ECG interpretation fundamentals and arrhythmia recognition principles

Robert M GowDepartment of PaediatricsUniversity of OttawaFaculty of Medicine

ECG fundamentals

R iReviewLead theory and wavefront activationU i l d bi l l dUnipolar and bipolar leadsDetermination of frontal plane axisD t i ti f h i t l l iDetermination of horizontal plane axisECG recognition of electrolyte imbalanceS t ti h t h th i itiSystematic approach to arrhythmia recognition

ECG fundamentals

HistoryyAugustus Waller (1856-1922)

First ECG 1887Labelled V1 V2 then ABCD

Willem Einthoven (1860-1927)PQRSTEinthoven’s triangle

Frank Wilson (1890 1952)Frank Wilson (1890-1952)Unipolar leads

ECG fundamentals

posteriorspine

left

anteriorElectrical field passes throughlungs, muscle, blood, fat

Detected at skinSpreads out from source (heart)

ECG fundamentals

•Skin resistance is critical to recording of electricalactivity

•Resistance due to dry, dead, horny layer of epidermis

•MegaOhm resistance at surface (1 million Ohms)

•Simple abrading of skin reduces resistance to 5000 Ohms

•Good electrodes reduce to 100 Ohms

ECG fundamentalsBipolar Unipolar

t

- +

remote zero

- + +

Records difference between-ve and +ve terminals

Records difference between+ve electrode and remote zerove and +ve terminals

-ve is subtracted from +veDifference is recorded

+ve electrode and remote zero

Impulse travelling towardselectrode causes a positiveelectrode causes a positivedeflection

ECG fundamentalsLead +ve -ve1 LA RA

RA LA ll LL RAlll LL LA

lead

LLGround True bipolar leadsActual voltage not knowng

ECG fundamentals

Einthoven’s Law

l +- l + lll = ll

lllll

--Simple check forl d i l tll

++

lead misplacement

++

ECG fundamentals

Chest lead exploring electrode - unipolar

ECG amplifierLARA

+ve

CG a p e

5000 Ohm - ve

Wilson central terminalLLLL

ECG fundamentals

Augmented limb leadsAugmented limb leads

unipolar limb leadsu po a b eadscompares one limb with the average of the other 2

For example lead aVRunipolar RA is positiveaverage of LA and LL is negativeaverage of LA and LL is negativeaVR equals the potential of right arm minus the mean of the potentials of left arm and left footp

ECG fundamentals

Lead theoryLead theoryVector: electrical forces have direction and magnitudeHeart vector - from activation and propagationp p gLead vector - orientation of leads to heart vector

3 vector conceptis an extremeis an extremesimplification

ECG fundamentals- - -

Frontal Plane+aVL -30o

+ aVR -150o

+ 1 0o150

-

--

+ ll 60o+lll 120o

+aVF 90o Hexaxial lead system

ECG fundamentals

Horizontal Plane Lead VectorsHorizontal Plane Lead Vectors

Posterior

V5

V6 0o

V3V4

V5

V1 V2V3

+ 90o

ECG fundamentals

vector away = negatived i i

2,3

1 3 1

vector towards = positiveequiphasic = 90o

1

2

11

2

ll

2 1 = septum2 = dominant LV3 = basal RV/LV

aVF1

31,2

3

ECG fundamentals

QRS axis estimationQRS axis estimation

actually estimating a “mean” or average electrical forcesmean force estimated by area under QRS complex

then these are plotted as vectors on the hexaxial graph

ECG fundamentals

QRS axis estimationQRS axis estimation

Thankfully, measuring the area is not necessary

The net amplitude of the QRS complexes can be usedi t dinstead.

+10 Resultant = (+10) + ( 4) = +6

-4

+10 Resultant = (+10) + (- 4) = +6

4

ECG fundamentals

Different methods for frontal plane QRS axisDifferent methods for frontal plane QRS axis

•Can take any 2 leads or all sixCan take any 2 leads, or all six

•Leads 1, aVF commonly used

•Lead with smallest or equiphasic complex identified, d i i i t l 90o t thi l dand axis is approximately 90o to this lead

ECG fundamentals

8

3

+5

153Lead l

15

6

aVF10

4

+6 aVF

Not perfect but good enough4Lead aVF

Not perfect, but good enough

ECG fundamentals

Horizontal Plane -Horizontal Plane

left t i

righti

V6 0o V6l fti ht

posteriorposterior- +

V4V5 left

anteriorrightanterior

V1 V2V3

V2+

ECG fundamentals

QRS axis

•affected by anatomic position of the hearty p

•properties of the cardiac conduction system

•developmental changes in RV/LV mass relationships

•conduction system most influential in mature heart

ECG fundamentalsElectrolyte abnormalities

HypokalaemiaHypokalaemiaST depressionlowering of T wavelowering of T waveincrease in U wave heightU > T suggests K < 2.7 mEq/l

