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ECG Basics

Leads-4 limb and 6 chest electrodes →electrical activity from 12 different viewpoints Limb leads- I , II , III , aVR , aVL , aVF -Frontal plane information Leads I , II and III are bipolar aVR , aVL , aVF are unipolar Precordial leads V1-6- provide information on horizontal plane 
( unipolar )

Rate –Check calibration ie 10 mm = 1mV ECG trace speed @ 25 mm/sec 1 mm (small sqaure )= 0.04 sec 5 mm ( big square) = 0.2 sec Ventricular rate → 300/RR interval (number of big squares bet successive R waves )
If rate irregular →Number of QRS in 30 squares multiplied by 10 Atrial rate →using number of squares bet P waves

Rhythm –Use 10 sec recording from lead II → to access rhythm
 Rate QRS Pattern-Regular / Irregular QRS Morphology ( Narrow or wide ) Atrial activity-P Waves ( absent / present ) < 0.08 sec P wave- QRS relationship Ventricular activity ? Broad , narrow or normal ?

Intervals and durations –PR interval 120-200 ms ( 3-5 small squares ) QRS complex 120 ms (3 small squares )
Narrow QRS complexes < 100 ms
Broad complexes > 100 ms QT interval →is inversely proportional to HR
Corrected QT interval ( QTC) estimates 
QT at HR of 60 bpm
QTC is prolonged if > 440ms in ♂ and > 460ms in ♀

QRS Voltage-Low voltage Amplitude of all QRS complexes in limb leads < 5 mm or Amplitude of all QRS complexes in precordial leads < 10mm High voltage Several criteria exist Can be normal- slim athletic build Commonly reflects LVH

Q Waves-Small Q waves can be normal in leads 
I , II , aVL , V5-6 ( called septal “q” waves ) Pathological Q waves if Pathological if
 > 2 small squares deep > 40 ms ( 1mm ) wide > 25 % of height of the following R wave in depth

Right atrial hypertrophy Peaked P waves ( P Pulmonale ) most pronounced in inferior leads P wave >= 2.5 mm lead II Left atrial hypertrophy -Bifid P waves → P Mitrale
best seen in lateral leads

Right ventricular hypertrophy-R>S in V1 and R in V1 > 5mm Is associated with
Rt axis deviation
Deep S wave in leads V5-V6

Left ventricular hypertrophy-Tall R waves I , aVL , V5-6 Several criteria exist- for eg measuring S wave in lead V3 and the R wave in lead aVL ( sum > 28 mm in ♂ and > 20 mm in ♀

1st degree heart block-Delay in conduction of impulse through AV node from atria to ventricles
 Prolonged PR Interval

Mobitz type 1 Each successive impulse from the atria finds ↑ difficult to pass via the AV node

Progressive prolonging of PR interval then failure to conduct a beat

Mobitz type 2-Normal AV conduction then complete loss of conduction ie a P wave without a QRS complex

2 :1 or 3: 1 or 4:1 varying block → number of P waves that are not 
followed by QRS complex

Complete heart block-No relationship between P waves and QRS complexes Broad complex ventricular escape rhythm

Right bundle branch block-QRS → RSR pattern in leads V1 -3 Broad QRS complexes (↑ than 3 small sq ) Wide slurred S wave in lareral leads ( I , aVL , V5-6 ) RSR pattern with normal width QRS complex → partial RBBB RBBB can be relatively normal finding in otherwise normal hearts Other causes → Rt ventricular h’trophy , PE , IHD , Cardiomyopathy , Atrial septal defect

Left bundle branch block-Normal direction of septal depolarisation is reversed QRS looks like W in V1 and an M in V6 QRS duration > = 0.12 sec Broad monophasic R wave in lateral leads ( I , aVL , V5-6 ) Q waves absent in leads V5-6 LBBB commonly associated with CAD , hypertensive heart disease or dilated cardiomyopathy ( Always abnormal )

Inverted T waves –Normal in children Ischaemia and infarction Ventricular hypertrophy ( strain pattern ) Bundle branch block PE Hypertrophic cardiomyopathy Digoxin toxicity

QT interval-Start of the Q wave to end of the T wave Requires correction according to heart rate Faster the HR shorter the QT interval An abnormally long QTc interval is associated with ↑ ed risk of ventricular arrythmias ( esp Torsades de Pointes ) QTc prolonged if > 440 ms in men and 460 ms in women QTc < 0.35 s consider- congenital QT syndromes , ↑ Ca , Digoxin

Read further about QT Interval –


University of Toledo Medical Center Basic Cardiac Rhythms Identification and Response learn about ECG- an excellent resource. Reading this fully will equip you with all the skills that you need to survive in primary care

You Tube – see HEARTSTART SKILLS Frasco a compilation of pathological ECG changes



Key references
 ECG interpretation made simple Dr Andrew Money-Kyrle Nov 2010 GP Online ECG in Practice-John R Hampton et al Churchill Livingstone Aug 2013 ECG Made Easy John R Hampton Churchill Livingstone Aug 2013 A General Practitioners guide to interpreting ECGs accessed via Making sense of ECG Andrew R Houghton , David Gray Hodder Education May 2008



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