A comparison of the overall first-pass kinetics of thallium-201 and technetium-99m MIBI in normoxic and low-flow ischaemic myocardium

The specific impact of ischaemia on the myocardial kinetics of thallium-201 and technetium-99m 2-methoxy-2-isobutylisonitrile (MIBI) remains a matter of debate. Using an isolated heart model perfused with red blood cell-enhan ced perfusate, we compared the overall first-pass kinetics of Tl-201 and MI BI under haemodynamically stable conditions of low-flow ischaemia (>50% red uction in normal coronary flow and greater than or equal to 20 mmHg fall in systolic contraction pressure, n=10) and normoxia (n=11). For both Tl-201 and MIBI, we found that under ischaemic conditions las compared with normox ia) there was a higher initial net extraction fraction (Tl-201: 0.78+/-0.03 vs 0.72+/-0.06, P=0.006; MIBI: 0.49+/-0.10 vs 0.39+/-0.11, P=0.03), a lowe r clearance rate in the 30 min following extraction (% decrease in cardiac uptake: Tl-201: 30=/-12 vs 47+/-14, P=0.02; MIBI: 5+/-5 vs 13+/-11, P=0.02) and a higher retention fraction at 30 min (Tl-201: 0.54+/-0.10 vs 0.39+/-0 .12, P=0.004; MIBI: 0.46+/-0.08 vs 0.33+/-0.12, P=0.01). Multivariate analy ses, however, revealed that all myocardial kinetic parameters of both trace rs were dependent only on coronary flow rates, without any additional signi ficant impact of the presence of ischaemia or states of contractility or ox idative metabolism. We conclude that the myocardial fractional retention of both Tl-201 and MIBI is strongly correlated with the decrease in coronary flow during ischaemia. This inverse relationship with coronary flow derives from both the flow-dependent increase in the initial myocardial extraction and the decrease in the subsequent myocardial washout of the tracers.