Second degree AV block is defined by

• A. Some atrial action potentials fail to conduct into ventricles, may be 2-3 atrial depolarizations per ventricular depolarization, ventricular bradycardia
• B. Complete dissociation between atrial and ventricular depolarizations
• C. Normal PR interval with occasional skipped beats
• D. Atrial fibrillation

Answer: (A)

Second-degree AV block occurs when not every atrial impulse conducts to the ventricles. You will see P waves that are not followed by a QRS complex, so the ventricular rhythm drops out intermittently and the ventricle-to-atrial activity ratio becomes uneven (for example, two atrial beats for every one ventricular beat, or three to one). This intermittent failure of conduction slows the overall ventricular rate, hence ventricular bradycardia.

This is different from complete AV block, where there is a complete lack of coordination between atrial and ventricular activity with no fixed relationship between P waves and QRS complexes. It’s also not atrial fibrillation, which involves chaotic, disorganized atrial activity and an irregularly irregular ventricular response. And a description like a normal PR interval with occasional skipped beats doesn’t capture the essential feature: that some atrial impulses fail to conduct to the ventricles, producing dropped ventricular beats.