The NST is based on the premise that the heart rate of a fetus that is not acidotic or neurologically depressed will temporarily accelerate with fetal movement. Heart rate reactivity is thought to be a good indicator of normal fetal autonomic function. Loss of reactivity is most commonly associated with a fetal sleep cycle but may result from any cause of central nervous system depression, including fetal acidemia.
The patient may be positioned in either the semi-Fowler position (sitting with the head elevated 30 degrees) or lateral recumbent position. In one small randomized study, it took less time to obtain a reactive NST when patients were placed in the semi-Fowler position 20. The FHR is monitored with an external transducer. The tracing is observed for FHR accelerations that peak (but do not necessarily remain) at least 15 beats per minute above the baseline and last 15 seconds from baseline to baseline. The NST should be conducted for at least 20 minutes, but it may be necessary to monitor the tracing for 40 minutes or longer to take into account the variations of the fetal sleep–wake cycle. Vibroacoustic stimulation may elicit FHR accelerations that are valid in the prediction of fetal well-being. Such stimulation offers the advantage of safely reducing the frequency of nonreactive NSTs by 40% and the overall testing time by almost 7 minutes without compromising detection of the acidotic fetus 21 22 23 24. To perform vibroacoustic stimulation, the device is positioned on the maternal abdomen and a stimulus is applied for 1–2 seconds. If vibroacoustic stimulation fails to elicit a response, it may be repeated up to three times for progressively longer durations of up to 3 seconds.
Nonstress test results are categorized as reactive or nonreactive. Various definitions of reactivity have been used. The most common definition of a reactive, or normal, NST is if there are two or more FHR accelerations (as previously defined) within a 20-minute period 25. A nonreactive NST is one that lacks sufficient FHR accelerations over a 40-minute period. The NST of the normal preterm fetus is frequently nonreactive: from 24 weeks to 28 weeks of gestation, up to 50% of NSTs may not be reactive 26, and from 28 weeks to 32 weeks of gestation, 15% of NSTs are not reactive 17 27 28. Thus, the predictive value of NSTs based on a lower threshold for accelerations (at least 10 beats per minute above the baseline and at least 10 seconds from baseline to baseline) has been evaluated in pregnancies at less than 32 weeks of gestation and has been found to sufficiently predict fetal well-being 29 30. Variable decelerations may be observed in up to 50% of NSTs 31. Variable decelerations that are nonrepetitive and brief (less than 30 seconds) are not associated with fetal compromise or the need for obstetric intervention 31. Repetitive variable decelerations (at least three in 20 minutes), even if mild, have been associated with an increased risk of cesarean delivery for a nonreassuring intrapartum FHR pattern 32 33. Fetal heart rate decelerations during an NST that persist for 1 minute or longer are associated with a markedly increased risk of both cesarean delivery for a nonreassuring FHR pattern and fetal demise 34 35 36. In this setting, the decision to deliver should be made with consideration of whether the benefits outweigh the potential risks ofI'll have to check the CPT book to get an actual code description for you.