• Atrial tachycardia (AT) arises when one or more atrial automaticity foci send(s) out irritable impulse(s) at a fast rate (> 100 bpm), which suppress(es) normal impulses from the sinus node.
• The underlying mechanisms of AT include ectopy (enhanced automaticity), afterdepolarization (triggered activity), and micro-reentry.
• Because the impulses conducted down the AV-His-Purkinje system, the resulting QRS complexes are normal and narrow.
• AT is further categorized into focal atrial tachycardia (commonly referred to as "atrial tachycardia or AT") and multifocal atrial tachycardia (MAT).
• Focal atrial tachycardia (or "atrial tachycardia or AT") arises when an atrial automaticity focus sends out an irritable impulse at a fast rate (> 100 bpm), which suppresses normal impulses from the sinus node.
• This results in new P’ waves that is morphologically different from normal "sinus" P waves and regular rhythm.
• Atrial rate often ranges from 100 to 250 bpm.
• AT often has irregularity at onset "warm up" and termination "warm down".
• Certain ECG features aid in localization of the ectopic atrial focus. For example, positive Ps in V1 and negative Ps in I, aVL suggest left atrium origin.
• When there is concurrent AV block (either fixed or variable), the resulting rhythm has an increased P to QRS ratio (more than 1 P wave before every QRS).
• This rhythm is often seen in digitalis toxicity.
• Multifocal atrial tachycardia (MAT) arises when irritable eletrical impulses are rapidly sent out (> 100 bpm) from multiple (3 or more) atrial automaticity foci.
• This results in P waves with different (3 or more) morphologies preceding narrow QRS complexes and irregular rhythm.
• This is often associated with chronic lung diseases and digoxin toxicity.
• MAT with normal rater is called wandering atrial pacemaker rhythm.
