In the clinical diagnosis of congenital or acquired heart disease, thepresence of electrocardiographic (ECG) abnormalities is often helpful What is the vectorial approach?The vectorial approach views the standard scalar ECG as three-dimensionalvector forces that vary with time.A vector is a quantity that possesses magnitude and direction; a scalar is aquantity that has magnitude only.A scalar ECG, which is routinely obtained in clinical practice, shows only themagnitude of the forces against time. However, by combining scalar leadsthat represent the frontal projection and the horizontal projections of thevector cardiogram, one can derive the direction of the force from scalarECGs. The limb leads (leads I, II, III, aVR, aVL, and aVF) provide informationabout the frontal projection (reflecting superior-inferior and right-to-leftforces), and the precordial leads (leads V1 through V6, V3R, and V4R)provide information about the horizontal plane that reflects forces that are rightto left and anterior-posterior It is important for the readers to becomefamiliar with the orientation of each scalar ECG lead. Once learned, thevectorial approach helps the readers to retain the knowledge gained andeven helps them recall what has been forgotten.
Figure 3-1 Hexaxial reference system (A) shows the frontalprojection of a vector loop, and horizontal reference system (B)
It is necessary to memorize the orientation of the hexaxial reference system Thehexaxial 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-leftrelationships of the electromotive forces. In this system, leads I and aVF cross at aright angle at the electrical center The bipolar limb leads (I, II, and III) are clockwisewith an angle between them of 60 degrees. Note that the positive poles of aVR, aVL,and aVF are directed toward the right and left shoulders and the foot, respectively.The positive pole of lead I is labeled as 0 degree and the negative pole of the samelead as ±180 degrees. The positive pole of aVF is designated as +90 degrees andthe negative pole of the same lead as -90 degrees. The positive poles of leads IIand III are +60 and +120 degrees, respectively, and so on.The lead I axis represents the left-right relationship with the positive pole on the left andthe negative pole on the right. The aVF lead represents the superior-inferiorrelationship with the positive pole directed inferiorly and the negative pole directedsuperiorly. The R wave in each lead represents the depolarization force directedtoward the positive pole; the Q and S waves are the depolarization force directedtoward the negative pole. Therefore, the R wave of lead I represents the leftwardforce and the S wave of the same lead represents the rightward force .The R wave inaVF represents the inferiorly directed force and the S wave the superiorly directedforce. By the same token, the R wave in lead II represents the leftward and inferiorforce and the R wave in lead III represents the rightward and inferior force. The Rwave in aVR represents the rightward and superior force and the R wave in aVLrepresents the leftward and superior force.
The horizontal reference system consists of precordial leads (leads V1through V6, V3R, and V4R) and provides information about theanterior-posterior and the left-right relationship. Leads V2 and V6cross approximately at a right angle at the electrical center of theheart. The V6 axis represents the left-right relationship and the V2axis represents the anterior-posterior relationship.The precordial leads V3R and V4R are at the mirror image points of V3and V4, respectively, in the right chest, and these leads are quitepopular in pediatric cardiology because right ventricular (RV) forcesare more prominent in infants and children.Therefore, the R wave of V6 represents the leftward force and the Rwave of V2 the anterior force. Conversely, the S wave of V6represents the rightward force and the S wave of V2 the posteriorforce.The R wave in V1, V3R, and V4R represents the rightward and anteriorforce and the S wave of these leads represents the leftward andposterior force The R wave of lead V5 in general represents theleftward force, and the R waves of leads V3 and V4 represent atransition between the right and left precordial leads.
Three major types of information are available in the commonly available form of a 12-lead ECGtracing1. The lower part of the tracing is a rhythm strip (of lead II).2. The upper left side of the recording gives frontal plane information and the upper rightside of therecording presents horizontal plane information. The frontal plane information is provided by the six limbleads (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 inleads II, III, and aVF, so-called inferior leads) and is equally anterior and posterior, judged by theequiphasic QRS complex in V2.3. There is also a calibration marker at the right (or left) margin, which is used todetermine themagnitude of the forces. The calibration marker consists of two vertical deflections of 2.5 mm width. Theinitial deflection shows the calibration factor for the six limb leads, and the latter part of the deflectionshows the calibration factor for the six precordial leads. With the full standardization, a 1- millivoltsignal introduced into the circuit causes a deflection of 10 mm on the record. With the halfstandardization, the same signal produces a 5-mm deflection. The amplitude of ECG deflections isread in millimeters rather than in millivolts. When the deflections are too big to be recorded, thesensitivity may be reduced to one fourth. With half standardization, the measured height inmillimeters should be multiplied by 2 to obtain the correct amplitude of the deflection.Thus, from the scalar ECG tracing, one can gain information about the frontal and horizontal orientationsof the QRS (or ventricular) complexes and other electrical activities of the heart as well as themagnitude of such forces.
