Atrial tachycardia with atrioventricular block

In this arrhythmia, an ectopic atrial focus is the dominant pacemaker and activates the atrial myocardium at a rate of 140 to 220 beats per minute. Due to changes in the electrophysiologic properties of the atrioventricular node, however, there is a slowing of impulse conduction through and prolongation of the refractory period of the AV node, resulting in failure of the node to conduct some of the atrial impulses. In this case, the atrial rate is 220 beats per minute and the failure to conduct occurs in a repeating 2:1 pattern.

The normal P wave in sinus rhythm is slightly notched since activation of the right atrium precedes that of the left atrium. The P wave is upright in a positive direction in leads I and II. A P wave with a uniform morphology precedes each QRS complex. The rate is between 60 and 100 beats per minute and the cycle length is uniform between sequential P waves and QRS complexes. In addition, the P wave morphology and PR intervals are identical from beat to beat.