A normal heartbeat begins with a group of specialized pacemaker cells called the sinoatrial node in...
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A normal heartbeat begins with a group of specialized pacemaker cells called the sinoatrial node in the heart's upper, right chamber, or atrium. Regular electrical impulses from these cells cause the atria to contract or squeeze, pushing blood into the lower chambers or ventricles. Traveling down to a junction called the atrioventricular, or AV node and on to the ventricles, these impulses then signal the ventricles to contract. This pumps blood out to the lungs and the rest of the body. Heat block means these electrical impulses can't pass from the atria to the ventricles. As a result, the contractions of these chambers may not be coordinated, which can cause a very slow heart rate.

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