Electrocardiogram (ECG/EKG)
• Is a recording of electrical activity of heart conducted thru ions in
body to surface
Normal conduction pathway:
SA node
-> atrial muscle
-> AV node
-> bundle of His
-> Left and Right
Bundle Branches
-> Purkinje fibers
-> Ventricular muscle
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
Normal ECG graph
The 12-Lead System
The most commonly used clinical ECG-system,
the 12-lead ECG, consists of the following
12 leads, which are:
 3 Augmented Leads
 3 Limb Leads
 6 Chest Leads
I , II , III
aVR , aVL , aVF
V1 ,V2 ,V3 ,V4 ,V5 ,V6
Chest Leads
Adapted from: www.numed.co.uk/electrodepl.html
Elements of the ECG:
1)
P wave: Depolarization of both atria;
 Shape and duration of P may indicate atrial enlargement
2)
3)
PR interval: from onset of P wave to onset of QRS
1)
Normal duration = 0.12-2.0 sec (120-200 ms) (3-4 horizontal boxes)
2)
Represents atria to ventricular conduction time (through His bundle)
3)
Prolonged PR interval may indicate a 1st degree heart block
QRS complex: Ventricular depolarization
• Larger than P wave because of greater muscle mass of ventricles
• Normal duration = 0.08-0.12 seconds
• It is useful in diagnosing cardiac arrhythmias, ventricular hypertrophy
and myocardial infarction.
4) ST segment:
• Connects the QRS complex and T wave
• Duration of 0.08-0.12 sec (80-120 msec)
5) T wave:
• Represents repolarization or recovery of ventricles
6) 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
Fig. 13.24b
Fig. 13.24c
Fig. 13.24d
Fig. 13.24g
The normal electrocardiogram