THE RESPIRATORY RESPONSE OF HEALTHY TERM INFANTS TO BREATH-BY-BREATH ALTERNATIONS IN INSPIRED OXYGEN AT 2 POSTNATAL AGES

We have studied the reflex respiratory responses to breath-by-breath a lternations in fractional inspired oxygen in a group of healthy term i nfants at two ages, 43 +/- 7 h (study 1) and 47 +/- 3 d (study 2). Res piration was measured noninvasively using inductance plethysmography. Responses to alternations of fractional inspired oxygen between 0.16 a nd 0.21 (test runs) were compared with responses to alternating the in spired gas between two lines each containing a fractional inspired oxy gen concentration of 0.21 (control runs). The respiratory response was measured as the mean percentage breath-by-breath alternation for insp iratory tidal volume (V-TI), expiratory tidal volume (V-TE), inspirato ry time (T-1), expiratory time (T-E), frequency (f), mean inspiratory flow (V-TI/T-I), mean expiratory flow (V-TE/T-E), timing (T-I.f), and ventilation. A significant chemoreflex response was present in the inf ants at the time of study 1, as shown by test runs that were significa ntly different from control for T-I, T-E, f, mean inspiratory flow, me an expiratory flow, timing, and ventilation (p < 0.05), and at study 2 for V-TI, V-TE, T-E, f, mean inspiratory flow, mean expiratory flow, timing, and ventilation (p < 0.05). When control and test runs were co mpared separately with respect to age, there were no significant diffe rences far 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 establis hed response, and this suggests that the ''resetting'' of chemorecepto r responses to hypoxia is essentially complete within approximately 24 -48 h of birth in humans.