Electrocardiography

Carol Jacobson

Electrocardiography is the graphic display of the changing potentials of the electrical field generated by the heart as recorded by electrodes placed on the body surface. Recording of the 12-lead electrocardiogram (ECG) is the most frequently used procedure for the diagnosis of heart disease. It is noninvasive, safe, simple to perform, reproducible, and relatively inexpensive. The 12-lead ECG can record changes indicative of primary myocardial disease such as coronary artery disease, cardiomyopathy, hypertension, or infiltrative diseases. It can also reflect changes associated with electrolyte abnormalities, metabolic disorders, drug effect, and other disease processes such as pulmonary embolism or pulmonary hypertension, renal failure, and central nervous system disease. The ECG is the gold standard for noninvasive diagnosis of cardiac arrhythmias and conduction abnormalities (see Chapter 16) and is a useful tool in evaluating function of implanted devices such as pacemakers and implantable cardioverter defibrillators.¹

This chapter discusses the electrocardiographic features of various cardiac conditions and other disease processes that may cause changes on the ECG. Specific information on the pathophysiology and treatment of cardiac disease and other medical conditions that may affect the ECG can be found in other chapters in this book or in medical textbooks.

ELECTRICAL CONDUCTION THROUGH THE HEART

The electrical impulse of the heart is the stimulus for cardiac contraction. The conduction system (Fig. 15-1) is responsible for the initiation of the electrical impulse and its sequential spread through the atria, atrioventricular (AV) junction, and ventricles.

The Cardiac Conduction System

The conduction system of the heart consists of the following structures.

Sinus Node

The sinus or sinoatrial (SA) node is a small group of cells in the high right atrium that functions as the normal pacemaker of the heart because it has the fastest rate of automaticity. The SA node normally depolarizes between 60 and 100 times per minute.

AV Node

The AV node is a small group of cells in the low right atrium near the tricuspid valve. The AV node has three main functions:

Its major job is to slow conduction of the impulse from the atria to the ventricles to allow time for the atria to contract and empty their blood into the ventricles.
The area around the AV node (junction) has automaticity at an impulse rate of 40 to 60 beats per minute and can function as a backup pacemaker if the SA node fails.
It screens out rapid atrial impulses to protect the ventricles from dangerously fast rates when the atrial rate is very rapid.

Bundle of His

The bundle of His is a short bundle of fibers at the bottom of the AV node leading to the bundle branches. Conduction velocity accelerates in the bundle of His, and the impulse is transmitted to both bundle branches.

Bundle Branches

The bundle branches are bundles of fibers that rapidly conduct the impulse into the right and left ventricles. The right bundle branch travels along the right side of the interventricular septum and carries the impulse into the right ventricle. The left bundle branch has two main divisions, the anterior fascicle and the posterior fascicle, which carry the impulse into the left ventricle.

Purkinje Fibers

The Purkinje fibers are hairlike fibers that spread out from the bundle branches along the endocardial surface of both ventricles and rapidly conduct the impulse to the ventricular muscle cells. Cells in the Purkinje system have automaticity at a rate of 20 to 40 beats per minute and can function as a backup pacemaker if all other pacemakers fail.

Origin and Spread of the Electrical Impulse Through the Heart

The impulse normally begins in the SA node, located in the high right atrium, because the SA node has the fastest rate of automaticity of all potential pacemaker cells in the heart. The impulse spreads from the SA node through both atria in an inferior and leftward direction, resulting in depolarization of the atrial muscle. When the impulse reaches the AV node, its conduction velocity is slowed before it continues into the ventricles. The slowing in the AV node is necessary to allow time for the atria to contract and empty their blood into the ventricles before the ventricles contract. The atrium’s contribution to ventricular filling is referred to as “atrial kick.” When the impulse emerges from the AV node, it travels rapidly through the bundle of His and down the right and left bundle branches into the Purkinje network of both ventricles, and results in depolarization of the ventricular muscle. The spread of this wave of depolarization through the heart produces the classic surface ECG, which can be recorded by an electrocardiograph (ECG machine) or monitored continuously on a bedside cardiac monitor.

