Clinical meaning
Chest physiotherapy (CPT) encompasses a group of airway clearance techniques designed to mobilize and remove retained pulmonary secretions, improve ventilation, and prevent or treat atelectasis. Understanding the normal mechanisms of mucus clearance and the pathophysiology of conditions that impair these mechanisms is essential for practical nurses who perform and assist with CPT.
The respiratory tract is lined with pseudostratified ciliated columnar epithelium from the trachea to the terminal bronchioles. This epithelium contains two critical components of the mucociliary escalator, the body's primary defense mechanism for clearing inhaled particles and pathogens from the airways. Goblet cells and submucosal glands produce mucus, a complex viscoelastic gel composed of water (approximately 95%), glycoproteins called mucins (which give mucus its sticky, gel-like consistency), electrolytes, immunoglobulins (IgA), lysozyme, and lactoferrin. Normal mucus production is approximately 100 mL per day. The mucus layer consists of two phases: the sol layer (a thin, watery layer closest to the epithelial surface in which the cilia beat) and the gel layer (a thicker, more viscous layer on top that traps particles). Ciliated cells have approximately 200 cilia per cell, each beating in a coordinated metachronal wave at 12-15 beats per second. During the effective stroke, the cilia tips engage with the gel layer and propel it toward the pharynx at approximately 1-2 cm per minute. During the recovery stroke, the cilia bend and move through the sol layer without contacting the gel layer. This continuous escalator moves trapped particles, bacteria, and debris upward from the lower respiratory tract to the pharynx, where they are swallowed or expectorated.
When the mucociliary escalator is impaired, secretions accumulate in the airways, creating several pathological consequences. Mucus plugging obstructs smaller airways, preventing ventilation of the alveoli distal to the plug. This creates areas of ventilation-perfusion (V/Q) mismatch where blood continues to flow through the pulmonary capillaries surrounding the collapsed alveoli but no gas exchange occurs (intrapulmonary shunting), leading to hypoxemia. Retained secretions provide an ideal medium for bacterial growth, increasing the risk of nosocomial pneumonia. Atelectasis (alveolar collapse) occurs when the obstructed alveoli absorb their trapped gas and collapse, further reducing the surface area available for gas exchange. In patients with chronic obstructive pulmonary disease (COPD) or cystic fibrosis, the mucus is abnormally thick and tenacious due to goblet cell hyperplasia, submucosal gland hypertrophy, and (in cystic fibrosis) defective chloride ion transport across the epithelium caused by mutations in the CFTR protein, which reduces the water content of the airway surface liquid.