Contraction stress test
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.
During uterine contractions, fetal oxygenation is worsened. Late decelerations in fetal heart rate occurring during uterine contractions are associated with increased fetal death rate, growth retardation and neonatal depression. This test asseses fetal heart rate in response to uterine contractions via electronic fetal monitoring. Uterine activity is monitored by tocodynamometer.
CST is performed weekly, as the fetus is assumed to be healthy after a negative test and should remain so for another week. This test is done in hospital or clinic setting. External fetal monitors are placed. Either nipple stimulation or IV pitocin (oxytocin) is used to stimulate uterine contractions.
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.
Oxytocin Challenge Test (OCT)
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.
|Positive||Presence of late decelerations with at least 50% of the contractions|
|Negative||No late or significant variable decelerations, with at least 3 uterine contractions (lasting 40 seconds) in 10 minute period.|
|Equivocal—Suspicious||Presence of late decelerations with fewer than 50% of contractions or significant variable decelerations. Requires repeat testing on following day.|
|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. Requires repeat testing on following day.|
|Equivocal—Unsatisfactory||Fewer than three contractions occur within 10 minutes, or a tracing quality that cannot be interpreted. Requires repeat testing on following day.|
Results & Efficacy
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-positive results in as many as 30% of cases. A positive CST indicates high risk of fetal death due to hypoxia and is a contraindication to labor. Patient's obstetricians usually consider operative delivery in such situations.
This "stress test" is usually not performed if there are any signs of premature birth, placenta praevia, vasa praevia, cervical incompetence, multiple gestation, previous classic caesarian section. Other contraindications include but are not limited to previous uterine incision with scarring, previous myomectomy entering the uterine cavity, and PROM. Any contraindication to labor is contraindication to CST.
The CST was the first antenatal surveillance test that was developed after the development of the cardiotocograph. The oxytocin challenge test was first described in 1972 and was standardised in 1975 when the parameters of contraction number and frequency were given.
- Danforth's obstetrics and gynecology (10th ed. ed.). Philadelphia: Lippincott Williams & Wilkins. 2008. p. 161. ISBN 9780781769372.
- Current diagnosis & treatment : Obstetrics & gynecology (10th ed. ed.). New York: McGraw-Hill. 2007. p. 255. ISBN 978-0-07-143900-8.
- III, Frances Talaska Fischbach, Marshall Barnett Dunning (2009). A manual of laboratory and diagnostic tests (8th ed. ed.). Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins. pp. 1030–31. ISBN 9780781771948.
- Munden, ed.: Julie (2005). Professional guide to diagnostic tests. Philadelphia: Lippincott Williams & Wilkins. p. 682. ISBN 9781582553047.
- American College of Obstetricians and Gynecologists (ACOG). (1999). Antepartum fetal surveillance (Practice Bulletin No. 9). Washington, DC: Author.
- Association of Women’s Health, Obstetric, and Neonatal Nurses (2005). Audrey Lyndon, Linda Usher Ali, ed. Fetal Heart Monitoring: Principles and Practices (3rd ed.). Dubuque, IA: Kendall/Hunt Publishing Co. ISBN 978-0-7575-6234-1.
- Evans, [edited by] Arthur T. (2007). Manual of obstetrics (7th ed. ed.). Philadelphia: Wolters Kluwer / Lippincott Wiliams & Wilkins. p. 587. ISBN 9780781796965.
- Anderson, edited by Jean R. (2005). A guide to the clinical care of women with HIV (2005 ed. ed.). Rockville, MD: U.S. Dept. of Health & Human Services, Health Resources & Services Administration, HIV/AIDS Bureau. p. 270. ISBN 9780160726118.
- Lagrew DC, Jr (March 1995). "The contraction stress test.". Clinical obstetrics and gynecology 38 (1): 11–25. doi:10.1097/00003081-199503000-00005. PMID 7796539.
- First aid for the family medicine boards. New York: McGraw-Hill Medical. 2008. p. 556. ISBN 0-07-159382-9.
- Ray M, Freeman R, Pine S, Hesselgesser R (September 1972). "Clinical experience with the oxytocin challenge test". Am. J. Obstet. Gynecol. 114 (1): 1–9. PMID 4637035.
- Contraction Stress Test at webmd.com