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Woman and Obstetrics

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Fetal Heart Rate Assessment

 

Fetal heart rate (FHR) assessment evaluates the fetal condition by identifying FHR patterns that may be associated with adverse fetal or neonatal outcome or are reassuring of fetal well-being.

Fetal monitoring techniques

Electronic fetal monitoring. The electronic fetal monitor determines the FHR and continuously records it in graphical form.
External fetal monitoring. The FHR is measured by focusing an ultrasound beam on the fetal heart. The fetal monitor interprets Doppler signals.
Internal fetal monitoring of FHR is an invasive procedure. A spiral electrode is inserted transcervically into the fetal scalp. The internal electrode detects the fetal (ECG) and calculates the fetal heart rate based upon the interval between R waves. This signal provides accurate measurement of beat-to-beat and baseline variability.
Biophysical profile. The biophysical profile (BPP) consists of electronic fetal heart rate evaluation combined with sonographically assessed fetal breathing movements, motor movement, gross fetal tone, and amniotic fluid volume.

Fetal heart rate patterns
The fetal heart rate pattern recorded by an electronic fetal monitor is categorized as reassuring or nonreassuring.
Reassuring fetal heart rate patterns
  • 1. A baseline fetal heart rate of 120 to 160 bpm
  • 2. Absence of FHR decelerations
  • 3. Age appropriate FHR accelerations
  • 4. Normal FHR variability (5 to 25 bpm).
Early decelerations (ie, shallow symmetrical decelerations in which the nadir of the deceleration occurs simultaneously with the peak of the contraction) and mild bradycardia of 100 to 119 bpm are caused by fetal head compression, and they are not associated with fetal acidosis or poor neonatal outcome.
The majority of fetal arrhythmias are benign and spontaneously convert to normal sinus rhythm by 24 hours after birth. Persistent tachyarrhythmias may cause fetal hydrops if present for many hours to days. Persistent bradyarrhythmias are often associated with fetal heart disease (eg, cardiomyopathy related to lupus), but seldom result in hypoxia or acidosis in fetal life.
FHR accelerations and mild variable decelerations are indicative of a normally functioning autonomic nervous system.

Nonreassuring fetal heart rate patterns

Nonreassuring FHR patterns are nonspecific and require further evaluation. The fetus may not be acidotic initially; however, continuation or worsening of the clinical situation may result in fetal acidosis.
Late decelerations are characterized by a smooth U-shaped fall in the fetal heart rate beginning after the contraction has started and ending after the contraction has ended. The nadir of the deceleration occurs after the peak of the contraction. Mild late decelerations are a reflex central nervous system response to hypoxia, while severe late decelerations suggest direct myocardial depression.

Sinusoidal heart rate is defined as a pattern of regular variability resembling a sine wave with a fixed periodicity of three to five cycles per minute and an amplitude of 5 to 40 bpm. The sinusoidal pattern is caused by moderate fetal hypoxemia, often secondary to fetal anemia.
Variable decelerations are characterized by the variable onset of abrupt slowing of the FHR in association with uterine contractions. Mild or moderate variable decelerations do not have a late component, are of short duration and depth, and end by rapid return to a normal baseline FHR. They are usually intermittent. Severe variable decelerations have a late component during which the fetal pH falls. They also may display loss of variability or rebound tachycardia and last longer than 60 seconds or fall to less than 70 bpm. They tend to become persistent and progressively deeper and longer lasting over time.

Fetal distress patterns

a. Fetal distress is likely to cause fetal or neonatal death or damage if left uncorrected. Fetal distress patterns are associated with fetal acidemia and hypoxemia.
b. Undulating baseline. Alternating tachycardia and bradycardia, often with reduced variability between the wide swings in heart rate.
c. Severe bradycardia. Fetal heart rate below 100 bpm for a prolonged period of time (ie, at least 10 minutes).
d. Tachycardia with diminished variability that is unrelated to drugs or additional non-reassuring periodic patterns (eg, late decelerations or severe variable decelerations).

Intrapartum fetal surveillance

Transient episodes of hypoxemia and hypoxia are generally well-tolerated by the fetus. Progressive or severe episodes may lead to fetal acidosis and subsequent asphyxia. One goal of intrapartum fetal surveillance is to distinguish the fetus with FHR abnormalities who is well compensated from one who is at risk for neurological impairment or death. Ancillary tests are useful for this purpose.

Ancillary tests

Fetal scalp stimulation.
Fetal scalp stimulation is similar to the vibroacoustic stimulation test used antepartum. Absence of acidosis (ie, fetal pH greater than 7.20) is confirmed by elicitation of a FHR acceleration when an examiner stimulates the fetal vertex with the examining finger. Fetal scalp sampling is recommended to further evaluate positive test results.

Fetal scalp blood sampling.
Capillary blood collected from the fetal scalp typically has a pH lower than arterial blood. A pH of 7.20 was initially thought to represent the critical value for identifying serious fetal stress and an increase in the incidence of low Apgar scores. The degree of technical skill required prohibits widespread use of this modality.

Management of nonreassuring FHR patterns during labor

Determine the cause of the abnormality (eg, cord prolapse, maternal medication, abruption placenta).
Attempt to correct the problem or initiate measures to improve fetal oxygenation (eg, change maternal position, administer oxygen and intravenous fluids, consider amnioinfusion or tocolysis).
If the nonreassuring pattern does not resolve within a few minutes, perform ancillary tests to determine the fetal condition.
Determine whether operative intervention is needed.

The presence of accelerations almost always assures the absence of fetal acidosis. Therefore, if such accelerations are not observed, they should be elicited by manual or vibroacoustic stimulation.
There is a 50 percent risk of fetal acidosis in fetuses in whom accelerations cannot be elicited, so further evaluation by fetal scalp sampling for pH is indicated to help clarify the fetal acid-base status. Serial evaluation every 20 to 30 minutes is necessary if the FHR pattern remains nonreassuring. Expeditious delivery is indicated for persistent nonreassuring FHR patterns.

Management of Variant Fetal Heart Rate Patterns

FHR Pattern Diagnosis Action
Normal rate normal variability, accelerations, no decelerations Fetus is well oxygenated None
Normal variability, accelerations, mild nonreassuring pattern (bradycardia, late decelerations, variable decelerations) Fetus is still well oxygenated centrally Conservative management
Normal variability, accelerations, moderatesevere nonreassuring pattern (bradycardia, late decelerations, variable decelerations) Fetus is still well oxygenated centrally, but the FHR suggests hypoxia Continue conservative management. Consider stimulation testing. Prepare for rapid delivery if pattern worsens
Decreasing variability, accelerations, moderate-severe nonreassuring patterns (bradycardia, late decelerations, variable decelerations) Fetus may be on the verge of decompensation Deliver if spontaneous delivery is remote, or if stimulation supports diagnosis of decompensation. Normal response to stimulation may allow time to await a vaginal delivery
Absent variability, no accelerations, moderate/severe nonreassuring patterns (bradycardia, late decelerations, variable decelerations) Evidence of actual or impending asphyxia Deliver. Stimulation or in-utero management may be attempted if delivery is not delayed


 


 
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