In the clinical diagnosis and management of congenital or acquired heart disease, the presence or absence of electrocardiographic (ECG) abnormalities is often helpful. Hypertrophies (of ventricles and atria) and ventricular conduction disturbances are the two most common forms of ECG abnormalities. The presence of other ECG abnormalities such as atrioventricular (AV) conduction disturbances, arrhythmias, and ST segment and T-wave changes, is also helpful in the clinical diagnosis of cardiac problems.
Throughout this chapter, the vectorial approach will be used whenever possible. The vectorial approach is preferred to “pattern reading,” which has infinite number of possibilities. The following topics will be discussed in the order listed.
- •
What is the vectorial approach?
- •
Comparison of pediatric and adult ECGs
- •
Basic measurements and their normal values that are necessary for correct interpretation of an ECG. The discussion will include rhythm, heat rate, QRS axis, P and T axes, and so on.
- •
Atrial and ventricular hypertrophy
- •
Ventricular conduction disturbances
- •
ST-segment and T-wave changes, including myocardial infarction (MI)
Cardiac arrhythmias and AV conduction disturbances will be discussed separately in Chapter 24, Chapter 25 .
What Is the Vectorial Approach?
The vectorial approach views the standard scalar ECG as three-dimensional vector forces that vary with time. Vector is a quantity that possesses magnitude and direction, but scalar is a quantity that has magnitude only. A scalar ECG, which is routinely obtained in clinical practice, shows only magnitude of the forces against time. However, by combining scalar leads that represent the frontal projection and the horizontal projections of the vectorcardiogram, one can derive the direction of the force from scalar ECGs.
The limb leads (leads I, II, III, aVR, aVL, and aVF) provide information about the frontal projection (reflecting superior-inferior and right-to-left forces), and the precordial leads (leads V1 through V6, V3R, and V4R) provide information about the horizontal plane, which reflects forces that are right-to-left and anterior–posterior ( Fig. 3-1 ). It is important for readers to become familiar with the orientation of each scalar ECG lead. After they have been learned, the vectorial approach helps readers retain the knowledge gained.
Hexaxial Reference System
It is necessary to memorize the orientation of the hexaxial reference system (see Fig. 3-1 , A ). The hexaxial reference system is made up by the six limb leads (leads I, II, III, aVR, aVL, and aVF) and provides information about the superoinferior and right–left relationships of the electromotive forces. In this system, leads I and aVF cross at a right angle at the electrical center (see Fig. 3-1 , A ). The bipolar limb leads (I, II, and III) are clockwise with the angle between them of 60 degrees. Note the positive poles of aVR, aVL, and aVF are directed toward the right and left shoulders and the foot, respectively. The positive limb of each lead is shown in a solid line and the negative limb in a broken line. The positive pole of each lead is indicated by the lead labels. The positive pole of lead I is labeled as 0 degree, and the negative pole of the same lead as ±180 degrees. The positive pole of aVF is designated as +90 degrees, and the negative pole of the same lead as −90 degrees. The positive poles of leads II and III are +60 and +120 degrees, respectively, and so on. The hexaxial reference system is used in plotting the QRS axis, T axis, and P axis.
The lead I axis represents the left–right relationship with the positive pole on the left and the negative pole on the right. The aVF lead represents the superior-inferior relationship with the positive pole directed inferiorly and the negative pole directed superiorly. The R wave in each lead represents the depolarization force directed toward the positive pole; the Q and S waves are the depolarization force directed toward the negative pole. Therefore, the R wave of lead I represents the leftward force, and the S wave of the same lead represents the rightward force (see Fig. 3-1 , A ). The R wave in aVF represents the inferiorly directed force, and the S wave the superiorly directed force. By the same token, the R wave in lead II represents the leftward and inferior force, and the R wave in lead III represents the rightward and inferior force. The R wave in aVR represents the rightward and superior force, and the R wave in aVL represents the leftward and superior forces.
An easy way to memorize the hexaxial reference system is shown in Figure 3-2 by a superimposition of a body with stretched arms and legs on the X and Y axes. The hands and feet are the positive poles of electrodes. The left and right hands are the positive poles of leads aVR and aVL, respectively. The left and right feet are the positive poles of leads II and III, respectively. The bipolar limb leads I, II, and III are clockwise in sequence for the positive electrode.
