This 45-year-old man with a longstanding history of diabetes, hypertension, dislipidemia and psychiatric illness is seen in the Emergency Department complaining of moderate retrosternal pain “on and off” for several hours, regardless of exertion:
 

• Sinus rhythm
• Borderline left axis deviation

The patient is kept under observation and monitored. His serum enzymes are normal. He complains of constant moderate discomfort in the anterior chest, worse on deep breathing. He is taken to the imaging department for a chest X-ray. Upon return to the Emergency, an ECG is recorded:

• Unusual P axis, suspect ectopic atrial rhythm
• Inferior-lateral infarction, new

Comment: Because of the changes of inferior myocardial infarction  the patient was transferred to the cardiac ward for further investigation. In fact, comparing ECG 1 and 2 a lead placement error is the most likely explanation of the unexpected changes (see: compare ECG 1 and ECG 2)

The patient is transferred to the Cardiology Ward for further investigation. A follow-up ECG is obtained:

• Compared with the previous tracing (ECG 2), there is no longer evidence of an inferior infarction.
• Sinus rhythm
• Left axis deviation

ECG 3 is similar to the ECG 1 recorded on arrival in the Emergency, and no longer shows an inferior infarction. Comparison of the unipolar limb leads confirms that the right arm and left leg cables had been reversed when ECG 2 was recorded. When an inferior infarction unexpectedly appears or disappears, consider the possibility of a lead placement error (usually left or right arm and left leg)