There are three P waves for every QRS complex. Two P waves between the QRS complexes are evident while one is ‘hidden’ over the T wave. Hence this is a 3:1 AV block. At an initial look, it may be called 2:1 AV block because there are only two obvious P waves between any pair of QRS complexes. But the clue is the PP interval, which is only one third of the RR interval. Then one has to look meticulously for the ‘hidden P waves’ over the T waves. The hidden P waves may not be very evident in some leads while it may be detected easily in other leads.
The QRS is not significantly widened, though there is rSr’ pattern in V1. Hence the level of the block is likely to be supraHisian. 3:1 AV block comes under advanced AV block (or high grade AV block) where two or more consecutive P waves are non conducted. Though it comes under second degree AV block in the simple classification, actually it may be considered midway between the usual second degree AV block and complete heart block.