What does the EKG typically show if a PAB fails to fully depolarize the AV node?

What does the EKG typically show if a PAB fails to fully depolarize the AV node?

• A. A missing P wave
• B. An early P wave with no QRS complex or T wave following it
• C. A very short QT interval
• D. A regular rhythm with occasional pauses

Answer: (B)

When a premature atrial beat (PAB) fails to fully depolarize the AV node, it typically manifests as an early P wave that is not followed by a QRS complex or T wave. This occurs because the PAB initiates depolarization in the atria prematurely, but if the AV node is not fully depolarized, the electrical impulse cannot travel through the node and reach the ventricles, resulting in a failure to produce a QRS complex and consequently a T wave that would normally follow. This phenomenon illustrates the concept of a blocked impulse where the timing of the beat disrupts the normal conduction pathway.

The presence of an early P wave is critical in identifying the origin of the beat as occurring in the atria. However, since the impulse fails to pass through the AV node, it effectively does not trigger the usual ventricular response, hence the absence of a subsequent QRS complex and T wave. This scenario helps reinforce the understanding of AV nodal conduction and the effects of ectopic beats in the atrial region on the overall cardiac rhythm.