How do I distinguish ventricular premature contractions (VPCs) from atrial premature contractions (APCs) on an ECG in an asymptomatic patient with occasional premature beats?

Distinguishing VPCs from APCs on ECG

The key distinction is that VPCs appear as premature abnormal QRS complexes NOT preceded by a premature P wave, while APCs present as premature P waves (with different morphology from sinus P waves) that may or may not conduct to the ventricles. 1

Primary ECG Features to Examine

For Ventricular Premature Contractions (VPCs):

• Look for a premature QRS complex that differs morphologically from the normal sinus QRS complex 1
• Confirm absence of a preceding premature P wave - this is the critical distinguishing feature 1
• The QRS will appear "abnormal" compared to sinus beats, though not necessarily wide (contrary to common teaching) 1
• VPCs are relatively uncommon in the same ECG strip as APCs, so if you see premature P waves with wide QRS complexes, search carefully for a premature P wave before each wide QRS before diagnosing both arrhythmias simultaneously 1

For Atrial Premature Contractions (APCs):

• Identify a premature P wave occurring before the next expected sinus P wave 1
• The premature P wave will have different morphology and mean vector compared to normal sinus P waves 1
• APCs may conduct in three ways: 1
  • Normally to the ventricles (normal QRS follows)
  • With ventricular aberration (wide or abnormal QRS follows)
  • Not conducted at all ("blocked" - no QRS follows)

Critical Pitfall to Avoid

Examine T waves meticulously for hidden premature P waves - blocked atrial bigeminy (where every other P wave is premature and blocked) can masquerade as sinus bradycardia, which has entirely different clinical implications 1, 2. This occurs when blocked APCs follow a regular pattern, creating what appears to be a slow heart rate when it's actually normal sinus rhythm with intervening blocked premature atrial beats 1.

Clinical Context for Asymptomatic Patients

If VPCs are identified:

• Multiple VPCs (≥2 on a single 12-lead ECG) warrant extensive evaluation including Holter monitoring, echocardiogram, and exercise stress testing, even in asymptomatic individuals 1, 3
• The burden matters significantly: ≥2,000 PVCs per 24 hours carries a 30% risk of underlying structural heart disease versus only 3% risk with <2,000 PVCs daily 1, 3
• PVCs that increase (rather than suppress) with exercise suggest underlying cardiac pathology and require cardiac MRI and possible electrophysiology study 1, 3

If APCs are identified:

• In structurally normal hearts, APCs are benign and require only observation 2
• Consider follow-up ECG at 1 month to document resolution or persistence 1, 2
• 24-hour Holter monitoring can quantify APC burden if clinically indicated 2

Algorithmic Approach

1. Identify the premature beat - occurs earlier than expected in the rhythm
2. Examine for a premature P wave:
   • Present with different morphology = APC 1
   • Absent = proceed to step 3
3. Examine the QRS morphology:
   • Different from sinus QRS without preceding premature P wave = VPC 1
   • Same as sinus QRS = likely sinus arrhythmia or other mechanism
4. Check T waves carefully for hidden P waves to avoid missing blocked APCs 1, 2
5. Count total premature beats - if ≥2 VPCs present, initiate workup regardless of symptoms 1, 3