The only ekg book youll ever need 5th ed by mik

During atrial depolarization and contrac tion, elec trodes plac ed on the s urface of the body record a sm all burs t of elec trical activity las ting a frac tion of a sec ond... Electri

Authors: Thaler, Malcolm S.

Title: Only EK G Book You'll Ever Need, The , 5th Edition

Co pyright ©200 7 Lippinco tt Williams & Wilkins

Authors: Thaler, M alcolm S.

Title: Only EK G Book You'll Ever Need, The , 5th Edition

Co pyright ©200 7 Lippinco tt Williams & Wilkins

> F r o nt o f Bo o k > D e d i c a t i o n

Dedication

For my mot her, who will always live in my heart, and for Nancy, Ali, and Jon,still and always the heart o f m y matter

Authors: Thaler, M alcolm S.

Title: Only EK G Book You'll Ever Need, The , 5th Edition

Co pyright ©200 7 Lippinco tt Williams & Wilkins

clinical disorders, from m ild palpitations and diz ziness to life-threatening heartattacks and arrhythmias The EKG predates relativity, quantum mechanics,molecular genetics, bebop, Watergate, and, well, you fill in the blank H ats o ff,then, to Willem Einthoven and his string galvano meter with which, in 19 05, herecorded the first elektrokardiogramm

So here we are, well into the next m illennium , and no w it is your turn to learnhow to use this amazing tool It is my hope that this little book (itself getting abit lo ng in the tooth, having first co me out in 1988 ) will make the pro cess funand easy Its goals remain the sam e as they did in the first editio n:

This boo k is about learning It's about keeping simple things simple and

complicated things clear, concise, and yes, sim ple, too It's about gett ing fromhere to there without scaring yo u to death, boring yo u to tears, or intim idatingyour socks off It' s about turning ignorance into knowledge, kno wledge intowisdom , and all with a bit o f fun

There is a lot o f new stuff in this fifth edition We have, among ot her things,updated the sections on basic elect rophysiolo gy, rhythm disturbances, andpacemakers, and included many new sample EK Gs at the end o f the t ext so youcan test your new, hard-won knowledge

Again I must thank Glenn Harper, M D., not o nly one of the world' s great

cardiologists, but also one of its really go od guys, for reviewing this book andmaking sure it is accurate and up to date To all the folks at Lippincott Williams

& Wilkins, thanks for o nce m ore producing a beautiful and readable text andmaking the whole process o f revising it so sim ple and enjoyable

And to you readers, I hope that The Only EKG Book Y ou'll Ever Need will onceagain give yo u everything you need—no more and no less— to read EKGs

quickly and accurately

Malco lm Thaler

P 2

P 3

Authors: Thaler, Malcolm S

Title: Only EKG Boo k Yo u'll Ev er Need, The, 5th Edition

Copyri ght ©2007 Li ppi ncott Wil l i ams & Wil ki ns

> T a bl e of C o nt e nt s > Ge t ti n g S t a r t ed

Getting Started

On the opposi te page i s a normal el ectrocardiogram, or EKG By the ti me you have

f i ni shed thi s book—and i t won' t take very much ti me at al l —you wil l be abl e to

recogni ze a normal EKG al most i nstantl y Perhaps even more i mportantl y, you wi ll

have learned to spot all of the common abnormal i ties that can occur on an EKG, andyou wil l be good at i t!

P 4

P 5

Some peopl e have compared l earni ng to read EKGs wi th l earni ng to read musi c In

both i nstances, one i s faced wi th a completel y new notati onal system not rooted i nconventi onal l anguage and ful l of unf ami l i ar shapes and symbol s

But there real l y i s no compari son The si mpl e l ub-dub of the heart cannot approachthe subtle complexi ty of a Beethoven stri ng quartet, the mul ti pl yi ng tonali ti es and

rhythms of Stravi nsky' s Ri te of Spri ng, or even the artl ess sal vos of a rock-and-rol lband

There' s just not that much goi ng on

The EKG i s a tool of remarkabl e cl i ni cal power, remarkabl e both f or the ease wi th

whi ch it can be mastered and f or the extraordi nary range of si tuations i n which i t

can provi de helpf ul and even cri tical i nf ormation One gl ance at an EKG can

di agnose an evol vi ng myocardi al i nfarcti on, i denti fy a potenti al ly l if e-threateni ng

arrhythmi a, pinpoi nt the chroni c ef f ects of sustained hypertensi on or the acute

ef fects of a massi ve pul monary embol us, or si mpl y provi de a measure of

reassurance to someone who wants to begi n an exercise program

P 6

Remember, however, that the EKG i s onl y a tool and, li ke any tool , i s onl y as

capabl e as i ts user Put a chi sel i n my hand and you are unl i kel y to get

Mi chel angel o's David

The nine chapters of thi s book wil l take you on an el ectri f yi ng voyage from

i gnorance to dazzl ing competence You wi ll amaze your f ri ends (and, more

i mportantl y, yoursel f ) The roadmap you wi l l f ol l ow l ooks l i ke thi s:

