Electrocardiography in Clinical Practice W Todd Gray DO

Electrocardiography in Clinical Practice W. Todd Gray, DO Cardiology Fellow Plaza Medical Center

Objectives o Systematically teach concepts of clinical electrocardiography n Normal and abnormal electrocardiograms in adults o n The Cardiac Arrhythmias o n Lectures 15 -21 Drug Effects, Electrolyte Imbalance, and Other Miscellaneous Conditions o n Lectures 1 -14 Lectures 22 -27 Traumatic Heart Disease, Cardiac Transplantation, Artificial Electronic Pacemakers, and Ambulatory Electrocardiogram o Lectures 28 -30

Objectives o o o Simultaneously read and discuss 10 EKG tracings weekly Correlate various EKG abnormalities with specific disease states Emphasize the value and limitations of the EKG, the differential diagnosis of EKG findings, and their correlation with the clinical and anatomic data

Objectives o Evaluate progress by examination at the end of each set of lectures: n n o Exam I: Lectures 1 -14 II: Lectures 15 -21 III: Lectures 22 -27 IV: Lectures 28 -30 A separate EKG interpretation exam will be given at course conclusion n Format to be announced

References o o Chou, Te-Chuan, Electrocardiography in Clinical Practice, Adult and Pediatric, 4 th Edition, W. B. Saunders Company, Philadelphia, 1996. Practice EKG tracings as provided from various sources weekly

The Normal Electrocardiogram W. Todd Gray, DO Cardiology Fellow Plaza Medical Center

P wave o Atrial activation begins in the SA node n n Spreads in radial fashion to depolarize the right atrium, interatrial septum, and then the left atrium (Fig. 1) The last area of the left atrium to be activated is the tip of the left atrial appendage n (Posterioinferior left atrium beneath the left inferior pulmonary vein)

Figure 1

P wave o Three pathways containing Perkinje fibers have been identified that connect the SA node to the AV node: n n n o Anterior internodal pathway Middle internodal pathway Posterior internodal pathway An interatrial pathway also exists n Bachmann’s bundle o Connects right and left atria

P wave o Early part of P wave: n o Late part of P wave: n o Represents electrical potential generated by the right atrium Represents electrical potential generated by left atrium Midportion of P wave: n Represents electrical potential generated by both atria and the interatrial septum

P wave o Normal Duration: n Varies from 0. 08 -0. 11 second o o Practically, P wave duration> 0. 11 secs is abnormal Normal Axis: n Directed inferiorly and leftward, correlating with the general direction of the spread of atrial excitation n Varies from 0 -75 degrees o o o Always upright in leads I and II Always inverted in a. VR May be upright, diphasic, or inverted in III n o If diphasic, initial deflection is positive and second component is negative Often diphasic in V 1 and V 2 n With positive-negative configuration

P wave o Normal amplitude: n n Seldom exceeds 0. 25 m. V normally in limb leads In precordial leads, positive component is normally less than 0. 15 m. V o In lead V 1, the begative deflection is normally < 0. 1 m. V n If the area of the negative component of the P wave in V 1 > 1 small square, it is considered abnormal

PR Interval o Measured from beginning of P wave to beginning of QRS complex n n Represents interval between the onset of atrial depolarization and onset of ventricular depolarization In normal AV conduction, it involves the time required for the activation impulse to advance from atria through the AV node, bundle of His, bundle branches, and the Purkinje fibers until ventricular myocardium begins to depolarize (Fig. 2) o Does not include duration of conduction from SA node proper to the right atrium

Figure 2

PR Interval o Normal PR Interval: n Adults: 0. 12 -0. 20 seconds o o n Generally shorter in children, longer in older persons May become shorter as sinus rate increases Should be taken from lead with the largest and widest P wave and longest QRS duration o Such selection avoids inaccuracies incurred by using leads in which the early part of the P wave or QRS complex is isoelectric

PR Interval o Most of the AV conduction time is consumed by impulse conduction proximal to the His bundle n Normal AH interval= 50 -130 msec o n o Time between intracavitary potential recorded from lower part of right atrium and His bundle spikes Normal HV interval= 35 -55 msec Longer AH interval is result of slower conduction through AV node

