cvs electrocardiogram.docx

7/27/2019 CVS ELECTROCARDIOGRAM.docx

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CARDIOVASCULAR SYSTEM

ELECTROCARDIOGRAM (ECG)

AP DR SWE SWE WIN

Physiology Unit

Division of Basic Sciences

Faculty of Medicine

Cyberjaya University Cyberjaya Medical College

No. 3410, Jalan Teknokrat 3, Cyber 4, 63000 Cyberjaya

Selangor, Malaysia


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ELECTROCARDIOGRAPHY

(ECG)

Topic Page

I BASIC THEORY ON ECGI.1 Functional Anatomy and Histology of the Heart

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I.2 The Electrical Activity of the Heart

I.2.1Characteristics of a Resting Ventricular Muscle

I.2.2 Production of Pacemaker Potential and Action Potential By the SA

Node

I.2.3 Action Potential of a ventricular fibre (Fast Response)

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I.3 Functional Organisation of the Conducting System 8

1.4 Background of ECG 11

II HISTORICAL BACKGROUND OF ECG

II. 1 Nature of ECG 13

II.2 Uses of ECG 15

III HOW TO PERFORM ECGThe Steps in Setting Up Basic ECG Measurement 54

IV INTERPRETING BASIC ECG 54

Approach for reading ECG

Determination ofIV.1 Heart rate 76

IV.2 Rhythm 79

IV.3 Cardiac Axis/Vectors 94

IV.4 Waves 96

IV.5 Intervals 129

IV. 6 Segments 145


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ELECTROCARDIOGRAPHY

(ECG)

I. BASIC THEORY ON ECGElectrocardiogram (ECG)

It is a standard clinical tool used to study the electrical activity of the heart.

It works by picking up and amplifying very small electrical potential changes betweendifferent points on the surface of the body caused by cyclical depolarisation and

repolarisation of the heart cells (cardiac myocytes).

The location of the electrodes and the conventional ways in which they are connectedenable the ECG to look at the heart from a series of different designated directions.

The cycle of electrical changes during a single heart beat is termed an ECG complex.

Different component of ECG complex reflect the activation of different parts of theheart.

Functional Anatomy of the Heart
https://upload.wikimedia.org/wikipedia/commons/f/f5/Wiki_Heart_Antomy_Ties_van_Brussel.jpghttps://upload.wikimedia.org/wikipedia/commons/f/f5/Wiki_Heart_Antomy_Ties_van_Brussel.jpg


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I.1 FUNCTIONAL ANATOMY AND HISTOLOGY OF THE HEART

The heart lies mainly in the lower part of left side of the chest

It is normally oriented so that the ventricles point diagonally downwards away fromthe atria which lie close to the midline.

I.2 THE ELECTRICAL ACTIVITY OF THE HEART

I.2.1 CHARACTERISTICS OF A RESTING VENTRICULAR MUSCLE CELL

(Concentrations in mEq/L)

ECF ion

concentration

IC ion

concentration

Equilibrium

potential

Permeability

K+ 4 135 -94 mV high

Na+ 145 10 +70 mV low

Ca2+

2 10- +132 mV low

MEMBRANE CHANNELS

Ungated Potassium Channels

Always open and unless the membrane potential reaches the potassium equilibriumpotential (-94 mV), a potassium flux (efflux) continues through these channels.

Voltage-gated (- Dependent) Sodium Channels

Closed under resting conditions.

Membrane depolarisation is the signal that causes these channels to quickly open andthen close.


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Because they open and close quickly, they are sometimes referred to as thefastchannels.

These channels have the same characteristics as the voltage-gated (VG) sodiumchannels in the neuron axon.

Once closed, they will not respond to a second stimulus until the cell repolarises.

Voltage-gated(VG) Calcium Channels

Closed under resting conditions.

Depolarisation is the signal that causes these channels to open, but they open moreslowly than the sodium channels.

Consequently, they are sometimes called theslow channels, because they allowsodium as well as calcium to pass, theslow calcium-sodium channel (as an

appropriate terminology).

The calcium entering the cell through these channels will participate in contraction

and will also be involved in the release of additional calcium from the sarcoplasmicreticulum.

If the fast Ca2+channels fail to open, depolarisation occurs via the entrance of calciumthrough these channels.

Voltage-Gated Potassium Channels

Open under resting conditions.

Depolarisation is the signal to close these channels.

They will be closing during the depolarisation phase and will be closed during themain part of the plateau phase.

They begin to reopen during the latter part of the plateau phase and continue to reopenduring repolarisation.

Thus, potassium conductanceo is exceptionally high under resting conditions,o decreases during depolarisation,o is at a minimum during the plateau phase, ando increases back toward the high resting level during repolarisation.


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I.2.2 PRODUCTION OF PACEMAKER POTENTIAL AND ACTION POTENTIAL

BY THE SA NODE

PACEMAKER POTENTIALS

I.2.3 ACTION POTENTIAL OF A VENTRICULAR FIBRE (FAST RESPONSE)


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Electrical Versus Mechanical Events (Excitation-contraction Coupling in Cardiac

Muscle)

I.3 FUNCTIONAL ORGANISATION OF THE CONDUCTING SYSTEM

The intrinsic rate of AP generation is fastest in SA node & slowest in BOH.

