Figure 3. A closer view of lead I and lead aVR of the 58-year-old man’s normal sinus rhythm electrocardiogram in which the automated reading misdiagnosed a postero-lateral myocardial infarction caused by a reversal of the upper extremity limb leads.

There are several salient features that establish that the left arm and right arm limb leads were reversed, which is best demonstrated by the findings in lead I. This lead represents the voltage difference between the electrodes placed on the left and right arms. Lead I (shown in A) is entirely negative (black oval); the P wave is negative (green arrows); the QRS complex has a negative deflection; and the T wave is negative (white arrows). In contrast, lead aVR (shown in B) has become positive (blue oval): the P wave is positive (purple arrows), the QRS complex has a positive deflection, and the T wave is positive (yellow arrows). In summary, the features suggestive of limb lead reversal are the positive P waves (purple arrows) in aVR (shown in B) and the negative P waves (green arrows) in lead I (shown in A); the highly unlikely possibility of a highly ectopic atrial focus originating from the left atrium, with the positive deflection (purple arrows) in lead aVR (shown in B) and the negative deflection (green arrows) in lead I (shown in A) helps establish the diagnosis that the limb leads have been reversed.