EKG Interpretation: The Basics

Humayun J. Chaudhry, DO, MS, FACP, FACOIAssistant Dean for Health Policy and Chairman, Department of MedicineN.Y. College of Osteopathic Medicine of NYITSeptember 29, 2005

Lecture Goals and Objectives

Understand the value of the EKG as a tool, and not as an end to itselfAppreciate the description and meaning of a normal 12-lead EKGLearn how the electrical current in the heart is generated and propagated through the four chambers of the heart“Yes, memorize some numbers and concepts”“Relax, proceed slowly and methodically”

Required Textbook

The Only EKG Book You’ll Ever Need, 4th edition, by Malcolm S. Thaler, M.D., Lippincott, Williams, & Wilkins; 2003; ISBN 0-7817-4176-9

Recommended Textbook

Fundamentals of Clinical Medicine, 4th edition, by Chaudhry, Grieco et al, Lippincott Williams & Wilkins; 2004; ISBN 0-7817-5192-6

Introduction

The EKG, or electrocardiogram (ECG), is nothing more than a recording of the heart’s electrical activityThe world’s first EKG was actually recorded in 1903 by William EinthovenIt is one of the most simple, useful, practical, economical, and available aids in the clinical evaluation of heart disorders - but it has limitations and pitfalls

Introduction

In the days before EKGs, The history and physical were everything “An intermission occurs when one or

more beats of the heart are dropped. Irregularity is the condition when the beats are unequal in volume and force, or follow each other at unequal distances.” -Sir William Osler, 1892

Indications for Ordering an EKG

The patient with cardiac diseaseThe patient with noncardiac complaints, to screen for occult cardiac abnormalities and to ensure absence of acute disease To obtain a baseline “picture” of the heart and its conducting system

Basic Cellular Electrophysiology

Cardiac cells in their resting state are electrically polarized (negative inside, positive outside)Resting state is maintained by membrane pumpsWhen cardiac cells lose their internal negativity, they become depolarized

Basic Cellular Electrophysiology

Depolarization is propagated from cell to cell, producing a wave that can be transmitted across the entire heartWhen a wave of depolarization is complete, cardiac cells cells are restored to their resting state in a process called repolarizationThis wave of depolarization followed by repolarization can be detected by electrodes placed on the surface of the body

Basic Cellular Electrophysiology

The heart consists of three types of cells essentially Pacemaker cells – the electrical power

source of the heart Electrical conducting cells – the hard

wiring of the heart Myocardial cells – the contractile

machinery of the heart

Pacemaker Cells

Small cells that depolarize by themselves over and over again, at a particular rateEach spontaneous depolarization serves as the source of a wave of depolarization, leading to cardiac contraction and relaxationThe dominant pacemaker cells in the heart are located high up in the right atrium, the Sinoatrial/Sinus (SA) Node, firing typically at 60-100 times per minute

Pacemaker Cells

Actually, every myocardial cell in the heart has some ability (an automatic ability) to behave like a pacemaker cellWhen dominant cells of the sinus node fail, this automatic ability may take over

Electrical Conducting Cells

Long, thin cells that are like the wires of an electrical circuitThey carry current rapidly and efficiently to distant regions of the heartThey exist mostly within the ventricles and, many argue, also in the atria

Myocardial cells

The major part of the heart tissue, these cells contain an abundance of actin and myosinResponsible for the heavy labor of repeatedly contracting and relaxing, thereby delivering blood to the rest of the bodyWhen a wave of depolarization hits the myocardial cell, calcium is released within the cell, causing the cell to contract (Excitation-Contraction Coupling)

Time and Voltage

The waves that appear on an EKG primarily reflect the electrical activity of the Myocardial cellsPacemaker cell activity and transmission by the Electrical Conducting cells are not seen generally on the EKG because the voltage is too low to be detected

Time and Voltage

EKG waves have three characteristics Duration (fractions of a second) Amplitude (millivolts, mV) Configuration (shape and

appearance; less objective; more subjective)

EKG Paper

Typically a long, continuous roll of graph paper, usually pink, with dark and light lines running vertically and horizontallyIn modern machines, all “12 leads” are represented on a piece of graph paper, as above, that is 8½ X 11 inchesEach small box/square is 1mm by 1mmEach large box/square is 5mm by 5mm

EKG Paper (Memorize This!)

