Voluntary changes in ventilation distribution in the lateral posture

To determine whether voluntary changes in the pattern of inspiratory muscle contraction influence topographical distribution of ventilation in the lateral decubitus posture during tidal breathing, we studied 4 normal subjects who breathed either naturally (N) or preferentially with intercostal and accessory muscles (IC), or with enhanced motion of the diaphragm and abdomen (Ab). We performed N2 as well as 133Xe washouts (after equilibration) which were measured at the mouth while recording regional count rates by external scintillation detectors. Ventilation per unit volume (delta V/Vo) in the nondependent lung regions was 0.55 +/- 0.05 (mean +/- 1 SD) and 0.42 +/- 0.02 of that in the dependent regions during natural and sustained Ab breathing, respectively. In contrast, during IC breathing this ratio was 0.99 +/- 0.17. Although N2 washout curves obtained during IC breathing more closely approached a monoexponential than did those from N and Ab runs, a two compartment analysis of washouts at the mouth did not demonstrate significant differences between breathing patterns. We conclude that in the lateral posture voluntary relaxation of the diaphragm during tidal breathing distributes the gas preferentially to the nondependent lung regions. Conversely, during N and Ab breathing the preferential ventilation of dependent regions is due to contraction of the diaphragm.