The Basics of ECG Interpretation Dr Tim Smith. Summary Cardiac conducting system and the ECG...

The Basics of The Basics of ECG ECG

InterpretationInterpretationDr Tim SmithDr Tim Smith

SummarySummary

Cardiac conducting system and the Cardiac conducting system and the ECG waveformECG waveform

The normal ECGThe normal ECG Abnormalities of conductionAbnormalities of conduction Heart RhythmsHeart Rhythms QT prolongationQT prolongation Normal VariantsNormal Variants

Cardiac DepolarisationCardiac Depolarisation

Originates in pacemaker cells (automaticity).Originates in pacemaker cells (automaticity). Spreads along defined pathways.Spreads along defined pathways. Causing co-ordinated muscular contraction.Causing co-ordinated muscular contraction.

Electrical signal from depolarisation Electrical signal from depolarisation detectable at body surface. (1 mV cf. 90 mV detectable at body surface. (1 mV cf. 90 mV @ cell)@ cell)

This is the ECG.This is the ECG.

The Cardiac Conducting The Cardiac Conducting SystemSystem

Anatomy of the ECGAnatomy of the ECG

The Limb LeadsThe Limb Leads

I, II, VL - L lateral I, II, VL - L lateral surfacesurface

VF, III - Inferior VF, III - Inferior surfacesurface

VR - R atriumVR - R atrium

Cardiac AxisCardiac Axis Lead II often used Lead II often used

for rhythm strips.for rhythm strips.

The Chest LeadsThe Chest Leads

VV1+21+2 - R - R VenticleVenticle

VV3+43+4 - Septum - Septum VV5+65+6 - L - L

VentricleVentricle

Bundle Branch Bundle Branch BlocksBlocks

Ischaemia Ischaemia (esp. V(esp. V55))

The Normal ECGThe Normal ECG

Characteristics of the Characteristics of the Normal ECGNormal ECG

RateRate 50-100 bpm50-100 bpm Sinus rhythmSinus rhythm Cardiac Axis -30º to +90ºCardiac Axis -30º to +90º P P <120 ms<120 ms PRPR <200 ms<200 ms QRSQRS <120 ms<120 ms QTQT Male <0.43 s Female <0.45 sMale <0.43 s Female <0.45 s STST isoelectricisoelectric

Calculating Heart RateCalculating Heart Rate

Standard ECG speed is 25 mm.sStandard ECG speed is 25 mm.s -1-1

Heart Rate = 300/# big squaresHeart Rate = 300/# big squares

300/3.8=79 bpm

Abnormalities of Abnormalities of conductionconduction

At the AVN = Heart BlockAt the AVN = Heart Block 1st degree1st degree 2nd degree2nd degree 3rd degree3rd degree

In the His/Purkinje system = Bundle In the His/Purkinje system = Bundle Branch BlockBranch Block

LBBBLBBB RBBBRBBB

First Degree Heart BlockFirst Degree Heart Block Prolonged PR interval >200 ms (5 small sq.)Prolonged PR interval >200 ms (5 small sq.)

Slow conduction through the AVNSlow conduction through the AVN Not itself importantNot itself important May indicate disease:May indicate disease:

RARA IHDIHD Dig. ToxicityDig. Toxicity

Second Degree Heart BlockSecond Degree Heart Block

Mobitz Type I (Wenkebach)Mobitz Type I (Wenkebach)

Increasing PR interval preceding unconducted Increasing PR interval preceding unconducted P.P.

Does not usually cause symptoms.Does not usually cause symptoms.• May indicate disease (RA, IHD, Dig.).May indicate disease (RA, IHD, Dig.).• Usually benignUsually benign

Second Degree Heart BlockSecond Degree Heart Block

Mobitz Type IIMobitz Type II

Fixed PR interval with some unconducted P Fixed PR interval with some unconducted P waves.waves.

May occur with fixed ratio eg. 2:1 blockMay occur with fixed ratio eg. 2:1 block Indicates underlying disease.Indicates underlying disease. May cause symptoms/precede complete block.May cause symptoms/precede complete block.

Third Degree Third Degree (Complete) (Complete) Heart BlockHeart Block No relationship between P & QRSNo relationship between P & QRS

QRS often wideQRS often wide

Atrioventricular dissociationAtrioventricular dissociation Impairs cardiac performance.Impairs cardiac performance.

Left Bundle Branch Left Bundle Branch BlockBlock

Characteristic, widened QRS in chest leadsCharacteristic, widened QRS in chest leads Will not cause symptoms itselfWill not cause symptoms itself Always indicates underlying heart diseaseAlways indicates underlying heart disease Makes ECG interpretation difficult or Makes ECG interpretation difficult or

impossibleimpossible

W i LL ia M

Right Bundle Branch Right Bundle Branch BlockBlock

Characteristic, widened QRS in chest leadsCharacteristic, widened QRS in chest leads May indicate right heart diseaseMay indicate right heart disease Can occur in normal individualsCan occur in normal individuals

Partial RBBB is always normal.Partial RBBB is always normal.

M a RR o W

Heart RhythmHeart Rhythm

Always ask:Always ask:

What is the QRS width?What is the QRS width? <120ms => Supraventricular source<120ms => Supraventricular source >120 ms => Ventricular source (or BBB)>120 ms => Ventricular source (or BBB)

Is there a P wave?Is there a P wave? Is the rhythm Is the rhythm

regular/irregular/irregularly regular/irregular/irregularly irregular?irregular?

Supraventricular Supraventricular Rhythms (1)Rhythms (1)

Normal Sinus RhythmNormal Sinus Rhythm


(Respiratory) Sinus Arrhythmia(Respiratory) Sinus Arrhythmia

Inspiration Expiration


Supraventricular EctopicSupraventricular Ectopic


Junctional Escape BeatJunctional Escape Beat


Sinus TachycardiaSinus Tachycardia


Junctional TachycardiaJunctional Tachycardia


Atrial FibrillationAtrial Fibrillation


Atrial FlutterAtrial Flutter

Ventricular Rhythms (1)Ventricular Rhythms (1)

Ventricular Ectopic and CoupletVentricular Ectopic and Couplet


Ventricular BigeminyVentricular Bigeminy


Ventricular TachycardiaVentricular Tachycardia


Ventricular FibrillationVentricular Fibrillation

QT ProlongationQT Prolongation QT represents the ventricular refractory periodQT represents the ventricular refractory period Normal <450 ms (ish)Normal <450 ms (ish) Risk of prolongation...Risk of prolongation...

Torsades de Pointes - potentially lethal.

QT ProlongationQT Prolongation

? How long is too long? How long is too long Affected by temperature, gender, Affected by temperature, gender,

heart rateheart rate Diurnal variation up to 70 msDiurnal variation up to 70 ms Numerous drugs:Numerous drugs:

CisaprideCisapride TerfenadineTerfenadine Total of 48 listed as causing TdP by FDATotal of 48 listed as causing TdP by FDA

Normal VariantsNormal Variants Always normal:Always normal:

Sinus ArrhythmiaSinus Arrhythmia Supraventricular ExtrasystolesSupraventricular Extrasystoles Partial RBBBPartial RBBB

Often normal:Often normal: Sinus Bradycardia (and pauses in athletes)Sinus Bradycardia (and pauses in athletes) First Degree Heart BlockFirst Degree Heart Block Ventricular ExtrasystolesVentricular Extrasystoles Left/Right Axis DeviationLeft/Right Axis Deviation RBBBRBBB

The Basics of ECG Interpretation Dr Tim Smith. Summary Cardiac conducting system and the ECG...

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