Fetal Monitoring and Fetal Heart Rate (FHR) Variations

How Do Uterine Contractions Affect Fetal Heart Rate? Uterine contractions can affect fetal heart rate by increasing or decreasing that rate in association with any given contraction. The three primary mechanisms by which uterine contractions can cause a decrease in fetal heart rate are compression of:

· Fetal head

· Umbilical cord

· Uterine myometrial vessels

· With each contraction, blood flow from the mother to the baby initially ceases as the uterine myometrial veins are compressed.

· At this point, the mother and baby are physiologically separated from each other for a few seconds.

· As the contraction begins to subside, the uterine myometrial arteries reopen, allowing blood carrying oxygen and nutrients to flow from the mother to the baby, and the uterine myometrial veins reopen, allowing blood carrying fetal waste products to flow from the baby to the mother.

Baseline Fetal Heart Rate. The baseline fetal heart rate is the heart rate range that occurs between contractions.

How Do You Determine Baseline Fetal Heart Rate? Look at the fetal heart rate occurring between contractions. This is the baseline fetal heart rate. Remember, the normal baseline heart rate can be anywhere between 110 and 160 beats per minute.

Fetal bradycardia is defined as a decrease in the baseline fetal heart rate to less than 100 beats per minute.

What are Causes of Fetal Heart Rate Bradycardia?

Fetal Bradycardia may result from the following:

· Fetal Hypoxia. Bradycardia is a late sign of fetal hypoxia (a continual lack of oxygen). The heart rate slows in response to a depression of heart muscle (myocardial) activity caused by this continued decrease in needed oxygen.

· Medications. Medications such as narcotics cause bradycardia by preventing receptor sites in the fetal heart muscle from accepting epinephrine, which works to increase heart rate.

· Epidurals cause vasodilation, which leads to an increase in the incidence of maternal hypotension during labor. Anesthetic agents such as medications used in epidurals cause bradycardia indirectly due to a reflex mechanism or as a result of hypotension, a potential complication for regional anesthesia. Anesthetic medications can produce bradycardia approximately 5 minutes following the block for as long as 10 minutes. The heart rate then usually returns to normal baseline rate.

· Synthetic Oxytocin (Pitocin) may produce bradycardia by causing a hyperstimulation of the uterine muscle (myometrium), resulting in hypoxia.

· Maternal Hypotension. Supine hypotension syndrome caused by pressure of the uterus and its contents on the inferior vena cava, when you lay on your back, results in decreased maternal blood pressure.

· Prolapsed Umbilical Cord or Prolonged Compression of Umbilical Cord. Either situation may activate the fetal regulatory mechanism, causing a stimulation of the vagal center, which is part of the parasympathetic nervous system. This results in bradycardia.

Tachycardia: Suspicious tachycardia is defined as being between 150 and 170 whereas a pathological pattern is above 170

What Are Causes Of Fetal Heart Rate Tachycardia? Fetal tachycardia may result from the following:

· Fetal Hypoxia. Tachycardia may be an early sign of hypoxia (fetal lack of adequate oxygen).

· Medications. Medications used to prevent/stop premature labor such as terbutaline, have a stimulating effect on the fetal heart, which increases the rate.

· Prematurity. A premature baby has an immature nervous system resulting in an increased heart rate.

· Maternal Anxiety. During periods of maternal stress and anxiety, epinephrine is released into the mother’s blood stream that crosses the placenta, resulting in an increase in fetal heart rate.

· Maternal Fever. Both the mother’s and the baby’s metabolism is increased, which results in an increased heart rate.

· Epidurals can cause maternal fever. Epidurals increase maternal core temperature but the physiologic pathways are not certain. It is most likely a blockade of the sympathetic nervous system so that the epidural analgesia limits perspiration (blocks skin's ability to regulate heat) and removes the stimulus for hyperventilation (ways our body get rid of heat).

· Fetal Infection. This may be an early sign of an intrauterine infection (a stress reaction to sepsis). Prolonged ruptured membranes may lead to maternal and fetal infection.

· Fetal Movement/Stimulation. Benign cause of fetal tachycardia.

Fetal heart rate variability has become one of the most important indicators in the clinical assessment of fetal well-being.

What Is Fetal Heart Rate Variability?

Fetal heart rate variability is the normal irregular changes and fluctuations in the fetal heart rate that shows as an irregular heart rate seen on the tracing instead of a smooth line. The baseline rate should vary by at least 10-15 beats over a period of one minute. This causes the tracing to appear as a jagged, rather than a smooth, line. Variability is indicative of a mature fetal neurologic system and is seen as a measure of fetal reserve. The loss of short-term variability is ominous, while the loss of long-term variability is probably not very significant. Do not confuse these two concepts.

· Short-term variability (beat-to-beat) from the fetal heart rate baseline. Either present or absent.

· Long-term variability is fluctuations around the fetal heart rate baseline. A decrease in long-term variability can be noted during fetal sleep. The baby sleeps in 20-40 minute cycles.

Long-term variability can be divided into the following categories:

· Decreased variability: minimal variability (0-5 bpm).

· Moderate variability: normal variability (6-25 bpm).

· Marked variability: saltatory variability (<25bpm).