A common form of a routine 12-lead scalar electrocardiogram. There are three types ofinformation available on the recording. Frontal and horizontal plane information isgiven on the upper part of the tracing. Calibration factors are shown on the right edgeof the recording. Rhythm strip (lead II) is shown at the bottom.
ECGs of normal infants and children are quite different from those of normal adults. Themost remarkable difference is RV dominance in infants. RV dominance is mostnoticeable in newborns, and it gradually changes to left ventricular (LV) dominanceof adults. By 3 years of age, the childs ECG resembles that of young adults. The age-related difference in the ECG reflects age-related anatomic differences; the rightventricle (RV) is thicker than the left ventricle (LV) in newborns and infants, and theLV is much thicker than the RV in adults.RV dominance of infants is expressed in the ECG by right axis deviation (RAD) and largerightward and/or anterior QRS forces (i.e., tall R waves in lead aVR and the rightprecordial leads [V4R, V1, and V2] and deep S waves in lead I and the left precordialleads [V5 and V6]), compared with the adult ECG.An ECG from a 1-week old neonate is compared with that of a young adult .The infants ECG demonstrates RAD (+140 degrees) and dominant R waves in the rightprecordial leads. The T wave in V1 is usually negative. Upright T waves in V1 in thisage group suggest right ventricular hypertrophy (RVH). Adult-type R/S progression inthe precordial leads (deep S waves in V1 and V2 and tall R waves in V5 and V6; israrely seen in the first month of life; instead, there may be complete reversal of theadult-type R/S progression, with tall R waves in V1 and V2 and deep S waves in V5and V6. Partial reversal is usually present, with dominant R waves in V1 and V2 aswell as in V5 and V6, in children between the ages of 1 month and 3 years.
The normal adult ECG demonstrates the QRS axisnear +60 degrees and the QRS forces directed tothe left, inferiorly and posteriorly, which ismanifested by dominant R waves in the leftprecordial leads and dominant S waves in the rightprecordial leads, the so-called adult R/Sprogression.The T waves are usually anteriorly oriented, resultingin upright T waves in V2 through V6 andsometimes in V1.
They are briefly discussed in the order listed. This sequence is one ofmany approaches that can be used in routine interpretation of anECG. The methods of their measurements are followed by theirnormal and abnormal values and the significance of abnormalvalues.1. Rhythm (sinus or nonsinus) by considering the P axis2. Heart rate (atrial and ventricular rates, if different)3. The QRS axis, the T axis, and the QRS-T angle4. Intervals: PR, QRS, and QT5. The P-wave amplitude and duration6. The QRS amplitude and R/S ratio; also abnormal Q waves7. ST-segment and T-wave abnormalities
Sinus rhythm is the normal rhythm at any age and is characterized by Pwaves preceding each QRS complex and a normal P axis (0 to +90degrees); the latter is an often neglected criterion. The requirementof a normal P axis is important in discriminating sinus from non sinusrhythm. In sinus rhythm, the PR interval is regular but is notnecessarily normal. (The PR interval may be prolonged as seen insinus rhythm with first degree AV block.)Because the sinoatrial node is located in the right upper part of theatrial mass, the direction of atrial depolarization is from the rightupper part toward the left lower part, with the resulting P axis in thelower left quadrant (0 to +90 degrees) .Some atrial (nonsinus) rhythms may have P waves preceding each QRScomplex, but they have an abnormal P axis. For the P axis to bebetween 0 and +90 degrees, P waves must be upright in leads I andaVF or at least not inverted in these leads; simple inspection of thesetwo leads suffices. A normal P axis also results in upright P waves inlead II and inverted P waves in aVR.
There are many different ways to calculate the heart rate, but they are all based on theknown time scale of ECG papers. At the usual paper speed of 25 mm/second, 1 mm= 0.04 second and 5 mm = 0.20 second . The following methods are often used tocalculate the heart rate.1. Count the R-R cycle in six large divisions (1/50 minute) and multiply itby 502. When the heart rate is slow, count the number of large divisionsbetween two R waves and dividethat into 300 (because 1 minute = 300 large divisions)3. Measure the R-R interval (in seconds) and divide 60 by the R-Rinterval. The R-R interval is 0.36: 60 ÷ 0.36 = 166.4. Use a convenient ECG ruler.5. An approximate heart rate can be determined by memorizing heartrates for selected R-R intervalsWhen R-R intervals are 5, 10, 15, 20, and 25 mm, the respective heart rates are 300,150, 100, 75, and 60 beats/minute.When the ventricular and atrial rates are different, as in complete heart block or atrialflutter, the atrial rate can be calculated using the same methods as described for theventricular rate; for the atrial rate, the P-P interval rather than the R-R interval isused.