Horizontal Reference System
The horizontal reference system consists of precordial leads (leads V1 through V6, V3R, and V4R) (see Fig. 3-1 , B ) and provides information about the anterior–posterior and the left–right relationship. The leads V2 and V6 cross approximately at a right angle at the electrical center of the heart. The V6 axis represents the left–right relationship, and the V2 axis represents the anterior–posterior relationship. The positive limb of each lead is shown in a solid line and the negative limb in a broken line. The positive pole of each lead is indicated by the lead labels (V4R, V1, V2, and so on). The precordial leads V3R and V4R are at the mirror image points of V3 and V4, respectively, in the right chest, and these leads are quite popular in pediatric cardiology because right ventricular (RV) forces are more prominent in infants and children.
Therefore, the R wave of V6 represents the leftward force and the R wave of V2 the anterior force. Conversely, the S wave in V6 represents the rightward force and the S wave of V2 the posterior force. The R wave in V1, V3R, and V4R represents the rightward and anterior force, and the S wave of these leads represents the leftward and posterior force (see Fig. 3-1 , B ). The R wave of lead V5 in general represents the leftward force, and the R waves of leads V3 and V4 represent a transition between the right and left precordial leads. Ordinarily, the S wave in V2 represents the posterior and thus the left ventricular (LV) force, but in the presence of a marked right axis deviation, the S wave of V2 may represent RV force that is directed rightward and posteriorly.
Information Available on the 12-Lead Scalar Electrocardiogram
There are three major types of information available in the commonly available form of a 12-lead ECG tracing ( Fig. 3-3 ):
- 1.
The lower part of the tracing is a rhythm strip (of lead II).
- 2.
The large upper left portion of the recording gives frontal plane information, and the upper right side of the recording presents horizontal plane information. The frontal plane information is provided by the six limbs leads (leads I, II, III, aVR, aVL, and aVF) and the horizontal plane information by the precordial leads. In Figure 3-3 , the QRS vector is predominantly directed inferiorly (judged by predominant R waves in leads II, III, and aVF, so-called inferior leads) and is equally anterior and posterior, judged by equiphasic QRS complex in V2.
- 3.
A calibration marker usually appears at the right (or left) margin, which is used to determine the magnitude of the forces. The calibration marker consists of two vertical deflections 2.5 mm in width. The initial deflection shows the calibration factor for the six limb leads, and the latter part of the deflection shows the calibration factor for the six precordial leads. With the full standardization, one millivolt signal introduced into the circuit causes a deflection of 10 mm on the record. With the ½ standardization, the same signal produces 5 mm of deflection. The amplitude of ECG deflections is read in millimeters rather than in millivolts. When the deflections are too big to be recorded, the sensitivity may be reduced to 1艠4. With ½ standardization, the measured height in millimeters should be multiplied by 2 to obtain the correct amplitude of the deflection. In Figure 3-3 , ½ standardization was used for the precordial leads.
Thus, from the scalar ECG tracing, one can gain information of the frontal and horizontal orientations of the QRS (or ventricular) complexes and other electrical activities of the heart as well as the magnitude of such forces.
Comparison of Pediatric and Adult Electrocardiograms
Electrocardiograms of normal infants and children are quite different from those of normal adults. The most remarkable difference is RV dominance in infants. RV dominance is most noticeable in newborns, and it gradually changes to LV dominance of adults. By 3 years of age, the child’s ECG resembles that of young adults. The age-related difference in the ECG reflects an age-related anatomic differences; the RV is thicker than the LV in newborns and infants, and the LV is much thicker than the RV in adults.
Right ventricular dominance of infants is expressed in the ECG by right axis deviation (RAD) and large rightward or anterior QRS forces (i.e., tall R waves in lead aVR and the right precordial leads [V4R, V1, and V2] and deep S waves in lead I and the left precordial leads [V5 and V6]) compared with an adult ECG.
A normal ECG from a 1-week-old neonate ( Fig. 3-4 ) is compared with that of a young adult ( Fig. 3-5 ). The infant’s ECG demonstrates RAD (+140 degrees) and dominant R waves in the right precordial leads. The T wave in V1 is usually negative. Upright T waves in V1 in this age group suggest right ventricular hypertrophy (RVH). Adult-type R/S progression in the precordial leads (deep S waves in V1 and V2 and tall R waves in V5 and V6; as seen in Fig. 3-5 ) is rarely seen in the first month of life; instead, there may be complete reversal of the adult-type R/S progression, with tall R waves in V1 and V2 and deep S waves in V5 and V6. Partial reversal is usually present, with dominant R waves in V1 and V2 as well as in V5 and V6, in children between the ages of 1 month and 3 years.
