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.

The cardiac function relies on synchronized electrical activity to maintain a regular rate and rhythm, ensuring effective hemodynamics.

Any deviation leads to cardiac arrhythmias  or dysrhythmias.

These deviations manifest as bradyarrhythmia, characterized by a slow heart rate, or tachyarrhythmia, a rapid heart rate. Their clinical symptoms include palpitations, dizziness, and unconsciousness.

Arrhythmias occur due to various interrelated mechanisms causing abnormal impulse formation or defective heart conductions.

For instance, electrical impulses may originate from structures other than the heart's natural pacemakers —the SA node. This phenomenon is termed ectopic pacemaker activity or enhanced automaticity.

Further, unusually high intracellular Ca²⁺ levels disrupt the action potential, causing delayed afterdepolarizations.

Sometimes, myocardial damage can alter the normal conduction pathway, resulting in the reentry and looping of electrical impulses, causing continuous cardiac stimulation.

Diseases of conducting structures, like AV node fibrosis, lead to different heart block degrees.

Arrhythmia diagnosis involves an ECG analysis to detect anomalies.