A normal, healthy fetal heart rate should possess average or moderate variability.

Decreasing variability is an indicator of possible stress. Decreased variability is especially ominous if associated with late persistent decelerations or repetitive severe, prolonged variable decelerations.

Decreased variability may occur in the following situations:

· Hypoxia and acidosis. The lack of oxygen and the build-up of acid in the fetal body depress the fetal heart and nervous system.

All central nervous system depressant medications, including narcotics and anesthetic agents, depress the fetal nervous system. Usually, variability increases as the drug is eliminated from the baby.

· Prematurity. The fetal nervous system in a premature baby cannot effectively control the heart rate.

· Fetal sleep (as noted above).

Persistent minimal or absent variability is considered an ominous pattern, requiring immediate delivery.

What Are Periodic\Nonperiodic Heart Rate Changes? When fetal heart rate changes are associated with contractions, they are termed periodic heart rate changes. When fetal heart rate changes are not associated with contractions, they are termed nonperiodic heart rate changes. They are documented as reassuring or nonreassuring.

Periodic\Nonperiodic Heart Rate Changes: They are classified as:

· Accelerations

· Decelerations

Þ Early

Þ Variable

Þ Late.

Þ Prolonged

Accelerations: The fetal heart rate will normally remain steady or accelerate during uterine contractions. Look at the fetal heart rate and what is happening with contractions.

Accelerations are defined as a transient increase in heart rate of greater than 15 bpm for at least 15 seconds (the 15x15 rule). Two accelerations in 20 minutes is considered a reactive trace. Accelerations are a reassuring sign as they show fetal responsiveness and the integrity of the mechanisms controlling the heart.

Early Decelerations: The early deceleration begins at the onset of the contraction and ends with the end of the contraction.

· Early deceleration is caused by vagal simulation from head compression, and is a reassuring pattern that may be prevented by avoiding early rupture of membranes.

· An early deceleration begins at or after the onset of a contraction and returns to the baseline rate by the time the contraction has finished and produces a mirror image of the contraction.

· Early decelerations are not a sign of fetal problems.

· Early decelerations occur most frequently in the following clinical situations:

Þ During sterile vaginal examinations

Þ In second stage of labor during pushing

Þ During application of internal FHR electrode

Þ With cephalopelvic disproportion

Þ After amniotic sac has ruptured

Þ With vertex presentations.

Late Decelerations: Late decelerations are transitory decreases in heart rate caused by uteroplacental insufficiency, a compromised blood flow to the baby that does not deliver the amount of oxygen needed to withstand the stress of labor.

· The late deceleration begins after the onset of the peak or middle of the contraction and ends after the contraction.

· A late deceleration begins during or after a contraction and has not recovered by the time that the contraction has ended. A late deceleration indicates decreased blood flow during uterine contraction. Note: Persistent late decelerations are ominous, especially if the decelerations are associated with loss of short-term variability.

Variable Decelerations: Variable decelerations are transitory decreases in fetal heart rate caused by umbilical cord compression.

· A variable deceleration is unrelated to contractions. They mean umbilical cord compression.

· They may appear V-shaped or U-shaped.

· If a woman could be monitored throughout the 9 months of her pregnancy, it would be apparent that variable decelerations occur transiently as the baby grabs the umbilical cord or the cord gets compressed between the baby and the uterine wall during fetal movement.

· As many as fifty percent of all monitored babies experience variable decelerations during labor.

· If the baseline fetal heart rate remains stable and the variability remains good, variable decelerations are not associated with poor fetal outcome.

· They indicate possible compromise if they become prolonged or are persistent.

Fetal heart rate patterns are called reassuring or nonreassuring and ominous.

Reassuring patterns are those associated with fetal well-being and positive outcomes.

Reassuring Patterns

· Mild variable decelerations (less than 30 seconds in duration with rapid return to baseline)

· Early decelerations (concurrent "mirror image" decrease with contraction)

· Accelerations without other changes

Nonreassuring, or "warning," patterns suggest decreasing fetal capacity to cope with the stress of labor.

Nonreassuring Patterns (Warning Signs)

· Decrease in baseline variability

· Progressive tachycardia (>160bpm)

· Decrease in baseline FHR

· Intermittent late decelerations with good variability

Ominous patterns suggest possible fetal compromise.

Ominous Patterns

· Persistent late decelerations, especially with decreasing variability

· Variable decelerations with loss of variability, tachycardia, or late return to baseline

· Absence of variability

· Severe bradycardia

If an ominous pattern appears to be present: Intrauterine Resuscitation

Have the mother lie on her left side (remember, lying on her back invites hypotension which affects baby’s oxygen supply) or in a knee chest position.

= To alleviate possible cord compression.

· Reduce or stop any oxytocin she may be receiving.

· Initiate tocolysis

= To decrease uterine activity and increase placental blood flow

· Increase IV fluid.

= To increase maternal blood flow volume

· Give her oxygen @ 10-12L to breathe by mask.

= To promote oxygenation across the placenta

· Apply an internal monitor to verify the accuracy of external monitor readings.

· Administer amnioinfusion

= To decrease pressure on cord.

If the heart rate is not restored to normal within 30 minutes, prompt delivery is needed. Cesarean section may then become necessary.