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). The key mechanisms for the development of this group include:
- Impaired normal ventilation of the alveoli with air.
- Perfusion disorders, meaning adequate blood supply to the lung tissue.
- Imbalance of ventilation-perfusion ratios (a mismatch occurs between the volume of incoming air and blood flow).
- Impaired gas diffusion directly across the alveolar-capillary membrane.
Extrapulmonary (extrapulmonary) causes are caused by pathologies of systems located outside the lungs but critically important for ensuring the act of respiration. Several groups of such disorders are distinguished:
- Disorders of the neurogenic regulation of external respiration. These occur when the conduction pathways of the central nervous system or the respiratory center itself are damaged (for example, due to birth injuries or brain tumors).
- Impairment of efferent regulatory influences. Associated with pathology of nerve impulse transmission in the neuromuscular synapses of the diaphragm and intercostal muscles. Typical examples: poliomyelitis, polyneuritis, myasthenia gravis.
- Decreased function of the respiratory muscles. Occurs with direct damage to the muscle tissue (myalgia, myodystrophy of the intercostal muscles).
- Restriction of respiratory excursions of the chest. The chest cage loses mobility due to injuries to the spine and ribs, as well as ankylosis of the rib joints.
- Systemic circulatory failure. Hemodynamic disorders in heart failure or anemia directly affect the transport of gases by the blood.
Classification by forms
Depending on changes in the gas composition of arterial blood, three main forms of respiratory failure are distinguished.
- Hypoxemic (parenchymal, type I).
Its main characteristic is the presence of hypoxemia alone (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 shifts in ventilation-perfusion ratios also lead to it. This form develops in severe lesions of the pulmonary parenchyma: shock, pulmonary edema, generalized infection, fluid aspiration, inhalation of toxic gases, as well as bronchial and bronchiolitis lesions.
- Hypercapnic (hypoventilatory, type II).
It is characterized by the simultaneous presence of hypoxemia and hypercapnia (elevated carbon dioxide levels). The main causes are alveolar hypoventilation and secondary impairment of ventilation-perfusion ratios. It is observed in obstructive airway diseases: bronchial asthma, bronchitis, bronchopneumonia, and bronchial tumors.
- Mixed form.
It naturally manifests as primary hypercapnia combined with hypoxemia. The main etiology includes acute and chronic lung diseases that lead to obstructive hypoventilation. These include 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 prerequisites:
- High position of the dome of the diaphragm.
- Narrow bronchial lumen.
- Pronounced tendency of the mucous membrane to edema and excessive secretion.
Among the general risk factors provoking the development of the syndrome at any age, the most commonly found are:
- Airway developmental anomalies (atresia, narrowing, or atresia of the nasal passages).
- Airway obstruction (blocking of the lumen by foreign bodies, gastric contents during regurgitation, or vomit masses).
- Glossoptosis (falling back of the tongue).
- Myasthenia gravis (pronounced weakness of the respiratory muscles).
- Chest trauma and structural anomalies.
- Pulmonary vasculitis.
- Various nerve pathway lesions.
Briefly: answers to topical questions
How is respiratory failure classified according to the rate of development of the pathological process?
According to the rate of development of the pathological process, respiratory failure is classified into two main forms:
- Acute respiratory failure — develops over several days, hours, or minutes; poses an immediate threat to life.
- Chronic respiratory failure — develops over months or years; may begin imperceptibly or form upon incomplete recovery from the acute form.
What compensatory-adaptive reactions of the respiratory and cardiovascular systems are triggered in acute respiratory failure?
In acute respiratory failure, compensatory mechanisms from the respiratory and circulatory systems do not have time to engage. This is due to the too rapid development of the pathological process, as a result of which the condition poses an immediate threat to life and requires intensive care.
To quickly recall the types of RF, focus on the gases: Type I — only one gas is affected (oxygen drops, hypoxemia occurs). Type II — two parameters are affected (oxygen drops and carbon dioxide rises, hypercapnia joins).
Class and exam questions
Is hypercapnia an obligatory symptom of respiratory failure?
No, hypercapnia occurs frequently, but not always. Only hypoxemia is an obligatory and constant sign of any form.
Why does the hypoxemic form develop specifically in shock or pulmonary edema?
These conditions cause severe damage to the pulmonary parenchyma. As a result, gas diffusion across the alveolar-capillary membrane is impaired, leading to a drop in oxygen levels.
What diseases cause the hypercapnic form?
First and foremost, these are pathologies leading to obstructive alveolar hypoventilation. These include bronchial asthma, various types of bronchitis, bronchopneumonia, and bronchial tumors.
What else to study on this topic
- Mechanisms of ventilation-perfusion ratio disorders.
- Pathogenesis of exogenous (hypo- and normobaric) hypoxia.
- The effect of heart failure and anemia on gas transport.
- The role of neuromuscular synapses in the development of extrapulmonary failure.
- Differences between partial and mixed gas exchange function disorders.
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