Basic and Bedside Electrocardiography, 1st Edition (2009)

Chapter 4. The Electrical Axis and Cardiac Rotation

The Frontal and Horizontal Planes

·   Figuring the direction or axis of the QRS complex (or any wave in the electrocardiogram [ECG]) requires a thorough understanding of the location of each of the different leads in the 12-lead ECG. This knowledge is crucial and provides the basic foundation for understanding electrocardiography. Before attempting to read this chapter, a review of the previous chapter is mandatory.

·   The ECG mirrors both frontal and horizontal planes of the body and is thus tridimensional.

o    Frontal plane: The frontal plane is represented by leads I, II, III, aVR, aVL, and aVF. It includes the left/right and superior/inferior orientation of the body (Fig. 4.1A). The electrical position of the heart in the frontal plane is described as axis deviation. Thus, the axis of the QRS complex may be normal or it may be deviated to the left, to the right or to the northwest quadrant.

o    Horizontal plane: The horizontal plane is represented by leads V1 to V6 (Fig. 4.1B). It includes left/right and anteroposterior orientation of the body. The position of the heart in the horizontal plane is described as rotation. Thus, the rotation of the heart may be normal or it may be rotated clockwise or counterclockwise.

Figure 4.1: The 12-Lead Electrocardiogram. The location of the different leads in the frontal (A) and horizontal (B) planes is shown.

The Frontal Plane

·   Frontal plane: Using the hexaxial reference system (Fig. 4.2), the frontal plane can be divided into four quadrants.

o    Normal quadrant: The left lower quadrant between 0° and +90° represents normal quadrant.

o    Left upper quadrant: The left upper quadrant between 0° and -90° represents left axis deviation.

o    Right lower quadrant: The right lower quadrant between +90° and +180° represents right axis deviation.

o    Right upper quadrant: The quadrant between -90° and -180° is either extreme right or extreme left axis deviation. Often, it is not possible to differentiate whether the axis has deviated extremely to the right or extremely to the left; thus, this axis is often called northwest axis.

·   Normal axis: The normal QRS axis depends on the age of the patient.

o    In newborns up to 6 months of age, the normal QRS axis is >+90° (vertical axis). With increasing age, the axis moves horizontally leftward toward 0°. It is rare in children to have a horizontal axis.

o    In adults, the normal axis extends horizontally from +90° to -30°. The axis -1° to -30° is located in the left upper quadrant and is left axis deviation. However, because the normal axis extends up to -30°, an axis of -1° to -30° is considered part of the normal axis (Fig. 4.2).

Figure 4.2: The Frontal Plane and the Hexaxial Reference System. The frontal plane is represented by the six limb leads. The position of the limb leads and the location of the different quadrants in the frontal plane are shown. Note that the leads are 30° apart. The normal axis in the adult extends from -30° to +90°, thus -1° to -30° is considered normal axis. LAD, left axis deviation; RAD, right axis deviation; L, left; R, right; SUP, superior; INF, inferior.

Figure 4.3: Leads I and aVF. Lead I and aVF are perpendicular to each other. The electrocardiogram deflection in lead I will register the tallest deflection if the current is directed toward the positive electrode (0°) as shown in A. It will record the deepest deflection if the current is directed toward 180° or the negative electrode as shown in B. The lead perpendicular to lead I will record an isoelectric deflection. Because aVF is perpendicular to lead I, aVF will record an isoelectric complex (C, D).

Figuring Out the Electrical Axis

·   Basic considerations: Before attempting to determine the axis of any deflection in the ECG, the location of all the six leads in the frontal plane as well as the location of the positive and negative terminals of each lead should be mastered. The ECG deflection is maximally upright if the flow of current is directed toward the positive side of the lead and is maximally inverted if the flow of current is directed toward the negative side. Thus, if the flow of current is parallel to lead I (0° to 180°), lead I will record the tallest deflection if the flow of current is directed toward 0° and the deepest deflection if the flow of current is directed toward 180° (Fig. 4.3A, B).

·   The lead perpendicular to lead I will record an isoelectric complex. Isoelectric or equiphasic implies that the deflection above and below the baseline are about equal. Since lead aVF is perpendicular to lead I, lead aVF will record an isoelectric deflection (Fig. 4.3C, D).

·   Determining the electrical axis: The electrical axis or direction of the QRS complex (or any wave in the ECG) can be determined by several methods. Although the area under the QRS complex provides a more accurate electrical axis, the area is not readily measurable. For convenience, the amplitude of the QRS complex is measured instead.

