The Biophysical Profile Score

Course

Introduction

The evaluation of the fetal condition began in the 1960's when Hon (1967) developed a method to continuously record the fetal heart rate. In the 1970's real time ultrasound became available . Dawes (1972) demonstrated in the lamb model that the fetal respiratory center was quite sensitive to hypoxemia. Real-time ultrasound observations of fetal movements were also shown to predict fetal health (Manning, 1979).

The biophysical profile score was developed as a method to integrate real-time observations of the fetus and his/her intrauterine environment in order to more comprehensively assess the fetal condition. These findings must be evaluated in the context of maternal/fetal history (ie, chronic hypertension, post-dates, intrauterine growth restriction, etc), fetal structural integrity (presence or absence of congenital anomalies), and the functionality of fetal support structures (placental and umbilical cord). For example, acute asphyxia due to placental abruption may result in an absence of the acute variables of the biophysical profile score (fetal breathing movements, fetal movement, fetal tone, and fetal heart rate reactivity) with a normal amniotic fluid volume. With post maturity the asphyxial event may be intermittent and chronic resulting in a decrease in amniotic fluid volume, but with the acute variables remaining normal.

Periodicity of Biophysical Parameters

The absence of a biophysical variable may reflect normal periodicity, an inability of the central nervous system to perform that function, or pathologic (hypoxic) depression. The normal periodicity of most biophysical variables is between 20-40 minutes (Patrick, 1978). The initiation, persistence, or inhibition of biophysical activity may be affected by a host of outside influences. For example, fetal breathing movements may be stimulated by caffeine and hyperglycemia. Hypoglycemia, maternal supine hypotension, cigarette smoking, alcohol, diazepam and meperidine will all inhibit fetal breathing movements (Natale, 1978; Gennser, 1976).

Hypoxemia decreases activity in central nervous system centers either as a direct effect (ie, as oxygen delivery falls, cell function is depressed) or indirectly due to diminished perfusion (Manning 1995a). Fetal adaptation to hypoxemia (ie, a loss of biophysical variables) conserves oxygen consumption by approximately 17% (Rurak 1983). There is also a reflex redistribution of cardiac output in favor of the brain and heart.

The variables evaluated in the biophysical profile score are controlled by different anatomic sites within the brain. From an embryologic standpoint those variables that develop first are the most resistant to anoxia. Fetal movement and fetal tone develop between 7.5 and 9 weeks' menstrual age (Fitzgerald, 1942)(Fig 1)

Fig 1a. Fetal tone documented with hand extension. Click for larger image.

Fig 1b. Fetal tone documented with flexion. Click for larger image.

.Fetal breathing movements (Fig 2) are detectable by, at least 17-18 weeks' gestation. Fetal acidosis can result in a reduction in amniotic fluid volume by 17.5 weeks' gestation (Manning, 1993). The non-stress test is most reliable between 32 weeks and term (Ware, 1994). The sonographically evaluated variables, in contrast to the non-stress test, are valid from before a gestational age at which the fetus would be considered viable.

Fig 2. Fetal breathing movements (arrows) documented utilizing M-mode. Click for larger image.

By evaluating variables with differing sensitivities to anoxia, the biophysical profile score can estimate the level of deterioration of fetal status. The non-stress test and fetal breathing movements are suppressed when the pH falls below 7.2. If the fetal pH falls below 7.10, fetal tone and fetal movements are abolished (Vintzileos, 1987). The cumulative effect of repetitive hypoxemia is progressive oligohydramnios and fetal growth restriction. Hence, the presence of oligohydramnios with all of the other variables of the biophysical profile being normal may reflect chronic uteroplacental insufficiency.

Fetal State

Fetal state plays an important role in the interpretation of the biophysical profile score. In state IF (quiet sleep), the average time to obtain a normal biophysical profile score is 26.3 minutes. This is in contrast to 3-5 minutes when the fetus is in either state 2F (rapid eye movement sleep) or 4F (active state) (Pillai, 1990). The biophysical profile score is, therefore, continued for a maximum of 30 minutes.

