I agree completely with Dr Röschl’s explanation for what is happening here but I wonder if 3:1 block is really the best description for what we see. Certainly, the first sinus impulse after each conducted beat is blocked because we would expect that one to conduct and it doesn’t. However, it is physiologically impossible for the next sinus impulse, occurring at the same time as, or immediately after, the escape beat, to conduct so perhaps that one should not be regarded as blocked. If the escape beat hadn’t occurred, maybe that sinus impulse would have conducted, and we would then have had 2:1 AV block, but we just don’t know. It’s even possible that if the escape beat hadn’t intervened there might have been 3:2 Wenckebach AV block, albeit with a big jump in PR interval from beat 1 to beat 2. I think the escape beats prevent us from defining exactly the degree of AV block we see and I’m always unsure how best to describe this phenomenon.

Wouldn't it be safe just to call this "high-grade AV block"? To me, this is a great descriptive term which means "some P waves are conducted, some are not", but there are more NOT conducted. I am more clinically-oriented, so, to me, a category is often enough, since the treatment, especially in the emergency setting, is determined by the rate and probable mechanism. I tell my students, "if a P wave is outside the refractory period, all alone, and does not conduct, we have a problem". Of course, we have to be able to discern escape beats, and not erroneously couple them with the non-conducted P waves.