88. Electrocardiogram

Introduction

• The electrocardiogram provides a graphical record of the heart’s electrical activity, reflecting the sequence of depolarization and repolarization. As a simple, noninvasive, and widely available investigation, it plays a crucial role in detecting arrhythmias, myocardial ischemia, conduction abnormalities, and electrolyte disturbances.
• The electrocardiogram is a graphical recording of the heart’s electrical activity obtained from electrodes placed on the body surface.
• Electrocardiography is the technique used to record the electrocardiogram.
• An electrocardiograph detects, amplifies, and records voltage changes generated by cardiac electrical activity over time.
• Body fluids act as effective conductors of electricity; therefore, electrical signals generated by the heart can be detected at the skin surface.
• The recorded tracing represents the summated electrical activity of the myocardium during the cardiac cycle.
• Electrocardiography is a simple, noninvasive, and widely used diagnostic investigation.
• Interpretation of an electrocardiogram should always be correlated with clinical findings and other relevant investigations.
• A normal electrocardiogram does not completely exclude structural heart disease.
• Conversely, minor electrocardiographic abnormalities may occasionally be present in healthy individuals.

Uses of ECG:

• It helps assess the electrical axis and anatomical orientation of the heart.
• It provides indirect evidence of atrial or ventricular enlargement.
• It is valuable for detecting cardiac arrhythmias and conduction abnormalities.
• It assists in identifying myocardial ischemia and monitoring its progression.
• It helps localize and assess the extent of myocardial infarction.
• It can reveal characteristic changes associated with electrolyte disturbances, particularly abnormalities of potassium and calcium levels.
• It is useful for monitoring the cardiac effects of certain medications, including digoxin.
• It aids in evaluating the function and performance of artificial cardiac pacemakers.
• Serial electrocardiograms are often useful for monitoring disease progression and response to treatment.

Technical Aspects  

• Modern electrocardiography uses electronic amplifiers to detect and record the heart’s electrical activity with high accuracy.
• The electrocardiogram is recorded on standardized graph paper that contains horizontal and vertical lines spaced 1 millimeter apart.
• The horizontal axis represents time, whereas the vertical axis represents voltage amplitude.
• Every five small squares form one large square measuring 5 × 5 millimeters.
• Electrocardiogram paper is heat-sensitive, and the tracing is produced by a thermal recording system.
• The standard recording speed is 25 millimeters per second.
• At this speed, one small square corresponds to 0.04 second and one large square corresponds to 0.20 second.
• In standard calibration, a voltage of 1 millivolt produces a vertical deflection of 10 millimeters.
• Accurate interpretation of wave duration and amplitude depends on proper paper speed and calibration.

ECG Leads

• An electrocardiographic lead represents a specific recording of electrical potential differences detected by electrodes.
• Leads provide information about the direction and magnitude of cardiac electrical activity.
• Leads are classified as direct or indirect leads.
• Direct leads are placed on the surface of the heart and are mainly used during certain cardiac procedures and research settings.
• Indirect leads record cardiac electrical activity from locations away from the heart, usually on the body surface.
• Routine clinical electrocardiography uses indirect leads.
• Indirect leads include limb leads, chest leads, and specialized leads such as esophageal leads.
• A standard 12-lead electrocardiogram is used for comprehensive cardiac assessment.
• Six limb leads evaluate electrical activity in the frontal plane.
• Six chest leads evaluate electrical activity in the transverse plane.
• These two perpendicular planes provide a three-dimensional assessment of cardiac electrical events and improve diagnostic accuracy.

Limb Leads

• Limb leads assess cardiac electrical activity in the frontal plane.
• They are classified into bipolar and unipolar limb leads.
• Bipolar leads record the electrical potential difference between two active electrodes placed at different sites.
• Unipolar leads record the potential difference between an exploring electrode and a reference electrode with near-zero potential.
• The reference potential is created by combining signals from multiple limb electrodes.

Bipolar Limb Leads

• Standard bipolar limb leads are designated as Leads I, II, and III.
• These leads provide information about the direction and magnitude of cardiac electrical activity in the frontal plane.
• Electrodes are attached to the right arm, left arm, and left leg.
• An additional electrode is placed on the right leg and serves as an electrical ground to minimize interference.
• The arrangement of the three recording electrodes forms Einthoven’s triangle, an imaginary triangle surrounding the heart.
• Lead I records the potential difference between the right arm (negative electrode) and the left arm (positive electrode).
• Lead II records the potential difference between the right arm (negative electrode) and the left leg (positive electrode).
• Lead III records the potential difference between the left arm (negative electrode) and the left leg (positive electrode).
• These three leads complement one another and provide a comprehensive frontal-plane view of cardiac electrical activity.
• Analysis of bipolar limb leads is important for evaluating cardiac rhythm, electrical axis, and conduction abnormalities.

Einthoven Triangle

• Einthoven triangle is an imaginary equilateral triangle used to analyze cardiac electrical activity in the frontal plane. Its vertices correspond to the right arm, left arm, and left leg electrode positions.
• The heart is considered to lie approximately at the center of the triangle.
• Each side of the triangle represents the axis of one bipolar limb lead: Leads I, II, and III.
• Because the body acts as a volume conductor, electrical activity recorded from the limbs reflects cardiac electrical events.
• The orientation of the triangle helps determine the direction of electrical forces generated by the heart.
• A depolarization wave moving toward the positive electrode produces an upward deflection on the electrocardiogram.
• The triangle provides a useful framework for understanding limb lead recordings. It is also important in determining the mean electrical axis of ventricular depolarization.

Unipolar Limb Leads

• Unipolar limb leads use one exploring electrode and one reference electrode.
• The exploring electrode records cardiac electrical activity from a specific limb.
• The reference electrode is created by combining electrical inputs from the other limb electrodes through resistive connections.
• Three augmented unipolar limb leads are used: aVR, aVL, and aVF.
• The prefix “a” indicates augmented, meaning that the recorded voltage is amplified compared with the corresponding unaugmented lead.
• The letter “V” denotes a unipolar voltage recording.
• The letters R, L, and F indicate the right arm, left arm, and left leg, respectively.
• aVR records electrical activity from the right arm, which serves as the positive exploring electrode.
• The reference for aVR is formed by combining signals from the left arm and left leg.
• aVL records electrical activity from the left arm as the positive electrode. Its reference is formed by the right arm and left leg electrodes.
• aVF records electrical activity from the left leg as the positive electrode. Its reference is formed by the right and left arm electrodes.
• Augmented limb leads provide additional frontal-plane views and complement the bipolar limb leads for comprehensive electrocardiographic assessment.

Chest Leads

• Chest leads, also called precordial leads, record cardiac electrical activity in the transverse plane.
• They provide detailed information about the anterior, lateral, and posterior aspects of the heart.
• Chest leads are classified into unipolar and bipolar chest leads.

Unipolar Chest Leads

• Six standard precordial leads, V1 to V6, are routinely used in clinical electrocardiography.
• These leads use a positive exploring electrode placed on the chest wall.
• The reference electrode is formed by combining signals from the right arm, left arm, and left leg, creating a near-zero potential known as Wilson’s central terminal.
• The right-leg electrode acts as a ground to reduce electrical interference.
  • V1: Fourth right intercostal space at the right sternal border.
  • V2: Fourth left intercostal space at the left sternal border.
  • V3: Midway between V2 and V4.
  • V4: Fifth left intercostal space at the midclavicular line.
  • V5: Same horizontal level as V4 at the anterior axillary line.
  • V6: Same horizontal level as V4 at the midaxillary line.
  • V7: Fifth intercostal space at the posterior axillary line.
  • V8: Fifth intercostal space at the midscapular line.
  • V9: Fifth intercostal space at the left paraspinal region.
• These additional leads are useful for detecting posterior wall myocardial infarction and other selected clinical conditions.

