Vectorial Analysis Degree Of Instantaneous Mean Vector Physiology Notes

Vector Introduction

The cardiac vector is the direction in which the electrical potential generated in the heart travels in an instant. It is also called the cardiac axis. The vector is represented by an arrow. The point of the arrowhead shows the direction of electrical potential The length of the arrow represents the amphth (magnitude or voltage) of the potential.

Instantaneous Mean Vector

• The current flows in all directions. The mean direction of the flow of electrical potential at one instance is known as the instantaneous mean vector or instantaneous summated vector.
• For example, when current flows through the inter-ventricular septum from the base of the ventricles toward the apex

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The electrical potential generated by the flow of current travels in different directions as follows:

• Electrical potential travels downwards through the interventricular septum towards the apical part, i.e. from the depolarized part of the septum towards the nondepolarized (polarized) part of the septum. This potential is strong
• Through the inner surface of the ventricles, the potential travels upwards from the apical part towards the base.
• The magnitude of this potential is very weak

• Through the outer surface of the heart, the electrical potential travels downwards.
• It has a higher magnitude. Though the potential travels in all directions in this instance, the potential flowing downwards (from base to apex of the heart) is much greater in magnitude than the potential flowing in the other directions.
• Thus, the mean direction of flow of electrical potential in this instance is downwards.
• This downward vector is called the instantaneous mean vector or instantaneous summated vector at this instance electrocardiogram in different limb leads, the degree of the vector is altered.
• The direction of the current flow is always from the negative point toward the positive point.
• When the electrical potential flows in a horizontal plane from the right side toward the left side of the heart, the degree of the vector is zero.

Degree Of Instantaneous Mean Vector

• While recording electrocardiograms in different limb leads, the degree of the vector is altered. The direction of the current flow is always from the negative point toward the positive point.
• When the electrical potential flows in a horizontal plane from the right side toward the left side of the. heart, the degree of the vector is zero (Fig. 95-2).

Degree Of Instantaneous Mean Vector At Different Limb Leads

Standard Limb Lead 1 (Right Arm and Left Arm)

• In this instance, the electrical potential travels from the right side (negative point) of the heart toward the left side (positive point) in the horizontal plane.
• So a degree of the vector is considered zero.

Standard Limb Lead 2 (Right Arm and Left Leg): The vector is from above downwards and slightly towards the left, i.e. at 60°.

Standard Limb Lead 3 (Left Arm and Left Leg): Here, the vector is from above downwards and slightly towards the right at 120°.

Lead aVR: The vector is from below towards the upper part of the heart and slightly towards the right at 210°.

Lead aVF: The vector is from above downwards at 90°.

Lead aVL: In this, the vector is from below, towards the upper part of the heart and slightly towards the left, at – 30° or at + 330°.

Calculated Vector Or Mean Qrs Vector

• The instantaneous mean vector cannot be determined by the recording of an electrocardiogram.
• However, another vector can be calculated by measuring the amplitude of the QRS complex from the ECG recorded in standard limb leads.
• It is called the calculated vector or mean QRS vector. It is also called the electrical axis of the heart or the cardiac vector.
• The calculated cardiac vector is useful in the diagnosis of heart diseases.

Calculation Of Mean Qrs Vector

• The calculation of the mean QRS vector depends upon the fact that if the amplitude of the QRS complex is determined from ECG recorded at any two standard limb leads, the amplitude of the QRS complex in the remaining lead can be known from the calculation.
• The amplitude is measured in mm. For determining the amplitude of the QRS complex, first, the height of the R wave is measured.
• From this value, the height of the negative wave Q or S (whichever is more) is deducted.

Steps for calculation of mean QRS vector:

• A scriptural triangle is drawn on plain paper. This triangle represents Einthoven’s triangle.
• The heart is presumed to lie in the center of Simhoven’s triangle.  The electrical potential generated from the heart appears simultaneously on the roots of the three limbs namely the left arm, right arm, and left leg.
• Each side of this triangle represents one standard limb lead
• From the midpoint of each side, a perpendicular line is drawn towards the center. The meeting point of the perpendicular lines represents the center of electrical activity in the heart.
• On each side of the triangle, the amplitude of the QRS complex is plotted from the mid-point towards the positive point of the lead.
• For example, the amplitude of the QRS complex in lead I is 10 mm, and in lead II, it is 16 mm. In the triangle, the upper side represents lead I and in this lead, the left is positive.
• So, a 10 mm line is drawn on the upper side from the midpoint towards the left (positive). This 10 mm distance along the axis of lead I is called the projected vector for lead. In the same way, the projected vector for Lead II is drawn on the right side of the triangle.
• From the positive end of each projected vector, another perpendicular line is drawn toward the interior of the triangle.
• Now an arrow is drawn between the center of electrical activity and the meeting point of perpendicular lines from the positive end of projected vectors.
• This arrow shows the vector. The arrowhead is drawn towards the positive end, i.e. downwards.
• The degree and the length of the arrow are measured. The degree denotes the direction of the vector and the length denotes magnitude.
• The amplitude of the QRS complex in lead 3
• According to Einthoven’s law, the amplitude of the QRS complex in one lead can be mathematically calculated by summing up or subtracting the amplitude in the other two leads depending upon the potentials of these leads.
• For example, in this case, the electrical potential at QRS in lead I is 1 mV, and lead II is 1.6 mV.
• Thus, the potential in lead III is 0.6 mV. It also can be measured from the triangle drawn to calculate the vector.

Vectorial Analysis

The mean QRS vector (cardiac axis) in normal conditions: Varies between – 30° and + 110°.

When the axis deviates towards the left, i.e. in anti- cT- was direction, away from – 30° it is called left axis deviation When the axis deviates towards the right (clockwise direction), away from + 110°, it is known as ugh; axis deviation.

Left axis deviation: The left axis deviation occurs in left ventricular hypertrophy, left bundle branch block, and posterior wall infarction.

Right axis deviation: The right axis deviation occurs due to right ventricular hypertrophy, right bundle branch block, and anterior wall infarction.

Vector Cardiogram

• From the recording of the electrocardiogram, only the calculated vector, i.e. cardiac axis is determined.
• The instantaneous mean vector cannot be determined by the electrocardiogram, but it can be determined by means of a vectorcardiogram.
• The vector cardiogram is the simultaneous recording of electrical potential in different axes across the heart above downward and sideward.
• It is obtained by using a cathode ray oscilloscope. The technique is equal to connecting the tops of all instantaneous mean vectors in the series of 3 loops.
• It is done by means of a sophisticated electronic device along with an oscilloscope.
• Each loop of electronic connection is used to record different vector cardiograms called P vector cardiogram, QRS vector cardiogram, and T vector cardiogram.