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The Basics of ECG Interpretation

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 prolongation Normal Variants. Cardiac Depolarisation. Originates in pacemaker cells (automaticity).

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The Basics of ECG Interpretation

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  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 @ 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 • V1+2 - R Venticle • V3+4 - Septum • V5+6 - 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 <120 ms • PR <200 ms • QRS <120 ms • QT Male <0.43 s Female <0.45 s • ST isoelectric

  10. Calculating Heart Rate • Standard ECG speed is 25 mm.s-1 • Heart Rate = 300/# big squares 300/3.8=79 bpm

  11. Abnormalities of conduction • At the AVN = Heart Block • 1st degree • 2nd degree • 3rd degree • In the His/Purkinje system = Bundle Branch Block • LBBB • RBBB

  12. First Degree Heart Block • Prolonged PR interval >200 ms (5 small sq.) • Slow conduction through the AVN • Not itself important • May indicate disease: • RA • IHD • Dig. Toxicity

  13. 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

  14. Second Degree Heart Block Mobitz Type II • Fixed PR interval with some unconducted P waves. • May occur with fixed ratio eg. 2:1 block • Indicates underlying disease. • May cause symptoms/precede complete block.

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

  16. 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

  17. Right Bundle Branch Block • Characteristic, widened QRS in chest leads • May indicate right heart disease • Can occur in normal individuals • Partial RBBB is always normal. M a RR o W

  18. Heart Rhythm Always ask: • What is the QRS width? • <120ms => Supraventricular source • >120 ms => Ventricular source (or BBB) • Is there a P wave? • Is the rhythm regular/irregular/irregularly irregular?

  19. Supraventricular Rhythms (1) • Normal Sinus Rhythm

  20. Supraventricular Rhythms (2) • (Respiratory) Sinus Arrhythmia Inspiration Expiration

  21. Supraventricular Rhythms (3) • Supraventricular Ectopic

  22. Supraventricular Rhythms (4) • Junctional Escape Beat

  23. Supraventricular Rhythms (5) • Sinus Tachycardia

  24. Supraventricular Rhythms (6) • Junctional Tachycardia

  25. Supraventricular Rhythms (7) • Atrial Fibrillation

  26. Supraventricular Rhythms (8) • Atrial Flutter

  27. Ventricular Rhythms (1) • Ventricular Ectopic and Couplet

  28. Ventricular Rhythms (2) • Ventricular Bigeminy

  29. Ventricular Rhythms (3) • Ventricular Tachycardia

  30. Ventricular Rhythms (4) • Ventricular Fibrillation

  31. QT Prolongation • QT represents the ventricular refractory period • Normal <450 ms (ish) • Risk of prolongation... Torsades de Pointes - potentially lethal.

  32. 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

  33. 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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