Unipolar Leads Are Used To Explore The Chest

A neutral reference lead can be made by connecting all three of the limb leads and then connecting this to the ''negative'' lead of the ECG amplifier, as shown in the bottom right panel of Fig. 3. This reference lead approximates the voltage at the center of the chest because it is the average of the two shoulders and the pubis. The ''positive'' lead is then free to explore the body surface. The positive electrode is moved from the anterior chest to the axilla in six equal steps to make leads V1 through V6. The V derives from the original designation as a voltage lead. Although the limb leads detect vectors only in the frontal plane, the chest leads can determine the front-to-back orientation of the mean electrical axis. This is called sense and normally the electrical sense is toward the patient's back.

The bottom left panel of Fig. 3 shows how the chest leads V1 through V6 are orientated around a cross section of the chest. A normal trace for each lead is also shown. Notice how lead V4 is isoelectric with as much positive deflection as negative. The mean axis must therefore be at right angles to lead V4. Because lead V1 is at right angles to lead V4 and shows a net-negative QRS complex (a deep S wave), the vector must point to the left rear of the chest. If the axis had been to the front, then an R wave would have been seen in lead V1 rather than the normal S wave. Note that the amplitudes are not comparable between the chest leads and the limb leads, and thus an accurate three-dimensional vector calculation is not possible.

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