Three Bipolar Limb Leads

Figure 11-6 shows electrical connections between the patient's limbs and the electrocardiograph for recording electrocardiograms from the so-called standard bipolar limb leads. The term "bipolar" means that the electrocardiogram 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 connecting from the body but a combination of two wires and their electrodes to make a complete circuit between the body and the electrocardiograph. The electrocardiograph in each instance is represented by an electrical 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, the negative terminal of 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 electrocardiograph records below the line.

Lead II. To record limb lead II, the negative terminal of the electrocardiograph is connected to the right arm and the 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, the negative terminal of 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, called Einthoven's triangle, is drawn around the area of the heart. 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 the electrical 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 in the 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 differences between the respective pairs of limbs.

Now, note that the sum of the voltages in leads I and III equals the voltage in lead II; that is, 0.5 plus 0.7 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 the electrocardiograms 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 diagnosis 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 cardiac muscle contraction or cardiac impulse conduction do

Normal electrocardiograms recorded from the three standard electrocardiographic leads.

Bipolar Limb Leads

Normal electrocardiograms recorded from the three standard electrocardiographic leads.

Flow Along Nephron
Figure 11-9

5000 ohms

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Responses

  • Erna
    How augmented voltage lead are connected with bipolar?
    3 years ago
  • Callimaco Palerma
    Which of the two limbs connected to leads in Lead1 bipolar standard lead system?
    3 years ago
  • Daniel
    Why bipolar limb lead 2 is recorded as long lead?
    1 year ago

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