Understanding the ECG
Andrew C Rankin
Professor of Medical Cardiology
BHF-Glasgow Cardiovascular Research Centre
University of Glasgow &
Glasgow Royal Infirmary
The 12-lead ECG
6 - Limb leads
3 bipolar
3 unipolar
6 - Chest Leads
The Physics
The Physics
Bipolar leads
• Measures the potential difference
between two points
Unipolar Leads
• Effectively measures the potential
variation at a single point
(reference = Wilson’s Central Terminal)
Bipolar leads
aVL
aVR
I
II
Unipolar Leads
III
aVF
The Physics
Bipolar leads
• Leads I, II and III
Unipolar Leads
• aVR, aVL, aVF (a = augmented)
• V1 – V6
Limb leads
Chest leads
ECG leads & the heart
• II, III, aVF = Inferior
• V1-V6
= Anterior
• I, aVL, V6 = Lateral
Lateral
Inferior
Anterior
Cardiac Cycle and the ECG
R
T
P
Q S
V1
r
q
V6
V1
S
R
V6
A systematic approach to the ECG
R
• Rate
• Rhythm
T wave
P
– regular or irregular?
• P waves (leads II, V1)
Q S
– specific rhythm diagnosis
• Intervals and durations – PR, QRS, QT
• QRS complexes (axis and morphology)
• ST segment / T wave changes
Rate
Rate?
Normal?
Fast?
Slow?
Reading ECG Squares
Intervals and Timing
•
•
•
•
Paper speed = 25mm/sec
5 large squares per second
Each large square = 200 ms
Each small square = 40 ms
Rate
• Paper speed = 25mm/sec
• 5 large squares per second
• Heart rate 60 bpm = 1 beat per second
• RR interval of 1 second = 60 bpm
• RR interval of 5 large squares = 60 bpm
Rate = 300 divided by the number of large
squares between each QRS complex
Rate
300 divided by the number of large
squares between each QRS complex
1 square - 300/min
2 squares - 150/min
3 squares - 100/min
4 squares - 75/min
5 squares - 60/min
6 squares - 50/min
OR - 1500 divided by the number of small
squares between each QRS complex
Rate
How fast is this rhythm?
RR interval = 2 squares
Rate = 300/2 = 150 bpm
Rhythm
Rhythm
Regular?
or
Irregular?
Rhythm
Regular?
or
Irregular?
P waves?
Sinus Bradycardia
Sinus Arrest
First Degree AV Block
• PR interval > 200 ms (1 large square)
• Delayed conduction through the conducting
system (AV node or distal)
- Example shows PR Interval = 320 ms
Second Degree AV Block – type I
Known as Wenckebach Block
Progressive prolongation of the PR interval until there
is failure to conduct and a ventricular beat is dropped

AV nodal block
Second Degree AV Block – type II
Dropped beats with constant preceding PR interval

Distal conduction system block
Third Degree AV Block
Complete Heart Block
No impulse conduction from the atria to the ventricles
Premature Atrial Contraction
Premature Ventricular Contraction
Atrial Fibrillation (AF)
• Irregular rhythm
• Absence of P waves
• Fibrillatory wave
Supraventricular tachycardia
• Narrow complex tachycardia
• Regular
Ventricular Tachycardia
• Broad complex tachycardia
• Regular
Ventricular Fibrillation
• Rapid irregular ventricular rhythm
• Cardiac arrest
Intervals & durations
Reading ECGs
Intervals and Timing
Normal Ranges in seconds:
PR Interval
QRS Complex
QT Interval
0.12 – 0.2 s
0.06 – 0.1 s
0.36 – 0.44 s
Reading ECGs
Intervals and Timing
Upper Normal Ranges in squares:
PR Interval <1 large square
QRS Complex < 3 small squares
QT Interval <12 small squares
QRS complexes
Cardiac axis
• Normal axis is towards
cardiac apex (-30° to 90º)
aVL
aVR
I + II positive = normal
• I +ve, II -ve
= LAD
• I -ve, II +ve
= RAD
I
II
III
aVF
http://www.blaufuss.org
Left anterior hemiblock
Left anterior
hemi-fascicle
Left posterior
hemi-fascicle
Left Bundle Branch Block
V6
V1
W
LBBB
WiLLiaM
M
Right Bundle Branch Block
V6
V1
M
RBBB
MaRRoW
W?
Broad Complex Tachycardia
Broad complex tachycardia
Bundle branch
block
SVT
OR
VT ?
ST segment
ST elevation
Acute Myocardial Infarction
“Current of injury”
ST elevation
Old Myocardial Infarction
“Myocardial window”
Q wave
Pathological Q wave >0.04s
ST depression
Left Ventricular Hypertrophy
ECG criteria for LVH e.g. SV1 + RV5 > 3.5 mV (35 mm)
A systematic approach to the ECG
R
• Rate
• Rhythm
T wave
P
– regular or irregular?
• P waves
Q S
– specific rhythm diagnosis
• Intervals and durations – PR, QRS, QT
• QRS complexes (axis and morphology)
• ST segment / T wave changes