introduction
An ECG is a recording of the electrical activity of the heart
Different “views” of the heart can be recorded using different electrodes
Electrodes
V1: Fourth intercostal space to the right of the sternum.
V2: Fourth intercostal space to the Left of the sternum.
V3: Directly between leads V2 and V4.
V4: Fifth intercostal space at midclavicular line.
V5: Level with V4 at left anterior axillary line.
V6: Level with V5 at left midaxillary line. (Directly under the midpoint of the armpit)
Electrode placement
Leads
12 possible leads
Six limb (extremity) leads
Bipolar leads: I, II, and III
Unipolar leads: aVR, aVL, aVF
Six chest (precordial) leads
lead i
LA = electrical voltages of the heart that are transmitted to the left arm
RA = electrical voltages of the heart that are transmitted to the right arm
The electrocardiograph sustracts RA from LA and the difference appears as lead I.
lead i
Lead one ‘travels’ horizontally.
Its left pole (LA) is postive and its right pole (RA) is negative.
Therefore, lead I = LA minus RA
Shows a positive wave when an impluses moves towards the left arm, negative wave when an impuse moves away from the left arm.
Lead II
Lead II points downward diagonally
Lower pole (LL) is positive and upper pole (RA) is negative.
Lead II = LL minus RA
Lead III
Lead III points downward diagonally
Lower pole (LL) is positive and upper pole (LA) is negative.
Lead III = LL minus LA
Unipolar or Augmented limb leads
Record the electrical voltages at one location rather than relative to the voltage at another electrode
Monitor leads
12 leads are not always necessary
Sample of a monitor lead
V1 (positive)
Right shoulder (negative)
Left shoulder (ground)
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