Conduction velocity is fastest in Purkinje system & slowest in AV node.


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The Heart and Its Electrical Conducting System

Ventricular Depolarisation

Proceeds from endocardium to epicardium.

The duration of the action potential of myocytes near the endocardium is longer than that of

outer myocytes. As a result, the outermost cells repolarise1st and EC current flow towards

V3-V6, which thus record a positive deflection (T wave).

Ventricular Repolarisation

Proceeds from epicardium to endocardium.

When cardiac cycle is completed, no myocytes are depolarised and ECG returns to baseline(isoelectric line).

Time Taken for Impulse Through Different Tissue

SA node

0.03 sec

AV node

0.16 secBundle branches

0.19 secPurkinje fibers

0.22secEndocardial and epicardial surface of ventricles
https://upload.wikimedia.org/wikipedia/commons/0/05/ConductionsystemoftheheartwithouttheHeart.pnghttp://4.bp.blogspot.com/_zb7BqrrxGWI/Swf5PAAep6I/AAAAAAAAAIs/tT_Va89y4io/s1600/Cardiac+conduction.gifhttps://upload.wikimedia.org/wikipedia/commons/0/05/ConductionsystemoftheheartwithouttheHeart.pnghttp://4.bp.blogspot.com/_zb7BqrrxGWI/Swf5PAAep6I/AAAAAAAAAIs/tT_Va89y4io/s1600/Cardiac+conduction.gifhttps://upload.wikimedia.org/wikipedia/commons/0/05/ConductionsystemoftheheartwithouttheHeart.pnghttp://4.bp.blogspot.com/_zb7BqrrxGWI/Swf5PAAep6I/AAAAAAAAAIs/tT_Va89y4io/s1600/Cardiac+conduction.gifhttps://upload.wikimedia.org/wikipedia/commons/0/05/ConductionsystemoftheheartwithouttheHeart.pnghttp://4.bp.blogspot.com/_zb7BqrrxGWI/Swf5PAAep6I/AAAAAAAAAIs/tT_Va89y4io/s1600/Cardiac+conduction.gif


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Sequence of Cardiac Excitation

Conduction Pathways and Velocity of Conduction

PathwaySequential spread of action potential from SA node atrial muscle AV node

(delay) Purkinje fibres ventricular muscle


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I.4 BACKGROUND OF ECG

Normally, the chambers of the heart beat in an ordered sequence.

The heartbeat occurs in response to electrical excitation of muscle fibres by propagatedaction potentials.

The origin of propagated action potentials is the dominant pacemaker of the heart-

normally the SA node of the right atrium. These electrical waves are transmitted via the conducting system of the heart to

contracting muscle fibres.

Excitation, then, contraction, of the atria (atrial systole) is followed by excitation, thencontraction, of the ventricles (ventricular systole).

Between these electrical events, and during diastole, all 4 chambers are relaxed.

Because our body fluids are good electrical conductors, the algebraic sum of actioncurrents from excited cardiac muscle fibres can be recorded as voltage signals on the

surface of the body.

In a volume conductor of these small currents, voltage difference may be derived by

electronically subtracting voltage signals from 2, strategically positioned, electrodes onthe body surface and, therefore, displaying these differences as an ECG.

When voltage signals are recorded from arms and leg, the limb act as linear conductorconnected to volume conductor.

Normal spread of electrical activity in the heart


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II. HISTORICAL BACKGROUND OF ECG

British physiologist, Augustus D. Waller was the pioneer of Electrocardiography andin 1887 published the first human Electrocardiogram.

In 1942, the Noble prize in Physiology / Medicine was awarded to a Dutchphysiologist, William Einthoven, who transformed the electrocardiogram into clinical

recording device that is still used today.

II.1 NATURE OF ECG

The recordof the potential fluctuations during the cardiac cycle is called electrocardiography.


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The machine used to record these potential fluctuations is calledElectrocardiograph(sensitive galvanometer). It has electrical filters that reduce the

electrical noise.

The recordingproduced by this non-invasiveprocedure is termed anelectrocardiogram (also ECG or EKG- German word = Elektro-kardiographie).

Moving limbs, breathing, coughing, shivering, and faulty contact between the skin

and the electrode produce artifacts on the recorded ECG.

ECG is a transthoracic (across the thorax or chest) interpretation of the electricalactivity of the heart over a period oftime, as detected by electrodes attached to the

surface of the skin and recorded by a device external to the body (Not a direct

recording of actual electrical activity of heart).

ECG recording represent overall spread of activity throughout heart duringdepolarisation & repolarisation. (Not a recording of a single action potential in a

single cell at a single point of time).

ECG refers to extracellular recording of the summed-up electrical events of all thecardiac muscle fibres generated with each heart beat.

ECG is the surface recording of the potential difference (algebraic sum of electricalcurrents and potential) detected by electrodes placed on the body surface (called

leads). It requires special amplifiers.

