Ectopic Atrial Rhythms

The term atrial tachycardia refers to any supraventricular tachycardia with P waves not of sinus origin (including atrial fibrillation, MAT, etc). However, atrial tachycardia is the common phrase used for an ectopic atrial tachycardia (aka paroxysmal atrial tachycardia). This rhythm occurs due to a single ectopic atrial focus (unlike MAT or atrial fibrillation with numerous atrial foci). It is an arrhythmia on the spectrum from sinus tachycardia with frequent PAC's to atrial fibrillation. Most commonly, it is a paroxysmal atrial rhythm from catecholamine excess, atrial scarring, digoxin toxicity, or congenital abnormalities.

Atrial tachycardia is defined as a HR>100bpm with a single P wave morphology not consistent with sinus origin. The P wave morphology is usually similar to that of atrial flutter, with inverted P waves in the inferior leads and upright in V1. This is because the origin of this ectopic atrial rhythm is usually in the lower portion of the atria (sometimes 'low atrial rhythm'). However, unlike atrial flutter, it has an isoelectric baseline. Additionally, the ventricular rate is not often a number divided by 300 (atrial flutter ventricular response rates usually either ~300bpm, 150bpm, 100bpm, or 75bpm) as the atrial rate is not 300bpm. An ectopic atrial rhythm is due to enhanced automaticity; atrial flutter is a reentrant arrhythmia that produces the sawtooth and consistent P wave pattern. In other words, atrial flutter is a complicated form of atrial tachycardia as the enhanced automaticity producing an ectopic atrial site triggers a macro-reentrant circuit, whereas atrial tachycardia does not cause a macro-reentrant circuit. An ectopic atrial rhythm has the above P wave changes but the HR is less than 100bpm. Because the ectopic pacemaker site is consistent, this will produce a regular narrow complex rhythm with consistent PP, PR, and QRS intervals. However, the PR interval is often quite different than that of a sinus rhythm (atrial usually shorter as pacemaker site usually closer to the AV node).

Ectopic atrial rhythms can mimic ischemia due to the atrial repolarization wave (discussed in further detail on atrial repolarization page). Essentially, the repolarization wave in a lower atrial rhythm will cause ST elevation in leads with a negative P wave (atrial repolarization is seen due to proximity of P wave to QRS, where it is normally hidden in the QRS in a sinus rhythm) and can mimic an OMI. In leads with an upright P wave, the atrial repolarization wave can cause ST depression.

Examples:

Source: LITFL

An ectopic atrial tachycardia around 120bpm. The P waves are inverted in the inferior leads and upright in V1, indicating that it is not of sinus node origin. However, the P waves do not march out at 300bpm that would be typical of atrial flutter.

Source: LITFL

Ectopic atrial rhythm around 95bpm. The morphology of the P waves is similar to the first example.

Source: Steve Smith's blog

Ectopic atrial rhythm mimicking an inferior OMI due to the atrial repolarization waves.