Figure 11–6 shows electrical connections between the patient’s limbs and the electrocardiograph for record-ing electrocardiograms from the so-called standardbipolar limb leads. The term “bipolar” means that theelectrocardiogram is recorded from two electrodes located on different sides of the heart, in this case, on the limbs. Thus, a “lead” is not a single wire connect-ing from the body but a combination of two wires and their electrodes to make a complete circuit between the body and the electrocardiograph. The electrocar-diograph in each instance is represented by an elec-trical meter in the diagram, although the actual electrocardiograph is a high-speed recording meter with a moving paper.
Lead I. In recording limb lead I, thenegative terminalof the electrocardiograph is connected to the right arm and the positive terminal to the left arm. Therefore, when the point where the right arm connects to the chest is electronegative with respect to the point where the left arm connects, the electrocardiograph records positively, that is, above the zero voltage line in the electrocardiogram. When the opposite is true, the elec-trocardiograph records below the line.
Lead II. To record limb lead II, thenegative terminal ofthe electrocardiograph is connected to the right arm andthe positive terminal to the left leg. Therefore, when the right arm is negative with respect to the left leg, the electrocardiograph records positively.
Lead III. To record limb lead III, thenegative terminalof the electrocardiograph is connected to the left arm and the positive terminal to the left leg. This means that the electrocardiograph records positively when the left arm is negative with respect to the left leg.
Einthoven’s Triangle. In Figure 11–6, the triangle, calledEinthoven’s triangle, is drawn around the area of theheart. This illustrates that the two arms and the left leg form apices of a triangle surrounding the heart. The two apices at the upper part of the triangle represent the points at which the two arms connect electrically with the fluids around the heart, and the lower apex is the point at which the left leg connects with the fluids.
Einthoven’s Law. Einthoven’s law states that if theelectrical potentials of any two of the three bipolar limb electrocardiographic leads are known at any given instant, the third one can be determined mathematically by simply summing the first two (but note that the positive and negative signs of the different leads must be observed when making this summation).
For instance, let us assume that momentarily, as noted in Figure 11–6, the right arm is -0.2 millivolt (negative) with respect to the average potential inthe body, the left arm is + 0.3 millivolt (positive), and the left leg is +1.0 millivolt (positive). Observing the meters in the figure, it can be seen that lead I records a positive potential of +0.5 millivolt, because this is the difference between the -0.2 millivolt on the right arm and the +0.3 millivolt on the left arm. Similarly, lead III records a positive potential of +0.7 millivolt, and lead II records a positive potential of +1.2 millivolts because these are the instantaneous potential differ-ences between the respective pairs of limbs.
Now, note that the sum of the voltages in leads I andIII equals the voltage in lead II; that is, 0.5 plus0.7 equals 1.2. Mathematically, this principle, called Einthoven’s law, holds true at any given instant while the three “standard” bipolar electrocardiograms are being recorded.
Normal Electrocardiograms Recorded from the Three Standard Bipolar Limb Leads. Figure 11–7 shows recordings of theelectrocardiograms in leads I, II, and III. It is obvious that the electrocardiograms in these three leads are similar to one another because they all record positive P waves and positive T waves, and the major portion of the QRS complex is also positive in each electrocardiogram.
On analysis of the three electrocardiograms, it can be shown, with careful measurements and proper observance of polarities, that at any given instant the sum of the potentials in leads I and III equals the potential in lead II, thus illustrating the validity of Einthoven’s law.
Because the recordings from all the bipolar limb leads are similar to one another, it does not matter greatly which lead is recorded when one wants to diagnose different cardiac arrhythmias, because diag-nosis of arrhythmias depends mainly on the time relations between the different waves of the cardiac cycle. But when one wants to diagnose damage in the ventricular or atrial muscle or in the Purkinje conducting system, it does matter greatly which leads are recorded, because abnormalities of cardiacmuscle contraction orcardiac impulse conduction do change the patterns of the electrocardiograms markedly in some leads yet may not affect other leads. Electrocardiographic interpretation of these two types of conditions—cardiac myopathies and cardiac arrhythmias—is discussed separately.
Often electrocardiograms are recorded with one elec-trode placed on the anterior surface of the chest directly over the heart at one of the points shown in Figure 11–8. This electrode is connected to the positive terminal of the electrocardiograph, and the negative electrode, called the indifferent electrode, is connected through equal electrical resistances to the right arm, left arm, and left leg all at the same time, as also shown in the figure. Usually six standard chest leads are recorded, one at a time, from the anterior chest wall, the chest electrode being placed sequentially at the six points shown in the diagram. The different recordings are known as leads V1, V2, V3, V4, V5, and V6.
Figure 11–9 illustrates the electrocardiograms of the healthy heart as recorded from these six standard chest leads. Because the heart surfaces are close to the chest wall, each chest lead records mainly the
electrical potential of the cardiac musculature imme-diately beneath the electrode. Therefore, relatively minute abnormalities in the ventricles, particularly in the anterior ventricular wall, can cause marked changes in the electrocardiograms recorded from indi-vidual chest leads.
In leads V1 and V2, the QRS recordings of the normal heart are mainly negative because, as shown in Figure 11–8, the chest electrode in these leads is nearer to the base of the heart than to the apex, and the base of the heart is the direction of electronegativity during most of the ventricular depolarization process. Con-versely, the QRS complexes in leads V4, V5, and V6 are mainly positive because the chest electrode in these leads is nearer the heart apex, which is the direction of electropositivity during most of depolarization.
Another system of leads in wide use is the augmentedunipolar limb lead. In this type of recording, two ofthe limbs are connected through electrical resistances to the negative terminal of the electrocardiograph, and the third limb is connected to the positive termi-nal. When the positive terminal is on the right arm, the lead is known as the aVR lead; when on the left arm, the aVL lead; and when on the left leg, the aVF lead.
Normal recordings of the augmented unipolar limb leads are shown in Figure 11–10. They are all similar to the standard limb lead recordings, except that the recording from the aVR lead is inverted. (Why does this inversion occur? Study the polarity connections to the electrocardiograph to determine this.)