Introduction to the Introduction to the 12-lead ECG12-lead ECG

Gustav SchellackGustav SchellackSASOCP WorkshopSASOCP Workshop

10 March 201210 March 2012

© G Schellack; 2012

Electrocardiogram (ECG)Electrocardiogram (ECG)

The ECG is a graphic recording of the The ECG is a graphic recording of the electrical activity of the heart, at a specific electrical activity of the heart, at a specific moment in time.moment in time.

Note:Note: ‘Electrical activity’ refers to the ‘Electrical activity’ refers to the depolarisation and repolarisation of the depolarisation and repolarisation of the atria and ventricles.atria and ventricles.

Note:Note: The ECG cannot tell us anything The ECG cannot tell us anything about the ‘mechanical activity’ (i.e. about the ‘mechanical activity’ (i.e. contraction and relaxation) of the contraction and relaxation) of the myocardium.myocardium.

Cardiac Conduction SystemCardiac Conduction System

(From: Yanowitz, FG. The Alan E Lindsay ECG Learning Centre in Cyberspace [homepage on the Internet]. c2012. Available from: http://library.med.utah.edu/kw/ecg/index.html.)

(Marquette Electronics, 1996)

Anatomical orientation:Anatomical orientation:

Figure 1:

Originates from the SA-node (in

the RA)

Originates from the SA-node (in

the RA)

Spreads through both atria [virtually]

simultaneously

RA

Spreads through both atria [virtually]

simultaneously

RA

The Cardiac ImpulseThe Cardiac Impulse

Spreads through both atria [virtually]

simultaneously

LA

Reaches the AV-node (where it is

momentarily delayed)

Reaches the AV-node (where it is

momentarily delayed)

Enters the Bundle of His and the right

and left bundle branches

LBB

Enters the Bundle of His and the right

and left bundle branches

LBB

Activates both ventricles via the His-Purkinje network; first the septum from (L) to (R), then the RV, then the LV, and finally the

apex

Activates both ventricles via the His-Purkinje network; first the septum from (L) to (R), then the RV, then the LV, and finally the

apex

Enters the Bundle of His and the right

and left bundle branches

RBBRepolarisation of the ventricles (note that the repolarisation of the atria is obscured by the depolarisation of the

ventricles)

The Cardiac ImpulseThe Cardiac Impulse

Figure 2: Diagram of the functional SA- and AV-nodes© G Schellack, 2012

AV-node

Bundle of His

Bundle branche

s

Bachmann’s bundle

Right atrium (RA)

Ca2+Ca2+

Internodal pathways

SA-node

Ca2+Ca2+

The Standard 12-lead The Standard 12-lead ECGECG

You have each received a copy of this ECG…

What is missing in this blank space?

This space should contain the following information:

•Patient identifiers (e.g. name, hospital

number, treating physician, etc.)

•Date and timetime that the ECG was

obtained

•Vital signs, including BP and HR

•Comment on any relevant symptoms

(e.g. chest pain), cardiovascular drugs

and special lead placements…

The The 12 leads12 leads are: are: Six Six limb leadslimb leads (I, II, III, aV (I, II, III, aVRR, aV, aVLL and aV and aVFF)) Six Six precordialprecordial (chest) (chest) leadsleads: V: V11 to V to V66

The 12 leads are displayed at a The 12 leads are displayed at a standardisedstandardised tracing speed of tracing speed of 25 mm 25 mm per secondper second, and with , and with 1 cm representing 1 cm representing 1.0 mV1.0 mV on the vertical axis. on the vertical axis.

The Standard 12-lead The Standard 12-lead ECGECG

Check the calibration Check the calibration markmark..

Criteria:Criteria: Standardisation: 10 mm in height = 1 mV; 25 mm per second.Standardisation: 10 mm in height = 1 mV; 25 mm per second. Double and half-standardisation may be required.Double and half-standardisation may be required. Sinus rhythm:Sinus rhythm:

Each P-wave must be followed by a QRS-complex, and Each P-wave must be followed by a QRS-complex, and vice vice versaversa..

