1675 LACK OF VOLUME-RELATED CONTROL OF VENTILATION IN NEONATES

Abstract

Breathing at large tidal volumes should result in inspiratory shortening if the Hering-Breuer reflex is active. This has been observed in human newborn infants by some investigators (Taeusch et al J. Ped.1976;88:102) but not by others (Boychuk et al J.Phys. London 1977;270:653). In the present study,24 pre-term infants were exposed to 0%,2%,5%,and 8% CO2 at 0,5,and 10 cm H2O of end expiratory pressure via endotracheal tube. The aim of this study was to increase tidal volume with CO2 stimulation at the same time lung volume was increased via PEEP. An average of 62% of the PEEP was transmitted at 5 cm H2O and 54% at 10 cm H2O. Using a computer-assisted program, recordings of tidal volume were made at each level of PEEP and CO2 after 3 minutes of CO2 breathing, and analyzed for inspiratory time (Ti), expiratory time (Te), minute volume (Ve), and tidal volume (Vt). Plots were then made of 60-100 breaths for each level of CO2 and PEEP. Despite a 3-4 fold increase in tidal volume which accompanied CO2 breathing, and a 2x increase in minute volume, Ti did not shorten with increasing Vt. In many cases a slight positive correlation was observed between Vt and Ti, and a negative correlation between Vt and Ti-1. These results confirm those of Boychuk, Rigatto, and Seshia and suggest that (1) vagal control of ventilation via Hering-Breuer afferents is deficient in the neonatal period, or (2) newborn infants breathe at tidal volumes and lung volumes below the vagal threshold.