RESPIRATORY MUSCLE FUNCTION DURING EMESIS IN AWAKE CANINES

Emesis requires a coordinated differential recruitment of gastrointest inal smooth muscle, upper airway muscles, and several muscles involved in respiration. In seven awake intact canines we measured the electri cal activity (electromyogram) and shortening of costal and crural diap hragm segments, parasternal intercostal, and transversus abdominis dur ing emesis that was induced by instillation of apomorphine into the lo wer conjunctival fornix. The process of emesis was tightly coordinated with ventilation and showed four respiratory phases: baseline ventila tion (Base), initial preemetic hyperventilation (Hyperv), prodromal ve ntilation associated with salivation and probable nausea (Prodrome), a nd finally retching and expulsion (Expel) of gastric contents. Ventila tion was suppressed during expulsive events, but a small inspiratory a irflow was interjected between expulsions. Resting electromyogram of a ll four muscles increased during the process of emesis, with costal an d crural segments showing a marked decrease in resting length through Prodrome and Expel. To produce an expulsive maneuver, both inspiratory and expiratory muscles were activated synchronously, unlike their usu al sequential activation during ventilation, with costal and crural se gments and transversus abdominis showing the most shortening. The crur al segment showed a biphasic length change with initial shortening and then lengthening to assist esophageal sphincter function during Expel . These results indicate a strong coordinated interaction between brai n stem centers responsible for control of respiration and of emesis.