Such detection is possible because body fluids contain electrolytes and therefore, aregood conductors of electricity (i.e. the body acts are a volume conductor).

ECG recording represents comparison in voltage detected by electrodes at differentpoints on the body (Not the action potential).

Because the movement of charge (i.e. the spreading wave of electrical activity of theheart) has both a three- dimensional direction and magnitude, the signal measured

on the ECG is a vector.

The system that clinicians use to measure the hearts 3-dimensional , time-dependentelectrical vector is simple to understand and easy to implement, but it can be

challenging to interpret.

The continuous electrical in the heart alternating between depolarisation andrepolarisation in all parts of the heart following transmission of impulses. Electrically

heart behaves as a dipole, i.e. two terminal batteries in which the excited part

(depolarised segment) forms a negative and non-excited forms the positive pole.
https://en.wikipedia.org/wiki/Non-invasive_(medical)https://en.wikipedia.org/wiki/Human_thoraxhttps://en.wikipedia.org/wiki/Electricityhttps://en.wikipedia.org/wiki/Hearthttps://en.wikipedia.org/wiki/Timehttps://en.wikipedia.org/wiki/Electrodehttps://en.wikipedia.org/wiki/Electrodehttps://en.wikipedia.org/wiki/Timehttps://en.wikipedia.org/wiki/Hearthttps://en.wikipedia.org/wiki/Electricityhttps://en.wikipedia.org/wiki/Human_thoraxhttps://en.wikipedia.org/wiki/Non-invasive_(medical)


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Depolarised segment

Non-excited segment

The Heart as a Dipole

Current flowing towards an electrode gives a positive deflection

Current flowing at right angles to an electrode gives no deflection

Current flowing away from an electrode gives negative deflection

II. 2 USES OF ECG

Electrocardiography (ECG orEKG) is used for detection, monitoring, confirmation of

heart rate (both atrial & ventricular)

heart rhythm (regularity of heartbeats)

site of origin and conduction of cardiac impulses (sinus, atrial, junctional / ventricular,accelerated, delayed or blocked; site of defect)

changes in myocardial perfusion (ischemia), structure (infarction, hypertrophy) orfunction (ventricular fibrillation, cardiac arrest).

changes in plasma electrolytes (K+, Ca2+) (K+ changes influences repolarisation,producing very tall slender peaked T waves in hyperkalaemia, and ST segment

depression and prominent U waves in hypokalaemia).

other systemic disorders the effects of drugs or devices used to regulate the heart, such as a pacemaker.

+ + + + + + + + + + + + + + + + + + + + + +

+ + + + + + + + + + + + + + ++ + + + +

+ + + + + + + + + + + +

+ + + + + + + + + + + ++ + + + + + + + + + +

++ + + + + + + + + + + ++ + + + ++ + + + +

- - - -- - -

-- - - -

+ +

---
https://en.wikipedia.org/wiki/Artificial_pacemakerhttps://en.wikipedia.org/wiki/Artificial_pacemaker


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III. HOW TO PERFORM ECG (RECORDING OF ECG)

The 4th electrode, connected to the

right leg, is used for electrical grounding.

RECORDING OF ECG

ECG is constructed as a system of leads in two planes that are perpendicular to eachother.

1. 6 limb leads(I, II, III, aVR, aVL, aVF) view the heart in vertical plane.2. 6 precordial /chest leads(V1- V6)view the heart in horizontal plane.

3. Anatomical relationships:o leads II, III, and aVF view the inferior surface of the heart;o leads V1 to V4 view the anterior surface;o leads I, aVL, V5, and V6 view the lateral surface; ando leads V1 and aVR look through the right atrium directly into the cavity of the

left ventricle.

Each lead is an axis in one of the two planes, onto which the heart projects itselectrical activity.

ECG recording from a single lead shows how that lead views the time-dependentchanges in voltage of the heart.

ECG Recording Electrodes (LEADS)

ECG leads have different viewpoints of the hearts electrical activity. ECG leads refer to the 2 electrodes which are placed on the body surface and

connected to ECG machine for measuring the potential fluctuations between only 2

points.

In a lead one electrode is treated as the positive side of a voltmeter and the other oneor more electrodes as the negative side.

Therefore, a lead records the fluctuation in voltage difference between positive andnegative electrodes.

By variation of which electrodes are positive and which are negative, a standard 12-lead ECG is recorded.
https://en.wikipedia.org/wiki/File:ECGcolor.svghttps://en.wikipedia.org/wiki/File:ECGcolor.svg


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Each lead looks from a unique angle and plane, i. e. from what is essentially its ownunique point of view.

ECG is recorded using 2 types of leads (12-lead ECG)o The bipolar leadso Unipolar leads

ECG leads record the fluctuationin difference between positive & negativeelectrodes.

