Effects of physical stress and pharmacologically induced coronary dilation on myocardial and non-myocardial 201Thallium-uptake.

Abstract

After subtraction of the 201Tl background radioactivity which was obtained from the paracardiac region, left ventricular (LV)-201Tl net uptake at rest correlated with (1) the LV-muscle mass and therefore with (2) the LV-oxygen consumption. In normal LV, after maximum coronary dilation with dipyridamole, LV-201Tl net uptake increased little more than LV-oxygen consumption but by factor of 2.27 less than was coronary vascular reserve. In coronary heart disease (c.h.d.), dipyridamole caused a significantly smaller increase of LV-201Tl uptake (factor 1.16) than in normal LV myocardium (factor 1.76). This correlated well with the severity of the disease. Therefore, in maximum coronary dilation passive transport mechanisms may be responsible for augmented uptake and changed LV distribution of 201Tl which depends on the perfusion pattern. In exercise, the 201Tl-background radioactivity decreased from 54% (at rest) to 35% in normal patients and to 44% in c.h.d. Stressed thigh muscles accumulated 201Tl 4.72-fold more than at rest. Therefore, it is still questionable if the myocardium during exercise receives enough 201Tl to be labelled maximum. Since the LV-201Tl count rates in c.h.d., measured over the maximum uptake area were increased to the same amount as in normal myocardium neither in coronary dilation nor during exercise, the validity of using the maximum LV-201Tl uptake as a reference for the minimum one under these conditions is problematic.