Bipolar Chest Leads

• Bipolar chest leads record the potential difference between a chest electrode and an extremity electrode.
• They are rarely used in modern electrocardiography because limb potentials can alter the recorded pattern.
• A specialized bipolar configuration, the Lewis lead, may be used to enhance atrial electrical activity. This lead is particularly helpful in the evaluation of certain atrial arrhythmias.

Esophageal Leads

• Esophageal leads are obtained using an electrode mounted on a catheter placed within the esophagus near the heart.
• Because the esophagus lies close to cardiac structures, these leads provide detailed recordings of atrial electrical activity.
• Lead designations such as E18 or E20 indicate the distance of the electrode from the incisor teeth in centimeters.
• Positions between approximately 15–25 centimeters primarily record right atrial activity.
• Positions between 25–35 centimeters record activity near the atrioventricular groove.
• Positions between 40–50 centimeters provide information from the posterior surface of the left ventricle.
• Esophageal leads are particularly useful in evaluating atrial arrhythmias and specialized electrophysiological studies.

Normal ECG

A normal electrocardiogram consists of characteristic waves, segments, and intervals that reflect the heart’s electrical activity.

ECG Waves

• Waves are positive or negative deflections from the isoelectric baseline.
• The principal waveforms are the P wave, QRS complex, T wave, and occasionally the U wave.

P Wave

• The P wave is normally the first positive deflection seen on the electrocardiogram. It represents depolarization of the atria before atrial contraction.
• Normal atrial activation begins in the sinoatrial node and spreads through both atria.
• Analysis of the P wave helps assess atrial rhythm and atrial enlargement.

QRS Complex

• The QRS complex represents ventricular depolarization and precedes ventricular contraction.
• The Q wave is the first negative deflection of the complex, when present.
• The R wave is the first positive deflection.
• The S wave is the negative deflection that follows the R wave.
• The duration and morphology of the QRS complex provide important information about ventricular conduction.

T Wave

• The T wave represents ventricular repolarization. It is normally upright in most electrocardiographic leads.

U Wave

• The U wave is a small positive deflection that may appear after the T wave. It is not consistently present in all individuals. Its exact origin remains uncertain, but delayed repolarization of Purkinje fibers and papillary muscles is considered a likely contributor.

ECG Segments

• Segments are portions of the electrocardiogram that lie on or near the isoelectric baseline between waves.
• The two principal segments are the PR segment and the ST segment.

PR Segment

• The PR segment extends from the end of the P wave to the beginning of the QRS complex. It primarily reflects impulse conduction through the atrioventricular node and His–Purkinje system.

ST Segment

• The ST segment extends from the end of the QRS complex to the beginning of the T wave.
• The junction between the QRS complex and ST segment is called the J point.
• Elevation or depression of the ST segment may indicate myocardial injury, ischemia, or infarction.

ECG Intervals

• Intervals are measured portions of the electrocardiogram that usually include one or more waves and segments.
• They provide important information about cardiac conduction and electrical activity.

PR Interval

• The PR interval extends from the beginning of the P wave to the beginning of the QRS complex. Its normal duration is 0.12–0.20 second.
• The interval may shorten slightly as heart rate increases. It represents atrial depolarization and conduction through the atrioventricular node and His–Purkinje system.

QRS Interval (QRS Duration)

• The QRS duration is measured from the beginning of the Q wave, or R wave if no Q wave is present, to the J point.
• The normal duration is 0.08–0.10 second. It represents ventricular depolarization.
• Atrial repolarization occurs simultaneously but is usually obscured by the QRS complex.

QT Interval

• The QT interval extends from the beginning of the QRS complex to the end of the T wave. It includes ventricular depolarization and repolarization.
• The normal duration is approximately 0.40–0.44 second, depending on heart rate. It corresponds to the period of ventricular electrical activity, often termed electrical systole.

ST Interval

• The ST interval extends from the J point to the end of the T wave. Its average duration is approximately 0.32 second. It mainly reflects ventricular repolarization.

PP Interval

• The PP interval is measured between two successive P waves. It is used to determine atrial rate and assess atrial rhythm regularity.

RR Interval

• The RR interval is measured between the peaks of two consecutive R waves. It is used to calculate ventricular rate, commonly referred to as heart rate.
• Variations in the RR interval can provide information about cardiac rhythm abnormalities.

Physiological Basis of ECG

• The electrocardiogram records voltage differences generated by electrical activity within the heart.
• These voltage differences arise from changing electrical dipoles produced during depolarization and repolarization of cardiac muscle.
• Electrocardiographic leads detect these electrical potentials at the body surface.
• The recorded tracing represents the summated electrical activity of the myocardium rather than the activity of individual cardiac cells.
• Understanding the concept of an electrical dipole is essential for interpreting the physiological basis of electrocardiographic waveforms.

Concept of a Dipole

• A dipole consists of two equal and opposite electrical charges separated by a small distance.
• Dipoles are fundamental to understanding the electrical basis of electrocardiography.
• An electrical field can be created when charged particles move within a conductive medium, such as a salt solution.
• When positive and negative electrodes are placed in such a medium, ions migrate toward electrodes of opposite charge.
• Positive ions move toward the negative electrode, whereas negative ions move toward the positive electrode. This movement establishes two poles and creates an electrical field between them.
• The region between the poles contains the greatest flow of electrical current. Together, the positive and negative poles form an electrical dipole.
• A dipole generates a measurable voltage difference within a conducting medium. This voltage can be detected by recording electrodes placed at different locations.
• The recorded voltage is greatest when the recording axis is aligned with the direction of the dipole.
• As the angle between the dipole and the recording axis increases, the measured voltage decreases.
• When the dipole is perpendicular to the recording axis, little or no voltage is recorded. Therefore, electrode position strongly influences the magnitude of the recorded signal.
• A dipole possesses both magnitude and direction and therefore behaves as an electrical vector.
• The magnitude of the vector reflects the strength of the electrical activity.
• The direction of the vector indicates the orientation of electrical charge movement.
• Both magnitude and direction determine the size and polarity of the recorded voltage.
• Electrocardiographic waveforms are produced by changing cardiac dipoles generated during depolarization and repolarization.
• Understanding dipole vectors is essential for interpreting electrocardiographic leads, electrical axis, and normal or abnormal patterns of cardiac electrical activity.

Heart as the Dipole

• The human body acts as a volume conductor, allowing cardiac electrical activity to spread to the body surface.
• During each heartbeat, numerous electrical dipoles are generated within the myocardium.
• The electrocardiogram records the resultant electrical activity produced by the combined effect of these dipoles. Therefore, the recorded signal represents the net electrical vector of the heart at a given moment.

How is the Dipole Created in the Heart?

• Cardiac muscle cells are electrically polarized at rest, with the cell interior negatively charged relative to the exterior.
• During depolarization, the electrical polarity reverses, making the cell interior positive and the exterior relatively negative.
• As excitation spreads through the heart, some regions become depolarized while adjacent regions remain at rest. This difference in electrical charge between activated and nonactivated regions creates an electrical dipole.
• The depolarized region acts as the negative pole on the external surface of the myocardium.
• The neighboring region that has not yet depolarized acts as the positive pole.
• As the wave of excitation advances, the position and orientation of the dipole continuously change. Thus, the heart behaves as a moving electrical generator during depolarization and repolarization.
• Electrical potentials generated by cardiac dipoles spread through body tissues and fluids.
• Electrodes placed on the skin detect these voltage differences.
• The electrocardiogram records changes in the magnitude and direction of the cardiac electrical vector over time.
• Each wave of the electrocardiogram reflects the movement of electrical activity through specific regions of the heart.
• Understanding the concept of the cardiac dipole is essential for interpreting electrocardiographic waveforms, electrical axis, and conduction abnormalities.

