Determinants of the first inspiratory volume and functional residual capacity at birth.

We have investigated the pattern of pressure and volume changes that occur in vaginally delivered, full-term infants during the onset of spontaneous respiration. Within a few seconds of delivery of the head, simultaneous measurements were made of stomach and esophageal pressure changes together with volume changes determined at the mouth. Values obtained for volume were very similar, but pressure changes were of a greater magnitude than previously reported. A significant correlation has been shown between first inspiratory volume and functional residual capacity (FRC) at the end of the first breath (p less than 0.004). No significant relationship was found between first inspiratory pressure and FRC. However, using a calculated index of inspiratory pressure and time ("inspiratory effort"), a significant relationship of this to FRC was observed (p less than 0.02).