Chapter 1: You wi l l l earn about the el ectrical events that generate the di f ferentwaves on the EKG, and—armed wi th thi s knowl edge—you wi l l be abl e to

recognize and understand the normal 12-l ead EKG

Chapter 2: You wi l l see how simpl e and predi ctabl e al terati ons i n certai n wavespermi t the di agnosi s of enl argement and hypertrophy of the atri a and

ventri cl es

Chapter 3: You wi l l become f ami l i ar wi th the most common di sturbances in

cardi ac rhythm and wi l l l earn why some are l i fe threatening whi le others are

merel y nuisances

Chapter 4: You wi l l l earn to i denti f y i nterrupti ons i n the normal pathways of

cardi ac conducti on and wi l l be i ntroduced to pacemakers

Chapter 5: As a compl ement to Chapter 4, you wil l learn what happens when

the el ectri cal current bypasses the usual channel s of conducti on and arri ves

more qui ckl y at its desti nati on

Chapter 6: You wi l l l earn to di agnose i schemi c heart di sease: myocardi al

i nf arcti ons (heart attacks) and angi na (i schemic heart pai n)

Chapter 7: You wi l l see how various noncardi ac phenomena can al ter the EKG.Chapter 8: You wi l l put all your newl y f ound knowl edge together i nto a simpl e11-step method f or readi ng al l EKGs

Chapter 9: A f ew practi ce stri ps wi l l l et you test your knowl edge and revel i n

your astoni shi ng i ntell ectual growth

P 7The whol e process is straightf orward and rather unsophi sti cated and shoul d not be

the l east bi t i nti mi dati ng Intri caci es of thought and great l eaps of creative l ogic arenot requi red

This i s not the ti me for deep thinki ng

P.11

A uthors : Thaler, M alcolm S

Title: Only EKG Book You'll Ever Need, The, 5th Edition

Copy right ©2007 Lippinc ott William s & Wilk ins

> Ta b l e o f C o nt e n t s > 1 - T h e B as i cs

1.

The Basics

Electricity and the Heart

Elec tric ity , an innate biologic al elec tric ity , is what mak es the heart go The EKG isnothing more than a rec ording of the heart's electric al ac tiv ity, and it is through

perturbations in the norm al electrical patterns that we are able to diagnose m any

different cardiac dis orders

All You Need to Know About Cellular Electrophysiology

in Two Pages

Cardiac cells, in their res ting s tate, are elec trically polarized, that is, their insides arenegativ ely c harged with res pec t to their outs ides T his elec trical polarity is m aintained

by m embrane pum ps that ens ure the appropriate dis tribution of ions (primarily

potass ium , sodium, c hloride, and calc ium ) nec es s ary to k eep the ins ides of thes e c ellsrelatively elec tronegativ e

The res ting c ardiac cell m aintains its elec tric al polarity by means of a m embrane

pump T his pum p requires a c ons tant supply of energy , and the gentlem an abov e,

were he real rather than a v is ual m etaphor, would soon be flat on his back

Cardiac cells c an los e their internal negativity in a proc ess called depol ari zation

Depolarization is the f undam ental electrical event of the heart

Depolarization is propagated from c ell to c ell, produc ing a wav e of depolarization that

c an be trans mitted across the entire heart T his wav e of depolarization repres ents a

flow of electric ity , an electric al c urrent, that can be detected by electrodes plac ed on

the s urface of the body

After depolarization is c om plete, the c ardiac c ells are able to res tore their res ting

polarity through a proces s c alled repol arizati on This, too, c an be sensed by rec ordingelec trodes

All of the different wav es that we see on an EKG are m anifestations of these two

proces ses : depolarization and repolarization

In A, a s ingle c ell has depolarized A wav e of depolarization then propagates from

cell to cell (B) until all are depolarized (C) Repolarization (D) then res tores eac h

cell's res ting polarity

The Cells of the Heart

From the standpoint of the elec troc ardiographer, the heart c ons is ts of three ty pes

of c ells :

