Thursday, February 5, 2015

The illusion of ST segment elevation in transmural myocardial infarction

To, understand the WHY of it all, we need to understand what is the electrical vector?

      1.Before the incoming of any impulse, the heart muscle is polarized- meaning the outside of heart muscles is more positive in compared to the inside of the heart muscle. This means that the ECF all over the heart has a positive charge

2.       With the incoming of the impulse from the S.A node, the heart muscle depolarizes..means positive ions are going inside. Which means the overlying ECF is becoming negative in compare to the surrounding area

3.       This change  in electrical charge, generates a current in the ECF ..this current flows from negative to positive and thus generates what is called the electrical vector
4.       The electrical vector changes in size and direction as the wave of depolarization spread

5.       Also, our body is a volume conductor..which means when the electrical energy flows..there is an electrical field generated around it. And different points on the field have a potential, based on their location from the electrical vector
6.       When we connect two potential in this field, and measure the potential difference between them, we get the electrocardiogram (ECG)

Next important point to be understood is how the strength of the vector is measured, whether the wave will be negative, or positive..and what will be its strength?

The axis of the electrical field is determined from negative to positive
Suppose there is a vector AB,  what we do to measure the value of this vector is project the same vector on this electrical field.

Hence, the strength of the voltage is +4 mv, and since the value is positive we get a positive wave
On the other hand, this vector has a value of -3mv, and it will be a negative wave

Whenever, there is an infarction in the myocardial tissue..the cell looses its polarity, meaning it becomes depolarized.
So even when the whole heart is in the resting stage..there is some amount of current flow from the infarcted tissue ( this is called funny currents-If)

Also remember, the ST segment is an isoelectic line, meaning there is no flow of charge in the heart muscle during this time..that is either the heart is completely polarized ( resting stage- all over positivity outside) or the heart is completely depolarized (all over negativity outside)
So now when you take an ecg of this heart..example with an infarction in the anterior wall, on the V1 lead, the overall voltage of the heart is reduced, this is because the constant flow of funny currents from anterior to posterior. This modifies the electrical vector ( with change in its direction and size)

But now as the wave of depolarization is completed (the QRS complex), the funny currents are abolished..cause they too are in the depolarized state (negativity outside). Hence, no flow of funny currents.
This makes the st segment isoelectric (that is 0mv)
When we see this graph, it seems the st segment has elevated, but in reality it is just an illusion, cause the st segment is right where it was supposed to be, what has changed is the voltage of the rest of the ECG

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