Complete heart block (CHB), also known as Third-Degree Atrioventricular Block, represents the most severe form of electrical signal disruption within the heart. This condition involves a complete failure of electrical impulses to pass from the heart’s upper chambers (atria) to the lower chambers (ventricles). This blockage leads to a profoundly slow and disorganized heart rhythm, preventing the heart from pumping blood effectively. It is a life-threatening medical emergency requiring immediate intervention.
Understanding the Heart’s Electrical System
The heart’s rhythmic contraction depends on a precisely timed electrical signal that begins in the sinoatrial (SA) node, the heart’s natural pacemaker, located in the right atrium. This impulse travels through the atria to the atrioventricular (AV) node, which normally acts as a gateway to delay the signal before it proceeds to the ventricles. The signal then moves through the Bundle of His and the Purkinje fibers, causing the ventricles to contract.
In complete heart block, the AV node or the conduction pathways immediately below it are entirely non-functional, meaning no atrial signal reaches the ventricles. This failure results in atrioventricular (AV) dissociation, where the atria and ventricles beat independently of one another. The atria continue to beat at their normal rate, governed by the SA node, but the ventricles are forced to rely on a secondary, slower pacemaker.
This backup mechanism is called an escape rhythm, which originates lower down in the heart’s electrical system, often in the Bundle of His or the ventricles themselves. These escape pacemakers are inherently slow and unstable, typically generating a ventricular rate of 30 to 40 beats per minute. This severely reduced rate is insufficient to sustain adequate blood flow.
Causes and Contributing Factors
Complete heart block often arises from damage to the heart muscle or the electrical tissue that occurs over time. One of the most common causes is age-related degenerative changes, such as fibrosis or sclerosis, which involve the progressive scarring of the heart’s conduction system. Ischemic heart disease, particularly a heart attack (myocardial infarction), can also damage the tissue responsible for conducting signals, especially if the damage occurs near the AV node. Infiltrative diseases like sarcoidosis or Lyme disease, though less common, can similarly disrupt the electrical pathways by causing inflammation or scarring.
Additionally, certain medications can induce or worsen the condition by slowing down the AV node’s function. These include high doses of antiarrhythmic agents, beta-blockers, and calcium channel blockers, which are commonly prescribed to manage blood pressure or other heart rhythm issues. Congenital heart block, while rare, can also occur due to an autoimmune process where maternal antibodies cross the placenta and damage the fetal AV node.
Recognizing the Immediate Symptoms
The profound drop in heart rate associated with complete heart block directly reduces the heart’s ability to circulate blood, leading to a host of immediate symptoms. Patients often experience severe fatigue and general weakness due to the lack of oxygenated blood reaching the muscles and organs. This reduced cardiac output can manifest as shortness of breath, particularly with activity, as the body struggles to meet its oxygen demand.
A sudden drop in blood pressure can cause lightheadedness, dizziness, or syncope (fainting). Some individuals may also report chest pain or pressure, which signifies that the heart muscle itself is not receiving enough blood flow.
Diagnostic Procedures
The definitive way to confirm complete heart block is through the use of an Electrocardiogram (ECG or EKG), which records the heart’s electrical activity. On the ECG tracing, the defining characteristic of this condition is complete AV dissociation. This electrical pattern shows the atrial electrical signals (P waves) and the ventricular signals (QRS complexes) occurring at their own regular but independent rhythms.
Specifically, the atrial rate, represented by the P waves, is faster than the ventricular rate, represented by the QRS complexes. There is no consistent timing relationship between the P wave and the QRS complex, meaning the P wave “marches through” the QRS complexes without any electrical connection. For patients whose symptoms are intermittent, continuous monitoring devices like a Holter monitor or an event recorder may be used to capture brief periods of block that might be missed during a standard ECG.
Management and Pacemaker Therapy
Management of acute complete heart block begins with immediate stabilization to raise the heart rate. In emergency situations, temporary measures such as administering medications like atropine or implementing temporary external pacing (transcutaneous pacing) are used to bridge the patient to a more permanent solution. Transcutaneous pacing involves using electrode pads on the chest to deliver electrical impulses directly to the heart muscle, forcing a contraction.
For most individuals with acquired complete heart block, a permanent pacemaker implantation is the established, definitive treatment. The pacemaker is a small, battery-powered device surgically placed under the skin near the collarbone, with insulated wires (leads) guided into the heart chambers. The device constantly monitors the heart’s electrical activity and, when it detects the complete failure of the AV node to conduct a signal, it delivers a small electrical impulse. This artificial signal stimulates the ventricles to contract at a programmed, reliable rate, thereby bypassing the non-functional AV node and restoring proper blood circulation.