RR-intervals must be constant (or ≤ than 0.12 s of physiological RR-intervals must be constant (or ≤ than 0.12 s of physiological variance).variance).

Normal heart rate (60-100 beats per minute).Normal heart rate (60-100 beats per minute). Normal PR-interval (0.12 to 0.2 s in duration).Normal PR-interval (0.12 to 0.2 s in duration).

P-wavesP-waves: Examine for normality in Lead I, II and V: Examine for normality in Lead I, II and V11.. QRS-complexesQRS-complexes: Must be ‘positive’ in Lead I, II, III, aVF and aVL, : Must be ‘positive’ in Lead I, II, III, aVF and aVL,

and ‘negative’ in aVand ‘negative’ in aVRR.. Narrow QRS-complexes (0.08 to 0.12 s in duration); Q-waves in VNarrow QRS-complexes (0.08 to 0.12 s in duration); Q-waves in V55

and Vand V66 are < 0.04 s and < 3 mm deep. are < 0.04 s and < 3 mm deep. R-S-wave progression from VR-S-wave progression from V11 to V to V66;; QT QTcc-interval is normal.-interval is normal. Normal cardiac axisNormal cardiac axis: Positive QRS-complexes in Lead I, III and aVF.: Positive QRS-complexes in Lead I, III and aVF. Normal ST-segments and T-waves.Normal ST-segments and T-waves.

The Standard 12-lead The Standard 12-lead ECGECG

Limb Lead Placement andLimb Lead Placement andEinthoven’s TriangleEinthoven’s Triangle

Lead I++--

-- --

++ + +

Lead II Lead III

aVR (+) aVL (+)

aVF

(+)

RA LA

LL

Limb Lead Placement andLimb Lead Placement andEinthoven’s TriangleEinthoven’s Triangle

(From: Yanowitz, FG. The Alan E Lindsay ECG Learning Centre in Cyberspace [homepage on the Internet]. c2012. Available from: http://library.med.utah.edu/kw/ecg/index.html.)

Limb Lead Placement andLimb Lead Placement andEinthoven’s TriangleEinthoven’s Triangle

(From: Yanowitz, FG. The Alan E Lindsay ECG Learning Centre in Cyberspace [homepage on the Internet]. c2012. Available from: http://library.med.utah.edu/kw/ecg/index.html.)

Placement of the Placement of the Six Chest LeadsSix Chest Leads

Antero-lateral view

Midclavicular

Anterior axillary Midaxillary

Transverse plane

Important clinical Important clinical considerationsconsiderations

Correct lead-placement and good skin contact are Correct lead-placement and good skin contact are essential.essential.

Avoid electrical interference (machine to be Avoid electrical interference (machine to be earthed).earthed).

Compare serial tracings, if available.Compare serial tracings, if available. Relate any changes to age, gender, clinical Relate any changes to age, gender, clinical

history, etc.history, etc. Consider co-morbidities and intercurrent illnesses Consider co-morbidities and intercurrent illnesses

that may have an effect on the ECG.that may have an effect on the ECG. For thermal paper: Obtain a photocopy for future For thermal paper: Obtain a photocopy for future

reference.reference. Interpret the ECG systematically to avoid errors.Interpret the ECG systematically to avoid errors.

The Standard 12-lead ECGThe Standard 12-lead ECG [5/5][5/5]

What/where are the 12 leads?

Calibration mark

3 Standard (bipolar)

limb leads[I, II and III]

3 Augmented (unipolar) limb leads[aVR, aVL and a VF]

6 Precordial (chest) leads[V1 to V6]

ECG Graph PaperECG Graph Paper

y

x

Voltage (millivolts; mV)

Time (seconds; s)

y’

x’

(1 1) mm2

0.04 s

0.1 mV

QuestionQuestion: What would the : What would the bigger square, i.e. the (5 bigger square, i.e. the (5 5) mm5) mm22, represent?, represent?

