Changes in respiratory frequency and end-expiratory volume accompanying augmented breaths in cats

We have studied the pattern of breathing before, during and after augmented breaths in spontaneously breathing, anesthetized cats with the larynx both in and out of the breathing circuit. Following augmented breaths we consistently observed increases in end-expiratory lung volume (EEV), end-expiratory transpulmonary pressure, dynamic lung compliance and respiratory frequency. These changes were of similar magnitude whether the larynx was in or out of circiut and were uninfluenced by section of the superior laryngeal nerves. Laryngeal resistance, measured under constant flow conditions with the larynx removed from the breathing circuit, showed an exaggerated inspiratory decrease during augmented breaths. Passive lung inflations, performed so as to mimic the pattern of augmented breaths, increased dynamic lung compliance but did not elicit changes in EEV or respiratory frequency. The results indicate that the increase in EEV cannot be attributed to increased lung compliance but results from a change in end-expiratory respiratory muscle tone. This change, and the change in respiratory frequency appear to be part of a reflexly evoked central response that includes the augmented breath itself. The larynx participates in the augmented breath, but its mechanical importance is small.