Antepartum Fetal Monitoring

•To learn a systematic approach for describing and interpreting the results of the fetal heart rate tracing.

•To learn the techniques for performing the components of the modified biophysical profile and the full biophysical profile.

•To review the perinatal mortality associated with each of the possible biophysical profile scores.

•To review the association between the presence or absence of each of the components of the biophysical profile score and acidosis.

•Learn the appropriate response for test results based on procedures and policy established by your department.

Objectives

•The goal of antepartum fetal surveillance is to prevent fetal death. •The MBPP is an abbreviated biophysical profile that includes only the nonstress test (NST) and amniotic fluid index (AFI). A normal result is a reactive NST with an AFI > 5.0 cm. •In one series the stillbirth rate, corrected for lethal congenital anomalies and unpredictable causes of demise was 0.8 per 1,000 in 54,617 modified BPPs, and the negative predictive value of the modified BPP was greater than 99.9%.

Antepartum Fetal Surveillance

The nonstress test (NST) is performed by auscultation of the fetal heart rate using an electronic monitor.

The Nonstress Test

The Nonstress Test

The ultrasound probe transmits the

fetal heart rate in beats per minute. •Each small vertical square is 10 beats. •Each small horizontal square is 10 seconds .•Each large horizontal square is 1 minute .

The pressure transducer

transmits the pressure generated

by uterine contractions in mm Hg.•Each small vertical square is 5 mm Hg •Each small horizontal square is 10 seconds . •Each large horizontal square is 1 minute . Pressure Transducer

Ultrasound Probe

The interpretation of the fetal heart rate tracing should follow a systematic approach with a full qualitative and quantitative description of the following:

•Baseline rate •Baseline fetal heart rate (FHR) variability •Presence of accelerations •Periodic or episodic decelerations •Changes or trends of FHR patterns over time •Frequency and intensity of uterine contractions

Interpretation of the Fetal Heart Tracing

The baseline FHR is the heart rate during a 10 minute segment rounded to the nearest 5 beat per minute increment excluding periods of marked FHR variability, periodic or episodic changes, and segments of baseline that differ by more than  25 beats per minute.

•The minimum baseline duration must be at least 2 minutes. • If minimum baseline duration is < 2 minutes then the baseline is indeterminate.

Baseline Fetal Heart Rate

Baseline Fetal Heart Rate

Two Minutes

Baseline Fetal Heart Rate

Baseline change: The decrease or increase in heart rate lasts for longer than 10 minutes.

Bradycardia

Mean FHR < 110 BPM A rate of 100-119 BPM in the absence of other non reassuring patterns is not usually a sign of compromise

Causes Heart block (little or no  variability) Occiput posterior or transverse positionSerious fetal compromise.

Patients with new onset bradycardia should be transferred to Labor and Delivery for further observation and physician notified

Tachycardia

Mean FHR>160 BPM In the presence of good variability tachycardia is not a sign of fetal distress

Causes:Maternal feverFetal hypoxiaFetal anemia AmnionitisNormal variant Fetal tachyarrhythmia (usually > 200 BPM with abrupt onset little to no variability)

SVT (200-240 BPM)Fetal heart failureDrugs • Beta sympathomimetics • Vistaril• PhenothiazinesRebound transient tachycardia

following a deceleration accompanied by decreased variability)

Tachycardia

FHR Variability

Fluctuation in baseline FHR > 2 cycles per minute.

No distinction is made between short-term variability (or beat-to-beat variability or R-R wave period differences in the electrocardiogram) and long-term variability.

Development of FHR Variability

Early in gestation the fetal heart rate is predominately under the control of the sympathetic nervous system and arterial chemoreceptors. 

As the fetus develops its heart rate decreases in response to parasympathetic (vagal stimulation) nervous system maturation and variability becomes more pronounced.

FHR Variability

Grades of fluctuation are based on amplitude range (peak to trough)– A sinusoidal pattern has

regular amplitude and frequency and is excluded in the definition of variability.

• The tracing to the right shows an amplitude range of ~ 10 BPM (moderate variability).

FHR Variability

Absent variability = Amplitude range undetectable

Minimal = < 5 BPM

Moderate = 6 to 25 BPM

Marked = > 25 BPM

How Hypoxia (Low Oxygen) Leads to Acidosis and Decreased FHR Variability

The fetus uses oxygen to "burn" molecules and release energy.

The reaction: glucose + oxygen >> carbon dioxide + water + energy

Poor blood flow from the uterus and placenta causes the fetus to constrict blood vessels in nonvital peripheral areas such as the arms and legs in order to supply more blood flow to vital organs such as the heart and brain.

How Hypoxia (Low Oxygen) Leads to Acidosis and Decreased FHR Variability

With a limited supplies of oxygen (hypoxia) the peripheral tissues can only partially break down the sugar and converts it to lactic acid.