AnswerAnswer: 0.2 s on the x-axis; : 0.2 s on the x-axis; 0.5 mV on the y-axis.0.5 mV on the y-axis.

Deflections on the ECGDeflections on the ECG

QRS-QRS-complecomplex (qRs)x (qRs)P-waveP-wave T-waveT-wave

U-waveU-wave

Q (q)Q (q)

S (s)S (s)

RR

Segments and Segments and Intervals on the Intervals on the

ECGECG

PR-PR-intervainterva

llQT-intervalQT-interval

ST-ST-segmensegmen

ttJ-pointJ-point

QRS-QRS-duratioduratio

n n

Isoelectric Isoelectric (base) line (base) line

[0 mV][0 mV]

The P-waveThe P-waveP-waveP-wave

Conduction through the Left Atrium Conduction through the Left Atrium (LA)(LA)

Conduction through the Right Atrium Conduction through the Right Atrium (RA)(RA) Conduction through the His-Purkinje Conduction through the His-Purkinje networknetwork

PR-intervalPR-interval

PR-intervalPR-interval:: Represents the depolarisation of Represents the depolarisation of the atria and the time it takes for the impulse the atria and the time it takes for the impulse to spread from the SA-node, through the AV-to spread from the SA-node, through the AV-junction, the bundle of His, and to the bundle junction, the bundle of His, and to the bundle branches.branches.

Drawing an Isoelectric Drawing an Isoelectric BaselineBaseline

Why do we need to do Why do we need to do this?this?

[A portion of the rhythm strip.]

Drawing an Isoelectric Drawing an Isoelectric BaselineBaseline

21

Next R-wave

Next R-wave

Rate Rate Determination:Determination:

First R-wave

First R-wave

22

Example:Example:What is the heart What is the heart rate ?rate ?

Count the number of squares between two consecutive R-waves…

•Are the RR-intervals constant?•RR-intervals are approximately 3 ‘big’ squares apart.•Thus, the heart rate is: 300 ÷ 3 = 100 beats per minute.

•Are the RR-intervals constant?•RR-intervals are approximately 3 ‘big’ squares apart.•Thus, the heart rate is: 300 ÷ 3 = 100 beats per minute.

Calculating Heart Rate (HR)Calculating Heart Rate (HR)

‘Counting squares’

‘Accurate measurement’

Calculating Heart Rate (HR)Calculating Heart Rate (HR)

Calculating Heart Rate (HR)Calculating Heart Rate (HR)

Measure the shortest and the longest distance in mm, and divide 1,500 by each measurement, for example:

•Shortest distance measured = 10 mm

•Longest distance measured = 15 mm

Then 1,500 10 = 150 and 1,500 15 = 100

Thus, the HR = 100-150 beats per minute

Correct answer:Correct answer: Shortest measurement = 16 mmShortest measurement = 16 mm Longest measurement = 19 mmLongest measurement = 19 mm

RememberRemember: Each small square represents 0.04 s, : Each small square represents 0.04 s, and 1,500 small squares represent one minute on and 1,500 small squares represent one minute on the horizontal axis.the horizontal axis.

ThereforeTherefore:: 1,500 1,500 16 = 93.75 (~ 94 beats per min.) 16 = 93.75 (~ 94 beats per min.) 1,500 1,500 19 = 78.95 (~ 79 beats per min.) 19 = 78.95 (~ 79 beats per min.)

Thus, this patient’s heart rate (HR) is Thus, this patient’s heart rate (HR) is 79 to 94 79 to 94 beats per minutebeats per minute..

Calculating Variance in HRCalculating Variance in HR

Following from the previous example, Following from the previous example, subtract the shortest distance measured, subtract the shortest distance measured, from the longest distance measured (i.e. from the longest distance measured (i.e. 150 mm – 100 mm = 50 mm).150 mm – 100 mm = 50 mm).

Then, multiply the answer by 0.04 s (i.e. Then, multiply the answer by 0.04 s (i.e. 50 50 0.04 = 2.0 seconds). 0.04 = 2.0 seconds).

