Type 2 Respiratory Failure
The term for combined hypoxemic and hypercapnic respiratory failure is Type 2 respiratory failure (also called hypercapnic respiratory failure or ventilatory failure). 1
Definition and Pathophysiology
Type 2 respiratory failure is defined by:
- PaCO2 > 6.1 kPa (45 mm Hg) - elevated carbon dioxide
- Often accompanied by hypoxemia (low PaO2 or SpO2 < 90%)
- pH < 7.35 when acute, indicating respiratory acidosis 1
The key distinguishing feature is that hypercapnia is present even when oxygen saturation may be in the normal range 1. This reflects the underlying problem: the respiratory system's inability to adequately ventilate and eliminate CO2, not just oxygenate blood.
Clinical Context
Type 2 respiratory failure indicates "pump failure" - the respiratory system cannot maintain adequate alveolar ventilation to meet metabolic demands 2, 3. This contrasts with Type 1 (hypoxemic) respiratory failure, which primarily reflects problems with gas exchange across the alveolar-capillary membrane.
Common Causes Include:
- COPD exacerbations - the most frequent cause 4, 5
- Obesity hypoventilation syndrome (OHS) - defined as BMI > 30 kg/m² with awake PaCO2 > 45 mm Hg 6
- Neuromuscular disease
- Chest wall disorders
- Central hypoventilation
Important Clinical Distinction
When both hypoxemia and hypercapnia coexist, this is sometimes referred to as "global respiratory failure" or "mixed respiratory failure" 2. However, the formal classification remains Type 2 respiratory failure when hypercapnia is the dominant feature.
Acute vs. Chronic Considerations:
- Acute Type 2 respiratory failure: pH < 7.35 with elevated PaCO2
- Chronic compensated: High PaCO2 with high bicarbonate and normal pH ("compensated respiratory acidosis") 1
- Acute-on-chronic: Sudden CO2 rise in patients with baseline chronic hypercapnia, causing acidosis despite elevated bicarbonate 1
Management Implications
The presence of hypercapnia fundamentally changes treatment approach:
For acute hypercapnic respiratory failure with acidemia (pH ≤ 7.35):
- Noninvasive ventilation (NIV) is strongly recommended as first-line therapy to reduce mortality and intubation rates 4, 5, 7
- High-flow nasal cannula (HFNC) may be considered only in mild hypercapnia with pH > 7.25, with close monitoring for escalation to NIV 4, 7
Critical pitfall: Using oxygen therapy alone in Type 2 respiratory failure can worsen hypercapnia by removing hypoxic respiratory drive, particularly in COPD patients. The primary treatment must address ventilation, not just oxygenation 1.