Online calculation of lung volumes by digital computer from a plethysmographic measurement of ventilatory flow

Abstract

The actual gas contents of lungs can be measured by a total plethysmographic technique. A classical way of obtaining thoracic gas volume (t.g.v.) is by fitting a straight line to the thoracic volume (V_t) and mouth pressure (P_m) changes occurring during a panting manoeuvre with airway occlusion at the mouth. In our lung-function laboratory, a minicomputer has been connected to a pressure-compensated flow body plethysmograph. We describe here both the plethysmographic technique of measuring thoracic flow (V′_t) and the computerised system developed to calculate on-line lung volumes in patients. By replacing V_t and P_m by their time derivatives, V′_t and P′_m, in the linear regression procedure, we significantly improve the correlation coefficient, namely the confidence which can be attributed to calculated t.g.v. The within-subject reproducibility of lung volumes is thus improved, and a closer comparison is found between the computer-derived and hand-calculated volumes, both in normal subjects and patients.