Overview
Airway Calibre, Pressure, and Flow
A patient who becomes breathless while exhaling is not necessarily losing the ability to move air in; the limiting problem may be narrowed or collapsible airways that prevent ai...
A patient who becomes breathless while exhaling is not necessarily losing the ability to move air in; the limiting problem may be narrowed or collapsible airways that prevent air from leaving efficiently. Airflow dynamics connects that bedside pattern with spirometry. Air moves from higher pressure in the alveoli toward lower pressure at the mouth. Airway resistance opposes this movement. Because resistance rises steeply as the airway radius decreases, a modest reduction in calibre from bronchoconstriction, mucosal oedema, secretions, or loss of elastic recoil can cause a disproportionate fall in expiratory flow. The result may be wheeze, prolonged expiration, air trapping, and hyperinflation. Use the same model to separate the major patterns. Obstruction limits flow, especially during expiration. Restriction limits the amount of air the respiratory system can contain. A low FEV1/FVC ratio supports obstruction; a low FVC with a preserved ratio requires measurement of total lung capacity before restriction can be confirmed. Symptoms, exposure history, variability, and the complete pulmonary function pattern determine whether the physiology is most consistent with asthma, COPD, parenchymal lung disease, chest wall disease, or neuromuscular...