ELECTROCARDIOGRAM PAPER. TIME IS MEASUREDON THE HORIZONTAL AXIS. EACH 1 MM EQUALS 0.04SECOND, AND EACH 5 MM(A LARGE DIVISION) EQUALS 0.20 SECOND. THIRTYMILLIMETERS (OR SIX LARGE DIVISIONS) EQUAL 1.2SECOND OR 1/50 MINUTE.HEART RATE OF 165 BEATS/MINUTE. THEREARE ABOUT 3.3 CARDIAC CYCLES (R-RINTERVALS) IN SIX LARGE DIVISIONS.THEREFORETHE HEART RATE IS 3.3 × 50 = 165 (BYMETHOD 1).BY METHOD 3, THE R-R INTERVAL IS 0.36SECOND; 60 ÷ 0.36 = 166.THE RATES DERIVEDBY THE TWO METHODS ARE VERY CLOSE.
HEART RATE OF 52 BEATS/MINUTE. THEREARE 5.8 LARGE DIVISIONS BETWEEN THETWO ARROWS. THEREFORE, THE HEARTRATE IS300 ÷ 5.8 = 52.QUICK ESTIMATION OF HEART RATE. WHENTHE R-R INTERVAL IS 5 MM, THE HEARTRATE IS 300 BEATS/MINUTE. WHEN THE R-RINTERVAL IS 10 MM, THE RATE IS 150BEATS/MINUTE, AND SO ONHeart rate according to age groupsNewborn 145 (90–180)6 months 145 (105–185)1 year 132 (105–170)4 years 108 (72–135)14 years 85 (60–120)
QRS AXIS, T AXIS, AND QRS-TANGLEQRS Axis.The most convenient way to determinethe QRS axis is the successiveapproximation method using thehexaxial reference system.The same approach is also used forthe determination of the T axis .For the determination of the QRS axis(as well as the T axis), one uses onlythe hexaxial reference system (or thesix limb leads), not the horizontalreference system.Successive ApproximationMethod.Step 1.Locate a quadrant, using leads I andaVF
In the top panel, the net QRS deflection of lead I is positive. This meansthat the QRS axis is in the left hemicircle (i.e., from –90 degreesthrough 0 to +90 degrees) from the lead I point of view. The netpositive QRS deflection in aVF means that the QRS axis is in the lowerhemicircle (i.e., from 0 through +90 degrees to +180 degrees) fromthe 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 +90degrees).Four quadrants can be easily identified based on the QRS complexes inleads I and aVF .Step 2.Among the remaining four limb leads, find a lead with an equiphasicQRS complex (in which the height of the R wave and the depth of theS wave are equal).The QRS axis is perpendicular to the lead with an equiphasic QRScomplex in the predetermined quadrant.
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 areupright in leads I and aVF.Step 2.The QRS complex is equiphasic in aVL. Therefore, the QRS axis is +60 degrees,which is perpendicular toaVL.Figure 3-12A, Set of six limb leads.B, Plotted QRS axis is shown.
Normal QRS Axis.Normal ranges of QRS axis vary with age. Newborns normallyhave RAD compared with the adult standard.By 3 years of age, the QRS axis approaches the adult meanvalue of +50 degrees.Table 3-1 -- Mean and Ranges of Normal QRS Axes byAgeAge 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)
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 ofnormal for thepatients age. LAD occurs with left ventricular hypertrophy (LVH), left bundle branch block (LBBB), and leftanterior hemiblock.2. RAD is present when the QRS axis is greater than the upper limit of normal for thepatients 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. Theoverlap withLAD should be noted. It may occur with left anterior hemiblock (in the range of –30 to –90 degrees, seen inendocardial cushion defect (ECD) or tricuspid atresia) or with RBBB. It is rarely seen in otherwise normalchildren.T Axis.The T axis is determined by the same methods used to determine the QRS axis. In normal children, includingnewborns, the mean T axis is +45 degrees, with a range of 0 to +90 degrees, the same as in normaladults. This means that the T waves must be upright in leads I and aVF. The T waves can be flat, but mustnot be inverted, in these leads. The T axis outside the normal quadrant suggests conditions withmyocardial dysfunction similar to those listed for abnormal QRS-T angle.QRS-T Angle.The QRS-T angle is formed by the QRS axis and the T axis. A QRS-T angle greater than 60 degrees is unusual,and one greater than 90 degrees is certainly abnormal. An abnormally wide QRS-T angle with the T axisoutside 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.
