Atrial Ectopic and Premature Atrial Complex (PAC)
These arise 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.
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).
ECG Features of Premature Atrial Complex (PAC)
PACs usually 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.
Classification of Premature Atrial Complex (PAC)
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 Premature Atrial Complex (PAC)
- 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:
- Excess caffeine.
- Digoxin toxicity.
- Myocardial ischaemia
- This rhythm strip displays the typical pattern of frequent PACs (arrows) separated by post-extrasystolic pauses.
- 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.
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.
LITFL Further Reading
- ECG Library Basics – Waves, Intervals, Segments and Clinical Interpretation
- ECG A to Z by diagnosis – ECG interpretation in clinical context
- ECG Exigency and Cardiovascular Curveball – ECG Clinical Cases
- 100 ECG Quiz – Self-assessment tool for examination practice
- ECG Reference SITES and BOOKS – the best of the rest
- Brady WJ, Truwit JD. Critical Decisions in Emergency and Acute Care Electrocardiography
- Surawicz B, Knilans T. Chou’s Electrocardiography in Clinical Practice: Adult and Pediatric
- Wagner GS. Marriott’s Practical Electrocardiography 12e
- Chan TC. ECG in Emergency Medicine and Acute Care
- Rawshani A. Clinical ECG Interpretation
- Mattu A. ECG’s for the Emergency Physician
- Hampton JR. The ECG In Practice, 6e