Pacemaker cel ls—the norm al elec tric al power s ourc e of the heart

Elec tri cal conducti ng cel ls —the hard wiring of the heart

Myoc ardial cel ls—the contractile m achinery of the heart

Pacemaker Cells

Pacemaker c ell s are sm all c ells approx im ately 5 to 10 µ m long T hes e cells are able todepolarize spontaneously ov er and ov er again, at a partic ular rate T he rate of

depolarization is determ ined by the innate elec trical charac teris tic s of the cell and by

ex ternal neurohormonal input Each s pontaneous depolarization s erv es as the sourc e

of a wave of depolarization that initiates one com plete c yc le of cardiac c ontraction andrelax ation

A pac emak er c ell depolarizing s pontaneous ly

If we rec ord one elec trical cy c le of depolarization and repolarization from a s ingle c ell,

we get an elec trical trac ing called an action potential With eac h s pontaneous

depolarization, a new ac tion potential is generated, whic h in turn s tim ulates

neighboring c ells to depolarize and generate their own action potential, and s o on and

on, until the entire heart has been depolarized

A typical action potential

The action potential of a c ardiac pac emak er c ell look s a little different from the

generic action potential s hown on the prev ious page A pac emak er c ell does not hav e

a true res ting potential Its electric al c harge drops to a m inim al negativ e potential

whic h it maintains for just a m oment (it does not rest there), and ris es gradually until

it reaches the thres hold for the sudden depolarization

that is an ac tion potential These ev ents are illustrated on the trac ing below:

The electrical depolarization-repolarization c yc le of a c ardiac pac emak er c ell

Point A is the m inim al negativ e potential T he gentle ris ing s lope between points A

and B repres ents a s low, gradual depolarization At point B, the thres hold is

cros s ed and the cell dramatically depolarizes ; i.e., an ac tion potential is

produced T he downslope between points C and D repres ents repolarization This

cy c le will repeat ov er and ov er for, let us hope, many , m any y ears

The dom inant pacem aker cells in the heart are located high up in the right atrium

This group of cells is c alled the s inoatrial (SA ) node, or s inus node for s hort T hes e

c ells ty pic ally fire at a rate of 60 to 100 tim es per m inute, but the rate can v ary

trem endous ly depending upon the activ ity of the autonomic nerv ous s y stem (e.g.,

s ym pathetic stimulation from adrenalin ac c elerates the sinus node, whereas v agal

s tim ulation slows it) and the dem ands of the body for inc reased cardiac output

(exercis e raises the heart rate, whereas a res tful afternoon nap lowers it)

The s inus node fires 60 to 100 tim es per minute, producing a regular series of

ac tion potentials , eac h of which initiates a wav e of depolarization that will s pread

through the heart

Every cell in the heart ac tually has the ability to behav e lik e a pac emak er c ell This

so-called autom atic ability is norm ally s uppres sed unles s the dominant c ells of the

sinus node fail or if s om ething in the internal or ex ternal environm ent of a c ell

(sy m pathetic s tim ulation, cardiac disease, etc ) s tim ulates its autom atic behav ior

This topic will as sum e greater im portance later on and is dis cus sed under Ectopi c

Rhythms in Chapter 3

Electrical Conducting Cells

Elec tri cal conducti ng cel ls are long, thin c ells L ike the wires of an electrical circuit,

thes e cells carry c urrent rapidly and effic iently to distant regions of the heart T he

elec tric al c onduc ting cells of the v entric les join to form dis tinc t electric al pathway s

The conduc ting pathway s in the atria hav e m ore anatomic variability ; prom inent

among these are fibers at the top of the intra-atrial septum in a region called

Bac hman's bundle whic h allow for rapid activ ation of the left atrium from the right

The hard wiring of the heart

Myocardial Cells

The myocardi al c ell s c ons titute by far the major part of the heart tis sue T hey are

res pons ible for the heav y labor of repeatedly c ontrac ting and relax ing, thereby

deliv ering blood to the rest of the body

Thes e c ells are about 50 to 100 µm in length and c ontain an abundance of the

c ontrac tile proteins actin and m y osin

When a wav e of depolarization reaches a m yoc ardial cell, c alc ium is releas ed within

the c ell, causing the c ell to c ontract T his process, in whic h c alcium play s the k ey

intermediary role, is c alled ex c itation–contracti on c oupl i ng

Depolarization c aus es c alc ium to be releas ed within a m y oc ardial c ell T his influx

of calc ium allows actin and m y os in, the c ontrac tile proteins, to interact, causing

the cell to contrac t (A) A res ting m y ocardial c ell (B) A depolarized, c ontrac ted

my ocardial c ell

My oc ardial c ells c an trans m it an electrical current just lik e elec tric al c onduc ting c ells ,

but they do it far less effic iently Thus, a wav e of depolarization, upon reac hing the

m yoc ardial cells, will spread s lowly ac ros s the entire m yoc ardium

Time and Voltage

T he waves that appear on an EKG prim arily reflect the elec tric al activ ity of the

myocardi al c ell s , whic h c ompos e the v ast bulk of the heart Pacem aker activ ity and

trans mis sion by the c onducting sy s tem are generally not seen on the EKG ; thes e

ev ents s imply do not generate s ufficient v oltage to be rec orded by s urface elec trodes The wav es produced by my ocardial depolarization and repolarization are recorded on

