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Difficulty: EasyObstetric Complications

A primigravid client is admitted to the maternity ward in spontaneous active labor at term. A continuous electronic fetal monitor is applied to evaluate fetal well-being. Throughout the observation period, the tracing displays multiple episodes where the fetal heart rate abruptly decreases by 20 to 30 beats per minute from a stable baseline of 145 beats per minute. These abrupt drops reach their nadir within 15 seconds, last for approximately 30 to 40 seconds, and then rapidly return to the baseline. The decreases occur inconsistently, showing no uniform relationship to the onset or peak of the concurrent uterine contractions. Which of the following is the most likely underlying physiological cause of this fetal heart rate pattern?

  1. Compression of the umbilical cordAnswer
  2. B
    Insufficient uteroplacental blood flow
  3. C
    Compression of the fetal head
  4. D
    Physiologic fetal sleep state
  5. E
    Maternal systemic inflammatory response

Answer

Compression of the umbilical cord
The correct answer is compression of the umbilical cord. The clinical vignette describes a classic pattern of variable decelerations. These are defined as abrupt decreases in the fetal heart rate (onset to nadir in less than 30 seconds) of at least 15 beats per minute, lasting at least 15 seconds, followed by a rapid return to the baseline. Unlike early or late decelerations, variable decelerations vary in their onset, depth, and duration, and they do not have a uniform relationship to uterine contractions. The physiological mechanism behind variable decelerations is the temporary compression of the umbilical cord, which acutely increases fetal peripheral vascular resistance. This triggers a baroreceptor-mediated vagal response that abruptly slows the fetal heart rate.

Step-by-Step Solution

1
Analyze the fetal heart rate pattern described in the clinical vignette.
The pattern shows abrupt decreases in heart rate that are independent of the timing of uterine contractions.
Identifying the precise characteristics of the deceleration (onset speed, depth, and relationship to contractions) is required to properly classify it.
2
Classify the type of deceleration based on standard obstetric definitions.
Abrupt decreases in heart rate with a rapid return to baseline, uncoupled from the contraction cycle, define variable decelerations.
Variable decelerations have a distinct morphology (a sharp 'V' or 'W' shape) compared to the gradual, symmetric curves of early and late decelerations.
3
Correlate the specific deceleration type with its known physiological etiology.
Variable decelerations are caused by umbilical cord compression.
Understanding the underlying pathophysiology is essential for determining if clinical interventions, such as maternal repositioning or amnioinfusion, are necessary.

Key Concept

Interpretation of intrapartum fetal heart rate monitoring, specifically identifying variable decelerations and their etiology.
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Topics

Obstetric Complications

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