PR Segment o Horizontal line between the end of the P wave and the beginning of the QRS complex n Duration depends on the duration of the P wave as well as the impulse conduction through the AV junction n Usually isoelectric, however it is often displaced in a direction opposite to the polarity of the P wave o o Depressed in most of the conventional leads except a. VR. Displacement is mainly due to atrial replarization Normally depression < 0. 8 mm, elevation < 0. 5 mm Taller P waves = greater PR changes; smaller P waves= lesser PR changes

QRS Complex o Represents resultant electrical forces generated from ventricular depolarization (Fig. 3) n n n Begins at the middle third of the left interventricular septal surface Spreads in a rightward direction Right ventricle begins to depolarize shortly after the initiation of left ventricular activation o Starts at the right septal surface near the base of the anterior papillary muscle and spreads leftward

Figure 3

QRS Complex o Soon after septal activation, the impulse arrives at most of the subendocardial layer of the myocardium of the apical and free wall of both ventricles through the Perkinje network and spreads in all directions n Impulse spreads in endocardial to epicardial direction

QRS Complex o o o Basal portion of septum and the posterobasal portion of the free wall of the LV are last areas of depolarization LV contributes most of the QRS forces due to larger muscle mass The polarity and amplitude of the QRS complex in the various leads are determined by the relation between these vectors and the lead axes

QRS Complex o o QRS duration represents duration of ventricular activation Should be measured from lead with widest QRS complex n o Traditionally measured from the limb leads, but V 1 or V 2 may have the widest complex Normal QRS varies between 0. 060. 11 second

QRS Complex o QRS Axis: n n Represents direction of the mean QRS vector in the frontal plane Determined using hexaxial reference system derived from the Einthoven equilateral triangle (Fig. 4) o Two methods: n n Find lead with isoelectric complex QRS is perpendicular to this lead, with positive terminus pointing toward limb lead with largest net positive deflection

QRS Complex n n o Results obtained from these two methods are not necessarily equal n o Use algebraic sum of the deflections in 2 leads (usually I and III, or I and a. VF) Plot out axis Why? § The electrical axes of the limb leads form a scalene triangle, not an equlateral triangle Normal QRS axis is -30 to 105 degrees n n n Axis is usually shifted leftward with age In individuals < 30, axis is seldom superior to 0 o Normal 0 -105 degrees In individuals > 40, axis is seldom to right of 90 o Normal -30 -90 degrees

Figure 4

QRS Complex o There is an association between QRS axis and body weight n n o Thinner persons tend to have more vertical axes (toward 90, or rightward) Obese persons tend to have more horizontal axes (toward 0, or leftward) There is no significant gender difference in the axis

QRS Complex o Morphology and amplitude are affected by constitutional variables: n n o Advancing age: amplitude decreases Men > women Blacks > whites Thin > obese In limb leads, morphology depends on the orientation and amplitude of the QRS vectors in the frontal plane (on the lead axis in question)

QRS Complex o Lead I: usually records a dominant R wave n o o o In younger subjects with more rightward axis, R/S ratio < 1 may be seen Lead II: invariably has prominent R wave since mean vector is always toward II if QRS axis is normal Lead a. VR: always records negative deflection Lead III: variable n Why?

Q wave o Q wave is inscribed in a lead when the initial QRS vectors are directed away from the positive electrode n n n When QRS is more vertical, Q waves are more likely to be seen in inferior leads When QRS is more horizontal, Q waves are more likely to be seen in I and a. VL. Q waves occur in one or more of inferior leads in more than half of normal adults o Seen in less than half in I and a. VL

Q wave o Duration is of considerable importance in the diagnosis of myocardial infarction n n With exception to leads III and a. VR, Q waves in limb leads are normally < 0. 03 sec Duration in lead III < 0. 05 sec o This is the lead from which most of the erroneous diagnoses of MI are made

Q wave o Amplitude in limb leads is small n n n Q wave < 4 mm in all limb leads except lead III (< 5 mm) Depth of Q wave < 25% of R wave in limb leads (lead III exception) Normal Q wave tends to be more prominent in younger individuals (Fig. 5) o Example: In lead I, Q < 1. 5 mm except in those < 30 years

