ECG for Beginners

8/20/2019 ECG for Beginners

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ECG for Beginners-Part IRate, Rhythm and Axis


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Aims

To provide an introduction to the basicsof the electrocardiogram and its

interpretation.


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Learning objectives

By the end of the session the studentswill be able to:

Describe the conduction system of theheart and its relation to the formation ofthe electrocardiogram (ECG).

Define the calibration of an ECG recording.

Calculate heart rate.

Calculate cardiac axis.


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What is an electrocardiogram?

A recording of the changing potentials of theelectrical field imparted by the heart.


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What's the point?

Fundamental part of CVS assessment.

Heart rhythm (Arrhythmias) E.g. SR cf AF, bradycardia, tachycardia

Heart Injury E.g. Ischaemia, infarction

Heart structure

E.g. LVHMetabolic disorders E.g. K


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Principles of ECG I

Myocardial cells

Contraction due to Depolarisation

Relaxation due to Repolarisation

The electrical changes associated with

de- and repolarisation are recorded andform the ECG


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Principles of ECG II

Depolarisation towards positive electroderesults in upward

deflection, away fromelectrode deflectiondownward

Size of deflection

reflects volume ofdepolarised muscle


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Conduction system of Heart

SA node Natural pacemaker

Initiates atrial

depolarisation

AV node Propagation of

impulse to ventricles

His-Purkinje system Conducting tissues of

ventricles


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Formation of ECG


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Lead placement – “chest leads”


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Lead placement – “chest leads”

Chest leads(Anterior leads)

V1-V6

View heart inhorizontal plane


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Lead placement - “limb leads”


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Lead placement - “limb leads”

Limb Leads

I, II, III

aVL, aVF, aVR

View heart in avertical plane


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Anatomical Relationship

Lead

II, III, aVF V1-V6

I, aVL, V5-V6

Heart surface

Inferior Anterior

Lateral


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Technical Aspects


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Technical Aspects I

ECG recorded on standard paper

Paper speed 25mm/s

Paper 1 Large square = 5mm = 0.2s

1 Small square = 1mm = 0.04s


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Technical Aspects II

Electrical aspect is measured in Millivolts

Machines are calibrated

Amplitude 1mV moves stylus 1cm vertically 0.1mV = 1mm = 1 small square


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Technical Aspects III

Amplitude of wave form is influenced by

Myocardial mass e.g. LVH

Thickness of intervening tissue Distance between electrode and

myocardium

Net vector of depolarisation


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Technical Aspects IV

Low amplitude

Obesity

COPD Pericardial Fluid

High amplitude

Thin patient

Left VentricularHypertrophy (LVH)


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QRS Complex

Direction of deflection of ECG Direction of electrical impulse

Towards electrode-upward deflection (Positive) Away from electrode-downward deflection

(Negative)

When the wave of depolarisation is at rightangles to the lead Equiphasic deflection isproduced.


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QRS Complex II


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Normal ECG


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Heart Rate


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Heart Rate

Tachycardia (fast)HR>100 beats/min

Bradycardia (slow) HR


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How to calculate heart rate


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R-R = 2 large squares

i.e. 300 / 2 = 150/ min


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R-R = 2 large squares

i.e. 300 / 2 = 150/ min

R-R = 6 largesquares

i.e. 300/6=50/min


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Heart Rhythm


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Rhythm

Use a rhythm strip, Lead II commonly used


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Sinus Rhythm

Normal cardiac rhythm

P wave precedes every QRS

P wave upright in leads I and IIHR 60min< RATE < 100 min

Rhythm originates in the SA node andconducts to ventricles


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Sinus Arrthymia

Common in healthy individuals

Beat-Beat variation in R-R interval with

respiration (constant PR interval)Rate Increases with Inspiration

Vagally mediated

Increased volume of blood returns to heartin Inspiration.


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Heart Rate II

Irregular Rhythm Heart rate calculated from rhythm strip I.e.

lead II

1 second to record 2.5cm

10 seconds to record 25cm

Count number of intervals between QRScomplexes X 6

At i l Fib ill ti Wh t i th


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Atrial Fibrillation-What is therate?


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Cardiac Axis


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Cardiac Axis

“… The average direction of spread of

the depolarisation wave through theventricles as measured from a zeroreference point…”


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Cardiac Axis-Vertical Plane


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Cardiac Axis II

Vertical plane = limbleads.