Normal physiological variance should be Normal physiological variance should be 0.12 seconds. 0.12 seconds.

Correct answer:Correct answer: Shortest measurement (RR-interval) = 16 Shortest measurement (RR-interval) = 16

mmmm Longest measurement (RR-interval) = 19 Longest measurement (RR-interval) = 19

mmmm

ThereforeTherefore:: 19 mm – 16 mm = 3 mm19 mm – 16 mm = 3 mm 3 mm 3 mm 0.04 s = 0.12 s (i.e. the upper limit 0.04 s = 0.12 s (i.e. the upper limit

of normal)of normal)

Normal physiological variance: Normal physiological variance: 0.12 s 0.12 s..

The ‘PQRSTU-complex’ The ‘PQRSTU-complex’ Summary:Summary: P-wave and PR-interval:P-wave and PR-interval:

Represents atrial depolarisation – Normal: Represents atrial depolarisation – Normal: ~~ 0.10 0.10 s in duration, with an amplitude of 0.50 to 2.50 s in duration, with an amplitude of 0.50 to 2.50 mm.mm.

PR-interval is measured from the beginning of P-PR-interval is measured from the beginning of P-wave to the beginning of the QRS-complex wave to the beginning of the QRS-complex (whether this a Q-wave or an R-wave) – Normal: (whether this a Q-wave or an R-wave) – Normal: 0.12 to 0.20 s in duration.0.12 to 0.20 s in duration.

Digoxin prolongs the PR-interval, effectively Digoxin prolongs the PR-interval, effectively causing a 1causing a 1stst-degree AV-block, which will worsen -degree AV-block, which will worsen in the event of digitalis toxicity.in the event of digitalis toxicity.

(Cont’d(Cont’d):):

QRS-complex and ST-segment:QRS-complex and ST-segment: Represents depolarisation of the ventricles Represents depolarisation of the ventricles

Ventricular contraction – Normal: 0.08 to 0.12 s Ventricular contraction – Normal: 0.08 to 0.12 s in duration; amplitude of 5 to 15 mm.in duration; amplitude of 5 to 15 mm.

ST-segment is the interval between the end of ST-segment is the interval between the end of the QRS-complex and the beginning of the T-the QRS-complex and the beginning of the T-wave – Normal: on the isoelectric line (‘electric wave – Normal: on the isoelectric line (‘electric silence’); deviation of < 1 mm on either side.silence’); deviation of < 1 mm on either side.

T-wave:T-wave: Should be a positive deflection (except in aVShould be a positive deflection (except in aVRR, ,

VV11 and V and V22).). Represents repolarisation of the ventricles. Represents repolarisation of the ventricles.

Normal: 0.10 to 0.25 s in duration; amplitude < Normal: 0.10 to 0.25 s in duration; amplitude < 5 mm.5 mm.

QT-interval:QT-interval: Is measured from the beginning of QRS-Is measured from the beginning of QRS-

complex to the end of T-wave; requires complex to the end of T-wave; requires rate-correctionrate-correction for heart rate (QT for heart rate (QTcc).).

Certain drugs may prolong the QTCertain drugs may prolong the QTcc (i.e. (i.e. cardiotoxicity) and elicit cardiotoxicity) and elicit torsades de torsades de pointespointes..

Example of QT-interval correction: Example of QT-interval correction: Bazett’s formulaBazett’s formula..

(Cont’d):(Cont’d):

U-wave:U-wave: ?? (not necessarily present on all (not necessarily present on all normal ECGs).normal ECGs).

RR-interval:RR-interval: Interval between two consecutive R-waves. Interval between two consecutive R-waves.

Each R-wave Each R-wave shouldshould immediately precede a immediately precede a contraction of the ventricles.contraction of the ventricles.

We use the RR-interval to calculate HR (heart We use the RR-interval to calculate HR (heart rate); heart monitors “beep” whenever R-waves rate); heart monitors “beep” whenever R-waves are recognised.are recognised.

