Certified Rhythm Analysis Technician (CRAT) Practice Exam

In a first degree heart block, what is the nature of the PR interval?

1. Constant and greater than 0.20 seconds in duration

2. Completely variable in duration

3. Gradually decreases in duration until a P wave appears without a QRS complex

4. Gradually lengthens until a P wave appears without a QRS complex

The correct answer is: Constant and greater than 0.20 seconds in duration

In first-degree heart block, the hallmark characteristic is a prolonged PR interval, which is consistently greater than 0.20 seconds. This duration indicates a delay in the conduction of electrical impulses from the atria to the ventricles, specifically at the level of the atrioventricular (AV) node. In this condition, the PR interval remains constant for each heartbeat, meaning that every P wave is followed by a QRS complex, but the timing of the P waves relative to the QRS complexes is consistent with a lengthened interval. Other options describe different types of heart block or arrhythmias. A completely variable PR interval reflects other conditions, such as atrial fibrillation. Gradual changes in PR interval lengthening or shortening relate to second-degree heart block (Mobitz types I and II), where there are patterns of dropping QRS complexes. Therefore, the defining feature of first-degree heart block is a constant, prolonged PR interval, confirming that the correct answer is that the PR interval is constant and greater than 0.20 seconds in duration.