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ECG Basics
What is an ECG?
The electrocardiogram (ECG) is a
representation of the electrical events of the
cardiac cycle.
Each event has a distinctive waveform, the
study of which can lead to greater insight
into a patient’s cardiac pathophysiology.
Electrocardiogram (ECG/EKG)
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ECG Leads
Leads are electrodes which measure the
difference in electrical potential.
They can be of two types:
1. Two different points on the body (bipolar leads)
2. One point on the body and a zero electrical potential.
EKG Leads
The standard EKG has 12 leads:
3 Standard Limb Leads
3 Augmented Limb Leads
6 Precordial Leads
The axis of a particular lead represents the viewpoint from
which it looks at the heart.
Standard Limb Leads
Types of ECG Recordings
Bipolar leads record
voltage between
electrodes placed on
wrists & legs (right leg is
ground)
Lead I records between
right arm & left arm
Lead II: right arm & left leg
Lead III: left arm & left leg
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Summary of Leads
Bipolar
Limb Leads
Precordial Leads
I, II, III
-
(standard limb leads)
Unipolar
aVR, aVL, aVF
(augmented limb leads)
V1-V6
Arrangement of Leads on the EKG
Precordial Leads
Adapted from: www.numed.co.uk/electrodepl.html
Precordial Leads
ECG Graph Paper
•one small block equals 0.04 s
• Five small blocks make up 1 large
block which translates into 0.20 s (200
msec)
Normal conduction pathway:
SA node -> atrial muscle -> AV node -> bundle
of His -> Left and Right Bundle Branches ->
Ventricular muscle
ECG
3 distinct waves are
produced during
cardiac cycle
P wave caused by
atrial depolarization
QRS complex
caused by
ventricular
depolarization
T wave results from
ventricular
repolarization
Elements of the ECG:
• P wave: Depolarization of both atria;
• Shape and duration of P may indicate
atrial enlargement
•Relationship between P and QRS helps
distinguish various cardiac
arrhythmias
•
Waveforms and Intervals
•QRS complex: Ventricular depolarization
• Larger than P wave because of greater
muscle mass of ventricles
• Normal duration = 0.08-0.12 seconds
• Its duration, amplitude, and morphology are
useful in diagnosing cardiac arrhythmias,
ventricular hypertrophy, MI, electrolyte
derangement, etc.
• Q wave greater than 1/3 the height of the R
wave, greater than 0.04 sec are abnormal and
may represent MI
•PR interval: from onset of P wave to onset of
QRS
• Normal duration = 0.12-2.0 sec (120-200 ms)
(3-4 horizontal boxes)
• Represents atria to ventricular conduction
time (through His bundle)
• Prolonged PR interval may indicate a 1st
degree heart block
ST segment:
• Connects the QRS complex and T wave
• Duration of 0.08-0.12 sec (80-120 msec
T wave:
• Represents repolarization
QT Interval
• Measured from beginning of QRS to the end of the
T wave
• Normal QT is usually about 0.40 sec
• QT interval varies based on heart rate
Determining the Heart Rate
Rule of 300
10 Second Rule
Rule of 300
Count the number of “big boxes” between
neighboring QRS complexes, and divide 300
by this number. The result will be
approximately equal to the rate
Although fast, this method only works for
regular rhythms.
What is the heart rate?
www.uptodate.com
(300 / 6) = 50 bpm
What is the heart rate?
www.uptodate.com
(300 / ~ 4) = ~ 75 bpm
What is the heart rate?
(300 / 1.5) = 200 bpm
The Rule of 300
It may be easiest to memorize the following table:
# of big
boxes
Rate
1
300
2
150
3
100
4
75
5
60
6
50
10 Second Rule
As most EKGs record 10 seconds of rhythm per
page, one can simply count the number of beats
present on the EKG and multiply by 6 to get the
number of beats per 60 seconds.
This method works well for irregular rhythms.
The QRS Axis
The QRS axis represents the net overall
direction of the heart’s electrical activity.
Abnormalities of axis can hint at:
Ventricular enlargement
Conduction blocks (i.e. hemiblocks)