BSI: EKG Interpretation II

• The axis is the mean or general direction of depolarization through the ventricles:
• - Vector tail is always at the AV node
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• - Normal Axis is ~ 40 °
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• - If the heart is displaced, the axis will also be displaced in the same direction
• 

Hypertrophy of a ventricle will shift the axis towards the hypertrophy because there is more electrical activity on the hypertrophied side of heart.

Myocardial infarction will shift the axis away from it because the infarcted area has no electrical activity.

• Lead I
• Lead AVF

• Lead I
• - Lead I runs through AV node from left to right and the AV node is center of sphere (see figure)
• - The left side of the sphere is positive and the right side is negative (left arm is positive and right arm is negative)
• - If the wave of depolarization through the ventricles is directed towards the left side of the heart (left side of sphere), the QRS in lead I will be positive (upward deflection)—remember: positive charges flowing towards positive electrode will cause positive deflection
• - If the wave of depolarization through the ventricles is directed towards the right side of the heart (right side of sphere), the QRS in lead I will be negative (downward deflection)

• Lead AVF
• - Lead AVF runs through AV node in a vertical line
• - If draw sphere with the center being the AV node, the top is negative and the bottom is positive (see figure)
• - If wave of depolarization through the ventricles is directed down, the QRS in Lead AVF will be positive (upward deflection)
• - If wave of depolarization through the ventricles is directed upward, the QRS in Lead AVD will be negative (downward deflection)

Therefore, a normal axis (~40 °) will show a positive QRS in leads I and AVF (normal = "two thumbs up!")

Recall, a normal axis (~40 °) will show a positive QRS in leads I and AVF

• Abnormal axis:
• 1) Right axis deviation (RAD): QRS is negative in lead I and positive in lead AVF
• 2) Extreme right axis deviation: QRS will be negative in lead I and lead AVF
• 3) Left axis deviation (LAD): QRS will be positive in lead I and negative in lead AVF

Ventricular hypertrophy of either ventricle will affect the QRS complex.

• Right ventricular hypertrophy
• - Axis will shift to right
• - Will see positive deflections in V₁ and V₂ that progressively
• 

• Left ventricular hypertrophy
• - Axis will shift left
• - QRS complexes will be in normal direction, just amplified
• - Will see huge QRS complexes in V₁-V₆
• - To diagnose, measure depth of QRS in Lead V₁ and height of QRS in V₅: if > 35 mm there is left ventricular hypertrophy
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• 
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• 1) Inverted T waves
• - Prolonged depolarization of ventricles during ischemia changes pattern of repolarization
• - Ventricles repolarize in opposite direction to normal repolarization
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• 2) Elevated or depressed S-T segments
• - Sign of acute injury
• 

• 3) Presence of significant Q waves
• - Elevated Q waves present in every lead except Lead AVR
• 

Every lead except Lead AVR will show elevated Q waves if infarction is present.