Advanced Cardiovascular Life Support (ACLS) Practice Exam

Which block is characterized by progressively lengthening PR intervals until a beat is dropped?

1. Normal sinus rhythm

2. Second degree AV block Type 1 Wenckebach

3. Asystole

4. Third degree AV block

The correct answer is: Second degree AV block Type 1 Wenckebach

The scenario described, wherein PR intervals progressively lengthen until one QRS complex is dropped, is specifically characteristic of Second-degree AV block, Type 1, also known as Wenckebach. This phenomenon occurs as a result of a repeated failure of conduction through the atrioventricular node (AV node). In this type of block, the electrical signals from the atria become progressively delayed as they pass through the AV node until it finally fails to conduct one of the impulses, leading to a dropped beat. This pattern is often seen in patients with transient heart block conditions, particularly in younger individuals and those without significant underlying heart disease, which supports its classification under Type 1. In contrast, a normal sinus rhythm would present with a consistent PR interval, while asystole is characterized by a complete absence of electrical activity without any cardiac output, evident on an ECG as a flat line. Third-degree AV block, also known as complete heart block, involves a total disassociation between atrial and ventricular activity, meaning there is not a consistent PR interval pattern as seen in Wenckebach. Therefore, the unique progressive lengthening of the PR intervals leading to a dropped beat clearly aligns with the features of Second-degree AV block, Type 1