Cardiac Vector Definition.
Cardiac vector is the direction at which electrical potential generated in the heart travels at an instant. It is also called cardiac axis. Vector is represented by an arrow. Arrowhead shows the direction of electrical potential. Length of the arrow represents the amplitude (magnitude or voltage) of the potential.
Instantaneous mean Vector.
Current flows in all directions. Mean direction of flow of electrical potential at one instance is known as instantaneous mean vector or instantaneous summated vector. For example, when current flows through interventricular septum from the base of ventricles towards apex, the electrical potential generated by flow of current travels in different directions as follows:
1. Electrical potential travels downwards through the interventricular septum, towards the apical part, i.e. from depolarized part of septum towards non-depolarized (polarized) part of septum. This potential is strong.
2. Through the inner surface of ventricles, the potential travels upwards from apical part towards the base. Magnitude of this potential is very weak.
3. Through the outer surface of 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 other directions. Thus, the mean direction of flow of electrical potential in this instance is downwards. This downward vector is called instantaneous mean vector or instantaneous summated vector at this instance.
Degree of Instantaneous Mean Vector.
While recording electrocardiogram (ECG) in different limb leads, the degree of vector is altered. Direction of current flow is always from negative point towards the positive point. When the electrical potential flows in a horizontal plane from right side towards left side of the
heart, the degree of vector is zero .
Degree of Instantaneous Mean Vector at different Limb Leads.
Standard Limb Lead I (Right Arm and Left Arm)
In this instance, the electrical potential travels from right side (negative point) of the heart towards the left side (positive point) in the horizontal plane. So, the degree of vector is considered as zero.
Standard Limb Lead II (Right Arm and Left Leg).
Vector is from above downwards and slightly towards left, i.e. at 60°.
Standard Limb Lead III (Left Arm and Left Leg).
Here, vector is from above downwards and slightly towards right at 120°.
Lead Augmented Vector Right (aVR).
Vector is from below towards upper part of the heart and slightly towards right at 210°.
Lead Augmented Vector Front (aVF).
Vector is from above downwards at 90°.
Lead Augmented Vector Left (aVL).
In this, the vector is from below, towards upper part of the heart and slightly towards left, at –30° or at +330°.
Calculated Vector or Mean QRS Vector.
Instantaneous mean vector cannot be determined by the recording of ECG. But, another vector can be calculated by measuring the amplitude of 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 cardiac vector. Calculated cardiac vector is useful in the diagnosis of heart diseases.
Calculation of Mean QRS Vector.
Calculation of mean QRS vector depends upon the fact that if the amplitude of QRS complex is determined from ECG recorded at any two standard limb leads, the amplitude of QRS complex in the remaining lead can be known from the calculation. Amplitude is measured in mm. For determining the amplitude of QRS complex, first the height of R wave is measured. From this value, height of negative wave Q or S (whichever is more) is deducted. The calculation is based on Einthoven triangle.
Steps for Calculation of Mean QRS Vector.
1. An equilateral triangle is drawn on a plain paper. This triangle represents Einthoven triangle. Each side of this triangle represents one standard limb lead.
2. From the midpoint of each side, a perpendicular line is drawn towards the center. Meeting point of the perpendicular lines represents center of electrical activity in the heart.
3. On each side of triangle, the amplitude of QRS complex is plotted from midpoint towards the positive point of the lead. For example, the amplitude of QRS complex in lead I is 10 mm and in lead II, it is 16 mm .
4. In the triangle, upper side represents lead I and in this lead, the left is positive. So, a 10 mm line is drawn on upper side from the midpoint, towards left (positive). This 10 mm distance along the axis of lead I is called projected vector for lead I.
5. In the same way, the projected vector for Lead II is drawn on the right side of the triangle.
6. From the positive end of each projected vector another perpendicular line is drawn towards interior of the triangle.
7. Now an arrow is drawn between center of electrical activity and the meeting point of perpendicular lines from positive end of projected vectors . This arrow shows the vector. Arrowhead is drawn towards positive end, i.e. downwards.
8. Degree and the length of the arrow are measured. Degree denotes the direction of vector and length denotes the magnitude.
Amplitude of QRS Complex in Lead III.
Amplitude (electrical potential) of QRS complex in lead III can be calculated by applying Einthoven law.
Einthoven law.
Einthoven law states that potential differences between the bipolar leads measured simultaneously will, at any given moment, have the values II = I + III. That is, the potential of any wave or complex in lead II of ECG is equal to the sum of potentials in lead I and lead III.
Einthoven law is the modification of Kirchhoff’s law of voltage.
Kirchhoff’s law of voltage.
According to Kirchhoff law, the algebraic sum of voltage rise in a closed circuit is equal to the algebraic sum of voltage drops.
Application of Einthoven law in calculating QRS complex.
