Pattern of chest wall motion and measurement of lung volume

The role of abdominal gas in the plethysmographic determination of thoracic gas volumes (TGV) may be evaluated using Boyle's Law from which: true TGV = -ΔV x (P(B) - PH₂0)/ΔPm -ΔPg x Vab/ΔPm, where ΔV is the change in volume during panting; ΔPm and ΔPg are the changes in mouth and abdominal pressures respectively; P(B) is the ambient and PH₂O the body water vapour pressure; and Vab the abdominal gas volume. Hence, true TGV = apparent TGV - ΔPb x Vab/ΔPm. For a given Vab, apparent TGV will overestimate true TGV when Δ Pg/ΔPm is positive and underestimate it when it is negative. In 4 normal subjects, FRC was measured during natural panting (BL(N)) and, during the same occlusion, when these subjects made inspiratory manoeuvers either with predominant abdominal motion (BL(ab)) or using mainly intercostal muscles (BL(ic)). Apparent TGV measured during BL(ic) consistently exceeded that during BL(ab) by 0.45 ± 0.17 l (mean ± 1SE). In 1 trained subject, the difference was 1.3 ± 0.3 l (n = 6). The value during BL(N) was intermediate between the other two. During BL(ic), ΔPg/ΔPm was +0.42 ± 0.06 and during BL(ab) it was -1.0 ± 0.23, whereas during BL(N) it was -0.18 ± 0.10. We conclude that the plethysmographic measurement of TGV is systematically influenced by different patterns of chest wall motion.