Electrocardiogram (ECG)

Last updated 23/09/18

Definition

• Graphical representation over time of cardiac electrical activity


Normal 12 lead ECG

Features

• Very common investigation
• 12 lead provides most information
• Standardised format
• 3 rows with 4 leads in each
• Top: I, aVR, V1, V4
• Middle: II, aVL, V2, V5
• Bottom: III, aVF, V3, V6
• Size
• 1mm cubes = small square
• Darker lines delineate 5mm sized cubes = large square
• Paper speed (X axis): 25mm/second
• 1mm (1 small square) = 0.04 seconds = 40 microseconds (ms)
• 5mm (1 large square) = 0.20 seconds = 200ms
• Voltage (Y axis): amplitude
• 5mm (1 large square) = 0.5mV
• 10mm (2 large squares) = 1mV


Measurements

• 3 lead also used

Indications

• Chest pain
• Palpitations
• Tachycardia
• Bradycardia
• Pre-operative assessment
• Cardiac stress testing
• Advanced Life Support (ALS)
• Cardiac arrest

Procedure

• Patient
• Chest exposed
• Shave chest is poor electrode contact
• Bra may be left on
• Semirecumbent or supine
• Relaxed with no movement
• 12 Lead
• Actually only uses 10 physical leads: 6 precordial and 4 limb
• 6 leads on chest (precordial): V1, V2, V3, V4, V5, V6
• V1: 4th intercostal space, right of sternum
• V2: 4th intercostal space, left of sternum
• V3: midpoint of V2 and V4
• V4: 5th intercostal space, midclavicular
• V5: 5th intercostal space, anterior axillary line, between V4 and V6
• V6: 5th intercostal space, midaxillary
• 4 leads on limbs: RA, RL, LA, LL
• RA: right arm
• RL: right leg
• LA: left arm
• LL: left leg


ECG lead placement


Limb lead colors and placement

• Produces 3 limb leads and 3 augmented (a) limb leads: I, II, II and aVF, aVL, aVR
• I
• II
• III
• aVF
• aVL
• aVR
• Leads should be placed equidistant


ECG Limb Leads

• +/- Posterior leads: V7, V8, V9
• All in same horizontal plane as V6
• V7: left post axillary line
• V8: tip of left scapula
• V9: left paraspinal


Posterior leads

• +/- Right side leads: V1R-V6R
• Mirror image to V1-V6
• V4R most important


Right side chest leads

• Record name, date, time, symptoms

Interpretation

• Rate, rhythm, axis, intervals, abnormalities
• Rate
• Machine generally calculates
• Calculate manually using lead II rhythm strip typically along bottom on ECG
• Each large square (5mm) = 200ms (0.2s)
• Total rhythm strip = 10 seconds (50 large squares)
• Total number of QRS x 6 = HR/minute
• Counting methods
• 300 / (number of large boxes between each QRS)
• 1 large box = 300 bpm
• 2 large boxes = 175 bpm
• 3 large boxes = 100 bpm
• 4 large boxes = 75 bpm
• 5 large boxes = 60 bpm
• 6 large boxes = 50 bpm
• 7 large boxes = 45 bpm


Box counting method for HR

• Adult
• <60 bpm: bradycardia
• >100 bpm: tachycardia
• Paediatric age dependant
• Rhythm
• Sinus rhythm
• P waves preceed each QRS complex
• Implies normal electrical conduction from sinus node to AV node and His-Purkinje system
• Regular rate
• HR <45: sinus bradycardia
• HR >60: sinus tachycardia
• Beat-to-beat variation in P-P interval: Sinus arrythmia
• Normal P wave axis
• Constant PR interval
• Narrow complex QRS (<120ms, 3 small squares)
• Normal Morphology
• Includes a P wave, QRS complex, T wave
• Arrythmias
• Supraventricular tachycardias (SVT): increased rate +/- irregular rhythm
• Ventricular tachycardia (VT): no P waves
• Ventricular fibrillation (VF): chaotic rhythm
• Axis
• Normal: between 30o to 90o
• If the main deflection of a QRS is positive then the overall axis lies within 90o either clockwise or anticlockwise from the lead
• If the main deflection of a QRS is negative then the overall axis lies more than 90o in both directions


ECG axis

• Assess from lead I and aVF
• Lead I +ve and aVF +ve: normal
• Lead I +ve and aVF -ve: left axis deviation caused by
• Left ventricular hypertrophy (LVH)
• Left anterior fascicular block (LAFB)
• Inferior acute myocardial infarction (AMI)
• Chronic obstructive pulmonary disease (COPD)
• Lead I -ve and aVF +ve: right axis deviation caused by
• Tall paitent, long thorax
• Situs inversus (dextrocardia)
• Left posterior fascicular block (LBBB)
• Lead I -ve and avF -ve: extreme left axis deviation or abnormal lead position
• Intervals