Waves, Complexes, and Intervals of the Cardiac Cycle

The ECG waves, complexes, and intervals are illustrated in Figure 15-2.

▪ Figure 15-1 Cardiac conduction system. (From Jacobson, C. [1991]. Cardiac arrhythmias and conduction abnormalities. In M. L. Patrick, S. L. Woods, R. F. Craven, et al. [Eds.], Medical-surgical nursing [2nd ed., pp. 648-693]. Philadelphia: J. B. Lippincott)

P Wave

The P wave represents atrial muscle depolarization. It is normally small, smoothly rounded, and no taller than 2.5 mm or wider than 0.11 second.

QRS Complex

The QRS complex represents ventricular muscle depolarization. The shape of the QRS complex depends on the lead being recorded and the ventricular activation sequence; not all leads record all waves of the QRS complex. A Q wave is an initial negative deflection from baseline and should be less than 0.03 second in duration and less than 25% of the R-wave amplitude. An R wave is the first positive deflection from baseline. An S wave is a negative deflection that follows an R wave. When a complex is all positive, it is just an R wave; when it is all negative, it is called a QS. Regardless of the shape of the complex, ventricular depolarization waves are called QRS complexes (Fig. 15-3). The width of the QRS complex represents intraventricular conduction time and is measured from the point at which it first leaves the baseline to the end of the last appearing wave. Normal QRS width is 0.04 to 0.10 second.

▪ Figure 15-2 Waves, complexes, and intervals of the cardiac cycle in leads II and V₁.

▪ Figure 15-3 Examples of various QRS complexes.

T Wave

The T wave represents ventricular muscle repolarization. It follows the QRS complex and is normally in the same direction as the QRS complex. The T wave is usually rounded and slightly asymmetric, rising more slowly than it descends. T waves are not normally taller than 5 mm in any limb lead or 10 mm in any precordial lead.

U Wave

The U wave is a small, rounded wave that sometimes follows the T wave and is most prominent in leads V₂-V₄. The U wave is normally in the same direction as the T wave but is only approximately 10% of its amplitude. The U wave is thought to be part of the ventricular repolarization process and may represent repolarization of the Purkinje network or certain cells in the deep subepicardial layer of the ventricle (M cells), or summation of ventricular afterdepolarizations.²,³

PR Interval

The PR interval is measured from the beginning of the P wave to the beginning of the QRS complex and represents the time required for the impulse to travel through the atria, AV junction, and Purkinje system. The normal PR interval is 0.12 to 0.20 second.

ST Segment

The ST segment represents the period of time when the ventricle is still depolarized. It begins at the end of the QRS complex (J point) and extends to the beginning of the T wave. The ST segment should be at the isoelectric line and gently curve up into the T wave.

QT Interval

The QT interval measures the duration of ventricular activation and recovery, and varies with age, gender, and heart rate. The QT interval is measured from the beginning of the QRS complex to the end of the T wave, and, because it varies inversely with the heart rate, it must be corrected to a heart rate of 60 beats per minute after measurement (QTc). Because the QT interval adjusts gradually to a change in heart rate, accurate measurement of the QTc can be done only after several regular and equal cardiac cycles. The normal QTc is usually less than half of the preceding R-R interval at normal heart rates, but a more accurate evaluation can be done using Bazett’s formula: QTc = QT/√R-R interval, where QT and R-R intervals are in seconds.⁴ The upper limit of normal QTc is generally considered to be <0.44 second in adult men and <0.45 second in adult women.²,³,⁵

BASIC ELECTROCARDIOGRAPHY

The ECG is the graphic record of the electrical activity of the heart. The spread of the electrical impulse through the heart produces weak electrical currents through the entire body, which can be detected and amplified by the ECG machine and recorded on calibrated graph paper. These amplified signals form the ECG tracing, consisting of the waveforms and intervals described previously, and are inscribed onto grid paper that moves beneath the recording stylus (pen) at a standard speed of 25 mm/s. The grid on the paper consists of a series of small and large boxes, both horizontally and vertically; horizontal boxes measure time, and vertical boxes measure voltage (Fig. 15-4). Each small box horizontally is equal to 0.04 second, and each large box horizontally is equal to 0.20 second. On the
vertical axis, each small box measures 1 mm and is equal to 0.1 mV; each large box measures 5 mm and is equal to 0.5 mV. In addition to the grid, most ECG paper places a vertical line in the top margin at 3-second intervals or places a mark at 1-second intervals.

