Abnormal heartbeat. The normal heartbeat is from the sinus, located in the top of the heart and is called normal sinus rhythm. An arrhythmia is anything other than sinus rhythm - supraventricular tachycardia (svt), atrial fibrillation, atrial flutter, ventricular tachycardia are all cardiac arrhythmias. They can cause fainting and worse, up to and including death. Water intoxication does not cause them. Alcohol can.
Water . Water intoxication is uncommon, but is a potential cause for dangerous and even fatal arrhythmias. It is very difficult to consume such high quantities of water that it becomes dangerous. Massively excessive water intake results in dilution of the blood's critical electrolytes. This can result in instability of the heart's electrical system, resulting in arrhythmia. Such cases have been reported in marathoners and other athletes who excessively hydrate during competition.