Augmented Limb Leads Provide Three More Limb Leads

Although any two leads should be sufficient to calculate the mean electrical axis in the frontal plane, in fact, six leads are routinely examined in a standard ECG. We have already considered leads I, II, and III. Using the unipolar lead system described earlier, it is possible to put the exploring electrode on any of the three limbs. Imagine that the exploring electrode is placed on the left foot. This lead would be oriented from the pubis to the center of the chest (the neutral lead) and thus have a vertical orientation, halfway between leads II and III. For technical reasons, eliminating the lead from the left foot to the reference connection augments the signal to a level comparable to that in leads I through III. Such a lead is termed an augmented unipolar lead from the foot, or aVF. Augmented unipolar leads can also be similarly made for the left and right arms, thus giving aVL and aVR, respectively. The advantage of the augmented leads is that they can be used along with leads I through III for calculating the electrical vectors. Figure 3 shows the connections for these leads and a normal trace for each of them.

The lower panel of Fig. 5 shows the orientation of the augmented limb leads with respect to leads I, II, and III. The augmented limb leads aVR, aVL, and aVF bisect the angle between standard limb leads I and II, I and III, and II and III, resp. Thus, there is a limb lead oriented every 30° in the frontal plane, which makes it much easier to calculate vectors. For example, lead I can now be plotted against aVF in the conventional Cartesian coordinate system because they are at right angles to each other. Note also that a patient with a mean electrical axis in the normal quadrant should have a QRS complex that is zero or net-positive in both lead I and aVF. If the QRS complex is net-negative in either of those, then the mean axis is outside the normal quadrant of 0° to 90°. Note that aVF is indeed net-negative in both Figs. 6 and 7. Also, with leads oriented at six equally spaced angles around the heart, one lead will invariably be nearly perpendicular to the mean electrical axis and hence be isoelectric. This usually allows the physician to estimate the mean electrical axis at a glance without even plotting the vectors. The direction of the vector is simply determined by noting the polarity of the axis, which is at right angles to the lead having the minimal deflection. Use the lower panel of Fig. 5 to determine the orientation and polarity of the six limb leads. Note that aVR is nearly isoelectric in Fig. 6. That would make the vector parallel to lead III. Because III is strongly negative the vector must be —60° rather than +120°.

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