Variable decelerations are characterized by an acute fall in FHR with a rapid down slope and a variable recovery. These have variable shapes, at times described as being "v," "u," or "w" (Fig. 21-13). They also have a variable relationship with contractions. Variable decelerations may be classified according to their depth and duration as mild when the
Figure 21-12 Fetal heart rate decelerations with contractions. Bradycardia occurred after the last contraction (only the initial segment is visible).
Box 21-9 Etiology of Fetal Bradycardia
Prolonged cord compression/cord prolapse Hypothermia
Tetanic uterine contractions Paracervical block Epidural and spinal analgesia Maternal seizure Rapid descent
Vigorous vaginal examinations Congenital heart disease Fetal heart block Severe hypoxia
Box 21-10 Etiology of Fetal Tachycardia
Parasympatholytic drugs (atropine, atarax)
Sympathomimetic drugs (terbutaline)
Idiopathic depth is greater than 80 beats/min and duration less than 30 seconds. Variable decelerations are considered moderate when the depth is 70 to 80 beats/min and duration is 30 to 60 seconds. Variable decelerations are severe when the depth is less than 70 beats/min and duration longer than 60 seconds.
Variable decelerations are the most commonly encountered pattern, occurring in 50% to 80% of all deliveries. They are almost always caused by umbilical cord compression. Variable decelerations are noted to be common with nuchal cord, short or prolapsed cord, or when the membranes have been ruptured. Segments of FHR accelerations just before and after the variable deceleration (shoulders) indicates a healthy response.
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