What are premature atrial complexes (PACs)?

Premature Atrial Complexes (PACs)

Premature atrial complexes (PACs) are early heartbeats that originate from the atria, appearing as premature P waves with different morphology and vector from normal sinus P waves, and may be conducted normally, with aberration, or blocked. 1

Characteristics and Identification

PACs have several distinguishing features:

• They appear as premature P waves that occur before the next expected P wave in normal sinus rhythm
• The P wave morphology differs from sinus P waves
• The premature P wave may be followed by:
  • Normal QRS conduction
  • Aberrant ventricular conduction (with either RBBB or LBBB pattern in infants)
  • No QRS (blocked PAC) 1

When PACs occur in a bigeminal pattern with blocked conduction (blocked atrial bigeminy), they can simulate sinus bradycardia. In these cases, careful examination of T waves for hidden P waves is crucial for correct diagnosis. 1

Clinical Significance

PACs are generally benign but can have important clinical implications:

• They are more common than junctional premature beats but less common than ventricular premature beats 2
• Frequent PACs (≥4/hour) are associated with:
  • Advanced age (OR = 1.030, CI 95% = 1.002-1.059)
  • Elevated BNP levels >20mg/dL (OR = 4.489, CI 95% = 1.918-10.507)
  • Intraventricular conduction blocks (OR = 4.184, CI 95% = 1.816-9.406)
  • Left atrial enlargement (OR = 1.065, CI 95% = 1.001-1.134) 3
• High PAC burden is associated with up to three-fold increased risk of new-onset atrial fibrillation 4

Differential Diagnosis

When evaluating PACs, it's important to distinguish them from other arrhythmias:

• Premature ventricular complexes (PVCs): Unlike PACs, PVCs appear as premature abnormal QRS complexes without preceding premature P waves 1
• Sinus arrhythmia: Shows gradual changes in P-P intervals rather than premature beats
• Atrial bigeminy: Regular pattern of normal sinus beat followed by a PAC

Evaluation

For patients with frequent PACs, consider:

1. Follow-up ECG at 1 month 1
2. 24-hour Holter monitoring to quantify PAC burden
3. Echocardiogram to assess for structural heart abnormalities, particularly left atrial enlargement 3
4. BNP level measurement, as elevated levels correlate with PAC frequency 3

Clinical Pitfalls and Caveats

1. Misdiagnosis pitfall: Blocked atrial bigeminy can mimic sinus bradycardia. Careful examination of T waves for hidden P waves is essential for correct diagnosis. 1

2. Aberrant conduction pitfall: In patients with both premature P waves and wide QRS complexes, search for a premature P wave preceding the wide QRS before concluding the patient has both PACs and PVCs. 1

3. Stroke risk assessment: Despite the association between PACs and atrial fibrillation, current evidence does not support that PACs alone increase the risk of recurrent stroke or death in patients with mild to moderate ischemic stroke. 5

4. Treatment decisions: For symptomatic patients with frequent PACs refractory to medical therapy, catheter ablation using ultra-high-density mapping has emerged as an effective treatment option. 6

Management

Management depends on symptom severity and underlying conditions:

• Asymptomatic patients with infrequent PACs generally require no specific treatment
• For symptomatic patients:
  1. Identify and treat underlying causes (e.g., caffeine, alcohol, stress)
  2. Consider antiarrhythmic medications for symptom control in highly symptomatic patients
  3. Catheter ablation may be considered for patients with refractory symptoms or those who cannot tolerate medical therapy 6

PACs are generally benign but warrant attention when frequent or symptomatic, particularly given their association with increased risk of developing atrial fibrillation. Proper identification and differentiation from other arrhythmias is essential for appropriate management.