ABOLITION OF BREATHING RHYTHMICITY IN LAMBS BY CO2 UNLOADING IN THE FIRST HOURS OF LIFE

The mechanisms responsible for the maintenance of regular breathing af ter 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 ho urs of life. A technique of graded CO2 removal through a veno-venous e xtracorporeal circuit was used in five unanesthetized lambs aged from 4 to 12 hours. In all lambs, CO2 unloading invariably resulted in sust ained central apnea, after a decrease in Pa-CO2 of 6.9 +/- 5.7 Torr. W e were unable to find a significant relationship between the decrease in Pa-CO2 and Pa-O2 (range 35-275 Torr) at onset of apnea. During apne a, the passage from behavioral quiet sleep to arousal or to active sle ep was marked by transient and weak breathing movements. We conclude t hat the CO2 drive, but not the behavioral states, is a major factor fo r maintaining breathing rhythmicity in lambs in the first hours of lif e. (C) 1997 Elsevier Science B.V.