A 70-year-old woman with a history of palpitation during the past 2 months is seen in the emergency department with recurrence of palpitation and dyspnea:

• Atrial Fibrillation – Ventricular rate 113/min
• Incomplete right bundle branch block (iRBBB) – unusual QRS pattern and ST elevation in leads I and aVL
• Suspect lead placement error: right arm-left arm lead reversal  
• ST depression in the inferior leads and V4-V6, consider inferior and anterolateral ischemia

Comments: the patient was in atrial fibrillation and negative P waves were not present in leads I and aVR, the computer’s interpretation missed the arm lead error. In fact it is obvious that the QRS in lead I is similar to V1 and to “lead V6 upside-down”.

A repeat tracing is obtained one hour after ECG 1 with special attention to correct lead placement

• Atrial fibrillation – Ventricular rate 97/min
• Right bundle branch block
• The unusual QRS  pattern in the previous tracing (ECG 1) was due to lead placement error (right arm-left arm lead reversal)
• The ST depression in the inferior leads and the chest leads has improved