We know from previous chapters that the limb lead with the tallest R wave is going to be the lead at which vector 3 is most directly pointed. So the easiest way to roughly determine electrical axis is simply to look for the limb lead

How to Determine Electrical Axis 25

Figure 5.1. The hexaxial reference system shaded to show regions of aVF normal axis and axis deviation.

Marked

Figure 5.1. The hexaxial reference system shaded to show regions of aVF normal axis and axis deviation.

Marked aVL

with the tallest R wave. We then know that vector 3 is pointed roughly in that direction or at least closer to that lead than to any other lead. Thus, as is illustrated in Figure 5.2A, if the tallest R wave is seen in lead II, we know that the axis is roughly +60 degrees. If, on the other hand, the tallest R wave is seen in lead aVL, as in Figure 5.2B, we know that the axis is roughly -30 degrees, and so on.

It is also easy to understand, from our previous discussion of the derivation of the normal ECG, that an alternate method of roughly determining axis would be to look for the lead with the deepest negative deflection or S wave. We would then know that vector 3 was going more directly away from that lead than from any other lead. Therefore, the axis of vector 3 should be in roughly the opposite direction, or 180 degrees from the lead with the deepest S wave. If aVR has the deepest S wave, then the electrical axis should be directly opposite on the hexaxial reference system, or roughly +30 degrees (Figure 5.2C).

The aforementioned two methods of determining axis provide us with only a rough indication of the direction of the main ventricular depolarization vector. To be more accurate, we will have to refine our methods.

A third, more accurate method of determining axis is to look for a QRS that is either equally biphasic (that is, the positive and negative deflections are of equal amplitude) or essentially isoelectric. We know from previous discussions that when we see a lead with an equally biphasic QRS, it means that vector 3 is perpendicular to that lead.

The ECG in Figure 5.2C shows an equally biphasic QRS in lead III. So we know that our vector is perpendicular to, or 90 degrees from, this lead. What we can't tell from looking solely at lead III, however, is whether the perpendicular vector is pointed toward the right lower quadrant or the left upper quadrant of our hexaxial reference system. To determine which quadrant, we simply look for the tallest R wave, which in this case we find in lead II. Now

Figure 5.2 A. Patient A. The tallest R wave is seen lead II, placing the axis at approximately +60 degrees. B. Patient B. The tallest R wave is seen in lead aVL, placing the axis at approximately -30 degrees. C. Patient C. The deepest S wave is seen in lead aVR, placing the axis roughly directly opposite aVR, or at approximately 30 degrees. An equally biphasic QRS in lead III places the axis perpendicular to lead III, or also at approximately 30 degrees, we can determine the axis to be approximately 90 degrees from lead III pointed toward the right lower quadrant, or, 30 degrees.

But what if we can't find a QRS that is exactly equally biphasic? In that instance we look for the QRS that comes closest to being equally biphasic. If it's a little more positive than negative, then we know that the vector is coming slightly more toward than away from that lead, and therefore the axis will be slightly <90 degrees from our biphasic QRS. If, on the other hand, the negative deflection is of slightly greater amplitude than the positive deflection, we know that the vector is pointed slightly more away from our lead than 90 degrees. Figure 5.2B shows a biphasic QRS in lead II that is slightly more negative than positive. Therefore, we know that the axis is actually slightly further counterclockwise from II than 90 degrees, perhaps approximately 95 degrees counterclockwise. A more refined determination would, thus, place the axis in Figure 5.2B at approximately -35 degrees.

Midway Between the Two Equally Tallest R Waves 27

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II | |||||||||

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A- |
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III |
aVF | ||||||||

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Figure 5.2 D. Patient D. Leads II and aVF are of equal amplitude, placing the axis midway between the two leads, or at 75 degrees,

E. Patient E. Lead II has a slightly taller R wave than lead aVF, shifting the axis slightly more toward II, or approximately 70 degrees,

F. Patient F. Lead aVR is equally biphasic, placing the axis at -60 degrees.

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