Prolonged Hypoxemia Enhances and Acute Hypoxemia Attenuates Laryngeal Reflex Apnea in Young Lambs

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To determine the influence of an altered carotid body function on the laryngeal chemoreflex (LCR) response, reflex apnea was induced by laryngeal water stimulation during normoxia or acute hypoxia in unanesthetized awake lambs in which the ventilatory response to acute hypoxia was attenuated by prolonged postnatal hypoxemia. Prolonged hypoxemia (H) was induced in seven lambs for 12 d after birth through exposure to 0.10 fraction of inspired oxygen. Five control lambs were kept in 0.21 fraction of inspired oxygen. Studies were performed repeatedly during the first 7 wk after birth. The ventilatory response to LCR stimulation, expressed as a percent decrease in minute ventilation, was tested in 0.21, 0.14, and 0.10 fraction of inspired oxygen. H after birth resulted in a markedly increased inhibition of ventilation in response to LCR stimulation and postponed the age-related decrease in LCR response. A potential failure to recover from apnea occurred only in the H lambs, and in these lambs there was a significantly greater requirement for mechanical ventilation after LCR stimulation. Acute hypoxemia preceding LCR stimulation significantly attenuated the ventilatory response in both control and H lambs, with a stronger effect in the H lambs. There was no difference between the two groups in heart rate response to LCR stimulation. Acute hypoxemia significantly augmented reflex bradycardia in the H lambs. These results show that there is a relationship between H immediately after birth—which is known to delay resetting of carotid chemoreceptors—and augmented ventilatory inhibition in response to LCR stimulation. They do not confirm the theory that acute hypoxia reinforces reflex apnea.

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