The electrical signals from the heart are measured with surface electrodes. The resulting electrode potential in the heart conducts to the body surface. Standardized electrode positions are used to record the ECG. The three types of electrode systems are
- Bipolar limb leads or standard leads
- Augmented unipolar limb leads
- Chest leads
Bipolar Limb leads – Standard Lead I, Lead II and Lead III
This lead system is also known as Einthoven lead system. Two electrodes record the ECG signal. As shown in figure from four body locations of our body namely Right Arm(RA), Left Arm(LA), Right Leg(RL) and Left Leg(LL) potentials are recorded.
Final output is the difference between the electrical potential generated between these two electrodes. Right leg is the ground reference electrode.
Lead I
Voltage drop occurs from left arm to right arm. Generated voltage is VI.
Lead II
Voltage drop occurs from left leg to right arm. Generated voltage is VII.
Lead III
Voltage drop occurs from left leg to left arm. Generated voltage is VIII.
Einthoven Triangle
Einthoven triangle is the closed path formed between right arm, left arm, left leg and right arm. Einthoven has defined that cardiac electric vector is two dimensional along the frontal plane of the body. Along the projections of the triangle, vector sums on three sides of triangle is zero. We can tell, by Kirchhoff’s law, the amplitude of R wave along the lead III is equal to the summation of amplitude of R wave along lead I and lead II.
Augmented Unipolar Leads
A person named Wilson introduced augmented unipolar limb lead system. In this system, ECG is recorded between single exploratory electrode and central terminal. Central terminal relates to the center of the body. Two equal and large resistors are used. Pair of limb electrodes is connected to the resistors. The center joint connection of this resistive network forms the central terminal. The remaining portion of the limb electrode forms the exploratory electrode. In this lead system, a very small increase in ECG voltage can be found. Three types of connections are used.
Lead aVR
Two resistors are connected to left arm and left leg. The middle point of the resistor connection is connected with negative terminal. Right arm is connected with the positive terminal of the amplifier. Here also right leg acts as a reference terminal.
Lead aVL
Two resistors are connected to right arm and left leg. The middle point of the resistor connection is connected with negative terminal. Left arm is connected with the positive terminal of the amplifier. Right leg acts as a reference terminal.
Lead aVF
Two resistors are connected to right arm and left arm. The middle point of the resistor connection is connected with negative terminal. Left leg is connected with the positive terminal of the amplifier. Here also right leg acts as a reference terminal.
Chest Leads
V1 – Fourth intercostal space of right sternal margin,
V2 – Fourth intercostal space at left sternal margin,
V3 – Midpoint between V2 and V4,
V4 – Fifth intercostal space at mid – clavicular line,
V5 – Same as V4 position but on anterior auxiliary line,
V6 – Same as V4 position but on mid auxiliary line.