Significant levels of acid in the blood (acidemia) may suppress the fetal nervous system and eventually lead

to cardiovascular collapse.

The Presence of Moderate FHR Variability is Reassuring .

•Persistently minimal or absent FHR variability appears to be the most significant intrapartum sign of fetal compromise.

•On the other hand  the presence of good FHR variability may not always be predictive of a good outcome (such as may occur with an abruption).

• Fetal metabolic acidosis

• CNS depressants

• Fetal sleep cycles

• Congenital anomalies

• Prematurity

Causes of Decreased Variability

•Fetal tachycardia•Preexisting neurologic abnormality •Normal variant•Betamethasone

Sinusoidal and Pseudosinusoidal Patterns

Sinusoidal pattern: A smooth, undulating pattern, lasting at least 10 minutes with a fixed period of three to five cycles per minute and an amplitude of 5-15 bpm.

Pseudosinusoidal:Usually caused by drugs such as Nubain or Stadol.

An acceleration is an abrupt increase in FHR above baseline with onset to peak of the acceleration less than < 30 seconds and less than 2 minutes in duration. The duration of the acceleration is defined as the time from the initial change in heart rate from the baseline to the time of return to the FHR to baseline.

• <32 weeks' : >10 BPM above baseline for >10 seconds [3]

• >32 weeks' : >15 BPM above baseline for > 15 seconds[3].

• Prolonged acceleration: Increase in heart rate lasts for  2 to 10 minutes.

• The absence of accelerations for more than 80 minutes correlates with increased neonatal morbidity.

Accelerations

A NST is considered reactivewhen two or more fetal heartrate accelerations peak (but donot necessarily remain) at least15 beats per minute above

the baseline and last 15 seconds from baseline to baseline within a 20 minute period with or

without fetal movementdiscernible by the woman.

Reactivity

If the pattern is nonreactive after 20 minutes of observation then vibroacoustic stimulation (VAS), using an artificial larynx, may be performed.

The acoustic stimulator should be positioned on the maternal abdomen and a stimulus of 3 seconds or less applied near the fetal head. If the NST remains nonreactive, the stimulus is repeated at 1-minute intervals up to three times.

Vibroacoustic stimulation (VAS)

• A reactive NST has been associated with a perinatal mortality of approximately 5/1,000.

• The false-positive rate (the test indicates fetal compromise when the fetus is actually O.K.) associated with the nonreactive NST is approximately 75 to 90 percent

• Malformed fetuses have a higher incidence of nonreactive NSTs.

How Reassuring is a Reactive NST?

• A nonreactive NST is one that lacks sufficient fetal heart rate accelerations as described above over a 40-minute period.

Overall, on initial testing, 85 percent of NSTs will be reactive and 15 percent will be nonreactive

Most fetuses exhibiting a nonreactive NST will not be compromised but will simply fail to exhibit heart rate reactivity during a 40-minute period of testing .

~50 percent of NSTs are nonreactive between 24 and 28 weeks' gestation.

~15 percent of NSTs remain nonreactive between 28 and 32 weeks.

Likelihood of a Nonreactive NST

A non‑reactive NST requires that a biophysical profile be done.

• Episodic patterns are those not associated with uterine contractions

• Periodic patterns are those associated with uterine contractions.

Early and late decelerations (with some exceptions-i.e., supine hypotension) are periodic.

Variables can also be periodic.

• Quantitated by the depth of the nadir in BPM below the baseline.

• Duration is quantitated in minutes and seconds from the beginning to the end of the deceleration.

– (Accelerations are quantitated similarly.)

Periodic or Episodic Decelerations

The type of the deceleration is distinguished on the basis of its waveform. Abrupt (variables) decrease in FHR below baseline

with onset to nadir < 30 seconds. Gradual decrease and return to baseline with time

from onset of the deceleration to nadir >30 seconds. Further subclassified based on their relation to

the contraction. Early decelerations: The nadir occurs with

the peak of a contraction. Late decelerations:The nadir occurs after the

peak of a contraction.

Decelerations

Abrupt  decrease in FHR of > 15 beats per minute measured from the most recently determined baseline rate.  The onset of deceleration to nadir is less than 30 seconds. The deceleration lasts   > 15 seconds and less than 2 minutes. A shoulder, if present, is not included as part of the deceleration.

Variable decelerations may be observed in up to 50% of NSTs. If nonrecurrent and <30 seconds, they are of no clinical significance.

Variable Deceleration

Gradual decrease in FHR with onset of deceleration to nadir >30 seconds. The nadir occurs with the peak of a contraction.

Early Deceleration

Gradual decrease in FHR with onset of deceleration to nadir >30 seconds. The nadir of the deceleration occurs after the peak of the contraction

Late Deceleration

These decelerations appear to be mediated by arterial chemo receptors in mild hypoxia.

