Characteristics and rate of occurrence of spontaneous and provoked augmented breaths

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Abstract
Tidal volume and corresponding efferent phrenic activity of spontaneously occurring and provoked augmented breaths (AB) and the subsequent post-augmented breaths were studied in cats anesthetized with pentobarbitone during hypercapnia and hypoxia. The augmentation phase (phase II) begins at, or close to, the crest of what appears as a normal inspiration (phase I). The amplitude and duration of phase II remained fairly constant; amplitude and the duration of phase I changed with the chemical drive, as in control breaths. The smaller amplitude and shorter duration of post-augmented breaths, compared to control breaths, seems to be due to both a lower-than-normal inspiratory off-switch threshold following the AB and an increased rate of rise of inspiratory activity. With increasing hypercapnia and hypoxia the time interval between AB and the refractory period following an AB during which a new AB cannot be provoked were reduced. Following bilateral vagotomy AB was temporarily abolished but reappeared after 1-2 h. The relatively low rates of occurrence after vagotomy still showed the same type of dependence on chemical stimuli. The refractory period was not abolished, although it was usually decreased, by gallamine paralysis or high thoracic spinalization.