RememberRemember: Electromechanical dissociation : Electromechanical dissociation (EMD)…(EMD)…

(Cont’d):(Cont’d):

Rate Correction of the QT-Rate Correction of the QT-intervalinterval

RR-interval (s)

[Measured] QT-interval (s)QTc (s)

=

Bazett’s Bazett’s Formula:Formula:

Correct answer:Correct answer: Measurement of the QT-interval = 9.5 mmMeasurement of the QT-interval = 9.5 mm ThereforeTherefore: QT: QTmm (s) = 9.5 mm (s) = 9.5 mm 0.04 s = 0.38 s 0.04 s = 0.38 s

Using Bazett’s formulaUsing Bazett’s formula:: Shortest RR-interval: 16 mm Shortest RR-interval: 16 mm 0.04 s = 0.64 s 0.04 s = 0.64 s Longest RR-interval: 19 mm Longest RR-interval: 19 mm 0.04 s = 0.76 s 0.04 s = 0.76 s

ThusThus:: QTQTcc (s) = 0.38 s (s) = 0.38 s 0.64 s = 0.475 s (~ 0.48 s) 0.64 s = 0.475 s (~ 0.48 s)

QTQTcc (s) = 0.38 s (s) = 0.38 s 0.76 s = 0.437 s (~ 0.44 s) 0.76 s = 0.437 s (~ 0.44 s)

A normal QT-interval (corrected for HR) should A normal QT-interval (corrected for HR) should measure between: measure between: 0.35 and 0.45 s0.35 and 0.45 s..

Cardiac Axis Cardiac Axis DeterminationDetermination

Complexes moving

towards one another

Complexes moving

towards one another

‘Positive’ complexes‘Positive’

complexes

Normal axisNormal axisNormal axisNormal axis

Right axis Right axis deviationdeviation

Right axis Right axis deviationdeviation

Complexes deflecting away from

one another

Complexes deflecting away from

one another

Left axis Left axis deviationdeviation

Left axis Left axis deviationdeviation

Atrial P-wave VariantsAtrial P-wave VariantsCardiac Axis DeterminationCardiac Axis Determination

NormalNormalRight Axis Right Axis DeviationDeviation Left Axis Left Axis

DeviationDeviation

IIII

VV11

QuestionsQuestions??

Back-up Slides…Back-up Slides…

Ventricular Ventricular HypertrophyHypertrophy

Ventricular muscle Ventricular muscle hypertrophyhypertrophy

Increased QRS-voltages Increased QRS-voltages in Vin V11 and V and V66, Lead I and , Lead I and aVaVLL

May need to change the May need to change the tracing to half of the tracing to half of the normal standardisationnormal standardisation

T-wave changes in the T-wave changes in the opposite direction to the opposite direction to the QRS-complexesQRS-complexes

Associated axis-deviationAssociated axis-deviation Associated atrial Associated atrial

hypertrophyhypertrophy

Blood Supply to the HeartBlood Supply to the Heart

LCALCA

RCARCA

LAD

LCX

RCA

The heart has four surfaces:The heart has four surfaces: Anterior surfaceAnterior surface: LAD, Left Circumflex (LCx): LAD, Left Circumflex (LCx) Left-lateral surfaceLeft-lateral surface: LCx, and partly via LAD: LCx, and partly via LAD Inferior surfaceInferior surface: RCA, terminal portion of the : RCA, terminal portion of the

LADLAD Posterior surfacePosterior surface: RCA, branches of the LCx.: RCA, branches of the LCx.

(R) and (L) coronary arteries arise from (R) and (L) coronary arteries arise from the root of the aorta (just above the the root of the aorta (just above the aortic valve opening).aortic valve opening).

Blood Supply to the Blood Supply to the HeartHeart

ST-segment AbnormalitiesST-segment Abnormalities

Source: Bing Images, at http://www.bing.com/images

Lead aVLead aVRR

Flip the Flip the ECG ECG verticallyvertically

Lead aVLead aVLL

??

????