A

Resting

Depolarized

Repolarized

FIGURE 2-14 A model of the way depolarization and repolarization of ventricular muscle results in voltage changes recorded by external electrodes. Ventricular muscle is placed in a conducting solution, and electrodes are located on either side of the muscle to record potential differences. (A) Resting (polarized) muscle has the same potential across the surface, as indicated by positive charges outside of the cells (relative to the negative cell interior; see text); therefore, the electrodes record no potential difference between them (0 voltage; i.e., isoelectric). (B) Muscle depolarizes beginning at the left side, and a wave of depolarization (arrow) travels from left to right across the muscle. The separation of charges in the partially depolarized muscle results in a positive voltage recording (analogous to the QRS complex). (C) All of the muscle is depolarized (all cells negative on the outside), so that there is no separation of charge and therefore no potential difference (isoelectric; analogous to the ST segment). (D) Partially repolarized muscle; the last cells to depolarize are the first to repolarize, resulting in a wave of repolarization (arrow) moving from right to left. The separation of charges results in a positive voltage recording (analogous to the T wave). (E) Muscle fully repolarized as in A.

entire muscle mass, all of the cells on the outside are negative, and once again, no potential difference exists between the two electrodes (i.e., isoelectric voltage) (Fig. 2-14, panel C). Because the movement of the wave of depolarization is time dependent, we initially see zero voltage (panel A) followed by a transient positive voltage deflection (panel B), ending once again at zero voltage (panel C). This pattern depicts in simplistic terms the process of atrial and ventricular depolarizations, and the way the P wave and QRS complex, respectively, are generated.

All of the cells are depolarized for only a brief period of time, after which they undergo repolarization. For this model, assuming that the last cells to depolarize are the first to re-polarize, a wave of repolarization would move from right-to-left (panel D). As repolarization occurs, cells on the right (nearest to the positive electrode) are the first to become positive again on the outside. This event results in a potential difference between the electrodes, with the positive electrode "seeing" a positive polarity and therefore recording a positive voltage. After the wave of repolarization sweeps across the entire mass and all the cells become repolarized, the entire surface is once again positive and no potential difference exists between the electrodes (i.e., isoelectric voltage) (Fig. 2-14, panel E). By convention, a wave of repolarization moving away from a positive electrode produces a positive voltage difference. This repolarization direction is what happens in the ventricle and explains why the T wave, which represents ventricular repolarization, is normally positive. If the wave of repolarization were to begin with the first cells that depolarized, the wave would travel toward the positive electrode, and a negative voltage deflection would be recorded. Therefore, by convention, a wave of repolarization moving toward a positive electrode produces a negative voltage deflection in the ECG. This repolarization direction is what happens in the atria. If atrial repolarization could be seen in the ECG, the waveform would have a negative voltage deflection.

Vectors and Mean Electrical Axis

The simplified model presented in Figure 2-14 depicts single waves of depolarization and repolarization. In reality, there is no single wave of electrical activity through the muscle. As illustrated for the atria in Figure 2-15, when the SA node fires, many separate depolarization waves emerge from the SA node and travel throughout the atria. These separate waves can be depicted as arrows representing individual electrical vectors. At any

FIGURE 2-15 Electrical vectors. Instantaneous individual vectors of depolarization (black arrows) spread across the atria after the sinoatrial (SA) node fires. The mean electrical vector (red arrow) represents the sum of the individual vectors at a given instant in time.

given instant, many individual vectors exist; each one represents action potential conduction in a different direction. A mean electrical vector can be derived at that instant by summing the individual vectors.

The direction of the mean electrical vector relative to the axis between the recording electrodes determines the polarity and magnitude of the recorded voltage (Fig. 2-16). If the mean electrical vector is pointing toward the positive electrode, the ECG displays a positive deflection (positive voltage). If at some other instant the mean electrical vector is pointing away from the positive electrode, there is a negative deflection (negative voltage). If the mean electrical vector is oriented perpendicular to the axis between the positive and negative electrodes, there is no net change in voltage.

