A 29-year-old G1P0 at 38 weeks gestation presents to the labor and delivery ward with pain… — USMLE Step 2 CK

A 29-year-old G1P0 at 38 weeks gestation presents to the labor and delivery ward with painful uterine contractions. Her prenatal course was uncomplicated. Her temperature is $37.0^\circ\text{C}$ ( $98.6^\circ\text{F}$ ), blood pressure is $118/76\text{ mmHg}$ , and pulse is $82/\text{min}$ . She is currently positioned in the left lateral decubitus position and receiving a continuous intravenous infusion of lactated Ringer solution. Cervical examination reveals dilation to $6\text{ cm}$ , $100\%$ effacement, and $+1$ station. The amniotic membranes ruptured spontaneously 3 hours ago, and the fluid was clear. Tocodynamometry shows uterine contractions occurring every 3 minutes and lasting 50 seconds. Fetal heart rate monitoring demonstrates a baseline of $145/\text{min}$ with moderate variability. Over the past 30 minutes, there have been recurrent, abrupt decreases in the fetal heart rate to $80/\text{min}$ that last for 40 seconds before rapidly returning to the baseline. These decelerations occur with most contractions but have a varied onset relative to the contraction peak. Which of the following is the most appropriate next step in the management of this patient?

AmnioinfusionAnswer
B
Cesarean delivery
C
Administration of oxytocin
D
Administration of terbutaline
E
Fetal scalp stimulation

Answer

Amnioinfusion

The correct answer is amnioinfusion. This patient is experiencing recurrent variable decelerations, which are defined as abrupt decreases in the fetal heart rate (onset to nadir <30 seconds) that drop at least

15/\text{min}

below baseline and last for at least 15 seconds. These occur due to umbilical cord compression, which is common after the rupture of amniotic membranes. Initial management of variable decelerations includes maternal repositioning (e.g., left lateral decubitus) to shift the uterus off the great vessels and cord, as well as intravenous fluid bolus administration. Because this patient is already situated in the left lateral decubitus position and receiving fluids, the next best step is an amnioinfusion. This procedure involves instilling isotonic fluid (like normal saline or lactated Ringer solution) into the uterine cavity via an intrauterine pressure catheter. The artificial fluid replaces the lost amniotic fluid volume, providing a cushion for the umbilical cord and relieving the compression during uterine contractions.

Step-by-Step Solution

Analyze the fetal heart rate tracing

Identified abrupt decreases in fetal heart rate lasting <2 minutes with varied onset relative to contractions.

Accurate pattern recognition is required to classify the abnormalities as variable decelerations.

Determine the underlying pathophysiology

Variable decelerations are caused by umbilical cord compression, commonly occurring after the rupture of membranes.

Understanding the cause directs the appropriate therapeutic intervention.

Identify the next indicated intervention

Because maternal repositioning and intravenous fluids are already in place without resolution of the decelerations, amnioinfusion is indicated.

Amnioinfusion instills fluid into the uterine cavity, relieving the physical compression on the umbilical cord during contractions.

Key Concept

Recurrent variable decelerations are managed initially with maternal repositioning and intravenous fluids; if these fail, amnioinfusion is the next appropriate step to relieve umbilical cord compression.

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