Mechanisms and causes of development
Normal gas exchange requires the coordinated work of two processes: ventilation (mechanical inflow of air into the alveoli) and oxygenation (release of carbon dioxide from venous blood and its saturation with oxygen via diffusion). Impairment of either stage triggers respiratory failure.
The process has a clear dynamics. First, the compensation stage occurs, when the external respiration system and the heart work in an enhanced mode, trying to balance the gas composition. Upon depletion of reserves, the decompensation stage develops with prominent clinical manifestations.
Based on the mechanism of origin, several groups of causes are distinguished:
- Exogenous factors: a drop in the partial pressure of oxygen in the inhaled air (e.g., in high-altitude hypoxia).
- Pulmonary causes: damage to the parenchyma and airways leading to alveolar hypoventilation.
- Extrapulmonary causes: chest deformities (kyphoscoliosis), trauma, depression of the central nervous system respiratory center, neuromuscular disorders, as well as endocrine disorders (obesity, hypothyroidism). This same group includes functional overloads, respiratory muscle fatigue, and pathological hyperventilation.
- Hypoxemia without hypoventilation: occurs with shunting (discharge of venous blood into the arterial bed bypassing the alveoli) or with heart failure and anemia.
Obstructive and restrictive types
Pulmonary causes of alveolar hypoventilation are traditionally divided into two pathogenetic patterns.
Obstructive respiratory failure (from Lat. obstructio — locking) is associated with impaired patency of the bronchial tree. This is the basis of chronic obstructive pulmonary disease and bronchial asthma. Clinically, the condition is given away by wheezing against the background of a clearly prolonged expiration. Pneumotachometry is used for diagnostics, and the Tiffeneau index is determined. Important: the use of bronchodilators before the study improves parameters, which confirms the reversibility of bronchospasm.
Restrictive respiratory failure (from Lat. restrictio — restriction) occurs when the airways are patent, but the alveoli cannot fully expand. The causes lie in diffuse tissue damage (pneumosclerosis, massive atelectasis, fibrosing alveolitis), pleural problems (pleurisy, pneumothorax, adhesions), or pronounced obesity (Pickwickian syndrome). In restriction, flow parameters (Tiffeneau index) are unchanged, but a decrease in volumetric parameters—vital capacity (VC)—is recorded.
Blood gas abnormalities
The mismatch between pulmonary ventilation and the metabolic demands of tissues is reflected in laboratory blood parameters.
- Hypoxemia — a drop in oxygen tension ($PaO_2$) below 75 mmHg. This can be a consequence of hypoventilation, impaired diffusion, or low oxygen content in the air. It manifests as fatigue, lethargy, altered consciousness, tachycardia, and arrhythmias.
- Hypercapnia — accumulation of carbon dioxide ($PaCO_2$ above 50 mmHg). It occurs in severe hypoventilation and pulmonary tissue pathologies (edema, pneumonia). It is characterized by drowsiness, headaches, arterial hypertension, and in severe cases leads to coma.
There is a dependence on the type of lesion. In pure restriction, hypoxemia without hypercapnia is usually observed. In obstruction, a combined disorder (both hypoxemia and hypercapnia) occurs. If blood shunts past ventilated alveoli, the patient develops cyanosis. Severe blood gas disturbances threaten irreversible organic lesions of the heart and brain.
Classification by severity
Assessment of the degree of respiratory failure is mandatory when formulating a diagnosis for any chronic pulmonary pathology. The physician focuses on the severity of cyanosis, tachycardia, and dyspnea. The main functional criterion is a decrease in exercise tolerance:
- Grade I: dyspnea appears exclusively upon physical exertion.
- Grade II: dyspnea occurs with minor physical exertion.
- Grade III: the patient experiences dyspnea even at complete rest.
Briefly: answers to topical questions
By what objective signs (examination, palpation, auscultation), apart from dyspnea, is the severity of respiratory failure assessed?
In addition to dyspnea, the severity of respiratory failure is assessed by cyanosis and tachycardia.
Assessment criteria include:
- Cyanosis.
- Tachycardia.
- Decreased exercise tolerance — a key functional criterion.
According to the severity of dyspnea, grades of RF are distinguished:
- Grade I — dyspnea only upon physical exertion.
- Grade II — dyspnea with minor physical exertion.
- Grade III — dyspnea at rest.
For quick memorization of types: Obstruction — Air outflow is difficult (narrowing, spasm, wheezing). Restriction — Expansion is impossible (fibrosis, pleural shell).
Class and exam questions
Why is a bronchodilator test performed in the obstructive type?
This test allows assessing the reversibility of bronchial narrowing. If functional parameters (e.g., Tiffeneau index) improve after drug inhalation, it means the obstruction is based on reversible spasm.
How do spirometry parameters change in restrictive pathology?
Airflow rate parameters remain normal because bronchial patency is not impaired. However, vital capacity (VC) drops sharply due to the inability to stretch the lung tissue.
What blood gas disorder is characteristic of pronounced obstruction?
A mixed pattern is formed: due to severe alveolar hypoventilation, primary hypoxemia is complemented by hypercapnia (accumulation of carbon dioxide).
What else to study on this topic
- Pathophysiology of hypercapnia development in pulmonary tissue damage.
- The influence of hemic and circulatory factors on tissue hypoxia.
- Acute processes and adult respiratory distress syndrome (ARDS).
- Functional factors: work of the respiratory muscles and an increase in aerodynamic resistance.
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