Active impedance of respiratory system in anesthetized cats.

We have assessed the validity of the method of Siafakas et al. (J. Appl. Physiol.: Respirat. Environ. Exercise Physiol. 51: 109-121, 1981) for determining active elastance (E'rs) and flow resistance (R'rs) of the respiratory system. In six cats anesthetized with pentobarbital sodium we have measured flow, volume, and tracheal occlusion pressure during spontaneous breathing. This allowed us to compute E'rs and R'rs. From these data and the occlusion pressure wave we predicted the time course of volume during inspirations with added linear flow resistances (delta R). These were compared to the actual loaded inspirograms. The agreement was generally good, except for small predictable discrepancies with the highest delta R values, which could be attributed to decompression of thoracic gas. These results indicate that the approach of Siakafas et al. to determine E'rs and R'rs is valid. In addition, we have quantified the "terminal inhibition" of inspiratory activity, which occurs toward the end of unoccluded breaths (both loaded and unloaded).