·   Method 1: Look for an isoelectric complex:When an isoelectric QRS complex is present in any lead in the frontal plane, the axis of the QRS complex is perpendicular to the lead with the isoelectric complex. The following leads in the frontal plane are perpendicular to each other.

o    Lead I is perpendicular to lead aVF (Fig. 4.4A).

§  When an equiphasic QRS complex is recorded in lead I (0°), the axis of the QRS complex is +90° or -90°.

§  Similarly, when an equiphasic QRS complex is recorded in lead aVF (+90°), the axis of the QRS complex is 0° or 180°.

o    Lead II is perpendicular to lead aVL (Fig. 4.4B).

§  When an equiphasic QRS complex is recorded in lead II (+60°), the axis of the QRS complex is -30° or +150°.

§  Similarly, when an equiphasic QRS complex is recorded in lead aVL (-30°), the axis of the QRS complex is +60° or -120°.

o    Lead III is perpendicular to lead aVR (Fig. 4.4C).

§  When an equiphasic QRS complex is recorded in lead III (+120°), the axis of the QRS complex is -150° or +30°.

§  Similarly, when an equiphasic QRS complex is recorded in lead aVR (-150°), the axis of the QRS complex is +120° or -60°.

Figure 4.4: Perpendicular Leads. In the frontal plane, the following leads are perpendicular to each other: Leads I and aVF (A), Leads II and aVL (B) and lead III and aVR (C).

Figuring Out the Electrical Axis when an Equiphasic Complex is Present

·   Lead I is equiphasic: If the QRS complex in lead I (0°) is equiphasic, the flow of current is toward lead aVF, because lead aVF is perpendicular to lead I. If the flow of current is toward +90°, which is the positive side of aVF, the tallest deflection will be recorded in aVF (Fig. 4.5A). If the flow of current is toward -90° away from the positive side of aVF, lead aVF will record the deepest deflection (Fig. 4.5B).

·   Figures 4.5C and D summarize the possible deflections of the other leads in the frontal plane if lead I is equiphasic.

·   Lead II is equiphasic: If lead II (+60°) is equiphasic, the flow of current is in the direction of lead aVL, because lead aVL is perpendicular to lead II. If the electrical current is directed toward -30°, the tallest deflection will be recorded in lead aVL (Fig. 4.6A) because this is the positive side of lead aVL. On the other hand, if the flow of current is toward +150°, lead aVL will record the deepest deflection, because this is away from the positive side of lead aVL (Fig. 4.6B).

·   Figures 4.6C and D summarize the possible deflections of the different leads in the frontal plane of the ECG if lead II is equiphasic.

·   Lead III is equiphasic: If the QRS complex in lead III (+120°) is equiphasic, the flow of current is in the direction of lead aVR, because lead aVR is perpendicular to lead III. If the electrical current is directed toward -150°, the tallest deflection will be recorded in lead aVR (Fig. 4.7A) because this is the positive side of lead aVR. On the other hand, if the flow of current is toward +30°, lead aVR will record the deepest deflection because this is the negative side of lead aVR (Fig. 4.7B).

·   Figures 4.7C and D summarize the possible deflections of the different leads in the frontal plane if lead III is equiphasic.

Figuring Out the Axis of the QRS Complex; Summary and Practice Tracings

·   When an isoelectric deflection is recorded in any lead in the frontal plane, the mean axis of the QRS complex can be easily calculated (Figs. 4.8,4.9,4.10,4.11,4.12,4.13).

Figure 4.5: Lead I is Equiphasic. If the QRS complex in lead I is equiphasic (A, B), lead aVF will register the tallest deflection if the current is directed toward the positive side of aVF at +90° (A) and the deepest deflection if the current is directed toward -90°, away from the positive side of aVF (B). The electrocardiogram configuration of the other leads if lead I is equiphasic is summarized in C and D.

Figure 4.6: Lead II is Equiphasic. If the QRS complex in lead II is equiphasic (A, B), lead aVL will register the tallest deflection if the current is moving toward -30°, which is the positive side of aVL (A). If the electrical current is moving away from the positive side of lead aVL or toward +150°, lead aVL will record the most negative deflection (B). The configuration of the electrocardiogram in the other leads is summarized in C and D.

