|- An abrupt (onset to
nadir less than 30 sec), visually apparent decrease in the FHR below
- The decrease in FHR is 15 beats per min or more, with a duration of
15 sec or more but less than 2 min
Variable decelerations appear
morphologically different from either early or late decelerations. They
are characterized by a sudden, abrupt drop in the FHT along with
usually a similarly abrupt return to baseline. The shape varies widely,
and they may appear as a V, U, or W. They may be either periodic or
episodic. Often, there are small accelerations immediately before and
immediately following the deceleration. These associated accelerations
are often referred to as shoulders.
The classic mechanism described as the cause of
variable decelerations is umbilical cord compression. Initial or mild
umbilical cord compression results in occlusion of the umbilical vein,
which is larger than the arteries and less rigid. This results in
decreased venous return resulting in reflex tachycardia to maintain
cardiac output. This explains the initial increase in heart rate
(shoulder) preceding the deceleration. Further compression of the cord
leads to occlusion of the umbilical artery, and the resulting increased
systemic resistance, sensed by the baroreceptors, results in a
protective reflex slowing of the heart rate. As the cord is
decompressed, this series of events is reversed, and an acceleration
may follow the deceleration (artery is decompressed but the vein is
still compressed) prior to returning to baseline.
Variable decelerations are classified as severe
when they last more than 60 seconds, fall below 70 beats/min, or have a
drop of 60 beats/min below the baseline rate.
While umbilical cord compression is typically
responsible for this pattern in the first stage of labor, it may also
result from head compression during the second stage of labor.
Variable decelerations may be seen in the
antepartum or early in labor if associated with oligohydramnios.
Similarly, they may be detected after rupture of membranes, and in this
setting they may portend cord prolapse, particularly if rupture of
membranes occurred prior to engagement of the presenting part. During
the descent phase of labors, typically between 8-10 cm cervical
dilation, variables may be associated with nuchal cords causing cord
stretch or compression, or with head compression associated with rapid
descent and maternal valsalva.
Variable decelerations in the presence of normal
FHR variability are not thought to represent hypoxia, but repetitive
severe variable decelerations with diminished or absent FHR variability
may indicate hypoxia.
See illustration below for an
example of variable deceleration. Please click to enlarge.
Variable Deceleration Video 1
Variable Deceleration Video 2
Variable Deceleration Video 3
Variable Deceleration Video 4
Variable Deceleration Video 5
Variable Deceleration Video 6
Variable Deceleration Video 7
Variable Deceleration Video 8
Variable Deceleration Video 9
Variable Deceleration Video 10
Prolonged Deceleration | Uterine