10.2: Mechanism of Cardiac Arrhythmias
Arrhythmias are irregular heart rhythms occurring when the heart's electrical impulses become abnormal. These disturbances can lead to various symptoms, depending on their severity and the underlying cause. Some common factors contributing to arrhythmias include hypoxia, ischemia, electrolyte imbalances, excessive catecholamine exposure, drug toxicity, and muscle overstretching. Arrhythmias can be classified into two main types based on the rate and site of origin of abnormal heart rhythms. Bradyarrhythmias, characterized by a slow heart rate typically less than 60 beats per minute, often result from dysfunction in the sinoatrial (SA) node or atrioventricular (AV) node. These nodes are essential in initiating and coordinating the heart's electrical activity. On the other hand, tachyarrhythmias are characterized by a rapid heart rate greater than 100 beats per minute. They can originate from the atria or the ventricles of the heart. Two main types of abnormal impulse formation mechanisms can lead to arrhythmias: ectopic pacemaker activity and reentrant circuits.
Ectopic pacemaker activity occurs when an area of the heart outside the regular pacemaker sites (SA node and AV node) generates abnormal electrical impulses. This can disrupt the typical sequence of electrical activation and lead to arrhythmias. Reentrant circuits, on the other hand, involve the circular movement of electrical impulses within the heart due to abnormal conduction pathways or areas with varying conduction properties. This can result in a self-sustaining loop of electrical activity, causing the heart to beat abnormally fast or irregularly. Diagnosis of arrhythmias often involves an electrocardiogram, which records the heart's electrical activity. Additional diagnostic tools may include Holter monitoring, event recorders, and electrophysiology studies. Treatment options for arrhythmias depend on the type, severity, and underlying cause. Some common treatments include medications that regulate heart rhythm, electrical cardioversion, implantable devices such as pacemakers or defibrillators, and, in some cases, surgical interventions.
