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:
- Anxiety
- Sympathomimetics
- Beta-agonists
- Excess caffeine
- Hypokalaemia
- Hypomagnesaemia
- Digoxin toxicity
- Myocardial ischaemia
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
Advanced Reading
Online
- Wiesbauer F, Kühn P. ECG Yellow Belt online course: Become an ECG expert. Medmastery
- Wiesbauer F, Kühn P. ECG Blue Belt online course: Learn to diagnose any rhythm problem. Medmastery
- Rawshani A. Clinical ECG Interpretation ECG Waves
- Smith SW. Dr Smith’s ECG blog.
Textbooks
- Mattu A, Tabas JA, Brady WJ. Electrocardiography in Emergency, Acute, and Critical Care. 2e, 2019
- Brady WJ, Lipinski MJ et al. Electrocardiogram in Clinical Medicine. 1e, 2020
- Straus DG, Schocken DD. Marriott’s Practical Electrocardiography 13e, 2021
- Hampton J. The ECG Made Practical 7e, 2019
- Grauer K. ECG Pocket Brain (Expanded) 6e, 2014
- Brady WJ, Truwit JD. Critical Decisions in Emergency and Acute Care Electrocardiography 1e, 2009
- Surawicz B, Knilans T. Chou’s Electrocardiography in Clinical Practice: Adult and Pediatric 6e, 2008
- Mattu A, Brady W. ECG’s for the Emergency Physician Part I 1e, 2003 and Part II
- Chan TC. ECG in Emergency Medicine and Acute Care 1e, 2004
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
ECG LIBRARY
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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