FIGURE 81.1 Reactive nonstress test. This nonstress test is reassuring, indicating fetal well-being.
FIGURE 81.1 Reactive nonstress test. This nonstress test is reassuring, indicating fetal well-being.
Fetal heart rate decelerations can also be appreciated on an NST or during continuous fetal monitoring during labor. Decelerations occur when the fetal heart rate falls below the baseline heart rate; they are classified according to their appearance and location in relation to uterine contractions. Different types of decelerations are caused by different mechanisms. Therefore, each type of deceleration has different implications for fetal status.
- Early decelerations: Early decelerations begin at the onset of uterine contractions and appear to mirror the contraction (Fig. 81.2) (29). They are believed to be caused by pressure on the fetal head. This pressure results in an alteration in cerebral blood flow and stimulation of the vagal center, causing parasympathetic stimulation and a subsequent decrease in the fetal heart rate. Early decelerations are thought to be benign and generally are not associated with fetal hypoxia, acidosis, or low Apgar scores.
- Variable decelerations: Variable decelerations are abrupt decreases in the fetal heart rate at least 15 bpm below baseline with the onset to nadir lasting less than 30 seconds. Variable decelerations do not necessarily correlate with contractions (Fig. 81.3) (29). They are thought to be caused by intermittent umbilical cord compression. “Shoulders” can be seen both preceding and following these variable decelerations, and should not be considered accelerations, as they are a manifestation of the increase in sympathetic nervous system stimulation during fetal heart rate decelerations. Mild or isolated variable decelerations are benign. Repetitive moderate or severe variable decelerations may indicate fetal compromise.
- Late decelerations: Late decelerations occur late in relation to the uterine contraction. Their onset begins after the contraction begins, and they resolve after the resolution of the contraction (29) (Fig. 81.4). These decelerations occur as a result of decreased uteroplacental oxygen delivery to the fetus, but they may not necessarily signify poor placental function—late decelerations may be caused by maternal hypotension or decreased uterine blood flow. Persistence of late decelerations, especially in the absence of baseline fetal heart rate variability, is an ominous sign of fetal compromise.
- Prolonged decelerations: A prolonged deceleration is any deceleration at least 15 bpm below the baseline that lasts for 2 to 10 minutes (29). Repetitive prolonged decelerations are cause for concern for fetal compromise. Any deceleration lasting longer than 10 minutes is considered a change in baseline if the new rate is greater than 110 bpm, or bradycardia if the new heart rate is less than 110 bpm.
The last aspect of the fetal heart rate tracing analyzed on an NST is heart rate variability, or the fluctuations in the fetal heart rate seen outside of accelerations or decelerations (29). Variability is described as absent (no fluctuation), minimal (<5 bpm), moderate (5 to 15 bpm), or marked (>25 bpm) (29). In addition to causing the NST to be nonreactive, fetal sleep can be a reason for decreased variability, but persistently minimal or absent fetal heart rate variability is the most significant sign of fetal compromise (30). When evaluating all of these aspects of an NST, a normal result is a reactive NST with a baseline between 110 and 160 bpm, no decelerations, and moderate variability.
The other (bottom panel) line on the NST represents the tocodynamometer, a strain gauge placed on the maternal abdomen used to measure uterine contractions. Though frequency and duration of contractions can be assessed with a tocodynamometer, the strength of a contraction cannot be assessed without an intrauterine pressure catheter (IUPC), placement of which requires ruptured amniotic membranes.
Contraction Stress Test
The contraction stress test (CST) is cardiotocography evaluated during spontaneous or induced contractions. It may be used as a follow-up to a nonreactive NST. The test requires at least three contractions during a 10-minute window and evaluates the fetal heart rate response to these contractions. Because of the necessity of uterine contractions, this test is contraindicated in various situations, including significantly preterm gestations and those in whom labor is contraindicated. The underlying premise for this test involves the idea that fetal oxygenation will transiently worsen in the presence of uterine contractions. In the already-compromised fetus, this will result in late decelerations. The CST is interpreted based on the presence or absence of late decelerations. A positive CST is one in which late decelerations occur with at least 50% of contractions and generally indicates that delivery is warranted. A negative test result (with no late decelerations) is highly reassuring, with a false-negative rate of only 0.3 in 1,000 (27) (Fig. 81.5). In practice, CST is infrequently utilized today, because the test is resource intensive and has more frequent contraindications than other forms of antenatal testing.
