The following points highlight the three main electrocardiographic leads of human body. The leads are:- 1. Standard Limb Leads 2. Augmented Unipolar Limb Leads 3. Chest Leads.
1. Standard Limb Leads:
The action current of the heart is recorded by placing two electrodes in two different positions in the body. (Fig. 7.62)
Such arrangement of lead is called bipolar leads. The bipolar leads (standard limb leads) which are in common use are listed in Table 7.4.
According to convention, the combination of the lead I will be made by connecting the left arm to the positive pole of the galvanometer and the right arm to the negative pole of the galvanometer. The combination of the lead II will be made by connecting the right arm to the negative pole and the left leg to the positive pole of the galvanometer. The combination of the lead III will be made by connecting the left arm to the negative pole and the left leg to the positive pole of the galvanometer.
It is an equilateral triangle drawn arbitrarily around the area of the heart (Figs 7.62 & 7.63).
Einthoven’s law states that if E.C.G. is recorded through standard limb leads then the sum total voltage of the QRS (ventricular complex) in leads I and II is equivalent to lead II. If the voltage of QRS complex in lead III is + 2 and in lead I is + 3, then the voltage of the same in lead II will be + 5. On the other hand, if the voltage of QRS in any two leads is known then the other one will be obtained after deducting the latter from other two (Fig. 7.63).
Normal E.C.G. Recorded in Standard Limb Leads:
E.C.G. recorded simultaneously in three leads shows mostly similar in shape, contour and other feature. P, R, T waves are positive waves in all the leads. The voltages of different complexes are different from lead to lead. For clinical purposes in diagnosis of the arrhythmia, it does not matter with which leads, the E.C.G. has been recorded. But for determining the extent of cardiac damage either in the atria or in the ventricles, it does matter with which leads the E.C.G. is taken.
Interpretation of Human Electrocardiogram (Fig. 7.64):
It shows the following five consecutive waves; P Q R S T. There are two isoelectric periods—the shorter one, between P and Q; the longer one, between S and T. P, R and T are upward deflections, while Q and S are downward waves. The waves are therefore alternately up and down. P is of atrial origin, hence called the atrial complex, while Q R S T being of ventricular origin, and are collectively known as the ventricular complex.
They are briefly described below:
i. P Wave:
This is the first upward deflection. It is a small but constant wave having a rounded or pointed top. It is depolarisation wave of the atria and takes place when the impulse spreads over the atria and therefore, caused by the passage of the action over the atria. Its average duration is about 0.1 sec. (same as atrial systole). Repolarisation wave of the atria is submerged within the ventricular complex.
The impulse arrives at the A.V. node, at about the summit of P. Normal P indicates that:
a. The impulse is originating at the S.A. node.
b. It spreads over the atria in the usual direction.
c. There is no defect of conduction.
d. The strength of contraction, the mass of atrial musculature and its nutrition, are normal. If the P is inverted, it indicates that S.A. node fails to initiate impulse and the atrial muscle depolarises by the impulse originating in A.V. node.
Any abnormality of atrial activity will be reflected by corresponding changes in the P wave. For instance, in atrial fibrillation it will be absent; in atrial hypertrophy, it will be large and may be notched; in nodal rhythm, the direction will be reversed.
ii. Q R S T Wave:
These four waves are caused by ventricular activity and are collectively known as the ventricular complex. QRS are depolarisation waves of the ventricle; and are the initial group of three waves—one following immediately upon the other. The average duration of QRS T is 0.43 sec.; that of Q R S is usually less than 0.08 sec. and should not exceed 0.1 sec.
iii. Q Wave:
As soon as the impulse arrives at the muscular part of the septum, the latter contracts producing the first wave Q. Hence, Q is caused by the activity of the septum. It is a small, negative wave and often inconspicuous deflection. It is not found in those animals which do not possess an interventricular septum (Reptiles and Amphibia). It is also absent in infants suffering from congenital patency of the septum. Prominent Q wave indicates old infarction.
