The fetal autonomic nervous system, the fetal heart rate and the beat-to-beat irregularity.

In the exteriorized fetal lamb an experimental study on the influence of the fetal autonomic nervous system upon the heart rate pattern is described by means of blockage of the cholinergic, alpha-adrenergic and beta-adrenergic system. The alpha-adrenergic system proved to have no effect upon heart rate. The beta-adrenergic system had a positive chronotropic effect and the parasympathetic system had a negative chronotropic effect. Beat-to-beat irregularity was independent of a functional alpha-adrenergic system, but at a high basic heart rate disappeared completely after cholinergic blockage and at a low rate after beta-adrenergic blockage. It is concluded that most probably the beat-to-beat irregularity is the result of the competition of the enhancing influence of the beta-adrenergic system and the inhibiting influence of the cholinergic system on fetal heart rate.