Depolarization and Repolarization

Objectives:

• Depolarization and repolarization of a single muscle cell
• Depolarization and repolarization of Atria
• Depolarization and repolarization of Ventricles
• intrinsicoid deflection
• sequence of ventricular activation

THINGS TO KEEP IN MIND

When do you get a Positive Deflection?

Positive charge moving towards +ve electrode = +ve deflection

Positive charge moving away from -ve electrode = +ve deflection

Negative charge moving towards -ve electrode = +ve deflection

Negative charge moving away from +ve electrode = +ve deflection

When do you get a Negative Deflection?

Positive charge moving towards -ve electrode = -ve deflection

Negative charge moving towards +ve electrode = -ve deflection

Basically, 

When opposite charges move towards oppositely charged electrodes = -ve deflection

When similar charges move towards similarly charged electrodes OR opposite charges move away from oppositely charged electrodes = + ve deflection.

Depolarization and Repolarization of a Single Muscle Cell

Muscle Cell at Rest and During Depolarization

• (A) Resting potential of a single muscle cell is approximately ~90 mV, with the inside of the cell more negative than outside
• (B) During depolarization, the activation wave travels from one end to the other end, changing the polarity inside the cell from negative to positive. Arrows point to the direction of depolarization.

Depolarization

• (A) The arrows indicate the direction of the activation wave, which is a zone of advancing positive charges. The front of the activation wave is circled. 
• (B) The wave of depolarization is represented as a moving dipole with the positive charge traveling in front and the negative charge behind. A recording electrode at position 1 will record a positive deflection because the dipole is traveling with the positive charge facing the electrode. A recording electrode at position 2 will record a negative deflection since the electrode is facing the negative charge of the moving dipole

Repolarization

• (A) The front of the repolarization wave is circled and is a zone of advancing negative charges. The direction of the repolarization wave is shown by the arrows. 
• (B) A recording electrode in front of the repolarization wave at position 1 will record an inverted T wave because the electrode is facing the negative charge of the moving dipole. A recording electrode behind the repolarization wave at position 2 will record a positive or upright T wave because the electrode is facing the positive charge of the moving dipole.

Depolarization and Repolarization of the Atria

• (A) Depolarization of the atria is represented as a P wave in the electrocardiogram. The impulse follows the length of the thin atrial muscle and spreads circumferentially. 
• (B) Repolarization is represented as a Ta wave  ( to differentiate it from T wave of ventricular depolarization) and follows the same direction as depolarization. Thus, the P wave and the Ta wave are inscribed in opposite directions. Arrows represent the direction of the spread of the electrical impulse.
• Ta wave is usually not visible because the wave is too small to be recorded.  When present, it usually coincides with the QRS complex in the ECG and is therefore obscured,

Depolarization and Repolarization of the Ventricles

Ventricular Depolarization

• Depolarization of both ventricles is synchronous and occurs from the endocardium to the epicardium because the Purkinje fibers are located subendocardially. A precordial lead such as V5 will record a positive deflection because the electrode is facing the positive end of the moving dipole.

Intrinsicoid Deflection

• Intrinsic deflection: If a recording electrode is experimentally placed directly over the epicardium of the left ventricle, an R wave will be recorded because the ventricles are activated from endocardium to epicardium. The abrupt turnaround from the peak of the R wave toward the baseline is called the intrinsic deflection. It indicates that the impulse has arrived at the site of the recording electrode.

• Intrinsicoid deflection: Clinically, the recording electrode is normally placed on the chest wall and not directly over the epicardium. What is recorded is not the intrinsic deflection but its equivalent, the intrinsicoid deflection. 
• The time it takes for the impulse to arrive at the recording electrode is the ventricular activation time and is measured from the onset of the QRS complex to the top of the R wave. 
• The abrupt downward deflection of the R wave that immediately follows is the intrinsicoid deflection. 
• When there is right ventricular hypertrophy, the onset of the intrinsicoid deflection is delayed in right-sided precordial leads V1 or V2 (normal,=<0.03 seconds). 
• When there is left ventricular hypertrophy, the onset of the intrinsicoid deflection is delayed in left-sided precordial leads V5 or V6 (normal, =<0.05 seconds).
• R peak time: When there is intraventricular conduction delay, use the term R peak time to indicate the onset of the intrinsicoid deflection and is measured from the onset of the QRS complex to the peak of the R or R’ wave.

