Mucociliary clearance and cough are the principal defense mechanisms against the inhalation of particles and infectious agents. The reduction or disappearance of these mechanisms contributes to the stagnation of bronchial secretions, favoring the formation of atelectasis.
Physiologically, tracheal and bronchial secretions comprise 20 to 80 ml of seromucous liquid daily. Under normal conditions, these secretions are spontaneously drained from the small bronchi towards the trachea by the vibratile cilia in the bronchial epithelium. This is known as mucociliary clearance.
Depending on the acute or chronic pathologies in the respiratory tract, the volume and quality of the secretions will vary, ranging from very fluid and abundant to viscous and sticky. Certain pharmacological agents, which modify mucociliary clearance by acting on the viscoelastic properties of the mucus and/or the vibratile activity of the cilia (Table 1), have similar effects.
Table 1 Pharmacological agents acting on mucociliary clearance
Secretions are then swallowed at the level of the pharynx. Coughing occurs at the level of the large bronchi in order to facilitate drainage and drive the secretions toward the larynx.
These defense mechanisms are altered during mechanical ventilation. The presence of an intubation or a tracheostomy tube short-circuits the upper airways and prevents evacuation of secretions. In addition, mechanical ventilation modifies the conditions for administration of inspired gases (temperature, humidity, oxygen concentration, pressure, etc.) which can affect mucociliary clearance, the only mechanism ensuring drainage of the peripheral bronchi, in the following ways.
1. The use of cold and dry gas reduces mucociliary clearance.
2. An increase in inspired fraction of oxygen FiO 2 reduces mucociliary clearance. It has been shown experimentally that it is totally inhibited in 30 min at FiO 2 = 1.
3. The cuff of the intubation tube blocks mucociliary clearance several centimeters above the carina.
4. Elevated insufflation pressures tend to crush and reverse the direction of movement of the vibratile cilia.
5. The dorsal decubitus position causes an accumulation of secretions.
6. Repeated tracheal aspirations produce abrasion of the tracheobronchial mucus, destroying the cilia over an extended surface.
7. Sedation and various anesthetic agents are probably responsible for a reduction in ciliary motility (see below).
Coughing is a complex phenomenon that is usually triggered by local 'irritation', making it possible to evacuate secretions from the central bronchi toward the larynx. It comprises an occlusion of the glottis, a sudden contraction of the abdominal muscles, a rapid elevation in airway pressure, and a sudden opening of the glottis. Under mechanical ventilation, this phenomenon is reduced because the glottis cannot close even if the resistance in the upper airways (intubation tube, ventilatory circuit) is sufficiently elevated to produce an increase in pressure. Coughing is suppressed during general anesthesia and can be significantly reduced by the administration of opioids. In a patient who is not intubated, diaphragmatic problems (medullar impairment, phrenic paralysis, myasthenia, Guillain-Barre syndrome, etc.), pain, or consciousness problems are responsible for 'ineffective' cough and tracheobronchial obstruction.
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