 

Figure 4.7: Lead III is Equiphasic. If the QRS complex in lead III is equiphasic (A, B), lead aVR will register the tallest deflection if the current is moving toward -150°, which is the positive side of aVR (A). If the current is moving away from the positive side of lead aVR toward +30°, lead aVR will record the most negative deflection (B). The configuration of the electrocardiogram in the other leads when lead III is equiphasic is summarized in C and D.

Figure 4.8: Isoelectric Deflection in Lead I. Lead I is isoelectric. Because lead I is perpendicular to aVF, and lead aVF has a tall complex, the axis is +90°.

Figure 4.9: Isoelectric Deflection in aVL. Lead aVL is isoelectric. Because lead aVL is perpendicular to lead II, and lead II shows the tallest deflection, the axis is +60°.

 

Figure 4.10: Isoelectric Deflection in aVR. Lead aVR is isoelectric. Because aVR is perpendicular to lead III, and lead III has the deepest complex, the axis is -60°. Note that tall R waves are present in aVL (-30°), which is beside the negative side of lead III.

Figure 4.11: Isoelectric Deflection in III. Lead III is isoelectric. Because III is perpendicular to lead aVR, and lead aVR has a negative complex, the axis is away from the positive side of aVR or +30°. This is substantiated by the presence of tall R waves in leads I and lead II. These leads flank the negative side of lead aVR.

Figure 4.12: Isoelectric Deflection in aVF. Lead aVF is isoelectric. Because aVF is perpendicular to lead I, and lead I has the tallest complex, the axis is 0°.

 

Figure 4.13: Isoelectric Deflection in aVR. Lead aVR is isoelectric. Because aVR is perpendicular to lead III, and lead III has the tallest complex, the axis is +120°.

·   The diagrams in Figure 4.14 summarize how to rapidly assess the axis of the QRS complex when an equiphasic complex is present.

Method 2

As shown in the previous examples, the axis of the QRS complex can be calculated rapidly using the “eyeball” technique when an isoelectric complex is present in any lead in the frontal plane. Not all ECGs, however, will have an isoelectric complex. If an isoelectric complex is not present, the mean QRS axis can be estimated just as rapidly by the following method.

·   Select the smallest QRS complex: The axis is obtained using the same method as calculating the axis when an isoelectric complex is present.

o    Thus, in Figure 4.15, lead aVL is selected because the complex is the smallest and is almost isoelectric. Lead aVL is perpendicular to lead II. Because lead II shows the tallest complex, the axis is approximately 60°. Adjustment has to be made to correct for the actual axis because the complex in lead aVL is not actually isoelectric.

o    Because aVL is negative (R < S), the axis is adjusted further away from 60°, thus the axis is approximately 70° rather than 60°.

o    Had aVL been positive (R > S), the axis is adjusted closer to 50° rather than 70°.

Method 3: Plotting the Amplitude of the QRS Complex using Two Perpendicular Leads

If there are no isoelectric complexes in the frontal plane, a simple way of calculating the axis is to select any pair of leads that are perpendicular to each other like leads I and aVF. The ECG in Figure 4.15 does not show any isoelectric QRS complex and will be used for calculation. The QRS complexes in leads I and aVF are shown in Figure 4.16.

·   Step 1: The total amplitude of the QRS complex in lead I is +4 units. This is measured by subtracting any upright deflection from any downward deflection (R +5 units, S -1 unit; total +4 units).

·   Step 2: The total amplitude of the QRS complex in lead aVF is +9 units.

·   Step 3: Perpendicular lines are dropped for 4 units from the positive side of lead I and for 9 units from the positive side of lead aVF until these two lines intersect (Fig. 4.16). The point of intersection is marked by an arrowhead and connected to the center of the hexaxial reference system. The line drawn represents a vector, which has both direction and magnitude. The direction of the vector is indicated by the arrowhead. Thus, the mean electrical axis of the QRS complex is +70°.

·   The diagram in Fig. 4.17 summarizes the different ECG deflections that will be recorded if several unipolar recording electrodes are placed along the path of an electrical impulse traveling from left to right toward 0°:

o    The electrode at 0° will record the most positive deflection.

o    The electrode at 180° will show the most negative deflection.

o    The electrode perpendicular to the direction of the impulse (+90° and -90°) will record an equiphasic or isoelectric complex.

o    Any recording electrode that is located within 90° of the direction of the electrical current (checkered area) will record a positive deflection (R > S wave).

o    Any electrode that is further away and is >90° of the direction of the electrical impulse will show a negative deflection (R < S wave).

