njit-etd2003-081 - New Jersey Institute of Technology

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The first deflection, the P wave, corresponds to the current flow during atrial
depolarization (contraction). The second deflection, the QRS complex, is a result of
ventricular depolarization. The third and final deflection is the T wave. The T wave is a
result of ventricular repolarization (relaxation). It should be noted that atrial
repolarization is usually not evident in the ECG because it occurs at the same time as the
QRS complex.
As mentioned earlier, the ECG is a measure of the electrical activity of the heart
measured on the skin. In order to measure the ECG, bipolar leads are required. The
bipolar method of acquiring ECG detects electrical variations at two different locations
on the skin and displays the difference to obtain one waveform. Figure 2.9 is an
illustration of the standard limb lead connections that form the Einthoven's triangle. In
addition, the diagram also shows the names of the respective leads. To record lead I, the
negative terminal of the ECG monitor is connected to the right arm (RA) and the positive
terminal is connected to the left arm (LA). To record lead II, the negative terminal of the
ECG monitor is connected to the right arm and the positive terminal is connected to the
left leg. To record lead III, the negative terminal of the ECG monitor is connected to the
left arm and the positive terminal is connected to the left leg (LL). The reference point
or ground is connected to the right leg (RL). It is important to realize that depending on
where the electrodes are attached, a different waveshape will be obtained for the same
electrical events occurring in the heart. In other words, leads I, II, and III all have a
different waveform shape.