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

• Unlike with a premature ventricular complexe (PVC), the post-extrasystolic pause is not equal to double the preceding RR interval

P wave morphology

• The P wave typically has a different morphology and axis to the sinus P waves
• 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)
• 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)

Aberrancy and “blocked” PACs

• 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

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-entry tachyarrhythmia — e.g. Atrial fibrillation, atrial flutter, AVNRT, AVRT

Causes of PACs

Frequent or symptomatic PACs may occur due to:

• Anxiety
• Sympathomimetics
• Beta-agonists
• Excess caffeine
• Hypokalaemia
• Hypomagnesaemia
• Digoxin toxicity
• Myocardial ischaemia

ECG Examples

Example 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 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: This rhythm strip is not recorded simultaneously

Related Topics

• Premature junctional complex (PJC)
• Premature ventricular complex (PVC)

Advanced Reading

Online

• Wiesbauer F, Kühn P. ECG Mastery: 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
• Smith SW. The ECG in Acute MI. 2002 [PDF]

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