A 42-year-old woman is admitted to the gynecology ward pre-operatively. An ECG is performed, which shows the following abnormality:

• Lead error: left arm electrode/cable detached/non-functional. This precludes the interpretation of the limb leads
• Sinus rhythm with sinus arrythmia
• The chest leads are normal

Leads III, aVR, aVL and aVF are derived from leads I and II, according to the Einthoven law (I + -II + III = 0). If the left-arm cable is disconnected or is non-functional, lead I is recorded as a straight line. The software uses lead II and this zero signal to calculate lead III and the augmented unipolar limb lead, that will therefore be incorrect.

A new tracing is obtained with a different ECG recorder:

• Sinus arrhythmia
• Otherwise a normal ECG