Consequently, pathological loss of fetal heart rate acceleration may be seen in conjunction with signiicantly decreased beat to-beat variability (Chap. 24, p. 462). Loss of such reactivity, however, is most commonly associated with sleep cycles. It also rette smoking Oansson, 2005).
The nonstress test is based on the hypothesis that the heart rate of a fetus that is not acidemic as a result of hypoxia or to fetal movement. Fetal movements during testing are identi ied by maternal perception and recorded. As hypoxia develops, these fetal heart rate accelerations diminish (Smith, 1988).
Gestational age inluences acceleration or reactivity ofthe fetal heart rate. Pillai and James (1990b) studied the development of fetal heart rate acceleration patterns during normal pregnancy. The percentage of body movements that is accompanied by accelerations and the amplitude of these accelerations both increase with gestational age (Fig. 17-5). Guinn and colleagues (1998) studied nonstress test results between 25 and 28 weeks' gestation in 188 normal fetuses. Only 70 percent of these normal fetuses demonstrated the required 15 beats per minute (bpm) or more of heart rate acceleration. Lesser degrees of acceleration, that is, 10 bpm, occurred in 90 percent ofthe fetuses.
The National Institute of Child Health and Human Development Fetal Monitoring Workshop defined normal acceleration based on gestational age (Macones, 2008). In fetuses at or beyond 32 weeks' gestation, the acceleration acme is 15 bpm or more above the baseline rate, and the acceleration lasts 15 seconds or longer but less than 2 minutes. Before 32 weeks, normal accelerations are defined as having an acme that is 10 bpm or more above baseline for 10 seconds or longer. Cousins and associates (2012) compared the Workshop criteria recommended before 32 weeks, that is, 10 bpm/10 seconds, with standard 15 bpm/15 seconds criteria in a randomized trial of 143 women. They found no diferences in perinatal outcomes.
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FIGURE 17-5 Percentage offetuses tlith at least one acceleration of 15 bpm sustained for 15 seconds concurrent with fetal movement. (Redrawn from Pillai M, James 0: The development offetal heart rate patterns during normal pregnancy, Obstet Gynecol. 1990 Nov;76(5 Pt 1):812-816.)
Criteria to define normal nonstress test results difer. They vary regarding the number, amplitude, and duration of accelerations and the test duration. he definition recommended by the American College of Obstetricians and Gynecologists (2016) requires two or more accelerations peaking at 15 bpm or more above baseline, each lasting 15 seconds or more, and all occurring within 20 minutes of beginning the test (Fig. 17-6). It is also recommended that accelerations with or without fetal movements be accepted, and that a 40-minute or longer tracing-to account for fetal sleep cycles-should be performed before concluding that fetal reactivity is insuicient. Miller and coworkers (1996b) reviewed outcomes in fetuses with nons tress tests considered as nonreactive because there was only one acceleration. hey concluded
FIGURE 17-6 Reactive nonstress test. In the upper panel, notice the increase offetal heart rate by more than 15 beats/min for longer than 15 seconds following fetal movements, which are indicated by the vertical marks (lower panen.