The normal adult ECG shown in Figure 3-5 demonstrates the QRS axis near +60 degrees and the QRS forces directed to the left, inferiorly and posteriorly, which is manifested by dominant R waves in the left precordial leads and dominant S waves in the right precordial leads, the so-called adult R/S progression. The T waves are usually anteriorly oriented, resulting in upright T waves in V2 through V6 and sometimes in V1.
Basic Measurements and Their Normal and Abnormal Values
In this section, basic measurements and their normal values that are necessary for routine interpretation of an ECG are briefly discussed in the order listed. This sequence is one of many approaches that can be used in routine interpretation of an ECG. The methods of their measurements will be followed by their normal and abnormal values and the significance of abnormal values.
- 1.
Rhythm (sinus or nonsinus) by considering the P axis
- 2.
Heart rate (atrial and ventricular rates, if different)
- 3.
The QRS axis, the T axis, and the QRS-T angle
- 4.
Intervals: PR, QRS, and QT
- 5.
The P wave amplitude and duration
- 6.
The QRS amplitude and R/S ratio; also abnormal Q waves
- 7.
ST-segment and T-wave abnormalities
Rhythm
Sinus rhythm is the normal rhythm at any age and is characterized by P waves preceding each QRS complex and a normal P axis (0 to +90 degrees); the latter is an often neglected criterion. The requirement of a normal P axis is important in discriminating sinus from nonsinus rhythm. In sinus rhythm, the PR interval is regular but does not have to be of normal interval. (The PR interval may be prolonged as seen in sinus rhythm with first-degree atrioventricular [AV] block.)
Because the sinoatrial node is located in the right upper part of the atrial mass, the direction of atrial depolarization is from the right upper part toward the left lower part, with the resulting P axis in the lower left quadrant (0 to +90 degrees) ( Fig. 3-6 , A ). Some atrial (nonsinus) rhythms may have P waves preceding each QRS complex, but they have an abnormal P axis ( Fig. 3-6 , B ). For the P axis to be between 0 and +90 degrees, P waves must be upright in leads I and aVF or at least not inverted in these leads; simple inspection of these two leads suffices. A normal P axis also results in upright P waves in lead II and inverted P waves in aVR. A method of plotting axes is presented later for the QRS axis.
Heart Rate
There are many different ways to calculate the heart rate, but they are all based on the known time scale of ECG papers. At the usual paper speed of 25 mm/sec, 1 mm = 0.04 second, and 5 mm = 0.20 second ( Fig. 3-7 ). The following methods are often used to calculate the heart rate.
- 1.
Count the R-R cycle in six large divisions (1/50 minute) and multiply it by 50 ( Fig. 3-8 ).
FIGURE 3-8
Heart rate of 165 beats/min. There are about 3.3 cardiac cycles (R-R intervals) in six large divisions. Therefore, the heart rate is 3.3 × 50 = 165 (by method 1). By method 2, the RR interval is 0.36 sec. 60 ÷ 0.36 = 166. The rates derived by the two methods are very close.
- 2.
When the heart rate is slow, count the number of large divisions between two R waves and divide that into 300 (because 1 minute = 300 large divisions) ( Fig. 3-9 ).
FIGURE 3-9
Heart rate of 52 beats/minute. There are 5.8 large (5 mm) divisions between the two arrows. Therefore the heart rate is 300 ÷ 5.8 = 52.
- 3.
Measure the R-R interval (in seconds) and divide 60 by the R-R interval. The R-R interval is 0.36 second in Figure 3-8 : 60 ÷ 0.36 = 166.
- 4.
Use a convenient ECG ruler.
- 5.
An approximate heart rate can be determined by memorizing heart rates for selected R-R intervals ( Fig. 3-10 ). When R-R intervals are 5, 10, 15, 20, and 25 mm, the respective heart rates are 300, 150, 100, 75, and 60 beats/min.
FIGURE 3-10
Quick estimation of heart rate. When the R-R interval is 5 mm, the heart rate is 300 beats/min. When the R-R interval is 10 mm, the rate is 150 beats/min, and so on.
When the ventricular and atrial rates are different, as in complete heart block or atrial flutter, the atrial rate can be calculated using the same methods as described for the ventricular rate; for the atrial rate, the P-P interval rather than the R-R interval is used.
