Polarization

In the body, ions spend a lot of time moving back and forth across cell membranes. As a result, a slight difference in the concentrations of charged particles across the membranes of cells is normal. Thus potential energy (voltage) exists because of the imbalance of charged particles. This imbalance makes the cells excitable.

Cell membranes contain pores or channels through which specific electrolytes and other small, water-soluble molecules can cross the cell membrane from the outside to the inside (Figure 3-2). When a cell is at rest, K⁺ leaks out of it. Large molecules such as proteins and phosphates remain inside the cell because they are too big to pass easily through the cell membrane. These large molecules carry a negative charge. This results in more negatively charged ions on the inside of the cell.

Figure 3-2 Cell membranes contain membrane channels. These channels are pores through which specific ions or other small, water-soluble molecules can cross the cell membrane from outside to inside.

When the inside of a cell is more negative than the outside, it is said to be in a polarized state (Figure 3-3). The voltage (difference in electrical charges) across the cell membrane is the membrane potential. Electrolytes are quickly moved from one side of the cell membrane to the other by means of pumps. These pumps require energy in the form of adenosine triphosphate (ATP) when movement occurs against a concentration gradient. The energy expended by the cells to move electrolytes across the cell membrane creates a flow of current. This flow of current is expressed in volts. Voltage appears on an ECG as spikes or waveforms. Thus an ECG is actually a sophisticated voltmeter.

Figure 3-3 Polarization. A, Resting. B, Inside negative.

Depolarization

For a pacemaker cell to “fire” (produce an impulse), a flow of electrolytes across the cell membrane must exist. When a cell is stimulated, the cell membrane changes and becomes permeable to Na⁺ and K⁺. Permeability refers to the ability of a membrane channel to allow passage of electrolytes once it is open. Na⁺ rushes into the cell through Na⁺ channels. This causes the inside of the cell to become more positive relative to the outside. A spike (waveform) is then recorded on the ECG. The stimulus that alters the electrical charges across the cell membrane may be electrical, mechanical, or chemical.

When opposite charges come together, energy is released. When the movement of electrolytes changes the electrical charge of the inside of the cell from negative to positive, an impulse is generated. The impulse causes channels to open in the next cell membrane and then the next. The movement of charged particles across a cell membrane causing the inside of the cell to become positive is called depolarization (Figure 3-4). Depolarization must take place before the heart can mechanically contract and pump blood. Depolarization occurs because of the movement of Na⁺ into the cell. Depolarization proceeds from the innermost layer of the heart (endocardium) to the outermost layer (epicardium).

Figure 3-4 Depolarization. A, Stimulated. B, Inside positive.

An impulse normally begins in the pacemaker cells found in the SA node of the heart. A chain reaction occurs from cell to cell in the heart’s electrical conduction system until all the cells have been stimulated and depolarized. This chain reaction is a wave of depolarization. The chain reaction is made possible because of gap junctions that exist between the cells. Eventually the impulse is spread from the pacemaker cells to the working myocardial cells. The working myocardial cells contract when they are stimulated. When the atria are stimulated, a P wave is recorded on the ECG. Thus the P wave represents atrial depolarization. When the ventricles are stimulated, a QRS complex is recorded on the ECG. Thus the QRS complex represents ventricular depolarization.

Repolarization

After the cell depolarizes, it quickly begins to recover and restore its electrical charges to normal. The movement of charged particles across a cell membrane in which the inside of the cell is restored to its negative charge is called repolarization. The cell membrane stops the flow of Na⁺ into the cell and allows K⁺ to leave it. Negatively charged particles are left inside the cell. Thus the cell is returned to its resting state (Figure 3-5). This causes contractile proteins in the working myocardial cells to separate (relax). The cell can be stimulated again if another electrical impulse arrives at the cell membrane. Repolarization proceeds from the epicardium to the endocardium. On the ECG, the ST segment and T wave represent ventricular repolarization.

Figure 3-5 Repolarization. A, Resting. B, Inside negative.

The action potential of a ventricular myocardial cell consists of five phases labeled 0 to 4. These phases reflect the rapid sequence of voltage changes that occur across the cell membrane during the electrical cardiac cycle. Phases 1, 2, and 3 have been referred to as electrical systole. Phase 4 has been referred to as electrical diastole. Figure 3-6 shows the action potential of a normal ventricular muscle cell.

Figure 3-6 Action potential of a ventricular muscle cell.

Sinoatrial Node

The normal heartbeat is the result of an electrical impulse that begins in the SA node. The heart’s pacemaker cells have a built-in (intrinsic) rate that becomes slower and slower from the SA node down to the end of the His-Purkinje system. The intrinsic rate of the SA node is 60 to 100 beats/min. The SA node is normally the primary pacemaker because it depolarizes more quickly than other pacemaker sites in the heart (Figure 3-8). Other areas of the heart can assume pacemaker responsibility if:

Figure 3-8 The conduction system.