Below a pO2 of 15-20 mm Hg chemoreceptors are triggered causing reflex alpha adrenergic stimulation leading to hypertension.

The hypertension stimulates a baroreceptor mediated vagal response (deceleration)

The onset of reflex late decelerations typically precedes the loss of accelerations

Late Decelerations

Late Decelerations associated with preservation of beat-to beat variability

Lates associated with no variability (where loss of variability has not been caused by drug administration) With progressing hypoxia, the decelerations become deeper.

As acidosis develops the brain stem reflexes become blunted and direct myocardial depression causes shallow decelerations [20,22].

If myocardial depression is severe enough, lates may be absent all together

CAUSES Excessive uterine contractions (hyperstimulation), maternal

hypotension, or maternal hypoxemia.

Reduced placental exchange as in hypertensive disorders, diabetes, IUGR, abruption.

Late Decelerations

Late Decelerations

From: Sweha A, Hacker TW, Nuovo J. Interpretation of the electronic fetal heart rate during labor.Am Fam Physician. 1999;59:2487-500

Late Decelerations

Management

• Place patient on side •Administer O2 by tight face mask •Discontinue oxytocin. •Correct any hypotension •IV hydration. •If hyperstimulation is present consider terbutaline 0.25 mg SC •If late decelerations persist for more than 30 minutes despite the above maneuvers, fetal scalp pH is indicated.

•Scalp pH > 7.25 is reassuring, pH 7.2-7.25 may be repeated in 30 minutes. •Deliver for pH < 7.2 or minimal baseline variability with late or prolonged decelerations and inability to obtain fetal scalp pH

These maneuvers are primarily intended to alleviate "reflex" lates.

Decelerations occur with > 50% of uterine contractions in any 20 minute segment.

– Recurrent variable decelerations (at least 3 in 20 minutes) may be observed. However, close follow up is recommended because cord accidents with subsequent fetal death may occur even in the presence of normal amounts of amniotic fluid.

– Recurrent late decelerations should lead to consideration of cesarean delivery unless the abnormal results are believed to be the result of a reversible maternal condition such as diabetic ketoacidosis or pneumonia with hypoxemia.

Recurrent Decelerations

• Maternal hypotension

• Uterine hyperactivity

• Cord prolapse

• Cord compression*

Prolonged Decelerations

A decrease in FHR of > 15 beats per minute measured from the most recently determined baseline rate. The deceleration lasts >= 2 minutes but less than 10 minutes.

Causes:

• Rapid descent of fetal head

• Abruption

• Artifact (maternal heart rate

• Maternal seizure

Prolonged Decelerations

Patients with a fall in the fetal hear rate of 15 PBM below the baseline for 1 minute or longer should be considered for transfer to Labor and Delivery for further observation and physician notified

The Contraction Stress Test (CST)

•The Contraction stress test is used by some antepartum testing centers to evaluate placental function under stress. The test is performed by placing transducers (ultrasound and toco), on patient's abdomen as with the nonstress test.

•The tracing is then observed for late decelerations.

•The test requires three contractions in 10 minutes to bepresent with the contractions lasting 40 to 60 seconds.

•If uterine activity is absent then oxytocin is infused or nipple stimulation is used.

The Contraction Stress Test (CST)

•The test is positive if late decelerations are consistent and present with more than 50% of the contractions.

•A positive CST has been has been associated with an increased incidence of intrauterine death, late decelerations in labor, low 5-minute Apgar scores, and intrauterine growth restriction.

The CST is equivocal or suspicious if there are intermittent late decelerations

The Contraction Stress Test (CST)

•Although the CST is not used routinely by the San Gabriel Valley Perinatal Medical Group a fetal tracing may fulfill the requirements for a positive or suspicious CST spontaneously.

•A suspicious or equivocal CST should be repeated in 24 hours

Amniotic Fluid Index (AFI)

•The amniotic fluid index is measured by dividing the uterus into four quadrants

•The linea nigra is used to divide the uterus into right and left halves.

•The umbilicus serves as the dividing point for the upper and lower halves.

•The transducer is kept parallel to patient’s longitudinal axis and perpendicular to the floor.

Amniotic Fluid Index (AFI)

The deepest, unobstructed, vertical pocket of fluid is measured in each quadrant

“Brief appearances of cord or an extremity are ignored, but aggregation of either one, to the exclusion of fluid, is not considered part of a fluid pocket.”

Add these numbers together and the sum represents the Amniotic fluid Index (AFI).

Interpretation of the AFl

10.1 to 24.0 cm Normal5.1 to 10.0 cm BorderlineLess than or equal 5.0 cm AbnormalGreater than 24.0 cm Abnormal

Risk of Oligohydramnios after "Low-normal AFI"

Gestational Age (weeks)

AFI Risk of Oligohydramnios after 4 days

>41 5-8 23.3%

37-40 5-8 17.8%

>41 >8 7.4%

37-40 >8 3.7%

Abnormal AFI Results

Notify attending physician for possible evaluation of functioning renal tissue and intact membranes for AFI less than or equal to 5.0.

