventilator settings is a better predictor of outcomes than the initial measurement 3
• SpO2/FiO2 ratio has limitations for classification and monitoring of ARDS, with a tendency to misclassify severity in 33% of cases 4
• For patients at risk of hypercapnic respiratory failure (e.g., COPD), target oxygen saturation should be 88-92% 2
• For most other patients with acute respiratory failure, target oxygen saturation should be 94-98% 2

Treatment Approach

• Initial oxygen therapy based on type of respiratory failure:
  • Type 1: Target SpO2 94-98% 2
  • Type 2 or at risk of hypercapnic failure: Target SpO2 88-92% 2
• Escalation of respiratory support based on response:
  1. Standard oxygen therapy
  2. High-Flow Nasal Oxygen (HFNO) if SpO2 remains < 93% despite standard oxygen 2
  3. Non-invasive ventilation for appropriate candidates 2
  4. Invasive mechanical ventilation if non-invasive methods fail 2

Remember that respiratory failure represents a critical condition requiring prompt recognition and intervention to prevent further deterioration and improve patient outcomes.