An electrocardiogram is also known as an ECG; the Dutch and German version of the word, elektrokardiogram, is shortened to EKG. It is a tool used to visualize, or “gram,” the electricity, or “electro,” that flows through the heart, or “cardio.” Specifically, an ECG tracing shows how the depolarization wave, which is a wave of positive charge, moves during each heartbeat by providing the perspectives of different sets of electrodes. This particular set of electrodes is called lead II; one electrode is placed on the right arm and the other on the left leg. Essentially, when the wave’s moving toward the left leg electrode, you get a positive deflection. This big, positive deflection corresponds to the wave moving down the septum. To read an ECG, there are a few key elements to keep in mind; one is to figure out the QRS transition.
The chest leads will have a mostly positive deflection, if a depolarization wave is moving towards them. The QRS transition zone refers to where the QRS complex switches from being mostly negative to mostly positive, from the point of view of the chest leads, V1 through V6, which “view” the heart through the horizontal plane. The QRS transition usually happens in lead V3 or V4, depending on factors such as chest lead placement and the exact anatomy of a person’s heart. So, the QRS transition tells us when the overall QRS vector is aligned in the direction of the chest leads; it’s a way of understanding what’s happening to the QRS axis in the horizontal plane.