Cardiac Medicine Certification (CMC) Practice Exam 2025 – Comprehensive All-in-One Guide for Exam Success!

Third-degree heart block, also known as complete heart block, is characterized by a complete absence of conduction between the atria and ventricles. In this condition, the electrical impulses generated in the sinoatrial (SA) node do not reach the atrioventricular (AV) node, which results in the atria and ventricles beating independently of one another.

In this scenario, the atrial pacing is intact with P waves occurring at regular intervals, but these P waves do not correlate with QRS complexes, which can lead to varying ventricular rates that may be slow and irregular. Since there is a complete block in conduction between the SA and AV nodes, the atrial and ventricular rhythms are dissociated, making this condition distinctly recognizable on an electrocardiogram (ECG).

In contrast to the other options, where P waves might still lead to QRS complexes in the case of a lesser degree of block or have a certain relationship with the QRS complexes, third-degree heart block is specifically defined by the total lack of communication between the SA and AV nodes, making the choice that describes this lack of conduction the correct one.