The Contraction Stress Test (CST) is a method to evaluate the viability of a fetus. It documents the ability of the placenta to provide an adequate blood supply to the fetus. The CST can be used to evaluate any high-risk pregnancy in which fetal well-being may be threatened. These pregnancies include those marked by diabetes, hypertensive disease of pregnancy (toxemia), intrauterine growth restriction, Rh-factor sensitization, history of stillbirth, postmaturity, or low estriol levels.The CST, frequently called the oxytocin challenge test (OCT), is a relatively noninvasive test of fetoplacental adequacy used in the assessment of high-risk pregnancy. Other tests used to evaluate the fetoplacental unit include Alpha-fetoprotein, Amniocentesis, Biophysical Profile, Estriol Excretion, Fetoscopy, Nonstress Test, Obstetric Ultrasound, and Pregnanediol.
For this study, a temporary stress in the form of uterine contractions is applied to the fetus after the intravenous (IV) administration of oxytocin. The reaction of the fetus to the contractions is assessed by an external fetal heart monitor. Uterine contractions cause transient impediment of placental blood flow. If the placental reserve is adequate, the maternal-fetal oxygen transfer is not significantly compromised during the contractions and the fetal heart rate (FHR) remains normal (a negative test). The fetoplacental unit can then be considered adequate for the next 7 days.
If the placental reserve is inadequate, the fetus does not receive enough oxygen during the contraction. This results in intrauterine hypoxia and late deceleration of the FHR. The test is considered to be positive if consistent, persistent, late decelerations of the FHR occur with two or more uterine contractions. False-positive results caused by uterine hyperstimulation can occur in 10% to 30% of patients. Thus, positive test results warrant a complete review of other studies (e.g., amniocentesis) before the pregnancy is terminated by delivery.
The test is considered to be unsatisfactory if the results cannot be interpreted (e.g., because of hyperstimulation of the uterus, excessive movement of the mother, or deceleration of unknown meaning [not associated with contractions]). In the case of unsatisfactory results, other means of evaluation should be considered (ultrasound or amniocentesis).
Two advantages of the CST are that it can be done at any time and that its results are available shortly afterward. Although this test can be performed reliably at 32 weeks of gestation, it usually is done after 34 weeks. The CST can induce labor, and a fetus at 34 weeks is more likely to survive an unexpectedly induced delivery than a fetus at 32 weeks. The Fetal Nonstress Test is the preferred test in almost every instance and can be performed more safely at 32 weeks; it can then be followed 2 weeks later by the CST if necessary. The CST may be performed weekly until delivery terminates pregnancy.
A noninvasive, alternative method of performing the CST is called the breast stimulation or nipple stimulation technique. Stimulation of the nipple causes nerve impulses to the hypothalamus that trigger the release of oxytocin into the mother’s bloodstream. This causes uterine contractions and may eliminate the need for IV administration of oxytocin. Uterine contractions are usually satisfactory after 15 minutes of nipple stimulation (gentle twisting of the nipples). Advantages of this technique include the ease of performing the test, shorter duration of the study, and elimination of the need to start, monitor, and stop IV infusions. If sufficient contractions do not result from nipple stimulation, the standard CST procedure is followed.
The CST is performed safely on an outpatient basis in the labor and delivery unit, where qualified nurses and necessary equipment are available. The test is performed by a nurse with a physician available. The duration of this study is approximately 2 hours. The discomfort associated with the CST may consist of mild labor contractions. Breathing exercises are usually sufficient to control any discomfort.
When not to Perform Contraction Stress Test
The following is a listing of conditions in which performing Contraction Stress Test may lead to harmful consequences:
- Moms pregnant with multiple fetuses, because the myometrium is under greater tension and is more likely to be stimulated to premature labor.
- Moms with a prematurely ruptured membrane, because labor may be stimulated by the CST.
- Moms with placenta previa, because vaginal delivery may be induced.
- Moms with abruptio placentae, because the placenta may separate from the uterus as a result of the oxytocin-induced uterine contractions.
- Moms with a previous hysterotomy, because the strong uterine contractions may cause uterine rupture.
- Moms with a previous vertical or classic cesarean section, because the strong uterine contractions may cause uterine rupture (The test can be performed, however, if it is carefully monitored and controlled.).
- Moms with pregnancies of less than 32 weeks, because early delivery may be induced by the procedure.
Causes of False Contraction
Contraction Stress Test may show false positive results of contraction in the cases of Hypertension.
Performing the Contraction Stress Test
- The mom should be taught about breathing and relaxation techniques.
- The mom’s blood pressure and the Fetal Heart Rates are both recorded before the test as baseline values.
- After the mom empties her bladder, she is placed in a semi-Fowler’s position and tilted slightly to one side to avoid vena caval compression by the enlarged uterus.
- Mom’s blood pressure should be checked every 10 minutes to avoid hypotension, which may cause diminished placental blood flow and a false-positive test result.
- Fetal heart tones need to be recorded by placing an external fetal monitor over the mom’s abdomen. An external tocodynamometer is also attached to the abdomen at the fundal region to monitor uterine contractions.
- The output of both the fetal heart tones and uterine contractions are recorded on a two-channel strip recorder.
- Both of the baseline Fetal Heart Rate and the uterine activity are monitored for 20 minutes.
- If uterine contractions are detected during the pretest 20 minutes period, oxytocin withheld and the response of the fetal heart tone to spontaneous uterine contractions is monitored.
- If no spontaneous uterine contractions occur, oxytocin (Pitocin) is administered by IV infusion pump.
- The rate of oxytocin infusion is increased until the mom is having moderate contractions, then the FHR pattern is recorded
- After the oxytocin infusion is discontinued, the FHR monitoring is proceeded for another 30 minutes until the uterine activity has returned to its preoxytocin state. The body metabolizes enough oxytocin in approximately 20 to 25 minutes.
Indication of Positive Contraction Stress Test
Fetoplacental Inadequacy: Any disease, trauma, or alteration in the fetoplacental unit will cause deceleration of the Fetal Heart Rate. This would include maternal causes, placental causes, or fetal diseases (or severe genetic defects).