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
Biopotential is an electric potential measured between living cells. The cells in the heart’s muscle tissue, the cardiomyocytes, carry electrical charge. When the cells are resting, the charge of the cell membrane is positive relative to that of the inside of the cell, and the heart is said to be polarised. The sinoatrial node, the hearts pacemaker, initiates a reaction where Na+ and Ca++ ions start flowing into the heart, making the electric potential decrease until a certain threshold is reached. The muscle cells then contract, and the charge of the membrane is now negative in respect to the inside of the cell; it is depolarised. The depolarisation will spread through the heart like a wave, though unevenly, before a wave of repolarisation will return the heart cells to rest.
The moment a wave of depolarisation is spreading through heart tissue, some of the tissue is negatively charged and the rest is still positively charged. A separation of charge between the cells creates an electric potential, a biopotential, or simply a voltage. As the depolarisation is spreading, the voltage increases and reaches its height when the amount of positively and negatively charged cells is the same. Once the entire tissue is depolarised, there is no longer a difference in potential, and there will be no voltage measured.
The following observations have been made of a volume conductor measured by a differential set of electrodes (CV Physiology Concepts):
- A wave of depolarization traveling toward a positive electrode results in a positive deflection in the ECG trace.
- A wave of depolarization traveling away from a positive electrode results in a negative deflection.
- A wave of repolarization traveling toward a positive electrode results in a negative deflection.
- A wave of repolarization traveling away from a positive electrode results in a positive deflection.
- A wave of depolarization or repolarization traveling perpendicular to an electrode axis results in a biphasic deflection of equal positive and negative voltages (i.e., no net deflection).
- The instantaneous amplitude of the measured potentials depends upon the orientation of the positive electrode relative to the mean electrical vector.
- The voltage amplitude is directly related to the mass of tissue undergoing depolarization or repolarization.
Considering the rules above and that the heart is a complex muscle, electrode setup and placement of the electrodes will be essential for the measurements and any diagnostics.