What is Variable Decelerations?
Problems with a newborn baby’s health can be due to various factors happening during childbirth, before birth, the functioning of the placenta, and blood flow to the womb. One method we use a lot during childbirth to monitor the baby’s health is called Electronic Fetal Monitoring. This helps to prevent harmful outcomes for the baby, such as lack of oxygen or conditions that could affect the baby’s brain function. However, understanding the baby’s heart rate patterns can be challenging because different doctors might interpret the same pattern differently. In fact, according to a study, doctors came to similar conclusions only 29% of the time when looking at the same heart rate patterns.
In 2008, experts held a meeting to standardize the language and symbols used in Electronic Fetal Monitoring. This meeting involved major organizations like the American College of Obstetricians and Gynecologists, the Society for Maternal-Fetal Medicine, and the Eunice Kennedy Shriver National Institute of Child Health and Human Development. During this meeting, they clearly defined what it means when the baby’s heart rate goes down quickly in a short span of time, which they called “variable decelerations.” To meet this definition, the heart rate must fall at least 15 beats per minute for 15 seconds within 30 seconds of starting but not last more than 2 minutes. These sudden drops can happen regularly, linked with contractions, or irregularly, not linked with contractions. Understanding these heart rate patterns helps in monitoring the baby’s health during childbirth.
What Causes Variable Decelerations?
Variable decelerations are sudden drops in a baby heart rate before it’s born. They can happen when a baby moves, especially if the baby is premature. For full-term babies, variable decelerations often occur due to the effect of the vagus nerve, which helps to control the heart. Scientists have some theories about why this happens, but it’s not completely understood.
Generally, a baby’s body adapts well to living in the womb. One special trait of babies is their unique form of hemoglobin, which is the substance in blood that carries oxygen. Babies have a sensitive reaction to the amount of oxygen in their blood, and their blood flow is normally high. They also have a way of directing oxygen-rich blood to important areas like the heart and brain. Due to these factors, even if a small or moderate amount of oxygen supply is blocked – like during a contraction – babies can still keep their organs working by using the stored oxygen.
We often think that variable decelerations happen because the umbilical cord, which links the baby to the mother’s blood supply, gets squeezed transiently. During the contraction, squeezing of the umbilical cord reduces blood flowing back to the baby, and a sensor (baroreceptor) in the baby’s blood vessels notices this and speeds up the baby’s heart rate. This is seen as a “shoulder” on the heart rate record. If the squeezing on the cord blocks it fully, the baby’s blood pressure goes up. This, in turn, gets the vagus nerve to swiftly reduce the baby’s heart rate. When the contraction ends, and the squeezing on the cord stops, the rapid heartbeat also returns to normal.
Alternatively, chemoreceptors -tiny sensors in the body that react to changes in the chemical makeup of the blood – might cause variable decelerations. They respond especially to low oxygen levels (hypoxia). When the womb contracts during childbirth, it can disrupt the supply of oxygen-rich blood (perfusion) from the placenta to the baby. If the lack of oxygen is mild, the baby might just sleep or move less to save oxygen. But, at a certain low level of oxygen, the chemoreflex gets triggered. This reaction causes a quick increase in certain activities in the baby’s body, which directs blood to vital organs, raises the blood pressure, and constricts blood vessels on the body’s surface. Because this increase in activities affects the heart more, it causes a quick drop in the baby’s heart rate.
Risk Factors and Frequency for Variable Decelerations
Electronic fetal monitoring is a standard procedure used during childbirth in the United States, done in about 85% of live births. This is a significant increase compared to 1980, when it was used in only about 45% of labors. The most common irregularity observed in fetal heart rate during labor is intermittent variable decelerations. These occur less than half the time with contractions and typically aren’t associated with any harmful outcomes for the baby, so they usually don’t need treatment. On the other hand, recurrent variable decelerations – where the decelerations happen with 50% or more of the contractions – are less frequent but might be more worrisome.
- Electronic fetal monitoring is performed in about 85% of live births in the U.S.
- The use of this procedure has risen since 1980, when it was used in about only 45% of labors.
- Intermittent variable decelerations are the most common fetal heart rate irregularity during labor.
- This condition is generally not harmful and often doesn’t need treatment.
- Recurrent variable decelerations, which occur with 50% or more of the contractions, are less common but more concerning.
Signs and Symptoms of Variable Decelerations
If you’re expecting a baby, there are several things that your doctor will consider when diagnosing or predicting variable decelerations, or changes in your baby’s heart rate. These usually appear as small changes in the heart rhythms of premature babies. For full-term babies, causes of low amniotic fluid need to be understood. These can include things like your water breaking or problems with the placenta.
