Sinus tachycardia is a rapid heartbeat or tachycardia that is caused by a sino-atrial node (S-A Node) that fires too rapidly.
The electrical system of the heart is made up of several parts that communicate with one another to signal the heart muscle fibers when to contract. The SA node starts the signal causing the atria to contract. This signal travels through the AV node and on to the bundle of His, bundle branches causing the ventricles to contract. The flow of electrical signals is what produces a normal heartbeat. A fast heart rate is called a tachycardia.
The sino-atrial node (or SA node) is the natural pacemaker of the heart and emits electrical signals that begin each heartbeat. If the SA node fires to rapidly this will cause the heart to beat at a very rapid rate. This is known as sinus tachycardia.