Functional immaturity of pulmonary irritant receptors and apnea in newborn preterm infants.

Absence of cough and gag reflexes has been noted in premature infants, but pulmonary irritant reflexes have not been studied. Irritant receptors respond to deflation, direct stimulation, or inhalation of irritant gases, resulting in an increased inspiratory effort. We have studied the responses of 28 intubated infants to direct bronchial mucosal stimulation with a fine catheter. Six of ten infants whose gestation was 35 weeks or more consistently showed a "mature" response, with an increase in inspiratory effort, while only one of 18 infants less than 35 weeks consistently showed this response. The premature infants showed variable responses. Fifty-eight of 93 stimulations produced an increased inspiratory effort, followed by marked slowing or apnea, while 20 stimulations resulted in slowing or apnea alone. These paradoxical responses to irritant stimulation may be related to immaturity of vagal myelination, and may be an important factor in the frequency of silent aspiration and apnea in premature infants.