Breathing movements in fetal lambs and the effect of hypercapnia.

1. A breath-by-breath analysis was made of tracheal pressure records from fetal lambs in utero at 117--130 days gestation. 2. Episodes of rapid irregular breathing were accompanied by diaphragmatic e.m.g. activity and were characterized by a gradual increase in breathing rate initially and a terminal decline. Within each episode breathing was very irregular. 3. Inspiratory time increased with increasing expiratory time rapidly as the rate fell to about 80/min, and thereafter more slowly. 4. The principal effects of hypercapnia (a rise in fetal Pa, CO2 by approximately 11 mmHg) were to increase the depth of fetal breathing, by nearly twofold, and to make breathing much more regular. A change in rate was doubtful because of the skew distribution of normal breath intervals.