Etiology: Pulmonary and Extrapulmonary Causes
The pathogenesis and etiology of respiratory failure are traditionally classified into two main groups: pulmonary and extrapulmonary.
Pulmonary (intrapulmonary) causes include all variants of impaired gas-exchange function of the lungs themselves (both partial and mixed). Key mechanisms in this group include:
- Impaired normal alveolar ventilation with air.
- Perfusion disorders, meaning inadequate blood supply to the lung tissue.
- Ventilation-perfusion ($V/Q$) mismatch (discrepancy between incoming air volume and blood flow).
- Impaired gas diffusion directly across the alveolar-capillary membrane.
Extrapulmonary (extrapulmonary) causes stem from pathologies of organ systems located outside the lungs that are critical for the act of breathing. These are divided into several groups:
- Neurogenic regulation failure. Occurs when central nervous system pathways or the respiratory center are damaged (e.g., due to birth trauma or brain tumors).
- Impaired efferent regulatory signaling. Associated with pathology of nerve impulse transmission at the neuromuscular junctions of the diaphragm and intercostal muscles. Typical examples include poliomyelitis, polyneuritis, and myasthenia gravis.
- Decreased function of respiratory muscles. Occurs with direct damage to muscle tissue (myalgia, intercostal muscle myodystrophy).
- Restriction of chest wall excursions. The thoracic cage loses mobility due to spinal and rib trauma, as well as ankylosis of the costal joints.
- Systemic circulatory failure. Hemodynamic disorders in heart failure or anemias directly affect the blood transport of gases.
Classification by Types
Depending on changes in the arterial blood gas composition, there are three main forms of respiratory failure.
- Hypoxemic (parenchymal, Type I).
Its main characteristic is isolated hypoxemia (decreased partial pressure of oxygen). The most frequent factor is impaired gas diffusion across the alveolar-capillary membrane. Perfusion disorders, exogenous hypoxia (normo- and hypobaric), and ventilation-perfusion mismatch also contribute. This form develops in severe lung parenchyma damage: shock, pulmonary edema, generalized infection, liquid aspiration, toxic gas inhalation, as well as bronchial and bronchiolar involvement.
- Hypercapnic (hypoventilatory, Type II).
Characterized by concurrent hypoxemia and hypercapnia (elevated carbon dioxide levels). The primary causes are alveolar hypoventilation and secondary ventilation-perfusion mismatch. It is observed in obstructive airway diseases: bronchial asthma, bronchitis, bronchopneumonia, and bronchial tumors.
- Mixed Form.
Manifests as primary hypercapnia combined with hypoxemia. The main etiology includes acute and chronic lung diseases leading to obstructive hypoventilation. This encompasses obstructive pulmonary emphysema, abscesses, pneumonia, bronchiectasis, as well as severe courses of asthma and bronchitis.
Risk Factors and Pediatric Features
In pediatric practice, respiratory failure occurs significantly more often than in adults. This is explained by anatomical predispositions:
- High positioning of the diaphragmatic dome.
- Narrow bronchial lumen.
- Pronounced mucosal tendency toward edema and excessive secretion.
Among the general risk factors provoking this syndrome at any age, the most common are:
- Congenital airway malformations (atresia or stenosis of the nasal passages).
- Airway obstruction (blockage of the lumen by foreign bodies, regurgitated gastric contents, or vomitus).
- Glossoptosis (tongue drop).
- Myasthenia gravis (marked respiratory muscle weakness).
- Trauma and structural anomalies of the chest wall.
- Pulmonary vasculitis.
- Various neural pathway lesions.