Why Has ECG Various Waveforms?

• The electrocardiogram displays different waveforms because the direction and magnitude of the heart’s electrical activity change continuously during the cardiac cycle.
• As depolarization and repolarization spread through the myocardium, multiple electrical dipoles are generated simultaneously.
• The electrocardiograph records the resultant or net electrical vector produced by these individual dipoles at any given moment.
• The recorded voltage depends on the relationship between the cardiac electrical vector and the recording lead.
• A positive deflection is recorded when the net electrical vector is directed toward the positive electrode of a lead.
• A negative deflection is recorded when the electrical vector moves away from the positive electrode.
• When electrical forces are balanced or perpendicular to the lead axis, little or no voltage is recorded, producing an isoelectric tracing.
• The amplitude of the recorded wave depends on the strength of the electrical vector and its alignment with the lead axis.
• Maximum voltage is recorded when the electrical vector is parallel to the lead axis.
• Minimal or zero voltage is recorded when the vector is perpendicular to the lead axis.
• The magnitude of the electrical vector is influenced by the amount of myocardium undergoing depolarization or repolarization.
• Larger masses of activated cardiac muscle generate larger electrical forces and greater waveform amplitudes.
• Different electrocardiographic leads view the heart from different angles. Therefore, the same cardiac event may appear as a positive, negative, or biphasic waveform in different leads.
• The sequence of atrial depolarization, ventricular depolarization, and ventricular repolarization produces the characteristic P wave, QRS complex, and T wave. Thus, variations in vector direction, vector magnitude, and lead orientation together create the diverse waveforms observed on a normal electrocardiogram.

P Wave

• The P wave represents atrial depolarization initiated by the sinoatrial node.
• The electrical impulse spreads through both atria and produces atrial excitation.
• The size of the atrial electrical vector depends on the amount of activated atrial muscle and the direction of depolarization.
• In Lead II, atrial depolarization is directed toward the positive electrode, producing an upward deflection. Therefore, the P wave is normally positive in Lead II.
• The P wave has a relatively small amplitude because the atrial muscle mass is much smaller than the ventricular muscle mass. Its smooth, rounded appearance reflects the gradual spread of depolarization through the atria.
• Once atrial depolarization is complete, the tracing returns to the isoelectric baseline.

PR Interval

• The PR interval extends from the beginning of the P wave to the beginning of the QRS complex. It includes both the P wave and the PR segment.
• The interval represents atrial depolarization and conduction through the atrioventricular node and His–Purkinje system.
• The PR segment is normally an isoelectric line.
• During this segment, the impulse travels slowly through the atrioventricular node and then enters the specialized ventricular conduction system.
• Electrical activity generated by these structures is too small to produce a detectable surface deflection. Therefore, the PR segment remains electrically silent on the electrocardiogram.

QRS Complex

• The QRS complex represents ventricular depolarization and precedes ventricular contraction. It is composed of three components: the Q wave, R wave, and S wave.
• The morphology of the QRS complex reflects the sequence and direction of ventricular activation.

Q Wave

• The earliest phase of ventricular depolarization occurs in the interventricular septum.
• Septal depolarization normally proceeds from left to right. This initial electrical vector is directed away from the positive electrode of Lead II, producing a small negative deflection. The resulting waveform is the Q wave.
• The Q wave is usually small because the septum contains relatively little muscle mass compared with the ventricular walls. Its brief duration reflects the rapid spread of excitation through the septum.
• In some normal electrocardiographic leads, a small physiological Q wave may be absent.

R Wave

• After septal activation, depolarization spreads rapidly through the ventricular myocardium.
• Excitation progresses from the endocardial surface toward the epicardial surface.
• Because the left ventricle contains the greatest muscle mass, the net electrical vector is directed predominantly toward the left and inferiorly.
• In Lead II, this produces a large positive deflection known as the R wave.
• The R wave is usually the largest waveform of the electrocardiogram. Its large amplitude reflects the substantial mass of ventricular muscle undergoing depolarization.
• The steep contour of the R wave indicates rapid ventricular activation through the His–Purkinje system.
• The tracing returns toward the baseline when most ventricular myocardium has been depolarized.

S Wave

• The final regions to depolarize include the posterobasal portion of the left ventricle and parts of the right ventricular outflow tract.
• The terminal electrical vector is directed away from the positive electrode of Lead II. This produces a small negative deflection called the S wave.
• The S wave is typically narrow and of lower amplitude than the R wave.
• The Q, R, and S waves together form the QRS complex.
• The duration, amplitude, and morphology of the complex provide important information about ventricular muscle mass and the speed of ventricular conduction.
• Abnormal changes may indicate conduction defects, ventricular hypertrophy, or myocardial injury.

ST Segment

• The ST segment begins at the end of the QRS complex (J point) and ends at the onset of the T wave. It is normally an isoelectric segment that lies close to the baseline.
• By this stage, ventricular depolarization is essentially complete, resulting in minimal net electrical voltage on the body surface.
• The ST segment corresponds to the early phase of ventricular repolarization and the plateau phase of the ventricular action potential.
• Elevation or depression of the ST segment may indicate myocardial ischemia, injury, or infarction.

T Wave

• The T wave represents ventricular repolarization.
• Repolarization restores the resting electrical state of ventricular muscle cells after depolarization.
• Although repolarization is electrically opposite to depolarization, the T wave is normally positive in many leads, including Lead II. This occurs because ventricular repolarization proceeds in a direction opposite to ventricular depolarization.
• Repolarization generally begins in the epicardial layers and progresses toward the endocardial layers.
• As epicardial cells repolarize first, the electrical polarity generated during repolarization produces a net vector directed similarly to the major depolarization vector.
• Consequently, a positive deflection is recorded on the electrocardiogram.
• The T wave is usually smooth, rounded, and broader than the waves of the QRS complex. Its longer duration reflects the relatively slower and less synchronized process of ventricular repolarization.
• Complete ventricular repolarization marks the end of the T wave.
• Delay in ventricular repolarization may prolong the QT interval, which can increase the risk of ventricular arrhythmias.
• The appearance of electrocardiographic waves depends on the relationship between the cardiac electrical vector and the recording lead.
• Different leads observe the same electrical activity from different angles. Therefore, waveforms may vary in polarity and amplitude among different leads.
• These variations help in assessing cardiac electrical axis, conduction pathways, and regional myocardial abnormalities.

