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.


CONCLUSIONS
We conclude that forced oscillation estimates of TGV are reliable in healthy subjects over a large part of the VC.
Peslin
R
R
Unité 14 de Physiopathologie Respiratoire, Institut National de la Santé et de la Recherche Médicale, Université H. Poincaré, Nancy I, Vandoeuvre-les-Nancy, France. rpeslin@u14.nancy.inserm.fr
Duvivier
C
C
D000328:Adult; D000368:Aged; D000403:Airway Resistance; D005260:Female; D006801:Humans; D008176:Lung Volume Measurements; D008297:Male; D008875:Middle Aged; D010993:Plethysmography, Whole Body; D015203:Reproducibility of Results; D015656:Respiratory Mechanics; D014797:Vital Capacity