A model for the conversion of respiratory neural output to mechanical output, and vice versa, is described. Neural output was expressed in terms of isometric pressure generated at passive functional residual capacity. The mechanical response time constant of respiratory muscle was assumed to be 0.06 s. The effect of volume and configuration on pressure output was modeled after the data of Grassino et al. (J. Appl. Physiol.: Respirat. Environ. Exercise Physiol. 44: 829-839, 1978). Equations were developed to examine the effect of different configurational pathways during inspiration. We utilized Hill's hyperbolic force-velocity relation to model the effect of flow on pressure output. The pressure asymptote of the hyperbola was considered to be similar to that in other skeletal muscles (0.25 isometric pressure). The flow asymptote was derived from data obtained during maximal voluntary inspiration. A major feature of the model is the dependence of volume, configuration, and flow-related pressure losses on level of inspiratory activity. The practical effect of potential errors and the overall accuracy of the model are presented in the two succeeding communications.
In the preceding two communications we described a model for the relation between respiratory neural and mechanical outputs. In the present report we test the accuracy of the model in predicting volume and flow from occlusion pressure wave forms, and vice versa. We performed single-breath airway occlusions in 21 unconscious subjects and determined the time course of occlusion pressure. We also measured the passive properties of the respiratory system. The time course of volume and flow was predicted from the occlusion pressure wave forms, and the results were compared with the spontaneous breaths immediately preceding occlusion. Inspiratory duration, shape and amplitude of occlusion-pressure wave forms, and the passive properties of the respiratory system varied widely among subjects. There was good agreement between predicted and observed values in all cases. Except for some prolongation of inspiration (Hering-Breuer reflex), diaphragmatic activity did not change during occlusion. Since occlusion pressure is proportional to inspiratory activity, we conclude that the model described provides a good approximation of the relation between inspiratory activity and spirometric output.
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