The Basics of ECG Interpretation Dr Tim Smith

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

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

The Cardiac Conducting System

Anatomy of the ECG

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

The Chest Leads V R Venticle V R Venticle V Septum V Septum V L Ventricle V L Ventricle Bundle Branch Blocks Bundle Branch Blocks Ischaemia (esp. V 5 ) Ischaemia (esp. V 5 )

The Normal ECG

Characteristics of the Normal ECG Rate bpm Rate bpm Sinus rhythm Sinus rhythm Cardiac Axis -30º to +90º Cardiac Axis -30º to +90º P <120 ms P <120 ms PR<200 ms PR<200 ms QRS<120 ms QRS<120 ms QTMale <0.43 s Female <0.45 s QTMale <0.43 s Female <0.45 s STisoelectric STisoelectric

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

Cardiac Axis (Einthoven’s Triangle) Average direction of depolarisation in the ventricles Average direction of depolarisation in the ventricles

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

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

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

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

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

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

Left Bundle Branch Block Characteristic, widened QRS in chest leads Characteristic, widened QRS in chest leads Will not cause symptoms itself Will not cause symptoms itself Always indicates underlying heart disease Always indicates underlying heart disease Makes ECG interpretation difficult or impossible Makes ECG interpretation difficult or impossible W i LL ia M

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

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

Supraventricular Rhythms (1) Normal Sinus Rhythm Normal Sinus Rhythm

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

Supraventricular Rhythms (3) Supraventricular Ectopic Supraventricular Ectopic

Supraventricular Rhythms (4) Junctional Escape Beat Junctional Escape Beat

Supraventricular Rhythms (5) Sinus Tachycardia Sinus Tachycardia

Supraventricular Rhythms (6) Junctional Tachycardia Junctional Tachycardia

Supraventricular Rhythms (7) Atrial Fibrillation Atrial Fibrillation

Supraventricular Rhythms (8) Atrial Flutter Atrial Flutter

Ventricular Rhythms (1) Ventricular Ectopic and Couplet Ventricular Ectopic and Couplet

Ventricular Rhythms (2) Ventricular Bigeminy Ventricular Bigeminy

Ventricular Rhythms (3) Ventricular Tachycardia Ventricular Tachycardia

Ventricular Rhythms (4) Ventricular Fibrillation Ventricular Fibrillation

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

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

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