Three important intervals are routinely measured in the interpretation ofan ECG: PR interval, QRS duration, and QT interval. The duration ofthe P wave is also inspected .PR Interval.The normal PR interval varies with age and heart rate . The older theperson and the slower the heart rate, the longer is the PR interval.A short PR interval is present in Wolff-Parkinson-White (WPW)preexcitation, Lown-Ganong-Levine syndrome, myocardiopathies ofglycogenosis, Duchennes muscular dystrophy (or relatives of thesepatients), Friedreichs ataxia, pheochromocytoma, and otherwisenormal children. The lower limits of normal PR interval are shownunder WPW preexcitationVariable PR intervals are seen in wandering atrial pacemaker andWenckebachs phenomenon (Mobitz type I second-degree AV block).
The QRS duration varies with age It is short in infants andincreases with age.The QRS duration is prolonged in conditions grouped asventricular conduction disturbances, which include RBBB,LBBB, preexcitation (e.g., WPW preexcitation), andintraventricular block (as seen in hyperkalemia, toxicity fromquinidine or procainamide, myocardial fibrosis, myocardialdysfunction of a metabolic or ischemic nature).Ventricular arrhythmias (e.g., premature ventricularcontractions, ventricular tachycardia, implanted ventricularpacemaker) also produce a wide QRS duration. Because theQRS duration varies with age, the definition of bundle branchblock or other ventricular conduction disturbances shouldvary with age.
The QT interval varies primarily with heart rate. The heart rate-corrected QT (QTc) interval is calculated bythe use of Bazetts formula:According to Bazetts formula, the normal QTc interval (mean ± SD) is 0.40 (± 0.014) second with theupper limit of normal 0.44 second in children 6 months and older. The QTc interval is slightly longerin the newborn and small infants with the upper limit of normal QTc 0.47 second in the first week of life and0.45 second in the first 6 months of life.Long QT intervals may be seen in long QT syndrome (e.g., Jervell and Lange-Nielsen syndrome, Romano- Wardsyndrome), hypocalcemia, myocarditis, diffuse myocardial diseases (including hypertrophic and dilatedcardiomyopathies), head injury, severe malnutrition, and so on. A number of drugs are also known to prolongthe QT interval. Among these are antiarrhythmic agents (especially class IA, IC, and III), antipsychoticphenothiazines (e.g., thioridazine, chlorpromazine), tricyclic antidepressants (e.g., imipramine, amitriptyline),arsenics, organophosphates, antibiotics (e.g., ampicillin, erythromycin, trimethoprim-sulfa, amantadine), andantihistamines (e.g., terfenadine).A short QT interval is a sign of a digitalis effect or of hypercalcemia. It is also seen with hyperthermia and inshort QT syndrome (a familial cause of sudden death with QTc 300 milliseconds).The JT interval is measured from the J point (the junction between the S wave and the ST segment) to the end ofthe T wave. A prolonged JT interval has the same significance as a prolonged QT interval. The JT interval ismeasured only when the QT interval is prolonged or when the QRS duration is prolonged as seen withventricular conduction disturbances. The JT interval is also expressed as a rate corrected interval (called JTc)using Bazetts formula. Normal JTc (mean ± SD) is 0.32 ± 0.02 second with the upper limit of normal 0.34second in normal children and adolescents.
P-WAVE DURATION AND AMPLITUDEThe P-wave duration and amplitude are important in the diagnosis ofatrial hypertrophy. Normally, the P amplitude is less than 3 mm. Theduration of P waves is shorter than 0.09 second in children andshorter than 0.07 second in infantsQRS AMPLITUDE, R/S RATIO, AND ABNORMAL Q WAVESThe QRS amplitude and R/S ratio are important in the diagnosis ofventricular hypertrophy. These values also vary with age , Because ofthe normal dominance of RV forces in infants and small children, Rwaves are taller than S waves in the right precordial leads (i.e., V4R,V1, V2) and S waves are deeper than R waves in the left precordialleads (i.e., V5, V6) in this age group.Accordingly, the R/S ratio (the ratio of the R-wave and S-wave voltages)is large in the right precordial leads and small in the left precordialleads in infants and small children.