HyperkalemiaTall thin T wavesTall thin T wavesPR prolongedQRS widensQP waves disappear

ECG fundamentals

Electrolyte abnormalities

ECG fundamentals

Electrolyte abnormalities

HypocalcaemiaProlongs QT interval (ST segment)

HypercalcaemiaSh t i f QT i t l (QT f T)Shortening of QT interval (QTa – apex of T)

Systematic approach to arrhythmia recognition

TachyarrhythmiasTachyarrhythmiasSupraventricularVentricular

Narrow complexWide complexWide complex

R l l / i lRegular, narrow complex/supraventricularprinciples behind the components of the algorithm

Regular narrow QRSt h ditachycardia

visible P wavesvisible P waves

atrial rate greater than i l

YesNo

ventricular rateyes no

atrial tachycardiaatrial flutter

identify RP interval

RP<PR RP>PR

RP < 70 msec RP> 70 msec atrial tachycardiaatypical AVRT/AVNRT

AVNRTAT with 1o AVB

AVNRTAVRT, AT

Systematic approach to arrhythmia recognition

Narrow complex/supraventricular (normal QRS duration, appearance)QRS l f 120 ( d l )

Cl ifi i h

QRS normal for age <120 msec (adults)

Classification schemesMechanistic

automaticity/ectopicautomaticity/ectopicreentrytriggeredgg

Anatomicalatrial, junctional, accessory pathway

AV l ti hiAV relationshipindependent, dependent

Systematic approach to arrhythmia recognition

Narrow complex/supraventricular

Combined anatomical/AV relationship

D dDependentthe arrhythmia requires both atrium and ventricleto continueto continue

Independentpthe arrhythmia continues regardless of the involvement of the other chamber

Systematic approach to arrhythmia recognition

Narrow complex/supraventricular

Direction of activationarrhythmia arises in the atriumythe atrial activity drives the ventricle

the atrium is activated by impulses that arise lowerdown (AV node, ventricle)

Sinus nodeatrial ectopic focus

Reentry circuitAtrial Flutter

P=QRS

p

AtriumP=QRSP<QRS

P=QRSP>QRS JET

Ventricle

Independent arrhythmiasCan have different rates in 2 chambersAtrium may be faster in one, may be slower in otherMore (or less) P waves than QRS complexes on ECG

AVNRT

atrium P=QRS

ventricleAVRT

ventricle

h h i d d b h h barrhythmia dependent on both chambersexpect a 1:1 relationshipone P for each QRS on ECGone P for each QRS on ECG

Systematic approach to arrhythmia recognition

Narrow complex/supraventricular

Anatomical Indep/dependentSNRT atrium independentSNRT atrium independentAtrial tach atrium independentAtrial flutter atrium independentAtrial fib atrium independentAVNRT Atr/AV node dependentAVRT Atr/Cond/vent dependentAVRT Atr/Cond/vent dependentJET AV node independent

Systematic approach to arrhythmia recognition

Locate P waveCalculate RP and PR intervals

R Rpseudo r’

P

RP/PR <1

pseudo

RP/PR <1

RP/PR>1RP=PR

RP/PR>1Long RP tachycardia

Systematic approach to arrhythmia recognition

2 explanations for same ECG

A (or P)(o )fast

slow

V (or R)1o AV block

Systematic approach to arrhythmia recognition

Regular, narrow complex tachycardia with no visible P wavesg , p y

Could be simple, uncomplicated AVNRTp p

Systematic approach to arrhythmia recognition

Concealed atrial activityadenosinecarotid sinus massage

Concealed atrial activity

Regular narrow QRSt h ditachycardia

visible P wavesvisible P waves

atrial rate greater than i l

YesNo

ventricular rateyes no

atrial tachycardiaatrial flutter

identify RP interval

RP<PR RP>PR

RP < 70 msec RP> 70 msec atrial tachycardiaatypical AVRT/AVNRT

AVNRTAT with 1o AVB

AVNRTAVRT, AT

Systematic approach to arrhythmia recognition

l h f f h h diIn general the common forms of SVT are short RP tachycardias (RP/PR<1), because the atrial activation is from rapid retrograde conduction to and then activation of the atriumconduction to, and then activation of, the atrium

Long RP tachycardias (those with a normal looking PR interval) g y ( g )are either tachycardias arising in the atrium, or unusual forms of SVT.

Sometimes it may be difficult to differentiate whether the atrium has been activated from the top or retrogradebeen activated from the top or retrograde.

Systematic approach to arrhythmia recognition

The analysis of the electrocardiogram in narrowThe analysis of the electrocardiogram in narrow complex SVT requires an understanding of the possible mechanisms, anatomical substrates and atrioventricular dependence.

Being aware of all these features will usually enable anBeing aware of all these features will usually enable an accurate diagnosis to be made on the electrocardiogram.