Electrocardiogram in atrial tachycardia suggesting left atrial origin of the arrhythmia

Leads aVL and V1 show a regular narrow QRS complex tachycardia with P waves present before each QRS complex. The P waves are abnormal when compared with those during sinus rhythm occurring after the tachycardia breaks (arrow). The P wave morphology - negative in aVL, positive in V1 - suggests a left atrial origin.