Differentiating LBBB-Pattern SVT From VT Using Initial Forces in Lead V1

Differentiating LBBB-Pattern SVT From VT Using Initial Forces in Lead V1

Based on the classic Kindwall Criteria (Kindwall et al., Circulation 1988)

Distinguishing supraventricular tachycardia (SVT) with left bundle branch block (LBBB) from ventricular tachycardia (VT) is often challenging because both can produce a broad-complex tachycardia with LBBB morphology.

Kindwall and colleagues (1988) published a landmark analysis identifying initial QRS forces in V1 as a powerful discriminator.

This article explains the why and how of these ECG clues.

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Background: Why V1 Initial Forces Matter

In true LBBB, ventricular activation starts normally in the right ventricle (RV) and then spreads slowly to the left ventricle (LV).

Thus, the initial portion of the QRS reflects rapid septal activation, which remains normal even if the overall QRS is wide.

In contrast, VT originating from the LV produces abnormal early activation, with slow cell-to-cell conduction from the ectopic focus → leading to slurred, delayed, or absent initial forces.

Kindwall et al. exploited this physiology to differentiate SVT with LBBB from VT.

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Kindwall Criteria (1988): Initial Forces in V1 to Diagnose VT

Kindwall et al. (Circulation, 1988) established three major criteria in lead V1 that strongly favor VT over SVT with LBBB.

All of these examine the first 40 ms of the QRS.

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1. RS Interval > 60 ms in V1

Measure from QRS onset to the first change of polarity (from R to S).

If > 60 ms → strongly suggests VT.

Rationale: slow initial activation implies myocardial (not His-Purkinje) conduction.

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2. Initial r Wave > 30 ms in V1

A small initial r should be short and sharp in true LBBB.

If the initial r is broad, slurred, or ≥ 30 ms → favors VT.

Rationale: early activation is slowed by muscle-to-muscle spread.

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3. Any Notching in the First 40 ms of QRS

Notching, slurring, or an irregular upstroke in the initial 40 ms indicates abnormal initial depolarization.

Presence of notching → consistent with VT.

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Physiological Logic Behind Kindwall's Criteria

Feature Why It Happens in SVT-LBBB Why It Suggests VT

Fast initial forces Septum and RV activate normally through His-Purkinje Initial activation occurs away from normal system

Sharp r in V1 Normal initial septal activation Delayed, slurred activation from myocardial focus

No early notching His–Purkinje still drives early depolarization Diseased myocardium causes slow, uneven conduction

Thus:

Normal → fast, clean onset.

VT → slow, wide, notched onset.

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Putting It Together: Practical Interpretation in V1

SVT with LBBB more likely when:

Initial r is thin, sharp, < 30 ms

RS interval < 60 ms

No notching in the initial upstroke

Overall morphology resembles “classical” LBBB pattern

Clinical context: prior LBBB on baseline ECG, young patient without structural heart disease

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VT more likely when:

Initial r wave in V1 is wide or slurred

Initial notch within 40 ms

RS interval > 60 ms

Dominant initial forces appear slow or broad

Elderly patient, structural heart disease, prior MI

Kindwall found that these criteria were highly specific for VT when any were present.

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Clinical Utility

These criteria remain useful even today and have influenced modern algorithms (Brugada, Vereckei, ACLS approaches).

They are especially helpful when a tachycardia looks like LBBB but the clinician must decide quickly between adenosine vs amiodarone, or rate-control vs defibrillation pathways.

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Conclusion

The Kindwall 1988 initial-forces criteria in V1 remain one of the most elegant and physiologically grounded ways to differentiate SVT with LBBB from true VT.

When the initial 40 ms of QRS in V1 is slow, wide, notched, or delayed, VT becomes the overwhelmingly likely diagnosis.