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Category: Cardiology

Topic: 3 Lead Rhythms originating in the Sinus Node and Atria

Level: Paramedic

Next Unit: Sinus Rhythms

5 minute read

Premature atrial complexes are premature heartbeats originating in the atria.

Abnormally early atrial depolarization causes a normal ventricular depolarization. Usually, the underlying rhythm is not affected and continues.

Atrial premature complexes are also referred to as atrial premature beats, premature atrial beats, premature atrial complexes, or atrial extrasystoles. They may originate from either a single ectopic pacemaker site or from multiple sites within the atria.


  • Rate: Depends on the underlying rhythm.
  • Rhythm: Irregular.
  • P Waves: Present in the PAC, but may have a different shape due to a non-SA node origin.
  • PR Interval: Varies in the PAC, otherwise the PRI is normal, 0.12 - 020.
  • QRS: 0.06 - 0.10.

When a single electrical impulse originates in the atria outside of the SA node, it does not initiate a normal appearing P wave. If the impulse is conducted through the atrioventricular node, it can also cause a QRS complex to occur before the next expected sinus beat, because the premature atrial complex depolarizes the SA node prematurely. This resets timing of the SA node.

The next expected P wave (of the underlying rhythm) appears earlier than it would have had the SA node had not been disrupted (a non-compensatory pause).


Timing of Premature Atrial Complexes

How premature a "premature" P wave is in relation to the expected time for its occurrence determines different outcomes:

  1. The PAC lands at the SA node while it is still refractory, causing a dropped QRS complex.
  2. The PAC lands at the SA node and makes it through the ventricles until it encounters a refractory portion of the bundle branches.

INCOMPLETE COMPENSATORY PAUSE: when a PAC resets the SA node with the subsequent beat occurring earlier than expected.

FULL COMPENSATORY PAUSE: due to a ventricular premature beat.

In a patient with heart disease, frequent PACs may prelude paroxysmal supraventricular tachycardia (PSVT), atrial fibrillation, or atrial flutter.