1. In infants, metabolic rate is highly variable. We investigated whether the strength of the vagal ventilatory inhibitory reflex was modified by the spontaneous changes in metabolic rate.

2. In 20 2-day-old infants, we measured the intensity of the Hering—Breüer expiratory-promoting reflex (inhibition of breathing during maintained lung inflation), the inspired and expired O2 and minute ventilation (V̇E), from which oxygen consumption (V̇o2) was calculated. The measurements were performed twice, a few hours apart [6 ± 1 h SEM)].

3. On average, V̇o2 differed by 39 ± 7% between the two measurements, with no correlation with the time of the day (morning/afternoon) or the time interval from a meal. V̇E was also significantly higher (19 ± 6%) when V̇o2 was higher, because of a larger tidal volume. The Hering—Breüer reflex (quantified as the apnoea time during lung inflation, either the absolute value or normalized by the airway pressure or lung volume) did not differ significantly between the two tests (<5% difference, P > 0.05), and did not correlate with the changes in V̇o2 or V̇E.

4. We conclude that, in the infant, pulmonary vagal inhibition is not sensitive to changes in metabolic drive, presumably because of the low ventilatory chemosensitivity of the newborn.

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