This is a modified part of a little report I wrote on the ECG project. Please have the slightly formal tone excused, and any mistakes that I might have made ;)
Einthoven, the father of ECG, devised a guide for electrode placement to map the heart activity from different perspectives, called Einthoven’s Triangle. Einthoven’s standard leads are defined in the following way:
- Lead I: V_I = φ_L −φ_R
- Lead II: V_II = φ_F −φ_R
- Lead III: V_III = φ_F −φ_L
Phi is the electric potential of the limb with respect to the right leg. We see that a "lead" is
nothing but the difference of potential between two points on the body. This is what is known as a bipolar setup. Not shown here is the Right Leg Drive electrode, a point of reference for establishing a common mode voltage level and reducing noise. Heartbeat Logger currently supports the measurement of one lead at a time in Einthoven's setup.
Einthoven's triangle. Source: bem.fi.
have evolved into 6 or 12-lead versions based on unipolar setups, such as Wilson Central Terminal, Einthoven’s Triangle is still used as a setup for simple heart monitors and in the classroom. For proper diagnostics, a 6 lead configuration is usuallypreferred.
Heartbeat Logger can be used with all Einthoven lead configurations. The main depolarisation vector in the heart is from top left to bottom right. Remembering the list of observations in The heart and the ECG -- Biopotential, it makes sense that the Lead II configuration will provide the most visible deflections, which is the configuration I'm using for my own recordings. The signal is then measured between electrodes placed at RA and LL - right arm and left leg. Our point of reference will be the commonly used RL - right leg.
However, as you can read in the project update about noise, bypassing the muscles in the arms and legs is a good idea to get a cleaner readout. I therefore place the two electrodes for Lead II on the right shoulder and upper left hip, and the Right Leg Drive electrode on the upper right hip.