▪ Figure 15-4 Time and voltage lines on ECG paper at standard paper speed of 25 mm/s. Horizontal axis measures time: each small box = 0.04 second, one large box = 0.20 second. Vertical axis measures voltage: each small box = 1 mm or 0.1 mV, one large box = 5 mm or 0.5 mV.

▪ Figure 15-5 (A) Heart rate determination for an irregular rhythm. Count the number or R-R intervals in a 6-second strip and multiply by 10. In (A) there are five complete R-R intervals in a 6-second strip; the heart rate is about 50 beats per minute. (B) Heart rate determination for a regular rhythm using the rate ruler. Count the number of large and small boxes between R waves on the rhythm strip. In (B) there are three large boxes and one small box between the R waves marked on the strip. On the rate ruler, the first R wave is represented by the thick line marked “A.” Each large box on the ECG paper is represented by a thick line on the rate ruler and is numbered at the top; each small box on the strip is represented by a thin line on the ruler. The number on the line on the ruler that corresponds to the second R wave on the strip represents the heart rate. In (B), count three large boxes at the top of the ruler and then one small box; the heart rate is 94 beats per minute. (Rate ruler in B from Marriott, H. J. L. [1988]. Practical electrocardiography [8th ed., p. 15]. Baltimore: Williams & Wilkins.)

The waveforms of the cardiac cycle can be recorded by a bedside cardiac monitor and displayed continuously on an oscilloscope or recorded on a rhythm strip, which consists of the same grid as described previously. The standard 12-lead ECG simultaneously records 12 different views of electrical activity as it travels through the heart and displays all 12 views on a full-page layout, which consists of the same grid. The 12 leads of the ECG are described in detail in following sections.

Determining Heart Rate on the ECG

Heart rate can be determined from the ECG strip by several methods. An easy method that can be used for both regular and irregular rhythms is to count the number of R-R intervals (not R waves) in a 6-second strip and multiply that number by 10, because there are ten 6-second intervals in 1 minute (Fig. 15-5A).

Another method that can be used only if the rhythm is regular is to count the number of large boxes between two R waves and divide that number into 300, because there are 300 large boxes in a 1-minute strip. The most accurate method to use for a regular rhythm is to count the number of small boxes between two R waves and divide that number into 1,500, because there are 1,500 small boxes in a 1-minute strip. The easiest way to do either of these methods is to use the rate ruler in Figure 15-5B.

Determining the Cardiac Rhythm on the ECG

The first step in interpreting a 12-lead ECG is to determine the cardiac rhythm. A rhythm strip should be analyzed in a systematic
manner to aid in rhythm interpretation until the learner is able to identify arrhythmias by scanning the strip. See Chapter 16 for detailed information on the normal cardiac rhythm and both basic and advanced arrhythmias. The following steps provide a systematic approach to rhythm interpretation:

Regularity: First determine if the rhythm is regular or irregular because this information determines the method of heart rate calculation. If the rhythm is irregular, determine if the irregularity is random or if it occurs in a pattern (i.e., repetitive groups of beats separated by a pause).

Rate: Determine the heart rate as described previously. Determine both atrial (P wave) and ventricular (QRS complex) rates if they are not the same.

P waves: Locate P waves and note their shape and relationship to QRS complexes. Determine if all P waves look alike and if they have a consistent relationship to QRS complexes (i.e., one P wave before every QRS, two or more P waves before each QRS) or if they occur randomly and are unrelated to QRS complexes.

PR interval: Measure the PR interval of several complexes in a row to determine if it is of normal duration and consistent for all QRS complexes.

QRS width: Measure the QRS complex and determine if it is normal or wide.

Determine the rhythm based on an analysis of the information obtained in these steps. See Chapter 16 for details on arrhythmia analysis.

