Electrocardiogram Leads

Standard Limb Leads: I, II, and III

The extremity leads are recorded first. In connecting a standard 12-lead electrocardiograph to a patient, electrodes are placed on the arms and legs. The right leg electrode functions solely as an electrical ground. As shown in Fig. 4.3 , the arm electrodes are usually attached just above the wrist and the leg electrodes just above the ankles.

The electrical voltages (electrical signals) generated by the working cells of the heart muscle are conducted through the torso to the extremities. Therefore an electrode placed on the right wrist detects electrical voltages equivalent to those recorded below the right shoulder. Similarly, the voltages detected at the left wrist or anywhere else on the left arm are equivalent to those recorded below the left shoulder. Finally, voltages detected by the left leg electrode are comparable to those at the left thigh or near the groin.

As mentioned, the limb leads consist of standard bipolar (I, II, and III) and augmented (aVR, aVL, and aVF) leads. The bipolar leads were so named historically because they record the differences in electrical voltage between two extremities.

Lead I, for example, records the difference in voltage between the left arm (LA) and right arm (RA) electrodes:

Lead II records the difference between the left leg (LL) and right arm (RA) electrodes:

Lead III records the difference between the left leg (LL) and left arm (LA) electrodes:

Consider what happens when the electrocardiograph records lead I. The LA electrode detects the electrical voltages of the heart that are 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 situation 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 Willem Einthoven (1860-1927), the Dutch physiologist/physicist and Nobel laureate who invented the electrocardiograph. Historically, the first “generation” of ECGs consisted only of recordings from leads I, II, and III. Einthoven’s triangle ( Fig. 4.4 ) shows the spatial orientation of the three standard limb 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 configured the leads differently. Because of the way he arranged them, the bipolar leads are related by the following simple equation:

In other words, add the voltage in lead I to that in lead III and you get the voltage in lead II. ^a

a Note: this rule of thumb is only approximate. It can be made more precise when the three standard limb leads are recorded simultaneously, as they are with contemporary multichannel electrocardiographs. The exact rule is as follows: The voltage at the peak of the R wave (or at any point) in lead II equals the sum of the voltages in leads I and III at simultaneously occurring points (since the actual R wave peaks may not occur simultaneously).

You can test this equation by looking at Fig. 4.2 . Add the voltage of the R wave in lead I (+9 mm) to the voltage of the R wave in lead III (+4 mm) and you get +13 mm, the voltage of the R wave in lead II. You can do the same with the voltages of the P waves and T waves.

Einthoven’s equation is simply the result of the way the bipolar leads are recorded; that is, the LA is positive in lead I and negative in lead III and thus cancels out when the two leads are added. Thus, in electrocardiography, “one plus three equals two.”

In summary, leads I, II, and III are the standard (bipolar) limb leads, which historically were the first invented. These leads record the differences in electrical voltage among extremities.

In Fig. 4.5 , Einthoven’s triangle has been redrawn so that leads I, II, and III intersect at a common central point. This procedure was done simply by sliding lead I downward, lead II rightward, and lead III leftward. The result is the triaxial diagram in Fig. 4.5 B. This diagram, a useful way of representing the three bipolar leads, is employed in Chapter 6 to help measure the QRS axis.

Augmented Limb Leads: aVR, aVL, and aVF

Nine leads have been added to the original three bipolar extremity leads. In the 1930s, Dr. Frank N. Wilson and his colleagues at the University of Michigan invented the unipolar extremity leads and also introduced the six unipolar chest leads, V ₁ through V ₆ . A short time later, Dr. Emanuel Goldberger invented the three augmented unipolar extremity leads: aVR, aVL, and aVF. The abbreviation a refers to augmented; V to voltage; and R, L, and F to right arm, left arm, and left foot (leg), respectively. Today 12 leads are routinely employed and consist of the six limb leads (I, II, III, aVR, aVL, and aVF) and the six precordial leads (V ₁ to V ₆ ).

