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Influence of Vagal Activity on the Neonatal Ventilatory Response to Hypoxemia

ABSTRACT: The sustained increase in ventilation (V1) that occurs during acute hypoxemia in adults is not characteristic of the neonate as V1 falls to or below baseline values soon after onset of the hypoxic stimulus. Associated with this decline in V1 is a decrease in tidal volume, lung compliance, inspiratory duration, and an increase in functional residual capacity and respiratory frequency. We hypothesized that hypoxemia induced small airway constriction and pulmonary time constant inequalities resulting in a frequency dependent fall in lung compliance and tidal volume and retention of lung volume. In seven newborn subhuman primates, responses to acute hypoxemia were measured prior to and after administration of atropine methyl bromide to prevent vagally mediated narrowing of peripheral airways. The increase in frequency and fall in inspiratory duration characteristic of the ventilatory decline during hypoxemia was eliminated by the drug but functional residual capacity and lung compliance were unaffected. Also, the initial rise in V1 was blunted or blocked in all subjects. Bilateral vagotomy caused V1 to fall significantly requiring oxygen supplementation but responses to hypoxemia were still biphasic in nature. These findings suggest that cholinergically mediated mechanisms in the airways do not alter effective lung distensibility related to respiratory rate. Acetylcholine may be important at the peripheral chemosensor since cholinergic blockade eliminated the initial ventilatory increase. Finally, although vagotomy radically affected respiratory pattern during eupnea and hypoxia, the ventilatory response to hypoxemia remained biphasic ruling out this pathway as a primary modulator in the newborn response.