Cooling the intermediate area of the ventral medullary surface affects tracheal responses to hypoxia

The intermediate area of the ventral medullary surface (VMS) influences changes in airway tone caused by hypercapnia and intrapulmonary irritant receptor activation. These studies evaluated the effects of cooling the intermediate area of the VMS on the reflex hypoxic responses of the trachealis smooth muscle and of the phrenic nerve. Anesthetized, paralyzed cats were hyperventilated with 100% oxygen to produce phrenic neural apnea. Tracheal tone was measured indirectly by evaluating pressure changes in an innervated tracheal segment and the phrenic electroneurogram was determined from the central end of a cut cervical root. Switching the inspired gas to 12% O2 increased tracheal pressure of 11 of 12 cats but caused phrenic activity to reappear in only 6 of the animals. Ventilation with 6% O2 significantly increased tracheal constriction prior to phrenic activity. After intravenous administration of atropine methyl nitrate tracheal responses to hypoxia were abolished but phrenic neural responses were unaltered. Neither the tracheal pressure nor the phasic phrenic electroneurogram responded to hypoxia after cutting the carotid sinus nerves. When the intermediate area of the VMS was cooled to 20 degrees C prior to ventilation with the hypoxic gases, both tracheal and phrenic responses were significantly diminished. While the cats were hyperventilated with 6% O2, cooling of the intermediate area significantly diminished tracheal pressure and phrenic nerve activity and both returned to the same levels after rewarming. Cooling of the intermediate area blunted tracheal and phrenic responses to carotid body stimulation by NaCN. However, the appearance of tracheal constriction prior to the onset of phasic phrenic activity may suggest that increased trachealis tone may occur independent of cyclical respiratory activity.