Cardiac Dipoles as Vectors

• Cardiac electrical activity generates numerous dipoles that continuously change in direction and magnitude during the cardiac cycle.
• The combined effect of these dipoles forms a net cardiac vector, which can be analyzed using electrocardiographic leads.
• The direction and magnitude of the net vector determine the appearance of electrocardiographic waveforms.
• Limb leads and chest leads provide information about cardiac vectors from different anatomical planes.
• The six limb leads assess cardiac electrical activity in the frontal plane.
• These include the three bipolar limb leads (Leads I, II, and III) and the three augmented limb leads (aVR, aVL, and aVF).
• Lead I records electrical activity along a horizontal axis extending from the right arm toward the left arm.
• Lead II is oriented approximately 60 degrees from the right arm toward the left leg.
• Lead III is oriented approximately 120 degrees from the left arm toward the left leg.
• The augmented limb leads are positioned between these axes and provide additional views of cardiac electrical activity.
• Together, the six frontal-plane lead axes form the hexaxial reference system, which is widely used for electrical axis determination.
• An upward deflection occurs when the net cardiac vector is directed toward the positive pole of a lead.
• A downward deflection occurs when the vector is directed away from the positive pole and toward the negative pole.
• The magnitude of the recorded wave increases when the vector is closely aligned with the lead axis.
• A minimal or isoelectric deflection occurs when the vector is oriented perpendicular to the lead axis. Therefore, both vector direction and vector magnitude influence the shape and amplitude of electrocardiographic waves.
• Analysis of cardiac vectors helps determine the electrical axis of the heart.
• Vector interpretation assists in identifying chamber enlargement, conduction abnormalities, myocardial infarction, and other cardiac disorders.
• Understanding vector orientation is essential for accurate interpretation of normal and abnormal electrocardiograms.

Normal 12 Lead ECG

• A standard 12-lead electrocardiogram provides a comprehensive assessment of cardiac electrical activity from multiple viewpoints.
• Interpretation of each lead is based on the direction of the cardiac electrical vector relative to the lead axis.
• A positive deflection is recorded when electrical activity moves toward the positive electrode of a lead.
• A negative deflection is recorded when electrical activity moves away from the positive electrode.
• A biphasic or nearly isoelectric waveform occurs when the electrical vector is approximately perpendicular to the lead axis.
• Atrial depolarization normally proceeds from the sinoatrial node downward and toward the left side of the heart. Therefore, the P wave is usually positive in Lead II.
• The P wave is normally negative in lead aVR because atrial activation moves away from its positive electrode.
• Ventricular depolarization begins in the interventricular septum and spreads from left to right. This initial activation produces a small septal q wave in left-sided leads such as V5 and V6. It also produces a small septal r wave in right precordial leads, particularly V1.
• As ventricular activation progresses, the larger left ventricular muscle mass dominates the electrical vector.
• The mean depolarization vector is directed toward the left ventricle.
• Consequently, tall R waves are seen in leads V5 and V6.
• Deep S waves are commonly observed in leads V1 and V2.
• Leads between these regions, especially V3 and V4, show a transitional pattern with gradual changes in R-wave and S-wave amplitudes.
• The appearance of limb-lead QRS complexes varies with the anatomical orientation of the heart.
• In a more vertical heart, qR patterns are often prominent in Leads II, III, and aVF.
• In a more horizontal heart, qR patterns are more commonly observed in Leads I and aVL.
• The T wave generally follows the overall direction of the QRS complex. It is normally positive in most left-sided chest leads and commonly positive in V2.
• In lead V1, the T wave may be either positive or negative in healthy individuals.

Systematic Interpretation of ECG

• A systematic approach helps ensure accurate and complete electrocardiogram interpretation.
• Determine the heart rate and assess both atrial and ventricular rates.
• Evaluate whether the cardiac rhythm is regular or irregular.
• Assess the mean electrical axis of the heart.
• Examine the morphology, amplitude, and duration of P waves.
• Verify whether each P wave is consistently related to a QRS complex.
• Measure the PR interval and assess atrioventricular conduction.
• Evaluate the duration, amplitude, and configuration of the QRS complex.
• Determine whether the ST segment remains isoelectric or shows elevation or depression.
• Examine the T waves for normal polarity, shape, and amplitude.
• Measure the QT interval and assess the corrected QT interval according to heart rate.
• Integrating all these findings improves the detection of rhythm disturbances, conduction defects, myocardial ischemia, and other cardiac abnormalities.

Heart Rate

• Heart rate assessment is the first step in electrocardiogram interpretation.
• Both atrial and ventricular rates should be evaluated.
• In routine practice, heart rate usually refers to the ventricular rate.
• At a paper speed of 25 millimeters per second, atrial rate can be calculated by dividing 1500 by the PP interval measured in millimeters.
• Ventricular rate can be calculated by dividing 1500 by the RR interval measured in millimeters.
• In normal sinus rhythm, the PP and RR intervals are usually equal.
• Atrial and ventricular rates may differ in certain conduction abnormalities and arrhythmias.
• When RR intervals are irregular, such as in atrial fibrillation, an average ventricular rate should be calculated.
• Count the number of QRS complexes in a 5-second rhythm strip (125 millimeters at a paper speed of 25 millimeters per second).
• Multiply this number by 12 to estimate the ventricular rate per minute.
• The normal resting heart rate ranges from 60 to 100 beats per minute.

Abnormalities of Heart Rate

Bradycardia

• Bradycardia is defined as a heart rate below 60 beats per minute.
• Common causes include sinus bradycardia, junctional rhythm, and complete atrioventricular block.
• Sinus bradycardia may occur in trained athletes due to high vagal tone.
• Other causes include sick sinus syndrome, beta-adrenergic receptor blockers, hypothyroidism, raised intracranial pressure, and severe cholestatic liver disease.

Tachycardia

• Tachycardia is defined as a heart rate greater than 100 beats per minute.
• Sinus tachycardia may occur with anxiety, fever, hypoxemia, hyperthyroidism, heart failure, or acute inflammatory cardiac conditions.
• Other causes include supraventricular tachycardia, atrial premature beats, atrial flutter, atrial fibrillation, ventricular premature beats, and ventricular tachycardia.
• Excess caffeine intake, certain medications, digitalis toxicity, coronary artery disease, valvular heart disease, and cardiomyopathies can also contribute to tachyarrhythmias.
• Recognition of abnormal heart rates is essential for diagnosing rhythm disorders and underlying cardiovascular disease.

Cardiac Rhythm

• Cardiac rhythm refers to the regularity of successive heartbeats.
• In normal sinus rhythm, cardiac cycles occur at nearly equal intervals.
• Rhythm is assessed by measuring consecutive RR intervals on the electrocardiogram.
• Small physiological variations in cycle length may occur in healthy individuals.
• A variation of up to approximately 10% between adjacent RR intervals is generally considered normal.
• Any disturbance in the regular generation or conduction of impulses is termed an arrhythmia.
• Sinus arrhythmia, commonly related to respiration, is a normal physiological variation in cardiac rhythm.

Waves and Intervals

P Wave

• The P wave represents atrial depolarization. Its normal duration is less than 0.12 second and its amplitude is usually not greater than 2.5 millimeters.
• P-wave morphology is best assessed in Leads II and V1.
• It is typically upright in Leads I, II, aVF, and V3–V6.
• It is normally inverted in aVR and may be upright, inverted, or biphasic in some other leads.
• Abnormal P waves may indicate atrial enlargement or intra-atrial conduction disturbances.
• Atrial enlargement commonly produces tall, peaked, or widened P waves.

PR Interval

• The PR interval normally measures 0.12–0.20 second (3–5 small squares).
• PR intervals should remain constant from beat to beat in normal sinus rhythm.

Short PR Interval:

• Pre-excitation syndromes such as Wolff–Parkinson–White syndrome.
• Junctional rhythm.
• Atrial premature beats.

Long PR Interval: (First‑degree AV block; discussed below)

• First-degree atrioventricular block.
• Inflammatory cardiac diseases.
• Effects of cardiac glycosides.
• Coronary artery disease.

QRS Complex

• The QRS complex represents ventricular depolarization.
• Its normal duration is less than 0.12 second.
• In limb leads, the total QRS amplitude is generally at least 5 millimeters.
• In chest leads, the amplitude is usually at least 10 millimeters.
• R waves are normally dominant in Leads I, II, and V4–V6.
• S waves are usually dominant in aVR, V1, and V2.
• The relative prominence of R and S waves may vary with the anatomical position of the heart.

