A complete heart block, also known as a third-degree atrioventricular (AV) block, is a severe disruption in the heart’s electrical signaling system where impulses from the upper chambers do not reach the lower chambers. As a result, the atria and ventricles beat independently, leading to a significantly slowed and often irregular heartbeat that can severely affect the heart’s ability to pump blood effectively throughout the body.
The Heart’s Electrical System
The heart’s ability to pump blood relies on a precisely coordinated electrical system. This system originates with the sinoatrial (SA) node, the heart’s natural pacemaker. Located in the upper right atrium, the SA node generates regular electrical impulses that initiate each heartbeat.
These impulses spread across the atria, causing them to contract and pump blood into the ventricles. The signal then reaches the atrioventricular (AV) node, located between the atria and ventricles. The AV node briefly delays the impulse, ensuring atria empty before ventricles contract.
After this brief delay, the electrical signal travels down the His-Purkinje network, a specialized pathway that distributes the impulse throughout the ventricles. This activation causes the ventricles to contract powerfully, pushing blood to the lungs and the rest of the body. The sequence then restarts with a new impulse from the SA node, maintaining a steady, rhythmic heartbeat.
Causes of Complete Heart Block
Complete heart block can develop from factors disrupting the heart’s electrical pathways. One common cause is age-related degeneration of the electrical conduction system, where the heart’s electrical connections scar and fail over time.
Heart conditions also contribute significantly, such as coronary artery disease, which can damage the electrical system. Weakened heart muscle (cardiomyopathies) or heart valve issues can also result in complete heart block.
Certain medications, especially those that slow heart rate, can induce this condition. Examples include beta-blockers, calcium channel blockers, and digoxin. Inflammatory conditions (e.g., sarcoidosis, Lyme disease) and electrolyte imbalances (e.g., high potassium) can also interfere with electrical conduction. Congenital heart block, present at birth, can stem from maternal autoimmune disorders or structural heart defects.
Recognizing the Symptoms
Symptoms of a complete heart block arise from the heart’s inefficient pumping, leading to reduced blood flow to the brain and other organs. Patients may experience severe fatigue, feeling unusually tired. Dizziness or lightheadedness are common, often with unsteadiness.
Fainting (syncope) can occur when the brain temporarily lacks sufficient blood supply, causing sudden loss of consciousness. Shortness of breath may develop as the heart struggles to meet oxygen demands. Chest pain or pressure may also be reported due to impaired heart function.
Diagnosing Complete Heart Block
Diagnosing a complete heart block primarily involves evaluating heart electrical activity. An electrocardiogram (ECG or EKG) is the main diagnostic tool, as it captures electrical signals. In a complete heart block, the ECG shows a complete dissociation between the electrical activity of the atria (P waves) and the ventricles (QRS complexes), showing independent beating.
For intermittent cases, extended monitoring is often used. A Holter monitor records heart rhythms for 24 hours or longer, while an event monitor captures less frequent, irregular episodes. An echocardiogram assesses heart function and structure, and blood tests can help identify underlying causes like electrolyte imbalances or medication toxicity.
Treating Complete Heart Block
Treating complete heart block typically involves restoring a regular heart rhythm, primarily through permanent pacemaker implantation. A pacemaker is a small, battery-powered device that sends electrical signals to the heart, ensuring a consistent rate. It coordinates heart chamber contractions, improving blood flow and alleviating symptoms.
Pacemaker implantation usually involves placing the device under the skin near the collarbone, with thin wires (leads) guided through veins into the heart chambers. These leads deliver impulses to stimulate the heart muscle, regulating its rhythm. Dual-chamber pacemakers, connecting to both the atrium and ventricle, are often preferred for synchronized beating.
In acute situations, such as when a patient is symptomatic or awaiting permanent implantation, temporary pacing options may be used. This can involve external transcutaneous pacing (electrodes on skin deliver impulses) or temporary transvenous pacing (lead inserted into vein and advanced to heart). Medications like atropine or isoproterenol may also be administered temporarily to increase heart rate, especially if the block is medication-induced or associated with an acute event. Managing underlying causes, such as adjusting medications or treating infections, is also important.