Cardiac contraction relies on the organized flow of electrical impulses through the heart. The electrocardiogram (ECG) is an easily obtained recording of that activity and provides a wealth of information about cardiac structure and function. This chapter presents the electrical basis of the ECG in health and disease and leads the reader through the basics of interpretation. To become fully adept at this technique and to practice the principles described here, you may wish to consult one of the complete electrocardiographic textbooks listed at the end of this chapter.
Electrical Measurement—Single-cell Model
This section begins by observing the propagation of an electrical impulse along a single cardiac muscle cell, illustrated in Figure 4.1. On the right side of the diagram, a voltmeter records the electrical potential at the cell’s surface on graph paper. In the resting state, the cell is polarized; that is, the entire out side of the cell is electrically positive with respect to the inside, because of the ionic distribution across the cell membrane, as described in Chapter 1. In this resting state, the voltmeter electrodes, which are placed on opposite outside surfaces of the cell, do not record any electrical activity, because there is no electrical potential difference between them (the myocyte surface is homogeneously charged).
Figure 4.1.Depolarization of a single cardiac muscle cell.
A. In the resting state, the surface of the cell is positively charged relative to the inside. Because the surface is homogeneously charged, the voltmeter electrodes outside the cell do not record any electrical potential difference (“flat line” recording). B. Stimulation of the cell initiates depolarization (shaded area); the outside of the depolarized region becomes negatively charged relative to the inside. Because the current of depolarization is directed toward the (+) electrode of the voltmeter, an upward deflection is recorded. C. Depolarization spreads, creating a greater upward deflection by the recording electrode. D. The cell has become fully depolarized. The surface of the cell is now completely negatively charged compared with the inside. Because the surface is again homogeneously charged, a flat line is recorded by the voltmeter. E. Notice that if the position of the voltmeter electrodes had been reversed, the electrical current would have been directed away from the (+) electrode, causing the deflection to be downward.
This equilibrium is disturbed, however, when the cell is stimulated (see Fig. 4.1B). During the action potential, cations rush across the sarcolemma into the cell and the polarity at the stimulated region transiently reverses such that the outside becomes negatively charged with respect to the inside; that is, the region depolarizes. At that moment, an electrical potential is created on the cell surface between the depolarized area (negatively charged surface) and the still-polarized (positively charged surface) portions of the cell. As a result, an electrical current begins to flow between these two regions.
By convention, the direction of an electrical current is said to flow from areas that are negatively charged to those that are positively charged. When a depolarization current is directed toward the (+) electrode of the voltmeter, an upward deflection is recorded. Conversely, if it is directed away from the (+) electrode, a downward deflection is recorded. Because the depolarization current in this example proceeds from left to right—that is, toward the (+) electrode—an upward deflection is recorded by the voltmeter.
As the wave of depolarization propagates rightward along the cell, additional electrical forces directed toward the (+) electrode record an even greater upward deflection (see Fig. 4.1C). Once the cell has become fully depolarized (see Fig. 4.1D), its outside is completely negatively charged with respect to the inside, the opposite of the initial resting condition. However, because the surface charge is homogeneous once again, the external electrodes measure a potential difference of zero and the voltmeter records a neutral “flat line” at this time.
Note that in Figure 4.1E, if the voltmeter electrode positions had been reversed, such that the (+) pole was placed to the left of the cell, then as the wave of depolarization proceeds toward the right, the current would be directed away from the (+) electrode and the recorded deflection would be downward. This relationship should be kept in mind when the polarity of ECG leads is described below.
Depolarization initiates myocyte contraction and is then followed by repolarization, the process by which the cellular charges return to the resting state. In Figure 4.2, as the left side of the cardiac muscle cell in our example begins to repolarize, its surface charge becomes positive once again. An electrical potential is therefore generated and current flows from the still negatively charged surface toward the positively charged region. Since this current is directed away from the voltmeter’s (+) electrode, a downward deflection is recorded, opposite to that which was observed during the process of depolarization.
Figure 4.2.Sequence of repolarization of a single cardiac muscle cell.