The SA node is richly supplied by sympathetic and parasympathetic nerve fibers. The fibers of the SA node directly connect with the fibers of the atria. As the impulse leaves the SA node, it is spread from cell to cell in wavelike form across the atrial muscle. As the impulse spreads, it stimulates the right atrium, the interatrial septum, and then the left atrium via Bachmann’s bundle, which is a small grouping of cells in the left atrium connected by one of four atrial conduction tracts. This results in contraction of the right and left atria at almost the same time.

Conduction through the AV node begins before atrial depolarization is completed. The impulse is spread to the AV node by three internodal pathways that consist of a mixture of working myocardial cells and specialized conducting fibers.

Atrioventricular Junction

The internodal pathways merge gradually with the cells of the AV node. Depolarization and repolarization are slow in the AV node, making this area vulnerable to blocks in conduction (AV blocks). The AV junction is made up of the AV node and the nonbranching portion of the bundle of His (Figure 3-9). This area consists of specialized conduction tissue that provides the electrical links between the atria and the ventricles. When the AV junction is bypassed by an abnormal pathway, the abnormal route is called an accessory pathway. An accessory pathway is an extra bundle of working myocardial tissue that forms a connection between the atria and the ventricles outside of the normal conduction system.

Figure 3-9 The atrioventricular (AV) junction consists of the AV node and the nonbranching portion of the bundle of His.

Right and Left Bundle Branches

The right bundle branch innervates the right ventricle. The left bundle branch spreads the electrical impulse to the interventricular septum and left ventricle, which is thicker and more muscular than the right ventricle. The left bundle branch divides into three divisions called fascicles, which are small bundles of nerve fibers allowing electrical innervation of the larger, more muscular left ventricle.

Purkinje Fibers

The right and left bundle branches divide into smaller and smaller branches and then into a special network of fibers called the Purkinje fibers. These fibers spread from the interventricular septum into the papillary muscles. They continue downward to the apex of the heart, making up an elaborate web that penetrates about one third of the way into the ventricular muscle mass. The fibers then become continuous with the muscle cells of the right and left ventricles. The Purkinje fibers have pacemaker cells that have an intrinsic rate of 20 to 40 beats/min (Table 3-2). The electrical impulse spreads rapidly through the right and left bundle branches and the Purkinje fibers to reach the ventricular muscle. The electrical impulse spreads from the endocardium to the myocardium, finally reaching the epicardial surface. The ventricular walls are stimulated to contract in a twisting motion that wrings blood out of the ventricular chambers and forces it into arteries.

TABLE 3-2 Normal Pacemaker Sites

Frontal Plane Leads

Frontal plane leads view the heart from the front of the body as if it were flat. Directions in the frontal plane are superior, inferior, right, and left (Figure 3-11). Six leads view the heart in the frontal plane. Leads I, II, and III are called standard limb leads. Leads aVR, aVL, and aVF are called augmented limb leads.

Figure 3-11 Frontal plane leads.

Standard Limb Leads

Leads I, II, and III make up the standard limb leads. If an electrode is placed on the right arm, left arm, and left leg, three leads are formed. The positive electrode is located at the left arm in lead I, while leads II and III both have their positive electrode located at the left leg. The difference in electrical potential between the positive pole and its corresponding negative pole is measured by each lead.

Lead I records the difference in electrical potential between the left arm (+) and right arm (−) electrodes. The positive electrode is placed on the left arm and the negative electrode is placed on the right arm. The third electrode is a ground that minimizes electrical activity from other sources (Figure 3-12A). Lead I views the lateral surface of the left ventricle.

Figure 3-12 Electrode placement on the patient’s limbs for (A) lead I, (B) lead II, and (C) lead III.

Lead II records the difference in electrical potential between the left leg (+) and right arm (−) electrodes. The positive electrode is placed on the left leg and the negative electrode is placed on the right arm (Figure 3-12B). Lead II views the inferior surface of the left ventricle. This lead is commonly used for cardiac monitoring because positioning of the positive and negative electrodes in this lead most closely resembles the normal pathway of current flow in the heart.

Lead III records the difference in electrical potential between the left leg (+) and left arm (−) electrodes. In lead III the positive electrode is placed on the left leg and the negative electrode is placed on the left arm (Figure 3-12C). Lead III views the inferior surface of the left ventricle. A summary of the standard limb leads can be found in Table 3-3.

TABLE 3-3 Standard Limb Leads

Augmented Limb Leads

Leads aVR, aVL, and aVF are augmented limb leads that record measurements at a specific electrode with respect to a reference electrode. The electrical potential produced by the augmented leads is normally relatively small. The ECG machine augments (i.e., magnifies) the amplitude of the electrical potentials detected at each extremity by about 50% over those recorded at the standard limb leads. The “a” in aVR, aVL, and aVF refers to augmented. The “V” refers to voltage and the last letter refers to the position of the positive electrode. The “R” refers to the right arm, the “L” to left arm, and the “F” to left foot (leg). Therefore, the positive electrode in aVR is located on the right arm, aVL has a positive electrode at the left arm, and aVF has a positive electrode positioned on the left leg (Figure 3-13).

Figure 3-13 View of the standard limb leads and augmented leads.

