We will begin with the extremity leads, since they are recorded first. In connecting a patient to an electrocardiograph, first place metal electrodes on the arms and legs. The right leg electrode functions solely as an electrical ground, so you need concern yourself with it no further. Attach the arm electrodes just above the wrist and the leg electrodes above the ankles.
The electrical voltages of the heart are conducted through the torso to the extremities. Therefore an electrode placed on the right wrist will detect electrical voltages equivalent to those detect below the right shoulder. Similarly, the voltages detected at the left wrist or anywhere else on the left arm will be equivalent to those detected below the left shoulder. Finally, voltages detected by the left leg electrode will be comparable to those at the left thigh or near the groin. In clinical practice the electrodes are attached to the wrists and ankles simply for convenience.
As mentioned, the extremity leads consist of two groups: the bipolar (I, II, and III) and the unipolar (aVR, aVL, and aVF) leads. The bipolar leads are so named because two extremities are recorded by them.
From lead I, for example, the difference in voltage is recorded between the left arm (LA) and the right arm (RA). Lead I = LA – RA
From lead II the difference is recorded between the left leg (LL) and the right arm (RA).
Lead II = LL- RA
From lead III the difference is recorded between the left leg (LL) and the left arm (LA).
Lead III = LL – LA
Consider then what happens when you turn on the electrocardiograph to lead I. The LA electrode detects the electrical voltages of the heart transmitted to the left arm, the RA electrode detects the voltages transmitted to the right arm. Inside the electrocardiograph the RA voltages are subtracted from the LA voltages and the difference appears at lead I. When lead II is recorded, a similar situation occurs between the voltages of LL and RA. When lead III is recorded, the same occurs between the voltages of LL and LA.
Leads I, II, and III can be represented schematically in terms of a triangle, called Einthoven’s triangle (after the Dutch physician who invented the electrocardiograph). At first the ECG consisted only of recordings from leads I, II, and III. Einthoven’s triangle shows the spatial orientation of the three bipolar extremity leads (I, II, and III). As you can see, lead I points horizontally. Its left pole (LA) is positive and its right pole (RA) is negative. Therefore lead I = LA – RA. Lead II points diagonally downward. Its lower pole (LL) is positive and its upper pole (RA) is negative. Therefore lead II = LL – RA. Lead III also points diagonally downward. Its lower pole (LL) is positive and its upper pole (LA) is negative. Therefore lead III = LL –LA.
Einthoven, of course, could have hooked the leads up differently; but because of the way he arranged them, the bipolar leads are related by the following simple equation: lead I + lead III = lead II. In other words, add the voltage in lead I to that in lead III and you get the voltage in lead II. You can test this equation by looking at the ECG. Add the voltage of the R wave in lead I to the voltage of the R wave in lead III and you get the voltage of the R wave in lead II. You can do the same with the voltages of the P waves and T waves.
It is a good custom to scan leads I, II, and III rapidly when you first look at a mounted ECG. If the R wave in lead II does not seem to be the sum of the waves in leads I and III, this may be a clue that the leads have been either recorded incorrectly or mounted improperly.
Unipolar Extremity Leads (aVR, aVL, aVF)
Following the invention of the three bipolar extremity leads nine additional leads were added. In the 1930s Dr. frank N. Wilson and his colleagues at the University of Michigan invented the unipolar limb leads and introduced the six unipolar chest leads, V1 through V6. shortly after this, one of the authors of this text (E.G.) invented the three augmented unipolar extremity leads, aVR, aVL, aVF. The abbreviation a refers to augmented; V, voltage R, L, and F, right arm, left arm, and left foot (leg) respectively. So, today, 12 leads are routinely employed.
A unipolar lead records the electrical voltages at one location relative to zero potential, rather than relative to the voltages at another extremity, as in the case of the bipolar extremity leads. The zero potential is obtained inside the electrocardiograph by joining the three extremity leads to a central terminal. Since the sum of the voltages of RA, LA, and LL equals zero, the central terminal has a zero voltage. The aVR, aVL, and aVF leads are derived in a slightly different way, because the voltages recorded by the electrocardiograph have been augmented 50% over the actual voltages detected at each extremity. This augmentation is also done electronically inside the electrocardiograph.
Just as we used Einthoven’s triangle to represent the spatial orientation of the three bipolar extremity leads. Note that each of the unipolar leads can be represented by a line (axis) with a positive and a negative pole. Since the diagram has three axes, it is also called a triaxial diagram.
The positive pole of lead aVR, the right arm lead, point upward and to the patient’s right arm as you would expect. The positive pole of lead aVL points upward and to the patient’s left arm. The position pole of lead aVF points downward toward the patient’s left foot.
Furthermore, just as leads I, II, and III are related by Einthoven’s equation, so leads aVR, aVL, and aVF likewise are related: aVR + aVL + aVF = 0. In other words, when the three unipolar extremity leads are recorded, they should total zero. Thus the sum of the P wave voltages is zero, the sum of the QRS voltages is zero, and the same holds for the T wave voltages. You can test equation by adding the sum of the QRS voltages in the three unipolar extremity leads, aVR, aVL, and aVF.
It is also a good custom to scan leads aVR, aVL, and aVF rapidly when you first look at a mounted ECG. If the sum of the waves in these three leads does not equal zero, this may also be a clue that these leads have either been recorded incorrectly or mounted improperly.
The ECG leads, both bipolar and unipolar, have two major features, which we have already described, they have axis of lead I is oriented horizontally while the axis of lead aVR point diagonally downward. The orientation of the bipolar leads is shown in Einthoven’s triangle.
The second major feature of the ECG leads, their polarity, can be represented by a line (axis) with a positive and a negative pole, as shown before. The polarity and spatial orientation of the leads are discussed further in Chapters 4 and 5 (when we describe the normal ECG patterns seen in each of the leads and the concept of electrical axis).
Do not be confused by the difference in meaning between ECG electrodes and ECG leads. And electrode is simply the metal plate used to detect the electrical currents of the heart in any location. An ECG lead, as we have been discussing, shows the differences in voltage detected by these electrodes. (For example, lead I presents the differences in voltage detected by the left and right arm electrodes.) Therefore, a lead is simply a means of recording the differences in cardiac voltages obtained by different electrodes.