Contraction Stress Test

The contraction stress test (CST), also known as the oxytocin challenge test, measures the fetus's ability to with stand a decreased oxygen supply and the stress of contractions induced before actual labor begins. An infusion of oxytocin is given to stimulate uterine contractions while the fetal heart rate (FHR) is assessed, to evaluate the placenta's ability to provide sufficient oxygen to the fetus.

Purpose

• To determine and evaluate the fetus's in utero risk of hypoxia or ability to with stand uterine contractions before labor begins
• To determine effects of maternal diabetes or hypertension on fetoplacental adequacy and fetal well-being
• To evaluate high-risk pregnancies
• To determine if cesarean delivery will place the fetus at risk
• To determine the need to terminate the pregnancy by labor induction and early birth when performed in conjunction with other diagnostic studies

Patient preparation

• Explain that the procedure takes about 2 hours and is performed within a controlled environment with specially trained nurses and a doctor nearby.
• Inform the patient that food and fluid will be restricted for 4 to 8 hours before the study.
• Review breathing and relaxation techniques the patient may have learned in Lamaze or other childbirth classes, or teach these techniques before the test.
• Inform the patient that she will experience mild contractions and that the fetus and she will be evaluated at all times.
• Vital signs, FHR, assessment, and history will be performed before the test.

Equipment

External monitor for FHR, oxytocin in ordered dosages, I.V. tubing, I.V. access device, dextrose 5% in water, for infusion, ultrasound transducer, tocodynamometer.

Procedure and posttest care

• Have the patient empty her bladder.
• Assist her into a semi-Fowler, sidelying position and drape her for privacy.
• Take the patient's blood pressure before and every 10 minutes during the procedure.
• Place the fetal monitor on her abdomen to monitor FHR and the tocodynamometer on the lower abdomen to monitor contractions. A baseline recording is made and then the FHR is monitored and recorded continuously for 20 minutes.
• An I.V. line is inserted and I.V. oxytocin (Pitocin) is administered via I.V. pump as ordered. The FHR and contractions are monitored and recorded, and the infusion is discontinued to determine the FHR response to the contractions.
• Encourage the patient to use deep breathing and relaxation skills while the oxytocin is administered and she feels the contractions.
• Continue to monitor FHR for 30 minutes as uterine movements return to normal.
• If premature labor begins, note the frequency, strength, and length of contractions, and prepare for labor or cesarean delivery.

Precautions

• This test is contraindicated in multiple pregnancy, previous cesarean birth or surgical hysterotomy, previous premature labor, premature ruptured membranes, abruptio placentae, placenta previa, and fetus of less than 34 weeks' gestation.

Normal findings

A negative result occurs when the FHR is within normal range, indicating that the fetus can tolerate the stress of labor.

Abnormal findings

Late decelerations in FHR during 2 or more contractions or late decelerations that are inconsistent indicate risk of fetal asphyxia.

Interfering factors

• Maternal hypotension (false-positive results due to diminished placental blood flow)
• Full bladder