Читать книгу Protocols for High-Risk Pregnancies - Группа авторов - Страница 84

The NST is currently the most common means of evaluation of fetal oxygenation status during the antepartum period. Less intensive than the CST in many regards, the NST evolved as an excellent means of fetal assessment following observations that the occurrence of two or more accelerations of the fetal heart during a CST most often predicted a negative CST while the absence of these accelerations of baseline fetal heart rate was associated with a positive test and poor perinatal outcome. The basic premise of the NST is that the fetal heart will accelerate its rate with fetal movement if the fetus is not acidotic or depressed neurologically.

A reactive NST is defined by the presence of two or more accelerations of the fetal heart rate of at least 15 beats per minute lasting for at least 15 seconds within 20 minutes. Other definitions of reactivity have been proposed with a requirement of two or more accelerations in as little as 10 minutes before the test is considered reactive. If such accelerations are not elicited either spontaneously or with repeated vibroacoustic stimulation within 40 minutes of monitoring, the NST is interpreted as nonreactive. Options for further management include admission to hospital for delivery or extended monitoring or, more commonly, some form of backup test (e.g., a CST or BPP) is performed immediately. If the variable decelerations are repetitive or prolonged (lasting greater than one minute), the test is read as equivocal and a back‐up test is indicated at that time.

The NST has a false‐negative rate of approximately 0.3% but this is influenced by indication for test and testing interval. Of note, most nonreactive NSTs have a normal back‐up test, which allows continuation of the pregnancy in most instances. The current recommendation is that the NST should be performed at least twice weekly.