Cardiac Path Online
Assessment of Ischemic Myocardial Damage
The key to successful
clinicopathological correlation in a case of ischemic myocardial damage is to take a 1cm thick transverse slice
through the short axis of both ventricles at mid-septal level.
Advantage of this approach:
Helps in localisation of ischemic changes and ventricular shape.
Allows any ischemic myocardial damage to be exactly localised with respect to the region involved : Antero-lateral ; Postero-septal etc. and thereby indicates the coronary artery likely to be involved.
A very small minority of old or recent infarcts will not reveal themselves in a mid-septal slice but higher and lower slices can always be made later.
Identification of ischemic damage:
The infarct of more than 48 hours duration which has become well delineated with a red rim and a yellow centre causes no difficulties either of recognition or of topographical localisations.
The use of enzyme staining methods does help in a number of ways and various and various techniques depend on the presence of dehydrogenase in normal heart muscle and their loss in infarcted muscle.
A range of variants of the technique are described, in some substrate is added to the incubation media improving the colour developed and enhancing the contrast between normal and ischemic myocardium.
Enzyme activity in the normal muscle is detected by a colour reaction developed with nitro blue tetrazolium.
These techniques are of some but limited value in detecting myocardial necrosis of less than 12 hours duration before naked-eye changes appear.
But these are of great use in delineating the margins of necrotic areas of 12-36 hours' duration and thus in aiding accurate descriptions of infarct shape.
Any regional infarct should be classified as non-transmural or transmural, a distinction that is only accurate using enzyme methods.
A further use of enzyme methods is in the accurate detection of widespread focal damage or diffuse subendocardial damage, for example after cardiac surgery.
Enzyme methods should be used with caution in demonstrating infarction that is not visible macroscopically to some extent.
The development of the colour reaction is not necessarily uniform in different parts of a normal myocardial slice and if the process is stopped before the full overall colour is developed a patchy appearance results. Interpretation of such uneven staining is difficult (it may indicate some enzyme loss and ischemic damage ; if the pattern fits the clinical presentation this interpretation may be valid ).
In some cases control hearts from road traffic deaths that are virtually instantaneous will develop the colour reaction at a slower rate in the subendocardial zone.
Following detection of a regional area of infarction the pathologist should be able to identify the reason for a failure of perfusion in the arterial supply to that region.
The cause should either be stated as thrombotic in relation to atheroma or accurate observations should be made on the artery indicating whether spasm or dissection are possibilities.
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