iv. R & S Wave:
R is the most constant and conspicuous wave having the tallest amplitude. It is the first positive deflection during ventricular depolarisation. It follows immediately upon Q. S is the next downward deflection, constant but often inconspicuous. In lead I, R is mainly caused by right ventricle and S due to left ventricle. In lead III, it is just the reverse. In abnormal conditions of ventricles, the shape, size and duration of RS alter. For instance, in bundle-branch heart block their duration is prolonged beyond 0.1 sec. and their relative amplitude varies.
v. T Wave:
R is followed by a long isoelectric period, after which comes the Last upward deflection T. It is a broad, smooth rounded deflection with an average duration of 0.27 sec. It is caused by the action current due to the contraction of the basal part of the ventricles. It is repolarisation wave of the ventricle. It is normally positive because the apex of the heart repolarises more earlier than the base of the heart.
In young adults, T is very prominent. In old age it is flattened. Exercise increases the amplitude in a healthy heart. It is sometimes inverted in lead III without apparent reason. It is also altered by stimulation of the vagus and sympathetic nerves, by digitalis and other poisons, by anoxia caused by coronary constriction and by the damage of myocardium from any cause.
Abnormalities of T wave in shape, size, direction, duration and reaction to exercise in leads I and II are of great prognostic significance. It indicates serious myocardial damage and is often associated with cardiac hypoxia.
vi. U Wave:
This wave is often seen just after the T wave. It is possibly due to slow repolarisation of the intraventricular conducting system.
vii. R-R Interval:
It is the interval between the two successive R waves. If the R – R interval in next successive stages are same then they indicate that the ventricle is depolarising rhythmically.
viii. P-P Interval:
It is the interval between the two successive P waves. Equal intervals in next successive stages indicate rhythmical depolarisation of the atrium.
ix. P-R Interval:
This is the interval from the onset of P to that of QRS. It measures the conduction time of the impulse from the S.A node to the ventricles. Normally, it varies from 0.13 to 0.16 sec. and should not exceed 0.2 sec. A longer interval shows impaired conduction through the bundle. Variable P-R interval in successive stages indicates A.V. dissociation.
This indicates ventricle to beat without the influence of S.A node. [True conduction time, i.e., from the A.V. node to the ventricles, should be measured from the summit of P to the beginning of Q. But since Q is often absent, P-R interval is taken as the conduction time for the sake of convenience.]
x. Q-R-S Interval:
It measures the total ventricular depolarisation time. It is measured from the onset of Q wave to the cessation of S wave. It varies from 0.08 to 0.1 sec.
xi. Q-T Interval:
It measures the ventricular total systolic time. It is generally measured from the onset of Q wave to the end of T wave. It is about 0.36 sec.
xii. RS-T Segment:
Elevation or sagging of this segment indicates myocardial damage or hypoxia.
xiii. T-P Interval:
Alteration of this interval indicates the alteration of the heart rate. It is measured from the end of T wave to the beginning of P wave. T-P interval actually measures the diastolic period of the heart. If the T-P intervals in successive stages are variable then it indicates atrioventricular dissociation.
Unipolar Limb Leads (Table 7.5):
Electrodes are placed respectively on the left arm, left leg and right arm. They are connected together to form a central terminal which passes through a suitable resistance (5,000 ohm) and is kept almost to zero potential. This is the indifferent electrode. Other electrode is put on different parts of the body surface. This is the exploring electrode. By this arrangement the indifferent electrode is kept at zero degree potential so that the exploring electrode records the local unmodified action current.
The following unipolar leads are in use:
i. Lead VR – Right arm.
ii. Lead VL – Left arm.
iii. Lead VF – Left leg (foot).
i. VR Lead:
Q wave is bigger which is followed by small R wave; small S wave and an inverted T wave (Fig. 7.65).
ii. VL Lead:
The waves in VL changes with the alteration of the position of the heart. When the heart is vertical, VL resembles with that of VR. R wave is small followed by big S wave. T wave is inverted.
iii. VF Lead:
There is a large R wave followed by S wave. T wave is erect. Through the unipolar limb leads VR, VL, VF, electrical activities of the ventricular cavity, upper part of the left side of the heart and the inferior surface of the heart respectively are known. QRS complex in VF resembles with that of chest lead V1 if the records are taken during expiration and with that of chest lead V6 if the records are taken during deep inspiration.
