The Normal ECG

Raja Selvaraj

Additional Professor, JIPMER

Introduction and History

Introduction to the workshop

  • ECG valuable diagnostic tool in a wide variety of situations
  • Important skill to acquire during the training period
  • Books / workshops / Online material serve as introduction / refresher
  • Constant practice is a must

Workshop objectives

  • Introduction to ECG - what it is, how to read, how to interpret
  • Not comprehensive coverage of all aspects
    • Congenital heart disease
    • Electrolyte abnormalities, poisoning
  • Try to cover aspects that are difficult to learn by other means

Workshop schedule

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Objectives of this talk

  • Genesis, recording and display of ECGs
  • Normal waveforms and intervals
  • Heart rate
  • Axis

But first, a little history. Roughly how old is the clinical ECG ?

  1. 30 years
  2. 50 years
  3. 100 years
  4. 200 years

Centenary celebrated in 2002

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Display of heart's electrical activity recorded with a galvanometer

1887 - British physiologist - Augustus Waller

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Waller's dog Jimmy

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1890s - Einthoven watched Waller's demonstration

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1902 - Einthoven makes significant improvements

  • Develops the string galvanometer
  • Correction formulae
  • Publishes first ECG recorded with the string galvanometer

Early rapid progress

  • Einthoven records 'telecardiograms' in 1905
  • Discusses commercial production
  • Available commercially in 1908

ECG recording in Einthoven's time - We have progressed so much

einthoven_recording.jpg

Or have we? - Recorded ECG

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Basic principles and recording

Activation wavefront and generation of electromagnetic force

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Genesis of deflections

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Electrocardiographic anatomy

Two chambers

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Activation pattern in atrium and ventricle

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Atrial activation

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Ventricular activation

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Recording the deflections

ECG leads

  • 12 conventional leads
  • Frontal plane leads / limb leads
  • Horizontal plane leads / precordial leads
  • Bipolar leads - I, II, III
  • Unipolar leads - aVR, aVL, aVF, V1-V6

Question - How many electrodes are used for recording the 12 leads?

  1. 12 electrodes
  2. 6 electrodes
  3. 10 electrodes

Electrode placement

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Frontal plane leads

frontal_leads.jpg

Lead planes

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Einthoven's triangle

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Question - The electrodes in the triangle are equidistant because

  1. The ends of the limbs are equidistant from the heart
  2. The electrodes are placed at equal distance from heart
  3. Beyond a certain distance, an electrode is considered equidistant

Hexaxial reference

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Horizontal plane leads

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Recording - calibration / speed

Paper

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Filters

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Normal waveforms

Normal P wave

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Normal P wave

  • Axis - inferior and left
  • Shape - smooth and rounded
  • Amplitude - less than 2.5 mm (0.25 mV)
  • Duration - less than 110 ms

Normal P wave

atrial_activation_pwave.jpg

Normal QRS

  • Image
  • Axis - left inferior, towards lead II
  • Duration - usually less than 100 ms
  • Shape - smooth, multiphasic, no notching
  • Precordial progression

Terminology

qrs_terminology.png

Normal QRS - Axis and progression

normal_qrs.jpg

Normal T wave

  • Axis
    • similar to QRS
  • Shape - asymmetric limbs, proximal shallower, blunt apex

T wave

normal_qrs.jpg

U wave

  • small rounded deflection after T wave
  • same direction as T
  • best seen V2-V4
  • Genesis - purkinje fibers? / M cells ?

PR interval

  • Normal 0.12 to 0.20 ms
  • Represents impulse conduction time from sinus node to ventricle

P-R_interval.jpg

QT interval measurement

tangent_method.jpg

QT interval correction

  • QT interval varies with heart rate
  • Comparison with a normal needs correction for this variation
  • Corrected QT = QT at heart rate 60 bpm
  • Methods - Bazett, Fridericia, Framingham
  • Estimation errors least with HR close to 60

Bazetts method

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Heart rate and Electrical axis

Heart rate

  • Important measurement from the ECG
  • Bradycardia and tachycardia
  • Constant practice

Deriving rate

  • 300 / large squares
  • 1500 / small squares

heart_rate_method.png

Heart rate ?

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Heart rate ?

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Heart rate ?

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Electrical axis

Concept of axis

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Question - How many leads are required at minimum to determine frontal axis

  1. One
  2. Two
  3. Three
  4. Six

Determine the axis

Determine the quadrant - Leads I and aVF

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Determining axis - perpendicular to most isoelectric lead

lead_vectors.png

Examples

Axis

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Axis

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Axis

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Axis

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Axis

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The Normal ECG

Reading an ECG

  • Rhythm
  • Rate
  • P wave
  • PR interval
  • QRS width
  • QRS axis
  • Transition

ECG

normal_ecg.jpg

Normal variants

Persistent juvenile pattern

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Early repolarization syndrome

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Summary

  • ECG remains one of the most resilient investigations across time
  • Knowing the basics of the genesis of the waveforms provides a better understanding of the abnormalities
  • Like every other skill, reading ECGs comes and improves with regular practice