This document provides an overview of electrocardiogram (ECG) interpretation. It discusses the components of a normal ECG tracing including rate, rhythm, axis, waveforms such as P, QRS, ST segments, and T waves. It describes how to systematically evaluate an ECG and identify abnormalities. Common abnormalities are outlined such as arrhythmias, conduction blocks, myocardial infarction, hypertrophy, and electrolyte disturbances. Causes of abnormalities in various ECG components are also reviewed. The goal is to provide clinicians a methodical approach to ECG interpretation.

1. ECG Interpretation
Dr S. A. Medway
SCGH EDCMEThursday 12/03/2015
Original Presentation Credit to Dr. James Wheeler

2. Resources
• Original Presentation – Dr JamesWheeler
• Erics Medical Lectures –YouTube
• LITFL
• emin5.com
• Time on the floor

3. What is an ECG?
• Graphical representation of the electrical activity of the myocardium over
time
• Standard 12 lead ECG assesses this in various planes (coronal & transverse)
to give a roughly 3D view of the heart
– 3 biploar leads I, II, III
– 9 unipolar leads aVR, aVL, aVF,V1-V6
– Can consider other unipolar lead placements
– V1-6R – look at RV
– V7-9 – look at post LV wall

4. Lead placement

5. Anatomical relationship

6. ANATOMICAL RELATIONSHIP OF LEADS

7. Considerations
• Methodical systematic approach
– “More is missed by not looking than by not
knowing"[Thomas McCrae, 1870-1935]
• Consider patients clinical condition
• Obtain previous ECG’s for comparison
• Limitations of ECG – Electrical activity, not
contractility etc…
• The “ normal” ECG
– All normal ECG’s do not look the same
– But they do have features in common

8. ECG helps in detection of:
– Ischaemic heart disease, acute or chronic
– Dysrrhythmias
– Electrolyte disturbances
– Conduction abnormalities (HB, BBB)
– CAD (exercise stress test)
– Cardiac structural abN (LVH, RVH)
– Cardiac manifestations of non-cardiac disease
(PE, metabolic disorders, lung disease…)

9. COMPONENTS OF ECG
• Rate
• Rhythm / Regularity
• Cardiac Axis
• Waveform (axis, amp, duration)
– P wave
– QRS complex
– T wave
– U wave
• Intervals
– PR interval
– QT interval
– RR interval
• Segments
– PR segment
– ST segment

10. ECG systematic evaluation
• Calibration / Speed
– N = 10mm/1mV, 25mm/sec
• Rhythm
– Regularity / QRS width / Rate
• Axis
– N / LAD / RAD
• P wave
– ?presence / relationship to QRS
– Axis
– Amp: RAA / LAA
• PR interval
– Duration / HB / Pre-excitation
• PR segment
– Depression
• QRS
– duration / amplitude / morphology
– Q waves
– R wave
• ST segment
– Depression / elevation / morphology
• T wave
– Axis / morphology / duration
• QT interval
• U wave
• Other waves (delta, osborne, epsilon, pacing
spikes, artifacts)
• ?other leads (V4R,V7-9, Lewis)

12. Normal ECG

13. ECG Rhythm
• Rate
– N / tachy / brady
• Regularity
– Regular / reg irreg / irreg irreg
• QRS morphology
– Narrow / wide (120ms)
• P waves
– Absent
– Present / morphology / relationship QRS / PR interval

14. ECG Rhythm
• Is it normal sinus rhythm (NSR)?
– Can you identify P waves
– P waves preceding all the QRS complexes
– QRS complexes after every P wave
– Normal PR interval
– Regular
• P waves most easily seen in II &V1
• If not – it’s not Sinus Rhythm. What is it??

15. Arrhythmias - Causes
• Abnormal impulse generation
– SA node
– Outside of the SA node
• Abnormal impulse conduction
– Damaged conduction pathways
– Re-entrant pathways

16. Note: there are many potential pacemaker sites in the heart!

21. AXIS (Coronal Plane)
• Axis of any ECG depolarisation or repolarisation
• QRS axis in the coronal (frontal) plane as determined
by the limb leads :
– bipolar (I, II, III) &
– unipolar (aVR, aVL, aVF)
• Normal axis determined largely by the relative size of
both the RV & LV, and the position of the heart
– may change in various pathological conditions

22. Determining QRS axis
• Normal QRS coronal axis
is (??) -30 to +90
degrees:
Both I & aVF +ve = normal
Both I & aVF -ve = EAD
lead I -ve & aVF +ve = RAD
lead I +ve & aVF -ve
lead II +ve = normal
lead II -ve = LAD
N
RAD
LAD
EAD

23. Normal ECG

24. Right axis deviation

25. Causes of RAD
• RVH (most common)
• Acute RH strain - Pulmonary embolus
• Dextrocardia
• Normal in children and tall thin adults
• Chronic lung disease even without pulmonary hypertension
• Left posterior hemiblock
• Lateral / apical AMI
• WPW syndrome - left sided accessory pathway
• Atrial septal defect
• Ventricular septal defect