Figure 5

R wave o R wave amplitude in any lead depends on the direction of the maximum QRS vector n Lead whose axis is most parallel to and same polarity records the tallest R wave o Normal values: n n n I < 15 mm a. VL < 10 mm II, III, a. VF < 19 mm § Larger amplitudes may be seen in younger subjects occasionally

S wave o Most prominent in a. VR n n o This lead has opposite polarity to main QRS forces Amplitude up to 16 mm may be seen in younger subjects o Large S wave may be seen in III and a. VL, depending on QRS axis o Usually < 9 mm o I, II, a. VF < 5 mm If amplitude of entire QRS complex < 5 mm in all limb leads, voltage is considered abnormally low

Precordial Leads o o QRS complexes represent projections of QRS vectors in the horizontal plane (Figure 6) As dominant left ventricular forces are directed leftward: n n Right precordial leads record negative deflections Left precordial leads record positive deflections

Figure 6

Precordial Leads o The early activation of the anterior wall and late excitation of the posterior wall explain initial positivity and terminal negativity in leads with axes directed anteriorly (V 1 -V 4) n n R wave progressively increases in amplitude from V 1 -V 6 S wave progressively decreases from right to left precordial leads (V 1 -V 6)

Transitional Zone o Represents the location of the lead having equal positive and negative deflections n Related to the direction of the QRS axis in the horizontal plane o Normally located between V 2 -V 4 n o If located to right of V 2: n o Lead V 3 most common Counterclockwise rotation If located to left of V 5: n Clockwise rotation

Q wave o Small Q waves are seen in left precordial leads in more than 75 % of normal subjects n Referred to as septal Q waves o o Generated from left to right septal activation Seen most frequently in lead V 6 n o Less seen in V 5 -V 4 Present more often in younger subjects n n Duration < 0. 03 second Amplitude < 4 mm

R wave o Increases in amplitude from right toward left precordium n Normal limits: o o V 1 < 6 mm V 5 -V 6 < 25 mm

S wave o o Deepest in right precordial leads Decreases in amplitude as the left precordium is approached Often absent in V 5 -V 6 S wave < 3 mm in V 1 abnormal: n Suggestive of RVH or posterior MI

Intrinsicoid Deflection o Represents the moment when the epicardial muscle lying under the electrode becomes depolarized n Beginning of the abrupt downstroke after the R wave reaches its peak o Time of onset measured from beginning of QRS to point of abrupt downstroke n n o Right precordial leads < 0. 035 sec Left precordial leads < 0. 045 sec Used mostly in diagnosis of ventricular hypertrophy and BBB when onset is delayed

ST Segment o Segment between end of QRS complex (J point or ST junction) and beginning of the T wave n Represents a state of unchanged polarization between the end of depolarization and the beginning of repolarization o Stage when terminal depolarization and starting repolarization are superimposed and cancel each other

ST Segment o Most important information regarding ST segment is presence or absence and degree of displacement from isoelectric line n As a rule, TP segment is used as reference baseline n n n Except with rapid rates, when PR segment is used Limb leads- elevation/depression<1 mm Precordial leads- elevation sometimes seen and normal in V 2 -V 3 (< 2 mm). Rarely > 1 mm in V 5 V 6 n Any ST depression in precordial leads is abnormal, since normal vector in horizontal plane is anterior and leftward

T Wave o Represents potential for ventricular repolarization n Proceeds in general direction of ventricular excitation o o Polarity of resultant T wave is similar to that of the QRS vector Always upright in I, II, V 5 -V 6 Always inverted in a. VR When inverted in 2 or more of right precordial leads, referred to as persistent juvenile pattern (Fig 7)

Figure 7

T wave o Limb leads: n Tallest in lead II o o o Normally < 6 mm in all limb leads Should never be < 0. 5 mm Precordial leads: n n n Tallest in V 2 -V 3 (average 6 mm) Smaller in left precordial leads See Figure 8

