The Basics of ECG Interpretation

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The Basics of ECG Interpretation

• Dr Tim Smith

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Summary

• Cardiac conducting system and the ECG waveform
• The normal ECG
• Abnormalities of conduction
• Heart Rhythms
• QT prolongation
• Normal Variants

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Cardiac Depolarisation

• Originates in pacemaker cells (automaticity).
• Spreads along defined pathways.
• Causing co-ordinated muscular contraction.
• Electrical signal from depolarisation detectable
  at body surface. (1 mV cf. 90 mV _at_ cell)
• This is the ECG.

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The Cardiac Conducting System
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Anatomy of the ECG
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The Limb Leads

• I, II, VL - L lateral surface
• VF, III - Inferior surface
• VR - R atrium
• Cardiac Axis
• Lead II often used for rhythm strips.

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The Chest Leads

• V12 - R Venticle
• V34 - Septum
• V56 - L Ventricle
• Bundle Branch Blocks
• Ischaemia (esp. V5)

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The Normal ECG
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Characteristics of the Normal ECG

• Rate 50-100 bpm
• Sinus rhythm
• Cardiac Axis -30º to 90º
• P lt120 ms
• PR lt200 ms
• QRS lt120 ms
• QT Male lt0.43 s Female lt0.45 s
• ST isoelectric

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Calculating Heart Rate

• Standard ECG speed is 25 mm.s-1
• Heart Rate 300/ big squares

300/3.879 bpm
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Cardiac Axis (Einthovens Triangle)

• Average direction of depolarisation in the
  ventricles

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Cardiac Axis (at a glance)
First look at lead I and aVF
Then look at lead II...
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Abnormalities of conduction

• At the AVN Heart Block
• 1st degree
• 2nd degree
• 3rd degree
• In the His/Purkinje system Bundle Branch Block
• LBBB
• RBBB

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First Degree Heart Block

• Prolonged PR interval gt200 ms (5 small sq.)
• Slow conduction through the AVN
• Not itself important
• May indicate disease
• RA
• IHD
• Dig. Toxicity

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Second Degree Heart Block

• Mobitz Type I (Wenkebach)
• Increasing PR interval preceding unconducted P.
• Does not usually cause symptoms.
• May indicate disease (RA, IHD, Dig.).
• Usually benign

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Second Degree Heart Block

• Mobitz Type II
• Fixed PR interval with some unconducted P waves.
• May occur with fixed ratio eg. 21 block
• Indicates underlying disease.
• May cause symptoms/precede complete block.

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Third Degree (Complete) Heart Block

• No relationship between P QRS
• QRS often wide
• Atrioventricular dissociation
• Impairs cardiac performance.

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Left Bundle Branch Block

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

W i LL ia M
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Right Bundle Branch Block
M a RR o W

• Characteristic, widened QRS in chest leads
• May indicate right heart disease
• Can occur in normal individuals
• Partial RBBB is always normal.

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Heart Rhythm

• Always ask
• What is the QRS width?
• lt120ms gt Supraventricular source
• gt120 ms gt Ventricular source (or BBB)
• Is there a P wave?
• Is the rhythm regular/irregular/irregularly
  irregular?

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Supraventricular Rhythms (1)

• Normal Sinus Rhythm

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Supraventricular Rhythms (2)
Inspiration
Expiration

• (Respiratory) Sinus Arrhythmia

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Supraventricular Rhythms (3)

• Supraventricular Ectopic

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Supraventricular Rhythms (4)

• Junctional Escape Beat

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Supraventricular Rhythms (5)

• Sinus Tachycardia

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Supraventricular Rhythms (6)

• Junctional Tachycardia

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Supraventricular Rhythms (7)

• Atrial Fibrillation

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Supraventricular Rhythms (8)

• Atrial Flutter

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Ventricular Rhythms (1)

• Ventricular Ectopic and Couplet

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Ventricular Rhythms (2)

• Ventricular Bigeminy

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Ventricular Rhythms (3)

• Ventricular Tachycardia

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Ventricular Rhythms (4)

• Ventricular Fibrillation

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QT Prolongation

• QT represents the ventricular refractory period
• Normal lt450 ms (ish)
• Risk of prolongation...

Torsades de Pointes - potentially lethal.
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QT Prolongation

• ? How long is too long
• Affected by temperature, gender, heart rate
• Diurnal variation up to 70 ms
• Numerous drugs
• Cisapride
• Terfenadine
• Total of 48 listed as causing TdP by FDA

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Normal Variants

• Always normal
• Sinus Arrhythmia
• Supraventricular Extrasystoles
• Partial RBBB
• Often normal
• Sinus Bradycardia (and pauses in athletes)
• First Degree Heart Block
• Ventricular Extrasystoles
• Left/Right Axis Deviation
• RBBB

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