This 58-year-old man is being assessed by the anesthesia staff prior to prostatic surgery. He has no cardiac complaints. A routine ECG is obtained:

• In lead III the QRS complex is tiny (“collapsed”) and the P wave “invisible”
• The PQRST pattern in leads I and II is virtually identical and unusual
• The precordial leads are normal
• The ventricular rate is 60/min
• Consider lead placement error (lead III is very likely measuring the potential difference between the legs….)

As a recording error is suspected, a second ECG is obtained, with particular attention to correct lead placement:

• Sinus rhythm, 60/min
• Minimal voltage criteria for left ventricular hypertrophy (R in aVL = 11 mm). It may be a normal variant
• A borderline ECG

Comment: The abnormalities in the previous tracings were due to a lead placement error A flat (“collapsed”) lead III is usually due to left arm-right leg lead reversal –  Lead II, however should should not be affected by the placement error. See NEXT: Explanation

In Left Arm – Right leg reversal lead II should not be affected and should remain unchanged! (see diagram) We need an explanation!