·   The diagrams in Fig. 4.18 summarize the location of the QRS axis when an equiphasic QRS complex is not present in the frontal plane (Fig. 4.18).

The Precordial Leads

·   Horizontal plane: The six precordial leads V1 to V6 are also called horizontal or transverse leads since they represent the horizontal or transverse plane of the chest. The horizontal plane includes the left/right as well as the anteroposterior sides of the chest (Fig. 4.19).

o    Leads V1 and V2: Leads V1 and V2 are right-sided precordial leads and are positioned directly over the right ventricle. The QRS complexes in V1 and V2 represent electrical forces generated from the right ventricle and generally show small r and deep S waves.

o    Leads V5 and V6: Leads V5 and V6 are left-sided precordial leads that directly overlie the left ventricle. The QRS complexes represent electrical forces generated from the left ventricle, which show small q waves followed by tall R waves.

o    Leads V3 and V4: The QRS complexes are equiphasic in leads V3 and V4 because these leads represent the septal area and is the transition zone between the deep S waves in V1 and V2 and the tall R waves in V5 and V6 (Fig. 4.19).

Figure 4.14: Diagrams Showing the Location of the QRS. Bold arrows point to the QRS axis when an equiphasic complex is present.

 

Figure 4.15: Figuring Out the Axis when no Isoelectric Complex is Present. Lead aVL is selected because the complex is the smallest and almost isoelectric. Lead aVL is perpendicular to lead II. Because lead II shows the tallest complex, the axis is close to 60°. Lead aVL, however, is not actually isoelectric but is negative (r < S); thus, the axis of the QRS complex is adjusted further away and is closer to 70° (dotted arrow) than 60°.

Figure 4.16: Figuring the QRS Axis. The electrocardiogram showing leads I and aVF. The total amplitude of the QRS complex of +4 units is identified on the positive side of lead I. The total amplitude of +9 units is also identified on the positive side of lead aVF. Lines are dropped perpendicularly from leads I and aVF until the line intersects. The lines intersect at +70°, which mark the axis of the QRS complex.

Figure 4.17: The Electrocardiogram Configurations of an Electrical Impulse Traveling Toward 0°. The diagram summarizes the different electrocardiogram configurations of an impulse traveling at 0° if several electrodes are placed along its path. Any lead within 90° of the direction of the electrical impulse (checkered area) will record a positive deflection. Any lead that is further away (>90°) will record a negative deflection. The most positive or tallest deflection is recorded by the electrode positioned at 0° and the most negative by the electrode at 180°.

 

Figure 4.18: Checkered Area shows the Location of the QRS Axis when an Equiphasic QRS Complex is Not Present. NW, northwest.

Cardiac Rotation

·   Cardiac rotation: In the horizontal plane, a change in the electrical position of the heart is described as rotation. The heart may rotate clockwise or counterclockwise (Fig. 4.20), resulting in a shift of the transition zone to the left or to the right of V3 or V4.

o    Clockwise rotation or delayed transition:When the heart rotates clockwise, the transition zone, which is usually in V3 or V4, moves to the left toward V5 or V6. This is called clockwise rotation, delayed transition, or late transition. When the apex of the heart is viewed from under the diaphragm, the front of the heart moves to the left, causing the right ventricle to move more anteriorly (Fig. 4.20A).

o    Counterclockwise rotation or early transition: When the heart rotates counterclockwise, the transition zone moves earlier, toward V1 or V2. This is called counterclockwise rotation or early transition. When the apex of the heart is viewed from under the diaphragm, the front of the heart moves to the right causing the left ventricle to move more anteriorly (Fig. 4.20C).

·   In cardiac rotation, it is important to recognize that the heart is being visualized from under the diaphragm looking up. This is opposite from the way the precordial electrodes are conventionally visualized, which is from the top looking down. Thus, in cardiac rotation, the anterior and posterior orientation of the body and the direction of cardiac rotation is reversed (compare Fig. 4.19 and 4.20).

·   Rotation of the heart is determined by identifying the transition zone where the QRS complex is equiphasic (Fig. 4.21). Rotation is normal if the transition zone is located in V3 or V4(Fig. 21A, B). Figures 4.21C and D show counterclockwise rotation or early transition, and Figures 4.21E and F show late transition or clockwise rotation. The transition zones are circled (Figs. 4.22,4.23,4.24).

