Electrode misplacement is a major source of error on the ECG. Some electrode misplacements are easier to spot than others. Comparing with an old ECG is often useful. Key signs of an electrode misplacement include an unusually flat lead (and the electrode hasn’t fallen off), unusually upside down leads, or two or more leads looking very similar. In the chest leads, swapping the electrodes around can cause a loss of normal R wave progression. If in doubt, repeat the ECG recording.
Total eclipse of the heart – 41 min later with crushing chest pain, just before arrest 26743