Assessment of thoracic gas volume by low-frequency ambient pressure changes in children

Abstract

The validity of a new method for measuring thoracic gas volume (TGV) was studied in 69 children, 4-16 yrs old, including twelve normal children and 57 children with an obstructive (n = 38) or restrictive (n = 19) respiratory disease. The method consisted of applying very slow (0.05 Hz) sinusiudal variations of ambient pressure around the body (ΔPam = 40 cmH2O peak to peak) and studying the relationship between ΔPam and the resulting gas displacement at the mouth (Vaw): TGVapc - Pb·ΔVaw/ΔPam·cosΦ, where Pb is barometric minus alveolar water vapour pressure and Φ the phase angle between Pam and Vaw. Functional residual capacities derived from TGVapc (FRCapc) were compared to the values obtained by plethysmography (FRCplet) and by helium dilution (FRCdil). FRCapc did not differ significantly from FRCplet in either the entire group (1.75 ± 0.62 l vs 1.79 ± 0.45 l) or in the patient subgroups. However, with the new method a trend to slightly lower FRCs was seen in patients with the most obstruction (p < 0.05). FRCdil was significantly lower than both FRCapc and FRCplet (p < 0.001), particularly in children with obstruction. Significant correlations were found between the three methods (p < 0.001). On the other hand, the method investigated requires that the subject breathe very regularly for a period of several minutes. This was rarely achieved, so that the reproducibility of the measurements was unacceptably low. At present, the method cannot be recommended for routine use in 4-16 yr old children.