Scenario: This 12-lead ECG reading was obtained from a 76-year-old African American woman who presented to the emergency department with shortness of breath, fatigue, and a cough for the past month. Prior medical history includes chronic obstructive pulmonary disease, hypertension, and cigarette smoking for more than 40 years.

Interpretation: Cannot evaluate limb leads because of misplacement of the right leg lead on upper extremity. Multifocal atrial rhythm with premature atrial complexes and with aberrant ventricular conduction.

One striking feature of this 12-lead ECG reading is the extremely low QRS amplitude in lead II. This pattern indicates reversal of the right leg and right arm electrodes, producing a “far-field” signal. This pattern is observed because the ECG machine is recording the electrical potential between the right and left leg, rather than recording the potential between the right arm and left leg. Because of electrode misplacement, the limb leads cannot be used...

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