Basic and Bedside Electrocardiography, 1st Edition (2009)

Chapter 6. Depolarization and Repolarization

Single Muscle Cell

·         Deflections in the electrocardiogram (ECG) including the P waves, QRS complexes, and T waves are due to depolarization and repolarization of the atria and ventricles. The following discussion will provide a basic understanding of how these ECG deflections are generated.

·         Every heartbeat is preceded by an electrical impulse that originates from the sinus node. This impulse is propagated from one cell to the next adjacent cell until the whole myocardium is depolarized. After it is discharged, the muscle cell immediately undergoes a process of repolarization that permits the cell to again depolarize at the arrival of the next impulse.

·         Single muscle cell: The resting potential of a single muscle cell is approximately -90 mV with the inside of the cell more negative than the outside (Fig. 6.1A). This difference in potential makes the cell capable of being discharged. When the myocardial cell is depolarized, the polarity reverses with the inside of the cell becoming more positive than the outside (Fig. 6.1B).

Depolarization of a Single Muscle Cell

·         Depolarization: During depolarization, the activation wave travels from one end of the myocardial cell to the other end (Fig. 6.2A).

o    Positive deflection: If a recording electrode is placed in front of the traveling impulse (at position 1, Fig. 6.2B), a positive deflection is recorded.

o    Negative deflection: If a recording electrode is placed behind the moving impulse (at position 2), a negative deflection is recorded.

·         Moving dipole: A positive deflection is recorded when the activation wave is advancing toward the recording electrode because the activation wave is traveling with the positive charge in front, which is facing the recording electrode. When the activation wave is moving away from a recording electrode, a negative deflection is recorded because the activation wave has a negative charge behind, which is facing the recording electrode. The activation wave in essence is a moving vector with opposite charges, one positive and the other negative. This moving vector with opposite charges is called a dipole (Fig. 6.2B). During depolarization, the dipole always travels with the positive charge in front and the negative charge behind.

Figure 6.1: Muscle Cell at Rest and During Depolarization. (A) The resting myocardial cell is negative inside the cell relative to the outside. (B) During depolarization, the activation wave travels from one end to the other end, changing the polarity inside the cell from negative to positive. Arrows point to the direction of depolarization.

Repolarization of a Single Muscle Cell

·         Repolarization: Repolarization restores the polarity of the cell to its original potential of -90 mV and is recorded as a T wave in the ECG. In a single myocardial cell, repolarization starts in the same area where the cell was first depolarized, because this part of the cell has had the most time to recover. The repolarization wave moves in the same direction as the wave of depolarization, only this time, the repolarization wave is a zone of advancing negative charges (Fig. 6.3A). Thus, the dipole is traveling with the negative charge in front and the positive charge behind (Fig. 6.3B).

o    Inverted T wave: During repolarization, the recording electrode positioned in front of the repolarization wave (position 1, Fig. 6.3B) will record an inverted T wave. This is because the electrode is facing the negative charge of the advancing dipole.

o    Upright T wave: The recording electrode placed behind the repolarization wave (position 2) will record an upright T wave because the electrode is facing the positive charge of the moving dipole.

·         Depolarization and repolarization of a muscle cell: Depolarization and repolarization of a single muscle cell travel in the same direction. Thus, the R wave and the T wave are inscribed in opposite directions.

Figure 6.2: Depolarization. (A) The arrows indicate the direction of the activation wave, which is a zone of advancing positive charges. The front of the activation wave is circled. (B) The wave of depolarization is represented as a moving dipole with the positive charge traveling in front and the negative charge behind. A recording electrode at position 1 will record a positive deflection because the dipole is traveling with the positive charge facing the electrode. A recording electrode at position 2 will record a negative deflection since the electrode is facing the negative charge of the moving dipole.

Figure 6.3: Repolarization of a Single Muscle Cell. (A) The front of the repolarization wave is circled and is a zone of advancing negative charges. The direction of the repolarization wave is shown by the arrows. (B) A recording electrode in front of the repolarization wave at position 1 will record an inverted T wave because the electrode is facing the negative charge of the moving dipole. A recording electrode behind the repolarization wave at position 2 will record a positive or upright T wave because the electrode is facing the positive charge of the moving dipole.