ECG

Leads

Purpose Recording

Electrodes

Type of

Leads

View/

Plane

Bipolar Record electrical difference

between 2 electrodes

Negative & positive

(Both active)

Standard

limb leads

LI, II, III

Vertical /

Frontal

Unipolar One electrode-

active/exploring/+ve

terminal

Next electrode-indifferent/zero/negative

terminal connecting two

other limbs.

aVR+ve = Right arm-ve = LA + LF

aVL+ve = Left arm

-ve = RA + LF

aVF+ve = Left foot-ve = LA + RA

Limb

leads

aVR (cavityof ventricles, -150),

aVL (lateralsurface of

ventricles, -

30),

aVF

(inferiorsurface ofventricles,

+90)

Vertical /

Frontal

+ve to one of the

6 differentlocation on chest

wall.

-ve toomiddle of

chest

oaverage of 3limb

electrodes

Chest

leadsV1V6( V7

V9 in

special

cases)

Transverse/

Horizontal

Normal ECG waves

P wave = 0 - + 75

QRS complex = -30 - + 90

T wave = Tangle < 45 Frontal

< 60 Precordial

1.BIPOLAR LEADS

These record the potential difference between 2 active electrodes.

In each bipolar recording, both the electrodes are active-

o one of the active electrode is connected to negative terminal of the ECG


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machine and

o the other active electrode is connective to the positive terminal.

Current flowing towards the positive electrode produces an upward deflection on theECG.

Clinically, 3 bipolar leads are used.

Standard limb Leads (3 limb leads)

Limb leads look at the heart in a vertical plane.

Three standard limb leads used in bipolar recording are based on Einthovenshypothesis that the body is like an electrically homogenous plate in which the right

and left shoulders and the pubic region form the corners of an equilateral triangle

with heart in its centre (EINTHOVENS TRIANGLE)

Einthoven law states that:

The sum of the potentials at the points of an equilateral triangle (LA, RA, and LF) with acurrent source (heart) in the centre is zero at all times.

If the electrical potentials of any two of the three bipolar limb ECG leads are known at anygiven instant, the third one can be determined mathematically from the first two by simply

summation the first two. (Note: the positive and negative signs of different leads must be

observed when making this summation= Voltage lead I + voltage lead III = voltage lead II)

That 2 active electrodes need to be placed at 2 corners of this triangle.

But for conveniences the electrodes are connected to the left are (LA), right arm (RA)and left foot (LF) instead of the shoulders and the pubic region (Figures below).

Practically, it does not make any difference whether the electrodes are placed inproximal or distal part of the extremities; because the current flows in the body fluids

and so the records obtained are similar.

These record the potential difference between 2 limbs.

If these electrodes are connected to a common terminal, an indifferent electrode thatstays near zero potential is obtained. This is because in a volume conductor, the sum

of the potentials at the points of an equilateral triangle (LA, RA, and LA) with a

current source (the heart) in the centre is zero at all times (Einthovens triangle). In 3 standard limb leads, the 2 active electrodes are connected as

Lead + ve electrode -ve electrode Defines an axis in the

frontal plane at

I Left arm (LA) RA= LARA 0 (degree)

II Left foot (LF) left Leg (LL) RA = LFRA + 60 (degrees)

III Left leg (LL) (LF) LA= LFLA + 120 (degrees)


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Normal30 to90

Normal Normal

Left axis

deviation

30 to90

May indicate left anterior fascicular

block or Q waves from inferiorMI.

Left axis deviation is considered

normal in pregnant women and

those with emphysema.

Right axis

deviation

+90 to

+180

May indicate left posterior

fascicular block, Q waves from

high lateral MI, or a rightventricular strain pattern

Right deviation is considered

normal in children and is a

standard effect ofdextrocardia.

Extreme right

axis deviation

+180

to 90Is rare, and considered an electricalno-mans land
https://en.wikipedia.org/wiki/Left_axis_deviationhttps://en.wikipedia.org/wiki/Left_axis_deviationhttps://en.wikipedia.org/wiki/Left_axis_deviationhttps://en.wikipedia.org/wiki/Myocardial_infarctionhttps://en.wikipedia.org/wiki/Emphysemahttps://en.wikipedia.org/wiki/Right_axis_deviationhttps://en.wikipedia.org/wiki/Right_axis_deviationhttps://en.wikipedia.org/wiki/Right_axis_deviationhttps://en.wikipedia.org/wiki/Myocardial_infarctionhttps://en.wikipedia.org/wiki/Dextrocardiahttps://en.wikipedia.org/wiki/File:Rapid_Axis_Vector.svghttps://en.wikipedia.org/wiki/File:Rapid_Axis_Vector.svghttps://en.wikipedia.org/wiki/Dextrocardiahttps://en.wikipedia.org/wiki/Myocardial_infarctionhttps://en.wikipedia.org/wiki/Right_axis_deviationhttps://en.wikipedia.org/wiki/Right_axis_deviationhttps://en.wikipedia.org/wiki/Emphysemahttps://en.wikipedia.org/wiki/Myocardial_infarctionhttps://en.wikipedia.org/wiki/Left_axis_deviationhttps://en.wikipedia.org/wiki/Left_axis_deviation


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Right leg (usually black) = earth lead
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2.UNIPOLAR LEADS

In unipolar recording, 1 electrode is active (exploring electrode)- placed on achosen site linked with and the other electrode- called indifferent electrodeat zero

potential.

