Effects of upper airway stimulation on swallowing, gasping, and autoresuscitation in hypoxic mice.

Severe hypoxia produces a state of neural depression known as hypoxic coma in which reflex activity is believed to be absent but from which spontaneous recovery ("autoresuscitation") still can occur. We evaluated the swallowing reflex during hypoxic coma by employing mechanical and chemosensory stimuli. BALB/c mice were given 97% N2-3% CO2 to breathe. At onset of coma, a 0.05-ml bolus of saline or water was infused into the pharynx. Unlike mechanical stimulation (sham infusion), fluid infusion usually was followed by rapid swallowing, more so with water than with saline. This model allowed examination of interactions among swallowing, hypoxic gasping, airway fluid removal, and autoresuscitation. Compared with sham infusion, saline and water reduced gasping rate equally. Saline, however, prolonged the process of autoresuscitation more than did water, an effect possibly related to an observed increased retention of saline in the airway. Occasionally, mice failed to swallow after infusions, in which case airway obstruction during gasping and autoresuscitation failure was repeatedly observed. These studies suggest that the swallowing component of the laryngeal chemoreflex is present during hypoxic coma and that swallowing facilitates autoresuscitation when upper airway fluid is present.