THE 12-LEAD ECG

The 12-lead ECG records electrical activity as it spreads through the heart from 12 different leads that are recorded through electrodes placed on the arms, legs, and specific spots on the chest. Each lead represents a different view of the heart and consists of two electrodes with opposite polarity (bipolar), or one electrode and a reference point (unipolar). A bipolar lead has a positive pole and a negative pole, with each contributing equally to the recording. A unipolar lead has one positive pole and a reference pole in the center of the chest that is algebraically determined by the ECG machine. The reference pole represents the center of the electrical field of the heart and has a zero potential, so only the positive pole of a unipolar lead contributes to the tracing.

The standard 12-lead ECG consists of six limb leads that record electrical activity in the frontal plane—traveling up/down and right/left in the heart—and six precordial leads that record electrical activity in the horizontal plane—traveling anterior/posterior and right/left. Limb leads are recorded by electrodes placed on the arms and legs, whereas precordial leads are recorded by electrodes placed on the chest (Fig. 15-6). For convenience in continuous bedside monitoring, arm electrodes can be placed on the shoulders and leg electrodes on the lower part of the rib cage rather than on the limbs without significantly altering the signals recorded.

A camera analogy makes the 12-lead ECG easier to understand. Each lead of the ECG represents a picture of the electrical activity in the heart taken by the camera. In any lead, the positive electrode is the recording electrode or the camera lens. The negative electrode tells the camera which way to “shoot” its picture and determines the direction in which the positive electrode records. When the positive electrode detects electrical activity traveling toward it, it records an upright deflection on the ECG. When the positive electrode detects electrical activity traveling away from it, it records a negative deflection (Fig. 15-7). If a positive electrode is positioned where electrical activity travels toward it and then away from it, a diphasic deflection is recorded. If the electrical activity travels perpendicular to a positive electrode, no activity is recorded. The 12-lead ECG records three bipolar frontal plane leads—lead I, lead II, and lead III; three unipolar frontal plane leads—aVR, aVL, and aVF; and six unipolar precordial leads: V₁, V₂, V₃, V₄, V₅, and V₆.

▪ Figure 15-6 Electrode placement for limb leads and precordial leads. Limb electrodes can be placed anywhere on the arms and legs. Chest electrodes are placed as follows: V₁ = fourth intercostal space at right sternal border; V₂ = fourth intercostal space at left sternal border; V₃ = halfway between V₂ and V₄ in a straight line; V₄ = fifth left intercostal space at midclavicular line; V₅ = fifth left intercostal space at anterior axillary line; V₆ = fifth left intercostal space at midaxillary line.

Bipolar Leads

Figure 15-8A illustrates the three bipolar frontal plane leads. In each lead, the camera represents the positive pole of the lead. In lead I, the positive electrode is on the left arm and the negative electrode is on the right arm. Any electrical activity in the heart that travels toward the positive electrode (camera lens) on the left arm is recorded as an upright deflection and any traveling away from it is recorded as a negative deflection. In lead II, the positive electrode is on the left leg and the negative electrode is on the right arm. Any electrical activity traveling toward the left leg electrode (camera lens) is recorded as an upright deflection and any traveling away from it toward the right arm electrode is recorded as a negative deflection. In lead III, the positive electrode is on the left leg and the negative electrode is on the left arm. Any electrical activity coming toward the left leg electrode (camera lens) is recorded as an upright deflection and any traveling away from it
toward the left arm is recorded as a negative deflection. The right leg electrode serves as a ground and does not contribute to the signals recorded. The electrical sum of the voltages in the three bipolar frontal plane leads equals zero potential and forms a virtual ground in the center of the triangle used by the unipolar leads as their reference point.

▪ Figure 15-7 A strip of cardiac muscle depolarizing in the direction of the arrow. A positive electrode at B sees depolarization coming toward it and records an upright deflection. A positive electrode at A sees depolarization going away from it and records a negative deflection. A positive electrode at C records a flat line because depolarization is traveling perpendicular to the electrode’s view.

▪ Figure 15-8 The 12 leads of the ECG. The camera represents the location of the positive, or recording, electrode in each lead. (A) Bipolar frontal plane leads I, II, and III. (B) Unipolar frontal plane leads aVR, aVL, and aVF. (C) Unipolar precordial leads V₁-V₆.

