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The Basics of ECG Interpretation

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The Basics of ECG Interpretation Dr Tim Smith Summary Cardiac conducting system and the ECG waveform The normal ECG Abnormalities of conduction Heart Rhythms QT ... – PowerPoint PPT presentation

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Title: The Basics of ECG Interpretation


1
The Basics of ECG Interpretation
  • Dr Tim Smith

2
Summary
  • Cardiac conducting system and the ECG waveform
  • The normal ECG
  • Abnormalities of conduction
  • Heart Rhythms
  • QT prolongation
  • Normal Variants

3
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.

4
The Cardiac Conducting System
5
Anatomy of the ECG
6
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.

7
The Chest Leads
  • V12 - R Venticle
  • V34 - Septum
  • V56 - L Ventricle
  • Bundle Branch Blocks
  • Ischaemia (esp. V5)

8
The Normal ECG
9
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

10
Calculating Heart Rate
  • Standard ECG speed is 25 mm.s-1
  • Heart Rate 300/ big squares

300/3.879 bpm
11
Cardiac Axis (Einthovens Triangle)
  • Average direction of depolarisation in the
    ventricles

12
Cardiac Axis (at a glance)
First look at lead I and aVF
Then look at lead II...
13
Abnormalities of conduction
  • At the AVN Heart Block
  • 1st degree
  • 2nd degree
  • 3rd degree
  • In the His/Purkinje system Bundle Branch Block
  • LBBB
  • RBBB

14
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

15
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

16
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.

17
Third Degree (Complete) Heart Block
  • No relationship between P QRS
  • QRS often wide
  • Atrioventricular dissociation
  • Impairs cardiac performance.

18
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
19
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.

20
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?

21
Supraventricular Rhythms (1)
  • Normal Sinus Rhythm

22
Supraventricular Rhythms (2)
Inspiration
Expiration
  • (Respiratory) Sinus Arrhythmia

23
Supraventricular Rhythms (3)
  • Supraventricular Ectopic

24
Supraventricular Rhythms (4)
  • Junctional Escape Beat

25
Supraventricular Rhythms (5)
  • Sinus Tachycardia

26
Supraventricular Rhythms (6)
  • Junctional Tachycardia

27
Supraventricular Rhythms (7)
  • Atrial Fibrillation

28
Supraventricular Rhythms (8)
  • Atrial Flutter

29
Ventricular Rhythms (1)
  • Ventricular Ectopic and Couplet

30
Ventricular Rhythms (2)
  • Ventricular Bigeminy

31
Ventricular Rhythms (3)
  • Ventricular Tachycardia

32
Ventricular Rhythms (4)
  • Ventricular Fibrillation

33
QT Prolongation
  • QT represents the ventricular refractory period
  • Normal lt450 ms (ish)
  • Risk of prolongation...

Torsades de Pointes - potentially lethal.
34
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

35
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

36
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