It is conducted through the atria and stimulates them to contract.
Problems can occur anywhere along the conduction system, causing various arrhythmias.
Problems can also occur in the heart muscle itself, causing it to respond differently to the signal to contract, also causing arrhythmias, or causing the ventricles to contract independently of the normal conduction system. Arrhythmias are classified as lethal if they cause a severe decrease in the pumping function of the heart. Lethal arrhythmias include ventricular fibrillation, also ventricular tachycardia that is rapid and sustained, or pulseless, and may include sustained episodes of other arrhythmias. 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.