This 81-year-old woman with chronic obstructive lung disease presents to the Emergency Department with increasing dyspnea and wheezing:

•  Multifocal atrial tachycardia 160/min
•  Occasional aberrantly conducted QRS complexes

This tracing illustrates the classical features of Multifocal Atrial Tachycardia (MAT), where the P waves originate from different foci.  The rhythm is irregularly irregular, and so MAT is frequently misdiagnosed as atrial fibrillation. P waves with varying morphology (3 or more different P wave patterns) precede each QRS complex. There is variation of the PP and PR intervals. Aberrantly conducted QRS complexes with right bundle branch block morphology are present. Notice the Ashman phenomenon: long-short RR interval sequence precedes aberrancy).