Pathophysiology and phases of the cough reflex
A cough effort occurs when specific receptors are irritated. Their highest density is observed in the region of the tracheal bifurcation and bronchial branching sites, as well as in the pleural leaves.
The process itself is divided into three consecutive stages:
- Irritation: activation of sensory endings leads to a deep inspiration.
- Compression phase (closed): the vocal cords close tightly. At this moment, the respiratory muscles and abdominal muscles contract sharply. Intra-thoracic pressure increases multifold. An interesting anatomical detail is that the posterior membranous part of the trachea bows inward into the lumen.
- Expulsion phase: the vocal cords open instantaneously. A sharp pressure drop generates a powerful air flow, allowing efficient expulsion of mucus and foreign bodies.
Main causes (etiology)
All factors triggering the reflex are divided into four major groups:
- Inflammatory: the most frequent cause. Caused by edema, hyperemia, and secretion accumulation. The process can affect both the airways and the alveolar tissue (in pneumonia or abscess).
- Mechanical: exogenous (dust inhalation, airway obstruction) and endogenous. Endogenous causes include external compression of the bronchi (tumors, aortic aneurysm, enlarged lymph nodes, or thyroid gland). This category also includes bronchospasm and cardiac pathologies — for example, an enlarged left atrium in heart valve disease can irritate the recurrent laryngeal nerve.
- Chemical: inhalation of caustic substances, cigarette smoke, or aspiration of gastric juice in reflux esophagitis.
- Thermal: reaction to extremely cold or hot air.
Non-productive (dry) cough
The absence of sputum is characteristic of early stages of inflammation or specific pathologies. Two main types are distinguished:
- Paroxysmal: exhausting, providing no relief, often accompanied by a feeling of chest tightness. It occurs in the early stages of viral pneumonia, pleurisy, and acute bronchitis. It is also characteristic of pulmonary embolism (PE), pulmonary infarction, the onset of an asthma attack, and foreign body aspiration.
- Non-paroxysmal (prolonged): monotonous and agonizing. Indicates a space-occupying process (endobronchial tumor), external airway compression, pulmonary fibrosis, or the development of congestive heart failure.
Clinical nuance: the appearance of a wheezing sound on inspiration between cough efforts (stridor) indicates acute airway narrowing. With the development of pulmonary edema, the cough is initially dry (interstitial stage), and then becomes moist with the release of pink foamy sputum (alveolar stage).
Productive cough and sputum expectoration
The transition from a dry to a wet cough indicates mucus hypersecretion or the formation of an inflammatory infiltrate.
- Chronic bronchitis: prolonged attacks occur predominantly in the morning upon waking and end with sputum expectoration.
- Cavitary lesions (abscesses, large bronchiectasis): sputum is often discharged all at once, "by the mouthful". The process is usually provoked by assuming a drainage position.
Patients with bronchiectasis often have to sleep on the affected side to prevent pus from flowing into healthy areas and triggering the reflex. Postural drainage is used for treatment — the discharge of secretions under the influence of gravity combined with forced expiration.
Objective signs and complications
Prolonged attacks exert a pronounced systemic effect:
- Hemodynamic shifts: due to high intra-thoracic pressure, venous return is hindered, which manifests as jugular vein engorgement and facial cyanosis.
- Pain: its appearance indicates involvement of the pleura. Pain typically intensifies on the deep inspiration that terminates the attack.
- Tissue barotrauma: high airway pressure can lead to air breakthrough into the mediastinum (pneumomediastinum) and subcutaneous emphysema. Rupture of the visceral pleura causes pneumothorax, which worsens compressive atelectasis of the lung with each successive breath.
Briefly: answers to topical questions
What types of sputum are distinguished by the nature of the discharge?
Based on the nature of the discharge, four main types of pathological airway secretions are distinguished.
- Mucous sputum — detailed characteristics are not provided in the materials.
- Serous sputum — is released as a foamy liquid, sometimes diffusely stained pink; occurs during congestion in the pulmonary circulation or increased vascular wall permeability.
- Mucopurulent sputum — has a viscous consistency, yellowish or greenish color, and may have a mild unpleasant odor.
- Purulent sputum — of liquid or semi-liquid consistency, greenish or yellowish color, often with an unpleasant putrid odor. When left to settle, it can separate into layers (two-layer in lung abscess or three-layer in lung gangrene).
Which nerves form the afferent link of the cough reflex?
The afferent link of the physiological arch of the cough reflex includes:
- Vagus nerve (n. vagus) — impulses travel via its sensory fibers from the bronchi; cough is indicated as the main pulmonary reflex of the vagus nerve.
- Glossopharyngeal nerve (n. glossopharyngeus) — impulses travel via its fibers from the larynx.
Impulses enter the medulla oblongata, where the cough reflex center is stimulated.
Class and exam questions
Why can a productive cough revert to a dry cough in acute laryngitis?
This is the phenomenon of "regression." It is associated not with the accumulation of secretions, but with hypersensitivity (a lowered excitation threshold) of cough receptors. In this phase, antitussives suppressing the reflex are indicated rather than expectorants.
What does the onset of acute chest pain during a cough indicate?
Pain syndrome indicates the involvement of the pleural leaves in the pathological process. Pain usually sharply intensifies on the deep inspiration that terminates the cough effort.
What forced posture is characteristic of bronchiectasis and why?
Patients prefer to lie on the affected side. This prevents purulent sputum from flowing into healthy bronchi and irritating receptors, thereby suppressing nocturnal cough attacks.
What else to study on this topic
- Mechanism of compression atelectasis development in pneumothorax
- Pathophysiology of cough syncope (bettalepsy) and hemodynamic shifts
- Principles and techniques of performing postural drainage
- Evolution of pulmonary edema: from the interstitial to the alveolar stage
- Differential diagnosis of sputum character in productive cough
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