Main QRS abnormalities manifest in the amplitude.

Low amplitude:

• Severe emphysema.
• Hypothyroidism.
• Pericardial effusion.
• Dilated cardiomyopathy

High amplitude:

Ventricular hypertrophy, reflecting increased ventricular muscle mass.

Q Wave

• The Q wave represents the initial phase of ventricular depolarization, particularly septal activation.
• Small physiological Q waves are commonly seen in Leads I, aVL, V5, and V6.
• A QS complex is frequently observed in lead aVR and may be a normal finding.
• Some healthy individuals may show a relatively deep Q wave in Lead III, which often becomes smaller during deep inspiration.
• Occasionally, isolated Q waves may be present in V1 or V2 without underlying disease.
• In most leads, the depth of a normal Q wave is less than 25% of the height of the following R wave.
• In aVL, a Q wave may occasionally reach up to 50% of the R-wave amplitude and still be physiological.

Pathological Q Waves

• A pathological Q wave is generally defined as a Q wave wider than 0.04 second or deeper than 25% of the amplitude of the succeeding R wave in the same lead.
• Pathological Q waves usually indicate loss of electrically active myocardium.
• Common causes include acute or previous myocardial infarction.
• They may also occur in dilated cardiomyopathy and hypertrophic cardiomyopathy.
• Persistent pathological Q waves often suggest previous myocardial damage and should be interpreted in conjunction with clinical findings and other electrocardiographic changes.

ST Segment

• The ST segment is normally isoelectric and lies close to the baseline.
• Mild ST depression of less than 0.5 millimeter may be a normal finding.
• Physiological ST elevation may occur up to 1 millimeter in limb leads and left precordial leads, and up to 2 millimeters in right precordial leads.
• Significant ST elevation commonly indicates acute myocardial injury, particularly acute myocardial infarction.
• ST elevation may also occur in acute pericarditis.
• ST depression is most often associated with myocardial ischemia.

T Wave

• The T wave represents ventricular repolarization.
• It is normally upright in Leads I, II, and V4–V6.
• It is typically inverted in aVR.
• In Leads III, aVL, aVF, and V1–V3, the T wave may be upright, inverted, or biphasic.
• T-wave abnormalities commonly present as increased amplitude or inversion.

Tall T wave:

• Tall, peaked T waves are characteristic of hyperkalemia.
• They may also occur during the early stages of acute myocardial infarction.

Inverted T wave:

A. Physiological

• T-wave inversion may be a normal finding in young children.
• It may occasionally occur during deep inspiration.
• Temporary inversion can rarely be seen after a heavy meal.

B. Pathological

• Ventricular hypertrophy with strain pattern.
• Bundle branch block.
• Effects of cardiac glycosides.
• Myocardial ischemia and other forms of myocardial injury.

QT Interval

• The QT interval extends from the beginning of the QRS complex to the end of the T wave.
• It represents the total duration of ventricular depolarization and repolarization.
• The upper normal limit is approximately 0.42 second in males and 0.43 second in females.
• QT measurement should be performed in the lead where the end of the T wave is most clearly visible.
• Because QT duration varies with heart rate, the corrected QT interval (QTc) is commonly used for clinical assessment.
• QTc is calculated using correction formulas, most commonly Bazett’s formula.

Abnormal QT Interval

QT interval may be either prolonged or shortened.

Prolonged QT interval:

• Congenital long QT syndromes.
• Certain antiarrhythmic medications.
• Hypokalemia and other electrolyte disturbances.
• Acute myocardial infarction and myocardial injury.

Shortened QT interval:

• Hypercalcemia.
• Less commonly, other conditions associated with accelerated ventricular repolarization.
• Recognition of QT abnormalities is important because marked prolongation increases the risk of serious ventricular arrhythmias.

Mean QRS Axis

• The mean QRS axis represents the average direction and magnitude of ventricular depolarization in the frontal plane.
• It reflects the overall electrical vector generated during activation of the ventricles.
• Determination of the QRS axis is an important part of electrocardiogram interpretation.

Normal Value

• The normal mean QRS axis usually lies between –30° and +110° in adults.
• An axis less than –30° is termed left axis deviation.
• An axis greater than +110° is termed right axis deviation.
• Significant axis deviation may indicate underlying cardiac or conduction abnormalities.

Factors Affecting Mean Cardiac Vector

• The anatomical position and orientation of the heart within the thorax affect the QRS axis.
• Conduction through the specialized cardiac conduction system influences the direction of ventricular activation.
• Electrical properties of the ventricular myocardium also contribute to the resultant vector.
• The relative muscle mass of the right and left ventricles significantly affects axis orientation.
• Assessment of the mean QRS axis provides valuable information about ventricular hypertrophy, conduction defects, and structural heart disease.
• Changes in the axis may help identify conditions affecting ventricular muscle mass or the ventricular conduction pathways.
• Therefore, QRS axis analysis is an important component of comprehensive electrocardiographic evaluation.

Measurement of Mean QRS Axis (Cardiac Vector)

• The mean QRS axis can be estimated using simple electrocardiographic methods or calculated more precisely using vector analysis.
• A rapid bedside assessment is commonly performed using Leads I and aVF.

Rough Estimation

• When the QRS complex is predominantly positive in both Lead I and lead aVF, the QRS axis is usually within the normal range.
• Right axis deviation is suggested when the QRS complex is predominantly negative in Lead I and predominantly positive in lead aVF.
• Left axis deviation is suggested when the QRS complex is predominantly positive in Lead I and predominantly negative in lead aVF.
• When the QRS complexes are predominantly negative in both Lead I and lead aVF, the axis lies in the extreme right superior quadrant, often termed an extreme axis deviation or indeterminate axis.
• Estimation of the QRS axis helps identify ventricular hypertrophy, conduction abnormalities, and structural cardiac disorders.
• Axis determination should always be interpreted along with other electrocardiographic findings and the clinical context.

Accurate Estimation

• The mean QRS axis can be determined accurately using either the Einthoven triangle method or the hexaxial reference system.
• Both methods estimate the average direction of ventricular depolarization in the frontal plane.

Using Einthoven’s Triangle:

• This method uses the net QRS amplitudes from any two standard limb leads.
• For each selected lead, the net QRS deflection is calculated by subtracting the largest negative deflection from the height of the R wave.
• The resulting values are plotted along the corresponding lead axes of the Einthoven triangle.
• Positive values are plotted toward the positive pole, whereas negative values are plotted toward the negative pole.
• Perpendicular lines are then drawn from the plotted points.
• The intersection of these perpendiculars identifies the resultant electrical vector.
• A line drawn from the center of the triangle to this intersection represents the mean cardiac vector.
• The direction of this vector indicates the mean QRS axis, and its length reflects the vector magnitude.
• This graphical method provides a reasonably accurate estimate of ventricular electrical activity.

Using Hexaxial Reference System:

• The hexaxial reference system is commonly used because it is simpler and faster.
• All six limb leads are examined to identify the lead with the most nearly isoelectric QRS complex.
• An isoelectric QRS complex has nearly equal positive and negative deflections, resulting in a net deflection close to zero.
• When a lead is isoelectric, the mean QRS vector is approximately perpendicular to that lead’s axis.
• The axis perpendicular to the isoelectric lead is then identified on the hexaxial reference diagram.
• The correct direction is determined by examining whether nearby leads show predominantly positive or negative QRS complexes.
• For example, if Lead III is nearly isoelectric, the mean QRS axis lies approximately perpendicular to Lead III.
• Depending on the polarity of adjacent leads, the axis is commonly estimated near +30 degrees or the opposite direction.
• Accurate axis determination assists in detecting ventricular hypertrophy, fascicular blocks, conduction abnormalities, and structural heart disease.
• It is an important component of comprehensive electrocardiographic interpretation.

