Thoracic dorsal rhizotomy in the anesthetized cat: maintenance of eupnic breathing.

In order to assess the level of participation of thoracic afferents in the determination of the eupnic breathing pattern, thoracic dorsal rhizotomies from T1 through T13 were performed in cats anesthetized with alpha-chloralose or sodium pentobarbital, with or without intact vagi. While recording several respiratory parameters, no consistent changes in the spontaneous breathing pattern could be demonstrated either 10--20 min or 90--100 min following disruption of thoracic afferent input. Thus, respiratory frequency, tidal volume and total ventilation were unaffected by rhizotomy. Inspiratory (TI) and expiratory (TE) durations and TI/TE ratios remained constant. Tracheal air flows during inspiration and expiration and intrapleural pressure were not altered. Finally, end-expiratory %CO2 and %O2 did not change after section of the thoracic dorsal roots. Only arterial blood pressure was found to fall as a function of time into the experiment. Since the results were identical in vagotomized and in vagi intact preparations, thoracic reflexes were not being masked by vagal dominance in eupnea. Rather, thoracic afferents, including intercostal muscle spindle afferents, appear to be ineffective in shaping the pattern of quiet breathing in anesthetized cats. This is unlike situations of loaded or stressed breathing where these afferents assume a more active role in the modulation of rate and depth mechanisms.