SMALL PRETERM INFANTS (LESS-THAN-OR-EQUAL-TO 1500 G) HAVE ONLY A SUSTAINED DECREASE IN VENTILATION IN RESPONSE TO HYPOXIA

The classic "biphasic" ventilatory response to 15% O2 was previously observed in preterm infants who were large compared with those in the intensive care nursery today. We hypothesized that in the smaller infant (less-than-or-equal-to 1500 g) the response might be closer to that of the fetus, with no initial increase in ventilation. Thus, we studied 14 healthy preterm infants less-than-or-equal-to 1500 g [birth weight 1220 +/- 63 g (mean +/- SEM); gestational age 29 +/- 0.4 wk; postnatal age 17 +/- 3 d] during rapid eye movement and quiet sleep. Ventilation was measured using a nosepiece and a flow-through system. Sleep states were defined using EEG, electro-oculogram, and body movements. After a control period in 21% O2 (3 min), infants breathed 15% O2 for 5 min. In rapid eye movement sleep, minute ventilation decreased from 0.186 +/- 0.020 (control) to 0.178 +/- 0.021 (30 s), to 0.171 +/- 0.017 (1 min; p = 0.03), to 0.145 +/- 0.016 (3 min; p = 0.002), and to 0.1 29 +/- 0.011 1 . min-1. kg-1 (5 min; p = 0.004). In quiet sleep, it decreased from 0. 1 73 +/- 0.019 (control) to 0.164 +/- 0.019 (30 s), to 0.166 +/- 0.019 (1 min), to 0.148 +/- 0.013 (3 min; p = 0.03), and to 0.146 +/- 0.012 1 . min-1. kg-1 (5 min; p = 0.04). These changes in ventilation were primarily related to a decrease in frequency in rapid eye movement [38 +/- 2 (control) versus 28 +/- 3 (5 min);p = 0.011 and in quiet sleep [36 +/- 5 (control) versus 27 +/- 3 (5 min); p = 0.02]. Changes in tidal volume were negligible. These findings suggest that the classic biphasic response to hypoxia is not observed in very small preterm infants. These infants show only a sustained decrease in ventilation with low O2. We speculate that the response reflects a more pronounced inhibitory mechanism induced by hypoxia at this gestational age, representing an intermediate profile between that observed in the fetus and that present in larger neonates.