Abnormal Axis Deviations

Right axis deviation:

Right axis deviation may occur in right ventricular hypertrophy, left posterior fascicular block, pre-excitation syndromes, and dextrocardia.

Left axis deviation:

• Left axis deviation may occur in left ventricular hypertrophy, left anterior fascicular block, inferior myocardial infarction, pre-excitation syndromes, and advanced chronic obstructive pulmonary disease.
• Axis deviation provides important clues to underlying conduction abnormalities and structural heart disease.

ABNORMAL ECG

• An abnormal electrocardiogram can reveal disturbances in cardiac rhythm, myocardial function, electrolyte balance, or systemic diseases affecting the heart.
• Major abnormalities detected on electrocardiography include disorders of impulse formation, impulse conduction, myocardial injury, and metabolic disturbances.

Cardiac Arrhythmias

• A cardiac arrhythmia is an abnormality in the generation, conduction, or rhythm of cardiac impulses.
• Clinically, arrhythmias are classified as bradyarrhythmias or tachyarrhythmias.
• Bradyarrhythmias are associated with a heart rate below the normal range.
• Tachyarrhythmias are associated with an increased heart rate.
• Disorders involving the sinoatrial node may alter the initiation of cardiac impulses.
• Atrial arrhythmias arise from abnormal electrical activity within the atria.
• Ventricular arrhythmias originate from the ventricular myocardium or ventricular conduction system.
• Conduction disorders result from impaired transmission of impulses through specialized conducting pathways.
• Recognition of these abnormalities is essential for diagnosing cardiac disease and guiding appropriate management.

Disorders of SA Node

• Common disorders of the sinoatrial node include sinus arrhythmia, sick sinus syndrome, sinus tachycardia, and sinus bradycardia.
• These conditions affect the initiation or rate of cardiac impulse generation.

Sinus Arrhythmia

• Sinus arrhythmia is a normal physiological variation in heart rate associated with respiration.
• Heart rate increases during inspiration and decreases during expiration.
• This phenomenon is known as respiratory sinus arrhythmia.
• Changes in autonomic nervous system activity contribute to this variation, with increased sympathetic influence during inspiration and increased parasympathetic influence during expiration.
• Increased venous return during inspiration stretches the right atrium and sinoatrial node, leading to a transient increase in heart rate.
• Activation of atrial stretch receptors also contributes to the inspiratory rise in heart rate.
• Respiratory-related influences on cardiovascular control centers further modulate this response.

Sick Sinus Syndrome

• Sick sinus syndrome results from dysfunction of the sinoatrial node.
• It commonly causes inappropriate slowing of heart rate and may be associated with pauses or alternating bradycardia and tachycardia.

Sinus Tachycardia

• Sinus tachycardia is defined as a heart rate greater than 100 beats per minute in adults.
• The rhythm originates normally from the sinoatrial node but at an increased rate.
• Common causes include anxiety, fever, hypoxemia, hyperthyroidism, heart failure, acute inflammatory cardiac conditions, and medications such as atropine.
• It is usually a physiological or secondary response rather than a primary arrhythmia.

Sinus Bradycardia

• Sinus bradycardia is defined as a heart rate below 60 beats per minute in adults.
• It results from reduced sinoatrial nodal discharge.
• It may occur physiologically in trained athletes due to increased vagal tone.
• Other causes include sick sinus syndrome, beta-adrenergic receptor blockers, hypothyroidism, raised intracranial pressure, and severe cholestatic liver disease.
• Clinical significance depends on the severity of bradycardia and the presence of symptoms.

Atrial Arrhythmias

• Atrial arrhythmias arise from abnormal impulse formation or conduction within the atria.
• Common atrial arrhythmias include atrial premature beats, paroxysmal supraventricular tachycardia, atrial flutter, and atrial fibrillation.

Atrial Premature Beats

• Atrial premature beats originate from an ectopic focus within the atrium.
• The ectopic impulse occurs earlier than the next expected sinus impulse.
• An abnormal P wave appears before the next normal sinus P wave.
• The shape of the ectopic P wave differs from that of a normal sinus P wave.
• These beats are also termed atrial extrasystoles.
• They may occur in healthy individuals during emotional stress or after excessive intake of caffeinated beverages.
• They are also associated with coronary artery disease, valvular heart disease, cardiomyopathies, and digitalis toxicity.

Paroxysmal Supraventricular Tachycardia

• Paroxysmal supraventricular tachycardia is characterized by sudden onset and termination of rapid supraventricular rhythm.
• It includes atrioventricular nodal re-entrant tachycardia and focal atrial tachycardia.

Nodal PST

• This arrhythmia commonly results from a re-entry circuit involving dual atrioventricular nodal pathways.
• Heart rate is usually between 170 and 250 beats per minute.
• Episodes may last from a few minutes to several hours.
• P waves may be hidden within the QRS complex because atrial and ventricular activation occur nearly simultaneously.
• The QRS complex is usually narrow and rhythm remains regular.
• It may be associated with pre-excitation syndromes and hyperthyroidism.
• Atrial premature beats often trigger the arrhythmia.

Atrial Tachycardia (Atrial PST)

• Atrial tachycardia arises from a rapidly discharging atrial focus.
• Atrial rates commonly exceed 200 beats per minute.
• It may occur in patients receiving cardiac glycosides.

Atrial Flutter

• Atrial flutter is usually caused by a large re-entry circuit within the atria.
• Electrocardiography shows characteristic flutter waves producing a saw-tooth baseline pattern.
• Atrial rates generally range from 250 to 350 beats per minute.
• Not all atrial impulses reach the ventricles because of atrioventricular conduction block.
• A 2:1 conduction ratio is most common, although other ratios may occur.
• It is associated with coronary artery disease, mitral valve disease, rheumatic heart disease, and hyperthyroidism.

Atrial Fibrillation

• Atrial fibrillation is characterized by rapid, irregular, and disorganized electrical activity within the atria.
• The atrial rate typically ranges from 300 to 500 beats per minute.
• Ventricular rate is variable and commonly ranges between 100 and 180 beats per minute when untreated.
• Electrocardiography shows an irregular baseline with small fibrillatory waves and absence of distinct P waves.
• The ventricular rhythm is characteristically irregularly irregular.
• QRS complexes are usually normal in shape unless an associated conduction abnormality is present.
• The arrhythmia commonly occurs in mitral valve disease, rheumatic heart disease, coronary artery disease, cardiomyopathies, and hyperthyroidism.
• It is usually caused by multiple re-entrant electrical wavelets circulating within the atrial myocardium.
• Recognition of atrial fibrillation is important because it increases the risk of thromboembolism and stroke.

Ventricular Arrhythmias

• Ventricular arrhythmias originate from abnormal electrical activity within the ventricles.
• Common forms include ventricular extrasystoles, ventricular tachycardia, and ventricular fibrillation.

entricular Extrasystole

• A ventricular extrasystole results from a premature impulse arising from an ectopic ventricular focus.
• The premature beat occurs earlier than the next expected sinus beat.
• The QRS complex is typically wide, bizarre, and distorted because ventricular activation occurs outside the normal conduction pathway.
• A preceding P wave is usually absent or not related to the premature ventricular complex.
• The premature beat is commonly followed by a compensatory pause.
• Ventricular extrasystoles may occur in healthy individuals or in patients with structural heart disease.

