Biophysical profile fetal ultrasounds

Introduction to Biophysical profile fetal ultrasounds

For the medical transcriptionist: Synonyms and related keywords: antenatal ultrasound fetal surveillance, prenatal ultrasound, BPP, fetal asphyxia, fetal heart rate monitoring, FHR monitoring, FHR, amniotic fluid volume, AFV, antepartum fetal surveillance, biophysical profile score, BPS, nonstress test, antepartum testing, antenatal testing, prenatal testing.

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The Biophysical Profile Fetal Ultrasounds - biophysical profile (BPP)

This is a noninvasive test that predicts the presence or absence of fetal asphyxia and, ultimately, the risk of fetal death in the antenatal period. When the BPP identifies a compromised fetus, measures can be taken to intervene before progressive metabolic acidosis leads to fetal death.

The BPP combines data from two sources, ie, ultrasound imaging and fetal heart rate (FHR) monitoring. Dynamic realtime B-mode ultrasound is used to measure the amniotic fluid volume (AFV) and to observe several types of fetal movement. The FHR is obtained using a pulsed Doppler transducer integrated with a high-speed microprocessor, which provides a continuously updated reading.

Originally described by Manning and colleagues, the BPP has become a standard tool for providing antepartum fetal surveillance. The BPP integrates 5 parameters to yield a biophysical profile score (BPS) and includes (1) the nonstress test (NST), (2) ultrasound measurement of the AFV, (3) observation of the presence or absence of fetal breathing movements, (4) gross body movements, and (5) tone. Table 1 describes specific criteria for the BPS.

The BPP allows 2 points for each parameter that is present, yielding a maximum score of 10; however, if all the ultrasound variable findings are normal, the FHR variable may be excluded because no change is made in the predicative accuracy of the BPP by including the FHR. If one or more ultrasound variable findings are abnormal, the NST should be performed.

A basic principle of antepartum testing is that a more accurate prediction of fetal wellness is achieved in direct proportion to the number of variables considered. The BPP is a clinical tool that integrates levels of dynamic biophysical activities into a useable standard. The BPP allows 2 points for each parameter that is present, yielding a maximum score of 10; however, if all ultrasound variables are normal, the FHR variable may be excluded because no change is made in the predictive accuracy of the BPP by including the FHR. If one or more ultrasound variable is abnormal, the NST should be performed.

Table 1. Criteria for Coding Biophysical Profile Fetal Ultrasounds Variables as Normal or Abnormal

Biophysical Variable	Normal (Score = 2)	Abnormal (Score = 0)
Fetal breathing movements	1 or more episodes of >20 s within 30 min	Absent or no episode of >20 s within 30 min
Gross body movements	2 or more discrete body/ limb movements within 30 min (episodes of active continuous movement considered as a single movement)	<2 episodes of body/limb movements within 30 min
Fetal tone	1 or more episodes of active extension with return to flexion of fetal limb(s) or trunk (opening and closing of hand considered normal tone)	Slow extension with return to partial flexion, movement of limb in full extension, absent fetal movement, or partially open fetal hand
Reactive FHR	2 or more episodes of acceleration of >15 bmp* and of >15 s associated with fetal movement within 20 min	1 or more episodes of acceleration of fetal heart rate or acceleration of <15 bmp within 20 min
Qualitative AFV	1 or more pockets of fluid measuring >2 cm in vertical axis	Either no pockets or largest pocket <2 cm in vertical axis

*Beats per minute

The nonstress test
The NST is a noninvasive method used to evaluate fetal well-being. The NST is derived from observations that a fetus that is not acidotic and has an intact normally functioning autonomic nervous system will have periodic accelerations of the FHR. Acceleration is defined as a rise in the FHR-baseline rate that peaks at least 15 bmp above the baseline and lasts for at least 15 seconds from the beginning of the rise until the return to the FHR baseline.

Accelerations almost always occur with fetal movement. Partial umbilical cord compression with transient occlusion of the umbilical vein also can cause accelerations. This occurs with normal autonomic function, which acts to preserve cardiac output by increasing heart rate in response to decreased blood return to the fetal heart.

NSTs are described as either reactive or nonreactive. An NST is considered reactive if at least 2 accelerations are present in a 20-minute period. Occasionally, the NST may require 40 minutes or more of FHR recording to account for variations of the fetal sleep-wake cycle. An NST is considered nonreactive if sufficient accelerations are absent within a 40-minute period.

