ECG basics and interpretations

8/13/2019 ECG basics and interpretations

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he electrical a)is is normally downward and to the left !ut we can estimate it more accurately inindividual patients if we understand from which direction* each recording lead measures the ECG.

Figure 2. !rientation of the li"# leads sho$ing the direction fro" $hich each lead%loo&s% at the heart

y convention, we record the standard surface ECG using /0 different recording lead directions,*though rather confusingly only /1 recording electrodes on the skin are re2uired to achieve this. 'i)of these are recorded from the chest overlying the heart + the chest or precordial leads . 3our arerecorded from the lim!s + the limb leads . It is essential that each of the /1 recording electrodes is

placed in its correct position, otherwise the appearance of the ECG will !e changed significantly,preventing correct interpretation.

he lim! leads record the ECG in the coronal plane, and so can !e used to determine the electricala)is "which is usually measured only in the coronal plane$. he lim! leads are called leads I, II, III,A, A&L and A&3. 3igure 0 shows the relative directions from which they look* at the heart. Ahori4ontal line through the heart and directed to the left "e)actly in the direction of lead I$ isconventionally la!elled as the reference point of 1 degrees "1 o$. he directions from which otherleads look* at the heart are descri!ed in terms of the angle in degrees from this !aseline.

he electrical a)is of depolarisation is also e)pressed in degrees and is normally in the range from%51 1 to 6 71 1 . A detailed e)planation of how to determine the a)is is !eyond the scope of this

article !ut the principles mentioned here should help readers to understand the concepts involved.

he chest leads record the ECG in the transverse or hori4ontal plane, and are called &/, &0, &5, &8,&9 and &: "see 3igure 5$.

Figure . 'ransverse section of the chest sho$ing the orientation of the six chest leadsin relation to the heart

oltage and ti"ing intervals

It is conventional to record the ECG using standard measures for amplitude of the electrical signaland for the speed at which the paper moves during the recording. his allows;

Easy appreciation of heart rates and cardiac intervals and


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wave inmost leads of an ECG, the > wave is in fact the sum of the electrical signals from the two atria,which are usually superimposed.

here is then a short, physiological delay as the atrioventricular "A&$ node slows the electricaldepolarisation !efore it proceeds to the ventricles. his delay is responsi!le for the ># interval, ashort period where no electrical activity is seen on the ECG, represented !y a straight hori4ontal or

isoelectric* line.

?epolarisation of the ventricles results in usually the largest part of the ECG signal "!ecause of thegreater muscle mass in the ventricles$ and this is known as the /* co"plex .

he @ wave is the first initial downward or negative* deflection he # wave is then the ne)t upward deflection "provided it crosses the isoelectric line and

!ecomes positive*$

he ' wave is then the ne)t deflection downwards, provided it crosses the isoelectric line to!ecome !riefly negative !efore returning to the isoelectric !aseline.


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In the case of the ventricles, there is also an electrical signal reflecting repolarisation of themyocardium. his is shown as the *' seg"ent and the ' $ave . he ' segment is normallyisoelectric, and the wave in most leads is an upright deflection of varia!le amplitude and duration"see 3igures 9 and :$.

Figure 0. 'he "a or $aves of a single nor"a

l ECG pattern

Figure . Exa"ple of a nor"al 12 lead ECG3 notice the do$n$ard deflection of all signalsrecorded fro" lead a /. 'his is nor"al+ as the electrical axis is directly a$ay fro" thatlead

4or"al intervals

he recording of an ECG on standard paper allows the time taken for the various phases ofelectrical depolarisation to !e measured, usually in milliseconds. here is a recognised normalrange for such intervals*;

-/ interval "measured from the !eginning of the > wave to the first deflection of the @#'comple)$. ormal range /01 + 011 ms "5 + 9 small s2uares on ECG paper$.

/* duration "measured from first deflection of @#' comple) to end of @#' comple) atisoelectric line$. ormal range up to /01 ms "5 small s2uares on ECG paper$.

' interval "measured from first deflection of @#' comple) to end of wave at isoelectricline$. ormal range up to 881 ms "though varies with heart rate and may !e slightly longer infemales$

Heart rate esti"ation fro" the ECG

'tandard ECG paper allows an appro)imate estimation of the heart rate "(#$ from an ECGrecording. Each second of time is represented !y 091 mm "9 large s2uares$ along the hori4ontala)is. 'o if the num!er of large s2uares !etween each @#' comple) is;

9 % the (# is :1 !eats per minute. 5 % the (# is /11 per minute.

0 % the (# is /91 per minute.


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iagrams (elo7 indicate 7hich part the heart is (eing affected and 7hat lead 7ouldsho7 the changes.