The biophysical profile (BPP) consists of an NST and an ultrasound examination. This test can be performed as a follow-up test to a nonreactive NST or can be used as a primary form of surveillance. Ultrasound is used to evaluate fetal tone, gross body movements, breathing movements, and amount of amniotic fluid present. The score is derived from these various assessments (Table 81.1). The false-negative rate of a BPP of 8/10 or 10/10 is 0.8 per 1,000 (27). However, as with all antenatal testing, many factors may alter the results of the BPP including maternal sedation, drug use, or hypoglycemia.
|TABLE 81.1 Components and Scoring of the Biophysical Profilea|
As an adjunct to antenatal testing with NST, CST, or BPP, Doppler ultrasound is used to address specific concerns in fetuses at high risk for stillbirth. In growth-restricted fetuses, Doppler evaluation of flow in the umbilical artery is utilized. Diminished diastolic flow signifies placental insufficiency and increased resistance to flow in the placenta. Absent end-diastolic flow indicates the need for delivery if the gestational age is 34 weeks or more. Reversed end-diastolic flow signifies even more critical resistance to flow and is an indication for delivery even earlier (31).
The other clinical situation in which Doppler is routinely used is for evaluation of fetuses at high risk for anemia. In this situation, elevated peak systolic velocity in the middle cerebral artery correlates with fetal anemia, and assessment of middle cerebral artery Doppler is used to help determine the need for percutaneous umbilical blood sampling. Umbilical blood sampling directly assesses fetal hematocrit and, if anemia is confirmed, intrauterine fetal blood transfusion can be done (32,33). Current recommendations are to plan for fetal blood sampling and possible transfusion when the peak systolic velocity is more than 1.55 multiples of the median for the given gestational age (34). Amniocentesis, to assess indirectly the bilirubin concentration in amniotic fluid, previously was the mainstay for monitoring fetuses at risk for anemia; middle cerebral artery Doppler interrogation has almost completely replaced amniocentesis for that indication.
Intrapartum Fetal Surveillance
Intrapartum fetal surveillance is cardiotocography during labor. The same parameters evaluated on an NST are used in an ongoing fashion for intrapartum fetal surveillance. However, fetal heart rate tracings in the intrapartum period are described according to three categories (35):
- Category I: Baseline fetal heart rate 110 to 160 bpm; moderate fetal heart rate variability accelerations may be present or absent; no late or variable decelerations; may have early decelerations
- Category II: All fetal heart rate patterns that are not category I or category III
- Category III: Absent variability with recurrent late decelerations, absent variability with recurrent variable decelerations, or bradycardia
These categories were developed to help determine the need for treatment. However, because most tracings are category II, utility of this classification system is limited, and some authors have advocated further stratification of category II (36,37). Consideration should also be given to the tocodynamometer (or IUPC tracing) during labor. Tachysystole is the term for more than five contractions in 10 minutes, averaged over 30 minutes. Tachysystole may occur as a result of oxytocin administered to stimulate contractions during labor inductions or augmentation of spontaneous labor. If tachysystole occurs spontaneously, it may signify placental abruption.
Abnormal Antenatal Testing
All tests used for antenatal testing (NST, CST, BPP) have low positive predictive values and high negative predictive values. That is, they all have a low rate of false-negative results but a high rate of false-positive results. Therefore, if antenatal testing is concerning, repeat testing is appropriate. Generally, an alternative test is used as the method of repeat testing. For example, if the NST was nonreactive, a BPP would be performed (Table 81.2). If the second test is reassuring, the likelihood of fetal acidemia is low, and repeat testing would be scheduled within the next few days to a week. However, if repeat testing also is nonreassuring, the approach generally would be to proceed with delivery. The mode of delivery would be determined by the degree of concern caused by antenatal testing results and obstetric factors. However, in some cases, fetal testing will be abnormal due to maternal disease, and these fetal testing abnormalities may resolve with maternal stabilization. Some specific examples of maternal situations where special consideration is required in interpreting fetal testing are addressed below:
- Maternal fever: Maternal fever, regardless of cause, generally causes fetal tachycardia. In this situation, fetal tachycardia may not be an indication for delivery but rather treatment of maternal infection and ongoing fetal monitoring.
- Sickle cell crisis: Sickle cell crisis can cause fetal hypoxia due to the decreased maternal oxygen–carrying capacity, and maternal treatment with oxygen, intravenous fluid resuscitation and blood transfusion will frequently improve fetal testing.
- Maternal hypoxia: Once maternal hypoxia is corrected, the fetus will frequently resuscitate in utero with return to reassuring fetal testing over an hour or two.
|TABLE 81.2 Biophysical Score and Recommended Interventions|