What are the definition and criteria for chronic respiratory failure and mechanical ventilation (MV) dependence?

Definition and Criteria for Chronic Respiratory Failure and Mechanical Ventilation Dependence

Chronic Respiratory Failure: Core Definition

Chronic respiratory failure is defined as persistent impairment of pulmonary gas exchange resulting in hypoxemia (PaO₂ <8 kPa or 60 mmHg) and/or hypercapnia (PaCO₂ >6 kPa or 45 mmHg) that develops gradually over time with compensatory mechanisms such as renal bicarbonate retention. 1

Type 1 (Hypoxemic) Chronic Respiratory Failure

• Diagnostic threshold: PaO₂ <8 kPa (60 mmHg) with normal or low PaCO₂ 1
• Results from ventilation-perfusion mismatch, intrapulmonary shunting, diffusion limitation, or alveolar hypoventilation 1
• Common causes include interstitial lung disease, chronic pulmonary edema, and advanced emphysema with vascular destruction 2

Type 2 (Hypercapnic) Chronic Respiratory Failure

• Diagnostic threshold: PaCO₂ >6 kPa (45 mmHg), often with concurrent hypoxemia 1, 3
• Represents failure of the ventilatory pump from alveolar hypoventilation where minute ventilation is insufficient relative to CO₂ production 3
• Key pathophysiological features in COPD include:
  • Dynamic hyperinflation with intrinsic PEEP (PEEPi) creating an inspiratory threshold load 2, 3
  • Inspiratory muscle dysfunction related to chronic hypercapnia 2, 3
  • Increased mechanical workload with greater energy consumption 2, 3
  • Severe V/Q abnormalities with low V/Q units representing partially blocked airways 2

Mechanical Ventilation Dependence: Criteria

MV-dependence is defined as the inability to maintain adequate gas exchange without mechanical ventilatory support, requiring either invasive or non-invasive ventilation for sustained periods. 4, 5

Indications for Long-Term Mechanical Ventilation

• Chronic hypercapnic respiratory failure with repeated decompensations despite optimal medical therapy 4
• Neuromuscular diseases (ALS, muscular dystrophy, myasthenia gravis) causing progressive ventilatory pump failure 1
• Chest wall disorders (severe scoliosis, thoracoplasty) with restrictive mechanics 1
• Obesity hypoventilation syndrome combining restrictive mechanics with central drive abnormalities 1

Specific Criteria for Domiciliary Mechanical Ventilation

• Patients with chronic hypercapnia who experience repeated hypercapnic decompensations requiring hospitalization 4
• Evidence of nocturnal hypoventilation with daytime symptoms despite optimal medical management 6
• Progressive decline in pulmonary function with inability to maintain adequate ventilation independently 6

Clinical Assessment Framework

Arterial Blood Gas Criteria

• For long-term oxygen therapy (LTOT): PaO₂ ≤7.3 kPa (55 mmHg) or PaO₂ 7.3-8.0 kPa (55-60 mmHg) with evidence of cor pulmonale or polycythemia 2
• For NIV initiation in chronic setting: Any elevation of PaCO₂ in neuromuscular disease may herald impending crisis 3
• For acute-on-chronic exacerbations: pH <7.35 with PaCO₂ >6-8 kPa (45-60 mmHg) and respiratory rate >24 breaths/min despite optimal medical therapy 2

Pulmonary Function Thresholds

• Significant hypoxemia or hypercapnia is rare with FEV₁ >1.0 L in COPD 2
• 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 2

Common Pitfalls to Avoid

• Do not withdraw oxygen from patients who previously qualified for LTOT but now have improved PaO₂, as this may negate reparative effects and cause deterioration 2
• Do not administer high-flow oxygen without CO₂ monitoring in Type 2 failure, as this can precipitate CO₂ narcosis and respiratory arrest 1
• Do not delay NIV initiation when pH <7.35 and PaCO₂ >6.0 kPa, as this misses the therapeutic window 1
• Recognize that standard chest radiographs are poor predictors of oxygenation severity in chronic respiratory failure 2

Monitoring Requirements for MV-Dependent Patients

• Regular arterial blood gas analysis while breathing room air before any changes in therapy 1
• 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 1
• Transcutaneous CO₂ monitoring can supplement arterial blood gas analysis when available 1