Uncomplicated variable deceleration.
During a contraction, the umbilical cord may be compressed.
The umbilical vein has thin walls and will collapse first leading to hypotension.
Fetal hypotension triggers
baroreceptors in the Aorta Kressel Receptors and carotid sinus.
These receptors send signals to the cardio accelerator centre via the vagus and glossopharyngeal nerves.
Stimulation of sympathetic nerves
causes the fetal heart rate to abruptly increase.
This is called an initial acceleration.
As the uterine contraction increases in strength, the umbilical arteries will also collapse
leading to hypertension.
Fetal hypertension triggers the baroreceptors to send signals to the cardioinhibitory centre.
Stimulation of the vagus nerve causes the fetal heart rate too abruptly decelerate to the nadir
This is called an uncomplicated variable deceleration.
As the contraction decreases in strength, the umbilical arteries are released
causing the fetus to return to a state of hypotension.
Again fetal hypotension triggers baroreceptors to send signals to the cardio accelerator centre.
Stimulation of sympathetic nerves causes the fetal heart rate to abruptly increase from the nadir
This is called a secondary acceleration.
Once the contraction ends, the umbilical vein is also released and
the fetal heart rate rapidly returns to base line.
When variable decelerations are associated with uterine contractions,
their onset, depth and duration
commonly vary with successive uterine contractions.
When uncomplicated variable decelerations occur occasionally,
the tracing is classified as normal. If they are repetitive,
three in a row, the tracing is classified as atypical.