Because of age-related differences in the heart rate, the definitions of bradycardia (<60 beats/min) and tachycardia (>100 beats/min) used for adults do not help distinguish normal from abnormal heart rates in pediatric patients. Operationally, tachycardia is present when the heart rate is faster than the upper range of normal for that age, and bradycardia is present when the heart rate is slower than the lower range of normal. According to age, normal resting heart rates per minute recorded on the ECG are as follows ( Davignon et al, 1979/1980 ).
| Newborn | 145 (90–180) |
| 6 mo | 145 (105–185) |
| 1 yr | 132 (105–170) |
| 2 yr | 120 (90–150) |
| 4 yr | 108 (72–135) |
| 6 yr | 100 (65–135) |
| 10 yr | 90 (65–130) |
| 14 yr | 85 (60–120) |
QRS Axis, T Axis, and QRS-T Angle
QRS Axis
The most convenient way to determine the QRS axis is the successive approximation method using the hexaxial reference system (see Fig. 3-1 , A ). The same approach is also used for the determination of the T axis (see later discussion). For the determination of the QRS axis (as well as T axis), one uses only the hexaxial reference system (or the six limbs leads), not the horizontal reference system.
Successive Approximation Method
Step 1: Locate a quadrant using leads I and aVF ( Fig. 3-11 ). In the top panel of Figure 3-11 , the net QRS deflection of lead I is positive. This means that the QRS axis is in the left hemicircle (i.e., from –90 degrees through 0 to +90 degrees) from the lead I point of view. The net positive QRS deflection in aVF means that the QRS axis is in the lower hemicircle (i.e., from 0 through +90 degrees to +180 degrees) from the aVF point of view. To satisfy the polarity of both leads I and aVF, the QRS axis must be in the lower left quadrant (i.e., 0 to +90 degrees). Four quadrants can be easily identified based on the QRS complexes in leads I and aVF (see Fig. 3-11 ).
Step 2: Among the remaining four limb leads, find a lead with an equiphasic QRS complex (in which the height of the R wave and the depth of the S wave are equal). The QRS axis is perpendicular to the lead with an equiphasic QRS complex in the predetermined quadrant.
Example: Determine the QRS axis in Figure 3-12 .
Step 1: The axis is in the lower left quadrant (0 to +90 degrees) because the R waves are upright in leads I and aVF.
Step 2: The QRS complex is equiphasic in aVL. Therefore, the QRS axis is +60 degrees, which is perpendicular to aVL.
Normal QRS Axis
Normal ranges of QRS axis vary with age. Newborns normally have RAD compared with the adult standard. By 3 years of age, the QRS axis approaches the adult mean value of +50 degrees. The mean and ranges of a normal QRS axis according to age are shown in Table 3-1 .
| Age | Mean (Range) |
|---|---|
| 1 wk–1 mo | + 110° (+30 to +180) |
| 1–3 mo | + 70° (+10 to +125) |
| 3 mo–3 yr | + 60° (+10 to +110) |
| Older than 3 yr | + 60° (+20 to +120) |
| Adult | + 50° (–30 to +105) |
Abnormal QRS Axis
The QRS axis outside normal ranges signifies abnormalities in the ventricular depolarization process.
- 1.
Left axis deviation (LAD) is present when the QRS axis is less than the lower limit of normal for the patient’s age. LAD occurs with left ventricular hypertrophy (LVH), left bundle branch block (LBBB), and left anterior hemiblock.
- 2.
RAD is present when the QRS axis is greater than the upper limit of normal for the patient’s age. RAD occurs with RVH and right bundle branch block (RBBB).
- 3.
“Superior” QRS axis is present when the S wave is greater than the R wave in aVF. The overlap with LAD and left anterior hemiblock should be noted. Left anterior hemiblock (in the range of –30 to –90 degrees is seen in congenital heart diseases such as endocardial cushion defect and tricuspid atresia) or with RBBB. It is rarely seen in otherwise normal children.
T Axis
The T axis is determined by the same methods used to determine the QRS axis. In normal children, including newborns, the mean T axis is +45 degrees, with a range of 0 to +90 degrees, the same as in normal adults. This means that the T waves must be upright in leads I and aVF. The T waves can be flat but must not be inverted in these leads. The T axis outside of the normal quadrant suggests conditions with myocardial dysfunction similar to those listed for abnormal QRS-T angle (see below).