Decelerations of the FHR may be seen in as many as 50% of NSTs. If decelerations are nonrepetitive and less than 30 seconds in duration, obstetric intervention is not needed; however, repetitive decelerations or decelerations that last longer than 60 seconds are associated with an increased risk of fetal demise and cesarean delivery for the diagnosis of nonreassuring FHR pattern.

The pathophysiologic basis of the biophysical profile
Hypoxemia and acidemia have been shown to interfere with measures of central nervous system (CNS) performance, such as FHR patterns, fetal movement, and tone, in both animals and humans. Most likely, oligohydramnios results from decreased fetal urine production, which is seen with fetal hypoxemia as a result of blood flow redistributed away from the fetal kidneys and viscera in favor of the brain, heart, and adrenal glands.

Each of the movements evaluated in the BPP results from efferent signals originating in different CNS centers, which mature at different gestational ages (see Table 2). When activities known to originate from each of these oxygen-dependent centers are observed, it can be assumed that brain function is normal and systemic hypoxia is not present. Conversely, if one or more of the BPP activities is not observed within the prescribed observation period of 30 minutes, hypoxemia must be assumed to be the cause of the absence of that activity.

Table 2. Maturation of Central Nervous System Regulatory Centers

Activity	Gestational Age of Maturation (wk)
Gross body movements	6
Breathing movements	12-14
FHR accelerations resulting from fetal movement	18-20
Sleep-wake cycles	18-22
Integrated behavioral patterns	28

However, the clinical reality is that hypoxemia is the least likely reason for the absence of a particular activity. In most fetuses, absence of a particular activity results from normal variations in fetal movements. Usually, this results from fetal sleep-wake cycles, which are approximately 20 minutes in length. The observation period of 30 minutes was chosen arbitrarily to exclude the effects of the fetal sleep-wake cycle on the majority of biophysical activities. Table 3 shows a variety of factors, other than hypoxemia, that have been shown to affect different BPP parameters.

Table 3. Factors Affecting the Biophysical Profile

Activity	FHR Accelerations	Tone	Gross Movement	Fetal Breathing	AFV
Fetal sleep	↓	↓	↓	↓
Early gestational age (<33 wk)	↓			↓
Late gestational age (>42 wk)	↓	↓		↓	↓
Maternal glucose ingestion	-		-	↑
Maternal alcohol ingestion		↓/-		↑/-
Maternal magnesium administration	↓			↓
Artifical rupture of membranes				↓	↓
Premature rupture of membranes					↓
Labor				↓

Key: A blank box indicates that no data are available for that parameter. Horizontal lines indicate that the parameter has been studied and that no change is demonstrated. Arrows indicate that the parameter has been studied and that it increases or decreases as shown.

The Biophysical Profile Procedure
The ultrasound portion of the BPP should begin by noting the starting time. The profile may be completed when all of the variables have been observed; however, a full 30 minutes must elapse before the profile is judged to be abnormal. The scan should start with a general survey, noting the position of the fetus and the presence of cardiac activity. Although not part of the BPP, surveying the placental position and grade and the fetal morphology is common practice during observation of fetal activity.

AFV is assessed as normal if at least one or more pockets of fluid are detected that measure at least 2 cm along the vertical axis. Oligohydramnios is present if the largest pocket measures less than 2 cm.

The pressure applied to the transducer by the sonographer is inversely proportional to the depth of the fluid pocket. Careful attention to transducer pressure is required to avoid a false diagnosis of oligohydramnios, which can result from excessive transducer pressure compressing the maternal abdomen.

The amniotic fluid index
The AFI is a semiquantitative method for evaluating the AFV. The AFI is derived by adding the largest vertically measured fluid pocket from each uterine quadrant. This method appears to be at least as accurate as the largest–pocket-of-fluid method and can be reasonably substituted as an alternate method for evaluating AFV in the BPP. Using this method, oligohydramnios is defined as an AFI of less than 5.

To obtain an AFI, the mother must be in the supine position and the linear ultrasound probe must be parallel to the maternal spine and perpendicular to the floor for all measurements. The abdomen is divided into 4 quadrants, with the umbilicus delineating the upper and lower halves and the linea nigra delineating the left and right halves. The largest pocket of fluid in each quadrant is measured along the vertical dimension, which is the dimension perpendicular to the ultrasound probe. The pockets must be free of umbilical cord or fetal extremities, although brief appearances of these are acceptable.