One being zero, the potential difference between the two represents the actual localpotential (potential flutuations occuring at the site of exploring electrode).

I n a volume conductor, the sum potenti als at the poin ts of an equil ateral tr ianglewith a current source at the center is zero at all times.

Therefore, if the 3 electrodes (placed on left arm, right arm and on the left leg) areconnected to a common terminal, through a resistance, an indifferent electrode that

stays near zero potential is obtained.

In clinical ECG, 2 types of unipolar leads are used.2.1 Unipolar limb Leads

2.2 Unipolar chest Leads

They (aVR, aVL, aVK, V1-V6) are called V leads because record valuesapproching meaningful voltages.

2.1 Unipolar limb Leads (3 leads)

These include VR(Right arm), VL (Left arm), VF (Left leg).

Lead Exploring electrodeVR Right arm

VL Left arm

VF Left foot

The exploring electrode is connected to the positive terminal of ECG. The indifferent electrode is placed over a limb and is connected to the negative

terminal of ECG.

These leads are not used and have been replaced by augmented limb leadsaVR, aVL, aVFWhen the amplitude of deflection in increased (a= augmented)

AugmentedUnipolar limb Leads

Generally the augmented unipolar limb leads designated as aVR, aVL, aVF are used.

In augmented leads, the size of the potential is increased by 50% without any changein configuration from the non-augmented record.

The active electrode is from one of the limbs, and the indifferent electrode is obtainedby connecting the other 2 limbs.

(Recordings between one limb and the other two limbs).

Lead Leads Exploring electrode Indifferent electrode Defines axis in

(+ve electrode)* Frontal planeaVR Right arm (usually red) LA + LF - 150

aVL Left arm (usually yellow) RA + LF - 30

aVF Left foot (usually green) RA + LA + 90

*(Negative electrode connection is electrotonically defined in the middle of the heart)


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Direction of augmented limb leads in vertical plane, and their orientation to the heart

and the cardiac vector in the centre of the triangle;

aVR- Lead reflects the activity of the cavity of the ventricles,irrespective of the position of

the heart.

- P wave, QRS complex, T wave are negativedeflection.

aVLLead reflects the electrical activity ofleft lateral surface of the heart- (formed

by the left ventricle)

- Like that of V6, QRS is predominately positive

aVFLead reflects the electrical activity of the inferior surface of the heart (formed by

left ventricles and interventricular septum)

- Like that of V3 & V4, QRS complex is predominantly biphasic.
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2.2 Unipolar Chest Leads (Precordial Leads)

They lie in the transverse plane, perpendicular to the plane of frontal leads (look at the

heart in a horizontal plane).o The positive connection is one of the six different locations on the chest wall,o The negative connection is electrotonically defined in the middle of the heart

by averaging of the 3 limb electrodes.

The resultant leads are named V1 to V6

V1 4th intercostals space (ICS), right sterna border/margin

V2 4th intercostals space (ICS), left sterna border/margin

V3 Equidistant between V2 and V3

V4 5th ICS, mid clavicular line

V5 5th ICS, anterior axillary line

V6 5th ICS, midaxillary line

(V7 5th ICS, posterior axillary line so forth up to V9) for posterior MI
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A Standard 12-lead ECG

V1 & V2 Reflects right ventricular activity:The main QRS complex is negative

V3 & V4Reflects activity ofInterventricular septum and anterior wall of the leftventricle:

The main QRS complex is biphasic

V5 & V6Reflects avtivity ofAnterior and lateral walls of the left ventricle:The main QRS complex is positive

R waveGradually incr eases in sizefrom V1 to V6 leads.in V1- R wave represents activity of right ventricle

V6- R wave represents activity of left ventricle

S wave- Gradually decreases in sizefrom lead V1 to V6.

in V1- S wave represents activity of left ventricleV6- S wave represents activity of right ventricle


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Red - 4th intercostals space (ICS), right sterna margin (V1)

Yellow- 4th (ICS), left sterna margin (V2)

Green - Between yellow & brown (V3)

BrownAnatomical position apex of the heart- 5th

ICS (V4) Black - 5th ICS, anterior axillary line (V5)

Purple - 5th ICS, mid axillary line (V6)

CARDIAC AXIS (VECTOR) of a wave

Cardiac axis refers to mean direction of the wave of ventricular depolarization in thevertical plane, measured from zero reference point.

The zero reference = same viewpoint as lead I.

An axis above line is negative number, and axis below line is positive number.

The normal range for cardiac axis is between 30 and 90. An axis 90 is termed right axis deviation. It is the determination of magnitude & direction (arrow) of current flow (potential

generated) through the heart.

By convention, the arrow head points towards the direction & length of the arrow isdrawn proportional to the voltage of the potential.

During most of the cycle of ventricular depolarisation direction of electrical potential(negative to positive) is from the base of the ventricles towards the apex. This

preponderant direction of potential during depolarisation is called the mean QRSvector(mean electrical axis of the heart) and is drawn through the centre of the

ventricles in a direction from the base of the heart toward the apex.

