Clinical meaning
The clinician applies respiratory airflow dynamics to assess and manage obstructive and restrictive lung disease. Airflow is governed by the Poiseuille equation: flow is proportional to the pressure gradient and the fourth power of the radius, meaning even small decreases in airway caliber dramatically increase resistance. Obstructive physiology (asthma, COPD, bronchiectasis): increased airway resistance from bronchospasm, mucosal edema, and mucus hypersecretion causes air trapping and hyperinflation. Spirometry shows reduced FEV1/FVC ratio (less than 0.70 defines obstruction per GOLD criteria), increased residual volume, and increased total lung capacity. Flow-volume loop demonstrates characteristic scooped-out expiratory limb with reduced peak expiratory flow. Reversibility testing with bronchodilator (greater than 12% AND greater than 200 mL improvement in FEV1 post-bronchodilator) distinguishes asthma from COPD, though overlap exists. Restrictive physiology (interstitial lung disease, chest wall disease, neuromuscular disease): reduced lung compliance or chest wall expansion limits lung volumes. Spirometry shows proportionally reduced FEV1 and FVC with preserved or increased FEV1/FVC ratio, reduced total lung capacity on plethysmography, and reduced DLCO in parenchymal disease (normal DLCO in chest wall/neuromuscular restriction). Upper airway obstruction produces characteristic flow-volume loop...