Other reasons for variable decelerations could include a tight umbilical cord around the baby’s neck, putting pressure on the cord during contractions. Twins sharing the same amniotic sac may also risk their cords getting tangled, which can cause changes in the heart rate. With more severe variability in the baby’s heart rate, a tear in the uterus could be the cause. If you’re in labor and your baby’s heart rate looks abnormal, a history of cesarean section or previous uterine rupture could warrant immediate further testing.
Moments during labor that need to be watched for include an umbilical cord that slips out ahead of the baby. If you’ve had a history of extra amniotic fluid, a baby positioned buttocks or feet first, or an advanced distance your cervix has opened before your water broke, these could increase the risk of a cord prolapse.
- Small heart rate changes often seen in premature babies
- Causes of low amniotic fluid
- Pressure on the umbilical cord (from things like neck wrapping or twins sharing the same sac)
- Severe variability in baby’s heart rate could hint at a uterine tear
- Previous cesarean section or uterine tear may demand immediate action when abnormal heart rates are observed
- Increased risk of umbilical cord slipping out ahead of the baby under certain conditions
The physical checkup during labor should also include an examination of the cervix for widening, the baby’s positioning, and checking for an umbilical cord that has slipped out. Abdominal examinations might help identify the presence of unusually frequent or strong contractions if the contraction monitor isn’t enough. The initial examination of a pregnant woman should also determine the number of fetuses. If you are carrying multiple babies, it’s essential to identify whether they are sharing the same sac or placenta.
Testing for Variable Decelerations
If you’re in labor and your baby’s heart rate slows down frequently, it’s crucial to figure out what’s going on. Healthcare professionals will evaluate this by looking at various factors such as how often these slowdowns (known as decelerations) happen, how severe they are, and how long they last. They’ll also monitor your contractions and other features of your baby’s heartbeat.
It’s also useful to perform a cervical exam to see how much you’ve dilated, where the baby’s head is positioned (fetal station), and to check for a prolapsed umbilical cord – when the cord drops down through the cervix before the baby. This can help with figuring out why there might be unusual fetal heartbeat patterns and what to do next.
In situations when you’re not in labor but these variable decelerations occur, further checks might be needed. You may also have to check the amount of amniotic fluid – the liquid surrounding and protecting the baby in the womb – and keep monitoring the situation closely.
Treatment Options for Variable Decelerations
In 2013, scientists proposed a plan for dealing with situations where a baby’s heart rate, monitored during labor, shows signs of possible distress. This is often known as a category II fetal heart tracing. Here’s how the plan works:
The first step is to find out if the baby’s heart rate shows signs of normal activity (called “accelerations”) or a steady rhythm (referred to as “moderate variability”). After that, it’s important to check if there are any serious dips in the heart rate, known as “significant decelerations”. They defined a significant deceleration as:
- When the baby’s heart rate drops more than 60 beats below its starting level and stays this low for more than 60 seconds.
- When the baby’s heart rate drops to fewer than 60 beats per minute no matter the starting rate and keeps this low for over 60 seconds.
- Any late decrease in the heart rate.
- Any prolonged decrease in heart rate that lasts for 2 minutes or longer.
If a baby’s heart rate shows repeated dips during labor, it could mean there is pressure on the umbilical cord. Changing the mother’s position can help to relieve this. Another method that has proven effective is using a process called amnioinfusion, where fluid is put back into the womb. This process has been shown to decrease the occurrences of heart rate drops and lower the need for c-section deliveries. A study in 2012 showed that amnioinfusion improved not only the outcome of deliveries and occurrences of heart rate drops, but also the newborn’s health right after birth, likelihood of maternal infections after birth, length of hospital stay for the mother, and blood oxygen levels in the baby.
When faced with a situation where a baby’s heart rate shows worrisome dips, the response should be guided by what might be causing these changes. Here are a few examples:
If during an examination, a doctor finds that the umbilical cord has slipped through the birth canal before the baby (this is known as a umbilical cord prolapse), the baby’s head should be gently lifted while preparing for an emergency c-section.
When the mother is in the final stage of labor, her efforts to push might cause the baby’s heart rate to drop with each push. Depending on the overall health of the baby, the mother might be asked to not push during every contraction. This gives the baby time to recover between pushes.
When a mother’s contractions are too frequent – a condition known as uterine tachysystole – reducing the number of contractions can help. This can be achieved by cutting back on medications that induce labor, such as oxytocin, or administering a medication that relaxes the uterus.
For a mother being monitored with early membrane rupture, repeated dips in the baby’s heart rate could suggest that the condition of the mother and baby is declining. In these circumstances, delivery is warranted due to concerns about the baby’s well-being, infections of the amniotic cavity, or separation of the placenta from the uterus.
If despite all measures, the baby’s heart rate remains abnormal, it may be necessary to expedite delivery. If the mother is in labor and the baby is ready to be born, assisted vaginal delivery, like use of forceps, might be considered. In situations where the delivery is not imminent, a c-section may be necessary. When the baby’s head is pointed downwards in the womb, inducing labor could be the next step in case of early membrane rupture. If the baby is in a feet-first position, known as breech presentation, a c-section may be required.
