Atrial Tachycardia with a Type I Block

What is the rhythm?

Image 1

Image 2

This is an irregular narrow complex rhythm. The first 2 sinus P waves (S) are of the same morphology with a PRI of about 0.20 sec and a rate of about 60 bpm (lead II). The tachycardia starts with a premature complex with different P wave morphology (blue arrow). The tachycardia is best observed in V1. The rate tends to increase (107 to about 170 bpm) then there is a non-conducted P wave (first red arrow). The first conducted P has a shorter PRI compared to the previous PRI. The tachycardia resumes with 1:1 conduction then a non-conducted P wave is seen (second red arrow). Thus, this is atrial tachycardia with a block (type I block).

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