Unipolar Leads

Figure 15-8B illustrates the three unipolar frontal plane leads, aVR, aVL, and aVF. The camera represents the location of the positive electrode: on the right shoulder for aVR, on the left shoulder for aVL, and at the foot (left leg) for aVF. The “negative
end” of the unipolar lead is the reference point in the center of the chest that is obtained as described previously. The same recording principles apply to unipolar leads: any electrical activity traveling toward the positive electrode is recorded as an upright deflection and any traveling away from it is recorded as a negative deflection. Figure 15-8C shows the six unipolar precordial leads recording from their locations on the chest and “shooting” toward the reference point in the center of the heart.

▪ Figure 15-9 (A) Electrode placement for standard precordial and right precordial leads. Only three right-sided leads are needed: V_4R, right fifth intercostal space at midclavicular line; V_5R, right fifth intercostal space at anterior axillary line; V_6R, right fifth intercostal space at midaxillary line. (B) Electrode placement for posterior leads: V₇, left posterior axillary line; V₈, tip of left scapula; V₉, left border of spine. All three are in the same horizontal plane of V₄ to V₆.

Right Chest and Posterior Leads

Additional leads can be recorded on the right chest or posterior thorax to gain additional information about right ventricular or posterior infarction, or right ventricular hypertrophy (RVH). Figure 15-9 shows lead placement for obtaining right chest leads and posterior leads.

The Hexaxial Reference System

Figure 15-10A shows the hexaxial reference system that is formed when the six frontal plane leads are moved together in such a way that they bisect each other in the center. Each lead is labeled at its positive end to make it easy to remember where the positive electrode, or camera, is. In Figure 15-10B, the hexaxial reference system is superimposed over a drawing of the heart to illustrate how each frontal plane lead views the heart. The reference system forms a 360-degree circle surrounding the heart with 180 positive degrees and 180 negative degrees. By convention, the positive end of lead I is designated 0 degrees and the six leads divide the circle into 30-degree segments, as labeled in the figure.

The 12 Views of the Heart

The normal sequence of depolarization through the heart and the resulting P, QRS, and T waves for each frontal plane lead are illustrated in Figure 15-11A. The impulse normally originates in the SA node high in the right atrium and spreads leftward through the left atrium and downward toward the AV node low in the right atrium. Leads I and aVL, with their positive electrode (camera lens) on the left side of the body, record this leftward electrical activity as an upright P wave because the positive electrode sees atrial depolarization coming toward it. Leads II, III, and aVF, with their positive electrode at the bottom of the heart, record the downward spread of atrial activity as upright P waves for the same reason. Lead aVR, with its positive electrode on the right shoulder, sees the electrical activity moving away from it and records a negative P wave.

▪ Figure 15-10 Hexaxial reference system (or axis wheel). Each lead is labeled at its positive end in both examples. (A) All six frontal plane leads bisect each other. The degrees of the axis wheel are shown. (B) The axis wheel superimposed on the heart to demonstrate each lead’s view of the heart. Leads I and aVL face the left lateral wall, leads II, III, and aVF face the inferior surface. Lead aVR does not face a ventricular surface.

As the impulse spreads through the AV node, no electrical activity is recorded because the AV node is too small to be recorded by surface leads. As the impulse exits the AV node, it moves through the bundle of His and enters the right and left bundle branches. The left bundle branch sprouts some Purkinje fibers high on the left side of the septum that carry the impulse into the septum and cause it to depolarize first in a left-to-right direction. The electrical impulse then enters the Purkinje system of both ventricular free walls simultaneously and depolarizes them from endocardium to epicardium (indicated by the small arrows through the ventricles in Fig. 15-11A). Millions of electrical impulses travel through the ventricles in three dimensions simultaneously, but, if averaged together, the main direction is downward, leftward, and posterior toward the large left ventricle, as indicated by the large arrow in the same figure. This large arrow represents the mean axis, which is the net direction of electrical depolarization through the ventricles when all the smaller arrows are averaged together.