The preceding discussion describes a mean electrical vector determined at a specific point in time (i.e., an instantaneous mean vector). If a series of instantaneous mean vectors is determined over time, it is possible to derive an average mean vector that represents all of the individual vectors over time. Figure 2-17 depicts the sequence of depolarization within the ventricles by showing four different mean vectors representing different times during depolarization. This model shows the septum and free walls of the left and right ventricles; each of the four vectors is depicted as originating from the AV node. The size of the vector arrow is related to the mass of tissue undergoing depolarization. The larger the arrow (and tissue mass), the greater the measured voltage. The electrode placement represents lead II (see the next section, ECG Leads). Early during ventricular activation, the interventricular septum depolarizes from left to right as depicted by mean electrical vector 1. This small vector is heading away from the positive electrode (to the right of a line perpendicular to the lead axis) and therefore records a small negative deflection (the Q

FIGURE 2-16 Recording of electrical vectors. Orientation of the mean electrical vector of depolarization relative to the recording electrodes determines the polarity of the recording. Arrow 1, which is heading directly toward the positive electrode, gives the greatest positive deflection. As the vector moves around the axis to the left, and therefore moves away from the positive electrode, the recorded voltage becomes less positive, and then negative as the vector heads away from the positive electrode. No net voltage is present when the vector is perpendicular to the axis between the two electrodes.

FIGURE 2-16 Recording of electrical vectors. Orientation of the mean electrical vector of depolarization relative to the recording electrodes determines the polarity of the recording. Arrow 1, which is heading directly toward the positive electrode, gives the greatest positive deflection. As the vector moves around the axis to the left, and therefore moves away from the positive electrode, the recorded voltage becomes less positive, and then negative as the vector heads away from the positive electrode. No net voltage is present when the vector is perpendicular to the axis between the two electrodes.

QRS Complex Lead II

FIGURE 2-17 Generation of QRS complex from vectors representing ventricular depolarization. Arrows 1-4 represent the time-dependent sequence of ventricular depolarization and the way these time-dependent vectors generate the QRS complex. The relationship of the positive and negative recording electrodes relative to the ventricle depicts lead II. See the text for more details.

QRS Complex Lead II

FIGURE 2-17 Generation of QRS complex from vectors representing ventricular depolarization. Arrows 1-4 represent the time-dependent sequence of ventricular depolarization and the way these time-dependent vectors generate the QRS complex. The relationship of the positive and negative recording electrodes relative to the ventricle depicts lead II. See the text for more details.

wave of the QRS). About 20 milliseconds later, the mean electrical vector points downward toward the apex (vector 2), and heads toward the positive electrode. This direction gives a very tall, positive deflection (the R wave of the QRS). After another 20 milliseconds, the mean vector is directed toward the left arm and anterior chest as the free wall of the ventricle depolarizes from the endocardial (inside) to epicardial (outside) surface (vector 3). This vector still records a small positive voltage in lead II and corresponds to a voltage point between the R and S waves. Finally, the last regions to depolarize result in vector 4, which causes a slight negative deflection (the S wave) of the QRS because it is pointed away from the positive electrode. If the four vectors in Figure 2-15 are summed, the resultant vector (red arrow) is the mean electrical axis. The mean electrical axis is the average ventricular depolarization vector over time; therefore, it is the average of all of the instantaneous mean electrical vectors occurring sequentially during ventricular depolarization. The determination of mean electrical axis is particularly significant for the ventricles. It is used diagnostically to identify left and right axis deviations, which can be caused by a number of factors, including conduction blocks in a bundle branch and ventricular hypertrophy.

It is important to note that the shape of the QRS complex can change considerably depending on the placement of the recording electrodes. For example, if the polarity of the electrodes were reversed in Figure 2-17, the QRS complex would be inverted: a small positive deflection, followed by a large negative deflection, and ending with a small positive deflection.

Based on the previous discussion, the following rules can be used in interpreting the ECG:

1. A wave of depolarization traveling toward a positive electrode results in a positive deflection in the ECG trace.

[Corollary: A wave of depolarization traveling away from a positive electrode results in a negative deflection.]

2. A wave of repolarization traveling toward a positive electrode results in a negative deflection. [Corollary: A wave of repolarization traveling away from a positive electrode results in a positive deflection.]

