RESPIRATORY MUSCLE FUNCTION DURING EMESIS IN AWAKE CANINES

Emesis requires a coordinated differential recruitment of gastrointestinal smooth muscle, upper airway muscles, and several muscles involved in respiration. In seven awake intact canines we measured the electrical activity (electromyogram) and shortening of costal and crural diaphragm segments, parasternal intercostal, and transversus abdominis during emesis that was induced by instillation of apomorphine into the lower conjunctival fornix. The process of emesis was tightly coordinated with ventilation and showed four respiratory phases: baseline ventilation (Base), initial preemetic hyperventilation (Hyperv), prodromal ventilation associated with salivation and probable nausea (Prodrome), and finally retching and expulsion (Expel) of gastric contents. Ventilation was suppressed during expulsive events, but a small inspiratory airflow was interjected between expulsions. Resting electromyogram of all four muscles increased during the process of emesis, with costal and 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 usual sequential activation during ventilation, with costal and crural segments and transversus abdominis showing the most shortening. The crural 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 brain stem centers responsible for control of respiration and of emesis.