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FETAL BRADYCARDIA |
Fetal bradycardia = heart rate of £100 beats/minute that are prolonged (episodes of 10 seconds and up to several minutes have been defined as pathologic) (1,2).
To make this diagnosis 1:1 atrioventricular concordance and some heart rate variability must be present. So synchrony between atrial and ventricular rates must be present. In cases of heart block there is a difference between ventricular and atrial rates.
Intermittent fetal bradycardia may be observed in the second trimester of pregnancy. Most episodes of acute abrupt bradycardia result from a vagal cardiovascular reflex. Explanations for this phenomenon include; fetal head compression (3), umbilical cord occlusion (3), hypoxia caused by myocardial depression (4), stimulation of the stretch receptors in the walls of the aortic arch and carotid sinuses (5) and increased intrauterine pressure caused by the ultrasound transducer on the maternal abdominal wall (6) (the pressure is transmitted throughout the uterine cavity on all parts of the fetus).
In the latter cause, rapid recovery of the bradycardia was observed after
removing the transducer. Studies on myelination of the vagus nerve confirms
that myelinization and organization of nerve fibres does increase from the
seventh to the twenty-third weeks of gestation, at which time the vagus nerve
develops it mature form (7).
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