EKG paper and, lik e any ty pe of wav e, have three chief c harac teristics :

Duration, meas ured in fractions of a sec ond

A typical wav e that might be s een on any EKG It is two large s quares (or 10

sm all s quares ) in am plitude, three large squares (or 15 s m all squares ) in

duration, and slightly as ym m etric in c onfiguration

EKG Paper

EKG paper is a long, c ontinuous roll of graph paper, usually pink (but any color will

do), with light and dark lines running v ertic ally and horizontally The light lines

c irc um s cribe sm all s quares of 1 X 1 mm ; the dark lines delineate large squares of 5 X

5 mm

P.19

The horizontal ax is meas ures time The dis tance ac ross one s mall s quare represents

0.04 sec onds The distance across one large s quare is fiv e tim es greater, or 0.2

s ec onds

The v ertical axis measures v oltage The dis tance along one sm all s quare repres ents

0.1 m V, and along one large s quare, 0.5 m V

You will need to mem orize these num bers at som e point, s o y ou m ight as well do it

now

Both waves are one large s quare in duration (0.2 s ec onds), but the sec ond wav e

is twic e the v oltage of the firs t (1 mV com pared with 0.5 mV) The flat segment

connec ting the two wav es is fiv e large squares (5 X 0.2 s ec onds = 1 sec ond) in

duration

P Waves, QRS Complexes, T Waves, and Some

Straight Lines

Let's follow one cy c le of c ardiac contrac tion (s y stole) and relax ation (dias tole),

foc using on the elec trical events that produc e the bas ic wav es and lines of the

s tandard EKG

Atrial Depolarization

The sinus node fires s pontaneous ly (an ev ent not v isible on the EKG ), and a wav e of

depolarization begins to spread outward into the atrial m y ocardium, m uc h as if a

pebble were dropped into a c alm , unruffled lak e Depolarization of the atrial

m yoc ardial cells results in atrial contrac tion

Eac h c y cle of norm al c ardiac contrac tion and relax ation begins when the sinus

node depolarizes s pontaneously T he wav e of depolarization then propagates

through both atria, c aus ing them to contrac t

During atrial depolarization and contrac tion, elec trodes plac ed on the s urface of the

body record a sm all burs t of elec trical activity las ting a frac tion of a sec ond T his is

the P wave It is a rec ording of the s pread of depolarization through the atrial

m yoc ardium from s tart to finis h

The EKG rec ords a sm all deflection, the P wav e

Bec aus e the sinus node is loc ated in the right atrium , the right atrium begins to

depolarize before the left atrium and finis hes earlier as well Therefore, the firs t part

of the P wav e predom inantly represents right atrial depolarization, and the s econd

part left atrial depolarization

O nc e atrial depolarization is com plete, the EKG again bec om es electric ally s ilent

The c omponents of the P wav e

A Pause Separates Conduction From the Atria to the

Ventricles

In healthy hearts, there is an elec tric al gate at the junc tion of the atria and the

v entricles The wav e of depolarization, hav ing com pleted its journey through the atria,

is prev ented from c om m unic ating with the v entricles by the heart v alves that separatethe atria and v entric les Electrical conduction m ust be funneled along the

interv entric ular

s eptum , the wall that separates the right and left v entricles H ere, a s truc ture c alled

the atri oventric ular (AV) node s lows c onduction to a crawl This paus e las ts only a

frac tion of a sec ond

This phys iologic delay in c onduction is es sential to allow the atria to finish contrac tingbefore the v entricles begin to contrac t This c lever elec trical wiring of the heart

perm its the atria to em pty their volume of blood c om pletely into the v entricles before

the v entricles c ontract

Lik e the sinus node, the AV node is also under the influence of the autonom ic nerv ous

s ys tem Vagal s tim ulation slows the c urrent ev en further, whereas sy m pathetic

s tim ulation acc elerates the current

(A) T he wav e of depolarization is briefly held up at the AV node (B) During this

pause, the EKG falls s ilent; there is no detec table elec trical activity

Ventricular Depolarization

After about one tenth of a s econd, the depolarizing wave esc apes the AV node and is

s wept rapidly down the ventric les along s pec ialized electrical conducting c ells

