Test for reversible cardiac ischemia aEUR’
Correct Answer: Thallium scan
Description: Thallium scan Thallium scanning Principle Thallium is a monovalent cation which acts as potassium analogue. It is taken up by ble cells - not dead ones. It is distributed to all pas of the body, largely in propoion to the regional blood flow. The concentration of thallium in the myocardium is propoional to the coronary blood flow over a wide range of flows. Once inside the myocardial cell, the thallium does not stay there. There is constant and continuous thallium exchange. Over a period of time thallium washes out of the normal cells and some thallium is extracted by the hypoperfusedareas. This unique propey is taken advantage of clinically during thallium stress testing and is the basis for the redistribution or late 4 hour image. Clinical application : Exercise is the most. frequently used form of stress in the testing of cardiac .function. During exercise myocardial blood flow normally increases by upto 3-4 folds to supply the increase in 0, demand. If coronary flow is limited by a stenosis there will be less thallium extracted .from the blood by the myocardium in the territoy of that aery compared with the normal, surrounding myocardium. This will cause a perfusion defect. Over 2-4 hours, this perfusion defect will redistribute or fill in as thallium washes out of the normally perfused cells and some more thallium is extracted by the hypoperfused myocardial cells Hence in ischemia the thallium perfusion defect disappears with time and is called a reversible defect. By contrast, areas of scar tissue or infarction do not change with time and appear as fixed defects. Clinical utility - interpretation No ischemia : In a patient without significant CAD, there will be uniform distribution of thallium in all myocardial segment's in the immediate post stress and 4 hour redistribution image. if the thallium scan is normal there is usually a very low probability, of angiographically significant CAD. Ischemia present : In a patient with significant CAD, (a stenosis of at least 70%), myocardial thallium uptake will be reduced causing a perfusion defect in the distribution of the stenosed vessel in the immediate poststress image. After 4 hours, the perfusion defect will 'fill in' or redistribute giving a unifonn thallium distribution, throughout the myocardium,, on reimagingo. Some perfusion defects only paially redistribute or do not "fill in' at all by 4 hours. These patient are now re-injected with a smaller dose of thallium and reimaged after 30 minutes or alternatively reimaged at 24 hours without reinjection. Following reinjection and /or delayed imaging patients with severe ischemia usually demonstrate ,full or paial redistribution. Fixed defects which do not show redistribution are interpreted as scar tissue from myocardial infarction or nonble myocardium, which would at not benefit from revascularisatioe Viability The question of whether myocardium is ble can also be addressed, using rest and redistribution imaging. Following thallium injection, coronary blood flow at rest is assessed immediately and again at 4 hour. In a patient with a critical stenosis there will be very low flow at rest, producing a perfusion defect of varying severity. If there is ble myocardium present, the defect will resdistribute over time.
Category:
Surgery
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