How can I differentiate unifocal from multifocal premature ventricular contractions on a 12‑lead ECG?

Distinguishing Unifocal from Multifocal PVCs on 12-Lead ECG

Unifocal PVCs have identical QRS morphology across all leads with the same frontal plane axis (within 15 degrees), while multifocal PVCs show different QRS configurations with frontal plane axes differing by more than 15 degrees, with or without morphologic differences in the horizontal (precordial) leads 1.

Key ECG Criteria for Differentiation

Unifocal PVCs

• Identical QRS morphology in all 12 leads across different beats
• Consistent frontal plane axis (variation ≤15 degrees)
• Same precordial lead progression pattern (V1-V6)
• Originate from a single ventricular focus

Multifocal PVCs

• ≥2 distinct QRS morphologies visible on the same ECG strip 2
• Frontal plane axes differ by >15 degrees between different PVC morphologies 1
• Different precordial lead patterns between PVC types
• Originate from multiple ventricular foci

Critical Pitfall: Multiformity ≠ Multifocality

A major caveat is that multiformity (varying QRS appearance) does not always mean multifocality 1. Unifocal PVCs can appear multiform when:

• Structural heart disease is present, particularly:

  • Left ventricular wall motion abnormalities (p<0.01) 1
  • Prior myocardial infarction (p<0.01) 1
  • Left ventricular ejection fraction <60% (p<0.05) 1

• Degree of prematurity varies (coupling interval changes) 1

• Site of origin within the same focus shifts slightly 1

In these scenarios, the same ectopic focus can produce different QRS configurations due to altered conduction through diseased myocardium.

Practical Assessment Algorithm

1. Compare QRS morphology in all 12 leads simultaneously (digital ECGs provide accurate temporal alignment within 10 ms) 3

2. Calculate frontal plane axis for each distinct PVC morphology:

   • Use leads I, II, III, aVR, aVL, aVF
   • If axes differ by >15 degrees → suggests multifocal 1

3. Examine precordial lead progression (V1-V6):

   • Different R/S wave transitions suggest different origins
   • RBBB morphology increases likelihood of underlying fibrotic substrate 2

4. Count distinct morphologies:

   • ≥2 different patterns strongly predict fibrotic substrate (p=0.01) 2
   • Associated with structural heart disease requiring advanced imaging

Clinical Implications

The presence of multiple PVC morphologies (≥2) significantly predicts myocardial fibrotic substrate on cardiac MRI 2, which fundamentally changes management from simple observation to consideration of ablation or advanced imaging. This finding is independent of left ventricular ejection fraction 2.

When multifocal PVCs are identified, this signals higher complexity for potential ablation procedures, requiring detailed pace-mapping strategies targeting multiple sites of origin 4.