The instant vectorrepresents the magnitude and direction of potential at a particular

instant during the cardiac cycle. The instantaneous vector of 5 different instantsduring the process (sequence) of ventricular depolarisation is shown below.


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It refers to the overall direction of the wave of ventricular depolarisation in thevertical plane measured from a zero reference point.

The normal range of axis of ventricular depolarisation in the frontal plane (cardiacaxis) is between -30 and + 90.

Lead Axis () The

electrode

lies in

plane

Positive

electrode

Negative

electrode

Algebraic sum

I 0 Horizontal

direction

Left arm Right arm Q (-3), R (+13),S (-5) =

-3+13 -5 = 5 mV

II 60 Horizontal Left leg Right arm Q (-1), R (+15), S (-0) =

-1 +150 = + 14 mVIII 120 Horizontal Left leg Left arm

aVF 90 Vertical Left leg Indifferent

electrodeRA + LA

aVR 210 Vertical Right arm LA + LF

aVL -30 Vertical Left arm RA + LF

Sequence of ventricular depolarisation

QRSAxisThere are many ways to determine QRS axis. The one described below combines simplicity

and efficiency.


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The limits of normal and abnormal QRS axis aresummarized in the diagram to the right.

QRS axis is the direction in which the mean QRS current flows.

The normal axis points mostly downward and to the left because

the more muscular left ventricle generates a stronger depolarizing

current that overwhelms that generated by the less bulky right.

Although both right axis deviation (RAD) and left axis deviation(LAD) are not necessarily associated with organic heart disease,

they are seen in a number of settings and their presence can provide

added evidence to support a clinical diagnosis.

RAD is seen in right ventricular (RV)

hypertrophy and in infarction involving the

left ventricle (LV). Right ventricular muscle

bulk is relatively larger than that of the left

in both conditions and generates a stronger

depolarizing current in its direction.

LAD is seen, but not always, in patients

with left ventricular hypertrophy. More

commonly the QRS axis is horizontal in

this condition.

It is only necessary to examine the QRS complexes in leads I and II to determine whether the

QRS axis in normal or deviated to the left or the right; a precise calculation of the QRS axis

is not required in clinical interpretation of the ECG.

It has been explained in a previous section that a current flowing in the direction of a

recording electrode (an ECG lead) registers a positive deflection and a current flowing away

registers a negative deflection.
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Therefore, the QRS in lead I would be positive if the QRS current flows in the direction of

lead I and negative if away. Similarly the QRS in lead II would be positive if the QRS axis

points in the direction of lead II and negative if away.

By overlapping the two circles representing leads I

and II, it can be seen that the QRS axis is between

+90o and -30o and normal if the QRS is positive

both in lead I and lead II.

QRS axis is between -30o and -90o or deviated to

the left (left axis deviation or LAD) if the QRS is

positive in lead I but negative in lead II.

QRS axis is between +90o and +150o or deviated

to the right (right axis deviation or RAD) if the

QRS is negative in lead I but positive in lead II.

On occasions the QRS complexes in all 6 limb leads are biphasic, neither positive nor

negative. In these instances the QRS axis is said to be indeterminate.


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In summary:

Normal QRS axis.

Left axis deviation

Right axis deviation


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TO CALCULATE CARDIAC AXIS,

The simplest method is inspection of leads I, II, and III.

Conditions for which determination of the axis is helpful in diagnosis Conduction defects for example, left anterior hemiblock Ventricular enlargement for example, right ventricular hypertrophy

Broad complex tachycardia for example, bizarre axis suggestive of ventricular origin Congenital heart disease for example, atrial septal defects Pre-excited conductionfor example, Wolff-Parkinson-White syndrome Pulmonary embolus

A more accurate estimate of axis achieved if all six limb leads examined.

The direction of current flow is towards leads with a positive deflection, away from leads

with a negative deflection, and at 90 to a lead with an equiphasic QRS complex.The axis is determined as follows:

Choose the limb lead closest to being equiphasic. The axis lies about 90 to right or left ofthis lead

Inspect QRS complexes in leads adjacent to equiphasic lead. If lead to the left is positive,then axis is 90 to the equiphasic lead towards the left. If the lead to the right side is positive,

then the axis is 90 to the equiphasic lead towards the right.

Schematic diagram used to determine the direction of electrical changes in the six limb

leads on an ECG

However, this axis can change in a number of pathological situations, includingo hypertrophy of one or both ventricular walls (a common sequel of prolonged

hypertension)

o conducting blocks in one or several of the ventricular conducting pathways.Mean Electrical Axis

o The mean electrical axis of the ventricles describes the net direction of currentmovement during ventricular depolarisation.

o It is affected by a number of factors, including the position of the heart, heart mass,and conduction time.

o It can be calculated by summing the depolarisation during the QRS complex in anytwo leads.

The axis is normal if leads I and II are positive.

A simple method to calculate the axis is by inspection of the QRS complex in leads I,II, III.

An axis more negative than -30 is termed left axis deviation.

An axis more positive than +90 is termed right axis deviation.


