Atrioventricular (AV) block is a condition characterized by a disruption in the heart’s electrical signals as they travel from the upper chambers (atria) to the lower chambers (ventricles). This impairment can result in a slowed heart rate, as the electrical impulses are either delayed or completely blocked from reaching the ventricles.
The Heart’s Electrical Conduction System
The heart functions as a pump, driven by a precise electrical conduction system that coordinates its contractions. This system originates with the sinoatrial (SA) node, often called the heart’s natural pacemaker, located in the upper right chamber. The SA node generates electrical impulses that spread across the atria, causing them to contract and pump blood into the ventricles.
Following atrial contraction, these electrical signals travel to the atrioventricular (AV) node, situated near the center of the heart. The AV node acts as a crucial gatekeeper, temporarily delaying the electrical impulse. This brief delay, approximately 0.09 seconds, ensures that the atria have fully emptied their blood into the ventricles before the ventricles begin to contract. From the AV node, the impulse then proceeds through specialized pathways, including the bundle of His and Purkinje fibers, to stimulate the ventricles, causing them to pump blood out to the body.
Common Cardiovascular Causes
Damage or disease affecting the heart’s structure can significantly impair the AV node’s function, leading to AV block. Coronary artery disease, a condition where blood vessels supplying the heart become narrowed, is a frequent contributor. A heart attack, or myocardial infarction, often results from severe coronary artery disease and can directly damage heart tissue, including parts of the electrical conduction system. Scar tissue formed after a heart attack can obstruct the normal flow of electrical signals.
Cardiomyopathy, a disease of the heart muscle, can also cause AV block by altering the heart’s structure and electrical properties. Myocarditis, which is inflammation of the heart muscle, can similarly disrupt the electrical pathways, as the inflammatory process and resulting cellular damage interfere with impulse conduction. Valvular heart diseases, especially those affecting valves near the AV node, can also contribute to AV block by causing structural changes or pressure on the conduction system.
Medications and Other Systemic Conditions
Beyond structural heart problems, several external and body-wide factors can induce AV block. Certain medications are known to slow heart rate and can inadvertently cause AV block as a side effect. Beta-blockers and calcium channel blockers, commonly prescribed for conditions like high blood pressure or angina, reduce the heart’s electrical activity and can depress AV nodal conduction. Digoxin, a medication used for heart failure and certain arrhythmias, increases vagal tone and directly slows conduction through the AV node. Some antiarrhythmic drugs, designed to regulate heart rhythm, can paradoxically cause AV block by affecting the heart’s electrical pathways.
Electrolyte imbalances, particularly high levels of potassium (hyperkalemia), can significantly disrupt the heart’s electrical signaling. Hyperkalemia can impair the depolarization of cardiac cells, slowing or blocking electrical signals as they pass through the AV node. Conversely, low potassium levels (hypokalemia), and imbalances in calcium and magnesium, can also affect cardiac cell repolarization and contribute to AV block.
Inflammatory diseases, such as sarcoidosis, can lead to granuloma formation and fibrosis within the heart, directly impacting the AV conduction system. Lyme disease and acute rheumatic fever, both inflammatory conditions, can also cause AV block through inflammation of cardiac tissues. Certain endocrine disorders can influence the heart’s electrical system, potentially leading to conduction abnormalities.
Age-Related Degeneration and Congenital Factors
The natural aging process is a common cause of AV block in older adults. Over time, the heart’s electrical conduction system can undergo degenerative changes, including fibrosis and sclerosis, which involve the hardening and scarring of tissues. These changes impede the efficient transmission of electrical impulses, making AV block a more prevalent condition with advancing age.
In some individuals, AV block can be present from birth, categorized as congenital factors. Congenital AV block can occur due to maternal autoimmune antibodies that cross the placenta and damage the developing fetal AV node. In other cases, it may be associated with structural congenital heart defects where the AV node or its connections are malformed. While rare, AV block can also emerge as a complication following cardiac surgery due to damage to the delicate conduction pathways during the procedure.