What else can Variable Decelerations be?
When experiencing recurrent variable decelerations, a range of issues could be causing it, including:
- Umbilical cord being squeezed within the womb due to
- position of the mother or baby
- low amounts of amniotic fluid around the baby
- cords wrapped around the baby’s neck or other complications
- an overly tight or short umbilical cord
- overly fast uterine contractions
- forceful pushing during the later stage of labor
- An increase in the acidity of the baby’s blood
- The umbilical cord slipping into the vagina ahead of the baby
- The uterus splitting open along the scar line from a prior C-section (uterine rupture)
Bear in mind, other types of fetal heart rate drops may be mistaken for variable decelerations. Although definitions for each type of deceleration are well known, readings of fetal heart rate traces can vary between observers. Understanding the difference between these decelerations is important because the treatment will vary. Here’s what you need to know about early and late decelerations:
- Early Deceleration:
- Visible slow rise and fall of the fetal heart rate linked with a uterine contraction.
- Time from start to lowest point is 30 seconds or more.
- The lowest heart rate is at the same time as the highest point of contraction.
- This is commonly a harmless result of pressure on the baby’s head.
- Late Deceleration:
- Visible slow rise and fall of the fetal heart rate linked with a uterine contraction.
- Time from start to lowest point is 30 seconds or more.
- The lowest heart rate comes after the peak of the contraction.
- This usually raises concern as it might indicate insufficient supply from the uterus to the placenta.
What to expect with Variable Decelerations
Periodic drops in the baby’s heart rate, also known as intermittent variable decelerations, are typically not harmful and don’t generally lead to adverse birth outcomes.
However, if these heart rate drops become frequent, they could signal poor outcomes for the baby, including a condition known as neonatal acidemia. Essentially, when the baby doesn’t get enough oxygen, their body responds by slowing down their heart rate even more. This worsening lack of oxygen results in more significant heart rate reductions.
This lack of oxygen and resulting acidosis in the fetus can be associated with several complications, including:
* Aspiration of meconium (a baby’s first feces which can cause lung complications)
* Disturbances in metabolic and blood conditions
* Issues with cognitive development
* Hypoxic-ischemic encephalopathy (a type of brain damage that occurs when the brain doesn’t receive enough oxygen)
* Cerebral palsy (a disorder that affects a person’s ability to move and maintain balance)
* Death
It’s important to note that the mere presence of these heart rate drops, even if they occur frequently, does not necessarily indicate a poor prognosis for the baby. For instance, if the baby’s heart tracing also shows moderate variability and/or accelerations, it might mean the baby’s pH level (acid-base status) can still be normal. Also, identifying and managing these heart rate drops properly, which might include going ahead with a cesarean delivery if necessary, usually means the prognosis for the baby can be good.
Possible Complications When Diagnosed with Variable Decelerations
Electronic fetal monitoring, which includes detecting changes in the baby’s heart rate, aims to prevent negative outcomes during childbirth like brain damage, cerebral palsy, or even death. However, the effectiveness of this constant monitoring is widely debated. Certain studies show that it increases the rates of cesarean section and instrumental deliveries while reducing neo-natal seizures. Despite this, the monitoring hasn’t been proven to reduce the risk of death or cerebral palsy in newborn babies.
It’s also important to mention that the interpretation of the baby’s heart rate often varies between different observers. During a particular study, obstetricians only agreed 22% of the time when reviewing the same heart rate records. Even reviewing the same records again two months later, they had different interpretations 21% of the time. Also, the accuracy metrics of electronic fetal monitoring are not very reliable. It correctly predicts cerebral palsy in just 0.14% of cases and records mistakes (false positives) more than 99% of the time.
Main Points:
- Electronic fetal monitoring aims to prevent negative birth outcomes.
- The effectiveness of continuous monitoring is debated.
- Monitoring can increase rates of cesarean and instrumental deliveries but reduces neo-natal seizures.
- Interpreting the baby’s heart rate often shows considerable variation between observers.
- The accuracy metrics of electronic fetal monitoring are poor.
Preventing Variable Decelerations
Identifying low fluid levels while a woman is pregnant can guide discussions about what to expect during labor. It can also help find those who may benefit from procedures such as amnioinfusion, which is adding fluid to the uterus for the wellbeing of the baby. Patients need to be informed about the purpose of electronic fetal monitoring, a tool used to check the baby’s heart rate during labor. Patients should understand their options regarding continuous or sporadic monitoring, as well as the possible outcomes of this monitoring, including the chance of a C-section.
Also, it’s necessary for patients to review the potential benefits of fetal monitoring. This understanding will help the patient make informed decisions and prepare them for different scenarios during their labor and delivery.