ECG Intervals

• PR Interval
• Represents conduction from atria to ventricles via AV node
• Commences at start of P wave and finishes at start of Q wave (or R)
• Normal: 0.12-0.20s (3-5 small squares)
• Prolonged: > 0.20s (5 small squares)
• Short: <0.12 (3 small squares): pre-excitation
• Wolf-Parkinson-White (WPW) syndrome
• Lown-Ganong-Levine syndrome
• AV nodal (junctional) rhythm
• QRS Complex
• Measured from start of Q wave to end of S wave
• Represents ventricular depolarisation
• Normal (narrow complex): <0.12s (3 small squares)
• Wide QRS: >120ms (3 small squares)


Normal QRS Complex

• ST Segment
• Should be isoelectric
• ST Depression
• Non ST elevated myocardial infarction (NSTEMI)
• Digoxin toxicity: reverse tick
• ST Elevation
• QT Interval
• Measured from start of Q wave to end of T wave
• Represents time of ventricular depolarisation and repolarisation
• Normal: <0.44s (11 small squares)
• Long: >0.44s (440ms): Long QT Interval
• Abnormalities
• Waves: P, Q, R, S, T, U
• P Waves
• First positive deflection on ECG
• Represents atrial depolarisation
• Summation of first right atrial followed by left atrial depolarisation
• Normal
• Should preceed each QRS
• Use lead II
• Smooth upright monophasic contour
• Amplitude (mV): <2.5mm (5 small squares)
• Duration: <0.12ms (3 small squares)
• May be biphasic in V1
• Upright in leads I, II
• Also usually in aVF, V3-V6
• Inverted in aVR
• Abnormal
• Absent
• Atrial fibrillation (AF)
• Hyperkalaemia
• Junctional rhythm
• Escape beat from AV node when SA node does not fire
• Normal QRS
• Negative P wave may appear in lead III after QRS complex
• Caused by
• Hypoxia
• Increased vagal tone
• Digoxin toxicity
• Saw tooth: atrial flutter
• Not associated with QRS: atrioventricular (AV) dissociation
• Inverted
• Axis deviation
• Lead reversal
• Atrial enlargement
• Left atrial: notched P wave with >0.12s (3 small squares)
• Normal amplitude <2.5mm (5 small squares)
• P mitrale: Mitral regurgitation (MR)


P Mitrale

• Right atrial: P wave amplitude >2.5mm (2.5 small squares) 0.25mV
• Use lead II
• Normal width <0.12s (3 small squares)
• Q Waves
• R Waves
• Upright: I, II, V4-V6
• Precordial (chest) leads
• Progressively taller from V1 to V6 (R wave progression)
• Transitional zone around V3-V4 where R and S are similar sizes
• Early transition = V1 or V2
• Late transition = V5 or V6
• Poor R Wave Progression
• R wave <3mm in V3
• Caused by
• Previous anteroseptal AMI
• LVH
• Right ventricular hypertrophy (RVH)
• Lead misplacement ie: V1 and V2
• Dilated cardiomyopathy
• Tension pneumothorax
• Dextrocardia
• Normal variant



• Abnormal
• Dominant R wave in V1
• Normal in children and young adults
• RVH
• RBBB
• Posterior AMI
• WPW
• Lead reversal
• Dextrocardia
• Dilated cardiomyopathy
• Dystrophy
• Dominant R wave in aVR
• Sodium channel blocker poisoning
• Dextrocardia
• Left arm/right arm lead reversal
• VT
• R/S (RS Ratio)
• >1: V1/V2: RBBB, RVH, normal variant in young children/adolescents
• S Waves
• Deep: aVR, V1-V2
• Progressively smaller from V1-V6
• R and S similar sizes V3 or V4 (transitional zone)
• Dominant S wave in V1
• Normal ECG
• If wide QRS: LBBB
• T Waves
• Same direction as QRS complex
• Normally round and symterical
• Upright in V2-V6
• T Wave Inversion (TWI)
• Normally inverted in aVR
• Peaked T waves
• Usually <5mm limbs, <10mm precordial
• Global: hyperkalaemia
• Regional: AMI subendocardial ischaemia
• De Winter T waves: STEMI
• Flat T waves
• Hypokalaemia +/- presence of U wave
• Hypercalcaemia
• Biphasic
• Brugada pattern/syndrome: risk of sudden cardiac death
• Wellens syndrome: critical LAD stenosis
• U Waves
• Wave which follows T wave and preceeds P wave
• Typically in same direction as T wave
• Not always seen
• Represents repolarisation of pappilary muscles or Purkinje fibers
• Caused by
• Normal variant particularly if young
• Hyperkalaemia
• Hypercalcaemia
• Thyrotoxicosis
• Digoxin
• Adrenaline
• Antiarrythmics
• Long QT syndrome
• Intracranial haemorrhage: subarachnoid Haemorrhage (SAH), subdural haematoma, epidural haematoma
• Inverted U waves: AMI
• Unless in aVR, III, aVF

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References

1. Cadogan M. The P Wave. Life in the Fast Lane. 17/04/2017.
2. ECGPedia.org: Rate. 15/03/2011.
3. Analysis and Interpretation of the Electrocardiogram: Intervals and segments. Queens School of Medicine.
4. Electrocardiography. Wikipaedia. 14/07/2018.