Left bundle branch block and right bundle branch block represent distinct patterns of electrical delay within the heart’s conduction system, and differentiating lbbb ecg vs rbbb is essential for accurate clinical interpretation. While both conditions involve failure of rapid ventricular depolarization, the underlying mechanisms, ECG signatures, and clinical implications diverge significantly. Recognizing these nuances allows clinicians to refine diagnosis, risk stratify patients, and avoid misattribution of underlying structural heart disease.
Anatomy and Physiology of the Bundle Branches
The heart’s specialized conduction system initiates each beat and coordinates sequential activation of the atria and ventricles. The bundle of His divides into the right and left bundle branches, which rapidly deliver electrical impulses to the respective ventricles. The right bundle branch is anatomically slender and possesses a longer refractory period, rendering it more susceptible to conduction delay. The left bundle branch fans out into a broad subendocardial network, so its failure typically indicates more extensive myocardial involvement or structural remodeling.
ECG Features of Right Bundle Branch Block
In rbbb ecg, the impulse reaches the left ventricle normally but reaches the right ventricle late, producing a characteristic triphasic pattern in the right precordial leads. The classic findings include a wide QRS duration exceeding 120 ms, an rSR’ morphology in V1 and V2 with a prominent terminal R wave, and a deep, wide S wave in the lateral leads such as I and V6. The ST segment and T wave typically deflect opposite to the terminal QRS vector, yielding secondary repolarization changes that reinforce the diagnosis.
Key Diagnostic Markers in rBBB
QRS duration ≥ 120 ms with a wide, slurred R wave in right precordial leads.
rsR’ or M-shaped R wave in V1 and V2 with rS pattern in V5 and V6.
Secondary ST-T changes consistent with discordant repolarization.
Normal or near-normal left ventricular activation sequence.
ECG Features of Left Bundle Branch Block
By contrast, lbbb ecg reflects delayed activation of the left ventricle, while the right ventricle is activated via interventricular septal conduction from left to right. This results in a broad, monomorphic QRS complex with a characteristic contour in the lateral and inferior leads. The absence of a septal q wave and delayed left ventricular activation produce a pattern that clinicians must interpret within the clinical context to avoid overdiagnosis of myocardial infarction.
Diagnostic Hallmarks of LBBB
QRS duration typically > 120 ms, often approaching 140–160 ms.
Broad, notched R waves in lateral leads (I, aVL, V5, V6) with delayed peak timing.
Absence of q waves in lateral leads and discordant ST-T changes relative to the QRS main vector.
Prolonged left ventricular activation time, often assessed with specific diagnostic criteria such as the width of the initial R wave in V6.
Clinical Implications and Differential Diagnosis
Identifying lbbb ecg vs rbbb extends beyond waveform recognition, as each pattern portends different considerations for management. RBBB is frequently observed in structurally normal hearts and may reflect benign conduction disease, whereas LBBB is more often associated with underlying cardiomyopathy, ischemic heart disease, or valvular pathology. In acute settings, misinterpreting a LBBB as a STEMI can lead to inappropriate reperfusion therapy, underscoring the importance of applying standardized diagnostic criteria for myocardial infarction in the presence of bundle branch block.
