PR interval is constant in:
**Core Concept**
The PR interval represents the time from the onset of the P wave to the start of the QRS complex in an electrocardiogram (ECG). It is a measure of the atrioventricular (AV) conduction time, reflecting the delay between atrial depolarization and ventricular contraction.
**Why the Correct Answer is Right**
The PR interval is constant in first-degree atrioventricular (AV) block, also known as first-degree heart block. In this condition, there is a prolongation of the PR interval, but it remains constant from beat to beat. This occurs due to a slowing of the electrical conduction through the AV node, which can be caused by various factors such as age, electrolyte imbalances, or intrinsic disease of the AV node.
**Why Each Wrong Option is Incorrect**
**Option A:** This is incorrect because in second-degree AV block, the PR interval is not constant; it is intermittently prolonged, resulting in dropped beats.
**Option B:** This is incorrect because in third-degree AV block, the PR interval is variable and the ventricles are not triggered by atrial contractions, leading to a complete dissociation between atrial and ventricular rhythms.
**Option C:** This is incorrect because in bundle branch block, the PR interval is normal, but there is a delay in ventricular depolarization, resulting in a widened QRS complex.
**Clinical Pearl / High-Yield Fact**
First-degree AV block is often asymptomatic, but it can be a marker for underlying cardiac disease or electrolyte disturbances. Regular monitoring of the ECG is essential to detect any progression to more severe forms of AV block.
**Correct Answer: A. First-degree AV block.**