Paroxysmal atrial tachycardia: Bouts of rapid, regular heart beating that originate in the upper chamber of the heart (atrium). Abbreviated PAT. PAT is caused by abnormalities in the atrioventricular (AV) node that lead to rapid firing of electrical impulses from the atrium that bypass the AV node under certain conditions, including excess alcohol consumption, stress, caffeine use, overactive thyroid or excessive thyroid hormone intake, and use of certain drugs. PAT is an example of an arrhythmia in which the abnormality is in the electrical system of the heart, while the heart muscle and valves may be normal. Also known as paroxysmal supraventricular tachycardia.