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Previous reports on the fetal hypoxic bradycardia in animals have indicated, that there is vagal influence, especially when asphyxia is induced by umbilical cord occlusion. In the present study hypoxia was induced via the mother, thus keeping the fetal circulation intact. The experiments were conducted on mature fetuses of three different species, namely 20 guinea-pig 3 cat and 3 lamb fetuses. The ECG was recorded continuoulsy and used for measuring the fetal heart rate. The vagal influence on the fetal hypoxic bradycardia was tested by comparing the time for the onset of the bradycardia with of without vagal activity. There was no indication of any vagal component in the fetal hypoxic bradycardia, which is therfore to be regarded rather as a sign of myocardial hypoxia and failing fetal circulation. The ECG recordings showed that the fetal bradycardia initially is an AV-block, type II, and that there are progressive changes in the S-T interval as an early sign of hypoxia.
Rosén et al. (1975) studied this question.