Limb Lead Reversal

The most common form of limb lead reversal is a switch of right arm electrode with left arm electrode. This form is quickly identified by inspection of aVR where the usual pattern of negativity (p, qrs, and t wave) is replaced by positivity. But how to quickly identify limb lead reversal where left leg and left arm are reversed? The right arm electrode is in its usual place, therefore aVR reads negative; aVL and aVF remain positive.

The trick lies in inspecting the p wave throughout the limb leads. Concordant p waves are the rule; rarely with ectopic atrial foci or tachycardia this may not be the case, but let's put that aside for the moment. In sinus rhythm the p waves should be positive in all limb leads. If one or two of the leads have a negative p wave then something is wrong with the electrode placement. (With right arm, left arm switch, lead I will have a negative p wave; with left arm, left leg switch, lead III will have a negative p wave).

So which form of limb lead reversal is present in the ECG below? (Thanks to Chris Nickson at lifeinthefastlane.com for the ECG). 

(download)

1. Limb Lead Reversal

2. Limb lead Reversal Reversed!

Of course, all is not lost when the patient has departed and the ECG is already taken. When you are certain which electrodes are switched, the ECG can be reconstructed (see picture 2) by an understanding of Einthoven's triangle – nice explanation of this at Christopher Watford's blog. Now that makes more sense!

When p waves are not visible, it becomes more difficult to spot limb lead reversal e.g. WCT. My method is to look at leads which are close by and see if they are recording roughly similar complexes. For example, aVF should not be so different from lead II or III. In this case however, this method may mislead as aVF and III do look similiar as there is lateral TWI present. The p wave in III is also biphasic, but predominantly negative where it is definitely positive everywhere else; and that is the vital clue.