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Electrode Placement
Anatomy
Heart Rate
PR Interval
QRS Interval
QT Interval
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Theoretical Basis of the ECG
An electrical current in the direction towards the positive end of a bipolar
electrode causes a positive deflection of the stylus of the ECG. If the number of
myocardial cells (dipoles) in this direction increases, the current will increase as well.
The greater the current, the more positive the voltage. An electrical current in the
direction away from the positive end of a bipolar electrode causes a negative deflection
of the stylus of the ECG. If the number of myocardial cells (dipoles) in this direction
increases, the current will increase as well. The greater the current, the more
negative the voltage.
ECG Leads
An electrocardiographic lead is a recording electrode or a pair of recording electrodes at
a specified location. In clinical practice, twelve leads are usually used in the
diagnostic ECG, although there is no limitation to the number of leads one may select for
special purposes. The leads are usually placed on the elbows and stifles. A right leg
electrode is used as the ground.
ECG Standard Leads
Leads I, II and III are all bipolar (i.e., they detect a change in electric
potential between two points) and detect an electrical potential change in the frontal
plane. Lead I is between the right arm and left arm electrodes, the left arm being
positive. Lead II is between the right arm and left leg electrodes, the left leg being
positive. Lead III is between the left arm and left leg electrodes, the left leg again
being positive. |