The QRS complex is recorded as the ventricles depolarize. Leads I and aVL, with their positive electrodes on the left side of the body, see the septum depolarizing away from them in a left-to-right direction and record a small negative deflection (Q wave).
They then see the large left ventricular free wall depolarizing toward them and record an upright deflection (R wave). Leads II, III, and aVF, with their positive electrodes at the bottom of the heart may not record septal activity at all. If these leads see septal activity coming slightly toward them, they record a positive deflection. They then see the forces moving downward through the left ventricle toward them and record an upright deflection (R wave). Lead aVR, positive on the right shoulder, sees all activity moving away from it and records a negative deflection (QS complex).

▪ Figure 15-11 (A) Normal sequence of depolarization through the heart as recorded by each of the frontal plane leads. (B) Cross section of the thorax illustrating how the six precordial leads record the normal ECG. In both examples, the small arrow (1) shows the initial direction of depolarization through the septum, followed by the mean direction of ventricular free wall depolarization, larger arrow (2).

The six precordial leads record electrical activity traveling in the horizontal plane. Figure 15-11B illustrates the position of the precordial leads and how they record electrical activity as it spreads through the ventricles in the horizontal plane. Lead V₁ is located on the front of the chest and records a small R wave as the septum depolarizes toward it from left to right. It then records a deep S wave as depolarization spreads away from it through the thick left ventricle. As the positive electrode is moved across the precordium from the V₁ to the V₆ position, it records progressively more left ventricular forces and the R wave gets progressively larger. Lead V₆ is located on the left side of the chest and usually records a small Q wave as the septum depolarizes from left to right away from the positive electrode, and a large R wave as electrical activity spreads toward the positive electrode through the thick left ventricle. Normal R-wave progression means that the R wave gets progressively larger from V₁ to V₆, or that V₆ is predominantly an R wave compared with V₁, which is predominantly an S wave. Often the largest precordial R wave is recorded in lead V₄ or V₅.

Many variations of the above patterns exist among individuals and represent normal variants in the ECG. Leads III and aVR may record larger Q waves because of their rightward orientation (Fig. 15-11A)⁶,⁷, lead III may record a large S wave if the heart sits horizontally in the chest, and lead aVL may record a large S wave if the heart sits more vertically in the chest.⁸ Variations in P-wave and T-wave morphology can also be normal variants depending on how the heart physically sits in the chest.

The Normal Adult 12-Lead ECG

Figure 15-12 shows a normal 12-lead ECG. Normal sinus rhythm is present at a rate of 70 beats per minute, and the axis is approximately +60 degrees. P waves are normal (they are flat in aVL, but this finding is a normal variant), and T waves are normal (flat or slightly inverted in lead aVL and V₁ is a normal variant). The QRS complex is normal (0.08 second wide), there are no abnormal Q waves, and R-wave progression is normal across the precordium. The ST segment is at baseline in all leads. This ECG can be used for comparison as abnormalities are discussed throughout this chapter.

AXIS DETERMINATION

Conduction of a wave of depolarization through the myocardium results in propagation of thousands of electrical potentials in multiple directions. More than 80% of these potentials are balanced by similar instantaneous charges moving in opposite directions. Balanced alterations in electrical potentials result in an algebraic “canceling out” of these instantaneous vectors. What remains as the detected and amplified ECG tracing is the net vector, which reveals the magnitude, direction, and polarity of the mean electrical force as it travels through the myocardium. Frontal plane axis can be determined for P waves, QRS complexes, and T waves. This section deals only with QRS axis determination.

The normal QRS axis is defined as −30 to +90 degrees because most of the electrical forces in a normal heart are directed downward and leftward toward the large left ventricle. Left axis deviation (LAD) is defined as −31 to −90 degrees and occurs when most of the forces move in a leftward and superior direction, as can happen in left ventricular hypertrophy (LVH), left anterior fascicular block (LAFB), inferior myocardial infarction (MI), left bundle-branch block (LBBB), several congenital defects, and some arrhythmias, especially ventricular tachycardia and Wolff-Parkinson-White syndrome. Right axis deviation (RAD) is defined as +91 to +180 degrees and occurs when most of the forces move rightward, as can happen in RVH, left posterior fascicular block (LPFB), right bundle-branch block (RBBB), dextrocardia, ventricular tachycardia, and Wolff-Parkinson-White syndrome. When most of the forces are directed superior and rightward between −91 and −180 degrees, the term indeterminate axis or extreme axis is used. This axis can occur with ventricular tachycardia and occasionally with bifascicular block.
Figure 15-13 shows the axis wheel divided into its normal, left deviation, right deviation, and indeterminate sections.

