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Old 11-21-2010
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Originally Posted by noone_123i View Post
Hi, I had a question about ECG. For which diseases (or better way of saying it is "when would"...) would you see a ST ELEVATION? And when would you see ST DEPRESSION?

Basically I am confused as to what does ST elevation and depression represent? And what causes these to happen? And what is the effect of these 2?

Please help!!!! Thanks
It's a simple concept of how the ions behave during myocardia injury. The EKG lead is recoding this change in ion inside and outside the myocardial cells. Remember not in a single cell but rather as an aggregate of whole. The EKG leads tells you where are the ion moving during a particular moment in time or during a particular cardiac cycle. All chest leads are positive leads so if a depolarizing current is moving toward it there is a positive deflection and the tracing moves up and if it is moving away from the positive lead then there is negative deflection e.g. Q is a negative wave and R is a positive wave. I hope you have a basic concept on EKG.

Now let's understand what is the deal with ST elevation and ST depression and then the Q wave.

Now all these ECG changes tell us that there is myocardial ischemia but the question is what type of ??? subendocardial ischemia, transmural ischemia or the infarction which is replaced by scar tissue.

In subendocardial infarction there is ST depression but why? Why not ST elevation?

Myocardium is divided into three layers the top most is the epicardium, then myocardium and then the endocardium. In subendocardial infarction endocardium is injured. Just picture there is a chest electrode facing this injured area.
In normal ST segment is a straight line on the EKG paper that represent completion of cardiac depolarization, as all the myocardial cells are now loaded with positive ions inside the cell and there is no net movement of ions in or out . This is why there is no positive or negative deflection on EKG. Now in subendocardial infarction, during this ST phase there is an influx of Cations from the surrounding area and area above it to the injured cells. So the chest electrode see there positive ions moving away from it (remember the chest electrode is directly above it)and makes a negative deflection. This negative deflection that occurs in ST phase is called ST depression.

In transmural (ALL 3 layers) infarction there is ST elevation why?

So again during the ST phase of depolarization normally all the cardiac cells are electrically silent as there is no net movement of ions but in transmural infarction during this ST phase there is an influx of Cations from the surrounding area to the injured cells. Because the infarction is all the way to the top layer and just above it is the chest electrode, the electrode sees cation rushing towards it, so it records the positive deflection during the ST phase which is ST elevation

Now why there is persistent Q wave in old myocardial infarction.

The old infarcted myocardial tissue is replaced by collagen fibers and this fiber does not conduct electrical current so it is electrically silent. Now think of the old infarcted myocardial tissues as a hole in the myocardium (going through all three layers), and there is a chest electrode directly above it. Now during depolarization, the scarred myocardial tissue is electrically silent (think of it as a hole) so the chest electrode sees directly opposite myocardial tissue electrical activity. During depolarization It sees the electrical current moving away from it hence you have the Q wave a negative wave. This is why in old infarct you have persistent Q wave.

I hope this clears your basic concept on cardiac electrical physiology

Good luck for USMLE

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