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Complete heart block is a disorder of the heart’s electrical system, which controls the rate and rhythm of heartbeats. Heart block occurs when there is a disruption, preventing the electrical signal from the upper chambers of the heart (the atria) from reaching the lower chambers (the ventricles).
Normally, the electrical signal passes through specialized conducting tissue known as the atrioventricular (AV) node. After the signal passes through the AV node and reaches the ventricles, it causes the heart to contract and pump blood. When this signal does not transmit properly, there is heart block or AV block. This does not mean that the flow of blood in the heart or that the blood vessels of the heart are blocked. It does mean that the electrical signal that spreads across the heart with each heartbeat is slowed or in some cases completely interrupted. This can limit the ability of the heart to pump blood to the rest of the body.
There are three types of heart block, depending on the extent of disruption of the electrical impulses: first degree, second degree, and third degree. Also known as complete heart block, third degree is the most severe and represents complete interruption of electrical communication between the atria and ventricles.
While all forms of heart block, including complete heart block, more commonly occur after birth, some babies are born with heart block. This is known as congenital and can be detected before or after a baby is born.
Pregnant women who have autoimmune diseases, such as lupus or Sjogren’s syndrome, are at an increased risk of having a baby with congenital heart block. Antibodies produced by the pregnant woman’s body in response to the autoimmune disease can cross the placenta, damage to the heart of the fetus, and lead to congenital heart block.
Congenital complete heart block usually occurs as an isolated anomaly but can be associated with other congenital problems with the structure of the fetal heart. A parent who has a congenital heart defect may be more likely than other parents to have a child with the same condition, so heredity may also be a factor. Often, however, the cause is unknown.
Congenital complete heart block is estimated to occur in about 1 in 10,000 fetuses. This is only an estimate based on newer technology that allows congenital complete heart block to be detected in utero.
Fetal echocardiograms are used to diagnose congenital complete heart block. Additional procedures, such as M-mode tracing and Doppler ultrasound can show the relationship between a faster heart rate in the atria and a slower heart rate in the ventricles. In fetuses with complete heart block, the rates will not be synchronized. These procedures can also reveal associated problems with the structure of the fetal heart.
The heart rate and other measurements of heart function are recorded at the time of diagnosis and usually repeated at intervals of two to four weeks to monitor the condition of the fetal heart.
Women who know they have antibodies in response to an autoimmune disease should have fetal echocardiography at 18 and at 22 weeks gestation. If these studies are normal, obstetricians may continue to follow the fetal heart rate, particularly if the mother previously had a child with complete congenital heart block. Congenital complete heart block usually develops between 18 and 30 weeks gestation.
The outcomes for fetuses diagnosed with congenital complete heart block depend on several factors, but especially significant is whether structural heart disease is also present. The prognosis for these fetuses is guarded, particularly if there are complex malformations. Hydrops, or swelling of the fetus, is an unfavorable sign and is usually associated with a very poor prognosis. Fetuses with complete heart block and very low heart rates are at increased risk for development of hydrops. The prognosis is better for fetuses diagnosed with congenital complete heart block without other structural defects of the heart. Treating hydrops in utero with steroids and other drugs may allow the pregnancy to continue and avoid premature delivery, which increases the newborn’s risk of sickness and death.
If the fetus is close to term and the complete heart block is considered to be in stable condition, a normal vaginal delivery may be done, but with close monitoring. If repeated measurements indicate that the fetal heart is deteriorating and that the fetus can no longer survive in utero, the baby will be delivered prematurely by caesarean section. In these cases, the newborn usually requires ventilation, a surfactant (a mixture that coats the air sacs in the lungs to prevent lung collapse), and a drug to increase the baby’s heart rate.
In some cases, a temporary pacemaker is connected to the newborn’s heart. A permanent pacemaker is implanted once the baby is stable, with no signs of infection, and weighs approximately 1.5 to 2 kg (3.3 to 4.4 lbs).
Some studies indicate that using steroids that cross the placenta may possibly prevent or reverse the degree of complete heart block in fetuses of women who previously had a child affected by complete heart block, but these studies need to be verified.
If you have questions regarding a fetal heart condition, or would like more information about the Fetal Heart Program, please contact: Dana McBride Administrative AssistantPhone: 513-636-9931 Toll free 1-800-344-2462 ext. 6-9931 Fax: 513-636-9747 Email: firstname.lastname@example.org
Watch: Fetal Pacemaker for Complete Heart Block.
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