A Third-Degree Atrioventricular (AV) Block, also known as complete heart block, is the most severe form of heart rhythm disorder, or arrhythmia. This condition involves a complete failure of the electrical signal to pass from the heart’s upper chambers (atria) to the lower chambers (ventricles). Because the heart’s rhythmic contraction relies on electrical impulses, this block causes a breakdown in communication. When this complete disconnect occurs, the heart rate drops significantly, severely impacting the body’s ability to circulate blood effectively.
Understanding the Electrical Disconnect
The healthy heart operates on an electrical circuit that begins with the sinoatrial (SA) node, the pacemaker located in the right atrium. This impulse travels across the atria, causing them to contract, and then converges at the AV node, which acts as a gatekeeper to the ventricles. The AV node normally introduces a brief delay before sending the signal down the His-Purkinje system, which triggers the contraction of the ventricles.
In a third-degree AV block, the electrical signal cannot pass through the AV node. This creates an absolute barrier, meaning the regular, rapid electrical signals from the atria are blocked from reaching the ventricles. The atria and ventricles are then forced to operate on their own independent rhythms, a state referred to as AV dissociation.
On an electrocardiogram (ECG), this electrical separation is visible, with the P-waves, representing atrial activity, occurring at a normal rate, and the QRS complexes, representing ventricular activity, occurring independently. Because the ventricles are deprived of the atrial signal, they must rely on a slower, backup pacemaker—an “escape rhythm”—originating lower down in the heart’s conduction system. This ventricular escape rhythm is slow, often between 30 to 40 beats per minute, which is insufficient to maintain proper blood flow and circulation.
Primary Causes and Risk Factors
The development of a third-degree AV block is often linked to underlying heart disease or external factors that damage the electrical pathways of the heart. One of the most common causes is chronic, age-related degeneration, where fibrous tissue infiltrates and scars the conduction system of the heart. This scarring process impairs the ability of the electrical signal to travel efficiently, eventually leading to a complete block.
Acute events, such as a myocardial infarction (heart attack), are a cause, particularly when the blockage affects the inferior wall of the heart and damages the nearby AV node. Severe coronary artery disease can also compromise the blood supply to the conduction system. Certain medications used to treat heart conditions, including beta-blockers, calcium channel blockers, and digoxin, can inadvertently slow conduction, causing a complete heart block.
Other risk factors include systemic inflammatory conditions, such as Lyme disease, sarcoidosis, and systemic lupus erythematosus. Structural heart problems, including congenital defects or damage resulting from heart surgery, such as aortic valve replacement, can also interrupt the electrical wiring of the heart. The presence of pre-existing conduction problems, like a bundle-branch block, also increases the likelihood of progressing to a complete AV block.
Recognizing Symptoms and Diagnostic Confirmation
The reduction in heart rate caused by a complete AV block often results in severe symptoms due to insufficient blood flow to the body and brain. Patients frequently experience syncope (fainting) or a feeling of near-fainting (presyncope), as the brain temporarily lacks oxygenated blood. Chronic symptoms include extreme fatigue and shortness of breath, even with minimal exertion, because the slow heart rate cannot meet the body’s metabolic demands.
Chest pain or pressure can also occur due to the heart muscle not receiving enough oxygen, and some individuals may report dizziness or lightheadedness. In severe cases, particularly if the ventricular escape rhythm is slow, patients can present with signs of hemodynamic instability, including low blood pressure and altered mental status. The severity of the symptoms is often directly related to how slow the ventricular escape rhythm is and the overall health of the patient’s heart.
Diagnostic confirmation relies on the 12-lead Electrocardiogram (ECG), which provides a tracing of the heart’s electrical activity. The characteristic finding of a third-degree AV block on the ECG is the complete electrical dissociation between the P-waves and the QRS complexes. Clinicians observe P-waves occurring regularly at a faster rate than the QRS complexes, with no apparent link between them, confirming the total conduction block.
For patients whose block is intermittent, a Holter monitor or an event recorder may be employed to capture the heart’s rhythm. These devices help capture transient episodes of complete heart block that may not be present during a brief ECG. Further testing may be done to identify the underlying cause, but the immediate electrical finding on the ECG is sufficient to confirm the presence of the block.
Treatment Protocols and Long-Term Management
The immediate treatment for a symptomatic third-degree AV block focuses on stabilizing the patient’s heart rate and circulation. In acute settings, this may involve temporary measures such as administering medications like atropine, although this is often ineffective if the block is located below the AV node. Transcutaneous pacing, which delivers electrical impulses through pads placed on the skin, is frequently used as a bridge to stabilize the heart rate until a more definitive solution can be implemented.
The definitive, long-term treatment for patients with a persistent third-degree AV block is the implantation of a permanent pacemaker. A pacemaker is a battery-powered device surgically placed beneath the skin, usually near the collarbone. Leads from the device are threaded through a vein to the heart, where they monitor the heart’s intrinsic rhythm.
If the pacemaker senses that the heart rate has dropped below a programmed threshold, or if a beat is missed due to the block, it delivers an electrical impulse to stimulate the ventricles and ensure a regular heartbeat. This effectively bypasses the damaged conduction system and restores coordinated pumping action, resolving the symptoms. Long-term management involves regular follow-up appointments to monitor the pacemaker’s function and battery life. Patients are also advised to take certain precautions, such as avoiding strong electromagnetic fields that could interfere with the device.