Baseline Fetal Heart Rate

Normal baseline FHR ranges from 110 to 160 beats/min. A baseline change is interpreted as one that persists for 10 minutes or more and occurs between or in the absence of contractions. An FHR of less than 110 beats/min is considered bradycardia. FHR is a function of the autonomic nervous system. The vagus nerve provides an inhibitory affect, whereas the sympathetic nervous system provides an excitatory influence. As the gestation advances, the vagal system gains dominance, resulting in a gradual decrease in the baseline. Stressful events such as hypoxia, uterine contractions, and head compression evoke a baroreceptor reflex, with resulting peripheral vasoconstriction and hypertension causing bradycardia. Stimulation of peripheral nerve receptors can cause acceleration of FHR (Fig. 21-10). An FHR baseline greater than 160 beats/min is defined as tachycardia. This is seen with certain maternal and fetal conditions, such as cho-rioamnionitis, maternal fever, and fetal tachyarrhythmias.

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