AEMCA Practice Exam

How does a left bundle branch block present on an ECG?

1. Narrow QRS complex

2. A wide QRS with a peaked R wave in an upward position

3. A wide QRS with peaked R wave in a downward position

4. Normal QRS duration with ST elevation

The correct answer is: A wide QRS with peaked R wave in a downward position

In the context of a left bundle branch block (LBBB), the presentation on an electrocardiogram (ECG) can be distinctly characterized. Specifically, LBBB typically manifests as a wide QRS complex, usually greater than 120 milliseconds, due to delayed conduction through the left bundle branch. The key feature of this ECG representation is the pattern of the QRS complex. In LBBB, there is often a significant change in the morphology of the QRS, which usually presents as a "broad" complex. This is due to the heart's left ventricle depolarizing later than the right ventricle. As a result, the morphology includes a peaked R wave that is oriented downward, which correlates with the electrical activity of the left ventricle being activated later than normal. Consequently, this characteristic downward peak of the R wave combined with the broad QRS complex is what identifies the presence of LBBB on an ECG. The other options do not appropriately describe the typical features associated with a left bundle branch block, such as the normal QRS duration or incorrect orientation of the QRS morphology.