Electrodes vs. Leads and Proper Placement
Key facts about the 10 electrodes, the 12 derived leads, chest landmarks, and misplacement errors.
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Questions Covered in This Set
10 cards to master
How many electrodes and how many leads make up a standard 12-lead EKG?
10 electrodes placed on the patient; the machine mathematically derives 12 leads (views).
Define 'electrode' vs. 'lead'.
An electrode is the physical adhesive sensor on the skin; a lead is a comparison (vector) between electrodes with a positive and negative pole.
What determines whether a waveform deflects up or down in a given lead?
Direction of the depolarization wave relative to that lead's positive pole: toward = upward, away = downward, perpendicular = small or biphasic.
Which leads view the frontal plane vs. the horizontal plane?
Frontal plane: I, II, III, aVR, aVL, aVF. Horizontal plane: V1–V6.
What are the four limb electrodes and their US color code?
RA = white (right arm), LA = black (left arm), LL = red (left leg), RL = green (right leg, ground/neutral only — not recording).
List the landmarks for V1–V6.
V1: 4th ICS right sternal border; V2: 4th ICS left sternal border; V4: 5th ICS midclavicular line; V3: midway between V2 and V4; V5: V4 level, anterior axillary line; V6: V4 level, midaxillary line.
How do you locate the 4th intercostal space?
Walk down from the sternal notch to the Angle of Louis (manubriosternal junction) where the 2nd rib attaches, then count spaces downward — never guess by 'nipple level'.
How are the bipolar limb leads defined?
Lead I = LA − RA, Lead II = LL − RA, Lead III = LL − LA.
Why is aVR normally negative?
Its positive pole points up and to the patient's right, away from the main direction of ventricular depolarization.
What does LA/RA reversal look like, and how is it distinguished from dextrocardia?
Lead I fully inverts (negative P, QRS, T) and aVR/aVL swap; in true dextrocardia the precordial leads V1–V6 also show abnormal/poor R-wave progression.
