Abolition of breathing rhythmicity in lambs by CO2 unloading in the first hours of life.

The mechanisms responsible for the maintenance of regular breathing after initiation of breathing at birth are still poorly understood. This study was designed to test the hypothesis that removing the chemical CO2 drive would abolish breathing rhythmicity in lambs in the first hours of life. A technique of graded CO2 removal through a veno-venous extracorporeal circuit was used in five unanesthetized lambs aged from 4 to 12 hours. In all lambs, CO2 unloading invariably resulted in sustained central apnea, after a decrease in Paco, of 6.9 +/- 5.7 Torr. We were unable to find a significant relationship between the decrease in PaCO2 and PaO2 (range 35-275 Torr) at onset of apnea. During apnea, the passage from behavioral quiet sleep to arousal or to active sleep was marked by transient and weak breathing movements. We conclude that the CO2 drive, but not the behavioral states, is a major factor for maintaining breathing rhythmicity in lambs in the first hours of life.