Thoracic gas volume measurements in chronic obstructive pulmonary disease by low frequency ambient pressure changes

The validity of a new method for measuring thoracic gas volume (Vtg) was studied in 18 bronchitic patients with mild to severe airway obstruction. The method entailed submitting the subject to very slow sinusoidal variations of ambient pressure (delta Pam) and studying the relationship between delta Pam and the resulting gas displacement at the mouth (delta Vaw): Vtgapc = PB.delta Vaw/delta Pam.cos phi, where PB is barometric minus alveolar water vapor pressure, and phi is the phase angle between Pam and Vaw. delta Pam of 40 cm H2O at 0.05 Hz were achieved by placing the subject in a 410-I body chamber connected to a large stroke volume reciprocating pump. Pam and Vaw were processed digitally by Fourier transform to obtain their amplitude ratio and phase angle at the frequency of interest. Vtg was also measured by body plethysmography (Vtgplet) during slow panting maneuvers (0.79 +/- 0.19 Hz) and also in order to detect any artifactual frequency dependence at a higher panting frequency (1.71 +/- 0.27 Hz); the agreement between the 2 estimates in all subjects (r = 0.975) suggested that Vtgplet could be taken as a valid reference. Functional residual capacities derived from Vtgapc and Vtgplet were not significantly different (5.113 +/- 1.198 versus 5.260 +/- 1.328 L) and were highly correlated (r = 0.915). Intermethod differences averaged 1.7 +/- 10.5% and were not significantly correlated to functional indices. We conclude that the new method provides accurate Vtg values in patients with chronic airway obstruction.