An obstetric unit receives a 24-year-old primigravida at 40 weeks gestation for an electiv… — USMLE Step 2 CK

An obstetric unit receives a $24$ -year-old primigravida at $40$ weeks gestation for an elective labor induction. Following cervical ripening, continuous administration of an intravenous uterine stimulant is initiated. Over the subsequent hour, uterine contractions intensify to a frequency of one every $2$ minutes. Between contractions, the patient mentions feeling mildly lightheaded and short of breath.

Her blood pressure is $118/76 \text{ mmHg}$ , pulse is $95 \text{ /min}$ , and respiratory rate is $20 \text{ /min}$ . Baseline laboratory values obtained upon admission demonstrate:
Hemoglobin: $11.2 \text{ g/dL}$
Leukocyte count: $14,000 \text{ /mm}^3$
Platelet count: $200,000 \text{ /mm}^3$

An arterial blood gas analysis on room air shows:
pH: $7.44$
PaCO $_2$ : $28 \text{ mmHg}$
PaO $_2$ : $102 \text{ mmHg}$
HCO $_3^-$ : $19 \text{ mEq/L}$

Continuous electronic fetal monitoring displays a baseline heart rate of $145 \text{ /min}$ with moderate variability. There are recurrent, symmetric decreases in the fetal heart rate that begin gradually, reach their nadir after the peak of each uterine contraction, and slowly return to the baseline after the contraction ends. The patient's cervix is $6 \text{ cm}$ dilated and $100\%$ effaced.

Which of the following is the most appropriate next step in the management of this patient?

A
Transport the patient to the surgical suite for an expedited operative delivery
Cease the uterine stimulant administration and turn the patient onto her sideAnswer
C
Instill warmed isotonic fluid into the uterine cavity via a transcervical catheter
D
Transfuse two units of cross-matched packed red blood cells
E
Instruct the patient to perform controlled hypoventilation into a closed receptacle

Answer

Cease the uterine stimulant administration and turn the patient onto her side

The fetal heart rate tracing describes recurrent late decelerations, which are characterized by a gradual decrease in heart rate with a nadir that occurs after the peak of the uterine contraction. This pattern indicates uteroplacental insufficiency. The patient is also experiencing uterine tachysystole (contractions every

2

minutes) due to the synthetic uterotonic infusion, which further compromises placental blood flow by shortening the critical relaxation period between contractions. The most appropriate initial step is to perform intrauterine resuscitation to restore fetal oxygenation. This includes discontinuing the uterine stimulant, repositioning the mother into the lateral decubitus position (to relieve aortocaval compression and increase venous return), administering intravenous fluid boluses, and providing supplemental oxygen.

Step-by-Step Solution

Analyze the fetal heart rate tracing description.

The gradual, symmetric decreases with nadirs occurring after the peak of contractions indicate late decelerations.

Identifying the specific type of deceleration is necessary to determine its underlying etiology (uteroplacental insufficiency).

Evaluate the patient's contraction frequency.

Contractions are occurring every 2 minutes, which indicates uterine tachysystole.

Tachysystole reduces the relaxation time between contractions, further compromising placental blood flow and exacerbating uteroplacental insufficiency.

Assess maternal laboratory and arterial blood gas values.

Hemoglobin of 11.2 g/dL and fully compensated respiratory alkalosis are normal physiological changes of pregnancy.

Recognizing these as normal expected findings prevents the implementation of unnecessary and potentially harmful interventions like blood transfusions or rebreathing techniques.

Determine the appropriate initial intervention for the fetal distress.

The first step is intrauterine resuscitation, which includes discontinuing uterotonics and repositioning the mother to the lateral decubitus position.

Conservative measures to improve placental perfusion must always be attempted before proceeding to an invasive surgical delivery.

Key Concept

Recurrent late decelerations indicate uteroplacental insufficiency and should initially be managed with intrauterine resuscitation, including maternal repositioning and discontinuation of uterotonics.

Estimated Time:1m 30s

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