BODY PLETHYSMOGRAPHIC MEASUREMENT OF THORACIC GAS VOLUME WITHOUT PANTING AGAINST A SHUTTER

When a subject breathes through a pneumotachograph in a body box, the measured value of specific airway resistance (sRaw(1)) is equal to the product of thoracic aas volume (TGV) and the sum of the airway resist ance (Raw) and the instrument resistance (Rins). If an additional resi stance (Radd) is put in the breathing path, the measured specific airw ay resistance (sRaw(2)) exceeds sRaw(1) by the product of TGV and Radd and can be used for determining TGV. With the use of a device increas ing Rins by a known amount (Radd) during normal breathing, sRaw(1) and sRaw(2) were measured in 3 normal subjects, 16 asthmatic patients, 2 patients with chronic obstructive pulmonary disease, and 1 patient wit h restrictive lung disease from the slopes of the x-y plots of airflow vs. box signals obtained before and after adding Radd. TGV was calcul ated by dividing (sRaw(2)-sRaw(1)) by Radd. We also determined subject s' TGV by the panting method of A. B. DuBois, S. Y. Botelho, G. N. Bed ell, and J. H. Comroe, Jr. (J. Clin. Invest. 35: 322-326, 1956) and fu nctional residual capacity by the helium-dilution method. The results of the new method were quite reproducible (coefficient of variation=5. 6) and equivalent to those obtained by the other two methods.