We have already discussed the two most common variations of reversed arm and leg leads. There are a number of other combinations and at least one more can be easily recognised on the 12-lead ECG; Right Arm-Right Leg Reversed.To summarise:

Major Footprint: Lead II is on the legs and thus isoelectric (red highlight).Minor Footprints: Leads I and III are mirror images (yellow highlight).Leads I and aVL are near identical (blue highlight)Leads aVR and aVL are near identical (green highlight)Lead III normal as leads are in normal position (brown highlight)Here is another example with the same colour scheme.

If the leg leads are placed on the abdomen, lead II is no longer isoelectric.

With abnormalities like right bundle branch block (note the ST changes)

and left bundle branch block, the changes are often missed.

Likewise for ventricular pacing:

Look carefully; biventricular pacing (brown highlight) with right ventricle paced before the left ventricle.Reversed limb leads are very easily confused with poor lead attachment:

Another combination which must be hard to create is reversing the left arm with the right leg.

Lead III is on the feet away from the heart.There are no other footprints.A low voltage or isoelectric lead III is not uncommon in the normal ECG and thus it is not possible to diagnose this abnormality without a previous ECG.There is another combination, I call the double twist, which is really a merry-go-round with all leads moving one clockwise position.Here is the result:

There are clues, but they are really minor footprints:Lead III is isoelectric because left leg and left arm are on the feet (red highlight).Lead I is really lead III (yellow highlight).Now go and take a tablet for that terrible headache!!Remember, keep your eyes open when reporting ECGs!Harry Mond