Paroxysmal Ventricular Tachycardia

• Ventricular tachycardia is a rapid ventricular rhythm arising from an ectopic ventricular focus or a re-entry circuit within the ventricles.
• It is characterized by a sequence of wide QRS complexes occurring at a rapid rate.
• Sustained ventricular tachycardia can significantly reduce cardiac output.
• Unlike most supraventricular tachycardias, ventricular tachycardia is often associated with serious underlying cardiac disease.
• It may progress to ventricular fibrillation if not treated promptly.

Ventricular Fibrillation

• Ventricular fibrillation is caused by multiple chaotic electrical wavelets or rapidly circulating re-entry circuits within the ventricles.
• Ventricular electrical activity becomes extremely rapid, irregular, and completely disorganized.
• Coordinated ventricular contraction is lost, making cardiac pumping ineffective.
• As a result, systemic blood flow ceases and circulatory collapse occurs.
• Ventricular fibrillation is a life-threatening medical emergency requiring immediate defibrillation and advanced resuscitative measures.
• It commonly develops during acute myocardial infarction and is a major cause of sudden cardiac death.

Conduction Disorders

• Conduction disorders result from abnormal transmission of electrical impulses through the cardiac conduction system.
• They may present as conduction delay, conduction block, or, less commonly, accelerated conduction.

duction Block (Heart Block)

• An atrioventricular block occurs when impulse conduction from the atria to the ventricles is impaired.
• It is commonly classified into first-degree, second-degree, and third-degree atrioventricular blocks.
• First-degree and second-degree blocks are considered incomplete heart blocks because some impulses still reach the ventricles.
• Third-degree block is a complete heart block, in which atrial impulses fail to conduct to the ventricles.

First‑Degree Heart Block

• In first-degree atrioventricular block, all atrial impulses are conducted to the ventricles.
• Conduction through the atrioventricular node is delayed, resulting in a prolonged PR interval greater than 0.20 second.
• The rhythm usually remains regular, and each P wave is followed by a QRS complex.

Second‑Degree Heart Block

• In second-degree atrioventricular block, some atrial impulses fail to reach the ventricles.
• Consequently, certain P waves are not followed by QRS complexes.
• This condition is divided into Mobitz type I and Mobitz type II blocks.
• Mobitz type I is characterized by progressive prolongation of the PR interval before a dropped ventricular beat.
• Mobitz type II shows intermittent failure of conduction without progressive PR interval prolongation and is generally more serious.
• Recognition of these conduction abnormalities is important because advanced blocks may progress to complete heart block and symptomatic bradycardia.

Mobitz Type I:

• In Mobitz type I, the PR interval progressively lengthens with successive beats.
• Eventually, an atrial impulse fails to conduct, resulting in a dropped QRS complex.
• After the missed ventricular beat, the PR interval returns to a shorter duration and the cycle repeats.
• This characteristic pattern is known as the Wenckebach phenomenon.
• The block usually occurs within the atrioventricular node.
• Common causes include acute inferior myocardial infarction, cardiac glycoside toxicity, and inflammatory cardiac disorders.

Mobitz Type II:

• In Mobitz type II, intermittent failure of atrioventricular conduction occurs without progressive prolongation of the PR interval.
• Some P waves are not followed by QRS complexes.
• Conduction patterns may appear as 2:1, 3:1, or higher-grade blocks, indicating that only one ventricular beat follows every two, three, or more atrial depolarizations.
• This form of block usually arises below the atrioventricular node within the His–Purkinje system.
• Common causes include acute anterior myocardial infarction and degenerative disease of the cardiac conduction system.
• Mobitz type II is clinically important because it carries a higher risk of progression to complete heart block and often requires close monitoring or permanent pacemaker therapy.

Third‑Degree Heart Block

• Third-degree atrioventricular block is a complete interruption of impulse conduction from the atria to the ventricles.
• Because atrial impulses cannot reach the ventricles, the atria and ventricles beat independently, a phenomenon known as atrioventricular dissociation.
• On electrocardiography, P waves occur regularly but have no fixed relationship to the QRS complexes.
• Complete heart block may result from disease of the atrioventricular node or damage to the conduction system below it.
• In many cases, an escape pacemaker within the His–Purkinje system or ventricular myocardium maintains ventricular activity.
• This idioventricular rhythm usually produces a ventricular rate of approximately 15–40 beats per minute.
• Severe bradycardia may markedly reduce cerebral blood flow, leading to transient loss of consciousness, known as Stokes–Adams syndrome.
• Common causes include myocardial infarction involving the conduction system, degenerative conduction disorders, cardiac glycoside toxicity, and surgical injury to conduction pathways.
• Permanent implantation of an electronic pacemaker is the standard treatment for symptomatic complete heart block.

Bundle Branch Block

• A bundle branch block occurs when conduction is interrupted in either the right or left bundle branch.
• Delayed ventricular activation causes widening of the QRS complex, usually exceeding 0.12 second.
• In right bundle branch block, characteristic changes include a widened QRS complex and secondary ST-segment depression with T-wave inversion in right precordial leads.
• In left bundle branch block, the R wave becomes broad, often notched, or displays a double-peaked appearance.
• Bundle branch blocks alter the normal sequence of ventricular depolarization and repolarization.

Acceleration of Conduction

• Abnormally rapid atrioventricular conduction occurs through accessory pathways that bypass part or all of the normal atrioventricular nodal delay.
• Important pre-excitation syndromes include Wolff–Parkinson–White syndrome and Lown–Ganong–Levine syndrome.

Wolff-Parkinson-White (WPW) Syndrome

• In Wolff–Parkinson–White syndrome, an accessory atrioventricular pathway, known as the bundle of Kent, connects the atria directly to the ventricles.
• This pathway conducts impulses faster than the atrioventricular node, producing premature ventricular activation.
• The electrocardiogram shows a shortened PR interval and a widened QRS complex.
• The initial part of the QRS complex is characteristically slurred, forming a delta wave.
• The interval from the beginning of ventricular activation to the end of the QRS complex remains essentially normal.
• This syndrome increases the risk of re-entrant supraventricular tachyarrhythmias and recurrent episodes of rapid heart rate.

Lown-Ganong-Levine (LGL) Syndrome

• Lown–Ganong–Levine syndrome is a pre-excitation disorder in which impulses bypass part of the normal atrioventricular nodal conduction pathway through an accessory pathway.
• Ventricular activation occurs through the normal His–Purkinje system; therefore, the QRS complex remains normal in duration and morphology.
• The PR interval is shortened because atrioventricular conduction time is reduced.
• The syndrome may predispose affected individuals to episodes of supraventricular tachycardia.
• Electrocardiographic recognition is based on a short PR interval with a normal QRS complex and absence of a delta wave.

Myocardial Abnormalities

Myocardial Ischemia

• Myocardial ischemia results from inadequate blood supply to a region of the myocardium.
• Reduced oxygen delivery alters cellular metabolism and electrical properties of affected myocardial cells.
• Ischemic cells become partially depolarized, producing a less negative resting membrane potential than surrounding healthy tissue.
• This change is partly related to disturbances in transmembrane ion gradients, particularly potassium ion distribution.
• Under normal conditions, myocardial cells remain electrically stable during the TP interval.
• In ischemic myocardium, a voltage difference develops between normal and injured tissue, creating an injury current during electrical diastole.
• This electrical gradient alters the apparent baseline of the electrocardiogram.
• During the ST segment, ventricular tissue is normally depolarized uniformly.
• In the presence of ischemia or myocardial injury, the ST segment may appear elevated or depressed relative to the baseline.
• ST-segment deviation is one of the most important electrocardiographic indicators of myocardial ischemia and acute coronary syndromes.
• ST-segment depression is commonly associated with subendocardial ischemia.
• ST-segment elevation often indicates acute transmural myocardial injury.
• Interpretation of ST-segment changes should always be correlated with clinical findings and other electrocardiographic abnormalities.
• Recognition of ischemic electrocardiographic patterns is essential for early diagnosis and management of coronary artery disease.

