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What is a Heart Block?
The heart has an electrical system that controls the rate and rhythm of heart beats. An electrical signal spreads across the heart from the two upper chambers (atria) to the two lower chambers (ventricles) with each heart beat causing the contraction of heart muscles. The result is pumping of blood from one chamber to the next, and into the general circulation of the body.
When the electrical impulses from an atrium to the ventricle are slowed down or disrupted, an atrio-ventricular heart block is said to occur. This results in an arrhythmia, or a disturbance in the rhythm or pattern of heart beats.
A heart block may be congenital or acquired during one’s lifetime. Congenital heart blocks often occur with congenital heart defects as a baby is born with, while acquired heart blocks are usually associated with heart diseases that develop as a person ages. Acquired heart blocks are more common than the congenital ones.
The severity of a heart block may be described according to three degrees.
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What is a First Degree Heart Block?
In the United States 0.65-1.6% of young adults experience a first degree heart block, and it is especially common in trained athletes. Its prevalence increases with age, although it is more common in African Americans than Caucasians.
First degree heart blocks are rarely symptomatic and don’t usually cause disturbances in a patient’s sense of well being. This is because despite the relatively slower conduction of impulses from the atrium to the ventricle compared to a normal heart, each atrial contraction is always followed by a ventricular contraction as in normal hearts. This may not be so for second or third degree heart blocks. Therefore, first degree heart blocks are usually benign and unreported, and may only be discovered as an incidental finding when an ECG is done.
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Causes of First Degree Heart Block
Many highly conditioned athletes with an increased vagal tone have a first degree heart block. Vagal tone is an intrinsic inhibition of heart beats by the vagal nerve that is responsible for slowing down the heart.
Other causes of first degree heart block are:
- Infectious disease – such as Lyme disease, which is associated with carditis (a heart complication of the disease) and usually found in children; viruses can also cause myocarditis or inflammation of the heart muscles
- Intrinsic disease of the atrioventricular node – part of the heart where electrical impulses are generated
- Myocardial infarction or an acute heart attack
- Disturbances in blood electrolytes such as increases in potassium and decreases in magnesium
- Drugs that can cause slowing of heart rate such as beta-blockers, calcium channel blockers, other antiarrhythmics and magnesium
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Diagnosis and Treatment of First Degree Heart Block
A first degree heart block is usually unrecognized except by an incidental finding with an ECG. No further work-up is necessary except if it is associated with other infections, cardiac disease, metabolic derangement or drug toxicity. Appropriate lab tests may be done to address the suspected contributing factor.
No specific treatment for the heart block itself is necessary. Usually correction of the causative factor such as infection or drug toxicity will make the aberration in electrical conduction disappear. Unlike more severe forms of heart blocks which may need pacemaker implantation to improve heart rate and rhythm, a first degree heart block can be left alone without fear of cardiac complications.
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NHLBI, “What Is Heart Block?" accessed 2/24/11
NHLBI, “Types of Heart Block" accessed 2/24/11
eMedicine, “Heart Block, First Degree" accessed 2/24/11