Vectors, Einthoven's Triangle & the Hexaxial System
Key concepts on cardiac vectors, limb lead axes, and quick QRS axis estimation.
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Questions Covered in This Set
11 cards to master
What is a net (mean) cardiac vector?
The sum of all instantaneous electrical events in the heart, expressed as an arrow with direction (where depolarization is heading) and magnitude (how much muscle is depolarizing).
What is the normal mean QRS axis range in an adult?
Between −30° and +90°, pointing down, left, and slightly posterior because the left ventricle dominates.
State the three deflection rules for vectors and leads.
Toward the positive electrode = upward deflection; away = downward deflection; perpendicular = biphasic or isoelectric complex.
What determines a deflection's amplitude mathematically?
The projection (dot product) of the vector onto the lead axis: amplitude ∝ |V| · cos(θ), where θ is the angle between the heart's vector and the lead axis.
Which electrodes form Einthoven's triangle, and what leads do the sides create?
Right arm, left arm, and left leg; the sides form the bipolar limb leads I, II, and III with the heart at the center.
Give the negative/positive electrodes and axis for leads I, II, and III.
I: RA(−)/LA(+) at 0°; II: RA(−)/LL(+) at +60°; III: LA(−)/LL(+) at +120°.
What is Einthoven's Law and why is it clinically useful?
Lead I + Lead III = Lead II. If the tracing doesn't add up, limb electrodes were likely swapped (RA/LA reversal makes lead I look inverted).
What are the axes of the augmented (unipolar) leads?
aVR = −150°, aVL = −30°, aVF = +90°; each uses one limb as positive and averages the other two as reference.
Describe the hexaxial reference system.
All six limb-lead axes shifted to pass through one center, forming a wheel with a spoke every 30°; 0° is at the patient's left, positive angles point downward, negative angles upward.
How does the two-lead method (I and aVF) estimate axis?
I+/aVF+ = normal axis; I+/aVF− = possible left axis deviation (confirm with lead II); I−/aVF+ = right axis deviation; I−/aVF− = extreme/northwest axis.
How do you refine the axis using an isoelectric lead?
Find the limb lead with the most equal up/down QRS; the axis is roughly perpendicular to it, pointing toward the direction that yields positive deflections in other leads.
