What is the most likely cause of an AV block classified as 3rd degree?

• A. Functional impairment in the AV node
• B. Increased vagal tone
• C. Electrolyte imbalances
• D. Complete failure of electrical communication

Answer: (D)

In a third-degree AV block, also known as complete heart block, there is a total failure of electrical communication between the atria and the ventricles. This condition occurs when the signals from the atria are completely blocked at the level of the AV node or below, meaning that the atria and ventricles beat independently of each other. As a result, there is no correlation between atrial and ventricular contractions, which can lead to significant bradycardia and potential hemodynamic instability.

The complete failure of communication is characterized by the absence of any conduction through the AV node, making it the primary cause of this type of block. In practice, this often results from structural heart disease, ischemia, or fibrosis affecting the conduction pathways.

Understanding this phenomenon helps in managing patients with third-degree AV block, often requiring interventions such as the placement of a permanent pacemaker to ensure adequate heart rate and cardiac output.