Inspiratory flow rate and dead space in dogs

It is generally accepted that a stationary concentration front is established in the tracheobronchial tree during the inspiratory phase of single- and multiple-breath washouts. The anatomic position of this front, which is determined by the balance between diffusive flux toward the airway opening and convective flux toward the periphery, is frequently used to predict the effects of molecular diffusivity and inspiratory flow rate on dead space. Although there is substantial experimental evidence supporting the predictive effect of molecular diffusivity, there is little evidence regarding the effect of convective flow. This study confirmed the predictions for the effects of molecular diffusivity but contradicted those for the effects of inspiratory flow. We measured dead space by multiple- and single-breath inert gas washout techniques and also measured physiological dead space in dogs for inspiratory flow rates of 10-71 ml.kg-1.s-1. None of the three measures of dead space increased over the entire range of flow rate, as predicted by contemporary gas transport models. A possible explanation for these findings is that axial dispersion coefficients in the anatomic region where stationary fronts are believed to develop (respiratory bronchioles and alveolar ducts) significantly increase with convective flow rate rather than remain equal to molecular diffusivity.