The excitation-conduction system of the heart is controlled by the sinoatrial (SA) node, which is located in the right atrium, near the junction between the atrium and the superior vena cava. The SA node emits rhythmic electrical impulses at a higher rate than the rest of the cardiac tissue, including the atrioventricular (AV) node, which fires at rates ranging from 40 to 60 beats per minute (bpm), and other segments of the conduction system and the myocardial tissue itself (20–40 bpm) (Fig. 5.1). As noted early, arrhythmias are anomalous cardiac rhythms caused by the abnormal formation and/or conduction of the electrical impulse. They can be broadly classified as tachyarrhythmias, characterized by rapid heart rates exceeding 100 bpm, and bradyarrhythmias, with slow rates of less than 60 bpm.