A contraction stress test (CST) is performed near the end of pregnancy to determine how well the fetus will cope with the contractions of childbirth. The aim is to induce contractions and monitor the fetus to check for heart rate abnormalities using a cardiotocograph.
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This is a procedure that relies on endogenous release of oxytocin following nipple stimulation, and is conducted by the patient. The nurse instructs the patient on the procedure, as follows. One nipple is massaged gently through clothing until a contraction begins, or for a maximum of 2 minutes. If at least 3 contractions in 10 minutes is not achieved, then the patient rests for 5 minutes and the other nipple is stimulated. [1][2]
If adequate contractions (at least 3 in 10 minutes) cannot be achieved with nipple stimulation, an oxytocin challenge test may be performed. It involves the intravenous administration of exogenous oxytocin to the pregnant woman. The target is to achieve around three contractions every ten minutes. [1][2]
*Positive: presence of late decelerations with at least 50% of the contractions
*Negative: no late or significant variable decelerations
*Equivocal—Suspicious: presence of late decelerations with fewer than 50% of contractions) or significant variable decelerations
*Equivocal—Tachysystole: Presence of contractions that occur more frequently than every 2 minutes or last longer than 90 seconds in the presence of late decelerations
*Equivocal—Unsatisfactory: Fewer than three contractions occur within 10 minutes, or a tracing quality that cannot be interpreted
The CST is used for its high negative predictive value. A negative result is highly predictive of fetal wellbeing and tolerance of labor. The test has a poor positive predictive value with false-posiive results in as many as 30% of cases. [3][2]
The CST was the first antenatal surveillance test that was developed after the development of the cardiotocograph.[2] The oxytocin challenge test was first described in 1972[4] and was standardised in 1975 when the parameters of contraction number and frequency were given.
Historically, a CST was done after a non reactive NST. Today, a biophysical profile (BPP) is usually performed.
This "stress test" is usually not performed if there are any signs of premature birth or placenta praevia. Other contraindications include but are not limited to previous uterine incision with scarring, previous myomectomy entering the uterine cavity, PROM and incompetent cervix.
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