A 78-year-old woman with moderately severe mitral regurgitation due to mitral valve prolapse is followed in the cardiology clinic. Her condition is stable and has no cardiac symptoms:

• Sinus rhythm, 84/min
• Normal ECG

Six months later the patient returns to the cardiac clinic. Her clinical condition is unchanged. She remains moderately active without discomfort:

The computer diagnosis:

•  Unusual P axis (negative P in III and aVF)
• Inferior infarct, new

The attending cardiologist’s diagnosis:

• Compared with the previous tracing of 6 months ago,
• the patterns of  aVL and aVF are reversed
• Lead placement error: Left arm – left leg reversal
• Please, record a new tracing, with particular attention to correct lead placement

 The QR pattern that was present in lead aVL in the previous ECG is now in aVF, causing the incorrect diagnosis of inferior infarction.
 Notice also that the P wave is wider and taller in lead I than in lead II (which is the opposite of the normal pattern).

A new tracing has been recorded:

• Sinus rhythm, 74/min
• Normal ECG

The previous tracing had been incorrectly recorded. While the Einthoven bipolar leads (I, II, III) measure the potential difference between two limbs, each unipolar lead explores the potential variations at one limb site. Therefore the misplacement of the cables of the Einthoven leads is more easily identified by observing aVR, aVL and aVF. (please, see the lead tutorial for more information)