A so-called unipolar lead records the electrical voltages at one location relative to the “central terminal,” an ensemble of electrodes with close to zero potential rather than relative to the voltages at a single locus, as in the case of the bipolar extremity leads. ^b

b Although “unipolar leads” (like bipolar leads) are represented by axes with positive and negative poles, the historical term unipolar does not refer to these poles; rather it refers to the fact that unipolar leads record the voltage in one location relative to an electrode (or set of electrodes) with close to zero potential.

The near-zero potential is obtained inside the electrocardiograph by joining the three extremity leads to the central terminal. Because the sum of the voltages of RA, LA, and LL equals approximately zero, the central terminal has about 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. ^c

c Augmentation was developed to make the complexes more readable.

Just as Einthoven’s triangle represents the spatial orientation of the three standard limb leads, the diagram in Fig. 4.6 represents the spatial orientation of the three augmented extremity leads. Notice that each of these unipolar leads can also be represented by a line (axis) with a positive and negative pole. Because the diagram has three axes, it is also referred to as a triaxial diagram.

As would be expected, the positive pole of lead aVR, the right arm lead, points upward and to the patient’s right arm. The positive pole of lead aVL points upward and to the patient’s left arm. The positive 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 are related:

In other words, when the three augmented limb leads are recorded, their voltages should total zero. Thus the sum of the P wave voltages is zero, the sum of the QRS voltages is zero, and the sum of the T wave voltages is zero. Using Fig. 4.2 , test this equation by adding the QRS voltages in the three unipolar extremity leads (aVR, aVL, and aVF).

(You can scan leads aVR, aVL, and aVF rapidly when you first look at a mounted ECG from a single-channel ECG machine. If the sum of the waves in these three leads does not equal zero, the leads may have been mounted improperly.)

It is important to understand Einthoven’s triangle and how the other limb leads are derived. When you reexamine Fig. 4.4 , you may see that every limb lead (with the exception of aVF) is configured by potential differences between the LA, RA, or LL. Lead misplacement, or reversal, will alter the configuration of the limb leads and may lead to misdiagnoses (this important source of artifact and error will be discussed in Chapter 22 ).

Orientation and Polarity of Leads

The limb and chest leads (discussed in the next section) have two major features, which have already been described. They all have both a specific orientation and a specific polarity.

Thus the axis of lead I is oriented horizontally, and the axis of lead aVR is oriented diagonally, from the patient’s right to left. The orientation of the three standard (bipolar) leads is shown in the represented Einthoven’s triangle (see Fig. 4.5 ), and the orientation of the three augmented (unipolar) extremity leads is diagrammed in Fig. 4.6 .

The second major feature of the ECG leads is their polarity, which means that these lead axes have a positive and a negative pole. The polarity and spatial orientation of the leads are discussed further in 5, 6 when the normal ECG patterns seen in each lead are considered and the concept of electrical axis is explored.

Relationship of Extremity Leads

Einthoven’s triangle in Fig. 4.5 shows the relationship of the three standard limb leads (I, II, and III). Similarly, the triaxial (three-axis) diagram in Fig. 4.6 shows the relationship of the three augmented limb leads (aVR, aVL, and aVF). For convenience, these two diagrams can be combined so that the axes of all six limb leads intersect at a common point. The result is the hexaxial (six axis) lead diagram shown in Fig. 4.7 . The hexaxial diagram shows the spatial orientation of the six extremity leads (I, II, III, aVR, aVL, and aVF).

The exact relationships among the three augmented extremity leads and the three standard extremity leads can be described mathematically. However, for present purposes, the following simple guidelines allow you to get an overall impression of the similarities between these two sets of leads.

As you might expect by looking at the hexaxial diagram, the pattern in lead aVL usually resembles that in lead I. The positive poles of lead aVR and lead II, on the other hand, point in opposite directions. Therefore the P–QRS–T pattern recorded by lead aVR is generally the reverse of that recorded by lead II: For example, when lead II shows a qR pattern, lead II shows an rS pattern. Finally, the pattern shown by lead aVF usually but not always resembles that shown by lead III.