Depolarization and Repolarization of the Atria

·         Atrial depolarization and repolarization: The atrial impulse originates from the sinus node and spreads within the thin atrial wall in a circumferential fashion until both atria are depolarized. Atrial depolarization and repolarization parallels that of a single muscle cell (Fig. 6.4).

o    Depolarization—P wave: Depolarization of the atria occurs longitudinally with the impulse spreading from one cell to the next adjacent cell. It is recorded as a P wave in the ECG. Any electrode in front of the advancing wave will record a positive deflection. Any electrode behind the advancing wave will record a negative deflection.

o    Repolarization—Ta wave: Repolarization of the atria is also represented by a T wave, but is more specifically called a Ta wave to differentiate it from the T wave of ventricular repolarization. Repolarization is similar to a single muscle cell and starts from the area that was first depolarized because these cells have had the longest time to recover. Any electrode in front of the repolarization wave will record a negative deflection. The Ta wave is usually not visible because the wave is too small to be recorded. When present, it usually coincides with the QRS complex in the ECG and is therefore obscured.

·         Depolarization and repolarization of the atria: Similar to a single muscle cell, depolarization and repolarization of the atria follow the same direction. Thus, the P wave and Ta wave are inscribed in opposite directions.

Figure 6.4: Atrial Depolarization and Repolarization. (A) Depolarization of the atria is represented as a P wave in the electrocardiogram. The impulse follows the length of the thin atrial muscle and spreads circumferentially. (B) Repolarization is represented as a Ta wave and follows the same direction as depolarization. Thus, the P wave and the Ta wave are inscribed in opposite directions. Arrows represent the direction of the spread of the electrical impulse.

Depolarization and Repolarization of the Ventricles

·         Ventricular depolarization: The ventricles consist of a thick layer of cells called the myocardium. The myocardium can be divided arbitrarily into three layers—the endocardium, which is the inner layer; the midmyocardium or middle layer; and epicardium or outer layer. Unlike the atria, the ventricles are depolarized by special conduction pathways called the intraventricular conduction system consisting of the bundle of His, bundle branches, and fascicles. The intraventricular conduction system terminates in a network of Purkinje fibers, which are subendocardial in location. Depolarization of both ventricles is synchronous and occurs from endocardium to epicardium because the Purkinje fibers are located subendocardially (Fig. 6.5). When the ventricles are depolarized, a QRS complex is recorded. If a recording electrode is placed on the chest wall immediately adjacent to the epicardium, an upright deflection (tall R wave) will be recorded.

Intrinsicoid Deflection

·         Intrinsic deflection: If a recording electrode is experimentally placed directly over the epicardium of the left ventricle, an R wave will be recorded because the ventricles are activated from endocardium to epicardium. The abrupt turnaround from the peak of the R wave toward baseline is called the intrinsic deflection. It indicates that the impulse has arrived at the site of the recording electrode.

·         Intrinsicoid deflection: Clinically, the recording electrode is normally placed on the chest wall and not directly over the epicardium. What is recorded is not the intrinsic deflection, but its equivalent, the intrinsicoid deflection. The time it takes for the impulse to arrive at the recording electrode is the ventricular activation time and is measured from the onset of the QRS complex to the top of the R wave. The abrupt downward deflection of the R wave that immediately follows is the intrinsicoid deflection (Fig. 6.6). When there is right ventricular hypertrophy, the onset of the intrinsicoid deflection is delayed in right-sided precordial leads V1 or V2 (normal, ≤0.03 seconds). When there is left ventricular hypertrophy, the onset of the intrinsicoid deflection is delayed in left sided precordial leads V5 or V6 (normal, ≤0.05 seconds).

·         R peak time: When there is intraventricular conduction delay, the working group of the World Health Organization/International Society and Federation for Cardiology prefers to use the term R peak time to indicate the onset of the intrinsicoid deflection and is measured from the onset of the QRS complex to the peak of the R or R′ wave.

Figure 6.5: Depolarization of the Ventricles. Depolarization of the free wall of the ventricles starts from the endocardium and spreads outward toward the epicardium(arrows) because the Purkinje fibers are located subendocardially. A precordial lead such as V5 will record a positive deflection because the electrode is facing the positive end of the moving dipole.

Figure 6.6: Intrinsicoid Deflection. (A) The ventricular activation time (VAT) starts from the onset of the QRS complex to the peak of the R wave. The intrinsicoid deflection is the downward deflection that immediately follows the peak of the R wave. (B) When there is bundle branch block, the R peak time is the preferred terminology to identify the onset of the intrinsicoid deflection and is measured from the onset of the QRS complex to the peak of the R′ wave.

Figure 6.7: Vector 1—Initial Activation of the Ventricles. (A) The earliest portion of the ventricles to be activated is the left side of the ventricular septum (arrow) at its mid-portion. The initial electrocardiogram for V1 and for V5-6 or leads I and aVL are shown. (B) In the horizontal and frontal planes, the direction of the initial vector is represented by the arrows indicated by the number 1. This initial impulse is directed to the right, anteriorly and inferiorly. LV, left ventricle; RV, right ventricle; R, right; L, left.

