What are premature atrial complexes (PACs)?

Premature Atrial Complexes (PACs)

Premature atrial complexes (PACs) are early beats originating from a focus in the atria other than the sinus node, characterized by premature P waves with different morphology from sinus P waves that may be conducted normally, with aberration, or blocked. 1

Electrocardiographic Features

Diagnostic Characteristics

• P wave characteristics: PACs appear as premature P waves with morphology different from sinus P waves 1
• Timing: Occurs before the next expected sinus P wave in normal sinus rhythm 1
• Conduction patterns:
  • Normal conduction to ventricles
  • Conduction with ventricular aberration (wide QRS)
  • Non-conducted or "blocked" PACs 1

Special Patterns

• Blocked atrial bigeminy: PACs occur in alternating pattern with sinus beats but are not conducted to ventricles
  • Can simulate sinus bradycardia
  • Careful examination of T waves needed to identify blocked P waves 1
• Aberrant conduction: In infants, PACs may conduct with either right or left bundle branch block pattern due to similar refractory periods of bundle branches 1

Clinical Significance

Prevalence and Risk

• Very common finding on Holter monitoring at all ages
• Incidence increases with age 2
• Generally benign in structurally normal hearts 2
• In patients with heart disease, may be a harbinger of more serious atrial tachyarrhythmias 2

Symptoms

• Often asymptomatic
• May cause palpitations described as "skipped beats" or "fluttering" 3
• Can cause symptomatic bradycardia when occurring in bigeminal pattern with blocked conduction 4

Evaluation

Initial Assessment

• 12-lead ECG to document PACs
• 24-hour Holter monitoring to quantify PAC burden 3
• Echocardiogram to assess for structural heart disease and ventricular function 3

Differential Diagnosis

• Distinguished from premature ventricular complexes (PVCs) by presence of premature P wave
• In cases with wide QRS, careful search for premature P wave preceding QRS is needed 1
• Dynamic auscultation maneuvers (Valsalva, position changes, carotid sinus massage) can help identify PACs 3

Management

When Treatment Is Not Required

• Asymptomatic patients with no structural heart disease generally do not require treatment 3
• For patients with mild symptoms:
  • Reassurance
  • Avoidance of triggers (caffeine, alcohol, stress)
  • Clinical follow-up 3

When Treatment Should Be Considered

• Symptomatic patients with palpitations or other symptoms
• PACs causing symptomatic bradycardia (blocked bigeminal PACs) 4
• Very frequent PACs with risk of developing tachycardia-induced cardiomyopathy

Treatment Options

1. First-line pharmacologic therapy:

   • Beta-blockers for symptomatic patients 3
   • Non-dihydropyridine calcium channel blockers as alternative when beta-blockers contraindicated 3

2. Second-line options:

   • Class IC antiarrhythmics (e.g., flecainide) in absence of structural heart disease 3
   • Avoid medications that may exacerbate underlying conditions (QT-prolonging drugs in long QT syndrome, etc.) 3

3. Interventional approach:

   • Catheter ablation for patients with:
     • Symptoms refractory to medical treatment
     • Intolerance to antiarrhythmic medications
     • PAC-induced ventricular dysfunction 3, 5
   • Ultra-high-density mapping has improved success rates for PAC ablation 5

Special Considerations

Athletes

• Athletes with symptomatic or exercise-induced atrial arrhythmias may need restriction from competitive sports except class IA sports 3

Follow-up

• For patients with frequent PACs, follow-up ECG at 1 month may be performed 1
• Important to distinguish blocked atrial bigeminy from severe sinus bradycardia, as the former is generally benign while the latter may indicate systemic illness 1

Relationship to Stroke

• Current evidence does not support an independent association between PAC burden and increased risk of recurrent stroke or death in patients with mild to moderate ischemic stroke 6
• PACs or non-sustained AF <30 seconds were not associated with higher risk of recurrent stroke/TIA or death in a large cohort study 6

Clinical Pitfalls

• Mistaking blocked atrial bigeminy for sinus bradycardia (examine T waves carefully for hidden P waves) 1
• Confusing PACs with aberrancy for PVCs (look for premature P waves) 1
• Overlooking PACs as a cause of symptomatic bradycardia, potentially leading to unnecessary pacemaker implantation 4