Paroxysmal atrial tachycardia with exit block.

A case of atrial tachycardia with A-V block is reported with a basic cycle length of 0.40 sec corresponding to an atrial rate of 150/min. Pauses of various length in atrial activity [see figure in the PDF file] were observed, and each was a multiple of 0.20 sec. These were attributed to the effect of an exit block. It was postulated that the actual atrial rate was 300/min, that is, double the manifest rate. In fact, doubling of the manifest atrial rate was temporarily produced by vagus stimulation and was observed also in spontaneous attacks which mimicked closely atrial flutter. These observations underline the difficulty of separating paroxysmal atrial tachycardia and flutter.