Electrocardiogram Principles I & II

• These slides are from MEDC2441: Cardiovascular System
• Lecture 23 covers the principles of the Electrocardiogram I, on March 4th 2025
• Lecture 24 covers the principles of the Electrocardiogram II, on March 5th 2025

Learning Objectives Overview

• Consult the Study Guide
• Keep the following in mind
• Describe how an ECG signal is generated from a single myocardial cell dipole
• Describe standard ECG leads, including bipolar limb, unipolar limb and pre-cordial, plus planes
• Describe the frontal myocardial depolarization axis and its calculation from standard ECG leads
• Describe the ECG wave components (P, QRS, T)
• Describe ECG interpretation rules for rate, rhythm, axis, intervals and ST changes
• Calculate the mean axis, and identify left/right axis deviations
• Relate ST segment changes and Q wave presence on ECG leads to the affected coronary artery
• Describe typical ECG changes in hyperkalemia

Historical Context of ECG

• Matteucci recorded electrical activity from a frog's heart in 1842
• Waller recorded the first electrical activity from a human heart in 1887
• Einthoven first used the term EKG in 1893
• EKG entered the U.S in 1901
• Einthoven won the Nobel Prize in 1924
• Wilson invented the central terminal, with precordial leads being born in 1934-1938
• Goldberg used the Central Terminal with Augmentation, where augmented unipolar leads were born in 1942
• The AHA standardized the 12-Lead EKG as known today, in 1954

Cardiac Conduction System

• The Sinoatrial Node (SAN) is located in the Right Atrium
• The Atrioventricular Node (AVN) receives signals from the Right Atrium
• The HIS Bundle transfers signals from the AVN to the ventricles
• The Left Bundle Branch (LBB) and Right Bundle Branch (RBB) conduct signals through the ventricles
• The Left Posterior Fascicle (LPS) and Left Anterior Fascicle (LAF) are branches of the LBB
• Purkinje Fibers (PF) distribute signals through the ventricular myocardium
• Cardiomyocytes contract
• Pacemaker cells: SA, AV
• Libroblasts: heart skeleton

ECG Basics

• A 12-Lead ECG is a surface-based recording of the heart's organized electrical impulses
• It is a method of recording electrical activity between electrodes in specific patterns
• A voltmeter records the potential difference between two poles in the single cell
• Convention 1: Electrical current flows from negatively to positively charged areas
• Convention 2: A depolarizing wave traveling towards the positive recording electrode produces a positive (upright) deflection on the voltmeter
• If the wave travels away or the electrode polarity is reversed, the deflection will be opposite or downward

Depolarization vs Repolarization

• Repolarization is slower than depolarization
• In ventricular repolarization, the T wave is normally upright, similar to the QRS complex
• Depolarization goes from endocardium to epicardium
• Repolarization goes from epicardium to endocardium
• The action potential is shorter in epicardial cells, meaning repolarization starts there

Planes of the Heart

• Frontal
• Horizontal

Standard ECG Leads

• Frontal leads represent the frontal plane and are either bipolar or unipolar limb leads
• Horizontal leads are unipolar chest (precordial) leads
• A complete 12-lead ECG records electrical activity between electrodes in specific patterns
• Bipolar leads: one limb electrode is positive, and another provides a negative reference
• Unipolar leads: one limb electrode is positive, and other limb electrodes are averaged for a negative reference

Limb Lead Placement

• Lead I: Left Arm (+) and Right Arm (-)
• Lead II: Left Leg (+) and Right Arm (-)
• Lead III: Left Leg (+) and Left Arm (-)
• aVR: Right Arm (+)
• aVL: Left Arm (+)
• aVF: Left Leg (+)

Chest Lead Placement

• Chest leads are all unipolar
• V1: 4th Intercostal Space (ICS), 2 cm to the right of the sternum
• V2: 4th ICS, 2 cm to the left of the sternum
• V3: Midway between V2 and V4
• V4: 5th ICS, left midclavicular line
• V5: 5th ICS, left anterior axillary line
• V6: 5th ICS, left midaxillary line

Normal Ventricular Depolarization

• As seen by aVF and aVL
• As seen by V1 and V2
• As seen by V1 to V6

ECG II - Sequence of Normal Cardiac Activation

• P wave represents atrial depolarization.
• QRS complex represents ventricular depolarization.
• T wave represents ventricular repolarization.