A. As repolarization commences, positive charges reemerge on the surface of the cell, and a current flows from the still negatively charged surface areas to the repolarized region (blue arrows). Because the current is directed away from the (+) electrode of the voltmeter, a downward deflection is recorded. B. Repolarization progresses. C. Repolarization has completed, and the outside surface of the cell is once again homogeneously charged, so that no further electrical potential is detected (flat line once again). D. Sequence of cardiac depolarization and repolarization as measured by an ECG machine at the skin surface. As described in the text, repolarization actually proceeds in the direction opposite to that of depolarization in the intact heart, such that the deflections of repolarization are inverted compared to the schematics presented in parts A–C of this figure. Therefore, the deflections of depolarization and repolarization of the normal heart are oriented in the same direction. Note that the wave of repolarization is more prolonged and of lower amplitude than that of depolarization.
Repolarization is a slower process than depolarization, so the inscribed deflection of repolarization is usually wider and of lower magnitude. Once the cell has returned to the resting state, the surface charges are once again homogeneous and no further electrical potential is detected, resulting in a neutral flat line on the voltmeter recording (see Fig. 4.2C).
The depolarization and repolarization of a single cardiac muscle cell have been considered here. As a wave of depolarization spreads through the entire heart, each cell generates electrical forces, and it is the sum of these forces, measured at the skin’s surface, that is recorded by the ECG machine.
It is important to note that in the intact heart the sequence by which regions repolarize is actually opposite to that of their depolarization. This occurs because myocardial action potential durations are more prolonged in cells near the inner endocardium (the first cells stimulated by Purkinje fibers) than in myocytes near the outer epicardium (the last cells to depolarize).
Thus, the cells close to the endocardium are the first to depolarize but are the last to repolarize. As a result, the direction of repolarization recorded by the ECG machine is usually the inverse of what was presented in the single cell example in Figure 4.2. That is, unlike the single cell model, the electrical deflections of depolarization and repolarization in the intact heart are usually oriented in the same direction on the ECG tracing (see Fig. 4.2D).
The direction and magnitude of the deflections on an ECG recording depend on how the generated electrical forces are aligned to a set of specific reference axes, known as ECG leads, as described in the next section.
Electrocardiographic Lead Reference System
When the first device to produce an ECG was invented over a century ago, the recording was made by dunking the patient’s arms and legs into large buckets of electrolyte solution that were wired to the machine. That process was likely fairly messy and fortunately is no longer necessary. Instead, wire electrodes are placed directly on the skin, held in place by adhesive tabs, on each of the four limbs and on the chest in the standard arrangement shown in Figure 4.3. The right-leg electrode is not used for the measurement but serves as an electrical ground. Table 4.1 lists the standard locations of the chest electrodes.
Figure 4.3.Placement of electrocardiogram (ECG) electrodes.
A. Standard positions. B. Close-up view of chest electrode placement, at the standard positions listed in Table 4.1.
Table 4.1.Positions of ECG Chest Electrodes
|V1||4th ICS, 2 cm to the right of sternum|
|V2||4th ICS, 2 cm to the left of sternum|
|V3||Midway between V2 and V4|
|V4||5th ICS, left midclavicular line|
|V5||5th ICS, left anterior axillary line|
|V6||5th ICS, left midaxillary line|
A complete ECG (termed a “12-lead ECG”) is produced by recording electrical activity between the electrodes in specific patterns. This results in six reference axes in the body’s frontal plane (termed limb leads) plus six in the transverse plane (termed chest leads). Figure 4.4 demonstrates the orientation of the six limb leads, which are electronically constructed as described in the following paragraphs.
The ECG machine records lead aVR by selecting the right-arm electrode as the (+) pole with respect to the other electrodes. This is known as a unipolar lead, because there is no single (?) pole; rather, the other limb electrodes are averaged to create a composite (?) reference. When the instantaneous electrical activity of the heart points in the direction of the right arm, an upward deflection is recorded in lead aVR. Conversely, when electrical forces are directed away from the right arm, the ECG inscribes a downward deflection in aVR.
Similarly, lead aVF is recorded by setting the left leg as the (+) pole, such that a positive deflection is recorded when forces are directed toward the feet. Lead aVL is selected when the left-arm electrode is made the (+) pole and it records an upward deflection when electrical activity is aimed in that direction.