Reciprocal +hanges

Region of ! levation Region of ! epression Anterior leads V#4V%) 8nferior true posterior)

8nferior leads 88$ 888$ aVF) Anterior leads V#4V3 or lateral lead#. aVL)Lateral leads 8$ aVF$ V&$

V') 8nferior leads 88$ 888$ aVF)

!rue Posterior Anterior leads V#4V3)#2 lead rapid assessment

erify a / is negative 8ssess rate and rhyth"

8xis deter"ination % Leads I and a&3

Conduction a#nor"alities9

L % seen in &/(ypertrophyAneurysm>ericarditis?rugs or ElectrolytesEarly repolari4ation

6sche"ia+ 6n ury+ 6nfarct signs9

%wave inversions' segment elevation'ignificant @ waves

8cute =6 pattern9

8nterior9

' elevation in &/, &0, &5, &8' depression in II, III, a&3

6nferior9 ' elevation in II, III, a&3

' depression in &/, &0, &5, or I, a&L

5ateral9 ' elevation in I, a&L, &9, &:

' depression in II, II, a&3


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*eptal $all9 ' elevation in I, a&L, &/, &0-osterior9 tall and wide # waves and ' depression in &/, &0/ight entricular9 ' elevations in &8#, &9#, &:#

"9 additional right chest wall electrodes placed on the chest in the same positions as theprecordial leads$

Clinical pressentaion 'reat"ent plan

Electrical Current9

lectricity al7ays flo7s from positive to negative. !he electrical current should flo7from negative to positive in the normal healthy heart. o$ if this pattern is disrupted (ya 0detour0 or as in the heart$ 0an infarct0 or 0in"ury0 the +: recording 7ill indicate thea(normal flo7 of current. =ith an infarcted heart$ the electrical current flo7s opposite

of 7here it is e1pected to flo7. ence$ the elevated or depressed ! segment . Forinstance$ an inferior infaction 7ill sho7 an elevation in lead 88$ 888 and aVF. !he normal

flo7 7ould (e 0isoelectrical0 and the ! segment 7ould (e e>ualized or level. 6ut$ sincethe flo7 is going (ac*7ards around the damaged heart muscle$ 7e see an elevation onthe record. 8t is this precise measurement that can dictate e1actly 7here the infarct islocated. 8f the ! segment is elevated in V2$ V3 and V%$ the infarct is anterior. !hese

vie7s are loo*ing at the front or anterior area of the heart muscle. !he current isflo7ing to7ard the positive electrode on the patients chest. =hen the current is

disrupted$ it 7ill sho7 as an elevation in the ! segment versus an isoelectric reading.

!hin* of it li*e this, An ?ray film is placed (ehind the heart at the area (et7een theVentrical eptum. !he ?4Ray machine shoots the picture from the anterior heart directly

a(ove the film. and the film catures the image. =e 7ould (e loo*ing at the area of theheart at the eptal region 7hich 7ould (e in +: terminology V3.

!he infract area 7ill have no electrical current. !he ! segment 7ill (e depressed !he in"ured area 7ill have ! elevations and 7ill release +ardiac 9uscle enzimes.

!hese enzimes are +@$ 9yo:lo(in and !roponin 8

!he ishemic area may have ! elevations and +: cahnges to include PV+ s$ PA+ sand PB+ s.

!he #2 Lead Photograph imulatneous a>uisition 2.& seconds per vie7$ #/

seconds for a complete study

8 aVR V# V%

88 aVL V2 V&

888 aVF V3 V'#2 lead +:C a real time vedio recording of the hearts electrical function.

!his record indicates a 0septal D anterior 8nfarct.0


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8f you can comprehend 7hich 7ay the current is e1pected to flo7 in !he ?A?8AL V8 =and !he PR +-R 8AL V8 = of the heart$ then you can diagnose 7hich area is effected ifit is an a(normal flo7...

ee the information (elo7.

#2 lead rapid interpretation +ommon +: Formation

6sceh"ia>6nverted ' $aves Inverted wave is symmetrical

waves are usually upright in leads I, II, and &0%&:6n ury>Elevated *' seg"ent

'ignifies an acute process ' returns to !aseline with time If ' elevation is diffuse and unassociated with @ wavesor reciprocal '

depression, consider pericarditis

Location of injury can !e determined in same manner as infarct location

Fsually associated with reciprocal ' depression in other leads6nfarction> $ave

'mall @s may !e normal in &9, &:, I and a&L A!normal @ must !e one small s2uare "1.18 sec$ wide

Also a!normal if @%wave depth is greater than one%third of @#' height inlead III

9a*ing the accurate Field iagnosis,

!here are elevations # mm )in t7o contegous connecting) leads,Leads ad"acent to eachother...

!here is at least one lead 7ith reciprocal changes..

8f the E 7ave is more than #D3 the size of the R 7ave...

!a(le (elo7 sho7s 7hat the +: 7ould loo* li*e in the Vector 7here the heart is (eingaffescted. All other areas 7ould loo* normal$ 7ithout elevation or depression. unless

there is an 0old 98.0 8n that case$ the prior damage 7ould sho7 up as a depressedsegment.


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Anterior 8nfarction ' elevation without a!normal @ wave

Fsually associated with occlusion of the left anterior decending !ranch ofthe left coronary artery "LCA$

Lateral 8nfarction ' elevation with without a!normal @ wave


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important to paramnedic in the pre%hospital phase !ecause the developement of the @%wave maytake hours and could easily !e missed in the field.

Eventually, the ' segment will return to its !aseline and the %wave resumes its normal position,leaving only the @%wave as evidence that an infarction has occurred. #ecent research and studieshave produced 79J accuracy in field diagnosis !y paramedics. >erhaps some reasons wouldinclude other indications for ' changes. hey would include simple angina, drug effects, andelectrolyte im!alance.

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ECG basics and interpretations