What are premature atrial contractions (PACs)?

Premature Atrial Contractions (PACs)

Premature atrial contractions are early beats that originate from the atria, characterized by premature P waves with different morphology and mean vector from sinus P waves, which may be conducted to the ventricles normally, with aberration, or may be blocked. 1

Definition and Electrocardiographic Features

PACs are characterized by:

• A premature P wave that occurs before the next expected sinus P wave
• Different P wave morphology compared to normal sinus beats
• May be followed by:
  • Normal QRS complex (normal conduction)
  • Aberrant QRS complex (abnormal conduction)
  • No QRS complex (blocked PAC) 1

When PACs occur in a bigeminal pattern (every other beat), they can create a pattern called "blocked atrial bigeminy" which may simulate sinus bradycardia. In these cases, careful examination of T waves for hidden P waves is important for correct diagnosis. 1

Mechanism

PACs result from enhanced automaticity, triggered activity, or re-entry mechanisms in the atrial tissue. They represent early depolarizations originating from atrial foci outside the normal sinus node. 1

The pathophysiological mechanisms include:

• Enhanced automaticity in ectopic atrial foci
• Triggered activity from afterdepolarizations
• Re-entrant circuits within atrial tissue 1

Clinical Presentation

Patients with PACs may experience:

• Palpitations described as "skipped beats" or "fluttering" 2
• Irregular heart rhythm sensation
• Pauses followed by stronger beats (compensatory pause)
• Sensation of "dropped beats" 2

Many patients with PACs are asymptomatic, with the arrhythmia discovered incidentally during routine ECG or monitoring.

Clinical Significance

Although previously considered benign, PACs have now been associated with:

• Increased risk of developing atrial fibrillation 3, 4
• Potential marker of atrial cardiomyopathy 3, 4
• Independent association with stroke and all-cause mortality 3
• Higher incidence of atrial fibrillation when originating from pulmonary veins compared to non-pulmonary vein origins 5

PACs with shorter coupling intervals (time between the PAC and preceding normal beat) are more likely to trigger atrial fibrillation than those with longer coupling intervals. 6

Diagnostic Evaluation

For symptomatic patients or those with frequent PACs:

• 12-lead ECG to document the arrhythmia
• 24-hour Holter monitoring to quantify PAC burden
• Echocardiogram to assess for structural heart disease
• Dynamic auscultation maneuvers (Valsalva, position changes) can help identify PACs 2

Management

Management depends on symptoms, PAC frequency, and underlying cardiac conditions:

1. Asymptomatic patients with infrequent PACs:

   • Reassurance
   • No specific treatment required 2

2. Symptomatic patients:

   • Lifestyle modifications:
     • Avoid triggers (caffeine, alcohol, stress)
     • Adequate sleep
     • Regular physical activity 2

3. Highly symptomatic patients or those with very frequent PACs:

   • Pharmacological therapy:
     • Beta-blockers as first-line therapy 2
     • Non-dihydropyridine calcium channel blockers as alternative 2
     • Class IC antiarrhythmics (e.g., flecainide) in absence of structural heart disease 2

4. Refractory cases:

   • Catheter ablation for symptomatic, frequent, drug-refractory PACs 5, 6
   • Pulmonary vein isolation for PACs originating from pulmonary veins 5, 6
   • Focal ablation for non-pulmonary vein origins 5

Prognosis and Follow-up

• Catheter ablation has shown high success rates (83-89%) for eliminating symptomatic PACs 5, 6
• Patients with frequent PACs should be monitored for development of atrial fibrillation
• Follow-up ECG at 1 month may be performed in patients with frequent PACs 1

Special Considerations

• PACs originating from pulmonary veins have higher association with atrial fibrillation than those from non-pulmonary vein sites 5
• Blocked atrial bigeminy may mimic sinus bradycardia but is generally benign, unlike severe sinus bradycardia which may indicate systemic illness 1
• The concept of atrial cardiomyopathy suggests that structural and functional changes in the atria may lead to both PACs and thromboembolic events, with atrial fibrillation potentially being an epiphenomenon rather than the direct cause 3, 4