In addition to these three unipolar leads, three bipolar limb leads are part of the standard ECG recording (see Fig. 4.4). Bipolar indicates that one limb electrode is the (+) pole and another single electrode provides the (?) reference. In this case, the ECG machine inscribes an upward deflection if electrical forces are heading toward the (+) electrode and records a downward deflection if the forces are heading toward the (?) electrode. A simple mnemonic to remember the orientation of the bipolar leads is that the lead name indicates the number of l’s in the placement sites. For example, lead I connects the left arm to the right arm, lead II connects the right arm to the left leg, and lead III connects the left arm to the left leg. Table 4.2 summarizes how the six limb leads are derived.
Figure 4.4.The six limb leads are derived from the electrodes placed on the arms and left leg.
Top, Each unipolar lead has a single (+) designated electrode; the (?) pole is an average of the other electrodes. Bottom, Each bipolar lead has specific (?) and (+) designated electrodes.
Table 4.2.Limb Leads
|Lead||(+) Electrode||(?) Electrode|
By overlaying these six limb leads, an axial reference system is established ( Fig. 4.5). In the figure, each lead is presented with its (+) pole designated by an arrowhead and the (?) aspect by dashed lines. Note that each 30° sector of the circle falls along the (+) or (?) pole of one of the standard six ECG limb leads. Also note that the (+) pole of lead I points to 0° and that, by convention, measurement of the angles proceeds clockwise from 0° as +30°, +60°, and so forth. The complete ECG recording provides a simultaneous “snapshot” of the heart’s electrical activity, taken from the perspective of each of these lead reference axes.
Figure 4.5.The axial reference system is created by combining the six limb leads shown in Figure 4.4.
Each lead has a (+) region indicated by the arrowhead and a (?) region indicated by the dashed line.
Figure 4.6 demonstrates how the magnitude and direction of electrical activity are represented by the ECG recording in each lead. This figure should be studied until the following four points are clear:
- An electrical force directed toward the (+) pole of a lead results in an upward deflection on the ECG recording of that lead.
- Forces that head away from the (+) electrode result in a downward deflection in that lead.
- The magnitude of the deflection, either upward or downward, reflects how parallel the electrical force is to the axis of the lead being examined. The more parallel the electrical force is to the lead, the greater the magnitude of the deflection.
- An electrical force directed perpendicular to an electrocardiographic lead does not register any activity by that lead (a flat line on the recording).
Figure 4.6.Relationship of the magnitude and direction of electrical activity to the ECG lead.
A. The electrical vector is oriented parallel to lead I and directed toward the (+) electrode; therefore, a tall upward deflection is recorded by the lead. B. The vector is still oriented toward the (+) electrode of lead I but not parallel to the lead, so that only a component of the force is recorded. Thus, the recorded deflection is still upward but of lower amplitude compared with that shown in A.C. The electrical vector is perpendicular to lead I so that no deflection is generated. D. The vector is directed toward the (?) region of lead I, causing the ECG to record a downward deflection.
The six standard limb leads examine the electrical forces in the frontal plane of the body. However, because electrical activity travels in three dimensions, recordings from a perpendicular plane are also essential ( Fig. 4.7A). This is accomplished by the use of the six electrodes placed on the anterior and left lateral aspect of the chest (see Fig. 4.3B), creating the chest (also termed “precordial”) leads. The orientation of these leads around the heart in the crosssectional plane is shown in Figure 4.7B. These are unipolar leads and, as with the unipolar limb leads, electrical forces directed toward these individual (+) electrodes result in an upward deflection on the recording of that lead, and forces heading away record a downward deflection.
Figure 4.7.The chest (precordial) leads.
A. The cross-sectional plane of the chest. B. Arrangement of the six chest electrodes shown in the cross-sectional plane. Note that the right ventricle is anterior to the left ventricle.
A complete ECG prints samples from each of the six limb leads and each of the six chest leads in a standard order, examples of which are presented later in this chapter (see Figs. 4.28, 4.29, 4.30, 4.31, 4.32, 4.33, 4.34, 4.35 and 4.36).