The next two chapters discuss abnormal cardiac rhythms. This chapter, chapter 7, discusses rhythms that may be of normal rate but originate from regions other than the sinus node, and rhythms that are abnormally rapid. These abnormally rapid rhythms are referred to as the tachyarrhythmias and include those that originate in the sinus node as well as those that originate at regions other than the sinus node. The second chapter, chapter 8, discusses rhythms that are abnormally slow, the bradyarrhythmias.
Some of the rhythm disturbances are physiologic, such as the increase in heart rate that occurs with exercise, the decrease in heart rate that occurs during sleep, and the variations in heart rate that occur with respiration. Others are pathologic and may be the cause of palpitations, syncope and/or sudden death. Some occur in hearts that are otherwise normal and represent isolated electrical abnormalities that may have a genetic basis, while other occur in hearts that have been damaged by a variety of cardiac diseases.
All rhythm disturbances, whether fast or slow, whether physiologic or pathologic, and whether occurring in normal or diseased hearts, can be attributed to abnormalities in impulse formation, to abnormalities in impulse propagation, or to both. From an electrocardiographic standpoint, the challenge is to determine the rhythm abnormality and to deduce its electrophysiologic cause. The ability to meet both of these challenges is important because the therapeutic approach, i.e. which drug or drugs to use and whether or not to insert a pacemaker or defibrillator often depends upon the interpretation of the body surface electrocardiogram.
In each of these chapters we will consider the mechanism(s) responsible for the various rhythm disturbances as well as their electrocardiographic presentations and interpretations.