Changes in rib cage shape during quiet breathing, hyperventilation and single inspirations

We measured the relative changes in upper and lower rib cage volume (delta RCU and delta RCL, respectively) using an induction coil plethysmograph (Respitrace) in seven normal seated subjects during relaxed passive expiration from total lung capacity, during quiet breathing, rapid breathing at 60 breaths/min (RB), during exercise-induced hyperventilation (EH) and during single fast inspirations. The plot of the RCU vs RCL was slightly curvilinear during relaxation from total lung capacity in all subjects. However, the inferred changes in rib cage shape were similar during RB and EH to those observed during quiet breathing and the relaxation manoeuvres. In contrast, single rapid inspirations were associated with marked and variable changes in rib cage shape, being most prominent in the first part of the breath. Our results suggest that during cyclic breathing respiratory muscle activity is so co-ordinated that the pattern of rib cage shape change is similar to that observed during relaxation. In contrast, single rapid inspirations are associated with markedly different and variable shape changes of the rib cage, presumably due to different patterns of inspiratory muscle recruitment. The results are consistent with the observation that during tidal breathing regional ventilation distribution is flow independent.