Disorders of Cardiac Rhythm
Abnormalities of cardiac rhythm can vary widely, from asymptomatic premature atrial complexes and mild sinus bradycardia to life-threatening ventricular tachycardia and fibrillation. Disorders of cardiac rhythm can be categorized into several groups, including bradyarrhythmias and conduction disturbances, ectopic or premature contractions, and tachyarrhythmias.
Although many rhythm and conduction disturbances are caused by underlying IHD, they are also attributable to valvular heart disease, myocarditis, cardiomyopathy, congenital aberrant conduction pathways, pulmonary disease, toxic or metabolic disorders, neurogenic causes, and cardiac trauma.
The electrical impulse that initiates each heartbeat normally begins in the sinoatrial (SA) node and is conducted down through the atria and ventricles, resulting in a coordinated series of contractions of these chambers. The SA node is the primary pacemaker of the heart. It controls the heart rate and is influenced by neural, biochemical, and pharmacologic factors. If the SA node function is depressed or absent, secondary pacemakers in the atrioventricular (AV) junction, the bundle of His, or the ventricular muscle can generate stimuli and maintain the heartbeat. Normally, stimulus formation in these secondary pacemaker sites is slower than in the SA node. However, abnormal stimuli can also be generated at any of these sites at a rapid pace, resulting in tachycardia.
Bradyarrhythmias and Conduction Disturbances
A bradyarrhythmia is any rhythm resulting in a ventricular rate of less than 60 beats per minute (bpm). Conduction block, or heart block, is a condition in which electrical signals are slowed or interrupted between the atria and ventricles. Bradyarrhythmias and conduction blocks are generally asymptomatic, although they may cause lightheadedness or syncope in rare cases. If the condition is linked to medication use, simply discontinuing the inciting medication may lead to resolution of the bradycardia. Treatment is generally unnecessary except in patients with syncope or hemodynamic instability. In those cases, placement of a cardiac pacemaker is usually the definitive treatment.
Excerpted from BCSC 2020-2021 series: Section 1 - Update on General Medicine. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.