Reliability of thoracic gas volume derived from mechanical impedance at different levels of the vital capacity.

BACKGROUND: Thoracic gas volume (TGV) may be estimated during spontaneous breathing by measuring simultaneously respiratory impedance (Zrs) and alveolar gas compression (Vpl) at several oscillation frequencies [Peslin and Duvivier: J Appl Physiol 1998;84:862-867]. OBJECTIVE: The aim of the study was to test the validity of that approach at different levels of the vital capacity (VC). METHODS: We measured Zrs and Vpl at frequencies ranging from 6 to 29 Hz in 10 healthy subjects rebreathing BTPS gas in a constant volume body plethysmograph. In a first series, the subjects were asked to breathe voluntarily at different levels of the VC and oscillation TGV (TGVos) was compared to standard plethysmographic TGV (TGVst) assessed immediately after TGVos measurements. In a second series, the subjects were asked to change stepwise their lung volume in the middle of the forced oscillation recording, and the changes in TGVos (DeltaTGVos) were compared to the changes in lung volume (DeltaV) computed from the integrated flow signal. RESULTS: In most subjects TGVos and TGVst were highly correlated and the slopes of the relationships did not differ significantly from unity. DeltaTGVos and DeltaV were also highly correlated both in individuals and in the group (r = 0.97), and their signless differences averaged 0.23 +/- 0.20 liter. CONCLUSION: We conclude that forced oscillation estimates of TGV are reliable in healthy subjects over a large part of the VC.