Horizontal axis measures timeEach small box/square is 0.04 seconds Therefore, 5 small boxes/squares

across equals how many seconds?

Vertical axis measures voltageEach small box/square is 0.1 mV Therefore, 5 small boxes/squares tall

equals how many millivolts?

One Cycle of Cardiac Contraction

The Sinoatrial/Sinus/SA Node fires spontaneously (not visible on the EKG)A wave of depolarization begins to spread outward into the atrial myocardium, leading to atrial contractionDuring atrial depolarization and contraction, EKG records a small burst of electrical activity lasting a fraction of a second (P wave)Right atrium depolarizes before left. Why?

One Cycle of Cardiac Contraction

From the atria, wave of depolarization is funneled along the interventricular septum, the wall that separates the left and right ventriclesHere, the Atrioventricular (AV) Node slows conduction to a crawl, lasting a fraction of a second – essential to allow atria to finish contracting (EKG falls silent here)Vagal nerve stimulation slows the current

Osteopathic Correlation

Suboccipital Release Patient supine or seated, operator at head

of table Operator “cups” patient’s occiput and

places 2nd-5th fingers on occiput Operator spreads fingers cephalad and

lateral to free up occipital musculature Rationale: by releasing the occiput, you are

normalizing the Vagus nerve (may abate hypotension in congestive heart failure)

Osteopathic Correlation

Suboccipital Release “freeing” the vagus nerve and causing normalizing of parasympathetic outflow

One Cycle of Cardiac Contraction

After the wave of depolarization escapes the AV Node, it is swept rapidly down the ventricles along the system of electrical conducting cells Bundle of His Bundle branches

Left Bundle Branch Right Bundle Branch

Purkinje fibers

One Cycle of Cardiac Contraction

Right Bundle Branch Carries current to apex of Right

Ventricle

Left Bundle Branch Divides into 3 Major Fascicles

Septal Fascicle, for interventricular septum Left Anterior Fascicle, for anterior surface

of Left Ventricle Left Posterior Fascicle, for posterior surface

of Left Ventricle

One Cycle of Cardiac Contraction

The beginning of ventricular depolarization, and contraction, is marked on the EKG by what is called the QRS Complex The amplitude of the QRS Complex is greater than that of the atrial P Wave because the ventricles are so much larger and more complex than the atria

The QRS Complex

A standard format for naming each component of the QRS Complex has been devised First downward deflection is a Q wave First upward deflection is an R wave If there is a second upward deflection it

is called an R’ wave (R-prime) The first downward deflection following

an upward deflection is called an S wave

The QRS Complex

The earliest part (Q wave) of the QRS Complex represents depolarization of the interventricular septum by the septal fascicle of the left bundle branchThe right and left ventricles then depolarize at about the same time, but most of what we see on the EKG represents left ventricular activation

One Cardiac Conduction Cycle

After myocardial cells depolarize, they pass through a brief refractory period during which they are resistant to further stimulation. They repolarize.The wave of repolarization of the ventricles is represented by the T Wave on the EKGN.B. Wave of atrial repolarization is hidden by the much more prominent QRS Complex

Naming the Straight Lines

The different straight lines connecting the various waves have been given namesA segment is a straight line connecting two waves (e.g. ST segment)An interval encompasses at least one wave plus the connecting straight line (e.g. PR interval, QT interval)

Normal Time Intervals (Memorize!)