The Modified Biophysical Profile
A modified BPP consisting of an NST and an AFI is used widely. If either the NST or the AFI is abnormal, a complete BPP or a contraction stress test (CST) is performed. The modified BPP, CST, and complete BPP have similarly low false-negative mortality rates, defined as the number of fetal deaths within 1 week of a normal test result. Nevertheless, no clear evidence exists that the 2 variables used in the modified BPP are better predictors than the other variables omitted from the BPP. Furthermore, this method requires 2 modalities for fetal evaluation, while normal ultrasound findings in a BPP eliminate the need for an NST.

Application of the biophysical profile
Antepartum testing using the BPP or any other method should not be performed earlier than the gestational age at which extrauterine survival or active intervention for fetal compromise is possible. Furthermore, no indications exist for testing in a fetus at term when likelihood of successful induction is high or when vaginal delivery is contraindicated for obstetric reasons. For patients with a low probability of successful induction, the BPP is a useful tool that can be used while waiting for cervical ripening. In these patients, the purpose of the BPP is to avoid the maternal morbidity resulting from failed induction followed by cesarean delivery.

The frequency of testing varies according to the clinical variables in each patient. In most high-risk pregnancies, testing plans start with weekly testing, although twice-weekly testing is the standard for pregnancies beyond 42 weeks and for patients with insulin-dependent diabetes. Frequency of testing increases in direct proportion to the severity of the maternal or fetal condition. In unstable pregnancies with progressive deterioration of the fetal condition, abnormal umbilical cord blood flow patterns occur first. Subsequently, FHR variation is reduced, followed by loss of breathing movements, while general fetal movements and tone are the last parameters to demonstrate abnormal results. Frequent assessment of fetal BPP movements may help prolong the pregnancy in fetuses with a marginally reduced FHR variation.

An abnormal BPS should prompt further evaluation or intervention depending on the circumstances. If an abnormal score occurs in a term fetus, preparation should be made for delivery. An abnormal score in a fetus who is remote from term requires conservative management, since the risk of fetal death is similar to the neonatal mortality rate resulting from prematurity. In these patients, daily testing often is performed. Table 4 provides general guidelines for treatment following the BPS.

Table 4. Recommended Fetal Treatment According to the Biophysical Profile Score

Result	Interpretation	Risk of Asphyxia* (%)	Risk of Fetal Death (per 1000/wk)	Recommended Treatment
10/10	Nonasphyxiated	0	0.565	Conservative
8/10 (normal AFV)	Nonasphyxiated	0	0.565	Conservative
8/8 (NST not performed)	Nonasphyxiated	0	0.565	Conservative
8/10 (decreased AFV)	Chronic compensated asphyxia	5-10 (estimate)	20-30	If mature (>37 wk), deliver If immature, serial testing (twice weekly)
6/10 (normal AFV)	Acute asphyxia possible	0	50	If mature (>37 wk), deliver If immature, repeat test in 24 h and if <6/10, deliver
6/10 (decreased AFV)	Chronic asphyxia with possible acute	>10	>50	Factor in gestational age If >32 wk, deliver If <32 wk, test daily
4/10 (normal AFV)	Acute asphyxia likely	36	115	Factor in gestational age If >32 weeks, deliver If <32 wk, test daily
4/10 (decreased AFV)	Chronic asphyxia with acute asphyxia likely	>36	>115	If >26 wk, deliver
2/10 (normal AFV)	Acute asphyxia almost certain	73	220	If >26 wk, deliver
0/10	Gross severe asphyxia	100	100	If >26 wk, deliver

*Umbilical venous blood pH less than 7.25

Reliability of the Biophysical Profile
The BPP is a reliable method of predicting fetal survival. Data have been collected on this and other antepartum testing procedures for more than 20 years. Testing methods usually are evaluated by comparing the false-negative mortality rate for each method. The false-negative mortality rate is defined as the number of fetal deaths, corrected for lethal congenital anomalies and unpredictable causes of demise, that occur within 1 week of a normal test result.

The BPP has a false-negative mortality rate of 0.77 deaths per 1000 tests. Furthermore, the BPS highly correlates with the antepartum fetal umbilical venous cord pH level. Cordocentesis performed immediately following a BPP demonstrated that a poor BPS was always associated with a pH of less than 7.20, while a score of 10 of 10 always yielded a pH of greater than 7.20. The false-negative mortality rate for NST alone is 1.9 per 1000 tests, more than twice that of the BPP. The modified BPP has a mortality rate of 0.8. The low false-negative rates of these testing methods depend on an appropriate response to an abnormal result. Intervention and retesting are the usual responses.