Tachycardia: A rapid heart rate, usually defined as greater than 100 beats per minute. The tachycardias include sinus tachycardia, paroxysmal atrial tachycardia (PAT), and ventricular tachycardia.
Sinus tachycardia is due to rapid firing of a normal structure called the sinoatrial (sinus) node which is the natural pacemaker of the heart. Sinus tachycardia occurs in response to exercise, exertion, excitement, pain, fever, excessive thyroid hormone, low blood oxygen (hypoxia), stimulant drugs (such as caffeine and amphetamines), etc.
Paroxysmal atrial tachycardia (PAT) consists of bouts of rapid, regular heart beating originating in the atrium (upper chamber of the heart). Often due to abnormalities in the AV node "relay station" that lead to rapid firing of electrical impulses from the atrium which bypass the AV node under certain conditions. These conditions include alcohol excess, stress, caffeine, overactive thyroid or excessive thyroid hormone intake, and certain drugs. PAT is an example of an arrhythmia where the abnormality is in the electrical system of the heart, while the heart muscle and valves may be normal.
Ventricular tachycardia is an abnormal heart rhythm that is rapid, regular and originates from an area of the ventricle, the lower chamber of the heart. Ventricular tachycardias are most commonly associated with heart attacks or scarring of the heart muscle from previous heart attacks and are life-threatening.