Effect on ventilation of change to the upright posture in newborn infants.

We have investigated the effect of postural change on tidal volume (VT), inspiratory (TI) and expiratory (TE) duration, minute ventilation (VI), and end-tidal PCO2 in a group of 11 healthy full-term sleeping infants, 2-4 days of age. During tilts from the supine to upright posture, the average volume increase was 4.3 +/- 4.4 (SD) ml or 1.2 ml/kg in the maneuvers unassociated with sighs. In the 20% of tilts in which sighs occurred, the average volume change was slightly higher. Transition from supine to upright posture resulted in statistically significant increases in VT (6.45 +/- 0.06 to 6.72 +/- 0.06 ml/kg), TI (554 +/- 7 to 604 +/- 7 ms), and TE (629 +/- 12 to 777 +/- 14 ms), and decrease in VI (328 +/- 5 to 288 +/- 4 ml X kg-1 X min-1) (all means +/- SE, P less than 0.0005). Return to supine position resulted in statistically significant changes in the opposite direction. Tilting to the upright posture brought about a small (0.4 +/- 0.1 Torr, means +/- SE) but consistent statistically significant increase in end-tidal PCO2, which persisted through the first minute when the infant was returned to the supine position. The expiratory prolongation observed with tilting appears to minimize changes in end-expiratory lung volume, obviating the need for an effective compensatory muscle response to defend ventilation. Thus it appears that infants, in contrast to adults, adopt a breathing strategy to limit the extent of change in absolute lung volume, rather than to defend ventilation at increased lung volume.