• Working Myocardium
• Propagation of Action Potential
• Electric Vector and ECG
• Depolarization and ECG
• Repolarization and ECG
• Depolarization and Repolarization of Myocardium
• Action Potential in Endocardium and Epicardium
• Action Potential and Contraction of the Cardiomyocyte
• Main Ventricular Vector
• Ventricular Vectors
• ECG and Ventricular Vectors

Working Myocardium

Action Potential and Working Myocardium

• In the image, there are cardiomyocytes of the working myocardium.
• At rest, all cardiomyocytes are polarized.
• Due to uneven ion distribution, the extracellular environment has a positive electrical charge.
• Extracellularly, there is a high concentration of Na⁺ ions.
• Cardiomyocytes form a syncytium.
• They are interconnected by intercalated discs through which ions rapidly transfer.
• Upon stimulation of a cardiomyocyte by an impulse, the action potential spreads to neighboring cells.
• Similar to throwing a stone into water (from the center outward).
• Cardiomyocytes depolarize.
• The extracellular environment becomes negative.
• The action potential spreads:
• Rapidly - longitudinally
• Slowly - transversely
• Because intercalated discs are located on the lateral sides of cardiomyocytes.

Electric Vector and ECG

Working Myocardium and Electric Vector

• The image shows a portion of the working myocardium (not a single cardiomyocyte)
• Volt is the voltage between 2 electrodes
• The ECG device has electrodes on the surface of the body
• Voltmeter shows electrical voltage if:
• One electrode is over depolarized (-) part
• The other is over repolarized (+) part of the myocardium
• The ECG device displays volts as waves and deflections
• Voltmeter shows 0 mV value
• If both electrodes are above depolarized (-) or repolarized (+) myocardium
• Voltmeter shows 0mV because there is no electrical voltage between the electrodes
• On the ECG, it represents the isoelectric line
• Electric Vector
• Always directs from negative (-) to positive (+) extracellular part of the myocardium
• If the electric vector is directed
• Towards the ECG lead - it creates a positive deflection (wave)
• Away from the ECG lead - it creates a negative deflection (wave)

Repolarization and ECG

Spread of Repolarization Wave and ECG

• Repolarization wave (extracellular change from - to +) spreads in the myocardial wall from epicardium to endocardium
• Because epicardium starts repolarizing earlier
• The wave has opposite direction compared to depolarization
• However, the electric vector always directs from (-) to (+) part of the myocardium
• The electric vector has the same direction during both depolarization and repolarization
• The repolarization wave spreads slowly
• Because calcium and potassium channels open slowly at the end of action potential in cardiomyocytes
• The repolarization wave is directed from the ECG electrode, but the electric vector directs towards the ECG electrode
• Repolarization occurs more slowly, resulting in a broad T wave
• The width of T wave represents the time during which the ventricles repolarize

Action Potential in Endocardium and Epicardium

Action Potential in Endocardium and Epicardium

• Action potential of cardiomyocytes in endocardium and epicardium has different electrical properties
• The conduction system activates myocardium in the endocardium
• Thus, depolarization begins in the endocardium
• Depolarization in endocardium and epicardium is nearly synchronous
• Therefore, the depolarization wave propagates rapidly through the myocardial wall
• Cardiomyocytes in the epicardium start repolarizing earlier than those in the endocardium
• Hence, repolarization wave begins in the epicardium
  
  
• The electrical vector of depolarization and repolarization, however, has the same direction

Main Ventricular Vector

Main Ventricular Vector

• The ventricles depolarize sequentially
1. Ventricular septum (Q wave)
2. Ventricles (R wave)
3. Base of the left ventricle (S wave)
• The depolarization wave has a specific direction and sequentially depolarizes the entire ventricles
• The direction of the resultant electrical vector changes during depolarization
• The ECG curve shows the resultant electrical vector over time
• At 50ms, the left and right ventricles are synchronously activated
• The electrical vectors diverge from each other
• The left ventricle vector is larger (due to the left ventricle being more massive)
• Thus, the resultant vector of ventricular depolarization is directed in the direction of the left ventricle
• Main cardiac vector
• It is the electrical vector of left ventricular depolarization (and right)
• The right ventricle vector is small and has little influence on the direction of the left ventricle vector
• It is the largest and creates the R wave on the ECG
• The size and direction of vectors over time is a vectorcardiogram

ECG and Ventricular Vectors

• The ventricles are activated sequentially:
1. Ventricular septum (V_S - septal vector)
2. Left and right ventricles (V_M - main vector)
3. Base of the left ventricle (V_T - terminal vector)
• QRS complex appears differently in each lead
• Because each lead "views" the vectors from a different angle
• If the vector is directed
• Towards the lead - creates a positive deflection
• Away from the lead - creates a negative deflection
  
  
• Ventricular depolarization (QRS complex) is shown in the images

Limb Leads

• "View" the vectors in the frontal plane

Chest Leads

• "View" the vectors in the horizontal plane