Electrocardiogram (ECG/EKG)
• Is a recording of electrical activity of heart conducted thru ions in
body to surface
Fig 13.22a
13-60
Normal conduction pathway:
SA node -> atrial muscle -> AV node -> bundle of His -> Left and
Right Bundle Branches -> 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
Fig 13.23
13-61
Fig. 13.22b
ECG Leads
The standard ECG 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.
The 12-Lead System
• The most commonly used clinical ECG-system, the
12-lead ECG system, consists of the following 12
leads, which are:
I , II , III
aVR , aVL , aVF
V1 ,V2 ,V3 ,V4 ,V5 ,V6
Standard Limb Leads
Augmented Limb Leads
Precordial Leads
Adapted from: www.numed.co.uk/electrodepl.html
All Limb Leads
Elements of the ECG:
• P wave: Depolarization of both atria;
• Relationship between P and QRS helps distinguish various cardiac
arrhythmias
• Shape and duration of P may indicate atrial enlargement
• 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
• 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
ST segment:
• Connects the QRS complex and T wave
• Duration of 0.08-0.12 sec (80-120 msec
T wave:
• Represents repolarization or recovery of ventricles
• Interval from beginning of QRS to apex of T is referred to
as the absolute refractory period
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
Elements of the ECG:
• P wave
• Depolarization of both atria;
• Relationship between P and QRS helps distinguish various
cardiac arrhythmias
• Shape and duration of P may indicate atrial enlargement
•QRS complex:
• Represents 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
T wave:
• Represents repolarization or recovery of ventricles
• Interval from beginning of QRS to apex of T is referred to
as the absolute refractory period
ST segment:
• Connects the QRS complex and T wave
• Duration of 0.08-0.12 sec (80-120 msec
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