The normal ST segment is isoelectric. However, in the limbleads, elevation or depression of the ST segment up to 1mm is not necessarily abnormal in infants and children.An elevation or a depression of the ST segment isjudged in relation to the PR segment as the baseline.Some ST-segment changes are normal (nonpathologic)and others are abnormal (pathologic). (See a latersection on nonpathologic and pathologic ST-T changesin this chapter.)Tall peaked T waves may be seen in hyperkalemia and LVH(of the volume overload type). Flat or low T waves mayoccur in normal newborns or with hypothyroidism,hypokalemia, pericarditis, myocarditis, and myocardialischemia.
GENERAL CHANGESVentricular hypertrophy produces abnormalities in one or more of the following: the QRS axis, the QRSvoltages, the R/S ratio, the T axis, and miscellaneous areas.1. Changes in the QRS axis. The QRS axis is usually directed toward the ventricle thatis hypertrophied. Although RAD is present with RVH, LAD is seen with the volume overload type,but not with the pressure overload type, of LVH. Marked LAD usually indicates ventricularconduction disturbances (e.g., left anterior hemiblock or “superior” QRS axis).2. Changes in QRS voltages. Anatomically, the RV occupies the right and anterioraspect, and the LV occupies the left, inferior, and posterior aspect of theventricular mass. With ventricular hypertrophy, the voltage of the QRS complex increases in thedirection of the respective ventricle.In the frontal plane ,LVH shows increased R voltages in leads I, II, aVL, aVF, and sometimes III,especially in small infants. RVH shows increased R voltages in aVR and III and increased S voltagesin lead I.In the horizontal plane,with RVH , tall R waves in V4R, V1, and V2 or deep S waves in V5 and V6 , WithLVH, tall R waves in V5 and V6 and/or deep S waves in V4R, V1, and V2 are present3. Changes in R/S ratio. The R/S ratio represents the relative electromotive force of opposingventricles in a given lead.In ventricular hypertrophy, a change may be seen only in the R/S ratio, without an increase in theabsolute voltage. An increase in the R/S ratio in the right precordial leads suggests RVH; adecrease in the R/S ratio in these leads suggests LVH.Likewise, an increase in the R/S ratio in the left precordial leads suggests LVH, and a decrease in theratio suggests RVH.
4. Changes in the T axis. Changes in the T axis are seen in severeventricular hypertrophy with relative ischemia of the hypertrophiedmyocardium. In the presence of other criteria of ventricular hypertrophy, awide QRS-T angle (i.e., >90 degrees) with the T axis outside the normalrange indicates a strain pattern. When the T axis remains in the normalquadrant (0 to +90 degrees), a wide QRS-T angle alone indicates a possiblestrain pattern.5. Miscellaneous nonspecific changesa. RVH1). A q wave in V1 (qR or qRs pattern) suggests RVH, although itmay be present in ventricular inversion.2). An upright T wave in V1 after 3 days of age is a sign ofprobable RVH.b. LVHDeep Q waves (>5 mm) and/or tall T waves in V5 and V6 are signs of LVH ofvolume overload type. These may be seen with a large-shunt ventricularseptal defect (VSD)
In RVH, some or all of the following criteria are present.1. RAD for the patients age2. Increased rightward and anterior QRS voltages (in the absence of prolonged QRSduration)a wide QRS complex with increased QRS voltages suggests ventricular conduction disturbances(e.g., RBBB) rather than ventricular hypertrophy.a. R waves in V1, V2, or aVR greater than the upper limits of normal for thepatients ageb. S waves in I and V6 greater than the upper limits of normal for the patients age3. Abnormal R/S ratio in favor of the RV (in the absence of bundle branch block)a. R/S ratio in V1 and V2 greater than the upper limits of normal for ageb. R/S ratio in V6 less than 1 after 1 month of age.4. Upright T waves in V1 in patients more than 3 days of age, provided that the T isupright in the left precordial leads (V5, V6); upright T waves in V1 are notabnormal in patients older than 6 years.5. A q wave in V1 (qR or qRs patterns) suggests RVH (the physician should ascertain thatthere is not asmall r in an rsR configuration).6. In the presence of RVH, a wide QRS-T angle with T axis outside the normal range(in the 0 to –90degree quadrant) indicates a strain pattern. A wide QRS-T angle with the T axis within the normalrange suggests a possible strain pattern.
