Cardiorespiratory Responses to Inhaled Laryngeal Irritants

Abstract

Numerous protective and defensive reflexes are elicited from the upper airway, but the extent to which receptors with myelinated and nonmyelinated fibers contribute to these reflex responses is not known. In this study we compared the effects of capsaicin (CAPS), a specific C-fiber receptor stimulant, and ammonia (NH₃), a nonspecific stimulant, on breathing pattern, heart rate, and blood pressure in anesthetized, spontaneously breathing rats and dogs. Nebulized CAPS and NH₃ vapor were added to a constant stream of air passing through the functionally isolated upper airway In the rat both CAPS and NH₃ prolonged the expiratory duration T_E (p <.05) and increased blood pressure (p <.05) and laryngeal resistance (p < .05). The respiratory and cardiovascular responses to CAPS occurred with a longer (5–7×) delay compared to NH₃ and were markedly reduced or abolished by superior laryngeal nerve section (SLN). In contrast, in the dog CAPS did not alter any of the parameters monitored, and NH₃ inhalation produced only modest shortening or lengthening of T_E We conclude that (1) marked species difference exists in the response to irritants administered into the upper airway; (2) these responses are mediated primarily by SLN afferents; (3) the C-fiber afferent supply to the dog's larynx may be scarce; and (4) the similarity of the responses elicited by capsaicin and ammonia from the rat's larynx may raise questions about the specificity of capsaicin as a C-fiber ending stimulant.