Antepartum and Intrapartum Surveillance of the Fetus and the Amniotic Fluid



Antepartum and Intrapartum Surveillance of the Fetus and the Amniotic Fluid


Maria Andrikopoulou

Michael G. Ross

Anthony M. Vintzileos



Introduction

The challenge of assessing the intrauterine fetus as a patient lies in the inability to perform a truly direct examination. Instead, one must rely on indirect modalities such as dynamic, high-resolution ultrasonography, electronic fetal heart rate (FHR) monitoring, and fetal Doppler velocimetry. Fortunately, technological and scientific advances over the years have permitted more specific examinations of the fetus and its behavior. One of the most important goals of perinatal medicine is to recognize fetal disease states, optimize fetal outcomes, and prevent perinatal morbidity and mortality by interventions and/or timing of delivery. Because fetal jeopardy has a diverse etiology, forms of testing must also be able to survey both acute and chronic disease states. Therefore, surveillance of the fetus during the antepartum and intrapartum periods is an important component of this process. In the past, both the method and the frequency of antepartum testing were arbitrary and generalized. However, it is apparent that tailoring testing to each disease-specific state is not only practical, but also more appropriate. By utilizing the information from fetal surveillance, conservative management may be possible and may allow continued maturation in utero, while reducing the potential of neonatal prematurity complications. This chapter will discuss the current techniques available to survey the fetus and amniotic fluid during both antepartum and intrapartum periods and present older and newer data and studies on antepartum and intrapartum fetal surveillance.


Antepartum Surveillance Techniques (Fetus and Amniotic Fluid)

There are multiple maternal and fetal indications to perform antepartum surveillance (Table 21.1), although these are not absolute. The common basis for selecting these patients are those who are at increased risk of perinatal mortality due to a variety of pathophysiologic processes, including uteroplacental insufficiency, intra-amniotic infection, fetal anemia, fetal heart failure, metabolic causes, and umbilical cord compression, to name a few. Many surveillance methods rely on natural fetal behavior. Although the mechanisms controlling sleep and activity cycles in the fetus are not well understood, knowledge of these behaviors is imperative to appropriately interpret FHR monitoring and fetal biophysical profile (BPP) activities.

In the near-term fetus, there are four behavioral states (occurring repeatedly and stable over time) that have been described: quiet sleep, active sleep, quiet awake, and active awake.1 Quiet sleep is characterized by limited eye or breathing movements, infrequent startle-type body movements, reduced FHR variability, and no accelerations. Active sleep is characterized by frequent gross body movements, rapid eye movements, breathing, normal FHR variability, and accelerations. The fetus predominantly spends its time in either a quiet or an active sleep state.2

The efficacy of the various antenatal fetal surveillance tests depends on the underlying pathophysiologic condition.3 No test is ideal for all high-risk fetuses. Therefore, multiparameter assessment or a
combination of different tests may often be the optimal strategy to determine the cause of suspected anomalies and make informed decisions regarding pregnancy management. Table 21.2 shows the various pathophysiologic processes, examples of maternal/fetal conditions, and the specific surveillance tests that may be the most appropriate.










Contraction Stress Test

Historically, a contraction stress test (CST) was the first antepartum fetal heart test used to detect uteroplacental insufficiency. This test is based on the response of the FHR to uterine contractions and relies on the premise that fetal oxygenation will be transiently worsened by contractions, which primarily occurs intrapartum. Therefore, in a suboptimally oxygenated fetus, the resultant intermittent worsening in oxygenation will, in turn, lead
to the FHR pattern of late decelerations.20 This test is rarely used today because it is cumbersome, is costly, has a high false-positive rate, and has been replaced by noninvasive tools such as the NST and Doppler velocimetry. However, the CST may be of particular value in preterm fetuses who demonstrate intermittent, “spontaneous” heart rate decelerations to assess the potential for uteroplacental insufficiency.








Although no longer standard practice, a brief mention of CST techniques and possible test results bears mention here.









Technique

Lying in a lateral recumbent position, the patient has an external fetal monitor recording both the FHR and uterine contractions simultaneously for a 20- to 30-minutes interval. If the patient is spontaneously contracting, the frequency is ≥3 contractions/10 minutes, and the duration of each contraction is ≥ 45 seconds, then uterine stimulation is not required. However, if these criteria are not met, then either nipple stimulation21 or exogenous oxytocin can be used to elicit contractions. Once adequate contractions are achieved, stimulation is discontinued.



