What are ectopic beats (premature ventricular contractions (PVCs) or premature atrial contractions (PACs))?

Understanding Ectopic Beats

Ectopic beats are premature cardiac contractions that originate from sites other than the sinoatrial node, most commonly classified as premature atrial contractions (PACs) or premature ventricular contractions (PVCs). 1

Types of Ectopic Beats

Premature Atrial Contractions (PACs)

• PACs are characterized by premature P waves that typically have a different morphology from sinus P waves 1
• PACs may be conducted to the ventricles normally, with ventricular aberration, or may not be conducted at all (blocked PACs) 1
• Blocked PACs can occur in a bigeminal sequence (blocked atrial bigeminy), which can simulate sinus bradycardia 1
• In infants, PACs may be conducted with either right or left bundle branch block patterns due to similar refractory periods of the bundle branches 1

Premature Ventricular Contractions (PVCs)

• PVCs appear on ECG as premature abnormal QRS complexes that are not preceded by premature P waves 1
• PVCs have a different morphology from the normal sinus QRS complex 1
• PVCs are common and increase in frequency with age, found in approximately 50% of all people on long-term monitoring 1
• The QRS duration may be normal or slightly prolonged, but the key diagnostic feature is the different morphology from sinus beats 1

Clinical Significance

PACs

• Most PACs are benign, especially in patients with structurally normal hearts 1
• Blocked atrial bigeminy should be distinguished from sinus bradycardia, as the former is generally benign while severe sinus bradycardia may indicate systemic illness 1
• Follow-up ECG at 1 month may be recommended for patients with frequent PACs 1

PVCs

• Isolated PVCs are generally benign in the absence of structural heart disease 2
• Risk stratification based on PVC burden:
  • 0% risk of underlying structural heart disease with <100 PVCs/24h 2
  • 3% risk with <2,000 PVCs/24h 2
  • Up to 30% risk with ≥2,000 PVCs/24h 2
• Very frequent PVCs (>10,000 to 20,000 per day) can be associated with depressed left ventricular function 1, 3
• When PVCs constitute >20% of total heartbeats, patients may develop cardiomyopathy and heart failure 4, 3

Evaluation

For PACs

• Careful examination of ECG to identify premature P waves and distinguish from PVCs 1
• In cases with both premature P waves and wide QRS complexes, search for premature P waves preceding wide QRS before diagnosing both PACs and PVCs 1

For PVCs

• Evaluation should include assessment for symptoms, underlying structural heart disease with echocardiogram, and exercise stress testing to assess if PVCs suppress with exercise 2, 1
• 24-hour Holter monitoring to quantify PVC burden 2
• QT interval should be measured carefully during periods of sinus rhythm 1
• For complex ventricular arrhythmias or frequent PVCs (≥2,000/24h), additional evaluation may include cardiac MRI 1, 2

Management

For PACs

• Generally no specific treatment is required for asymptomatic PACs 1
• Follow-up ECG at 1 month for frequent PACs 1

For PVCs

• Asymptomatic patients with normal ventricular function require only clinical surveillance without specific treatment 2
• No antiarrhythmic therapy is indicated for asymptomatic patients with normal ventricular function 2
• Treatment should be considered for:
  • Symptomatic patients 2, 5
  • Patients with very frequent PVCs (>10-20% of total beats) 4, 3
  • PVCs associated with structural heart disease 5
• Treatment options include antiarrhythmic medications or catheter ablation 3

Special Considerations

• PVCs that increase rather than suppress during exercise warrant further evaluation 2
• PVCs originating from the right ventricular outflow tract are generally benign but may indicate early arrhythmogenic right ventricular cardiomyopathy if QRS duration exceeds 160ms 1, 2
• In patients with ischemic heart disease, the frequency and complexity of PVCs is associated with increased mortality 5
• Exercise can sometimes suppress cardiac arrhythmias that are present at rest due to overdrive suppression of ectopic impulse formation 1

Pitfalls and Caveats

• Do not confuse blocked atrial bigeminy with sinus bradycardia, as their clinical significance differs significantly 1
• It is not possible to distinguish PVCs from PACs with aberrancy based solely on QRS morphology 1
• Detraining is not recommended as a diagnostic or therapeutic measure in athletes with PVCs 1, 2
• Not all patients with frequent PVCs develop cardiomyopathy, suggesting differential susceptibility 3
• The relationship between PVC morphology and site of origin is not exact enough to predict which ventricle is causing the arrhythmia 1