The Normal Sequence of Ventricular Activation

·         The normal QRS complex: When the conduction system is intact, the sequence of ventricular activation occurs in a predictable fashion that can be broken down into three stages; vector 1 depolarization of the ventricular septum, vector 2 depolarization of the free walls of both ventricles, and vector 3 depolarization of the posterobasal wall of the left ventricle and posterobasal septum.

o    Vector 1—depolarization of the ventricular septum: When the sinus impulse finally arrives at the ventricles, the first portion of the ventricle to be activated is the middle third of the left side of the ventricular septum. This is because the left bundle branch is shorter than right bundle branch. The septum is activated from left to right as represented by the arrows in Figure 6.7A. Any electrode located to the right of the ventricular septum (such as V1) will record a positive deflection (small r wave) because the impulse is traveling toward the positive side of the electrode. Any electrode located to the left of the septum (such as precordial leads V5, V6, and limb leads I and aVL) will record a negative deflection (small q wave) because the impulse is traveling away from the positive side of these electrodes. This small q wave is often called septal q wave to indicate that the initial vector of the QRS complex is due to septal activation. The total duration of the normal septal q wave should not exceed 0.03 seconds.

o    Vector 2—depolarization of both ventricles: Depolarization of the free wall of both ventricles occurs simultaneously, beginning within the endocardium adjacent to the subendocardial Purkinje fibers and spreading outward toward the epicardium. Activation of the remaining ventricular septum occurs on both sides of the septum simultaneously, which cancels each other. Activation of the free wall of both ventricles also occurs in opposite directions, and similarly neutralizes one another. Because the right ventricle is thinner than the left ventricle, a certain portion of the forces generated by the thicker left ventricle will remain unopposed. Additionally, apical depolarization forces are not neutralized because the area opposite the apex is occupied by the non-muscular mitral and tricuspid valves. Taken together, these two forces manifest in a vector 2 that is directed to the left and slightly posteriorly, either inferiorly or superiorly, and corresponds to the mean axis of the QRS complex. A downward deflection (deep S) is recorded in V1 and an upward deflection (tall R) is recorded in V5-V6 (Fig. 6.8).

o    Vector 3—terminal portion of the QRS complex: Depolarization of the ventricles occurs in an apex to base direction. Thus, the last portion of the ventricles to become depolarized includes the posterobasal wall of the left ventricle and posterobasal portion of the ventricular septum. These structures are located superiorly in relation to the other structures of the heart. Thus, the late forces are directed superiorly and posteriorly (Fig. 6.9).

·         Vectors one through three are oversimplifications of the complex process of ventricular depolarization and are summarized in Figure 6.10. These vectors differ both spatially and temporally and produce a unique QRS complex that is contingent on the location of the recording electrode.

Figure 6.8: Vector 2 or Depolarization of the Free Walls of Both Ventricles. (A) Both ventricles are depolarized from endocardium to epicardium in an outward direction (small arrows). The mean direction of vector 2 is represented by the large arrow, which is toward the left, posteriorly and superiorly or inferiorly. (B) The mean direction of vector 2 is shown in the horizontal and frontal planes. Vector 2 corresponds to the mean axis of the QRS complex, which is -30° to +90° in the frontal plane. In the above example, the frontal plane vector is close to +60° and is inferior. RV, right ventricle; LV, left ventricle; R, right; L, left.

Ventricular Repolarization

·         Repolarization: Unlike the situation in the single muscle cell or the atria where depolarization and repolarization travel in the same direction, depolarization and repolarization of the ventricular myocardium occur in opposite directions. Thus, depolarization starts from endocardium to epicardium (Fig. 6.11A) and repolarization is reverse, occurring from epicardium to endocardium (Fig. 6.11B). This causes the QRS complex and T wave to be inscribed in the same direction. Thus, precordial electrodes V5 and V6 will record a positive deflection (tall R wave) during depolarization and also a positive deflection during repolarization (upright T wave) because these precordial electrodes are facing the positive end of the moving dipole.

·         Several explanations have been offered as to why the epicardial cells recover earlier than the endocardial cells even if they are the last to be depolarized. More recently, it has been shown that the action potential duration of endocardial cells is longer when compared with epicardial cells. This is most probably the main reason why the epicardial cells recover earlier than endocardial cells causing repolarization to start from epicardium to endocardium.

