An arrhythmia is an abnormal heart rhythm that, when severe or long-lasting, can prevent the heart from pumping enough blood to the body. Damage to the heart muscle from a heart attack can make someone prone to arrhythmias, as can some congenital heart conditions. A variety of minerals, such as potassium, magnesium and calcium, with high or low concentrations in the blood and tissue can cause arrhythmias. So can alcohol, cigarettes and recreational drugs.