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DETERMINATION OF QRS COMPLEX FROM ECG

QRS complex

Direction of depolarization (vector) of the QRS complex

1. The left ventricle is thicker so the mean QRS vector is down and to the left. (Theorigin of the vector is the AV node with the left ventricle being down and to the left of

this).

2. The vector will point toward hypertrophy (thickened wall) and away from the infarct(electrically dead area).

Figure: Axis nomenclature.

Normal axis -30 to +90 degrees

Left axis deviation -30 to -90 degrees

Right axis deviation +90 to +/-180 degrees
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Indeterminate (extreme) axis deviation -90 to +/-180 degrees

Since lead I and aVF are perpendicular to each other, you can use those two leads to quickly

determine axis.

Lead I runs from right to left across a patient's body, positive at the left hand:

If the QRS in lead I is positive (mainly above the baseline), the direction of depolarization

will be in the positive half (right half) of the circle above.

Lead aVF runs from top to bottom across a patient's body, positive at the feet:

If the QRS in lead aVF is positive (mainly above the baseline), the direction of depolarization

will be in the positive half (lower half) of the circle above.

To find the axis overlap the two circles. The common shaded area is the quadrant in which

the axis lies. In this example, the axis lies in the normal quadrant, which on a patient, pointsdown and to the left.

You can repeat this process for any two leads, but I and aVF are the classic places to look. If

you realize that there are two leads to consider and a positive (+) or (-) orientation for each

lead, there would be four possible combinations. Memorize the following axis guidelines.


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Lead ILead

aVF

1. Normal axis (0 to +90 degrees) Positive Positive

2. Left axis deviation (-30 to -90) Also check lead

II. To be true left axis deviation, it should also be

down in lead II. If the QRS is upright in II, the axis

is still normal (0 to -30).

Positive Negative

3. Right axis deviation (+90 to +180) Negative Positive

4. Indeterminate axis (-90 to -180) Negative Negative

Figure:Normal axis

Figure:Left axis deviation


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Figure:Right axis deviation.

The bottom line is, if the axis is shifted out of the normal quadrant, evaluate the reasons for

this.

Differential Diagnosis

Left axis deviation LVH, left anterior fascicular block, inferior wall MI

Right axis deviation RVH, left posterior fascicular block, lateral wall MI

The electrical axis represents the average direction of the total force produced by the right

and left ventricular depolarization. A three step process can be used when determining axis:

1. Out of leads I, II, III, aVF, AVL, AVR, determine which lead is has roughly equal

amplitude of positive and negative deflections. This is the "isoelectric" lead.

2. Find the lead that is perpendicular to this isoelectric lead.

3. Identify the direction of the QRS axis in the lead identified in step 2. If the QRS

deflection is positive, then the axis is the same as the positive pole of that lead. If

negative, then the axis is the same as the negative pole.
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The normal axisis generally between -30 and +90 degrees.

Righ t axis deviationis defined as axis located between 90 degrees and 180 degrees.

Left axi s deviationis defined as the axis located between -30 degrees and -90 degrees.Indeterminate (or extreme) axis deviation is defined as an axis in the arc between 180 to -90

degrees.

Another quick way to approximate the axis is to look first at leads I and aVF. The following

table can be used to determine the axis rapidly although this method is not as rigorous as

above.


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Right Axis Deviation

Right Axis Deviation

Definition

QRS axis between + 90 and + 180 degrees
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Right axis deviation: +90 to +180 degrees

How to recognise right axis deviation (RAD)

QRS is positive (dominant R wave) in leads III and aVF

QRS is negative (dominant S wave) in leads I and aVL

Right axis deviation: leads III and aVF are positive; leads I and aVL are negative
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Causes of RAD

Left posterior fascicular block

Lateral myocardial infarction

Right ventricular hypertrophy

Acute lung disease (e.g. PE)

Chronic lung disease (e.g. COPD)

Ventricular ectopy

Hyperkalaemia

Sodium-channel blocker toxicity

WPW syndrome

Normal in children or thin adults with a horizontally positioned heart

Left Axis Deviation

Left ventricular hypertrophy
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Left Axis Deviation

Definition

QRS axis between -30 and -90 degrees

Left axis deviation: -30 to -90 degrees
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How to recognise left axis deviation (LAD)

QRS is positive (dominant R wave) in leads I and aVL

QRS is negative (dominant S wave) in leads II and aVF

Left Axis Deviation: leads I and aVL are positive; leads II and aVF are negative

Causes of left axis deviation (LAD)

Left anterior fascicular block

Left bundle branch block

Left ventricular hypertrophy

Inferior MI

Ventricular ectopy

Paced rhythm

Wolff-Parkinson White syndrome

LEFT AXIS DEVIATION(LAD)

is said to be present when the MEA lies between -30 and - 90 (or 2 and 12Oclock).

Lead I is positive and lead II and III negative.
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Left axis deviation

Causes of left axis deviation:

A block of the anterior bundle of the main left bundle conducting system.

