Question

1. Describe the axis of an ECG lead and the vector of electrical current.

2. Identify the waves, complexes, segments and intervals on an ECG.

3. Identify the limb, augmented limb, and precordial leads.

4. Identify the negative and positive electrodes for the different lead configurations.

5. Describe the process of analyzing an ECG strip or recording.

Answer 1

1. 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.This potential is 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.

* Vectoral 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.

2. Normal ECG consists of waves, complexes, intervals and segments. Waves of ECG recorded by limb lead II are considered as the typical waves. Normal electrocardiogram has the following waves, namely P, Q, R, S and T .

*Major Complexes in ECG :-

1. ‘P’ wave, the atrial complex

2. ‘QRS’ complex, the initial ventricular complex

3. ‘T’ wave, the final ventricular complex

4. ‘QRST’, the ventricular complex.

•P wave = ‘P’ wave is a positive wave and the first wave in ECG. It is also called atrial complex.

Cause- ‘P’ wave is produced due to the depolarization of atrial musculature. Depolarization spreads from SA node to all parts of atrial musculature. Atrial repolarization is not recorded as a separate wave in ECG because it merges with ventricular repolarization (QRS complex)

Duration- Normal duration of ‘P’ wave is 0.1 second.

Amplitude - Normal amplitude of ‘P’ wave is 0.1 to 0.12 mV.

Morphology- ‘P’ wave is normally positive (upright) in leads I, II, aVF, V4, V5 and V6. It is normally negative (inverted) in aVR. It is variable in the remaining leads, i.e. it may be positive, negative, biphasic or flat

* QRS Complex= QRS’ complex is also called the initial ventricular complex. ‘Q’ wave is a small negative wave. It is con-tinued as the tall ‘R’ wave, which is a positive wave. ‘R’ wave is followed by a small negative wave, the ‘S’ wave.

Cause- ‘QRS’ complex is due to depolarization of ventricular musculature. ‘Q’ wave is due to the depolarization of basal portion of interventricular septum. ‘R’ wave is due to the depolarization of apical portion of interventricular septum and apical portion of ventricular muscle. ‘S’ wave is due to the depolarization of basal portion of ventricular muscle near the atrioventricular ring.

Duration- Normal duration of ‘QRS’ complex is between 0.08 and 0.10 second.

Amplitude- Amplitude of ‘Q’ wave = 0.1 to 0.2 mV.Amplitude of ‘R’ wave = 1 mV.Amplitude of ‘S’ wave = 0.4 mV.

Morphology - ‘Q’ wave is normally small with amplitude of 4 mm or less. It is less than 25% of amplitude of ‘R’ wave in leads I, II, aVL, V5 and V6. In the remaining leads, its amplitude is < 0.2 mm. From chest leads V1 to V6, ‘R’ wave becomes gradually larger. It is smaller in V6 than V5. ‘S’ wave is large in V1 and larger in V2. It gradually becomes smaller from V3 to V6.

* T wave= T’ wave is the final ventricular complex and is a positive wave.

Cause- ‘T’ wave is due to the repolarization of ventricular musculature.

Duration- Normal duration of ‘T’ wave is 0.2 second.

Amplitude- Normal amplitude of ‘T’ wave is 0.3 mV.

Morphology - ‘T’ wave is normally positive in leads I, II and V5 and V6. It is normally inverted in lead aVR. It is variable in the other leads, i.e. it is positive, negative or flat

* P-R interval= P-R’ interval is the interval between the onset of ‘P’ wave and onset of ‘Q’ wave.

‘P-R’ interval signifies the atrial depolarization and conduction of impulses through AV node. It shows the duration of conduction of the impulses from the SA node to ventricles through atrial muscle and AV node.‘P’ wave represents the atrial depolarization. Short isoelectric (zero voltage) period after the end of ‘P’ wave represents the time taken for the passage of depolarization within AV node.

Duration- Normal duration of ‘P-R interval’ is 0.18 second and varies between 0.12 and 0.2 second. If it is more than 0.2 second, it signifies the delay in the conduction of impulse from SA node to the ventricles. Usually, the delay occurs in the AV node. So it is called the AV nodal delay.

