It is characterized as an intermittend complete impairment of conduction from the sinoatrial node to the atrial myocardium. For example, only every 2nd, 3rd, or n-th firing of the sinoatrial node spreads to the atrial myocardium. The resulting rhythm is half, a third, or a fifth of the actual sinoatrial heart rate. Since the body usually reacts to bradycardia-induced hypotension and reduced tissue perfusion with an increase in the sinus node rate, it is impossible to determinethe exact rate of conduction on the surface ECG. If the heart rate falls below the spontaneous depolarization rate of the secondary pacemaker centers, a periodic replacement rhythm typically arises.