The physiologic mechanisms of variable decelerations.
OBJECTIVE
The purpose of this study was to determine the cause and physiologic consequences of variable decelerations.


METHODS
Previous studies of heart rate changes in human and experimental animals were critically reviewed with respect to the autonomic nervous system component, the cause of the increased vagal activity, and the role of cord compression or other stimuli in these heart rate changes.


RESULTS
There is abundant evidence from experimental and human studies that variable decelerations can be reproduced by either cord compression or head compression. The vagal reflex produced is probably caused by a combination of chemoreflex (earlier in the deceleration) and baroreflex (later). The variable deceleration is accompanied by an acidosis, primarily respiratory, and probably hypoxemia. Cord compression results in decreased umbilical blood flow.


CONCLUSIONS
Recent Doppler velocimetry studies suggest that even though the variable decelerations may be similar in duration and depth, the reduction of umbilical blood flow may be greater when the prime cause is cord compression than when the prime cause is vagal reflex from another source.
Ball
R H
RH
University of California San Francisco, Department of Obstetrics, Gynecology and Reproductive Sciences 94143-0550.
Parer
J T
JT
D000653:Amniotic Fluid / Q000502:physiology; D001145:Arrhythmias, Cardiac / Q000209:etiology*; D001341:Autonomic Nervous System / Q000502:physiology; D001794:Blood Pressure; D002302:Cardiac Output; D003250:Constriction; D005260:Female; D005318:Fetal Heart; D005333:Fetus / Q000502:physiology; D005740:Gases / Q000097:blood; D006340:Heart Rate, Fetal; D006801:Humans; D011247:Pregnancy; D011312:Pressure; D014465:Ultrasonics; D014470:Umbilical Cord
D005740:Gases