Left Bundle Branch Block (LBBB)

Electrocardiographic Criteria and Characteristics

Left bundle branch block is characterized by a significant reorganization of the activation and recovery patterns within the left ventricle, resulting in a widened QRS complex and secondary ST-T wave abnormalities [7].

Diagnostic Criteria

The diagnostic requirements for LBBB vary by the stringency of the criteria applied:

• Classic Criteria: These include a QRS duration of $\ge 120$ msec, the presence of broad and typically notched R waves in leads I, aVL, and the left precordial leads, and narrow r waves followed by broad, deep S waves in the right precordial leads [7]. Septal q waves are usually absent [7].
• Stricter Criteria: These require a QRS duration exceeding 140 msec in males and 130 msec in females, along with slurring or notching of the mid-QRS in at least two left-facing leads [7]. These stricter parameters correlate more closely with abnormal LV mechanical function and the potential benefit of resynchronization pacemaker therapy [7].
• R-Wave Peak Time: Some criteria define LBBB by a prolonged time to the R-wave peak ($\ge 60$ msec) in the left precordial leads [7].

The following table summarizes the common diagnostic criteria for complete and incomplete LBBB:

ST-T Wave Characteristics

In most instances, the ST segment and T wave are discordant with the QRS complex [7]. Specifically, T waves are inverted and ST segments are depressed in leads with positive QRS waves (such as I, aVL, V5, and V6), while ST segments are elevated and T waves are typically upright in leads with predominantly negative QRS complexes (such as V1 and V2) [7].

Etiology and Mechanisms

LBBB may result from lesser degrees of conduction delay within the left bundle system in the case of incomplete LBBB [7]. In more severe forms, LBBB is associated with the following:

• Myocardial Infarction: LBBB can develop as a complication of myocardial infarction (MI) [5]. It is more frequently associated with anterior infarction than with inferoposterior infarction [4].
• Myocardial Necrosis: The occurrence of LBBB (as a form of bifascicular block) is linked to extensive myocardial necrosis [4].

Clinical Implications and Prognosis

Mechanical and Diagnostic Impact

LBBB causes extensive reorganization of left ventricular activation [2]. This leads to abnormal LV mechanical function [7]. Furthermore, LBBB significantly complicates the electrocardiographic diagnosis of MI because the resulting secondary ST-T changes can either mask or mimic the signs of ischemia or infarction [5].

Prognosis and Survival

The presence of LBBB has several implications for patient outcomes:

• Mortality in STEMI: Patients presenting with ST-elevation myocardial infarction (STEMI) and bundle branch blocks exhibit higher long-term and in-hospital mortality rates, as well as lower ejection fractions and higher peak biomarker levels, compared to those without such conduction defects [12].
• Sudden Cardiac Death (SCD): In patients with a low ejection fraction, syncope is considered a risk factor for death [15]. For patients with LBBB and a left ventricular ejection fraction (LVEF) $< 35%$, the implantation of a defibrillator or a CRT-D is indicated to prevent SCD [15].
• Disease Progression: There is a notable rate of progression for those with incomplete LBBB, with up to one-third of these patients developing complete LBBB within a two-year period [7].

---

References [1] International guideline (EU), Section 5.3.1.3 [2] International guideline (EU), Mechanisms for the Electrocardiogram Abnormalities [3] International guideline (EU), Table 12.7 [4] International guideline (EU), Bifascicular Block, Including Left Bundle Branch Block [5] International guideline (EU), Electrocardiogram Diagnosis of Myocardial Infarction with Bundle Branch Blocks [7] International guideline (EU), Electrocardiogram Abnormalities [12] International guideline (EU), Intraventricular Block [15] International guideline (EU), Section 5.3.1.1