Myocardial Infarction

• Myocardial infarction occurs when prolonged interruption of coronary blood flow causes irreversible myocardial cell death.
• Acute ischemia may initially produce T-wave abnormalities, particularly T-wave inversion or hyperacute T waves.
• Electrocardiography is valuable for diagnosing infarction and determining its location and extent.
• In acute transmural infarction, ST-segment elevation is typically seen in leads facing the injured myocardium.
• Reciprocal ST-segment depression may appear in leads opposite the infarcted region.

Physiological Basis of ECG Changes in AMI

• The characteristic electrocardiographic feature of acute myocardial infarction is ST-segment elevation.
• Injured myocardial cells exhibit altered membrane potentials and abnormal electrical behavior compared with healthy tissue.
• Shortly after infarction, damaged myocardial cells repolarize differently from surrounding viable myocardium.
• These differences generate injury currents that shift the ST segment above the baseline in leads overlying the infarcted area.
• Loss of cellular integrity and potassium imbalance reduce the resting membrane potential of injured myocardial cells.
• Voltage differences between injured and normal tissue create diastolic injury currents.
• These currents contribute to the apparent elevation of the ST segment on the electrocardiogram.
• Conduction through injured myocardium becomes slow and nonuniform.
• Delayed depolarization and repolarization further accentuate electrical differences between normal and infarcted regions.
• These abnormalities contribute to persistent ST-segment changes during the acute phase.
• As infarction progresses, necrotic myocardium loses its ability to generate or conduct electrical impulses.
• The infarcted region becomes electrically silent and no longer contributes to the normal ventricular depolarization vector.
• This change often produces pathological Q waves, which indicate loss of viable myocardial tissue.
• Pathological Q waves may persist long after the acute event and can serve as evidence of a previous myocardial infarction.
• As infarction progresses, necrotic myocardium loses its ability to generate or conduct electrical impulses.
• The infarcted region becomes electrically silent and no longer contributes to the normal ventricular depolarization vector.
• This change often produces pathological Q waves, which indicate loss of viable myocardial tissue.
• Pathological Q waves may persist long after the acute event and can serve as evidence of a previous myocardial infarction.

Ventricular Hypertrophy

• Ventricular hypertrophy is commonly identified by changes in the mean QRS axis and increased QRS voltage on the electrocardiogram.
• In right ventricular hypertrophy, the mean QRS axis shifts to the right.
• A tall R wave is typically seen in lead V1 because of increased right ventricular muscle mass.
• In left ventricular hypertrophy, the mean QRS axis may shift leftward, although it can remain within the normal range.
• Increased left ventricular muscle mass produces tall R waves in leads V5 and V6 and deep S waves in right precordial leads.
• Voltage criteria are supportive but should be interpreted with clinical findings and imaging studies.

Myocarditis and Cardiomyopathies

• Myocarditis and cardiomyopathies may produce a variety of electrocardiographic abnormalities.
• Findings are often nonspecific and may include rhythm disturbances, conduction defects, ST-segment changes, or T-wave abnormalities.
• Therefore, electrocardiographic findings should be correlated with clinical assessment and other diagnostic investigations.

Effects of Electrolyte Disturbances

Alterations in potassium, calcium, and sodium concentrations significantly affect cardiac electrical activity and function.

Alteration in Plasma K+

Hyperkalemia

• Electrocardiographic changes in hyperkalemia depend on the severity of potassium elevation.
• When plasma potassium reaches approximately 7 milliequivalents per liter, tall, peaked T waves commonly appear.
• At levels around 8.5 milliequivalents per liter, the QRS complex becomes widened and slurred, while T waves remain tall and narrow.
• Severe hyperkalemia, usually above 9 milliequivalents per liter, can produce serious conduction disturbances and malignant ventricular arrhythmias.
• Advanced cases may lead to ventricular tachycardia, ventricular fibrillation, or cardiac arrest if not treated promptly.
• Recognition of these electrocardiographic changes is important for early diagnosis and emergency management of hyperkalemia.

Hypokalemia

• Electrocardiographic changes in hypokalemia become more prominent as plasma potassium concentration decreases.
• At approximately 3 milliequivalents per liter, ST-segment depression and a prominent U wave may appear after the T wave.
• At approximately 2 milliequivalents per liter, the PR interval may become prolonged.
• Severe potassium deficiency can also produce ST-segment depression, T-wave inversion, and marked U-wave prominence.
• These changes reflect delayed ventricular repolarization and increased susceptibility to cardiac arrhythmias.

Alteration in Plasma Ca++

• Hypercalcemia usually produces minimal electrocardiographic changes.
• A shortened QT interval may occur because ventricular repolarization is accelerated.
• Severe hypercalcemia can impair myocardial relaxation and contractile function.
• Hypocalcemia prolongs the ST segment, resulting in a prolonged QT interval.

Alteration in Plasma Na+

• Hypercalcemia usually produces minimal electrocardiographic changes.
• A shortened QT interval may occur because ventricular repolarization is accelerated.
• Severe hypercalcemia can impair myocardial relaxation and contractile function.
• Hypocalcemia prolongs the ST segment, resulting in a prolonged QT interval.

HIS Bundle Electrogram

• A His bundle electrogram is an intracardiac recording of electrical activity from the atrioventricular conduction system, particularly the His bundle.
• It is used alongside surface electrocardiography, especially lead II, to evaluate conduction abnormalities and localize the site of heart block.
• The recording is obtained by advancing a catheter-mounted electrode through a peripheral vein into the right atrium near the tricuspid valve and His bundle region.
• His bundle electrography provides detailed assessment of atrioventricular and intraventricular conduction.
• A deflection represents electrical activation of the atrial tissue and atrioventricular nodal region.
• H deflection represents conduction of the impulse through the His bundle.
• V deflection represents ventricular depolarization.
• Analysis of the intervals between these deflections helps identify whether conduction delay occurs within the atrioventricular node, His bundle, or distal conduction pathways.
• This technique is particularly useful in the evaluation of complex atrioventricular blocks and certain cardiac arrhythmias.

HBE Intervals

• During His bundle electrogram recording, lead II electrocardiography is recorded simultaneously on the same time scale.
• Correlation of the two recordings allows precise assessment of conduction through different parts of the cardiac conduction system.
• Three important intervals are measured: PA, AH, and HV intervals.

PA Interval

• The PA interval extends from the beginning of the P wave on lead II electrocardiography to the A deflection on the His bundle electrogram.
• It represents conduction time from the sinoatrial node through the atria to the atrioventricular node.
• The normal value is approximately 27 milliseconds.

AH Interval

• The AH interval is measured from the A deflection to the H deflection on the His bundle electrogram.
• It reflects conduction through the atrioventricular node.
• The normal value is approximately 92 milliseconds.
• Prolongation of this interval usually indicates delayed atrioventricular nodal conduction.

HV Interval

• The HV interval extends from the H deflection to the onset of the QRS complex on the electrocardiogram.
• It represents conduction through the His bundle and bundle branches to the ventricular myocardium.
• The normal value is approximately 43 milliseconds.
• A prolonged HV interval suggests disease of the His–Purkinje conduction system.
• Measurement of these intervals helps localize conduction delays and diagnose different types of heart block with greater accuracy.