The diagnosis of RVH in newborns is particularly difficult because of thenormal dominance of the RV during this period of life. Helpful signs inthe diagnosis of RVH in newborns are as follows.1. S waves in lead I that are 12 mm or greater2. Pure R waves (with no S waves) in V1 that are greaterthan 10 mm3. R waves in V1 that are greater than 25 mm or R waves inaVR that are greater than 8 mm4. A qR pattern seen in V1 (this is also seen in 10% ofnormal newborns)5. Upright T waves seen in V1 after 3 days of age6. RAD with the QRS axis greater than +180 degrees
In LVH, some or all of the following abnormalities are present.1. LAD for the patients age2. QRS voltages in favor of the LV (in the absence of a prolonged QRS duration forage)a. R waves in leads I, II, III, aVL, aVF, V5, or V6 greater than the upper limits ofnormal forageb. S waves in V1 or V2 greater than the upper limits of normal for ageIn general, the presence of abnormal forces to more than one direction (e.g., to the left, inferiorly,and posteriorly) is a stronger criterion than the abnormality in only one direction.3. Abnormal R/S ratio in favor of the LV: R/S ratio in V1 and V2 less than the lowerlimits of normalfor the patients age4. Q waves in V5 and V6, greater than 5 mm, as well as tall symmetrical T waves inthe same leads(“LV diastolic overload”)5. In the presence of LVH, a wide QRS-T angle with the T axis outside the normal rangeindicates a strain pattern; this is manifested by inverted T waves in lead I or aVF. A wideQRS-T angle with the T axis within the normal range suggests a possible strainpattern.The R waves in leads I, aVL, V5, and V6 are beyond the upper limits of normal, indicating abnormalleftward force. The QRS duration is normal. The T axis (+55 degrees) remains in the normalquadrant. This tracing
BVH may be manifested in one of the following ways.1. Positive voltage criteria for RVH and LVH in the absence of bundlebranch block or preexcitation (i.e., with normal QRS duration)2. Positive voltage criteria for RVH or LVH and relatively large voltages forthe other ventricle3. Large equiphasic QRS complexes in two or more of the limb leads andin the mid-precordial leads (i.e., V2 through V5), called the Katz-Wachtelphenomenon (with normal QRS duration)It is difficult to plot the QRS axis because of large diphasic QRS complexes in limb leads.The R and S voltages are large in some limb leads and in the mid precordial leads(Katz-Wachtel phenomenon). The S waves in leads I and V6 are abnormally deep(i.e., abnormal rightward force), and the R wave in V1 (i.e., rightward and anteriorforce) is also abnormally large, suggesting RVH. The R waves in leads I and aVL (i.e.,leftward force) are also abnormally large. Therefore, this tracing shows BVH.
Conditions that are grouped together as ventricular conduction disturbances haveabnormal prolongation of the QRS duration in common. Ventricular conductiondisturbances include the following:1. Bundle branch block, right and left2. Preexcitation (e.g., WPW-type preexcitation)3. Intraventricular blockIn bundle branch blocks (and ventricular rhythms), the prolongation is in the terminalportion of the QRS complex (i.e., “terminal slurring”). In preexcitation, theprolongation is in the initial portion of the QRS complex (i.e., “initial slurring”),producing “delta” waves. In intraventricular block, the prolongation is throughout theduration of the QRS complex ( Fig. 3-19 ). Normal QRS duration varies with age; it isshorter in infants than in older children or adults . In adults, a QRS duration greaterthan 0.10 second is required for diagnosis of bundle branch block or ventricularconduction disturbance. In infants, a QRS duration of 0.08 second meets therequirement for bundle branch block.By far the most commonly encountered form of ventricular conduction disturbance isRBBB. Although uncommon, WPW preexcitation is a well-defined entity that deservesa brief description. LBBB is extremely rare in children, although it is common inadults with ischemic and hypertensive heart disease.Intraventricular block is associated with metabolic disorders and diffuse myocardialdiseases.
