Myocardial Infarction

Question 1

What is the aetiology of MI?

Answer 1

Occlusive thrombus overlying a disrupted plaque. Unstable plaques are seen in 90% of patients who have coronary angiography within first hour of symptoms.

Question 2

What is the basic pathogenesis of MI?

Answer 2

Occlusive thrombus in a coronary artery leading to ischemia throughout the affected vessels region

Question 3

After what time period does infarction begin in the subendocardial zone?

Answer 3

After 20-30mins of ischemia infarction begins in the subendocardial zone, due to the increased vulnerability of the subendocardial zone.

Question 4

Why is the subendocardial zone vulnerable?

Answer 4

Vulnerable because of less collateral perfusion and increased compressive forces

Question 5

How does infarction progress?

Answer 5

Begins in the subendocardial zone after 20-30mins of ischemia then extends like a wave front through the full thickness of the wall over the next 3-6hrs, by which time it becomes transmural.

Question 6

Why might the eventual size of the infarct be smaller in thickness and width?

Answer 6

This is due to the modifying effect of collateral vessels which lead to retrograde filling of occluded vessel and /or from thrombolysis (endogenous or treatment) leading to restablishment of antegrade flow.

Question 7

What is stunned myocardium?

Answer 7

In zones of acute ischemic damage, short of infarction, viable myocytes which have been re-perfused remain non-contractile for hours or days due to calcium overload and free radical damage.

Question 8

What vessels are commonly affected in a MI?

Answer 8

LAD 50%
RCA 30%
Circumflex 20%

Question 9

What sort of people are said to have left dominant coronary circulation?

Answer 9

20% of people have their circumflex artery supply the posterior descending coronary artery.