In second-degree heart block, not every impulse reaches its
destination.
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Interruption of the impulses in the heart's conduction system is
known as heart block. Heart block may be caused by scar tissue from
a heart attack, among other things.
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First-degree heart block or AV (arterioventricular) block, or PR prolongation, is a disease of the electrical conduction system of the heart in which the PR interval is lengthened beyond 0.20 seconds.
First-degree heart block rarely causes any symptoms or problems and normally remain undiagnozed.
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Third-degree block, also called complete heart block, is the
most serious.