In RBBB, delayed conduction through the right bundle branch prolongs the time requiredfor a depolarization of the RV. When the LV is completely depolarized, RVdepolarization is still in progress.This produces prolongation of the QRS duration, involving the terminal portion of the QRScomplex, called terminal slurring and the slurring is directed to the right andanteriorly because the RV is located rightward and anteriorly in relation to the LV.In RBBB (and other ventricular conduction disturbances), asynchronous depolarization ofthe opposing electromotive forces may produce a lesser degree of cancellation of theopposing forces and thus result in greater manifest potentials for both ventricles.Consequently, abnormally large voltages for both RV and LV may result even in theabsence of ventricular hypertrophy. Therefore, the diagnosis of ventricularhypertrophy in the presence of bundle branch block (or WPW preexcitation orintraventricular block) is insecure.Criteria for Right Bundle Branch Block1. RAD, at least for the terminal portion of the QRS complex (the initialQRS force is normal)2. QRS duration longer than the upper limit of normal for the patientsage .3. Terminal slurring of the QRS complex that is directed to the right andusually, but not always, anteriorly:a. Wide and slurred S waves in leads I, V5, and V6b. Terminal, slurred R in aVR and the right precordial leads (V4R, V1, andV2)4. ST-segment shift and T-wave inversion are common in adults but not inchildren
Figure 3-20 is an example of RBBB. The QRS duration is increased (0.11 second), indicating aventricular conduction disturbance. There is slurring of the terminal portion of the QRScomplex, indicating a bundle branch block, and the slurring is directed to the right (slurred Swaves in leads I and V6 and slurred R waves in aVR) and anteriorly (slurred R waves in V4R andV1), satisfying the criteria for RBBB. Although the S waves in leads I, V5, and V6 are abnormallydeep and the R/S ratio in V1 is abnormally large, it cannot be interpreted as RVH in thepresence of RBBB.
Two pediatric conditions commonly associated with RBBB are ASD andconduction disturbances after open heart surgery involving rightventriculotomy. Other congenital heart defects often associated with RBBBinclude Ebsteins anomaly, COA in infants younger than 6 months, ECD, andPAPVR; it is also occasionally seen in normal children. Rarely, RBBB is seenin myocardial diseases (cardiomyopathy, myocarditis), muscle diseases(Duchennes muscular dystrophy, myotonic dystrophy), and Brugadasyndrome.In ASD, the prolonged QRS duration is the result of a longer pathway through adilated RV rather than an actual block in the right bundle. Rightventriculotomy for repair of VSD or tetralogy of Fallot disrupts the RVsubendocardial Purkinje network and causes prolongation of the QRSduration without necessarily injuring the main right bundle, although thelatter may occasionally be disrupted.Some pediatricians are concerned with the rsR pattern in V1. Although it isunusual to see this in adults, the rsR pattern in V1 not only is normal but isexpected to be present in infants and small children, provided that the QRSduration is not prolonged and the voltage of the primary or secondary Rwaves is not abnormally large. This is because the terminal QRS vector isnormally more rightward and anterior in infants and children than adults.
LEFT BUNDLE BRANCH BLOCKLBBB is extremely rare in children. In LBBB, the duration of the QRS complex isprolonged for age and the slurred portion of the QRS complex is directedleftward and posteriorly. A Q wave is absent in V6. A prominent QS pattern isseen in V1 and a tall R wave is seen in V6. LBBB in children is associatedwith cardiac disease or surgery in the LV outflow tract, septalmyomectomy, and replacement of the aortic valve. Otherconditions rarely associated with LBBB include LVH, progressiveconduction system disease, myocarditis, cardiomyopathy,myocardial infarction, and aortic valve endocarditis.LBBB alone may rarely progress to complete heart block and sudden death, butthe prognosis is more dependent on associated disease than on the LBBBitself.INTRAVENTRICULAR BLOCKIn intraventricular block, the prolongation is throughout the duration of the QRScomplex (see Fig. 3-19D ). This usually suggests serious conditions such asmetabolic disorders (e.g., hyperkalemia), diffuse myocardialdiseases (e.g., myocardial fibrosis, systemic diseases withmyocardial involvement), severe hypoxia, myocardial ischemia,or drug toxicity (quinidine or procainamide).
WPW preexcitation results from an anomalous conduction pathway (i.e., bundle of Kent)between the atrium and the ventricle, bypassing the normal delay of conduction inthe AV node. The premature depolarization of a ventricle produces a delta wave andresults in prolongation of the QRS duration .Criteria for Wolff-Parkinson-White Syndrome1. Short PR interval, less than the lower limit of normal for the patientsage.2. Delta wave (initial slurring of the QRS complex)3. Wide QRS duration beyond the upper limit of normalPatients with WPW preexcitation are susceptible to attacks of paroxysmalsupraventricular tachycardia (SVT) When there is a history of SVT, the diagnosis ofWPW syndrome is justified.WPW preexcitation may mimic other ECG abnormalities such as ventricular hypertrophy,RBBB, or myocardial disorders. In the presence of preexcitation, the diagnosis ofventricular hypertrophy cannot be safely made.The most striking abnormalities are a short PR interval (0.08 second) and a wide QRSduration (0.11 second). There are delta waves in most of the leads. Some deltawaves are negative, as seen in leads III, aVR, V4R, and V1. The ST segments and Twaves are shifted in the opposite direction of the QRS vector, resulting in a wideQRS-T angle. The leftward voltages are abnormally large, but the diagnosis of LVHcannot safely be made in the presence of WPW preexcitation.