▪ Figure 15-12 Normal 12-lead ECG.

The mean frontal plane QRS axis can be determined in a number of ways. The most accurate method is to average the forces moving right and left with those moving up and down because this method represents the frontal plane. Because lead I is the most direct right/left lead and lead aVF is the most direct up/down lead, it is easiest to use these two perpendicular leads to calculate the mean axis. Figure 15-14A shows the frontal plane leads of a 12-lead ECG. In Figure 15-14B, leads I and aVF are shown enlarged along with the axis wheel with small hash marks along the axes of lead I and lead aVF. These hash marks represent the small 1-mV boxes on the ECG paper. To determine the mean QRS axis, follow these steps:

Look at the QRS complex in lead I and count the number of positive and negative boxes. Mark the net vector along the appropriate end of lead I on the axis wheel. In Figure 15-14B, the QRS complex in lead I is eight boxes positive with no significant negative deflections. Count eight hash marks toward the positive end of lead I and put a mark on the axis wheel at that spot.
Look at the QRS complex in aVF and follow the same procedure as before. In this example, the QRS complex in aVF is 14 boxes positive with no significant negative deflections. Count 14 hash marks along the positive end of the aVF axis and place a mark at that spot.
Draw a perpendicular line down from the mark on the lead I axis and a perpendicular line across from the mark on the aVF axis.
Draw a line from the center of the axis wheel to the spot where these two perpendicular lines meet. This line is the mean QRS axis—approximately +60 degrees.

▪ Figure 15-13 Normal axis = −30° to +90°, LAD = −31° to −90°, RAD = +91° to +180°, indeterminate axis = −91° to −180°.

A quick but less accurate method of axis determination is to place the axis in its proper quadrant of the axis wheel by looking at leads I and aVF, because these leads divide the wheel into four quadrants. As illustrated in Figure 15-15, if the QRS in both of these leads is positive, the axis falls in the normal quadrant, 0 to +90 degrees. If the QRS in lead I is positive and the QRS in aVF is negative, the axis falls in the left quadrant, 0 to −90 degrees. If the QRS in lead I is negative and the QRS in aVF is positive, the axis falls in the right quadrant, +90 to +180 degrees. If both leads are negative, the axis falls in the indeterminate quadrant or “no-man’s land,” −90 to −180 degrees. Locating the correct quadrant is often adequate but, because the portion of the left quadrant between 0 and −30 degrees is considered normal, it is necessary to determine more precisely whether the axis is less than or greater than −30 degrees. To do this quickly, look at Lead II: if the QRS in lead II is positive, the axis is less than −30 degrees; if the QRS in Lead II is negative, the axis is more negative than −30 degrees indicating LAD.

Using the ECG in Figure 15-16A, first place the axis in the appropriate quadrant by using leads I and aVF. Lead I is upright and aVF is negative, placing the axis in the left quadrant. However, because 30 degrees of the left quadrant is considered normal, we need to fine-tune the axis to determine where in the left quadrant it actually falls. Look at lead II: the QRS in lead II is mostly negative indicating that the axis is left of −30 degrees and that LAD is present. The axis wheel shows how to count boxes to get a more precise axis. The QRS in lead I is six boxes positive with no negative deflections; count six hashmarks along the positive end of lead I axis and place a mark. The QRS in aVF has an R wave 4 boxes positive and an S wave 16 boxes negative, for a net direction of −12 boxes; count 12 hashmarks along the negative end of aVF and place a mark. The axis is about −70 degrees, indicating LAD.

