Premature Atrial Complex (PAC)

A premature atrial complex (PAC) is a premature beat arising from ectopic pacemaking tissue within the atria. There is an abnormal P wave, usually followed by a normal QRS complex. AKA: Atrial ectopics, atrial extrasystoles, atrial premature beats, atrial premature depolarisations.

ECG features of PACs
  • Abnormal (non-sinus) P wave followed by a normal QRS complex (< 120 ms)
  • PACs arising close to the AV node (“low atrial” ectopics) cause retrograde activation of the atria, 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
  • Similiarly, PVCs 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
  • 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 premature junctional complex (PJC)

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 (PAC), junctional (PJC) or ventricular (PVC)

Classification of PACs

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 of PACs
  • 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. Atrial fibrillation, atrial flutter, AVNRT, AVRT

Causes of Premature Atrial Complex (PAC)

Frequent or symptomatic PACs may occur due to:

ECG Examples
Example 1
Premature Atrial Complex (PAC) 1
  • This rhythm strip displays the typical pattern of frequent PACs (arrows) separated by post-extrasystolic pauses

Example 2
Blocked Premature Atrial Complex (PAC) 1

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
Premature Atrial Complex (PAC) 2

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.

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Emergency Physician in Prehospital and Retrieval Medicine in Sydney, Australia. He has a passion for ECG interpretation and medical education | ECG Library |

MBBS (UWA) CCPU (RCE, Biliary, DVT, E-FAST, AAA) Emergency Medicine Advanced Trainee in Melbourne, Australia. Special interests in diagnostic and procedural ultrasound, medical education, and ECG interpretation. Editor-in-chief of the LITFL ECG Library. Twitter: @rob_buttner

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