A third-degree heart block represents a severe disruption in the heart’s electrical system, demanding immediate medical attention. This condition occurs when the electrical signals from the upper chambers of the heart, the atria, are completely unable to reach the lower chambers, the ventricles.
Understanding 3rd Degree Heart Block
Third-degree heart block, also known as complete heart block, involves a total dissociation between the electrical activity of the atria and the ventricles. The sinoatrial (SA) node, the heart’s natural pacemaker located in the right atrium, continues to generate electrical impulses at its regular rate. However, these impulses are entirely blocked from passing through the atrioventricular (AV) node, which normally serves as the sole electrical connection to the ventricles.
When atrial signals cannot reach the ventricles, they must initiate their own independent electrical impulses to contract. This ventricular “escape rhythm” originates from a site lower in the conduction system, often within the AV node or ventricular muscle. This intrinsic rhythm is significantly slower and less reliable than the normal heart rate, typically ranging from 20 to 40 beats per minute.
How It Manifests in the Body
The reduced heart rate and uncoordinated pumping in third-degree heart block lead to various symptoms. Individuals may experience profound dizziness or lightheadedness due to insufficient blood flow to the brain. This can progress to syncope, or fainting, as the brain temporarily lacks oxygen.
Extreme fatigue is common, as the body’s tissues and organs do not receive enough oxygenated blood. Shortness of breath may develop, particularly with exertion, because the heart cannot effectively pump blood. Some individuals may also report chest pain, or angina, from the heart muscle not receiving enough oxygen.
A slow heart rate can also lead to confusion or altered mental status due to reduced oxygen delivery to the brain. The severity of symptoms often correlates directly with how slow the ventricular rate becomes and and how effectively the heart can compensate.
The ECG Picture
An electrocardiogram (ECG) provides a clear picture of third-degree heart block. The most defining characteristic is complete atrioventricular (AV) dissociation, meaning the P waves (atrial depolarization) and the QRS complexes (ventricular depolarization) occur independently. The atrial rate, indicated by the P-P interval, remains regular, usually at a normal or slightly elevated pace.
The ventricular rate, represented by the R-R interval, also remains regular, but it is significantly slower than the atrial rate, typically between 20 and 40 beats per minute. This uncoupling means that P waves can be seen “marching through” the tracing, appearing at various points relative to the QRS complexes, sometimes before, within, or after a QRS. There is no consistent PR interval, which would normally indicate the time delay for electrical signals to travel from atria to ventricles.
The appearance of the QRS complex itself offers clues about the origin of the ventricular escape rhythm. If the escape rhythm originates from a site within the AV junction, the QRS complexes tend to be narrow, similar to normal QRS complexes. However, if the escape rhythm arises from the ventricles themselves, the QRS complexes will appear wide and often bizarre, reflecting the slower, abnormal conduction pathway within the ventricular muscle. These distinctive ECG findings are diagnostic for complete heart block.
Underlying Causes
Third-degree heart block can arise from various underlying conditions that impair the heart’s electrical conduction system. Myocardial infarction, commonly known as a heart attack, is a frequent cause, particularly if the damage affects the inferior wall of the heart and disrupts blood supply to the AV node. Degenerative changes within the heart’s conduction system, such as Lenègre’s disease (progressive fibrosis of the His-Purkinje system) or Lev’s disease (calcification of the fibrous cardiac skeleton), can also lead to complete block over time.
Certain medications can induce third-degree heart block, especially when taken in high doses or in sensitive individuals. These include beta-blockers, calcium channel blockers, and digoxin, which all slow AV nodal conduction. Electrolyte imbalances, particularly severe hyperkalemia (high potassium levels), can profoundly affect cardiac electrical activity. Inflammatory conditions, such as Lyme disease or acute rheumatic fever, can also cause inflammation and damage to the AV node, leading to conduction block.
What Happens Next
Third-degree heart block constitutes a medical emergency that necessitates immediate hospitalization. Upon diagnosis, initial interventions often focus on stabilizing the patient and temporarily increasing the heart rate. This frequently involves temporary transcutaneous or transvenous pacing, where electrical impulses are delivered to the heart to ensure adequate ventricular contractions until a more definitive solution can be implemented.
The standard long-term treatment for persistent third-degree heart block is the implantation of a permanent pacemaker. This small device is surgically placed under the skin, usually near the collarbone, with wires threaded into the heart chambers. The pacemaker continuously monitors the heart’s electrical activity and delivers precisely timed electrical impulses to ensure a regular and adequate heart rate, thereby preventing the symptoms and complications associated with severe bradycardia. With timely and appropriate intervention, particularly permanent pacemaker implantation, the prognosis for individuals with third-degree heart block generally improves significantly.