Nonstress Test

The purpose of the NST is to identify healthy fetuses and prolong preterm pregnancies, as well as identify those in jeopardy so that timely intervention can improve outcomes. This testing modality is based on the premise that the heart rate of the fetus that is not acidemic or neurologically depressed will temporarily accelerate with fetal movement. FHR reactivity is speculated to be a good indicator of normal fetal autonomic function and well-being; it relies on normal brainstem neurological development and normal integration of the central nervous system (CNS) control of FHR. In contrast, loss of reactivity is most commonly associated with a sleep cycle, but can result from any cause of CNS depression, including fetal acidemia, infection, or anemia. Compared with the CST, the NST is faster, is easier to interpret, lacks contraindications, and is most commonly used in contemporary practice.36


Technique

FHR is monitored using a Doppler ultrasound transducer, while a tocodynamometer may be used to record any uterine contractions. Fetal activity is also recorded on the strip; however, the patient does not need to document fetal movement for the test to be interpreted. Less than 1% of NSTs provide unsatisfactory results owing to inadequate recording of the FHR tracing.37 Technical difficulties that may be encountered include obesity, fetal hiccups, excessive fetal activity, and polyhydramnios.29



Vibroacoustic Stimulation

Because the majority of nonreactive NSTs occur in healthy fetuses secondary to a physiologically normal sleep state, fetal stimulation has been used as a way to distinguish normal fetal sleep from fetal compromise. Vibroacoustic stimulation (VAS) is an effective technique to improve the efficiency of antepartum FHR testing because it safely reduces the time needed to perform an NST without compromising detection of the sick fetus.60


Technique

To perform a VAS, an artificial larynx is positioned on the maternal abdomen over the fetal vertex with a stimulus of 1 to 2 seconds. This procedure may be repeated up to three times (at 1-minute intervals) for progressively longer durations of up to 3 seconds to elicit FHR accelerations. The increases in intrauterine sound decibels created by VAS are thought to be safe and harmless to the fetus.



Biophysical Profile

The BPP is unique in that it assesses both acute (ie, FHR reactivity, fetal breathing movements, fetal movements, fetal tone) and chronic (ie, AFV) markers of fetal condition. It is a noninvasive modality with a high negative predictive value for adverse perinatal outcomes.34,64 When used in conjunction with other surveillance and testing measures, the BPP helps to assess fetal hypoxia and acidemia, presence of infection in patients with premature rupture of membranes (PROM), placental dysfunction, and stillbirth.

The BPP is based on the premise that the biophysical activities developed last in utero are the first to become abnormal in the presence of fetal acidemia or infection.65 This theory, the gradual hypoxia concept, was proposed by Vintzileos et al in 1983. At about 7.5 weeks, the CNS center controlling fetal tone is the first to develop, followed by development of body movement at 8.5 to 9.5 weeks. The center controlling regular breathing movements develops after 20 to 21 weeks, and the
center controlling FHR reactivity functions by the end of the second/beginning of the third trimester. Therefore, in accordance with the gradual hypoxia concept, early stages of compromise are revealed by abnormalities in FHR reactivity and breathing, whereas movement and tone are not abolished until much later stages of compromise.


Technique

The BPP is performed using real-time ultrasonography to assess multiple fetal biophysical activities as well as AFV. Sonographic observation is continued until either normal activity is seen or after 30 consecutive minutes of scanning have elapsed. The interval of BPP testing frequency (1-2 per week) is arbitrary; however, it is often a matter of individual clinical judgment, training, preferences, and experience. An advantage of the BPP over the NST is that observations of fetal movement and breathing (or their absence) are unequivocal, and usually there are no interobserver discrepancies in interpretation.

The modified BPP is composed of the NST (an acute indicator of fetal acid-base status) and AFV (indicator of chronic uteroplacental function). It is used by many centers as a primary mode of surveillance. Using the modified BPP to assess perinatal morbidity/mortality compares favorably with using the BPP alone. Algorithm 21.1 shows our suggested protocol for the modified BPP. By using this protocol in 17,211 tests, we had only four deaths of normal fetuses, with a false-negative rate of 0.02%.66 Another study compared the outcomes of high-risk patients whose last antepartum assessment was a negative CST or negative modified BPP.67 The incidence of adverse perinatal outcome, after reassuring testing, was significantly less in those managed by the modified BPP than in the CST group (5.1% vs 7.0%).








Amniotic Fluid Volume Assessment

Amniotic fluid (AF) is essential to pregnancy, providing an environment for normal development, growth, and movement of the fetus. AFV is a chronic marker of fetal well-being, and a normal AFV also protects the fetus from cord compression during fetal activity or uterine contractions. This volume changes during pregnancy: at 22 weeks, the average AFV is 630 mL, and this increases to 770 mL at 28 weeks87 (Figure 21.3). Between 29 and 37 weeks, there is little change in volume, which averages 800 mL. Beyond 39 weeks, AFV decreases sharply (average 515 mL at 41 weeks). At postdates, there is a 33% decline in AFV per week, consistent with clinical observations of an increased incidence of oligohydramnios in postterm gestations.88 Surveillance of the AFV is crucial, as fluid disorders may not only represent but also lead to fetal disease states.

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Jun 19, 2022 | Posted by in OBSTETRICS | Comments Off on Antepartum and Intrapartum Surveillance of the Fetus and the Amniotic Fluid
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