Third-degree atrioventricular (AV) block, also called complete heart block, is a serious medical condition involving a total failure of the heart’s electrical communication system. This interruption means the upper chambers (atria) cannot send their electrical signal to the lower chambers (ventricles) to trigger a coordinated heartbeat. When this complete blockage occurs, the heart rate drops significantly, leading to severe symptoms and compromised blood pumping. Immediate medical intervention is necessary because this condition carries a high risk of cardiovascular collapse or sudden cardiac arrest if left untreated.
How the Heart’s Electrical System Works
The heart functions as an efficient pump, governed by a specialized electrical conduction system. The process begins in the right upper chamber (atrium) at the sinoatrial (SA) node, which is the heart’s natural pacemaker. This node generates an electrical impulse that causes the atria to contract, typically at a rate between 60 and 100 times per minute, pushing blood into the lower chambers.
The electrical signal then travels to the atrioventricular (AV) node, a relay station between the upper and lower chambers. The AV node briefly delays the impulse, ensuring the atria have fully emptied their blood into the ventricles. Finally, the signal passes through the His-Purkinje system, which rapidly distributes the impulse to the ventricles, causing them to contract and eject blood out to the body and lungs. This coordinated sequence is essential for maintaining a normal heartbeat.
The Mechanics of Complete Heart Block
A Third-Degree AV Block signifies a complete failure of the electrical impulse to pass from the atria to the ventricles through the AV node. None of the electrical signals generated by the SA node reach the lower chambers, though the atria continue to beat at their normal rate.
Because the ventricles are completely cut off, they must rely on backup pacemaker cells located lower in the conduction system. This results in complete atrioventricular dissociation, where the upper and lower heart chambers beat independently. The heart’s rhythm is dictated by a slower, less reliable “escape rhythm” originating from the AV junction or the ventricles themselves.
This escape rhythm is significantly slower than the normal heart rate, often falling below 50 beats per minute. If the impulse originates deep within the ventricles, the rate may drop to 20 to 40 beats per minute. This slow rate and lack of coordination severely compromises the heart’s pumping efficiency, causing the severe symptoms experienced by patients.
Recognizing the Symptoms and Identifying the Cause
The profound slowing of the heart rate leads to a lack of sufficient oxygenated blood reaching the body’s tissues, causing a distinct set of symptoms. Patients often experience severe fatigue and weakness because the circulatory system cannot meet metabolic demands. Dizziness, lightheadedness, and shortness of breath are common due to inadequate blood flow to the brain and lungs.
Syncope, or fainting, is a particularly concerning symptom that occurs when the slow heart rate momentarily interrupts blood flow to the brain. Symptom severity relates directly to how slow and unreliable the ventricular escape rhythm is. In the most serious cases, the escape rhythm may fail entirely, leading to sudden loss of consciousness and cardiac arrest.
The development of a Third-Degree AV Block is often linked to underlying heart disease or age-related changes. The most common cause in older adults is idiopathic fibrosis, involving the progressive degeneration and scarring of the electrical conduction pathways. Ischemic heart disease, especially following a heart attack (myocardial infarction), can also damage the AV node tissue.
Certain medications, including beta-blockers, calcium channel blockers, and digoxin, can slow conduction enough to precipitate a complete block. Other causes include congenital heart defects, inflammatory conditions like Lyme disease or sarcoidosis, and electrolyte imbalances. Identifying the specific cause is important because some blocks, such as those related to drug toxicity or acute infection, may be reversible.
Immediate and Permanent Treatment Options
Managing a Third-Degree AV Block is a medical emergency focused on stabilizing the patient’s heart rate and blood pressure. Immediate interventions involve temporary pacing to artificially increase the heart rate. This is initiated using transcutaneous pacing, where external pads on the chest deliver electrical impulses through the skin to stimulate the heart muscle.
Physicians may attempt to use medications like atropine, which can sometimes temporarily improve conduction by acting on the AV node. However, atropine is often ineffective because the block is usually located lower in the conduction system. If extended temporary pacing is required, transcutaneous pacing is replaced by transvenous pacing, which uses a temporary wire threaded into the heart to deliver a more reliable stimulus.
For patients whose block is not due to a temporary, reversible cause, the definitive long-term treatment is a permanent pacemaker. This small device is placed under the skin near the collarbone, with leads guided into the heart chambers. The pacemaker constantly monitors the heart’s rhythm and delivers a precisely timed electrical impulse to the ventricles when the heart rate drops below a safe threshold. This intervention ensures a regular and adequate heart rate, preventing the severe symptoms and risks associated with complete heart block.