There is a delay in central nervous system development with growth restriction. Hence, the distribution of fetal breathing movements, fetal movements and fetal eye movements (an assessor of fetal state) are different than in control fetuses. These differences are particularly striking when there is an absence of diastolic flow in the umbilical artery (Rizzo 1987).

Components of the Biophysical Profile Score

The results obtained by evaluating multiple fetal biophysical variables yields better results than single-variable monitoring. The high false-positive rate of any variable is greatly reduced when all the variables are combined (Manning 1980).

While the 5 components of the biophysical profile score have remained unchanged since 1980 (Manning, 1980), the definitions of a normal and abnormal parameter have evolved with increasing experience.

In 1984 the definition of oligohydramnios was increased from < 1cm pocket of fluid to < 2.0 x 1.0 cm pocket. Oligohydramnios is now defined as a pocket of amniotic fluid < 2.0 x 2.0 cm (Manning, 1995a)(Fig 3).

Fig 3. Single pocket of amniotic fluid of 4.0 cm (between graticules), indicating adequate amount of amniotic fluid. Click for larger image.

It is now recognized that the association between fetal breathing movements and the condition of the fetus is whether breathing movements are present or absent, not the duration of the activity (Table I). Fetal breathing movements may, therefore, be considered normal after any episode of clearly recognizable breathing movements. (Manning 1995c, Platt 1978).

If the four ultrasound variables are normal, the accuracy of the biophysical profile score was not found to be significantly improved by adding the non-stress test. As a result, in 1987 the profile score was modified to incorporate the non-stress test only when one of the ultrasound variables was abnormal (Manning 1987). Table I outlines the current definitions for quantifying a variable as present or absent.

Each of the 5 biophysical variables is currently scored as present (2 points) or absent (0 points). The total biophysical score obtained is either normal, equivocal, or abnormal (Table II). One advantage of the biophysical profile score as an antepartum test is the high percentage of normal test results. A normal biophysical profile score (8-10) occurs in 97.5% of cases (Table II)(Manning 1990b). There are fewer false positive tests with the biophysical profile score than with the non-stress test.

Perinatal Mortality and the Biophysical Profile Score

Studies evaluating the efficacy of the biophysical profile score have, in general, compared the management of high-risk patients with the biophysical profile score versus historical controls. The corrected perinatal mortality for 55,661 patients managed with the biophysical profile score was 1.86/1000 (Table III) versus 7.69/1000 for 104,337 historical controls - a decrease of 76% (Manning 1990a, Manning 1995b). The perinatal mortality within 1 week of a normal biophysical profile has remained around 0.8/1000 for over 10 years. It should be noted that the use of historical controls may lead to conclusions that are not subsequently supported by properly controlled comparative studies. Even when stringent criteria are established, differences can arise between groups selected at different points in time. Approximately 50% of studies using historical controls lead to an incorrect assumption that a particular treatment regime is effective (Sacks 1983). There have not been any randomized controlled trials comparing the biophysical profile score to controls that did not have antepartum testing. Four randomized trials evaluating a total of only 2,839 patients have compared the fetal biophysical profile to other types of antepartum fetal assessment. The use of the biophysical profile did not have a significant beneficial or deleterious effect on pregnancy outcome. The number of patients involved in this meta-analysis are insufficient to reach any definitive conclusions concerning the efficacy of the biophysical profile (Alfirevic 2000).

An evaluation of the perinatal mortality associated with a normal biophysical profile score revealed that 66.6% were due to congenital anomalies, 7.5% to severe Rh disease, and 25.9% were in structurally normal fetuses. Of the 8 structurally normal fetuses, 3 were growth restricted, 3 mothers had diabetes mellitus, and 2 women were hypertensive with postmature pregnancies (Manning 1985). In a more recent study Dayal (1999) determined the cause of stillbirth in 27 structurally normal fetuses that had a normal biophysical profile score within 1 week of fetal demise. The maternal and fetal causes of stillbirth (Table IV) were random events that would not be detected by the biophysical profile score. When Dayal (1999) compared results from two different facilities, the false negative rate (fetal death within 1 week of a normal biophysical profile score) was 1/10th the institution's perinatal mortality. The false-negative rates at the two facilities were 0.7/1000 and 2.3/1000. On an individual basis, the greater the maternal and fetal risk factors, the higher the false negative rate.

