The respiratory response of healthy term infants to breath-by-breath alternations in inspired oxygen at two postnatal ages.

We have studied the reflex respiratory responses to breath-by-breath alternations in fractional inspired oxygen in a group of healthy term infants at two ages, 43 +/- 7 h (study 1) and 47 +/- 3 d (study 2). Respiration was measured noninvasively using inductance plethysmography. Responses to alternations of fractional inspired oxygen between 0.16 and 0.21 (test runs) were compared with responses to alternating the inspired gas between two lines each containing a fractional inspired oxygen concentration of 0.21 (control runs). The respiratory response was measured as the mean percentage breath-by-breath alternation for inspiratory tidal volume (VTI), expiratory tidal volume (VTE), inspiratory time (TI), expiratory time (TE), frequency (f), mean inspiratory flow (VTI/TI), mean expiratory flow (VTE/TE), timing (TI.f), and ventilation. A significant chemoreflex response was present in the infants at the time of study 1, as shown by test runs that were significantly different from control for TI, TE, f, mean inspiratory flow, mean expiratory flow, timing, and ventilation (p < 0.05), and at study 2 for VTI, VTE, TE, f, mean inspiratory flow, mean expiratory flow, timing, and ventilation (p < 0.05). When control and test runs were compared separately with respect to age, there were no significant differences for any respiratory variable between study 1 and study 2. Thus, we did not observe significant maturation of respiratory chemoreflex responses to hypoxia after an age at which we could detect an established response, and this suggests that the "resetting" of chemoreceptor responses to hypoxia is essentially complete within approximately 24-48 h of birth in humans.