Clinical
ECG Interpretation Part 2: Determination of Bundle Branch and Fascicular Blocks

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QRS Complex: Normal Morphology

The duration of the QRS complex is determined by the beginning and end of ventricular depolarization. Normally, the right and left ventricles depolarize simultaneously. The right bundle branch depolarizes the right ventricle, whereas because of its larger muscle mass, the left bundle branch splits into 2 fascicles to enable synchronized depolarization of the left ventricle.

In any given lead of the 12-lead ECG, the Q, R, or S wave may be present.4 A Q wave is the first initial deflection below

Right Bundle Branch Block

The presence of a right bundle branch block (RBBB) following ACS indicates greater damage to the myocardium and increased risk of mortality.1 New onset RBBB may suggest worsening coronary artery disease or congestive heart failure.

The conduction defect is determined to be an incomplete RBBB when the ECG changes are observed in the presence of QRS duration of 0.08 to 0.11 seconds.1 Incomplete RBBB may be a normal variant or may be associated with right ventricular hypertrophy or right

Left Bundle Branch Block

Patients in whom left bundle branch block (LBBB) develops in association with ACS are at increased risk of hemodynamic instability and of having complete atrioventricular block develop and have reduced long-term survival.7 The ST–T complex changes associated with LBBB imitate those usually associated with STEMI, making ECG interpretation difficult. Failure to diagnosis STEMI in the presence of LBBB in a presenting ECG may result in delays in revascularization. As with any ECG changes suggestive

Fascicular Blocks

Fascicular blocks (also known as hemiblocks) occur either in the anterior or posterior fascicles of the left bundle branch (Figure 2). The right bundle branch does not have fascicles. The anterior fascicle is thin, has only one blood supply, and lies within the turbulent outflow tract, making it more prone to blockage.1

Summary

This article has identified the defining characteristics of bundle branch and fascicular blocks on the 12-lead ECG. The ability to recognize these ECG patterns is important for ED nurses. Accurate ECG interpretation will assist with appropriate triage and timely intervention for patients. All patients who present to the emergency department with any suspicion of cardiac disease should have an ECG recording taken as soon as practicable. Also, ED nurses should be educated in ECG interpretation to

Louise Rose is Lawrence S. Bloomberg Limited Tenure Professor in Critical Care Nursing, Lawrence S. Bloomberg Faculty of Nursing, University of Toronto, Toronto, ON, Canada.

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Louise Rose is Lawrence S. Bloomberg Limited Tenure Professor in Critical Care Nursing, Lawrence S. Bloomberg Faculty of Nursing, University of Toronto, Toronto, ON, Canada.

Lisa Kuhn, is Research Assistant and PhD Student, St Vincent’s/ACU National Centre for Nursing Research, Australian Catholic University (St. Patrick’s Campus), Melbourne, Australia.

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