Figure 6.9: Vector 3—Terminal Portion of the QRS Complex. (A) The posterobasal portion of the septum and left ventricle are the last segments to be depolarized. The terminal vector is directed superiorly and posteriorly. (B) The direction of vector 3 in the horizontal and frontal planes is shown. L, left; R, right; LV, left ventricle; RV, right ventricle.

Figure 6.10: Summary of the Sequence of Ventricular Activation. The initial vector (A) represents depolarization of the left side of the ventricular septum at its mid-portion, which is directed to the right, anteriorly and inferiorly. (B) Vector 2 is directed to the left, posteriorly and superiorly or inferiorly corresponding to the mean axis of the QRS complex. (C) Vector 3 is directed superiorly and posteriorly.

Figure 6.11: Ventricular Repolarization. (A) Diagram showing depolarization of the ventricular myocardium, which starts from endocardium to epicardium. This causes the QRS complex to be upright since the depolarization wave is advancing toward the recording electrode. Arrows point to the direction of depolarization. (B) Repolarization of the ventricular myocardium is from epicardium to endocardium. Because the repolarization wave is moving away from the recording electrode, the recording electrode is facing the positive side of the moving dipole. Thus, a positive deflection (upright T wave) is recorded. Arrows point to the direction of repolarization.

Depolarization and Repolarization of the Atria and Ventricles

Depolarization and Repolarization

·         Single muscle cell: In a single muscle cell, depolarization and repolarization travel in the same direction. Thus, the R wave and the T wave are normally inscribed in opposite directions. If the QRS complex is upright or positive, then the T wave is normally inverted, and if the QRS is negative or inscribed downward, then the T wave is normally upright.

·         Atria: The direction of depolarization and repolarization of the atria is similar to that of a single muscle cell. The sinus impulse spreads longitudinally from right atrium to left atrium. Repolarization occurs in the same direction. Thus, if an electrode records an upright P wave, the repolarization or Ta wave is inverted and if the electrode records an inverted P wave, the Ta wave is upright.

·         Ventricles: Depolarization and repolarization of the ventricles occur in opposite directions. Thus, if the QRS complex is upright or positive, then the T wave is also upright. If the QRS complex is negative, then the T wave is inverted.

Mechanism

·         Single muscle cell: In a single muscle cell, depolarization and repolarization occur in the same direction because the area that is first depolarized has had a longer time to recover.

·         Atria: The atria consist of a thin layer of cells. Unlike the ventricles, the atria do not have a special conducting system. Thus, the impulse is spread from one muscle cell to the next muscle cell longitudinally until both atria are depolarized. Depolarization and repolarization is similar to a single muscle cell and occur in the same direction.

·         Ventricles: The ventricles consist of a thick layer of muscle cells and are depolarized from endocardium to epicardium because the Purkinje fibers are located subendocardially. Unlike the atria and the single muscle cell where depolarization and repolarization occur in the same direction, depolarization and repolarization of the ventricles occur in opposite directions. The reason as to why the epicardial cells recover earlier than the endocardial cells despite being the last to be depolarized may be due to the following reasons.

o    The endocardial cells have longer action potential duration compared with epicardial cells.

o    Myocardial perfusion occurs mainly during diastole when the ventricles are relaxed and the pressures within the cavities are lowest. Because repolarization (T wave) occurs during systole when the myocardium is mechanically contracting, there is no significant myocardial perfusion within the subendocardial layer because it is subjected to a much higher tension than the epicardium.

o    The endocardium has a higher rate of metabolism as compared with the epicardium and thus requires more oxygen than the epicardium.

o    The subendocardial layer is the deepest part of the myocardium. Because the coronary arteries are anatomically epicardial in location, the subendocardial areas are the farthest from the coronary circulation, making the endocardium relatively ischemic as compared with the epicardium.

Suggested Readings

Burch GE, Winsor T. Principles of electrocardiography. In: A Primer of Electrocardiography. 5th ed. Philadelphia: Lea & Febiger; 1966;1-66.

Dunn MI, Lipman BS. Basic physiologic principles. In: Lipman-Massie Clinical Electrocardiography. 8th ed. Chicago: Yearbook Medical Publishers; 1989;24-50.

Marriott HJL. Genesis of the precordial pattern. In: Practical Electrocardiography. 5th ed. Baltimore: Williams & Wilkins Co.; 1972;44-55.

Sgarbossa EB, Wagner GS. Electrocardiography. In: Topol EJ ed. Textbook of Cardiovascular Medicine. 2nd ed. Philadelphia: Lippincott Williams & Wilkins; 2002:1330-1383.

Willems JL, Robles de Medina EO, Bernard R, et al. Criteria for intraventricular conduction disturbances and pre-excitation. J Am Coll Cardiol. 1985;1261-1275.