The signal then travels through the 2 atria and stimulates them to contract. The signal passes through another node (the AV node), and finally travels through the ventricles and stimulates them to contract.

Problems can occur anywhere along the conduction system, causing various arrhythmias. There can be a problem in the heart muscle itself, causing it to respond differently to the signal, or causing the ventricles to contract independently of the normal conduction system.

Arrhythmias include "tachycardias" (the heartbeat is too fast), "bradycardias" (the heartbeat is too slow) and "true" arrhythmias (a disturbed rhythm).

Arrhythmias are classified as lethal if they cause a severe decrease in the pumping function of the heart. When the pumping function is severely decreased for more than a few seconds, blood circulation is essentially stopped, and organ damage (such as brain damage) may occur within a few minutes. Lethal arrhythmias include ventricular fibrillation; ventricular tachycardia that is rapid and sustained, or pulseless; and sustained episodes of other arrhythmias.

Other arrhythmias include atrial fibrillation/flutter, multifocal atrial tachycardia, paroxysmal supraventricular tachycardia, Wolff-Parkinson-White syndrome, sinus tachycardia, sinus bradycardia, bradycardia associated with heart block, sick sinus syndrome, and ectopic heartbeat.

People who have a history of coronary artery disease, heart valve disorders, or other cardiac disorders, and people with imbalances of blood chemistries, are at higher risk for arrhythmias and complications from arrhythmias.

Arrhythmias are also caused by some drugs. These include anti-arrhythmics, beta blockers, caffeine, cocaine, psychotropics, and sympathomimetics.

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