3. A wave of depolarization or repolariza-tion oriented perpendicular to an electrode axis has no net deflection.

4. The instantaneous amplitude of the measured potentials depends upon the orientation of the positive electrode relative to the mean electrical vector.

5. Voltage amplitude (positive or negative) is directly related to the mass of tissue undergoing depolarization or repolarization.

The first three rules are derived from the volume conductor models described earlier. The fourth rule takes into consideration that, at any given point in time during depolarization in the atria or ventricles, many separate waves of depolarization are traveling in different directions relative to the positive electrode. The recording by the electrode reflects the average, instantaneous direction and magnitude (i.e., the mean electrical vector) for all of the individual depolarization waves. The fifth rule states that the amplitude of the wave recorded by the ECG is directly related to the mass of the muscle undergoing depolarization or repolarization. For example, when the mass of the left ventricle is increased (i.e., ventricular hypertrophy), the amplitude of the QRS complex, which largely represents left ventricular depolarization, is sometimes increased (depending on the degree of hypertrophy).

ECG Leads: Placement of Recording Electrodes

The ECG is recorded by placing an array of electrodes at specific locations on the body surface. Conventionally, electrodes are placed on each arm and leg, and six electrodes are placed at defined locations on the chest. Three basic types of ECG leads are recorded by these electrodes: standard limb leads, augmented limb leads, and chest leads. These electrode leads are connected to a device that measures potential differences between selected electrodes to produce the characteristic ECG tracings. The limb leads are sometimes referred to as bipolar leads because each lead uses a single pair of positive and negative electrodes. The augmented leads and chest leads are unipolar leads because they have a single positive electrode with the other electrodes coupled together electrically to serve as a common negative electrode.

ECG Limb Leads

Standard limb leads are shown in Figure 2-18. Lead I has the positive electrode on the left arm and the negative electrode on the right arm, therefore measuring the potential difference across the chest between the two arms. In this and the other two limb leads, an electrode on the right leg is a reference electrode for recording purposes. In the lead II configuration, the positive electrode is on the left leg and the negative electrode is on the right arm. Lead III has the positive electrode on the left leg and the negative electrode on the left arm. These three limb leads roughly form an equilateral triangle (with the heart at

FIGURE 2-18 Placement of the standard ECG limb leads (leads I, II, and III) and the location of the positive and negative recording electrodes for each of the three leads. RA, right arm; LA, left arm; RL, right leg; LL, left leg.

the center), called Einthoven's triangle in honor of Willem Einthoven who developed the ECG in 1901. Whether the limb leads are attached to the end of the limb (wrists and ankles) or at the origin of the limbs (shoulder and upper thigh) makes virtually no difference in the recording because the limb can be viewed as a wire conductor originating from a point on the trunk of the body. The electrode located on the right leg is used as a ground.

When using the ECG rules described in the previous section, it is clear that a wave of depolarization heading toward the left arm gives a positive deflection in lead I because the positive electrode is on the left arm. Maximal positive deflection of the tracing occurs in lead I when a wave of depolarization travels parallel to the axis between the right and left arms. If a wave of depolarization heads away from the left arm, the deflection is negative. In addition, a wave of repolarization moving away from the left arm is seen as a positive deflection.

Similar statements can be made for leads II and III, with which the positive electrode is located on the left leg. For example, a wave of depolarization traveling toward the left leg gives a positive deflection in both leads II and III because the positive electrode for both leads is on the left leg. A maximal positive deflection is obtained in lead II when the depolarization wave travels parallel to the axis between the right arm and left leg. Similarly, a maximal positive deflection is obtained in lead II when the depolarization wave travels parallel to the axis between the left arm and left leg.

If the three limbs of Einthoven's triangle (assumed to be equilateral) are broken apart, collapsed, and superimposed over the heart (Fig. 2-19), the positive electrode for lead I is defined as being at zero degrees relative to the heart (along the horizontal axis; see Figure 2-19). Similarly, the positive electrode for lead II is +60° relative to the heart, and the positive electrode for lead III is +120° relative to the heart, as shown in Figure 2-19. This new construction of the electrical axis is called the axial reference system. Although the designation of lead I as being 0°, lead II as being

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