This v entric ular c onducting sy s tem has a complex anatom y but es sentially c ons ists of

The bundl e of H is emerges from the AV node and alm ost im m ediately div ides into rightand left bundle branches The ri ght bundle branch c arries the c urrent down the right

s ide of the interv entric ular septum all the way to the apex of the right v entricle T he

l eft bundl e branc h is m ore com plicated It div ides into three m ajor fas cic les :

Septal fasc ic le, whic h depolarizes the interv entric ular s eptum (the wall of mus cle

s eparating the right and left v entricles ) in a left-to-right direc tion

The right bundle branch and the left bundle branch and its fas c ic les term inate in

c ountles s tiny Purki nje fi bers, which resem ble little twigs c om ing off the branc hes of atree These fibers deliv er the elec tric al c urrent into the v entricular m yoc ardium

The v entric ular c onduction s ys tem, shown in detail Below the bundle of H is, the

conduction s y stem div ides into right and left bundle branc hes The right bundle

branc h rem ains intac t, whereas the left div ides into three separate fasc icles

Ventric ular my ocardial depolarization—and, henc e, ventricular c ontrac tion—is mark ed

by a new deflection on the EKG called the Q RS c omplex T he am plitude of the Q RS

c om plex is muc h greater than that of the atrial P wav e bec aus e the v entric les hav e s o

m uc h more m usc le m ass than the atria T he Q RS com plex is als o m ore c om plicated

and v ariable in s hape, reflec ting the greater intricac y of the pathway of v entric ular

depolarization

(A) Ventricular depolarization generates (B) a c omplic ated wav eform on the EKG

called the QRS complex

The Parts of the QRS Complex

The Q RS com plex c ons ists of sev eral dis tinc t wav es , eac h of which has a nam e

Bec aus e the precis e c onfiguration of the Q RS com plex c an v ary s o greatly , a standard

form at for naming eac h c om ponent has been dev ised It may seem a bit arbitrary to

y ou right now, but it ac tually m akes good sens e

If the firs t deflection is downward, it is called a Q wave

T he firs t downward deflec tion following an upward deflec tion is c alled an S wave

T herefore, if the firs t wav e of the com plex is an R wav e, the ens uing downward

deflection is c alled an S wav e, not a Q wave A downward deflec tion c an only be

c alled a Q wav e if it is the first wav e of the c omplex Any other downward

4

deflection is c alled an S wav e

If the entire c onfiguration c ons ists s olely of one downward deflec tion, the wav e is

c alled a Q S wave

5

H ere are s ev eral of the mos t c om m on Q RS configurations , with each wav e com ponent

named

The earliest part of the QRS complex represents depolarization of the interv entricular

s eptum by the s eptal fas cicle of the left bundle branc h The right and left v entricles

then depolarize at about the sam e time, but mos t of what we see on the EKG

repres ents left

v entricular activ ation because the m usc le m ass of the left v entricle is about three

tim es that of the right v entricle

The initial part of the QRS c omplex represents s eptal depolarization Sometimes ,

this s eptal depolarization m ay appear as a s mall, dis c rete, negative deflection, a

Q wave

Repolarization

After m y ocardial c ells depolarize, they pas s through a brief refrac tory period during

whic h they are res is tant to further s tim ulation They then repol ari ze; that is, they

res tore the elec tronegativ ity of their interiors so that they can be res tim ulated

Jus t as there is a wav e of depolarization, there is also a wav e of repolarization T his ,

too, can be seen on the EKG Ventric ular repolarization insc ribes a third wave on the

EKG , the T wave

Note: T here is a wav e of atrial repolarization as well, but it c oincides with v entric ulardepolarization and is hidden by the muc h m ore prom inent Q RS c om plex

