Nonstress test

Premise

The premise of the NST is that a well-oxygenated, non-acidemic fetus will spontaneously have temporary increases, termed "accelerations," in the fetal heart rate (FHR).

Vibroacoustic stimulation can wake the fetus, and is sometimes used to speed up the test or to facilitate further evaluation of a non-reactive nonstress test.

Interpretation

A nonstress test can be classified as normal, atypical, or abnormal. A normal nonstress test will show a baseline fetal heart rate between 110 and 160 beats per minute with moderate variability (5- to 25-interbeat variability) and 2 qualifying accelerations in 20 minutes with no decelerations.

"Reactive" is defined as the presence of two or more fetal heart rate accelerations within a 20-minute period. Each acceleration must increase the heart rate 15 beats per minute above the baseline rate, and last for at least 15 seconds when the fetus is above 32 weeks' gestation, or 10 beats per minute over 10 seconds when the fetus is at or below 32 weeks' gestation.

"Non-reactive" is defined as fewer than two adequate accelerations during a prolonged period, which may be over an hour.

Confounding factors

An NST may be non-reactive for reasons unrelated to fetal oxygenation status. These include fetal sleep, exposure to central nervous system depressants, exposure to beta-blockers, or chronic smoking during the pregnancy.

Utility

If the nonstress test is reactive, the rate of fetal death in the following week is approximately 2 per 1000. A nonreactive nonstress test has a false positive rate of 75%-90%, and therefore is followed by more definitive testing, which may include a contraction stress test or a biophysical profile.

References

References

1. (2013). "Neonatology : management, procedures, on-call problems, diseases, and drugs".
2. (1986). "Intrapartum assessment of fetal well-being: a comparison of fetal acoustic stimulation with acid-base determinations". Am J Obstet Gynecol.
3. (2006). "Textbook of Perinatal Medicine, Second Edition (Two Volumes)". Informa Healthcare.
4. Association of Women’s Health, Obstetric, and Neonatal Nurses. (2005). "Fetal Heart Monitoring: Principles and Practices". Kendall/Hunt Publishing Co.
5. (October 2012). "Nonstress testing at ≤ 32.0 weeks' gestation: a randomized trial comparing different assessment criteria". Mosby, Inc..