Each of the 5 components of the biophysical profile score do not have equal significance. Fetal breathing movements, amniotic fluid volume, and the non-stress test are the most powerful variables. For example, when the biophysical profile score is 2, the perinatal mortality varies between 428/1000 with only fetal movement present to 66/1000 if the non-stress test is reactive and all of the ultrasound parameters are absent (Manning 1990b). Some authors have, therefore, proposed utilization of a modified biophysical profile that incorporates only the non-stress test and amniotic fluid volume (Miller 1996). Although the positive predictive value of these 2 tests is equivalent to a biophysical profile score of 6, the perinatal mortality is still increased over a normal test score of 8 or 10 (Manning 1990b). The false positive rate with the modified biophysical profile score is also substantially higher.

A biophysical profile score of 0 is a rare event with a prevalence of approximately 1/1000 tests. In view of a reported perinatal mortality of 483/1000, a biophysical profile score of 0 should be considered a perinatal emergency (Manning 1990c). Perinatal death has occurred within 30 minutes to 11 days after a last biophysical profile score of 0 (Manning 1985).

Perinatal Morbidity and the Biophysical Profile Score

Perinatal morbidity, defined as a 5 minute Apgar score of < 7, intrauterine growth restriction, delivery for fetal distress, or an umbilical vein pH at delivery of < 7.2, increases in progressively larger increments as the biophysical score decreases (Manning 1995b). The utilization of the biophysical profile score in antepartum assessment has been associated with a significant reduction in the incidence of cerebral palsy when compared to an untested population (Manning 1998).

Biophysical Profile Score with Oligohydramnios

Rupture of the membranes does not alter the short-term sonographic variables of the biophysical profile in the healthy fetus. A low biophysical profile score predicts impending infection and/or fetal compromise. The presence of fetal breathing movements has the highest specificity in predicting an absence of fetal infection (Vintzileos 1985).

Biophysical Profile Score with Congenital Anomalies

The negative predictive value of a normal biophysical profile score is not as high with an anomalous fetus, in contrast to a structurally normal fetus. Sudden fetal deaths have been reported with a normal biophysical profile score in fetuses with gastroschisis, omphalocele, and diaphragmatic hernia. The observation of an abnormal biophysical profile in an anomalous fetus does not correlate very well with the presence of hypoxia. In fetuses with a chromosomal abnormality, brain disorganization may affect the normal function of the regulatory centers. With a central nervous system anomaly (holoprosencephaly, hydrocephalus, etc), the absence of a biophysical variable may be due to altered perfusion of a regulatory center. The biophysical profile score cannot be used in fetuses with congenital muscular diseases or central nervous system conditions that would affect muscular function. If an anomalous fetus had a previously normal biophysical profile score, a decreasing score should be considered an indication of compromise (Manning 1995d).

Biophysical Profile Score with Rh Sensitization

In severe Rh sensitization there is cardiomegaly and an increased cardiac output. Just as with growth restricted fetuses, the umbilical vein pH falls significantly as the biophysical profile score decreases. In the severely anemic fetus, the effect of transfusion and increased oxygen delivery to the fetal brain is a return of the short-term sonographic variables (breathing movements, fetal movements, fetal tone) within minutes of initiating the transfusion (Manning 1990c, Manning 1993). A deteriorating biophysical profile score, despite intravascular transfusion, should prompt repeat cordiocentesis since unrecognized bleeding from the puncture site may have occurred.

Conclusions

The fetus expresses its well being or compromised status through a number of different biophysical activities that are controlled by different central nervous system centers. The utilization of the biophysical score for antepartum surveillance in high-risk patients has resulted in a reduction in perinatal mortality when compared to historical controls. The appropriate management of the viable fetus with an abnormal biophysical profile score may also decrease long-term neurologic morbidity (Manning 1998).

It is unlikely that in the future additional variables will be added to the biophysical profile score. However, perhaps the incorporation of the fetal state (ie, eye movements) and Doppler flow studies of specific fetal vessels (umbilical artery, middle cerebral artery, ductus venosus) will be incorporated into a complete assessment of the fetal condition.

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