SIMULATED UTERINE CONTRACTIONS FACILITATE FETAL AND NEWBORN RESPIRATORY BEHAVIOR IN RATS

We tested the hypothesis that sensory and nonsensory factors associate d with birth stimulate respiratory behavior in the fetal and newborn r at. Late gestation (Day 21) rat fetuses were externalized from the ute rus with intact umbilical connections to the dam and exposed to stimul i normally associated with labor and delivery. Onset and maintenance o f respiratory movements were monitored. In the first experiment, rat f etuses were exposed to either: (i) simulated uterine contractions; (ii ) cooling (26 degrees C); (iii) umbilical cord occlusion; or (iv) air heated to intrauterine temperature (37.5 degrees C). Subjects were vid eotaped for 1 h, and respiratory movements counted during tape review. Fetuses in each group showed some respiratory behavior although compr ession significantly elevated respiratory rate compared to other exper imental conditions. All subjects in each group were respiring after 1 h, except for pups that received umbilical cord occlusion. The 100% at trition rate of the cord occlusion-alone group was reversed by combini ng cord occlusion with compression, or with compression and cooling, b ut not by combining cord occlusion with cooling. Simulated birth pups (i.e., those exposed to compression, cooling and umbilical cord occlus ion) and normal, vaginally delivered pups breathed at identical rates and showed a similar pattern of postpartum breathing. These results su ggest that mechanical stimulation of the fetus associated with uterine contractions plays a critical role in the maintenance of continuous r espiration at birth. Possible mechanisms for the facilitative effects of compression on perinatal breathing are discussed.