External Fetal Monitoring, Contraction Stress Test (CST) and Oxytocin Challenge Test (OCT)

Norm of External Fetal Monitoring, Contraction Stress Test (CST) and Oxytocin Challenge Test (OCT)

Fetal heart rate (FHR) and variability normal.

FHR	110–160 bpm
FHR variability	5 bpm: minimal variability
	6–25 bpm: moderate variability
	<25 bpm: pronounced variability

 

Usage of External Fetal Monitoring, Contraction Stress Test (CST) and Oxytocin Challenge Test (OCT)

To stimulate labor, to evaluate fetal responses to contractions. CST/OCT administered as a result of nonreactive non–stress test (NST). CST is one measure used to evaluate the quality of placental perfusion and fetal well-being. This test is usually conducted after 32 weeks of gestation.

 

Description of External Fetal Monitoring, Contraction Stress Test (CST) and Oxytocin Challenge Test (OCT)

A noninvasive procedure in which the fetus is electronically monitored and uterine contractions are stimulated. A regular contraction pattern is achieved for the purpose of evaluating fetal responses and making predictions regarding fetal outcome to labor.
Interpretation of CST/OCT:

Negative = No decelerations during entire procedure; long-term viability (LVT) is present. Uterine contractions have no adverse effect on fetus (deceleration). Is predictive of continued fetal well-being for 7 days.

Positive = At least half of contractions obtained are accompanied by late decelerations. Prognosis is poor in the presence of decreased LTV. Vigorous management is indicated.

Equivocal = Test needs to be repeated within 24 hours.

Hyperstimulation = >4 contractions in 10 minutes, or contractions lasting longer than 90 seconds, or <60 seconds between contractions.

Suspicious = Late decelerations present in less than half of contractions. LTV usually present.

Unsatisfactory = Quality of tracing too poor to provide accurate interpretation.

 

Professional Considerations of External Fetal Monitoring, Contraction Stress Test (CST) and Oxytocin Challenge Test (OCT)

Consent form is NOT required, but maternal permission should be obtained. Client needs to be informed that the effect of oxytocin may induce labor.

Contraindications
Preterm labor, placenta previa.

 

Preparation

1. Complete a 20-minute monitor strip for a baseline FHR and uterine activity. Observe fetal movement and changes in FHR.
2. Test must be conducted in an intrapartal unit in the event of nonreassuring outcomes that require immediate interventions.

 

Procedure

1. For exogenous oxytocin release by oxytocin (Pitocin) infusion, begin an intravenous infusion of lactated Ringer's, dextrose with saline, or normal saline. Add oxytocin piggyback IV infusion at 0.5 milliunit (mU)/minute. Test period conducted until client achieves three uterine contractions, each lasting 40 seconds, within a10-minute period. Increase oxytocin rate per protocol until contraction pattern is achieved.
2. For endogenous oxytocin release, see Client and Family Teaching.

 

Postprocedure Care

1. Continue to observe and document fetal patterns and changes in uterine activity. Watch for uterine hyperstimulation.
2. Discontinue oxytocin; monitor client's blood pressure and pulse rate until contractions have subsided.

 

Client and Family Teaching

1. For endogenous oxytocin release by nipple stimulation, instruct client to brush or massage a nipple until contractions begin, or for 10 minutes. If there are no contractions after 10 minutes, ask client to brush or massage other nipple.

 

Factors That Affect Results

1. CST/OCT has a <2% false-negative rate and a >50% false-positive rate.

 

Other Data

1. CST/OCT is more sensitive to fetal oxygen reserves than is NST.
2. In the presence of variable decelerations, regardless of the outcome of the test, amniotic fluid infusion may be indicated.