Notify attending physician for possible evaluation of fetal structural abnormalities or diabetes for AFI greater than 24

Patients with an AFI less than or equal to 5.0 should be transferred to Labor and Delivery for further observation and physician notified

The Biophysical Profile (BPP)

•Between 24 and 28 weeks' gestation, approximately 50 percent of NSTs are nonreactive.

•In contrast sonographically evaluated variables are valid early in gestation and account for three of the five components of the biophysical profile.

•The biophysical profile may be used to verify fetal well being when the nonstress test is not reactive.

The Biophysical Profile (BPP)

•Fetal movement and fetal tone develop between 7.5 and 9 weeks’ menstrual age.

•Fetal breathing movements are detectable by, at least 17-18 weeks’ gestation.

•Amniotic fluid may be reduced as early as 17.5 weeks by fetal acidosis.

•The components of the biophysical profile develop sequentially. In order of appearance: tone, movement, breathing, reactivity.

The Biophysical Profile (BPP)

•Fetal state (wake-sleep cycle) plays an important role in the interpretation of the biophysical profile score.

•In quiet sleep the average time to obtain a normal biophysical profile score is 26.3 minutes.

•The biophysical profile score is, therefore, continued for a maximum of 30 minutes.

The Biophysical Profile (BPP)

•The sonographic variables that develop last in gestation are the most sensitive to acidosis and would be the first components of the BPP to become abnormal.

•The NST, breathing, and amniotic fluid volume are the most significant variables.

The Biophysical Profile (BPP)

•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 become abolished

•The presence of oligohydramnios with all of the other variables of the biophysical profile being normal may reflect chronic uteroplacental insufficiency

Components of the Biophysical Profile Score

Component Definition

Non-stress test

Two or more fetal heart rate accelerations peak (but do notnecessarily remain) at least 15 beats per minute above thebaseline and last 15 seconds from baseline to baseline withina 20-minute period with or without fetal movement discernibleby the woman.

Amniotic fluid volume

A single 2 cm x 2 cm pocket is considered adequate or AFIgreater than 5.0 cm .

Fetal breathing movements

One or more episodes of rhythmic fetal breathing movementsof 30 seconds or more within 30 minutes.Hiccups are considered breathing activity.

Fetal movementsAt least three discrete body or limb movements. Episodes of continuous movement are considered as a singlemovement.

Fetal toneOne or more episodes of extension of a fetal extremity ortrunk with return to flexion, or opening or closing of a hand

Perinatal Mortality and the Biophysical Profile Score

Score Interpretation Perinatal Mortality/1000

8- 10 Normal 1.86*

6 Equivocal 9.76

4 Abnormal 26.3

2 Abnormal 94.0

0 Abnormal 285.7

*The perinatal mortality is 0.8/1000 for structurally normal fetuses with a normal test within 7 days.

Maternal and Fetal Causes of Stillbirth within One Week of a Normal Biophysical Profile Score

Maternal Fetal

Placental abruptionDiabetic ketoacidosis Sickle cell crisis Drug overdose Motor vehicle accident Acute myocardial infarction

Acute alcohol poisoning

Fetomaternal hemorrhage

Cord prolapse Ruptured membranes Vase previa Cord entanglement Umbilical artery thrombosis

Interpretation of an Equivocal or Abnormal BPP

The absence of a biophysical variable may reflect:

•Normal fetal activity and sleep cycles•An inability of the central nervous system to perform that function•Hypoxia •External influences•Fetal breathing movements may be

•Stimulated by caffeine and hyperglycemia.•Inhibited by hypoglycemia, maternal supine hypotension, cigarette smoking, alcohol, diazepam and meperidine.

Action for Equivocal or Abnormal BPP

• The term fetus is generally delivered for a score of 6. • However, a score of 6 in a preterm fetus is usually repeated in 12 to 24 hours. In the interim, antenatal steroids may be given for pregnancies of less than 34 weeks of gestation.

• Delivery is usually indicated for BPP score of 4 or less.

• Oligohydramnios always requires further evaluation.

Patients with a biophysical profile score of 6 or less should be considered for transfer to Labor and Delivery for further

observation or delivery and physician notified.

Special Considerations

•Rupture of the membranes does not alter the short-term sonographic variables of the biophysical profile in the healthy fetus.

•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 following a normal biophysical profile score in fetuses with gastroschisis, omphalocele, and diaphragmatic hernia.

Special Considerations

•The observation of an abnormal biophysical profile in an anomalous fetus does not correlate very well with the presence of hypoxia.

•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