Myocardial Infarction

Immediately after an acute coronary occlusion, blood

flow ceases in the coronary vessels beyond the occlusion

except for small amounts of collateral flow from

surrounding vessels.The area of muscle that has either

zero flow or so little flow that it cannot sustain cardiac

muscle function is said to be infarcted. The overall

process is called a myocardial infarction.

Soon after the onset of the infarction, small amounts

of collateral blood begin to seep into the infarcted area,

and this, combined with progressive dilation of local

blood vessels, causes the area to become overfilled with

stagnant blood. Simultaneously the muscle fibers use

the last vestiges of the oxygen in the blood, causing

the hemoglobin to become totally de-oxygenated.

Therefore, the infarcted area takes on a bluish-brown

hue, and the blood vessels of the area appear to be

engorged despite lack of blood flow. In later stages, the

vessel walls become highly permeable and leak fluid;

the local muscle tissue becomes edematous, and the

cardiac muscle cells begin to swell because of diminished

cellular metabolism.Within a few hours of almost

no blood supply, the cardiac muscle cells die.

Cardiac muscle requires about 1.3 milliliters of

oxygen per 100 grams of muscle tissue per minute just

to remain alive.This is in comparison with about 8 milliliters

of oxygen per 100 grams delivered to the

normal resting left ventricle each minute.Therefore, if

there is even 15 to 30 per cent of normal resting

coronary blood flow, the muscle will not die. In the

central portion of a large infarct, however, where

there is almost no collateral blood flow, the muscle

does die.

Subendocardial Infarction. The subendocardial muscle

frequently becomes infarcted even when there is no

evidence of infarction in the outer surface portions of

the heart. The reason for this is that the subendocardial

muscle has extra difficulty obtaining adequate

blood flow because the blood vessels in the subendocardium

are intensely compressed by systolic contraction

of the heart, as explained earlier. Therefore, any

condition that compromises blood flow to any area of

the heart usually causes damage first in the subendocardial

regions, and the damage then spreads outward

toward the epicardium.