12-lead electrocardiogram

Regarding misplacement of limb leads prior to recording a 12-lead electrocardiogram, the following misplacements would mimic the stated condition EXCEPT which one?

a) Left-arm electrode and right-arm electrode switch – dextrocardia

b) Right-leg electrode and right-arm electrode switch – pericardial effusion

c) Left-arm electrode and left-leg electrode switch – inferior myocardial infarction

d) Right-leg electrode and left-leg electrode switch – true posteriormyocardial infarction

e) Clockwise rotated limb leads (with right leg correctly sited) – extra-nodal atrial rhythm

Answer: D

Explanation

When making a diagnosis from any investigation, recognition of an artefact is as

important as identification of a pathology. A 12-lead electrocardiogram (ECG) uses

ten wires and electrodes: six chest leads plus four limb leads (the right leg of which is

the ground or indifferent electrode). As the display of this electrical activity uses a

positive deflection to represent current in the direction of that electrode and from

which territory of pathology is deduced, it is easy to see why lead misplacement can

cause misleading changes. In this question the false statement is recognisable without

knowledge of the correctness of the others. True posterior myocardial infarctions

provoke ‘mirror’ changes in the anterior chest leads (V1-V3) and do not (unless there

is an inferior component) produce changes in the limb leads. For this reason limb lead

misplacement and misleading changes in the limb leads could not lead to erroneous

diagnosis of posterior myocardial infarction. It is intuitive why switching right- and

left-arm leads would mimic dextrocardia as the heart axis would appear grossly

deviated to the right. Switching the right-arm and right-leg electrode would involve

putting the indifferent electrode at the main origin of activity so all complexes would

be reduced in magnitude as seen in pericardial effusion. Switching left-arm and left-leg

would produce Q waves in II, III and aVf. If the indifferent electrode remains on the

right leg, clockwise rotation of the limb leads would produce p-wave inversion,

suggesting an extra-nodal origin of rhythm.

Reference

Vardan S, Vardan S, Mookherjee D, et al. Guidelines for the detection of ECG limb

lead misplacements. Resid Staff Physician 2008; 54(1). Online at www.residentandstaff.com/issues/2008-01.asp (Accessed 30 October 2009)