2. Augmented Unipolar Limb Leads:
In these types of leads there are three leads widely used. These are aVR, aVL and aVF. In the unipolar limb leads (VR, VL, VF) the amplitudes are small but in augmented unipolar limb leads (aVR, aVL, aVF) the amplitudes are increased by 50%.
In these types of recordings, one limb is connected to the positive terminal and the other two limbs are connected together to the negative terminal. When the right arm is connected to the positive terminal and the left arm and left leg together to the negative terminals, then the combination will be a VR (denoting block R as right arm).
Similarly when the left arm is connected to the positive terminal and the right arm and left leg together to the negative terminals, the combination will be a VL (denoting L as left arm). When the left leg is connected to the positive terminal, and the right arm and left arm to the negative terminals, then the combination will be a VF (denoting F as foot).
Normal E.C.G. recorded in these leads (Fig. 7.66) shows more or less similar except that the recording in a VR is inverted. This is because the side of the heart nearest to the right arm is negative in respect to the rest of the heart.
The augmented unipolar limb leads are most valuable for:
(a) Determining the position of the heart,
(b) Confirming the significance of Q and T waves in the standard leads, and
(c) Confirming the evidence of ventricular damage or hypertrophy.
iii. Abnormalities in E.C.G. may not always indicate heart disease, because these abnormalities may be functional, congenital or induced by drugs or by disease.
iv. The clinical importance of Electrocardiogram is obviously very great. It gives fairly accurate information as to the condition of atria and ventricles—almost in all of its functional aspects. In cardiac abnormalities characteristic variations occur in the Electrocardiogram along with side by-side clinical findings may act as a dependable guide to the diagnosis, prognosis and treatment.
3. Chest Leads (Unipolar):
There are six chest leads, viz., V1, V2, V3, V4, V5, and V6. In the chest leads (Fig. 7.66), one electrode is placed on the anterior surface of the chest and connected to the positive pole of the galvanometer and the other electrode— the indifferent electrode placed anywhere in the body is connected to the negative pole of the galvanometer. One such combination actually makes one lead.
The conventional precordial positions used are as follows:
Fourth intercostal space at 2.54 cm (1 inch) away from the right sternal border.
Fourth intercostal space at 2.54 cm (1 inch) away from the left sternal border.
At the midpoint between V2, and V4.
Fifth intercostal space at left midclavicular line.
At a point where the anterior axillary line intersects perpendicularly the horizontal line extended from V4.
At a point where midaxillary line intersects perpendicularly the same horizontal line extended from V4.
Normal E.C.G. Recorded in Chest Leads (Fig. 7.66):
In leads V1 and V2 the QRS complex is normally negative, because the chest electrodes in these leads are nearer to the base of the heart which is the direction of electronegativity during most of the ventricular depolarisation phase. On the other hand, in V4, V5 and V6 the QRS complex is positive because these chest leads are nearer to the apex of the heart and are electropositive during depolarisation. Localisation and the extent of damage can be determined by using the chest leads.
Normally V1-V3 indicates the activity of the right ventricle and is characterised by increasing height of R wave from V1 and a progressively decreasing S wave. The height of R wave increases from V1 to V4 or V5. V5 -V6 denotes the activity of the left ventricle and is characterised by a very small Q wave and a tall R wave. T wave is generally upright in all positions except sometimes in V1.
Significance of Various Leads and their Limitations:
i. In general the standard limb leads are most valuable for diagnosis of arrhythmia and also for preliminary studies of the functional abnormalities of the heart.
ii. The precordial chest leads are very important for diagnosis.
(a) Localisation of the recent or old ventricular damage,
(b) Bundle-branch block, and
(c) Detection of ventricular hypertrophy.