P.28(A) Ventricular repolarization generates (B) a T wav e on the EKG

Naming the Straight Lines

T he different straight lines c onnec ting the v arious wav es have also been giv en

names T hus , we speak of the PR i nterval, the ST s egment, the Q T i nterval , and so

on

What differentiates a segment from an interv al? A segm ent is a s traight line

c onnecting two wav es , whereas an interv al enc om pas ses at least one wav e plus the

c onnecting straight line

The PR i nterval inc ludes the P wav e and the s traight line connec ting it to the Q RS

c om plex It therefore m easures the time from the start of atrial depolarization to the

s tart of v entric ular depolarization

The PR s egm ent is the s traight line running from the end of the P wav e to the start of

the Q RS com plex It therefore m easures the time from the end of atrial depolarization

to the s tart of v entricular depolarization

The ST segment is the straight line c onnecting the end of the Q RS c om plex with the

beginning of the T wav e It m eas ures the tim e from the end of ventric ular

depolarization to the s tart of v entric ular repolarization

The Q T i nterval inc ludes the Q RS c om plex , the ST segm ent, and the T wav e It

therefore meas ures the tim e from the beginning of ventricular depolarization to the

end of v entricular repolarization

The term Q RS i nterval is used to des cribe the duration of the Q RS com plex alone

without any c onnecting s egm ents O bv ious ly , it m easures the duration of v entric ular

depolarization

P.31

Summary The Waves and Straight Lines of the EKG

Each cy c le of cardiac c ontrac tion and relax ation is initiated by s pontaneous

depolarization of the s inus node This ev ent is not s een on the EKG

1

T he P wav e rec ords atrial depolarization and c ontrac tion T he firs t part of the P

wave reflec ts right atrial ac tiv ity ; the s econd part reflects left atrial activity

2

T here is a brief pause when the electric al c urrent reac hes the AV node, and the

EKG falls silent

3

T he wave of depolarization then s preads along the ventric ular conduc ting s y stem

(H is bundle, bundle branc hes , and Purk inje fibers) and out into the ventric ular

m y oc ardium T he first part of the v entric les to be depolarized is the

interv entric ular septum Ventricular depolarization generates the Q RS c om plex

4

T he T wav e records v entric ular repolarization Atrial repolarization is not s een

5

Various segments and intervals desc ribe the tim e between thes e ev ents :

T he PR interv al m easures the time from the start of atrial depolarization to

the s tart of v entricular depolarization

a

T he PR s egm ent m easures the tim e from the end of atrial depolarization to

the s tart of v entricular depolarization

b

T he ST segment rec ords the time from the end of v entricular depolarization

to the start of v entricular repolarization

c

T he Q T interv al m eas ures the tim e from the s tart of v entricular

depolarization to the end of v entric ular repolarization

d

T he Q RS interval meas ures the tim e of v entric ular depolarization

e

6

P.33

Making Waves

Elec trodes can be plac ed any where on the s urfac e of the body in order to rec ord

the heart's electric al ac tiv ity If we do this , we quick ly disc ov er that the wav es

rec orded by a pos itiv e electrode on the left arm look v ery different from thos e

rec orded by a pos itiv e electrode on the right arm (or right leg, left leg, etc )

It's eas y to see why this oc c urs A wav e of depolarization mov ing toward a positiv e

elec trode c aus es a posi tive deflection on the EKG A wav e of depolarization mov ing

away from a positiv e electrode c auses a negati ve deflec tion

Look at the figure below T he wav e of depolarization is m ov ing left to right, toward

the electrode T he EKG records a pos itiv e deflec tion

A wav e of depolarization m ov ing toward a positive electrode records a positiv e

deflec tion on the EKG

Now look at the following figure T he wave of depolarization is m oving right to left;

the electrode is placed s o that the wav e of depolarization is mov ing away from it T he

EKG therefore rec ords a negativ e deflec tion

A wav e of depolarization m ov ing away from a positive elec trode records a

negativ e deflec tion on the EKG

What will the EKG rec ord if the pos itiv e elec trode is plac ed in the m iddle of the cell?Initially , as the wav efront approaches the electrode, the EKG rec ords a positivedeflection

Depolarization begins, generating a positiv e deflection on the EKG

Then, at the precis e m oment that the wav e reac hes the elec trode, the pos itiv e andnegativ e charges are balanc ed and ess entially c anc el each other out T he EKG

rec ording returns to bas eline

The wav efront reaches the electrode T he pos itiv e and negativ e c harges are

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As the wave of depolarization recedes , a negative deflection is insc ribed

The wav e of depolarization begins to rec ede from the electrode, generating a

negativ e deflec tion

The EKG finally returns to baseline once again when the entire mus cle is depolarized

The c ell is fully depolarized, and the EKG once again returns to baseline

The final ins cription of a depolarizing wav e m ov ing perpendic ularly to a pos itiv e

elec trode is therefore a bi phasi c wave

What would the trac ing look lik e if the rec ording elec trode were placed ov er a s ection

of pacem ak er cells ? The tracing would s how a downward, negativ e deflec tion, s inc e allthe c urrent is m ov ing away from the origin where y ou are rec ording

