Standard Limb Leads: I, II, and III

The extremity leads 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. As shown in Figure 3-3, the arm electrodes are attached just above the wrist and the leg electrodes are attached above the ankles.

Figure 3-3 Electrodes (usually disposable paste-on) are attached to the body surface to take an ECG. The right leg (RL) electrode functions solely as a ground to prevent alternating-current interference. LA, left arm; LL, left leg; RA, right arm.

The electrical voltages of the heart 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. In clinical practice the electrodes are attached to the wrists and ankles simply for convenience.

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 the Dutch physiologist (1860-1927) who invented the electrocardiograph. At first the ECG consisted only of recordings from leads I, II, and III. Einthoven’s triangle (Fig. 3-4) 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.

Figure 3-4 Orientation of leads I, II, and III. Lead I records the difference in electrical potentials between the left arm and right arm. Lead II records it between the left leg and right arm. Lead III records it between the left leg and left arm.

Einthoven, of course, could have hooked the leads up differently. Yet 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.^∗

You can test this equation by looking at Figure 3-2. 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.

It is a good practice 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 R waves in leads I and II, this may be a clue that the leads have been recorded incorrectly or mounted improperly.

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 Figure 3-5, Einthoven’s triangle has been redrawn so that leads I, II, and III intersect at a common central point. This was done simply by sliding lead I downward, lead II rightward, and lead III leftward. The result is the triaxial diagram in Figure 3-5B. This diagram, a useful way of representing the three bipolar leads, is employed in Chapter 5 to help measure the QRS axis.

Figure 3-5 A, Einthoven’s triangle. B, The triangle is converted to a triaxial diagram by shifting leads I, II, and III so that they intersect at a common point.

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, V₁ through V₆. 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 (V₁ to V₆).

A so-called unipolar lead records the electrical voltages at one location relative to an electrode with close to zero potential rather than relative to the voltages at another single 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. Because 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 Einthoven’s triangle represents the spatial orientation of the three standard limb leads, the diagram in Figure 3-6 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 called a triaxial diagram.

Figure 3-6 Triaxial lead diagram showing the relationship of the three augmented (unipolar) leads (aVR, aVL, and aVF). Notice that each lead is represented by an axis with a positive and negative pole. The term unipolar was used to mean that the leads record the voltage in one location relative to about zero potential, instead of relative to the voltage in one other extremity.

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 Figure 3-2, 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.

The 12 ECG leads have two major features, which have already been described. They have both a specific orientation and a specific polarity.

Thus, the axis of lead I is oriented horizontally, and the axis of lead aVR points diagonally downward. The orientation of the standard (bipolar) leads is shown in Einthoven’s triangle (see Fig. 3-5), and the orientation of the augmented (unipolar) extremity leads is diagrammed in Figure 3-6.

The second major feature of the ECG leads, their polarity, can be represented by a line (axis) with a positive and a negative pole. (The polarity and spatial orientation of the leads are discussed further in Chapters 4 and 5 when the normal ECG patterns seen in each lead are considered and the concept of electrical axis is explored.)

Do not be confused by the difference in meaning between ECG electrodes and ECG leads. An electrode is simply the the paste-on disk or metal plate used to detect the electrical currents of the heart in any location. An ECG lead shows the differences in voltage detected by electrodes (or sets of electrodes). For example, lead I records the differences in voltage detected by the left and right arm electrodes. Therefore, a lead is a means of recording the differences in cardiac voltages obtained by different electrodes. For electronic pacemakers, discussed in Chapter 21, the terms lead and electrode are used interchangeably.

Relationship of Extremity Leads

Einthoven’s triangle in Figure 3-4 shows the relationship of the three standard limb leads (I, II, and III). Similarly, the triaxial diagram in Figure 3-7 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 lead diagram shown in Figure 3-7. The hexaxial diagram shows the spatial orientation of the six extremity leads (I, II, III, aVR, aVL, and aVF).

Figure 3-7 A, Triaxial diagram of the so-called bipolar leads (I, II, and III). B, Triaxial diagram of the augmented limb leads (aVR, aVL, and aVF). C, The two triaxial diagrams can be combined into a hexaxial diagram that shows the relationship of all six limb leads. The negative pole of each lead is now indicated by a dashed line.

The exact relationships among the three augmented extremity leads and the three standard extremity leads can also 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 aVR usually shows an rS pattern

Finally, the pattern shown by lead aVF usually but not always resembles that shown by lead III.

The 12-Lead ECG: Frontal and Horizontal Plane Leads

You may now be wondering why 12 leads are used in clinical electrocardiography. Why not 10 or 22 leads? The reason for exactly 12 leads is partly historical, a matter of the way the ECG has evolved over the years since Dr. Willem Einthoven’s original three extremity leads were developed around 1900. There is nothing sacred about the “electrocardiographer’s dozen.” In some situations, for example, additional leads are recorded by placing the chest electrode at different positions on the chest wall. Multiple leads are used for good reasons. The heart, after all, is a three-dimensional structure, and its electrical currents spread out in all directions across the body. Recall that the ECG leads were described as being like video cameras by which the electrical activity of the heart can be viewed from different locations. To a certain extent, the more points that are recorded, the more accurate the representation of the heart’s electrical activity.

The importance of multiple leads is illustrated in the diagnosis of myocardial infarction (MI). An MI typically affects one localized portion of either the anterior or inferior portion of the left ventricle. The ECG changes produced by an anterior MI are usually best shown by the chest leads, which are close to and face the injured anterior surface of the heart. The changes seen with an inferior MI usually appear only in leads such as II, III, and aVF, which face the injured inferior surface of the heart (see Chapters 8 and 9). The 12 leads therefore provide a three-dimensional view of the electrical activity of the heart.

