Revisiting the Strauss Criteria for the Diagnosis of Left Bundle Branch Block

Left bundle branch block (LBBB) is a condition frequently identified on electrocardiograms (ECGs), and cardiac resynchronization therapy (CRT) has shown significant benefits for patients with complete LBBB, but proper selection of CRT candidates remains a challenge. The traditional criterion for diagnosing LBBB may not be sufficient, and new proposals suggest adjusting the criteria to better select patients. In this post, we explore the Strauss criteria for the diagnosis of LBBB and how they may influence the effectiveness of CRT.

Traditional Criteria for Diagnosing LBBB

The conventional diagnosis of LBBB includes three main criteria:

1. QRS duration: ≥120 ms.
2. QRS morphology: QS or rS in V1.
3. Absence of Q waves: in V6 and I.

In addition to these criteria, guidelines recommend the presence of broad, notched R waves in leads such as I, aVL, V5, and V6, as well as occasional RS patterns in V5 and V6. However, these criteria may not fully capture the complexity of LBBB and its relationship with CRT.

Proposed Improvements to the Diagnosis

Strauss suggests that the definition of LBBB may need adjustment to improve patient selection for CRT. The new proposal includes:

1. QRS duration: a stricter cutoff of ≥140 ms for men and ≥130 ms for women. These cutoffs are based on the idea that men have larger hearts and therefore a longer depolarization time.
2. QRS notching and slurring: mid-QRS notching or slurring in at least two of the following leads: V1, V2, V5, V6, I, and aVL.

Rationale for the New Criteria

The new criteria are based on computer simulations showing that QRS notches reflect the time needed for electrical depolarization to spread through the left ventricle. Research also showed that QRS duration can be significantly prolonged in complete LBBB, which is not fully captured by the traditional criteria. In addition, the new criteria help differentiate complete LBBB from conditions such as left ventricular hypertrophy and left anterior fascicular block, which can mimic LBBB.