The effec ts of repolarization on the EKG are sim ilar to thos e of depolarization, ex cept

that the c harges are rev ersed A wav e of repolarization mov ing toward a positiv e

elec trode insc ribes a negati ve deflec tion on the EKG A wave of repolarization m oving

away from a positiv e electrode produces a posi tive deflection on the EKG A

perpendic ular wav e produces a bi phas ic wave; howev er, the negativ e deflec tion of thebiphas ic wave now precedes the pos itiv e deflec tion

A wav e of repolarization m ov ing through mus cle tis sue is recorded by three

different pos itiv e elec trodes (A) Early repolarization (B) L ate repolarization (C)

Repolarization is com plete

We c an easily apply thes e c onc epts to the entire heart Elec trodes plac ed on the

s urface of the body will rec ord wav es of depolarization and repolarization as they

s weep through the heart

If a wav e of depolarization pass ing through the heart is m oving toward a surface

elec trode, that elec trode will rec ord a pos itiv e deflection (elec trode A) If the wav e of

depolarization is mov ing away from the elec trode, the elec trode will rec ord a negativ edeflection (elec trode B) If the wav e of depolarization is m ov ing perpendic ularly to theelec trode, the elec trode will record a biphasic wave (electrode C) T he effec ts of

repolarization are prec is ely the opposite of those of depolarization, as y ou would

ex pec t

A wav e of depolarization m ov ing through the heart (large arrow) Electrode A

rec ords a positive deflection, elec trode B records a negative deflection, and

elec trode C rec ords a biphas ic wave

The 12 Views of the Heart

If the heart were as sim ple as a s ingle m y ocardial c ell, a c ouple of rec ording

elec trodes would giv e us all the inform ation we need to des cribe its elec trical activ ity

H owev er, as we hav e already seen, the heart is not so sim ple—a burden to y ou, a

boon to authors of EKG book s

The heart is a three-dim ens ional organ, and its electrical ac tiv ity m ust be understood

in three dim ens ions as well A couple of elec trodes are not adequate to do this , a fact

that the original elec trocardiographers rec ognized m ore than a century ago when theydev ised the firs t limb leads Today , the standard EKG c ons is ts of 12 leads, with eac h

lead determ ined by the plac em ent and orientation of various electrodes on the body

Each lead v iews the heart at a unique angle, enhancing its sensitivity to a partic ular

region of the heart at the expens e of others The more v iews, the more inform ation

prov ided

To read an EKG and ex trac t as m uch information as pos s ible, y ou need to unders tand

the 12-lead s y stem

Three obs erv ers get three very different im press ions of this c ons umm ate ex ample

of the Lox odonta afri cana One observ er s ees the trunk , another s ees the body,

and the third sees the tail If y ou wanted the best des cription of the elephant,

who would y ou ask ? All three, of c ourse

To prepare a patient for a 12-lead EKG , two elec trodes are placed on the arms and

two on the legs These prov ide the bas is for the six li mb leads , whic h inc lude the

three s tandard l eads and the three augmented leads (these term s will m ake m ore

s ens e in a m oment) Six elec trodes are als o placed ac ros s the c hes t, form ing the s ix

prec ordi al l eads

The precis e rec ordings will vary som ewhat depending on the prec ise plac ement of the

elec trodes T herefore, adherence to s tandard positioning protoc ols is very im portant

to allow com paris on between EKG s tak en at different tim es in different s ettings

The Six Limb Leads

The lim b leads v iew the heart in a vertic al plane called the frontal pl ane T he frontal

plane c an be env isioned as a giant c irc le s uperim pos ed on

the patient's body T his circle is then m ark ed off in degrees The lim b leads view

elec tric al forc es (wav es of depolarization and repolarization) m ov ing up and down andleft and right through this c irc le

The frontal plane is a coronal plane The lim b leads view elec trical forces mov ing

up and down and left and right on the frontal plane

To produc e the s ix leads of the frontal plane, eac h of the elec trodes is v ariably

des ignated as pos itiv e or negativ e (this is actually done automatically by c irc uitryins ide the EKG m achine)

Each lead has its own s pec ific view of the heart, or angle of orientati on T he angle ofeach lead can be determ ined by drawing a line from the negativ e elec trode to thepos itiv e electrode T he resultant angle is then ex pres sed in degrees by s uperimpos ing

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it on the 360° circle of the frontal plane

The three standard lim b leads are defined as follows :

L ead I is c reated by m ak ing the left arm positiv e and the right arm negativ e Its

