PLETHYSMOGRAPHIC DETERMINATION OF THE VOLUME OF GAS TRAPPED IN THE LUNGS 1

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Abstract
Thoracic gas volume, which includes all the compressible gas in the lungs and thorax whether in communication with the airway or not, was measured plethys-mographically at the end of normal expiration in 36 patients with cardiopulmonary or thoracic disorders. The volume of gas in the lungs in communication with the airway was measured in the same patients by the dilution or "washout" method of Darling and associates. Thoracic gas volume minus "washout" volume is the volume of "trapped" gas. In patients with pneumothorax, pulmonary cysts, emphysema and intrathoracic or pulmonary tumors (all conditions in which "trapping" of gas is known or thought to occur), the thoracic gas volume exceeded the "wash-out" volume by a significant amount (more than 0.44 1); in one patient with pulmonary cysts and emphysema the excess volume (i.e., trapped gas volume) was 3.67 1. In patients with other car-diopuln onary or thoracic disorders, no significant amount was found. The combined use of these two methods for measuring gas volumes provides a method for studying the phenomenon of gas trapping and for extending our knowledge regarding true residual volume and total lung capacity in patients with certain types of pulmonary disease.