Chronic respiratory failure - etiology, pathogenesis, compensatory mechanisms, and clinical manifestations.
Chronic respiratory failure is a long-standing failure of the respiratory system to maintain normal gas exchange, developing gradually over weeks to months, allowing time for compensatory mechanisms to adapt.
- PaO₂ < 60 mmHg (hypoxemia), and/ or
- PaCO₂ > 50 mmHg (hypercapnia),
- with compensated (near-normal) pH due to renal adaptation.
Etiology
Type I (hypoxemic CRF)
- Advanced interstitial lung diseases (pulmonary fibrosis).
- Severe COPD with emphysema predominance.
- Chronic pulmonary vascular diseases (pulmonary hypertension, chronic thromboembolism).
Type II (hypercapnic CRF)
- Severe COPD with chronic bronchitis phenotype.
- Obesity hypoventilation syndrome (Pickwickian syndrome).
- Neuromuscular diseases (muscular dystrophy, ALS).
- Severe kyphoscoliosis.
Pathogenesis
Type I (hypoxemic)
- Chronic V/ Q mismatch (areas poorly ventilated or perfused).
- Progressive diffusion impairment (e.g., interstitial fibrosis).
- Development of pulmonary hypertension → worsening gas exchange.
Type II (hypercapnic)
- Chronic alveolar hypoventilation → CO₂ retention.
- Gradual decrease in respiratory drive or muscle strength.
Compensatory mechanisms
Renal compensation
- Increased bicarbonate (HCO₃ ) reabsorption to buffer chronic respiratory acidosis → near-normal pH.
Hematologic compensation
- Increased erythropoietin production → secondary polycythemia → improves oxygen- carrying capacity.
Cardiovascular adaptation
- Right ventricular hypertrophy → initially helps maintain pulmonary circulation (can lead to cor pulmonale).
Clinical manifestations
General symptoms
- Chronic dyspnea on exertion → progresses to rest dyspnea.
- Fatigue, reduced exercise tolerance.
Hypoxemia signs
- Central cyanosis.
- Digital clubbing.
- Polycythemia (ruddy complexion).
Hypercapnia signs
- Morning headache (CO₂ retention overnight).
- Daytime sleepiness.
- Cognitive impairment ("CO₂ narcosis").
- Asterixis (flapping tremor in severe cases).
Right heart failure signs ( cor pulmonale)
- Peripheral edema.
- Hepatomegaly.
- Jugular venous distension.
Summary table