The three augmented lim b leads are created som ewhat differently A s ingle lead is

c hos en to be positiv e, and all the others are m ade negativ e, with their average

ess entially s erv ing as the negative electrode (c om mon ground) T hey are c alled

augmented l eads bec aus e the EKG m achinery mus t am plify the tracings to get an

adequate rec ording

L ead AVL is c reated by m aking the left arm positiv e and the other lim bs negativ e.Its angle of orientation is -30°

1

L ead AVR is c reated by m ak ing the right arm pos itive and the other lim bs

negativ e Its angle of orientation is -150°

2

L ead AVF is c reated by m ak ing the legs pos itiv e and the other limbs negativ e Itsangle of orientation is +90°

3

In the figure below, all six leads of the frontal plane are indic ated with their

appropriate angles of orientation Just as our three obs erv ers each look ed at the

elephant from his or her own unique perspectiv e, s o that each lead perceiv es the

heart from its own unique point of v iew

Leads II, III, and AVF are called the inferi or leads bec aus e they m ost effec tiv ely v iew

the inferior surface of the heart The inferior s urfac e, or wall, of the heart is an

anatomic term for the bottom of the heart, the portion that res ts on the diaphragm

Leads I and AVL are often called the left lateral leads bec aus e they hav e the bes t v iew

of the left lateral wall of the heart

AVR is pretty m uc h a loner, and y ou c an c all it whatev er y ou lik e

Memorize thes e s ix leads and their angles

Of six lim b leads , three are s tandard (I, II, and III), and three are augm ented

(AVR, AVL, and AVF) Eac h lead v iews the heart from its own partic ular angle of

orientation

The Six Precordial Leads

The six precordial leads , or chest leads, are ev en easier to unders tand T hey are

arranged ac ros s the c hes t in a horizontal pl ane as illus trated below Whereas the

leads of the frontal plane v iew electrical forces mov ing up and down and left and

right, the prec ordial leads rec ord forces mov ing anteriorly and pos teriorly

To c reate the s ix prec ordial leads , each ches t electrode is made pos itiv e in turn, and

the whole body is tak en as the c omm on ground The six positive elec trodes , creating

the prec ordial leads V1 through V6, are pos itioned as follows:

V1 is plac ed in the fourth intercostal space to the right of the s ternum

V2 is plac ed in the fourth intercostal space to the left of the sternum

V3 is plac ed between V2 and V4

V4 is plac ed in the fifth interc os tal s pac e in the midclav ic ular line

V5 is plac ed between V4 and V6

V6 is plac ed in the fifth interc os tal s pac e in the midaxillary line

The prec ordial leads define a horizontal or transv ers e plane and v iew electric al

forces mov ing anteriorly and pos teriorly

Jus t lik e the lim b leads , eac h prec ordial lead has its own particular line of sight and

region of the heart that it v iews bes t

Note that the right v entric le lies anteriorly and medially within the body c av ity , and

the left v entricle lies posteriorly and laterally L eads V1 and V2 lie direc tly over the

right v entricle, V3 and V4 ov er the interv entric ular s eptum , and V5 and V6 over the

left v entricle

Leads V1 through V4 are often referred to as the anteri or l eads, and V5 and V6 join I

and AVL as left lateral leads

Leads G roup

V1, V2, V3, V4 Anterior

I, AVL , V5, V6 L eft lateral

II, III, AVF Inferior

A Word About Vectors

It is im portant to recognize that each EKG elec trode records only the av erage

c urrent flow at any given mom ent Thus, although tiny s wirls of c urrent m ay

s im ultaneous ly be going off in every direc tion, each lead rec ords only the

ins tantaneous av erage of thes e forces In this way , out of c haos , s ome v ery s im ple

patterns em erge

This c onc ept is really quite s im ple; an analogy m ay be helpful During the c ourse of a

s oc cer matc h, a goalie may k ic k or tos s the ball m any different tim es to v arious

m em bers of his team Some balls will go left, others right, still others s traight down

the field H owev er, by the end of the gam e, the average direc ti on of all of the goalie's

k ick s and toss es is lik ely to be s traight ahead, toward the oppos ing net This av erage

m ov em ent can be repres ented by a s ingle arrow, or vec tor

This v ec tor is precis ely what our EKG elec trodes rec ord when m easuring the elec trical

flow within the heart The vec tor's angle of orientation repres ents the av erage

di rection of c urrent flow, and its length represents the v oltage (ampli tude) attained

At any given mom ent, the elec tric al forc es m ov ing within the heart c an be represented

by a single v ec tor, and this v ector is translated by eac h of the 12 leads into the

s im ple wav e patterns that we see on the EKG