*Q- T interval= Q-T’ interval is the interval between the onset of ‘Q’ wave and the end of ‘T’ wave.

‘Q-T’ interval indicates the ventricular depolarization and ventricular repolarization, i.e. it signifies the electrical activity in ventricles.

Duration- Normal duration of Q-T interval is between 0.4 and 0.42 second.

* S-T segment = S-T’ segment is the time interval between the end of ‘S’ wave and the onset of ‘T’ wave.

It is an isoelectric period.

J point-  The point where ‘S-T’ segment starts is called ‘J’ point. It is the junction between the QRS complex and ‘S-T’ segment.

Duration of ‘S-T’ Segment- Normal duration of ‘S-T’ segment is 0.08 secs.

*R-R interval=  is the time interval between two consecutive ‘R’ waves. ‘R-R’ interval signifies the duration of one cardiac cycle. Duration Normal duration of ‘R-R’ interval is 0.8 second.

3. ECG is recorded by placing series of electrodes on the surface of the body. These electrodes are called ECG leads and are connected to the ECG machine. Electrodes are fixed on the limbs. Usually, right arm, left arm and left leg are chosen. Heart is said to be in the center of an imaginary equilateral triangle drawn by connecting the roots of these three limbs. This triangle is called Einthoven triangle.

**Einthoven Triangle and Einthoven Law:- Einthoven triangle is defined as an equilateral triangle that is used as a model of standard limb leads used to record electrocardiogram. Heart is presumed to lie in the center of Einthoven triangle. Electrical potential generated from the heart appears simultaneously on the roots of the three limbs, namely the left arm, right arm and the left leg.

ECG is recorded in 12 leads, which are generally classified into two categories. I. Bipolar leads II. Unipolar leads.

•BIPOLAR LIMB LEADS- Bipolar limb leads are otherwise known as standard limb leads. Two limbs are connected to obtain these leads and both the electrodes are active recording electrodes, i.e. one electrode is positive and the other one is negative .Standard limb leads are of three types: a. Limb lead I b. Limb lead II c. Limb lead III.

Lead I - Lead I is obtained by connecting right arm and left arm. Right arm is connected to the negative terminal of the instrument and the left arm is connected to the positive terminal.

Lead II - Lead II is obtained by connecting right arm and left leg. Right arm is connected to the negative terminal of the instrument and the left leg is connected to the positive terminal.

Lead III - Lead III is obtained by connecting left arm and left leg. Left arm is connected to the negative terminal of the instrument and the left leg is connected to the positive terminal.

•UNIPOLAR LEADS- Here, one electrode is active electrode and the other one is an indifferent electrode. Active electrode is positive and the indifferent electrode is serving as a composite negative electrode. Unipolar leads are of two types: 1. Unipolar limb leads 2. Unipolar chest leads. 1.

* Unipolar limb leads - Unipolar limb leads are also called augmented limb leads or augmented voltage leads. Active electrode is connected to one of the limbs. Indifferent electrode is obtained by connecting the other two limbs through a resistance. Unipolar limb leads are of three types: i. aVR lead ii. aVL lead iii. aVF lead.

i) aVR lead - Active electrode is from right arm. Indifferent electrode is obtained by connecting left arm and left leg.

ii). aVL lead - Active electrode is from left arm. Indifferent electrode is obtained by connecting right arm and left leg.

iii). aVF lead- Active electrode is from left leg (foot). Indifferent electrode is obtained by connecting the two upper limbs.

*Unipolar chest leads- Chest leads are also called ‘V’ leads or precardial chest leads. Indifferent electrode is obtained by connecting the three limbs, viz. left arm, left leg and right arm, through a resistance of 5000 ohms. Active electrode is placed on six points over the chest . This electrode is known as the chest electrode and the six points over the chest are called V1, V2, V3, V4, V5 and V6.

V indicates vector, which shows the direction of current flow.

Position of chest leads:-

V1 : Over 4th intercostal space near right sternal margin

V2 : Over 4th intercostal space near left sternal margin.

V3 : In between V2 and V4

V4 : Over left 5th intercostal space on the mid clavicular line

V5 : Over left 5th intercostal space on the anterior axillary line

V6 : Over left 5th intercostal space on the mid axillary line.