Introduction
After conception, the myocardium (the muscular layer of the heart) begins rhythmic contractions within three weeks. With further growth and maturation, the normal fetal heart rate reaches 110 bpm (beats per minute) within five to six weeks. In a healthy fetus (prenatal stages that follow the embryo stage), the heart rate is regular and remains between 110 and 180 bpm. Abnormalities in the fetal rhythm are seen in two percent of pregnancies. Fetal bradycardia is one of the types of fetal rhythm abnormalities characterized by a slower heart rate.
What Is Fetal Bradycardia?
Fetal bradycardia or bradyarrhythmia is a rare condition in which the developing baby has a sustained heart rate slower than 110 bpm. In some cases, a slower heart rate may be accompanied by an irregular rhythm. Fetal bradycardia usually resolves and does not cause any harm to the fetus. If it persists and does not resolve, it requires treatment before and after birth.
What Causes Fetal Bradycardia?
A slower fetal heart rate can be due to problems in the electrical system of the heart, which sends out impulses that stimulate the heart muscles to beat or contract. Problems can occur in the sinus node (the natural pacemaker of the heart, which generates the electrical impulses) or the conduction system that transmits signals from the heart’s upper chamber to the lower chamber. Sinus bradycardia is the most common reason for fetal bradycardia, which can be due to vagal stimulation due to compression during ultrasound examinations, fetal distress, and maternal autoimmune disorders. Fetal bradycardia can be due to various fetal and maternal causes, which include the following:
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Chromosomal anomalies (a disorder in which alterations occur in the chromosomes resulting in congenital abnormalities).
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Congenital heart defects (presence of heart problems at birth).
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Conduction abnormalities (a problem in the electrical system of the heart that controls heart rate and rhythm).
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Cordocentesis (a prenatal test that detects abnormalities in the fetal blood sample from the umbilical cord).
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Vagal cardiovascular reflex (vagal nerve compression causes cardiovascular inhibition) due to umbilical cord compression, fetal head compression, hypoxia caused by myocardial depression, and stimulation of stretch receptors in the aortic arch or carotid sinus walls.
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Lupus erythematosus in the mother.
What Are the Types of Fetal Bradycardia?
The types of fetal bradycardia include the following:
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Sinus Bradycardia: Sinus bradycardia is a condition in which the fetal heart rate is slower than 110 bpm, though the heart’s electrical system is functioning normally. Sinus bradycardia can be temporary and can occur during an ultrasound examination because of the pressure on the uterus. In some cases, sinus bradycardia can be due to fetal distress caused by hypoxia (lack of oxygen) or sinus node damage or malfunction. Mild sinus bradycardia does not require treatment if any abnormalities are not present.
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Atrioventricular Block: Atrioventricular block (first-degree heart block) occurs when there is a delay in the transmission of impulses between the upper and lower chambers of the heart. Atrioventricular block does not require treatment if any abnormalities are not present.
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Partial Heart Block: A partial heart block (second-degree heart block) is also known as a partial atrioventricular block. It occurs when the signal transmission from the upper chamber to the lower chamber of the heart is intermittently blocked. It can cause a slower and irregular heartbeat.
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Complete Heart Block: Complete heart block (third-degree heart block) occurs when the electrical impulses from the upper chambers do not reach the lower chambers of the heart, and can result in a slower and unreliable heart rate. A complete heart block is a life-threatening condition as it causes persistent bradycardia in the fetus. It causes complications such as hydrops (fluid build-up in the baby’s tissues and organs, resulting in extensive swelling) and heart failure. A complete block is usually caused by structural heart defects in the fetus or maternal autoimmune disorders such as lupus.
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Blocked Premature Atrial Contractions: Blocked premature atrial contractions are a type of bradycardia, and if it occurs in the womb, it is called fetal bradycardia. It occurs when premature atrial contractions occur early and are not conducted to the lower chambers of the heart due to a refractory atrioventricular node or bundle of his.
How Is Fetal Bradycardia Diagnosed?
Fetal bradycardia is usually diagnosed during a routine ultrasound examination. If an abnormally slow heart rate is detected, a comprehensive ultrasound examination is performed to determine fetal well-being, including amniotic fluid levels, movements, and muscle tone, and to evaluate if the baby is in distress and requires emergency delivery. Additional tests, such as a fetal echocardiogram to evaluate the heart structure and function and to determine the cause of fetal bradycardia, amniocentesis (amniotic fluid removal and examination), and chromosomal analysis are performed to determine genetic abnormalities.
How Is Fetal Bradycardia Treated During Pregnancy?
Treatment depends on the type of bradycardia, the gestational age of the baby, associated conditions, and the overall health of the mother and the baby. The treatment strategies include the following:
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In mild cases, treatment is not required but the baby’s heart rate is closely monitored until it resolves.
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Medications are given to the mother to improve the fetal heart rate.
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If preterm delivery is expected, steroids are given to promote the baby’s lung growth.
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If any underlying causes are present, treatment is required.
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In cases of fetal distress, emergency delivery is performed.
How Is Fetal Bradycardia Treated After Birth?
After delivery, babies with fetal bradycardia require treatment in advanced care units such as neonatal intensive care units or cardiac intensive care units. Treatment for fetal bradycardia after birth varies depending on the type of bradycardia and associated conditions. Mild cases of bradycardia may increase to a normal rate without any treatment. However, all babies are closely monitored until it resolves. In some cases, if fetal bradycardia is persistent, medications and sometimes a pacemaker may be required after birth to improve the speed of the heart rate.
Conclusion
Fetal bradycardia is a condition in which the heart rate of the developing baby is slower than 110 bpm. Mild cases of fetal bradycardia usually resolve spontaneously and require close monitoring until it resolves completely. Severe cases of bradycardia require neonatal cardiac intensive care management and pacemaker therapy with life-long care. Early diagnosis during an ultrasound examination, proper and timely interventions, and management of underlying conditions can help in good outcomes. In addition, delivery and postnatal care are carefully planned with a coordinated team to manage fetal bradycardia during delivery and after birth.
