Nonstress Test
last authored: July 2011, David LaPierre
last reviewed:
Introduction
The nonstress test (NST) is an ultrasound assessment of fetal heart rate, measured through the mother's abdomen using a Doppler ultrasound. A tocometer is also used to measure uterine contractions.
The fetal heart rate (FHR) increases and decreases largely due to autonomic signals from the brainstem, which also controls variability from beat to beat (Matsurra et al, 1996). These are measured and recorded.
The NST is the most widely-used test for fetal well-being. In many countries it is carried out routinely as term approaches. Testing was initially carried out one week apart, but this frequency has been increased in women who are postterm, carrying multiple fetuses, have diabetes or gestational hypertension, or have demonstrated fetal growth restriction. It is also indicated acutely if uteroplacental insufficiency or fetal distress is suspected.
Normal (Reactive) NST
A normal acceleration is defined as 15 bpm above baseline, lasting between 15-120 seconds in a fetus older than 32 weeks. For younger than 32 weeks, it is defined as 10 bpm above baseline for at least 10 seconds.
A normal NST has at least two accelerations within 20 minutes. However, the test should proceed for at least 40 minutes before being labelled abnormal, in order to rule out sleep.
Abnormal (Nonreactive) NST
An abnormal (nonreactive) response is less than two accelerations over 20 minutes.
While normal NSTs are reassuring, 'insufficient acceleration' has a very high rate of false-negatives - up to 90%, by some studies (Devoe et al, 1986). However, it also warrants further testing via a biophysical profile, if available.
A lack of FHR accelerations can occur:
- sleep cycles (the fetus can be stimulated using fundal pressure or acoustic or vibratory stimulation).
- sedative/narcotic medications
- cigarette smoking
- acidemia
- fetal growth restriction
- oligohydramnios
- meconium
- placental infarction
- fetal CNS abnormalities
While false-positives are common, there are some rhythms that reliably signify fetal distress. A beat-to-beat variation in less than 5 bpm, absent accelerations, and late decelerations following uterine contractions is very worrying.
Decelerations are normally seen with fetal movement, and need not be worrisome. However, repetition of at least declerations in 20 minutes does appear to care increased risk. Decelerations lasting >1min are even more ominous.
Monitoring During Labour
FHR monitoring is done in over 80% of births in North America.
FHR tracing should be reviewed:
- if no complications, every 30 minutes in the first stage of labour & every 15 minutes in the second stage
- if complications, every 15 minutes in the first stage of labour & every 5 minutes in the second stage
Common patterns include:
- acceleration (fetal movements)
- early deceleration (head compression from uterine contraction)
- late decleration (uteroplacental insufficiency and fetal hypoxemia)
- variable decleration (umbilical cord compression)
- bradycardia (congentital heart disease, severe hypoxemia)
- tachycardia (hypoxia, maternal fever, anemia)
Resources and References
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