Normal variation (physiologic, often with age)

Mechanical shifts, such as expiration, high diaphragm (pregnancy, ascites, abdominal

tumour) Left ventricular hypertrophy
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Left bundle branch block

Left anterior fascicular block

Wolff-Parkinson-White syndrome

Congenital heart disease (e.g. atrial septal defect)

Emphysema

Hyperkalaemia Ventricular ectopic rhythms

Preexcitation syndromes

Anterolateral myocardial infarction

Inferior myocardial infarction

Obesity

Pacemaker rhythm

Left Atrial Enlargement

AKA: Left atrial hypertrophy, left atrial abnormality.

Background

Left atrial enlargement (LAE) is due to pressure or volume overload of the left atrium.

It is often a precursor to atrial fibrillation.

Electrocardiographic Criteria

LAE produces a broad, bifid P wave in lead II (P mitrale) and enlarges the terminal negative

portion of the P wave in V1.

Diagnostic criteria are as follows:

In lead II Bifid P wave with > 40 ms between the two peaks

Total P wave duration > 110 ms

In V1 Biphasic P wave with terminal negative portion > 40 ms duration

Biphasic P wave with terminal negative portion > 1mm deep

Causes

In isolation:

Classically seen with mitral stenosis

In association withleft ventricular hypertrophy:

Systemic hypertension

Aortic stenosis

Mitral incompetence

Hypertrophic cardiomyopathyExamples

Broad (>110ms), bifid P wave in lead II (P mitrale) with > 40ms between the peaks
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P wave terminal portion > 40 ms duration in V1

P waves with terminal portion > 1mm

Right Atrial Enlargement

AKA: Right atrial hypertrophy, right atrial abnormality


Right atrial enlargement produces a peaked P wave (P pulmonale) with amplitude:

> 2.5 mm in the inferior leads (II, III and AVF)

> 1.5 mm in V1 and V2Causes of right atrial enlargement

The principal cause is pulmonary hypertension due to:

Chronic lung disease (cor pulmonale)

Tricuspid stenosis

Congenital heart disease (pulmonary stenosis, Tetralogy of Fallot)

Primary pulmonary hypertension

Examples
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Right atrial enlargement: P wave amplitude > 2.5mm in leads II, III and aVF

Right atrial enlargement: P wave amplitude > 1.5 mm in V2

RIGHT AXIS DEVIATION (RAD) is said to be present when the mean electrical axis (MEA) lies between +120 and +180 (7 & 9 Oclock position).

Lead 1 is negative and lead III positive.
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Right axis deviation

Causes of right axis deviation:

Normal variation (vertical heart with an axis of 90)


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Mechanical shifts, such as inspiration and emphysema

Right ventricular hypertrophysecondary to chronic lung disease or pulmonary valve

stenosis.

Right bundle branch blockcausing delayed activation of the right ventricle

Left posterior fascicular block

Dextrocardia Ventricular ectopic rhythms

Wolff-Parkinson-White syndrome

Preexcitation syndromes

Posterior (or inferior) myocardial infarction

Lateral wall myocardial infarction

Right ventricular load, for example Pulmonary Embolism or Cor Pulmonale (as in

COPD

Biatrial EnlargementDefinition

Biatrial enlargement is diagnosed when criteria for both right and left atrial enlargement arepresent on the same ECG.

The spectrum of P-wave changes in leads II and V1 with right, left and bi-atrial enlargement

is summarised in the following diagram:
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Reproduced from Wagner et al. (2007)


The diagnosis of biatrial enlargement requires criteria for LAE and RAE to be met in either

lead II, lead V1 or a combination of leads.

In lead IIBifid P wave with:

Amplitude 2.5mmand

Duration 120 msIn V1Biphasic P waves with:

Initial positive deflection 1.5mm tall

andTerminal negative deflection 1mm deepand

Terminal negative deflection 40 ms durationCombination criteriaP wave positive deflection 1.5 mm in leads V1 or V2and

Notched P waves with duration >120 ms in limb leads, V5 or V6

Causes

Combination of both left and right atrial enlargement.

Right atrial enlargement

Pulmonary hypertension due to:

Chronic lung disease (cor pulmonale)
http://cdn.lifeinthefastlane.com/wp-content/uploads/2011/12/P-wave-morphologies.png


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Tricuspid stenosis

Congenital heart disease (pulmonary stenosis, Tetralogy of Fallot)

Primary pulmonary hypertension

Left Atrial Enlargement

Mitral valve disease

Aortic valve disease Hypertension

Aortic stenosis

Mitral incompetence

Hypertrophic cardiomyopathy (HOCM)

Example ECGs

Example 1

Biatrial enlargement due to idiopathic cardiomyopathy:

Biphasic P waves in V1 with a very tall positive deflection (almost 3 mm in height!)

and a negative deflection that is both deep (> 1 mm) and wide (> 40 ms).Example 2
http://cdn.lifeinthefastlane.com/wp-content/uploads/2011/12/biatrial-enlargement018.jpg


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Biatrial enlargement: P waves in lead II are tall (> 2.5mm) andwide (> 120 ms).

P waves in V2 are tall (> 1.5 mm), while the terminal negative portion of V1 is deep

(> 1mm) and wide (> 40 ms).
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