Chronic Respiratory Failure and Mechanical Ventilation Dependence

Definition and Diagnostic Criteria

The European Respiratory Society defines chronic respiratory failure as persistent impairment of pulmonary gas exchange resulting in hypoxemia and/or hypercapnia that develops gradually over time with compensatory mechanisms, with a diagnostic threshold of PaO₂ <8 kPa or 60 mmHg and/or PaCO₂ >6 kPa or 45 mmHg 1
Type 1 (hypoxemic) chronic respiratory failure is characterized by a diagnostic threshold of PaO₂ <8 kPa (60 mmHg) with normal or low PaCO₂, resulting from ventilation-perfusion mismatch, intrapulmonary shunting, diffusion limitation, or alveolar hypoventilation 1
Type 2 (hypercapnic) chronic respiratory failure represents failure of the ventilatory pump from alveolar hypoventilation, with a diagnostic threshold of PaCO₂ >6 kPa (45 mmHg), often with concurrent hypoxemia 1, 3

Key pathophysiological features in COPD include dynamic hyperinflation with intrinsic PEEP (PEEPi) creating an inspiratory threshold load, inspiratory muscle dysfunction related to chronic hypercapnia, increased mechanical workload with greater energy consumption, and severe V/Q abnormalities with low V/Q units representing partially blocked airways 2, 3

The American Thoracic Society recommends long-term mechanical ventilation for patients with chronic hypercapnic respiratory failure, neuromuscular diseases, chest wall disorders, and obesity hypoventilation syndrome 1

The European Respiratory Society recommends arterial blood gas criteria for long-term oxygen therapy (LTOT) of PaO₂ ≤7.3 kPa (55 mmHg) or PaO₂ 7.3-8.0 kPa (55-60 mmHg) with evidence of cor pulmonale or polycythemia 4
The European Respiratory Society also recommends criteria for NIV initiation in chronic setting, including any elevation of PaCO₂ in neuromuscular disease, and for acute-on-chronic exacerbations, including pH <7.35 with PaCO₂ >6-8 kPa (45-60 mmHg) and respiratory rate >24 breaths/min despite optimal medical therapy 3, 4, 5

Significant hypoxemia or hypercapnia is rare with FEV₁ >1.0 L in COPD, and flow-limited expiration during tidal breathing initially occurs with exercise, then progresses to rest 2
Vital capacity <15 mL/kg suggests need for mechanical ventilation consideration 6

Regular arterial blood gas analysis while breathing room air before any changes in therapy, and if PaO₂ <7.3 kPa on discharge in COPD, repeat measurement after at least 3 weeks 1
Pulmonary function testing every 6 months in neuromuscular disease to guide NIV timing, and transcutaneous CO₂ monitoring can supplement arterial blood gas analysis when available 1