What are the cut-offs for type 1 and type 2 respiratory failure in terms of PaO2 (partial pressure of oxygen) and PaCO2 (partial pressure of carbon dioxide) levels?

Cut-offs for Type 1 and Type 2 Respiratory Failure

Type 1 respiratory failure is defined as a PaO2 < 8.0 kPa (60 mmHg) with normal or low PaCO2, while Type 2 respiratory failure is defined as a PaO2 < 8.0 kPa (60 mmHg) with PaCO2 ≥ 6.5 kPa (50 mmHg) and pH < 7.35. 1

Detailed Definitions

Type 1 Respiratory Failure

• PaO2 < 8.0 kPa (60 mmHg) on room air
• Normal or low PaCO2 (< 6.0 kPa or 45 mmHg)
• Primarily a failure of oxygenation
• Causes: V/Q mismatch, shunt, diffusion limitation, or low inspired oxygen tension

Type 2 Respiratory Failure

• PaO2 < 8.0 kPa (60 mmHg) on room air
• PaCO2 ≥ 6.5 kPa (50 mmHg)
• pH < 7.35 (indicating acute respiratory acidosis)
• Represents failure of ventilation
• Causes: alveolar hypoventilation, increased dead space ventilation, or increased CO2 production

Important Considerations in Assessment

Using PaO2/FiO2 Ratio

• Simple PaO2 measurement can be confounded when patients are on oxygen therapy
• PaO2/FiO2 ratio provides a more accurate assessment of respiratory failure when patients are receiving supplemental oxygen 2
• A PaO2/FiO2 ratio < 300 mmHg indicates respiratory failure

Role of Alveolar-arterial (A-a) Gradient

• Using PaCO2 alone to classify respiratory failure can be misleading
• A-a gradient helps distinguish between Type 1 and Type 2 respiratory failure more accurately
• A normal A-a gradient with hypoxemia suggests hypoventilation (Type 2)
• An increased A-a gradient suggests V/Q mismatch or shunt (Type 1) 2

Hypercapnia Thresholds

• The British Thoracic Society recognizes hypercapnia as PaCO2 ≥ 6.0 kPa 3
• For clinical intervention with NIV, a higher threshold of PaCO2 ≥ 6.5 kPa is recommended 3
• For patients with PaCO2 between 6.0-6.5 kPa, NIV should be considered but is not mandatory 3

Clinical Implications

NIV Initiation Criteria

• NIV should be started when:
  • pH < 7.35
  • PaCO2 ≥ 6.5 kPa
  • Respiratory rate > 23 breaths/min
  • These conditions persist after one hour of optimal medical therapy 3

Oxygen Therapy Targets

• For standard patients: target SpO2 94-98%
• For patients at risk of hypercapnic respiratory failure (e.g., COPD): target SpO2 88-92% 4
• Uncontrolled oxygen therapy can be harmful to patients at risk of hypercapnic respiratory failure, especially if PaO2 is raised above 10 kPa 3

Monitoring Considerations

• Arterial blood gases should be monitored within 30-60 minutes of starting oxygen therapy
• Continuous pulse oximetry is recommended for patients with respiratory failure
• Be alert for rebound hypoxemia if oxygen therapy is suddenly withdrawn in patients with hypercapnic respiratory failure 3

Pitfalls to Avoid

1. Misclassification: Using PaCO2 alone to classify respiratory failure can lead to misdiagnosis in up to 39% of cases 2

2. Overlooking mixed respiratory failure: Both Type 1 and Type 2 respiratory failure can co-exist 2

3. Sudden oxygen withdrawal: Abrupt cessation of oxygen therapy in hypercapnic patients can cause dangerous rebound hypoxemia 3

4. Overtreatment with oxygen: High-concentration oxygen therapy can worsen hypercapnic respiratory failure and increase mortality risk in certain patient groups 3

5. Delayed intervention: Waiting for PaCO2 to exceed 6.5 kPa before considering NIV may delay necessary treatment in deteriorating patients 3