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 patterns: fixed obstruction (flattened inspiratory and expiratory loops), variable extrathoracic (flattened inspiratory limb), variable intrathoracic (flattened expiratory limb). The clinician interprets spirometry in clinical context, classifies disease severity, and adjusts treatment accordingly.