ECG e-learning in video



Placement of electrodes

Myocardial cells action potential

Welcome to this series of tutorials concerning ecg 's reading


- there are 2 types of cardiac cells
        -Myocardial cells- working or mechanical cells
        -Pacemaker cells- responsible for spontaneous generation and conduction of impulses

In this chapter, we're gone study Myocardial Cells Electrophysiology

- myocardial cells at rest are polarized
- if you introduce an electrode inside the cell, you will mesure a potentential difference between the inside and the outside of the cell, measured at about -90mV  with an internal negativity
- this resting potential membrane is the consequence of three phenomenes:
          - the inside of the cell is rich in proteins, phophates and organic acids, all negatively             charged             
K+ can easily circulate through the cell membrane from the inside to the outside, but            Na+ cannot enter easily
         - and the most important fact is the existence of  a membrane Na/K- ATPase pump. This           pump exchange 3 K+ against 2 Na+. So the electrical neutrality is not respected.
- So there is a resting potential membrane measured at -90 mV. But this potential membrane is unstable. If the myocardic cell is electrically stimulated (even by a little electric stimulation) and reachs the trigger zone (measured at - 70 mV), then is created an action potential. An action Potential is a succession of depolarization and repolarization phases.
- the cardiac action potential is composed with four phases:
         - Phase 0- rapid depolarization; sodium moves rapidly into the cells, by openning of Na             channels, voltage dependant
          - Phase 1- early repolarization; the flow of sodium into the cell is slowed down, by closing           the Na channels, voltage dependant
          - Phase 2- repolarization (plateau phase); there is still a slow inward movement of calcium            into the cells and a continued outward movement of potassium from the cells
          - Phase 3- rapid repolarization; the outward movement of potassium from the cells is fast
          - Phase 4- resting membrane potential; the sodium-potassium pump actively transports            sodium out of the cell and potassium back in the cell
- So the external resting membrane is negatively polarized. When depolarization occures the external membrane gets positively polarized. The depolarization progress from cells to cells. But at the surface of the cells, there is an electic field between depolarized regions and not yet depolarized regions.
- if you place 2 electodres at the surface of the body, you can can record this electic field and its variations. It's the  principle of the electrocardiogram

    Following tutorials coming soon....
      Just beginning this part of the site !