An ECG was performed as part of a routine cardiac evaluation. Based on the findings, which of the following is the most likely cause of the abnormal pattern?
Show Hint
Always check the P wave in Lead I and aVR first. An inverted P wave in Lead I is abnormal and immediately suggests either LA/RA lead reversal, dextrocardia, or an ectopic atrial rhythm originating from the left atrium.
Concept:
The question requires the interpretation of an abnormal 12-lead ECG, focusing on identifying a common technical error versus true anatomical pathology. Explanation: • The most common technical error in clinical electrocardiography is the accidental reversal of the Left Arm (LA) and Right Arm (RA) limb leads.
• Normally, electrical depolarization of the heart moves from the top right (SA node) down towards the bottom left (apex). Therefore, lead I (which looks from right to left) and aVL (which looks at the high left) should record positive deflections (upright P wave, upright QRS, upright T wave).
• Lead aVR (which looks at the upper right) normally records negative deflections, as electricity is moving away from it.
• If the LA and RA leads are physically swapped on the patient's body, the electrical axis appears perfectly reversed in the frontal plane.
• Consequently, Lead I becomes a mirror image of itself: it shows an inverted P wave, a negative QRS complex, and an inverted T wave.
• Lead aVR becomes falsely positive, and aVL becomes negative.
• Dextrocardia (Option B) (a heart situated on the right side of the chest) will also produce inverted P, QRS, and T waves in Lead I. However, in dextrocardia, the precordial leads (V1-V6) will also be highly abnormal, showing progressively smaller R waves because the leads are placed over the left chest while the heart is on the right.
• In LA-RA limb lead reversal, the precordial leads remain completely normal (normal R wave progression), which decisively differentiates it from true dextrocardia.
• The answer key confirms Limb lead reversal as the diagnosis for the provided ECG. Final answer:
The presence of a negative P, QRS, and T axis in lead I with normal precordial progression indicates a technical Limb lead reversal.
