• Ventricular Depolarization
• Electrical Axis of the Heart
• ECG Leads
• Limb ECG Leads
• Electrical Axis of ECG Leads
• Positive and Negative ECG Deflections
• Size of the ECG Deflection
• Isoelectric ECG Deflection
• Neutral Plane
• Positive and Negative Hemispheres of ECG Leads
• QRS Complex and Electrical Heart Axis
• I Lead
• aVF Lead
• aVR Lead
• 12-Lead ECG and the Main Heart Vector

Ventricular Depolarization

• The ventricular myocardium depolarizes gradually
• Depolarization creates an electrical vector
• Which is captured by ECG leads
• The more cardiomyocytes depolarize
• The larger the electrical vector becomes
  
  
• Summation Electrical Vector
• Each cardiomyocyte creates a vector during depolarization
• The common direction of vectors during depolarization
• Is referred to as the summation electrical vector
• Summation Vector of the Left Ventricle:
• Is the resultant direction of depolarization vectors
• Of all cardiomyocytes in the left ventricle
• Is the largest vector
• Because it involves the depolarization of the largest mass of myocardium
• Its direction determines the electrical axis of the heart
• The summation vector during ventricular depolarization
• Rotates around the electrical center of the heart
• (Wilson's central terminal)

ECG Leads

• An ECG recording has 12 leads
• Limb leads (I, II, III, aVF, aVL, aVR)
• Chest leads (V1-V6)
• Each lead "views" the vectors from a different angle
• Leads "view" the electrical center of the heart
• At the Wilson central terminal
• If the electrical vector points
• Toward the lead on the surface - a positive deflection occurs
• Away from the lead on the surface - a negative deflection occurs
  
  

Electrical Axis of ECG Leads

• Each ECG lead has its own electrical axis
• Between the central terminal (-) and the surface electrode (+)
• The main cardiac vector projects onto the electrical axes of all ECG leads
• Each electrical vector of the heart projects onto the electrical axes of ECG leads
• However, the cardiac axis is determined solely by the main electrical vector (from the depolarization of the left ventricle)
  
  
• When determining the electrical axis of the heart, we only consider
• the limb ECG leads (I, II, III, aVF, aVR, aVL)

Size of the ECG Deflection

• The size of the ECG deflection
• depends on the angle between the vector and the ECG lead axis
  
  
• The smaller the angle
• the more directly the vector points towards the electrode (+)

Neutral Plane

• When a perpendicular electrical vector
• Is rotated in 3D space around the electrical axis of the lead
• It creates the neutral plane of the given ECG lead
• Every vector in the neutral plane
• Projects onto the electrical axis of the ECG lead as a single point
• No deflection appears on the ECG

Negative Hemisphere and Negative Deflection

• Negative deflection occurs
• Whenever the vector points away from the (+) electrode
• The electrical axis of the lead continues beyond the electrical center of the heart (-)
• Size of the deflection
• Depends on the size of the vector that is projected onto the electrical axis of the ECG lead
• Does not depend on the distance of the vector from the (+) electrode
• Both vectors in the image produce the same ECG deflection

QRS Complex and Electrical Heart Axis

• During ventricular depolarization, 3 main vectors are formed
• Initial vector (Q wave)
• Depolarization of the septum
• Main vector (R wave)
• Depolarization of the left ventricle
• Terminal vector (S wave)
• Depolarization of the base of the left ventricle
• The vectors are simultaneously "viewed" by the ECG leads
• However, each lead "views" the vectors from a different angle
• Each vector is projected onto the electrical axis of the ECG leads
  
  
• The electrical axis of the heart is determined by the direction of the main electrical vector

aVF Lead

aVF Limb Lead

• The white semicircle represents the positive hemisphere
• aVF lead "views" vectors from an angle of 90°
• Vectors are formed sequentially
1. Septal (directed towards aVF lead)
2. Main (directed towards aVF lead)
3. Terminal

12-Lead ECG and the Main Heart Vector

• The main heart vector creates the largest deflection on the 12-lead ECG
• If it points toward the lead (R wave)
• If it points away from the lead (S wave)
• The electrical axis of the heart is determined by the direction of the main electrical vector in the limb leads
  
  
• The QRS complex on the ECG shows the depolarization of the ventricles
• In each ECG lead, it looks different
• Because each lead "views" the vectors from a different angle
• The vectors are always projected onto the electrical axis of each ECG lead
• The images show ventricular depolarization (QRS complex)

Limb Leads

• "View" the vectors in the frontal plane

Chest Leads

• "View" the vectors in the horizontal plane