Premature Atrial Complex (PAC)

AKA atrial ectopics, atrial extrasystoles, atrial premature beats, atrial premature depolarisations. 


  • A premature beat arising from an ectopic focus within the atria.

Origin Of Ectopic Beats

  • Groups of pacemaker cells throughout the conducting system are capable of spontaneous depolarisation.
  • The rate of depolarisation decreases from top to bottom: fastest at the sinoatrial node; slowest within the ventricles.
  • Ectopic impulses from subsidiary pacemakers are normally suppressed by more rapid impulses from above.
  • However, if an ectopic focus depolarises early enough — before the arrival of the next sinus impulse — it may “capture” the ventricles, producing a premature contraction.
  • Premature contractions (“ectopics”) are classified by their origin — atrial (PACs), junctional (PJCs) or ventricular (PVCs).

Premature ventricular complex (PVC) cardiac conduction system Atrial Ectopics

  • These arise from ectopic pacemaking tissue within the atria.
  • There is an abnormal P wave, usually followed by a normal QRS complex.

Electrocardiographic Features

PACs have the following features:

  • 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.


PACs may be either:

  • Unifocal – Arising from a single ectopic focus; each PAC is identical.
  • Multifocal – Arising from two or more ectopic foci; multiple P-wave morphologies.


PACs often occur in repeating patterns:

  • Bigeminy — every other beat is a PAC.
  • Trigeminy — every third beat is a PAC.
  • Quadrigeminy — every fourth beat is a PAC.
  • Couplet – two consecutive PACs.
  • Triplet — three consecutive PACs.

Clinical Significance

  • PACs are a normal electrophysiological phenomenon not usually requiring investigation or treatment.
  • Frequent PACs may cause palpitations and a sense of the heart “skipping a beat”.
  • In patients with underlying predispositions (e.g. left atrial enlargement, ischaemic heart disease, WPW), a PAC may be the trigger for the onset of a re-entrant tachydysrhythmia — e.g. AF, flutter, AVNRT, AVRT.


Frequent or symptomatic PACs may occur due to:

ECG Examples

Example 1

  • This rhythm strip displays the typical pattern of frequent PACs (arrows) separated by post-extrasystolic pauses.


Example 2

Blocked PAC:

  • This hidden PAC gives a peaked appearance to the T wave (circled).
  • The PAC is not not followed by a QRS complex, indicating that it has not been conducted to the ventricles (“blocked PAC”).
  • It is followed by a compensatory pause.


Example 3

Normally and aberrantly-conducted PACs:

  • There is an aberrantly conducted PAC, best seen in aVL and aVF (circled).
  • This could be mistaken for a ventricular ectopic — however, it is clearly preceded by an abnormal P wave.
  • A normally-conducted PAC is also present on the rhythm strip (circled).

NB. The rhythm strip is not recorded simultaneously.


Related Topics

Further Reading

Author Credits


  • Chan TC, Brady WJ, Harrigan RA, Ornato JP, Rosen P. ECG in Emergency Medicine and Acute Care. Elsevier Mosby 2005.
  • Surawicz B, Knilans T. Chou’s Electrocardiography in Clinical Practice (6th edition), Saunders 2008.
  • Wagner, GS. Marriott’s Practical Electrocardiography (11th edition), Lippincott Williams & Wilkins 2007.
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  1. Saleh Awdaly says

    The problem arising occasionally in clinical practice when there is a berrantly conducting PAB in the setting of atrial fibrillation and /or flutter , because no P wave ; therefore , it is difficult to discriminate between atrial or ventricular beats , though the absence or short compensatory pause and the direction with the same QRS complex May helpful suggest atrial aberrancy . Thank you so much providing such incredible information .