The QRS Complex

• R wave is the initial upward deflection
• Q wave is any downward deflection BEFORE the R wave
• S wave is any downward deflection AFTER the R wave
• R' (prime) is the second upward deflection

ECG Graph Paper

• Lines are spaced 1 mm apart in both horizontal and vertical directions
• Each fifth line is made heavier to facilitate measurement
• The vertical axis measures voltage in millivolts (mV)
• Each 1-mm line represents 0.1 mV
• The horizontal axis represents time
• At a standard recording speed of 25 mm/sec, each 1 mm division represents 0.04 seconds, and each heavy line (5 mm) represents 0.2 seconds

Voltage Calibration

• A normal calibration is 1 mV
• Normally, a 1 mm vertical box equals 0.1 mV

Rate Calculation

• Heart rate (bpm) = (25 mm/sec x 60 sec/min) / Number of mm between beats
• Heart rate (bpm) = 1,500 / Number of small boxes between two consecutive beats

Rhythm Interpretation

• Sinus rhythm conditions:
• Every P wave is followed by a QRS
• Every QRS is preceded by a P wave
• P wave is upright in leads I, II, and III
• Heart rate in sinus rhythm between 60 and 100 bpm indicates normal sinus rhythm
• Less than 60 bpm is sinus bradycardia
• Greater than 100 bpm is sinus tachycardia

ECG Intervals

• PR interval represents the time for the electrical impulse to propagate through the atria into the AV node
• A prolonged PR interval can indicate a slow SA node, larger atria or mitral stenosis
• Diseased His/Purkinji system means it takes longer for the ventricles to depolarize; wider QRS
• Hypertension and a thicker left ventricle leads to taller QRS
• Ventricular cycle indicates depolarization and repolarization

QT Interval and Bazett's Formula

• To calculate the corrected QT interval (QTc)
• QTc interval = QT interval / √RR interval

ECG Intervals

Axial Reference for Frontal ECG Leads

• Axis is where the heart forces are pointing normally

Mean QRS Axis Deviation

• Average electrical forces during ventricular depolarization in the frontal plane
• A normal axis usually falls between -30 and 90 degrees

Left axis deviation

• Inferior wall myocardial infarction
• The Lead II is –ve (negative)
• Left anterior fascicular block
• Left ventricular hypertrophy (sometimes)

Right axis deviation

• Right ventricular hypertrophy
• Acute right heart strain
• Left posterior fascicular block

QRS Axis Orientation

• Look at leads I and II
• Both need to be positive

Right Bundle Branch Block

• Widened QRS
• RSR' in V1 ("rabbit ears")
• Prominent S in V6

Left Bundle Branch Block

• Widened QRS
• Broad, notched R in V6
• Absent R and prominent S in V1

Abnormalities of the P Wave

Right Ventricular Hypertrophy

• QRS is Tall
• R>S in Lead V1
• Right axis deviation
• RVH = V, mostly high

Left Ventricular Hypertrophy

• LVH = V5 & V6 high
• S in V1 plus R in V5 or V6 ≥ 35 mm
• R in aVL > 11 mm
• R in lead I > 15 mm

ECG Evolution During ST Elevation Myocardial Infarction

• Normal ECG exhibits typical wave patterns
• Acute phase shows ST elevation
• Hours later, ST elevation persists, R wave decreases and Q wave begins to appear
• The T wave begins to invert, and the Q wave deepens by day 1-2
• In the following days, the ST segment normalizes and the T wave inverts
• Weeks later, ST and T waves normalize, but the Q wave persists

Identifying Q Waves

• Physiologic Q waves:
• Found in V6 and aVL due to initial septal depolarization
• Less than 0.04 sec (1 small box)
• Low amplitude (< 25% of QRS amplitude)
• Pathologic Q waves indicate myocardial infarction
• Wider (more than 1 small box)
• Deeper (> 25% of QRS amplitude)

ECG Localisation of Myocardial Infarction

Conditions Altering ST/T Wave Morphology