26. Left axis deviation

27. Causes of LAD
• ?LVH
• Left anterior hemiblock
• LBBB
• Q waves of inferior myocardial infarction
• Artificial cardiac pacing
• Emphysema
• WPW syndrome - right sided accessory pathway
• Tricuspid atresia
• Ostium primum ASD

28. Causes of extreme axis deviation
• Emphysema
• Hyperkalaemia
• Lead transposition
• Artificial cardiac pacing
• Ventricular tachycardia

29. Transverse axis of QRS / Rotation
• Normal transverse axis is leftward and posterior:
– Hence usually a progression in R wave height fromV1 to
V6
• Transitional lead (where R = S)
– usuallyV3 orV4
• Displacement of transition:
– Right (V1,V2) = counterclockwise rotation
– Left (V5,V6) = clockwise rotation

30. Normal ECG

31. WAVE MORPHOLOGY
• Shape
• Axis
• Duration
• Amplitude
• Progression

32. P wave - Normal
• Normal P wave
– Represents atrial depolarisation
– Axis 0 – 75 deg
– Upright monophasic I, II, III and AVF
– Inverted in AVR
– Biphasic inV1 (RA bf LA)
– <2.5mm (0.25mV) in height limb leads
– <1.5mm in praecordial leads
– <3mm (120ms sec) in width
Lead II

33. Normal ECG

34. P wave - Abnormalities
• Morphology
• RAH (Pulm HTN) increased amplitude
• LAH (MS, HTN) notched in I & II, deep -ve inV1
• A Fib/Flut no clear P waves / sawtooth
• Hyperkalaemia reduced amplitude
• Ectopic atrial rhythm
• Axis
• Retrograde
• Dextrocardia right displacement
• Pulmonary disease inferior displacement
• Congenital heart disease left displacement
• Rhythm
• Ectopic atrial rhythm
• A Fib / flutter
• Multifocal atrial tachycardia
• SVT (retrograde P waves)

35. P wave – RAE / LAE

37. PR Interval - Normal
• Start of the P wave - start of the QRS
• Normal = 120 – 200ms
= 3 - 5 small squares
• Increases with age
• Is rate dependent
• Delay at AV node
» Protect ventricles
» Allow for ventricular filling

38. PR interval - Abnormal
• Short
– Preexcitation of ventricles (WPW) & other SVT’s
– AV junctional rhythm
– Exercise induced tachycardia
• Long
– AV Block
– Hyperkalaemia
– CHD
– Drugs (Digoxin, B-Blockers, Quinidine)
– Hypothermia
– Hypothyroidism

40. PR segment
• Normally isoelectric using theTP (not ST) segment as a
baseline
• Abnormal:
– Elevation
• Myocarditis (aVR,V1)
• Atrial infarction
– Depression
• Myo/pericarditis
• Atrial infarction
• Exercise induced tachycardia

42. QRS parameters
• Q waves
– ?abnormal Q’s
• R wave
– Progression
– Dominant R wave in aVR /V1
• QRS amplitude
• QRS duration (<100ms)

43. QRS waveform nomenclature
R r qR qRs Qrs QS
Qr Rs rS qs rSr’ rSR’

44. QRS complex – Q waves
• Normal Q waves
– Due to rightward septal depolarisation
– May occur in I, II,III, aVR, aVF, aVL,V4-V6
– Size <0.4mV (4 squares) or 25% of R wave height
– Duration <0.03sec (under 1 square)
– May be slightly greater in III & aVR (normal variant)
– No Q waves inV1-V3
• Abnormal Q waves
– >0.4mV (4 squares) or 25% of R wave height
– >0.03sec (ie. 1 square or greater)
– Presence inV1-3
– Suggest: evolved AMI / ventricular enlargement / rotation of heart /
HOCM / abnormal conduction / lead misplacement

45. Normal ECG

47. QRS – R wave Abnormalities
• Dominant R waveV1
– normal in children / young adults / persistent juvenile pattern
– RVH / PE / L to R shunt
– RBBB
– Post AMI (= Q wave)
– WPW type A
– Dextrocardia
– HOCM
– Muscular dystrophy
– Incorrect lead placement
• Dominant R wave aVR (>3mm, or R/S ratio >0.7)
– Na channel blockade
– Dextrocardia
– RVH
– Incorrect lead placement
• Poor R wave progression
– Prior anteroseptal AMI / LVH / incorrect lead placement / dextrocardia / normal variant

51. QRS abnormal amplitude:
• Increased
• LVH (S inV1 + R inV5 orV6 >35mm sensitive but not specific)
• BER
• Hyperthyroidism
• Normal / athletes
• Decreased (<5mm limb leads, <10mm precordial)
• Pericardial / Pleural effusion
• Hypothyroidism
• Pneumothorax
• Restrictive cardiomyopathy
• COPD
• Haemochromatosis
• Fluctuating / Electrical alternans