Figure 4.19: Precordial Leads V1 to V6. The location of the precordial leads and the expected normal configuration of the QRS complexes from V1 to V6 are shown. The QRS complex is equiphasic in V3, which is circled. V3 and V4 represent the transition zone between the deep S waves in V1 and V2 and the tall R waves in V5 and V8. LV, left ventricle; RV, right ventricle.

Figure 4.20: Clockwise and Counterclockwise Rotation. Rotation of the heart is viewed from under the diaphragm. (A) Clockwise rotation. The front of the heart moves to the left as shown by the arrow causing the right ventricle to move more anteriorly. (B) Normal rotation. (C) Counterclockwise rotation. The front of the heart moves to the right causing the left ventricle to move more anteriorly. A, anterior; CCW, counterclockwise rotation; CW, clockwise rotation; L, left; LV, left ventricle; P, posterior; R, right; RV, right ventricle.

Tall R Waves in V1

·   R wave taller than S wave in V1: In children, the R wave may be taller than the S wave in V1. This is unusual in adults (Figs. 4.25A and 4.26, normal ECG). When R wave is taller than the S wave in V1, the following should be excluded before this finding is considered a normal variant.

o    Right bundle branch block (RBBB)

o    Right ventricular hypertrophy

o    Pre-excitation or Wolff Parkinson White (WPW) ECG

o    Straight posterior myocardial infarction (MI)

o    Pacemaker rhythm

o    Ventricular ectopic impulses

·   RBBB: In RBBB, the QRS complexes are wide measuring ≥0.12 seconds (Figs. 4.25B and 4.27). This is the most important feature distinguishing RBBB from the other entities with tall R waves in V1. Terminal R' waves are also present in V1 and wide S waves are present in V5 and V6 or lead I (see Chapter 10, Intraventricular Conduction Defect: Bundle Branch Block).

Figure 4.21: Transition Zones. (A, B) Normal transition where the R and S waves are equiphasic in V3 or V4. (C, D) Early transition or counterclockwise rotation with the transition zone in V1 or V2. (E, F) Late transition or clockwise rotation with the equiphasic QRS complex in V5 or V6. The transition zones are circled. A, anterior; P, posterior; R, right; L, left.

Figure 4.22: Normal Rotation. Precordial leads V1 to V6 are shown. There is gradual progression of the R waves from V1 to V6. V4 is equiphasic (circled), representing the normal transition zone.

 

Figure 4.23: Counterclockwise Rotation or Early Transition. There is early transition of the QRS complexes with the equiphasic zone in V1. This represents counterclockwise rotation or early transition.

Figure 4.24: Clockwise Rotation or Late Transition. There is gradual progression of the R wave from V1 to V6 until the QRS complex becomes equiphasic in V6. This represents clockwise rotation or late transition.

 

Figure 4.25: Tall R Wave in V1. (A) Normal electrocardiogram. The R wave is smaller than the S wave. (B) Right bundle branch block.(C) Right ventricular hypertrophy. (D) Pre-excitation. (E) Straight posterior myocardial infarction. (F) Pacemaker-induced ventricular complex. (G) Ectopic ventricular complexes from the left ventricle.

Figure 4.26: Normal Electrocardiogram. Note that the R waves are smaller than the S waves in V1.

Figure 4.27: Right Bundle Branch Block. The QRS complexes are wide and tall terminal R waves are present in V1.

 

Figure 4.28: Right Ventricular Hypertrophy. When right ventricular hypertrophy is the cause of the tall R waves in V1, right axis deviation of ≥90° is almost always present. The diagnosis of right ventricular hypertrophy is unlikely if the axis is not shifted to the right.

·   Right ventricular hypertrophy: In right ventricular hypertrophy, a tall R wave in V1 is almost always associated with right axis deviation of approximately ≥90° (Figs. 4.25C and 4.28). The diagnosis of RVH is uncertain unless there is right axis deviation in the frontal leads.

·   Pre-excitation or WPW ECG: In WPW syndrome, evidence of pre-excitation is present in the baseline ECG with short P-R interval and presence of a delta wave. The R waves are tall in V1 when the bypass tract is left-sided (Figs. 4.25D and 4.29).

·   Posterior MI: Straight posterior MI is usually seen in older patients, not in children or young adults. It is often associated with inferior MI with pathologic q waves in leads II, III, and aVF (Figs. 4.25E and 4.30) or history of previous MI.

