Question

How are leads I, II, III determined; why is there an interaction; and how can you determine if you have a good connection using the interaction of these leads?

Answer 1

Ans) Lead I is constructed by comparing the left arm (as positive) to the right arm's electrode (as negative) (as shown in the next figure). The zero point is in the center of the lead (indicated by the hash mark). Any current flowing to the left (towards the left arm's electrode) produces a positive deflection on the ECG, while any current flowing to the right produces a negative deflection. In essence, we are viewing the heart as if from the zero point (indicated by the little man on the picture). Lead I gives us a very good view of what is going on from left to right in the heart, but a poor view of events moving up or down (perpendicular to the lead I axis).

- There are two additional bipolar leads (the name "bipolar" refers to the fact that one electrode is defined as positive, while a second electrode provides the negative).

- Lead II (pictured below, on left) connects the left leg as positive to the right arm's negative.

- Lead III connects the left leg as positive to the negative left arm (pictured below, right). Each of these provides a view of the heart essentially from the zero point, as with lead I.

- There are several ways in which you can remember the different bipolar leads (both what connects to what and which is positive vs. negative. These are summarized in the table:

Remembering the connections

Right arm always - Lead I 1 "L": Left arm to right arm

Left leg always + Lead II 2 "L's": Left Leg to right arm

Left arm whatever's left Lead III 3 "L's": Left Leg to Left arm