54. QRS prolonged duration:
• Causes
– BBB (may be rate related)
– Ventricular origin /Ventricular pacing
– Hyperkalaemia
– Na channel blockade
– Pre-excitation
– Hypothermia

56. ST Segment - Normal
• Represents period between depolarisation & repolarisation
(J point toT wave)
• Normally isoelectric with respect toTP & PR segment
• Abnormal if:
– Elevated
– Depressed

57. Normal ECG

58. ST segment - Elevation
• Morphology
– Concave - some non-AMI causes
– Convex - suggestive of AMI
• Distribution
– Widespread - ?non-AMI
– Localised to anatomically contiguous leads - suggestive of AMI
• Magnitude
– Larger suggestive of AMI
• QRS morphology
– Large amplitude - consider LVH
– Widening - consider BBB or vent. paced rhythm

59. ST segment elevation causes:
• AcuteCoronary Syndrome
• Acute pericarditis
• Benign early repolarisation (BER)
• LV aneurysm
• LVH
• BBB
• Left - R to mid precordial leads
• Right - lateral leads
• Cardiomyopathy
• Acute myocarditis
• Hypothermia
• Hyperkalaemia
• Myocardial contusion
• CNS injury
• Brugarda Syn
• Paced vent. Rhythms
• Post-electrical cardioversion

61. BER
- Prominent, assymetrical, concordant (with QRS)T waves
-Widespread concave ST segmentsV2-5
- J point elevation (notching / slurring)
- <25%T wave height inV6 (usually <2mm precordial)
- No reciprocal ST depression
- Stable over time

64. ST Segment - Depression
• Morphology
– Flat or downsloping with ACS
• Distribution
• QRS morphology
– Large amplitude - consider LVH
– Wide complex - BBB, vent. Pacing, LVH

65. ST segment depression causes:
• ACS (>1mm depression at J point, diffuse, horizontal or downsloping, not
localising)
• AMI (NSTEMI, Post MI, reciprocal)
• BBB
• LVH
• Ventricular pacing
• Digoxin effect
• Rate related
• Myocardial contusion
• Metabolic
• Post-electrical cardioversion

68. T wave - Normal
• Magnitude
– No clearly defined range (<5mm limb, <15mm precordial)
– General rule - 1/2 the height of preceding QRS
• Axis
– Largely dependent onQRS (concordant)
– Positive in I, II,V3-V6
– Negative in aVR,V1
– Variable in III, aVF, aVL, &V2

69. Normal ECG

70. T wave - Abnormalities
• Prominent
• AMI (hyperacute), Hyperkalaemia, BER, Myopericarditis (concave STE & PR
depression), BBB (esp. LBBB inV1-V3), LVH (similar to LBBB)
• Inverted
• ACS (all stages) NB may be flattened
• Wellen’s Syn. (critical prox LAD stenosis without MI) NB may be biphasic
• Past MI
• Pericarditis
• Children / Persisting juvenile pattern (V1-V3)
• CNS injury/ICH
• BBB
• PE, RVH, LVH, HOCM, Digoxin, toxic, metabolic

77. U wave
• Positive deflection that occasionally occurs afterT wave
• Most visible at slow HR’s inV1-V4
• The origin is uncertain (repol of the IVS)
• Presence or absence does not signify pathology
• Abnormal if:
– >1/3 precedingT wave height (N 1-2mm)
– Disconcordant withT wave

78. U wave - Abnormalities
• Increased amplitude
• Athletes
• Bradycardia
• HypoK+ / HypoCa2+ / HypoMg2+
• CNS event
• Hypertension
• Hypothermia
• Medications (digoxin, quinidine)
• Inversion
• Ischaemia
• LV overload
• RV overload
• PE

81. QT interval - Normal
• Measured from the start of the QRS to the
end of theT wave
– Measure in II,V 5-6
– use slope
– include U waves if continuous withT wave
• Represents the time required for ventricular
activation & recovery
• Range 330 - 440ms (460ms women)
• Rule of thumb < half RR interval
• QT interval corrected to the heart rate

82. QTc calculation
• Multiple formulas
• Bazett’s
– QTc = measured QT interval
 R-R interval
• Hence QTc = QT at HR of 60bpm

83. QTc - Abnormal Duration
• Short
– Hypercalcaemia / Digoxin / Congenital
• Long
– lengthened repolarisation time
– ventricle more susceptible to early after depolarisation which can precipitate a variety of potentially fatal arrhythmias (VF, VT, torsades de
pointes)
– Congenital long QT syndromes
– CNS disease
– Metabolic syndromes (HYPO: K+, Ca++,T4)
– Hypothermia
– Drugs:
• erythromycin, cisapride, amiodarone, haloperidol, quinidine…

86. Overview
• Rate & rhythm
• Axis
• Wave morphology
• Intervals
• Segments
• In the context of:
– Look at the patient (hear the story)
– Compare to old ECG’s
– Get a second opinion