What are common causes of third-degree AV block?

• A. Hypertension, diabetes, and old age
• B. Degenerative changes of the conduction system, digitalis, heart surgery, and acute MI
• C. Hyperkalemia and severe dehydration
• D. Coronary artery disease and Cardiomyopathy

Answer: (B)

Third-degree AV block, also known as complete heart block, occurs when there is a complete interruption of electrical signals between the atria and ventricles. This condition can lead to a dissociation between the atrial and ventricular activities. Option B encompasses common causes of this phenomenon, including degenerative changes in the conduction system, which can occur with aging or due to underlying conditions that affect the heart's electrical pathways.

Digitalis toxicity, a risk for patients on certain cardiac medications, can also lead to third-degree AV block by interfering with the normal electrical impulses. Heart surgery can disrupt the conduction pathways, potentially leading to blockages. Additionally, an acute myocardial infarction (MI) can damage the heart tissue, including the conduction system, resulting in third-degree AV block due to ischemia or necrosis in critical areas of the heart involved in electrical conduction.

Each of the other options mentions conditions that may have some association with heart issues but are not directly linked to the occurrence of third-degree AV block in the way that option B outlines.