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. Ventricular Fibrillation, or Vfib, means the electrical impulse in the heart is no longer coordinated. Normal contraction of the ventricles is replaced with heart muscle quivering, and when contraction of the heart muscles is not coordinated, blood can't be pumped to the body.

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