Thoracic gas volume at functional residual capacity measured with an integrated-flow plethysmograph in infants and young children

Thoracic gas volume (TGV) was measured with an integrated flow plethysmograph in 15 infants aged 2-34 months. End-expiratory (TGVe) and end-inspiratory (TGVi) airway occlusions were compared, after correction of TGV for the occluded volume above functional residual capacity (FRC). The relationship between pressure at the airway opening (Pao) and volume displaced from the box during airway occlusion (Vg) was studied numerically by: 1) an algorithm including a correction for the drift of Vg and linear regression analysis (LR); and 2) Fourier analysis of the signals (FFT). TGVe was significantly higher than TGVi (256 vs 237 ml, 20.4 (square root of residual variance; p less than 0.002). The correlation coefficient of the Pao-Vg relationship was slightly but significantly higher for TGVi than for TGVe: 0.9968 (0.9937-0.9995) vs 0.9947 (0.9840-0.9990) (means and range). No difference was observed between LR and FFT, although the intra-individual coefficient of variation was lower for LR than FFT: 5.2% (1.6-11.3) vs 7.9% (1.9-21.0) (means and range). Model simulations suggested that the difference between TGVe and TGVi could be mainly attributed to gas compression in the instrumental deadspace and upper airway wall motion and/or to uneven distribution of alveolar and pleural pressure associated with chest wall distortion.