Vectors Can Be Determined From The Limb Leads

As action potentials are conducted through the heart, the resulting dipoles are constantly changing in both magnitude and orientation. Because of the physical orientation of the limb leads, they are sensitive to the orientation of the dipoles generated by the heart. Consider a vertically oriented dipole whose positive pole points down toward the patient's foot. This would cause the pubis to be positively charged with respect to either shoulder. Thus, the pen would be deflected upward in leads II and III because the positive lead of the ECG is attached to the left foot in both of those leads. On the other hand, lead I would show no deflection because this lead is oriented at right angles to the dipole and hence parallel to the dipole's isoelectric plane.

Figure 4 shows the wave of conduction spreading across the heart at eight time points. As the wave of conduction spreads, it sets up a dipole in the heart that is the net sum of all potential differences in the heart at that moment. Note how the orientation and magnitude of the dipole are continuously changing throughout the cardiac cycle. Each lead will be influenced by the vector component of the dipole that is parallel to that lead at each time point. Conversely, we could reverse the process and reconstruct the entire history of the heart's electrical dipole by simply having a computer analyze the ECG traces from several leads and determine the dipole's magnitude and orientation at all times throughout the cardiac cycle. This is the basis of vector cardiography. However, such an intricate analysis has been found to be of little diagnostic value. It is easier and more useful to just determine the mean electrical axis of the ventricles.

The mean electrical axis is simply the average direction of the dipole throughout the QRS complex. It can easily be determined by estimating the area under the QRS complex in any two leads and plotting those quantities on the triaxial system shown in Fig. 5. A number of approaches exist for estimating the area under a QRS complex, but the simplest is to assume that the areas are proportional to the height of the deflections. One then simply sums all positive deflections and subtracts all negative deflections from a given trace to determine the net deflection. The units for measuring the deflections are not important because only the angle of the vector—not its magnitude—is of interest. When plotting vectors one must be careful to observe the polarity, however. Deflections above the baseline are plotted toward the positive end of the lead and those below the baseline toward the negative end of the lead. Refer to Figs. 3 and 5 to determine the polarity of each lead configuration.

Figure 5 shows how to plot a vector on the triaxial system. For each of the three sides of the triangle, draw a line perpendicular to that side bisecting it at its midpoint (see dashed lines in Fig. 5). Do this for all three sides. All three lines should intersect at the center of the triangle as shown in Fig. 5. That intersection is the tail of the vector. Next plot the magnitude of the deflection for each of the three leads along the leg of the triangle corresponding to that lead. Start at the center of the leg and measure a distance along it equal to the net deflection of the QRS complex for that lead and make a mark at that point. If the deflection is positive, make the measurement toward the positive end of the leg (+). If the deflection is negative, the measurement should be toward the negative end (—). Then on each of the three legs draw a second perpendicular line going through the mark on that leg. Where those three new lines intersect is the head of the vector. Because all three lines should converge at a single point, all that is really needed to plot a vector is a calculation from any two leads.

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