Nonstress Testing

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Chapter 188 Nonstress Testing

THE CHALLENGE

Fetal health may be assessed using the nonstress test (NST). This test is the simplest of the antenatal tests to perform and often represents the first line in managing an at-risk pregnancy.

Scope of the Problem: Of pregnancies, 3% to 12% are at risk because of gestations that extend beyond term. More pregnancies may be compromised by maternal disease states that affect fetal health or placental function (e.g., hypertension, diabetes) resulting in abnormalities of fetal growth or amniotic fluid volume and other problems.

Objectives of the Test: To assess fetal health and reserve.

TACTICS

Relevant Pathophysiology: The NST is based on the premise that when fetal oxygenation is only marginally adequate, the fetus will not possess the normal ability to alter heart rate in response to fetal movement. A normal (reactive) NST has two or more accelerations in fetal heart rate that peak at 15 beats/min (although not necessarily remaining at that level), lasting for 15 seconds in a 20-minute period. Loss of reactivity is most often associated with fetal sleep but may result from any cause of central nervous system depression, including fetal acidosis. It may be necessary to continue the tracing for 40 minutes or longer to take into account the fetal sleep–wake cycle. Acoustic stimulation may also be used to startle the fetus and induce a heart rate increase.

Strategies: The NST is considered reactive (normal) if there are two or more fetal heart rate accelerations (as defined previously) within a 20-minute period, with or without fetal movement felt by the mother. Accelerations with fetal activity are reassuring. A nonreactive NST is one that does not meet these criteria (lacks sufficient fetal heart rate accelerations) over a 40-minute period. A reactive NST is a good predictor of adequate fetal oxygenation, and most reactive fetuses do well for at least another week.

Patient Education: Reassurance; American College of Obstetricians and Gynecologists Patient Education Pamphlet 098 (Special Tests for Monitoring Fetal Health), AP015 (Fetal Heart Rate Monitoring During Labor).

IMPLEMENTATION

Special Considerations: To increase the reliability of the NST, the patient should not have smoked recently. The NST is more easily performed than the contraction stress test, but it has the highest false-positive rate (up to 90% of positive tests) and the highest risk of a false-negative test result (1.4 of 1000). (The NST of the healthy preterm fetus is frequently nonreactive: from 24 to 28 weeks of gestation, up to 50% of NSTs may not be reactive, and from 28 to 32 weeks of gestation, 15% of NSTs are not reactive.) Variable decelerations may be observed in up to 50% of NSTs. If sporadic and brief (<30 seconds), they are inconsequential and do not indicate fetal compromise or the need for intervention. (Repetitive variable decelerations [at least three in 20 minutes], even if mild, have been associated with an increased risk of cesarean delivery for fetal indications.) Because of these factors, the interpretation of a nonreactive test must be made in the perspective of other information about the mother and fetus, including the results of other tests such as the contraction stress test or biophysical profile. (See also “Antepartum Testing.”)

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