Using the ECG in Figure 15-16B, place the axis in the appropriate quadrant. Because lead I is negative and aVF is positive, the axis is in the right quadrant. The axis wheel shows how to count boxes to obtain a more precise axis. The QRS in lead I is two boxes positive and five boxes negative for a net of three boxes negative; mark this spot on the negative end of lead I on the axis wheel. The QRS in aVF is two boxes negative and 12 boxes positive for a net of +10 boxes; mark this spot on the positive end of lead aVF on the axis wheel. The axis is about +110 degrees, indicating RAD.

▪ Figure 15-14 Calculating the mean QRS axis. (A) The six frontal plane leads of an ECG. (B) Lead I and lead aVF enlarged. See text for instructions on calculating the axis using leads I and aVF on the axis wheel.

▪ Figure 15-15 The four quadrants of the axis wheel. (A) If the QRS in lead I is positive and the QRS in aVF is negative, the axis is in the left quadrant. (B) If the QRS is positive in both leads I and aVF, the axis is normal. (C) If the QRS in lead I is negative and the QRS in aVF is positive, the axis is in the right quadrant. (D) If the QRS is negative in both leads I and aVF, the axis is indeterminate.

INTRAVENTRICULAR CONDUCTION ABNORMALITIES

The intraventricular conduction system consists of the right bundle branch and the left main bundle branch, which fans out into septal fascicles, an anterior fascicle, and a posterior fascicle. There are numerous individual anatomic variations, but the intraventricular conduction system is generally regarded to consist of three major fascicles that diverge from the bundle of His: (1) the right bundle branch, (2) the anterior division of the left bundle branch (left anterior fascicle), and (3) the posterior division of the left bundle branch (left posterior fascicle⁹; Fig. 15-17). Block may occur in any part of this conduction system. Monofascicular block involves block in only one of the three major fascicles. The term bifascicular block is most commonly used to describe the combination of RBBB and either LAFB or LPFB. Trifascicular block means block in all three major divisions.

Bundle-Branch Block

When one of the bundle branches is blocked, the ventricles depolarize asynchronously. Bundle-branch block is characterized by a delay of excitation to one ventricle and abnormal spread of electrical activity through the ventricle whose bundle is blocked. This delayed conduction results in widening of the QRS complex to
0.12 second or greater and a characteristic pattern best recognized in precordial leads V₁ and V₆ and limb leads I and aVL.

▪ Figure 15-16 (A) Frontal plane leads demonstrating LAD. See text for explanation. This is an example of LAFB. (B) Frontal plane leads demonstrating RAD. See text for explanation. This is an example of LPFB.

▪ Figure 15-17 Intraventricular conduction system. Right bundle branch carries impulse into right ventricle. Left main bundle branch divides into anterior and posterior fascicles, which carry impulse into left ventricle.

Normal ventricular depolarization as recorded by leads V₁ and V₆ is illustrated in Figure 15-18. The positive electrode for V₁ is located on the front of the chest at the fourth intercostal space to the right of the sternum, close to the right ventricle. The positive electrode for V₆ is located in the left midaxillary line at the fifth-sixth intercostal space, close to the left ventricle. Lead V₁ records a small R wave as the septum depolarizes from left to right toward the positive electrode. It then records a negative deflection (S wave) as the main forces travel away from the positive electrode toward the left ventricle, resulting in the normal rS complex in V₁. Lead V₆ records a small Q wave as the septum depolarizes left to right away from the positive electrode. It then records a tall R wave as the main forces travel toward the left ventricle, resulting in the normal qR complex in V₆. When both ventricles depolarize together, the QRS width is less than 0.12 second.

Right Bundle-Branch Block

Figure 15-19A illustrates the spread of electrical forces in the ventricles when the right bundle branch is blocked. Three separate forces occur:

▪ Figure 15-18 Normal ventricular activation as recorded by leads V₁ and V₆.

▪ Figure 15-19 (A) Ventricular depolarization with right bundle branch block as recorded by leads V₁ and V₆. Septal activation occurs first (arrow 1) causing an R wave in V₁ and Q wave in V₆; left ventricular activation occurs second (large arrow 2) causing an S wave in V₁ and an R wave in V₆; right ventricular activation occurs last and slowly (curved arrows 3) causing an R′ in V₁ and a wide S wave in V₆. (B) Three commonly seen variations of RBBB pattern. (C) 12-lead ECG illustrating RBBB.

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