a. Resting ECG:
The patient lies down for few minutes while a record is made. In this type of ECG, disks are attached to the patient’s arms and legs as well as to the chest.
b. Exercise ECG (stress test):
The patient exercises either on treadmill machine or bicycle while connected to the ECG machine. This test tells whether exercise causes arrhythmias or makes them worse or whether there is evidence of inadequate blood flow to the heart muscle (ischemia).
c. 24-Hour ECG (Holter) monitoring:
This is a sophisticated type of ECG. The patient goes about his or her usual daily activities while wearing a small, portable tape recorder that connects to the disks on the patient’s chest. Overtime this test shows changes in rhythm (or ischemia) that may not be detected during a resting or exercise ECG . The most commonly used ECG is the Resting ECG. For less frequent or hard to predict arrhythmias, loop recorders are used. These are worn by the patient for weeks to even as long as month. When the patient feels an uncomfortable heart rhythm start up he/she presses a button which records his/ her ECG for a fixed duration.
ECG supplies are used when monitoring a patient for a resting or diagnostic ECG analysis, stress testing and exercise monitoring, ambulatory monitoring and other imaging procedures. Also, gels, wipe pads and paper are used during an ECG measurement.
The word lead has two meanings in electrocardiography: it refers to either the wire that connects an electrode to the electrocardiograph, or (more commonly) to a combination of electrodes that form an imaginary line in the body along which the electrical signals are measured.
An electrocardiogram is obtained by measuring electrical potential between various points of the body using a biomedical instrumentation amplifier. A lead records the electrical signals of the heart from a particular combination of recording electrodes which are placed at specific points on the patient’s body. When a depolarization wave front (or mean electrical vector) moves towards a positive electrode, it creates a positive deflection on the ECG in the corresponding lead.
When a depolarization wave front moves away from a positive electrode, it creates a negative deflection on the ECG in the corresponding lead. When a depolarization wavefront moves perpendicular to a positive electrode, it creates an equiphasic (or isoelectric) complex on the ECG [24]. In standard ECG recording there are five electrodes connected to the patient:
1. Right arm, RA
2. Left arm, LA
3. Left leg, LL
4. Right leg, RL
5. Chest, C
Depending how the electrodes pairs are connected to the ECG sensor different waveforms and amplitudes can be obtained. Each pair contains unique information of the heart activity that cannot be obtained from another pair of leads.
The different leads are divided into groups depending how they are connected to the ECG amplifier. There are two types of leads-Unipolar and Bipolar. The former have an indifferent electrode at the center of the Einthoven’s triangle (which can be likened to the ‘neutral’ of a wall socket) at zero potential. The direction of these leads is from the center .
The bipolar type, in contrast, has both electrodes at some potential, with the direction of the corresponding lead being from the electrode at lower potential to the one at higher potential, e.g. in limb lead I, the direction is from left to right. These include the limb leads-I, II and III.
Lead I, II and III are the so-called limb leads because at one time, the subjects of electrocardiography had to literally place their arms and legs in buckets of salt water in order to obtain signals for Einthoven’s triangle. Eventually, electrodes were invented that could be placed directly on the patient’s skin. Even though the buckets of salts water are no longer necessary, the electrodes are still placed on the patient’s arms and legs to approximate the signals obtained with the buckets of salt water. They remain the first three lead of modern 12 lead ECG.
Lead I is a dipole with the negative electrode on the right arm and the positive electrode on the left arm.
Lead II is a dipole with the negative electrode on the right arm and the positive electrode on the left leg.
Lead III is a dipole with the negative electrode on the left arm and the positive electrode on the left leg.
Leads aVR, aVL, and aVF are augmented limb leads. They are derived from the same three electrodes as leads I, II and III. However, they view the heart from different angles ( or vectors) because the negative electrode for these leads is a modification of Wilson’s central terminal, which is derived by adding leads I, II ,III together and plugging them into the negative terminal of the ECG machine. This zeroes out the negative electrode and allows the positive electrode to become the “exploring electrode” or a unipolar lead. This is possible because Einthoven’s Law states that
I+(-II)+III=0.....................equation (i)
I+III=II..............................equation (ii)
Where, I, II, III are leads.
It is written this way instead of I+II+III=0 because Einthoven reversed the polarity of lead II in Einthoven’s triangle, possibly because he liked to view upright QRS complexes. Wilson’s central terminal paced the way for the development of the augmented limb leads aVF, aVL, aVF and the precordial leads V1, V2, V3, V4, V5 and V6.
Lead aVR or “augmented vector right” has the positive electrode on the right arm. The negative electrode is a combination of the arm electrode and the left leg electrode, which “augments” the signal strength of the positive electrode on the right arm.
Lead aVL or “augmented vector left has the positive electrode on the left arm. The negative electrode is a combination of the right arm electrode and the left leg electrode, which “augments” the signal strength of the positive electrode on the left arm. Lead aVF or “augmented vector foot” has the positive electrode on the left leg. The negative electrode is a combination of the right arm electrode and the left arm electrode, which “augments” he signal of the positive electrode on the left leg.
The augmented limb lead a VR, aVL, and aVF are amplified in this way because the signal is too be useful when the negative electrode is Wilson’s central terminal. Together with lead I, II, III augmented limb leads a VR, aVL, and a VF form the basis of the hexaxial reference system, which is used to calculate the heart’s electrical axis in the frontal plane .
The precordial leads V1, V2, V3, V4, V5 and V6 are placed directly on the chest. Because of their close proximity to the heart, they do not require augmentation. Wilson’s central terminal is used for the negative electrode, and these leads are considered to be unipolar. The precordial leads view the heart’s electrical activity in the so-called horizontal plane. The heart’s electrical axis in the horizontal plane is referred to as the Z-axis.
Leads V1, V2 and V3 are referred to as the right precordial leads and V4, V5 and V6 are referred to the left precordial leads.
The QRT complex should be negative in lead V1 and positive in lead V6. The QRT complex should show a gradual transition from negative to positive between leads V2 and V4. The equiphasic lead is referred to as the transition lead. When the transition occurs earlier than lead V3, it is referred to as an early transition. When it occurs later than lead V3, it is referred to as late transition. There should also be a gradual increase in the amplitude of the R wave between leads V1 and V4. This is known as T wave progression. Poor R wave progression is a non-specific finding. It can be caused by conduction abnormalities, myocardial infarction, cardiomyopathy, and other pathological conditions .
Lead V1 is placed in the fourth intercostal space to the right of the sternum.
Lead V2 is placed n the fourth intercostal space to the left of the sternum.
Lead V3 is placed directly between leads V2 and V4.
Lead V4 is placed in the fifth intercostal space in the midclavicular line (even if the apex beat is displaced).
Lead V5 is placed horizontally with V4 in the anterior axillary line.
Lead V6 is placed horizontally with V4 and V5 in the mid axillary line.
An additional electrode is present in modern three-lead and twelve-lead ECGs. This is the ground lead and is placed on the right leg by convention, although in theory it can be placed anywhere on the body. With a three-lead ECG, when one dipole is viewed, the remaining lead becomes the ground lead by default.