Figure 8

T wave o Normally asymmetrical n n n First half has more gradual slope than second half First portion has upward concavity if T-wave is upright and downward concavity if T-wave is inverted In right precordial leads, if T-wave is diphasic, the first portion is upright and second portion inverted o Negative-positive diphasic T-wave is abnormal in leads V 1 -V 3

QT Interval o o Represents duration of ventricular electrical systole Measured from beginning of QRS complex to end of the T wave n n Lead with a large T wave and distinct termination is used Leads V 2 -V 3 are usually best for this specific measurement

QT Interval o Varies with heart rate (Table 1) n n o o Lengthens as heart rate decreases Shortens as heart rate increases Increases slightly with age Diurnal variation of QTI has been documented n Longer during sleep than during waking hours o Sleep prolongs QTI by 18 msec with HR=60 o Sleep prolongs QTI by 21 msec with HR=50

Table 1

QT Interval Normal QTI= K x square root of RR interval K= 0. 37 for men k= 0. 40 for women QT corrected= measured QTI divided by square root of RR interval

QT Interval o Normals: n 0. 40 second for HR=70 o For every 10 -beat increase or decrease of the rate, 0. 02 second is deducted or added to the QTI, respectively

U wave o Small, low-frequency deflection that appears after the T wave n n Genesis is controversial o Afterpotentials of ventricular myocardium o Repolarization of the Perkinje fibers Amplitude is proportional to T wave o Usually 5 -25% of T wave voltage o Largest in leads V 2 -V 3 o Prominent during slower heart rates o Initial portion is normally steeper than terminal portion

Normal Variants o S 1 S 2 S 3 Pattern: (Figure 9) n n n Terminal negative deflection is present in QRS complex of all standard limb leads in a significant number of normal individuals o S waves are recorded when the terminal QRS vectors are originated from the outflow tract of the right ventricle or the posterobasal septum Also seen in patients with RVH or pulmonary emphysema Must be distinguished between abnormal left axis deviation o S 3 > S 2

Figure 9

Normal Variants o RSR’ Pattern in V 1: n RSR’ pattern in V 1 with QRS duration < 0. 12 sec is found in 2. 4% of normals o n Secondary R wave has been attributed to late activation of the crista supraventricularis of the right ventricular outflow tract Also seen in those with organic heart disease, including those with RVH o In healthy persons, R’ is usually smaller than R wave

Normal Variants o Early Repolarization Syndrome: n n (Figs. 10 -11) Some degree of ST elevation is commonly seen in healthy individuals (V 2 -V 5) o Degree is minimal, but may be more pronounced o Attributed to early ventricular repolarization o Characteristics: n n Elevated takeoff of the ST segment at the J point Distinct notch or slur on the downslope of the R wave Upward concavity of the ST segment Symmetrically limbed T waves that are often of large amplitude

Figure 10

Figure 11

Normal Variants o Early Repolarization Syndrome: n ST/T ratio can be used to distinguish early repolarization from acute pericarditis o If the ST/T ratio is equal to or greater than 0. 25, the ST segment elevation is most likely due to acute pericarditis n Most reliable lead is V 6, but V 5, V 4, or I are also useful

Normal Variants o Poor R-Wave Progression in Right Precordial Leads: (Fig. 12) n In younger adults, small R waves may be present in V 1 -V 3 in the absence of cardiac or pulmonary disease

Figure 12

Normal Variants o Athlete Heart: n EKG of trained athletes often presents findings that are considered abnormal by usual standards o Physiological adaptations of the heart to the prolonged and intensive training may result in functional and structural changes that mimic those seen in organic heart disease n n Sinus bradycardia Sinus pauses First degree AV block Second degree AVB (Type I)

Normal Variants n Echocardiographic studies: o Increased left and right ventricular mass and cavity dimensions n o o Ventricular dilatation is seen mostly when the training is primarily isotonic rather than isometric Increased LV wall thickness LA enlargement

Normal Variants n EKG studies: o o Increased P wave amplitude with notching Increased QRS voltage n o o o May occur after only a few months of training, decreasing after deconditioning ST segment elevation (early repolarization) Hypertrophic T waves (precordial leads) Inverted or biphasic T waves n Asymmetric ventricular repolarization has been suggested etiology