ABSTRACT
Although nonnal tidal volumes are under 1000 mL, an average cough involves the expulsion of 2.3 to 2.5 L of air. To achieve these volumes the vital capacity (VC) should, therefore, be at least 2.5 L (l). Nonnal peak cough flows (PCFs) are 6 to 16 LIsee and minimum PCFs of 160 LIm are required to cough out airway secretions (2). To achieve nonnal cough flows, the abdominal, internal intercostal, and accessory expiratory muscles nonnally generate over 200 cm H20 of thoracoabdominal pressure to create the forces required to expel air at these flows upon glottic opening. Although patients with chronic obstructive pulmonary disease (COPD) can usually attain adequate inspiratory volumes for coughing, the inspissation of airway secretions and the collapse of airways with the generation of high thoracoabdominal pressures often prevent them from adequately expelling airway mucus. Very often, airway collapse in these patients prevents PCFs from attaining 160 LIm, the minimum flow necessary to eliminate airway secretions (2). For the majority of patients with the conditions listed in Table I, however, the lungs and airways are nonnal, but inspiratory and expiratory muscle weakness results in failure to autonomously attain adequate PCFs to effectively eliminate airway secretions.