Specifically, the six limb leads (I, II, III, aVR, aVL, and aVF) record electrical voltages transmitted onto the frontal plane of the body (Fig. 3-10). (In contrast, the six precordial leads record voltages transmitted onto the horizontal plane.) For example, if you walk up to and face a large window, the window is parallel to the frontal plane of your body. Similarly, heart voltages directed upward and downward and to the right and left are recorded by the frontal plane leads.

Figure 3-10 Spatial relationships of the six limb leads, which record electrical voltages transmitted onto the frontal plane of the body.

The six chest leads (V₁ through V₆) record heart voltages transmitted onto the horizontal plane of the body (Fig. 3-11). The horizontal plane cuts your body into an upper and a lower half. Similarly, the chest leads record heart voltages directed anteriorly (front) and posteriorly (back), and to the right and left.

Figure 3-11 Spatial relationships of the six chest leads, which record electrical voltages transmitted onto the horizontal plane.

The 12 ECG leads are therefore divided into two sets: the six extremity leads (three unipolar and three bipolar), which record voltages on the frontal plane of the body, and the six chest (precordial) leads, which record voltages on the horizontal plane. Together these 12 leads provide a three-dimensional picture of atrial and ventricular depolarization and repolarization. This multilead display is analogous to having 12 video cameras continuously recording cardiac electrical activity from different angles.

Bedside Cardiac Monitors

Up to now, only the standard 12-lead ECG has been considered. However, it is not always necessary or feasible to record a full 12-lead ECG. For example, many patients require continuous monitoring for a prolonged period. In such cases, special cardiac monitors are used to give a continuous beat-to-beat record of cardiac activity from one monitor lead. Such ECG monitors are ubiquitous in emergency departments, intensive care units, operating rooms, and postoperative care units, as well in a variety of other inpatient settings.

Figure 3-12 is a rhythm strip recorded from a monitor lead obtained by means of three disk electrodes on the chest wall. As shown in Figure 3-13, one electrode (the positive one) is usually pasted in the V₁ position. The other two are placed near the right and left shoulders. One serves as the negative electrode and the other as the ground.

Figure 3-12 A and B, Rhythm strips from a cardiac monitor taken moments apart but showing exactly opposite patterns because the polarity of the electrodes was reversed in the lower strip (B).

Figure 3-13 Monitor lead. A chest electrode (+) is placed at the lead V₁ position (between the fourth and fifth ribs on the right side of the sternum). The negative (–) electrode is placed near the right shoulder. A ground electrode (G) is placed near the left shoulder. This lead is therefore a modified V₁. Another configuration is to place the negative electrode near the left shoulder and the ground electrode near the right shoulder.

When the location of the electrodes on the chest wall is varied, the resultant ECG patterns also vary. In addition, if the polarity of the electrodes changes (e.g., the negative electrode is connected to the V₁ position and the positive electrode to the right shoulder), the ECG shows a completely opposite pattern (see Fig. 3-12).

Ambulatory ECG Technology: Holter Monitors and Event Recorders

The cardiac monitors just described are useful in patients primarily confined to a bed or chair. Sometimes, however, the ECG needs to be recorded, usually to evaluate arrhythmias, in ambulatory patients over longer periods. A special portable system, designed in 1961 by N.J. Holter, records the continuous ECG of patients as they go about their daily activities (Box 3-2).

Most of the Holter monitors currently in use consist of electrodes placed on the chest wall and lower abdomen interfaced with a special digital, portable ECG recorder. The patient can then be monitored over a long, continuous period (typically 24 hours). Two ECG leads are usually recorded. The digital recording can be played back, and the P-QRS-T complexes are displayed on a special screen for analysis and annotation. The recording can also be digitally archived, and selected sections can be printed out.

The limitations of Holter monitors have led to the development and widespread use of miniaturized ECG monitors, called event recorders. These event recorders are designed with replaceable electrodes so that patients can be monitored for prolonged periods (typically up to 2-3 weeks) as they go about their usual activities. The ECG is continuously recorded and then can be automatically erased unless the patient presses an event button (this type of event recorder is called a loop recorder).

When patients experience a symptom (e.g., lightheadedness, palpitations, chest discomfort), they can push a button so that the ECG obtained around the time of the symptom is stored. The saved ECG also includes a continuous rhythm strip just (e.g., 45 sec) before the button was pressed, as well as a recording after the event mark (e.g., 15 sec). The stored ECGs can be transmitted by phone to an analysis station for immediate diagnosis. Contemporary event recorders also have automatic settings that will record heart rates above or below preset values even if the patient is asymptomatic.

Event recorders can also be used to monitor the ECG for asymptomatic drug effects and potentially important toxicities (e.g., excessive prolongation of the QT interval with drugs such as sotalol, quinidine, or dofetilide) or to detect other potentially proarrhythmic effects (Chapter 19) of drugs.

In recent years, event recorders have been extended to include mobile continuous telemetry (MCT). This type of continuous home recording provides wireless transmission if the ECG rhythm exceeds predesignated values (auto-trigger mode) or if the patient pushes a button (patient trigger option). The transmitted ECGs are then evaluated at a specialized analysis center.

Finally, in some cases, life-threatening arrhythmias (e.g., intermittent complete heart block or sustained ventricular tachycardia) may be so rare that they are not readily detected by any of the preceding ambulatory devices. In such cases, a small monitor can be surgically inserted under the skin of the upper chest (insertable cardiac monitor) where it records the ECG and saves records when prompted by the patient (or family member if the patient faints, for example) or when activated by an automated algorithm.