NONSTRESS TEST

The nonstress test (NST) assesses the frequency of fetal movements using an
external fetal heart rate (FHR) monitoring device to detect the presence or
absence of accelerations. These are abrupt increases in FHR above the baseline
lasting <2 min and are unrelated to contractions. The criteria vary by gestational
age:

They are mediated by the sympathetic nervous system and always occur in
response to fetal movements. Interpretation: accelerations are always
reassuring.

<32 weeks,  the increase    should  be  ≥10 beats/min   lasting ≥10 s

>32 weeks,  the increase    should  be  ≥15 beats/min   lasting ≥15 s

Reactive    NST requires    the presence    of  2   accelerations   in  a   20  min window  of

time    meeting the above   criteria.   This    is  reassuring  and highly  predictive  for

fetal   well-being. Fetal   death   rate    is  only    3   per 1,000   in  the next    week.

Management  is  weekly  NST.

Nonreactive NST is  diagnosed   when    any criteria    for reactivity  are not met:

either  the number  of  accelerations   in  20  min or  the amplitude   or  duration    of

the acceleration.   80% of  nonreactive NSTs    are false   positives   (meaning    the

fetus   is  not hypoxemic). Nonhypoxemic    causes  include fetal   sleep,

prematurity,    drug    effects,    and CNS anomalies.  Management  is  fetal

vibroacoustic   stimulation to  see whether this    results in  reactivity. If  the NST is

persistently    nonreactive,    perform a   biophysical profile.