Two other forms of preexcitation can also result in extreme tachycardia.1. Lown-Ganong-Levine syndrome is characterized by a short PR interval and normal QRSduration. inthis condition, James fibers (which connect the atrium and the bundle of His) bypass the upper AVnode and produce a short PR interval, but the ventricles are depolarized normally through the His-Purkinje system. When there is no history of SVT, the ECG tracing should simply be read asshowing a short PR interval rather than Lown-Ganong-Levine syndrome.2. Mahaim-type preexcitation syndrome is characterized by a normal PR interval andlong QRSduration with a delta wave. There is an abnormal Mahaim fiber that connects the AV node and oneof the ventricles, bypassing the bundle of His, and “short-circuits” into the ventricle.
Two common ECG abnormalities in children, ventricular hypertrophy andventricular conduction disturbances, are not always easy todistinguish; both arise with increased QRS amplitudes. The followingapproach may aid in the correct diagnosis of these conditions ( Fig. 3-22 ). An accurate measurement of the QRS duration is essential.1. When the QRS duration is normal, normal QRS voltagesindicate a normal ECG. Increased QRS voltages indicateventricular hypertrophy.2. When the QRS duration is clearly prolonged, a ventricularconduction disturbance is present whether the QRS voltagesare normal or increased. Additional diagnosis of ventricularhypertrophy should not be made.3. When the QRS duration is borderline prolonged,distinguishing these two conditions is difficult.Normal QRS voltages favor a normal ECG or a mild (right or leftventricular) conduction disturbance. An increased QRS voltage favorsventricular hypertrophy.
ECG changes involving the ST segment and the T wave are common in adults butrelatively rare in children. This is because of a high incidence of ischemic heartdisease, bundle branch block, myocardial infarction, and other myocardial disordersin adults. Some ST-segment changes are normal (nonpathologic) and others areabnormal (pathologic).NONPATHOLOGIC ST-SEGMENT SHIFTNot all ST-segment shifts are abnormal. Slight shift of the ST segment is common innormal children. Elevation or depression of up to 1 mm in the limb leads and up to 2mm in the precordial leads is within normal limits. Two common types ofnonpathologic ST-segment shifts are J-depression and early repolarization. The Tvector remains normal in these conditions. J-Depression. J-depression is a shift ofthe junction between the QRS complex and the ST segment (J-point) withoutsustained ST segment depression ( Fig. 3-23 A ). The J-depression is seen moreoften in the precordial leads than in the limb leads .
Early Repolarization.In early repolarization, all leads with upright T waves have elevated ST segments, and leads withinverted T waves have depressed ST segments (see Fig. 3-24 ). The T vector remains normal. Thiscondition, seen in healthy adolescents and young adults, resembles the ST-segment shift seen in acutepericarditis; in the former, the ST segment is stable, and in the latter, the ST segment returns to theisoelectric line.
PATHOLOGIC ST-SEGMENT SHIFTAbnormal shifts of the ST segment are often accompanied by T-wave inversion. A pathologic ST-segmentshift assumes one of the following forms:1. Downward slant followed by a diphasic or inverted T wave (see Fig. 3-23B )2. Horizontal elevation or depression sustained for more than 0.08 second (see Fig. 3-23C )Pathologic ST-segment shifts are seen in left or right ventricular hypertrophy with strain (discussed underventricular hypertrophy); digitalis effect; pericarditis, including postoperative state; myocarditis ;myocardial infarction; and some electrolyte disturbances (hypokalemia and hyperkalemia).T-WAVE CHANGEST-wave changes are usually associated with the conditions manifesting with pathologic ST-segment shift.Twave changes with or without ST-segment shift are also seen with bundle branch block andventricular arrhythmias. Pericarditis.The ECG changes seen in pericarditis are the result of subepicardial myocardial damage or pericardialeffusion and consist of the following:1. Pericardial effusion may produce low QRS voltages (QRS voltages <5 mm in every oneof the limbleads).2. Subepicardial myocardial damage produces the following time-dependent changes inthe ST segmentand T wave ( Fig. 3-25 ):a. ST-segment elevation occurs in the leads representing the left ventricle.b. The ST-segment shift returns to normal within 2 to 3 days.c. T-wave inversion (with isoelectric ST segment) occurs 2 to 4 weeks after the onset ofpericarditis.