In this article we will discuss about the meaning and significance of electrocardiogram recording.
Meaning of Electrocardiogram Recording (ECG/EKG):
Pacemaker of the heart automatically and rhythmically gets depolarized and repolarized. This inturn depolarizes and repolarizes both atrial and ventricular muscle fibers. The potential changes produced can be recorded by placing pair of electrodes over the myocardium itself or suitable points on the body surface.
The points that are commonly selected are the right arm, the left arm and the left leg. When these points are connected to each other it forms an equilateral triangle, the heart is said to lie in the center of the triangle.
The electrical potentials generated by the heart spread towards these points by the volume conduction principle. Placing a pair of electrodes and connecting these electrodes to a galvanometer or to a cathode ray oscilloscope can record the potential changes.
The summated potential recorded this way is known as an ECG recording. The recordings are made on a standard, calibrated paper. Along the horizontal axis is the time scale, the smallest division is equal to 0.04 seconds. And along the vertical scale the amplitude-voltage is recorded in millivolt (mV)— one small division is equal to 0.1 mV.
Spread of potential through conducting system to muscle mass in different parts in heart has been shown in Fig. 3.17.
Lead System:
A lead is nothing but a pair of electrodes placed in a definite position on the body surface, connected to the negative and to the positive terminals of an oscilloscope.
Standard Bipolar Limb Lead System (Fig. 3.18):
Records the potential difference between the two electrodes. Accordingly, there are three leads namely lead one (L I), lead two (L II), and lead three (L III).
In L I, the right arm is connected to negative and the left arm to positive terminals.
In L II, the right arm is connected to negative and the left leg to positive terminals.
In L III, the left arm is negative and the left leg to positive terminals.
It has been mathematically shown that the potential recorded in lead two (L II) is usually the biggest and potential at lead two is always equal to the sum of the potentials recorded at lead I and lead III (Fig. 3.19).
This is because, when the electrical axis of the heart is parallel to the axis of a lead, maximum recording will be obtained. On the other hand, if the electrical axis of the heart is perpendicular to the axis of a lead the recording will be the smallest.
Unipolar Lead System:
It has been shown by Einthoven that when the three points on the body surface are connected to a common terminal by means of resistance wires of 5000 ohms, since lead II potential is equal to lead I + III potentials, a zero potential is recorded. This is used as an indifferent electrode, and connected to the negative terminal of the recording device.
The other electrode is used as an active electrode and connected to the positive terminal of the recording device. Unlike the bipolar recording, the unipolar recording gives us the potential that exists over a particular area on the body surface.
Under the unipolar lead system (Fig. 3.20):
1. VR VL and VF
2. aVR, aVL and aVF
3. Chest leads V1 to V6 to V9
To interpret the details of an ECG recording, the standard bipolar limb lead II recording is usually considered.
Normal Recording:
(In the limb lead II recording) (Fig. 3.21):
i. The recording shows three positive deflections and two negative deflections.
ii. P, R and T are the positive deflections and Q and S are the negative deflections.
iii. PQ and ST are the segments.
iv. PQ or PR, QT, PP, RR are the intervals.
v. Rarely following a T wave a U wave may be seen.
Cause for the Various Waves (standard bipolar limb lead II is considered for explanation):
P Wave:
i. P wave is due to atrial depolarization.
ii. It is an upward deflection.
iii. The duration of the P wave is 0.08 seconds and the amplitude is 0.1 to 0.3 mV.
iv. Clinical importance: Increase in the amplitude is suggestive of atrial hypertrophy, as it can occur in mitral or tricuspid stenosis. It may be absent in atrial fibrillation and being replaced by fibrillatory waves. The configuration of the wave is also altered in mitral stenosis, due to hypertrophy of the left atrium.
QRS Complex:
i. Represents the depolarization of the interventricular septum and the ventricles.
ii. In lead II, the normal value is 0.08 to 0.1 sec.
iii. Normal height of the R wave is 1.2 to 1.3 mV.
iv. If the height of the R wave is more than 2.5 mV, it is suggestive of ventricular hypertrophy.
v. The normal duration should not exceed 0.1 seconds.
T Wave:
i. It is due to ventricular repolarization.
ii. The amplitude of the wave is 0.1 to 0.3 mV.
P-Q/R Interval:
i. Denotes the time taken for the impulse to spread from the SA node to the Purkinje fibers.
ii. Normal duration is 0.12 to 0.16 seconds.
iii. Measured from the beginning of P wave to the beginning of Q/R wave.
iv. If it exceeds 0.2 seconds, it is suggestive of a first degree heart block.
v. If it is less than normal, it is suggestive of an ectopic foci situated nearer to the AV node, acting as the pacemaker.
Q-T Interval:
i. It is the time interval between the beginning of the Q wave and to the end of the T wave.
ii. It includes depolarization and repolarization of the ventricular muscle.
iii. The normal duration is 0.37 to 0.43 seconds.
P-P or R-R interval denotes the time taken for one cardiac cycle.
The electrical axis of the heart:
The normal axis is plus 59 degrees. The normal range is from minus 30 to plus 110 degrees. If it is more negative, it is known as left axis deviation as seen in left ventricular hypertrophy, and left bundle branch block. If it is more positive, it is known as right axis deviation which occurs in right ventricular hypertrophy and right bundle branch block.
Clinical Significance of Electrocardiogram Recording (ECG):
1. Anatomical orientation of the heart, could be vertical or horizontal (Fig. 3.22). Left or right axis deviation of heart could be made out.
2. Detect different types of arrhythmias (conduction blocks, fibrillations, flutters, varieties of tachycardias)
3. Hypertrophy of various chambers
4. Detection of myocardial infarction
5. Electrolyte imbalances particularly of K+, Na+, and Ca++
6. Monitoring administration of digoxin group of drugs
7. Hyperkalemia (plasma K+ ±7 mEq/L), there is tall slender T wave.
8. Hyperkalemia (plasma K+ ±8.5 mEq/L) P wave is absent; QRS complex is broad and slurred. The T wave remains tall and slender.
9. Hypokalemia (plasma K+ ±3.5 mEq/L) PR interval is almost 0.2 sec, prominent U wave.
10. Hypokalemia (plasma K+ ±2.5 mEq/L) PR interval is very much prolonged, ST segment depressed and T wave is inverted, prominent U wave.
11. Some of the normal ECG recordings have been shown in Fig. 3.23.
