The effects of pneumothorax or pleural effusion on respiratory function as measured by the commonly applied tests were investigated by studying 13 patients (six with pneumothorax, seven with effusion) with and, as far as possible, without air or fluid in the pleural cavity. Measurements included spirometric volumes, carbon monoxide transfer factor (TLCO), and KCO by the single breath method, maximum expiratory flow-volume curves, and subdivisions of lung volume estimated by both inert gas dilution and body plethysmography. In patients with pneumothorax "pleural volume" was estimated as the difference between lung volumes measured by dilution and thoracic gas volume measured by plethysmography. In patients with effusion the change in "pleural volume" was equated with the volume of fluid subsequently aspirated. "Total thoracic capacity" (TTC) was estimated by adding total lung capacity (TLC) measured by dilution and "pleural volume." Both effusion and pneumothorax produced a restrictive ventilatory defect with reductions of vital capacity, functional residual capacity, and TLC. In the patients with effusion TTC fell after aspiration, suggesting that the pleural fluid produced relative expansion of the chest wall as well as compression of the lung. In patients with pneumothorax, however, there was no difference in TTC with and without air in the pleural space. In the presence of pleural air or fluid there was a slight decrease in TLCO and increase in KCO, with a small but significant increase in the rate of lung emptying during forced expiration.
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