P-R Interval: 0.12-0.20 secondsQRS Duration: 0.06-0.10 secondsQ-T Interval: varies with heart rate

EKG Leads

The conventional EKG records 12 leads 6 Limb/Extremity Leads

3 Bipolar Standard limb leads: I, II, and III 3 Unipolar limb leads: aVR (for right arm),

aVL (for left arm), aVF (for left leg) 6 Chest/Precordial Leads

V1, V2, V3, V4, V5, V6 These are placed on the chest in designated

areas

Limb Lead Electrode Placement

Electrodes are placed on the right arm (RA), left arm (LA), left leg (LL), and right leg (RL). The electrode on each extremity records electrical forces from the heart as viewed from the junction of that extremity with the body

Chest Lead Electrode Placement

V1: 4th intercostal space, RSBV2: 4th intercostal space, LSBV3: midway between V2 and V4V4: 5th or 6th interspace (where apex of heart is felt), MCLV5: ant. axillary line, same level as V4V6: midaxillary line, same level as V4

Chest Lead Electrodes

V1 and V2 overlie the R VentricleV4, V5, and V6 overlie the L VentricleV3 overlies the interventricular septum

Einthoven Triangle

The 3 Bipolar Standard Limb Leads (I, II, and III) may be transposed into an equilateral triangle called the Einthoven TriangleConcept postulates that the 3 Limb Leads form an equilateral triangle with the heart in the center of the triangleWhen the 3 sides of the triangle are transposed to a common central point, a triaxial reference system is produced

Hexaxial Reference System

The 3 Unipolar leads may be transposed onto the triaxial reference system to form a hexaxial reference systemThe figure to be shown in class shows the limb leads in the hexaxial reference system with their designated positive endsThis system is very important in determining the axis (or major direction) of the heart’s electrical forces (mainly the ventricles)

The QRS Axis

QRS Axis is determined from the EKG by projecting the magnitude, direction, and polarity of the QRS deflection in the EKG lead onto its axis lead in the hexaxial reference systemIf the majority of the QRS deflection on the EKG is above the baseline, the vector will point toward the positive end

The Mean QRS AxisIf the QRS deflection in lead I is primarily positive, the vector points towards the positive side of lead I (toward 0°)In the same EKG, if the QRS deflection in lead aVF is primarily positive, the vector points toward +90°When averaged, the Mean QRS Axis is somewhere between 0° and +90° To localize more accurately, find the EKG with the smallest or biphasic QRS deflection and take its perpendicular on the hexaxial system

Causes of Left Axis Deviation

Left anterior hemiblock (LAHB)Left ventricular hypertrophy (LVH)Chronic Obstructive Pulmonary Disease (COPD)Inferior Wall Myocardial Infarction (IWMI)Wolff-Parkinson White (WPW) SyndromeOld ageNormal Variant

Causes of Right Axis Deviation

Left posterior hemiblock (LPHB)Right ventricular hypertrophy (RVH)Chronic Obstructive Pulmonary Disease (COPD)Lateral Wall Myocardial Infarction (LWMI)Wolff-Parkinson White (WPW) Syndrome Younger ageNormal variant

R Wave Progression

Chest electrodes, when placed in their proper positions, record depolarization of the ventricles (QRS Complexes) that become progressively more positive from lead V1 to lead V6This is known as R wave progressionNormally, the transition zone where the negative deflections become positive deflections occurs in leads V3 or V4

R Wave Progression

Poor R Wave Progression Anterior (wall) myocardial infarction

(AWMI) Left bundle branch block (LBBB) Left ventricular hypertrophy (LVH) Chronic Obstructive Pulmonary

Disease (COPD)

Heart Rate Determination

(Memorize!)

Standard recording speed of the EKG paper is 25 mm/secHeart rate is beats/minuteA rapid method of determining heart rate involves memorizing a sequence of numbers (300, 150, 100, 75, 60, 50, etc.) with each number corresponding to a heavy black line, which is one large box/squareInterval between two R waves, the R-R interval, determines ventricular rate

End of Lecture #1

Questions?