What defines a 3rd degree heart block?

A third-degree heart block, also known as complete heart block, is characterized by a complete dissociation between the atria and ventricles. In this condition, electrical signals originating in the sinoatrial (SA) node do not reach the atrioventricular (AV) node, leading to a lack of communication between the two chambers. As a result, the atria and ventricles beat independently of one another.

This complete block primarily occurs between the SA node and the AV node, preventing any impulses from reaching the AV node for conduction down to the ventricles. Consequently, the atria may contract regularly under the influence of the SA node, while the ventricles rely on a separate, slower intrinsic pacemaker for their contractions. This important distinction is what defines the third-degree block, leading to symptoms such as bradycardia, possible syncope, or other signs of inadequate cardiac output.

Understanding the characteristics of various heart blocks helps healthcare providers quickly identify and manage these conditions appropriately. The other options reflect different types of heart blocks or ECG interpretations that do not align with the definitions and mechanisms of a complete heart block. For instance, a fixed PR interval points to first-degree AV block, which is not the same, while dropping beats with