Flow dependence of gas distribution and the pattern of inspiratory muscle contraction

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Abstract
Regional distribution of Xe-133 boli at 0.25, 0.75 and 1.5 l/s were measured in 4 normal seated subjects during inspirations performed predominantly with intercostal and accessory muscles (IC) or with the diaphragm, accompanied by outward abdominal motion (Ab). In 6 additional subjects the topographical distribution of He boli was inferred during similar breaths and flow rates from the slope of the alveolar plateau recorded during a slow expiration (< 0.5 l/s). Distribution of the He boli was studied during natural as well as IC and Ab inspirations. At each of the flow rates IC breaths distributed relatively more of the inspired bolus to upper lung regions than did Ab inspirations. Natural breaths at 0.25 l/s resulted in distributions similar to those of Ab inspirations, whereas at 1.5 l/s the distribution approached that of IC inspirations. A 3-compartment model, representing upper, middle and lower lung regions, was used to simulate bolus distribution. Substantial departure from predictions based on regional time constants alone was shown. Additional small differences in applied pressure (< 0.50 cmH2O) between the regions satisfactorily accounted for the gas distribution.