·   Pacemaker rhythm: When the rhythm is induced by an artificial pacemaker, a pacemaker artifact always precedes the QRS complex. Generally, a pacemaker-induced QRS complex has a QS or rS configuration in V1 because the right ventricle is usually the chamber paced. However, when the R wave is tall in V1 and is more prominent than the S wave (R or Rs complex), left ventricular or biventricular pacing should be considered as shown in Figures 4.25F and 4.31 (see Chapter 26, The ECG of Cardiac Pacemakers).

·   Ventricular ectopic impulses: Ventricular ectopic impulses may show tall R waves in V1. This can occur when the ectopic impulses originate from the left ventricle (Figs. 4.25G and4.32).

·   Normal variant: The ECG is shown (Fig. 4.33).

Figure 4.29: Pre-excitation (Wolff Parkinson White Electrocardiogram). A short P-R interval with delta wave (arrows) from pre-excitation is noted. In pre-excitation, the R waves are tall in V1when the bypass tract is left-sided.

 

Figure 4.30: Posterior Myocardial Infarction. In posterior myocardial infarction (MI), tall R waves in V1 is usually associated with inferior MI. Note the presence of pathologic q waves in leads II, III, and aVF.

Figure 4.31: Pacemaker-induced QRS Complexes. Tall R waves in V1 from pacemaker-induced rhythm. Arrows point to the pacemaker artifacts.

Figure 4.32: Wide Complex Tachycardia with Tall R Waves in V1. Tall R waves in V1 may be due to ectopic impulses originating from the left ventricle.

 

Figure 4.33: Normal Variant. The electrocardiogram shows tall R waves in V1 and V2 in a patient with completely normal cardiac findings. Before considering tall R waves in V1 and V2 as normal variant, other causes should be excluded.

Clockwise Rotation

·   Clockwise rotation: In clockwise rotation or late transition, the transition zone of the QRS complexes in the precordial leads is shifted to the left of V4 resulting in deep S waves from V1 to V5 or often up to V6 (Fig. 4.34). Clockwise rotation is usually the result of the following:

o    Left ventricular hypertrophy: This can be due to several causes, including dilated cardiomyopathy or left-sided valvular insufficiency.

o    Right ventricular hypertrophy: Depending on the cause of the right ventricular hypertrophy, clockwise rotation may be present instead of a tall R in V1. This often occurs when there is mitral stenosis, pulmonary hypertension and chronic obstructive pulmonary disease (see Chapter 7, Chamber Enlargement and Hypertrophy).

o    Biventricular hypertrophy: Both ventricles are enlarged.

o    Chronic obstructive pulmonary disease: In chronic obstructive pulmonary disease such as emphysema or chronic bronchitis, the diaphragm is displaced downward causing the heart to rotate clockwise and become vertically oriented (Fig. 4.34).

o    Acute pulmonary embolism: See Chapter 7, Chamber Enlargement and Hypertrophy.

o    Left anterior fascicular block: See Chapter 9, Intraventricular Conduction Defect: Fascicular Block.

o    Other causes: Cardiac rotation resulting from shift in mediastinum or thoracic deformities including pectus excavatum.

Figure 4.34: Clockwise Rotation. In clockwise rotation, the transition zone is shifted to the left, resulting in deep S waves from V1 to V6. Note that the R waves are smaller than the S wave in V5 and in V6 because of a shift in the transition zone to the left of V6. The electrocardiogram also shows right axis deviation; peaked P waves in II, III, and aVF; and low voltage in lead I. The cardiac rotation is due to chronic obstructive pulmonary disease.

Suggested Readings

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Burch GE, Winsor T. Precordial leads. In: A Primer of Electrocardiography, 5th ed. Philadelphia: Lea and Febiger; 1966: 146-184.

Dunn MI, Lippman BS. Basic ECG principles. In: Lippman-Massie Clinical Electrocardiography, 8th ed. Chicago: Yearbook Medical Publishers; 1989:51-62.

Marriott HJL. Electrical axis. In: Practical Electrocardiography, 5th ed. Baltimore: Willliams & Wilkins; 1972:34-43.

Wagner GS. Cardiac electrical activity. In: Marriott's Practical Electrocardiography, 10th ed. Philadelphia: Lippincott Williams & Wilkins; 2001;2-19.

Wagner GS. Recording the electrocardiogram. In: Marriott's Practical Electrocardiography, 10th ed. Philadelphia: Lippincott Williams & Wilkins; 2001;26-41.