By applying Einthoven law, Amplitude (electrical potential) of QRS complex in one lead can be mathematically calculated, by summing up or subtracting the amplitude in other two leads, depending upon the potentials of these leads. For example, amplitude of QRS in lead II = I + III
and the amplitude of QRS in lead III = II – I. Thus, Amplitude of QRS in lead I is 1 mV and lead II is 1.6 mV. Thus, the amplitude of QRS in lead III is 0.6 mV. It can also be measured from the triangle drawn to calculate the vector.
Vector Analysis.
Mean QRS vector (cardiac axis) in normal conditions is at about +59°. It varies between –30° and +110°. When the axis deviates towards the left, i.e. in anticlockwise 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 right axis deviation.
Left axis deviation.
Left axis deviation occurs in left ventricular hypertrophy, left bundle-branch block and posterior wall infarction.
Right axis deviation.
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. Instantaneous mean vector cannot be determined by the electrocardiogram, but it can be determined by means of vector cardiogram. Vector cardiogram is the simultaneous recording of electrical potential in different axis 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 oscilloscope. Each loop of electronic connection is used to record different vector cardiogram called P vector cardiogram, QRS vector cardiogram and T vector cardiogram.
Effect of Changes in Electrolyte Concentration on heart .
Distribution of electrolytes in extracellular fluid and intracellular fluid is responsible for the electrical activity of the tissues including myocardium. Thus, any change in the concentration of any electrolyte will definitely alter the electrical activity of cardiac muscle.
Effect of changes in Sodium ion Concentration.
Normal sodium ion concentration in blood is 135 to 145mEq/L. Change in concentration of sodium ion does not alter the electrical activity of heart severely. Only the low level of sodium ion in body fluids reduces the electrical activity of cardiac muscle and electrocardiogram (ECG) shows low-voltage waves. Changes in the concentration of potassium and calcium ions have significant effects on heart.
Effect of Changes in Potassium ion Concentration.
Normal potassium ion concentration in blood is about 3.5 to 5 mEq/L. Changes in ECG appear when the potassium level increases to 6 mEq/L (hyperkalemia) or when it decreases to 2 mEq/L (hypokalemia).
Effect of Hyperkalemia.
Hyperkalemia decreases:
1. Resting membrane potential, leading to hyperpolarization.
2. Excitability of the muscle.
Effects of hyperkalemia on the excitability of cardiac muscle, depend upon the severity of hyperkalemia.
Changes in ECG When Potassium Level increases to 6 or 7 mEq/L.
T wave is tall and tented. P-R interval and QRS complex are normal.
Changes in ECG When Potassium Level increases to 8 mEq/L.
P-R interval and the duration of QRS complex are prolonged because, hyperkalemia decreases the rate of conduction. P wave may be small.
Changes in ECG When Potassium Level increases beyond 9 mEq/L.
Severe hyperkalemia makes the atrial muscle unexcitable. So, P wave is absent in ECG. QRS complex merges with T wave. This condition is fatal because, it leads to ventricular fibrillation or stoppage of heart in diastole, due to the lack of excitability.
Effect of Hypokalemia.
Hypokalemia decreases the sensitivity of heart muscle.
Changes in ECG When Potassium Level Falls to 2 mEq/L.
1. S-T segment is depressed.
2. T wave is small, flat or inverted.
3. U wave appears. Sometimes, the U wave merges with T wave. Because of this, the Q-T interval is mistaken for being prolonged.
Changes in ECG When Potassium Level Falls below 2 mEq/L.
1. Depression of S-T segment below the isoelectric baseline.
2. Inversion of T wave.
3. Appearance of prominent U wave.
4. Prolongation of P-R interval.
Effect of Changes in Calcium ion Concentration.
Normal concentration of calcium ion in blood is 9 to 11mg/dL (4.5 to 5.5 mEq/L). Mostly, hypocalcemia affects the heart, rather than hypercalcemia.
Effect of Hypercalcemia.
Hypercalcemia is the elevation in blood calcium level. It increases the excitability and contractility of the heart muscle. In clinical conditions, the effect of hypercalcemia is very rare.
Changes in ECG.
1. Shortening of duration of S-T segment.
2. Shortening of QT interval.
3. Appearance of U wave.
Calcium Rigor.
Stoppage of the heart in systole, due to hypercalcemia is called the calcium rigor. It can be demonstrated in experimental animals by infusing large quantity of calcium. Calcium rigor is a reversible phenomenon and the heart starts functioning normally, when the calcium ions are washed.
Effect of Hypocalcemia.
Hypocalcemia is the reduction in blood calcium level. It reduces the excitability of the cardiac muscle.
Changes in ECG.
1. Prolongation of S-T segment.
2. Prolongation of Q-T interval.
3. Appearance of a prominent U wave.
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