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.
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.
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