Lead aVR views the heart from the right shoulder (the positive electrode) and views the base of the heart (primarily the atria and the great vessels). This lead does not view any wall of the heart. Lead aVL combines views from the right arm and left leg, with the view being from the left arm and oriented to the lateral wall of the left ventricle. Lead aVF combines views from the right arm and the left arm toward the left leg; it views the inferior surface of the left ventricle from the left leg. A summary of augmented leads can be found in Table 3-4.

TABLE 3-4 Augmented Limb Leads

Lead	Positive Electrode	Heart Surface Viewed
aVR	Right arm	None
aVL	Left arm	Lateral
aVF	Left leg	Inferior

Horizontal Plane Leads

Horizontal plane leads view the heart as if the body were sliced in half horizontally. Directions in the horizontal plane are anterior, posterior, right, and left. Six chest (precordial or “V”) leads view the heart in the horizontal plane (Figure 3-14). This allows a view of the front and left side of the heart.

Figure 3-14 Horizontal plane leads.

Right Chest Leads

Other chest leads that are not part of a standard 12-lead ECG may be used to view specific surfaces of the heart. Right chest leads are used to evaluate the right ventricle (Figure 3-15). The placement of right chest leads is identical to the placement of the standard chest leads except that it is done on the right side of the chest. If time does not permit obtaining all of the right chest leads, the lead of choice is V₄R. A summary of the right chest leads can be found in Table 3-6.

Figure 3-15 Placement of the left and right chest leads.

TABLE 3-6 Right Chest Leads and Their Placement

Lead	Placement
V1R	Lead V2
V2R	Lead V1
V3R	Midway between V2R and V4R
V4R	Right midclavicular line, fifth intercostal space
V5R	Right anterior axillary line; same level as V4R
V6R	Right midaxillary line; same level as V4R

Posterior Chest Leads

On a standard 12-lead ECG, no leads look directly at the posterior surface of the heart. Additional chest leads may be used for this purpose. These leads are placed farther left and toward the back (Figure 3-16). All of the leads are placed on the same horizontal line as V₄ to V₆. Lead V₇ is placed at the posterior axillary line. Lead V₈ is placed at the angle of the scapula (i.e., the posterior scapular line) and lead V₉ is placed over the left border of the spine.

Figure 3-16 Posterior chest lead placement.

ACLS Pearl

Fifteen- and 18-lead ECGs are being used with increasing frequency to help spot infarctions of the right ventricle and the posterior wall of the left ventricle. The 15-lead ECG uses all of the leads of a standard 12-lead ECG plus leads V₄R, V₈, and V₉. An 18-lead ECG uses all of the leads of the 15-lead ECG plus leads V₅R, V₆R, and V₇. When obtaining a 15- or 18-lead ECG, first obtain a standard 12-lead ECG. Then, move the electrodes (and corresponding wires) to the desired position for the additional leads (such as V₅R, V₆R, and V₇) and run a second ECG. Because the machine will not know that you have repositioned the electrodes, it will be necessary for you to handwrite the electrode position/lead onto the ECG paper to properly indicate the origin of the tracing. The machine’s computer-generated interpretation also must be disregarded in the event the cables are moved.

What Each Lead “Sees”¹

Think of the positive electrode as an eye looking in at the heart. The part of the heart that each lead “sees” is determined by two factors. The first factor is the dominance of the left ventricle on the ECG and the second is the position of the positive electrode on the body. Because the ECG does not directly measure the heart’s electrical activity, it does not “see” all of the current flowing through the heart. What the ECG sees from its vantage point on the body’s surface is the net result of countless individual currents competing in a tug-of-war. For example, the QRS complex, which represents ventricular depolarization, is not a display of all the electrical activity occurring in the right and left ventricles. It is the net result of a tug-of-war produced by the many individual currents in both the right and left ventricles. Since the left ventricle is much larger than the right, the left overpowers it. What is seen in the QRS complex is the additional electrical activity of the left ventricle, that is, the portion that exceeds the right ventricle. Therefore, in a normally conducted beat, the QRS complex primarily represents the electrical activity occurring in the left ventricle.

The second factor, position of the positive electrode on the body, determines which portion of the left ventricle is seen by each lead. You can commit the view of each lead to memory, or you can easily reason it by remembering where the positive electrode is located. The view of each lead is listed in Table 3-7, while Figure 3-17 demonstrates the portion of the left ventricle that each lead views. Please note that aVR is not included in Table 3-7 or Figure 3-17.

TABLE 3-7 What Each Lead “Sees”

Figure 3-17 The position of the positive electrode on the body determines the portion of the heart “seen” by each lead. A, Leads II, III, and aVF each has its positive electrode positioned on the left leg. From the perspective of the left leg, each of them “sees” the inferior wall of the left ventricle. B, From their vantage point on the left arm, leads I and aVL “look” in at the lateral wall of the left ventricle. C, Leads V₅ and V₆ also “view” the lateral wall because they are positioned on the axillary area of the left chest. D, Leads V₃ and V₄ are positioned in the area of the anterior chest. From this perspective, these leads “see” the anterior wall of the left ventricle. E, The septal wall is “seen” by leads V₁ and V₂, which are positioned next to the sternum.