Patterns of development in fetal breathing activity in the latter third of gestation of the baboon

Patterns of fetal breathing activity were examined in a longitudinal study of the fetal baboon over the latter third of gestation. More than 1400 h of recorded tracheal fluid pressure in 16 or 24 h records from seven fetuses over a range in gestation from 121 to 172 days (term, 175-180 days) were analyzed. In these 81 records, there was a high degree of variability in the percent of time spent breathing by the fetuses (range, 14-83%) with no apparent influence of gestational age (mean ± S.D., 45.6 ± 17.6%). Nonetheless, the mean amplitude of fetal breaths increased with gestation from absolute values of about 5-10 mmHg (r = 0.73, P < 0.001) and the mean inspiratory time interval increased from about 0.45-0.55 s (r = 0.40, P < 0.001). During epochs of breathing, the mean rate decreased from about 42-36 breaths per min (r = -0.54, P < 0.001) and the indices of both short term (r = -0.54, P < 0.001) and long term (r = -0.73, P < 0.001) variability in rate decreased. These results demonstrate a clearly defined pattern of development in the breathing activity of the fetal baboon which is comparable to the pattern described for the human fetus in the third trimester of gestation. These similarities suggest that the progressive functional maturation of the mechanisms generating respiratory patterns are comparable among primate species.