Premature Atrial Complex

Lead II




Interpretation Rules

  • An abnormal (non-sinus) P wave is followed by a QRS complex.
  • The P wave typically has a different morphology and axis to the sinus P waves.
  • The abnormal P wave may be hidden in the preceding T wave, producing a “peaked” or “camel hump” appearance — if this is not appreciated the PAC may be mistaken for a PJC.
  • PACS arising close to the AV node (“low atrial” ectopics) activate the atria retrogradely, producing an inverted P wave with a relatively short PR interval ≥ 120 ms (PR interval < 120 ms is classified as a PJC).
  • PACs that reach the SA node may depolarise it, causing the SA node to “reset” — this results in a longer-than-normal interval before the next sinus beat arrives (“post-extrasystolic pause”). Unlike with PVCs, this pause is not equal to double the preceding RR interval (i.e. not a “full compensatory pause”).
  • PACs arriving early in the cycle may be conducted aberrantly, usually with a RBBB morphology (as the right bundle branch has a longer refractory period than the left). They can be differentiated from PVCs by the presence of a preceding P wave.
  • Similarly, PACs arriving very early in the cycle may not be conducted to the ventricles at all. In this case, you will see an abnormal P wave that is not followed by a QRS complex (“blocked PAC”). It is usually followed by a compensatory pause as the sinus node resets.


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