QRS-T Angle
The QRS-T angle is formed by the QRS axis and the T axis. A QRS-T angle of greater than 60 degrees is unusual, and one greater than 90 degrees is certainly abnormal. An abnormally wide QRS-T angle with the T axis outside the normal quadrant (0 to +90 degrees) is seen in severe ventricular hypertrophy with “strain,” ventricular conduction disturbances, and myocardial dysfunction of a metabolic or ischemic nature.
Intervals
Three important intervals are routinely measured in the interpretation of an ECG: PR interval, QRS duration, and QT interval. The duration of the P wave is also inspected ( Fig. 3-13 ).
PR Interval
The normal PR interval varies with age and heart rate ( Table 3-2 ). Davignon et al’s data are unsuitable for clinical use because they are presented separately according to age and heart rate. The PR interval is longer in older individuals and with a slower heart rate.
| Rate | 0–1 mo | 1–6 mo | 6 mo–1 yr | 1–3 yr | 3–8 yr | 8–12 yr | 12–16 yr | Adults |
|---|---|---|---|---|---|---|---|---|
| <60 | 0.16 (0.18) | 0.16 (0.19) | 0.17 (0.21) | |||||
| 60–80 | 0.15 (0.17) | 0.15 (0.17) | 0.15 (0.18) | 0.16 (0.21) | ||||
| 80–100 | 0.10 (0.12) | 0.14 (0.16) | 0.15 (0.16) | 0.15 (0.17) | 0.15 (0.20) | |||
| 100–120 | 0.10 (0.12) | (0.15) | 0.13 (0.16) | 0.14 (0.15) | 0.15 (0.16) | 0.15 (0.19) | ||
| 120–140 | 0.10 (0.11) | 0.11 (0.14) | 0.11 (0.14) | 0.12 (0.14) | 0.13 (0.15) | 0.14 (0.15) | 0.15 (0.18) | |
| 140–160 | 0.09 (0.11) | 0.10 (0.13) | 0.11 (0.13) | 0.11 (0.14) | 0.12 (0.14) | (0.17) | ||
| 160–180 | 0.10 (0.11) | 0.10 (0.12) | 0.10 (0.12) | 0.10 (0.12) | ||||
| >180 | 0.09 | 0.09 (0.11) | 0.10 (0.11) |
Prolongation of the PR interval (i.e., first-degree AV block) is seen in myocarditis (rheumatic, viral, or diphtheric), digitalis or quinidine toxicity, certain congenital heart defects (endocardial cushion defect, atrial septal defect, Ebstein’s anomaly), some myocardial dysfunctions, hyperkalemia, and otherwise normal heart with vagal stimulation.
A short PR interval is present in Wolff-Parkinson-White (WPW) preexcitation, Lown-Ganong-Levine syndrome, myocardiopathies of glycogenosis, Duchene’s muscular dystrophy (or relatives of these patients), Friedrich’s ataxia, pheochromocytoma, and otherwise normal children. The lower limits of normal PR interval are shown under the topic of WPW preexcitation (see later discussion).
Variable PR intervals are seen in the wandering atrial pacemaker and the Wenckebach phenomena (Mobitz type I second-degree AV block).
QRS Duration
The QRS duration varies with age ( Table 3-3 ). It is short in infants and increases with age.
| 0–1 mo | 1–6 mo | 6–12 mo | 1–3 yr | 3–8 yr | 8–12 yr | 12–16 yr | Adults | |
|---|---|---|---|---|---|---|---|---|
| Seconds | 0.05 (0.07) | 0.055 (0.075) | 0.055 (0.075) | 0.055 (0.075) | 0.06 (0.075) | 0.06 (0.085) | 0.07 (0.085) | 0.08 (0.10) |
The QRS duration is prolonged in conditions grouped as ventricular conduction disturbances, which include RBBB, LBBB, preexcitation (e.g., WPW preexcitation), and intraventricular block (as seen in hyperkalemia, toxicity from quinidine or procainamide, myocardial fibrosis, and myocardial dysfunction of a metabolic or ischemic nature). Ventricular arrhythmias (e.g., premature ventricular contractions, ventricular tachycardia, implanted ventricular pacemaker) also produce a wide QRS duration. Because the QRS duration varies with age, the definition of bundle branch block (BBB) or other ventricular conduction disturbances should vary with age (see the section on ventricular conduction disturbances).
QT Interval
The QT interval varies primarily with heart rate. The heart rate–corrected QT (QTc) interval is calculated by the use of Bazett’s formula:
QTc = QT / RR interval
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