An irregular fetal heart rhythm- too slow, fast, or abnormal- is considered fetal arrhythmia. Usually, it is diagnosed during a routine ultrasound or fetal heart monitoring with the help of a Doppler device.
Fetal arrhythmias are usually benign. However, the condition still requires constant monitoring; otherwise, it can lead to fluid buildup in the baby’s body organs (hydrops fetalis), premature delivery, or even death. The sinus rhythm of a fetal heart is anywhere between 110 and 160 beats per minute (variable from beat to beat). When a baby’s heart rate is slower or faster than the normal range or skipping beats, it indicates an issue that needs further evaluation.
Risk Factors of Fetal Arrhythmias:
Fetal arrhythmias are detected in around 2% of pregnancies. You may be at higher risk if you have the following:
- Pregnancy through in vitro fertilization
- Pregnancy with monochromic twins
- Fetal heart block in a previous pregnancy
- Have been taking teratogenic drugs
- Infections in the first trimester include cytomegalovirus, rubella, and parvovirus B19.
- Having autoantibodies to Ro/SSA and La/SSB, which are usually found in people with certain autoimmune diseases, like Lupus or Sjogren’s disease
- Pre-existing diabetes or gestational diabetes.
- Family history
- Chromosomal abnormalities such as Down syndrome or Turner syndrome
- First-degree relative having heart defects
- Electrolytes imbalance
- Non-structural defects like Inflammation
Types of fetal Arrhythmias:
Before we discuss the types of fetal arrhythmias, here is a brief overview of heart physiology. The human heart comprises four chambers, two atria, and two ventricles. The SA node in the right atrium generates and regulates the heartbeat. The impulses then pass through the AV node between the atria and ventricles.
Lastly, the impulses reach the network of Purkinje fibers in the ventricles leading to ventricular contraction. The atria and ventricles work together to pump blood to the lungs and the rest of the body. The SA node’s firing rate determines the heart’s normal rhythm. When the heart rate falls below 110 bpm or above 170 bpm, it is diagnosed as fetal arrhythmia.1https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3727373/ There are a few types of fetal arrhythmias based on the part or function of the heart affected.
Extrasystoles or premature contractions (PCs):
These are the common types of fetal arrhythmias usually observed during routine auscultation during the second or third trimester of pregnancy. The baby has extra heartbeats in this condition, typically originating from the atria or ventricles. However, in most cases, these arrhythmias are supraventricular in origin. This condition is generally self-limiting and occurs in an isolated form. It does not require specific treatment and may resolve independently when the baby is born.
When a baby’s heart rate is more than 160 beats per minute, it’s considered tachycardia. Sustained tachycardia occurs more than 50% of the time. A faster heart rate can lead to hydrops fetalis (swelling in an unborn or newborn baby), heart failure, or too much accumulation of amniotic fluid (polyhydramnios).
The most commonly seen tachyarrhythmias are:
Atrial flutter and supraventricular tachycardia (heart rate between 220 and 300 beats per minute.
An abnormally slow heart rate of fewer than 110 beats per minute is considered bradycardia. If the baby’s heart rate remains low for 10 minutes or more when monitored is called sustained bradycardia.
Bradyarrhythmia can be mild or fatal depending on the underlying cause, the gestational, and the associated complications. Shorter periods of slow heart rate called transient fetal decelerations are benign and usually occur in the second trimester. Different types of bradyarrhythmia are:
- Sinus Bradycardia – a slower heart rate with a regular rhythm
- Atrioventricular (AV) block or first-degree heart block happens when the conduction between the upper and lower chambers of the heart is prolonged.
- Second-degree or partial heart block – in this type, the impulse conduction is intermittently blocked, resulting in skipped beats. It is also called a partial AV block.
- Complete or third-degree heart block – As the name suggests, the impulse conduction between the atria and ventricles is completely blocked. The impulses are generated irregularly by another natural pacemaker. The result is a much slower and more irregular heartbeat. It is a life-threatening condition.
How is Fetal Arrhythmia Diagnosed?
Fetal arrhythmias can be detected during a routine ultrasound. If an arrhythmia is suspected, then a fetal echocardiogram is recommended.
A fetal echocardiogram (FECG) is a safe and noninvasive procedure that allows a pediatric cardiologist to see the structures and functions of the heart. The test can take anywhere between 45 to 120 minutes, depending on the complexity of the fetal heart.
Fetal magnetocardiography (FMCG) is not yet widely available but provides a more accurate image of a fetus’ heart than FECG. Doctors can use FMCG to help diagnose heart arrhythmias in fetuses at around twenty gestational weeks or older. Some other tests are also carried out, such as:
- High-level fetal ultrasound
- Fetal magnetic resonance imaging (fMRI)
- Amniocentesis (a medical procedure used in the prenatal diagnosis by taking out a small sample of amniotic fluid).
- Chromosomal analysis
How is Fetal Arrhythmia treated?
The cases of fetal arrhythmia are variable, and not all fetal arrhythmias require treatment. Many will resolve on their own. Treatment depends on the cause of the fetal arrhythmia, the pregnancy stage, the fetus, or the mother’s health.
Hospital treatment for most arrhythmias consists of detailed monitoring of the fetus and the mother until the condition gets resolved. Two groups of medications are prescribed to improve the fetal heart rate and the causative conditions.
- Steroids such as Dexamethasone and Betamethasone are administered if premature delivery is expected. They help in lung maturity by increasing the production of surfactants and reducing the amount of fluid in the lungs.
- Cardiovascular drugs are administered to improve and normalize the fetal heart rate. Common CVS drugs administered in fetal heart arrhythmias include:
Digoxin is usually the first-line drug. In cases where digoxin fails to treat tachycardia, flecainide, sotalol, and amiodarone can also be used.
Digoxin is rarely successful in a hydropic fetus due to its narrow therapeutic range and high drug-to-drug interactions (DDIs), which cause digoxin toxicity. After childbirth, the baby’s heart rate is monitored carefully, and different medications are recommended.
Fetal arrhythmia is rare and occurs in 2 to 3 percent of pregnancies. Although it is benign, it is necessary to properly understand arrhythmia, treatment possibilities, awareness of drugs being used, and possible side effects. An early diagnosis can highly improve the prognosis. Intervention is rarely indicated. However, the risks of premature delivery, premature rupture of membranes, placental abruption, or stillbirth remain. Hence, complete prenatal diagnosis is imperative for appropriate prenatal and postnatal treatments.