Arrhythmia Recognition: The Art of Interpretation

Now, notice that the dividing line between each positive and negative half of a lead happens to fall on a lead that is at an exact 90° angle from it. This lead is referred to as the isoelectric lead, meaning that it is neither positive nor negative along that line (red labels in Figure 5-9). In other words, each lead has a corresponding isoelectric lead; I is isoelectric to aVF, II is isoelectric to aVL, III is isoelectric to aVR, and vice versa. This concept of the isoelectric lead will be very useful to us later on when we want to isolate the lead to within 10°.

On the ECG, any positive vector will be represented as taller or more positive. Any negative vector will appear as a deeper or more negative complex (Figure 5-10). A lead is considered positive if it is even a smidgen more positive than negative. (Webster’s defines smidgen as a little teeny bit.) Likewise, it is considered negative if it is even a smidgen more negative than positive.

A lead is isoelectric when it is exactly the same distance positive as it is negative. There is only one isoelectric limb lead on the ECG because there is only one ventricular axis. All the other leads are either positive or negative.

When we plot the vector on the hexaxial system, a vector that is even slightly positive will be found on the positive half of the circle. By the same token, any negative complex has to be on the negative half of the circle. If it is exactly isoelectric, then it will fall directly along the isoelectric lead. If that occurs, you have a slight problem. The vector can point in one of two directions, either toward the negative or toward the positive pole. Note that both of these directions will be exactly isoelectric to the lead in question. How can we resolve this dilemma? At this point, you need to go back to the ECG and take a look at the complexes that are found in that isoelectric lead. If those complexes are positive, then the vector will point in the direction of the positive pole of the isoelectric lead. If the complexes are negative, then the vector will point toward the negative pole. This is your first introduction into how we will always use two leads to isolate the vector. This is an important, but difficult, point to understand. In Figure 5-11, for instance, vectors A, B, and C are all positive in lead I, and vectors D, E, and F are all negative in lead I.

Can you now see how the vector and the ECG are related? Because the vector cannot be seen, we use the complexes, and their relative positivity or negativity in each lead, to calculate the exact direction of the ventricular axis. Let’s begin by seeing how to shorten the direction from 360° down to one of four, 90° quadrants.

When we look at a 12-lead ECG, we do not know where the axis is pointing. To start isolating that direction, we want you to look at leads I and aVF (notice that these leads are isoelectric to each other). First, look at lead I and figure out whether it is positive or negative. Don’t worry about how positive or negative right now; you only care about which half of the circle it falls into. If it is positive, it would have to be on the blue or positive side of the lead; if negative, it will fall in the white or negative half of the lead, as shown in Figure 5-12A. Next, look at lead aVF. Repeat the same thought process. Is it positive or negative in aVF? Place it in either the yellow or the white half, as referenced in Figure 5-12B. Because we know that yellow and blue make green, by overlapping these two circles, we create a new circle with four quadrants: one white, one blue, one yellow, and one green, as seen in Figure 5-12C.

Instead of saying positive or negative, we find it useful to say that a positive lead is taller than it is deep, hence ↑. A negative lead is deeper than it is tall, thus ↓. Using this system, you do not have to add the heights of the complex’s components algebraically.

Suppose a 12-lead has a positive lead I and a positive lead aVF. The only quadrant that matches this pattern is the normal quadrant (Figure 5-13). See how easy? Next, we will isolate the axis to within 10°, but for now let’s just take baby steps in determining the quadrant.

We have seen that by looking at only leads I and aVF, we were able to break the hexaxial system down into four quadrants. We are going to name these four quadrants to make them easier to identify: normal, left, right, and extreme right. These are shown in Figure 5-13.

These quadrants will be very useful to us when we get ready to calculate the true axis as closely as possible (see Figure 5-14). For now, we can just state that any axis that falls outside the normal quadrant should be considered abnormal. (In reality, the normal quadrant extends from −20° to +100°, not 0° to +90°, but the latter is close enough for now.) If the axis falls into the left quadrant, it is considered to have a left axis deviation. If it falls into either the right or extreme right, it has a right axis deviation.

QUICK REVIEW

Let’s do some examples.

10 sets of rhythm strips. — Figure 5-14 Sample ECG waves from which to calculate quadrants.

From Arrhythmia Recognition: The Art of Interpretation, courtesy of Tomas B. Garcia, MD.

1. Normal. 2. Left. 3. Right. 4. Extreme right. 5. Normal. 6. Left (−90°).
7. Extreme right. 8. Normal (90°). 9. Left. 10. Right (180°).