Zero reference point

= lead INormal range -30 to + 90

aVL-aVF

Axis above Negative

Axis below Positive


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Cardiac Axis III

Normal cardiac axis 11o clock- 5 o clock.

Right Axis Deviation(RAD)

Axis beyond +90

Left Axis Deviation(LAD)

Axis beyond –30


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How to measure axis?

Normal RAD LAD

I

II

III or


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What is the axis?


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Hexaxial approach

Choose limb leadclosest to beingequiphasic

Axis lies 90 toequiphasic lead

Examine adjacentleads


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Why measure Axis?

Determination of axis is helpful indiagnosis of:

Conduction defects Broad complex tachycardias

Pre-excited conduction

PE


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Break


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Quiz


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True/False

The following are true of the cardiacconducting system AV node is anterior to AV septum

Right bundle of HIS divides into 2 fascicles Intrinsic rate of AV node higher than that of

ventricles.

AV node is under influence of vagus.

A pacemaker in right ventricle gives a similar QRSmorphology to LBBB

(F, F, T, T, T)


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True/False

In a normal person during inspirationthe following statements are correct Increased blood flow through SVC

Right ventricular systole is prolonged.

Aortic blood flow is reduced.

Heart rate is increased.

JVP is elevated.

(T, T, T, T, F)


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True/False

The following are normal Paper speed 25mm/s

1 large square = 10mm 1 small square = 1mm = 0.04s

Amplitude 1mV = 1cm

Low amplitude QRS complexes are

common(T, F, T, T, F)


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True/False

The following statements are correct

The SA node initiates atrial repolarisation

The SA node lies in the left atrium

The left bundle consists of 2 fascicles

The HIS-Purkinje system is the conducting tissuesof the ventricles

QRS complex is due to ventricular depolarisation(F, F, T, T, T)


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True/False

Heart Rate Tachycardia = HR


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True/False

Cardiac axis

To calculate cardiac axis you use the chest leads

Normal cardiac axis +30 - -90

Cardiac axis may be abnormal in broad complextachycardia

Axis –60 = LAD

Axis + 150 = RAD(F, F, T, T, T)


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Summary


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Conduction System

SA Node-Natural pacemaker in rightatrium

AV Node-Propagation of impulse toventricles

His Purkinje system-Conducting tissuesto ventricles

Left bundle branch consists of 2fascicles


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Calibration

Standard paper speed = 25mm/s

1 large square = 5mm = 0.2s

Amplitude 1mV moves stylus 1cmvertically

1 small square = 1mm = 0.04s


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Heart Rate

Tachycardia HR>100 beats/min

Bradycardia HR


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Cardiac Axis

Calculated using limb leads (verticalplane)

Normal axis –30 - +90Zero reference point = lead I

LAD axis beyond –30

RAD axis beyond +90


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Intervals

PR interval measured from beginning ofP wave to first deflection of QRS.

Normal PR interval 0.12-0.2s.QRS


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Nomenclature

Q Wave: Any INITIAL negativedeflection.

R Wave: Any positive deflection.S Wave: Any negative deflection afteran R wave.

T Wave: Ventricular repolarisation.

Ventricular Hypertrophy


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Ventricular HypertrophySokolow and Lyon

Precordial Leads (one or more) SV1 + RV5 or RV6

> 35mm if > 30yr of age > 40mm if 20 – 30 yr

> 60mm if 16 – 19 yr RV5 or RV6 > 26 mm SV1 or SV2 > 26 mm

Limb Leads (one or more) RI ≥ 14 mm

RaVL ≥ 12 mm RaVR ≥ 15 mm

(Cornell Criteria- most accurate) R aVL + SV3 > 24 mm in males > 20 mm in females


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Other Enlargements

Rt atrium: P > 2.5 in II, III, aVF (p pulm) or> 1.5 in V1 Causes: in V1 COPD, pulm HTN, thin habitus

Lt atrium: Terminal neg P in V1 ≥ 1 mm and0.04s. Notched P ≥ 0.12s (P mitrale) in II, IIIand aVF. Inter-atrial conduction disturbance

Rt vent: R/S in V1 > 1or R/S in V5 or 6 ≥ 1RV1 ≥ 7mm

Combined hypertrophy

≥


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Recommended Reading

The ECG made easy

J Hampton

ABC of electrocardiography

BMJ 2002


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Any Questions?

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ECG for Beginners