In an ECG, a QRS complex axis of -30° to -90° (left axis deviation) is seen in?

Appeared in: ISRO 2024

Explanation

Left Axis Deviation (LAD) is defined as a QRS axis between -30° and -90°.
Left ventricular hypertrophy (LVH) involves an increase in the muscle mass of the left ventricle.
The larger, thicker left ventricle generates a stronger electrical potential and takes longer to depolarize.
This increased electrical dominance of the left ventricle shifts the heart's mean electrical axis to the left, causing LAD.

Option B: Right ventricular hypertrophy (RVH) causes the electrical axis to shift towards the right, resulting in Right Axis Deviation (RAD), typically an axis greater than +90°.
Option C: Complete heart block is a disorder of atrioventricular (AV) conduction, leading to a dissociation between P waves and QRS complexes. It does not directly cause a shift in the QRS axis.
Option D: Left atrial enlargement primarily affects the P wave, causing it to become broad and notched (P mitrale). It does not affect the QRS axis, which represents ventricular depolarization.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain ECG Interpretation: QRS Axis Deviation as background academic context rather than a clinical decision trigger.
Identifying the QRS axis is a fundamental step in ECG interpretation. It provides clues to underlying structural heart disease.
Left axis deviation is a significant finding that should prompt a search for underlying causes, most commonly hypertension leading to LVH, ischemic heart disease, or conduction system disease.
What if? If the axis was +110°, the most likely cause among the options would be Right Ventricular Hypertrophy, as this indicates Right Axis Deviation.

How to Approach the Question

First, identify the key information in the question: the ECG finding is a QRS axis of -30° to -90°.
Recognize that this range defines Left Axis Deviation (LAD).
Recall or deduce the common causes of LAD. Think about what conditions would make the left side of the heart electrically dominant.
Evaluate each option: Left ventricular hypertrophy increases the left ventricle's muscle mass, making it electrically dominant and causing LAD.
Eliminate the other options by recalling their typical ECG manifestations: RVH causes RAD, complete heart block affects AV conduction, and left atrial enlargement affects the P wave.

Concept Tested & Keywords

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E6 Medicine Harrison 22e Part 1 pp. 1912-1914, 1913-1915

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