We demonstrate how to record the Einthoven leads and then how to construct the mean electrical axis from the recording. The Einthoven leads are differential leads because our differential amplifier is directly connected to the limbs. For example Einthoven I is measured between the left and right arm. The third electrode is the ground or neutral which can be connected to any place of the body but is usually placed on the right foot. The ground electrode is required because internally the differential amplifier first measures its two inputs +/- against GND and then subtracts them internally from each other. Some setups use an active GND which injects a small voltage into the patient which aims to reduce the mains interference but state of the art instrumentation amplifiers won't need to this trick. As long as the mains contamination is identical on both +/- channels and won't saturate the inputs they will cancel out the mains and will produce a clean ECG.
We also show in this clip how to estimate the angle of the mean electrical vector of the heart. In the next clip about the augmented leads we will show that constructing the angle with I and aVF is much easier.
Presenter: Vasso Georgiadou http://biosignals.berndporr.me.uk
