A Lewis Lead is a modified EKG lead used to detect atrial flutter

Atrial flutter

Atrial flutter is an abnormal heart rhythm that occurs in the atria of the heart. When it first occurs, it is usually associated with a fast heart rate or tachycardia , and falls into the category of supra-ventricular tachycardias. While this rhythm occurs most often in individuals with...

 waves when atrial flutter is suspected clinically but not definitely demonstrated on the standard 12 lead EKG. In order to create the Lewis Lead, the right arm electrode is moved to the right, second intercostal space

Intercostal space

The intercostal space is the space between two ribs . Since there are 12 ribs on each side, there are 11 intercostal spaces, each numbered for the rib superior to it.-Structures in intercostal space:* several kinds of intercostal muscle...

 adjacent to the sternum. Then the left arm electrode is moved to the right, fourth intercostal space adjacent to the sternum. The Lewis Lead is then read as Lead I on the EKG and, since in most patients it will be roughly perpendicular to the wave of atrial depolarization

Depolarization

In biology, depolarization is a change in a cell's membrane potential, making it more positive, or less negative. In neurons and some other cells, a large enough depolarization may result in an action potential...

, flutter waves may be more apparent.