The Normal Sequence of Ventricular Activation

• Can be divided into three stages:
• vector 1: depolarization of the ventricular septum (directed right, anteriorly, and inferiorly)
• vector 2: depolarization of the free walls of both ventricles (directed left, posteriorly, and superiorly or inferiorly)
• vector 3: depolarization of the posterobasal wall of the left ventricle and posterobasal septum (directed superiorly and posteriorly)

Vector 1

• Part of the heart that is activated
• the middle third of the left side of the ventricular septum
• this is the first portion of the ventricle to be activated because the left bundle branch is shorter than the right bundle branch
• Direction of the vector
• Right, anteriorly, and inferiorly ( as the septum is activated from left to right)
• Representation in QRS 
• Any electrode located to the right of the ventricular septum (such as V1) will record a positive deflection (small r wave) because the impulse is traveling toward the positive side of the electrode. Any electrode located to the left of the septum (such as precordial leads V5, V6, and limb leads I and aVL) will record a negative deflection (small q wave) because the impulse is traveling away from the positive side of these electrodes. 
• This small q wave is often called a septal q wave to indicate that the initial vector of the QRS complex is due to septal activation.

Vector 2

• Part of the heart that is activated
• major part of the ventricles
• Direction of the vector
• Because the right ventricle is thinner than the left ventricle, a certain portion of the forces generated by the thicker left ventricle will remain unopposed. Additionally, apical depolarization forces are not neutralized because the area opposite the apex is occupied by the non-muscular mitral and tricuspid valves. Taken together, these two forces manifest in vector 2 which is directed to the left and slightly posteriorly, either inferiorly or superiorly.
• activation of the remaining ventricular septum occurs on both sides of the septum simultaneously, which cancels each other. Hence no net vector is generated from the septum at this point
• also, remember, depolarization occurs from endocardium to epicardium (unlike in repolarization, discussed later)
• Representation in QRS 
• represents the mean axis of the QRS complex
• A downward deflection (deep S) is recorded in V1, and an upward deflection (tall R) is recorded in V5–V6

Vector 3

• Part of the heart that is activated
• posterobasal wall of the left ventricle and posterobasal portion of the ventricular septum
• Direction of the vector
• this is the last portion of ventricles to be depolarized as the process of depolarization occurs in an apex-to-base direction. 
• these structures are located superiorly in relation to the other structures of the heart. Thus, the late forces are directed superiorly and posteriorly
• Representation in QRS 
• terminal portion of QRS

Ventricular Repolarization

• Depolarization and Repolarization occur in opposite direction
• Unlike the situation in the single muscle cell or the atria, where depolarization and repolarization travel in the same direction, depolarization and repolarization of the ventricular myocardium occur in opposite directions. Thus, depolarization starts from endocardium to epicardium and repolarization is reverse, occurring from epicardium to endocardium. 
• This causes the QRS complex and T wave to be inscribed in the same direction. Thus, precordial electrodes V5 and V6 will record a positive deflection (tall R wave) during depolarization and also a positive deflection during repolarization (upright T wave) because these precordial electrodes are facing the positive end of the moving dipole
• Reason
• Several explanations have been offered as to why the epicardial cells recover earlier than the endocardial cells, even if they are the last to be depolarized. More recently, it has been shown that the action potential duration of endocardial cells is longer when compared with epicardial cells. This is most probably the main reason why the epicardial cells recover earlier than endocardial cells causing repolarization to start from epicardium to endocardium.

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