Myocardial Uptake of 99m Tc-N-NOET and 201 Tl During Dobutamine Infusion

Background —The myocardial uptake of 99m Tc-sestamibi is attenuated by dobutamine stress, resulting in underestimation of ischemia. N -Ethyl- N -ethoxy-dithiocarbamato- N - 99m Tc ( 99m Tc-N-NOET) is a new 99m Tc-labeled perfusion agent that is highly extracted by the myocardium by a mechanism different from that defined for 99m Tc-sestamibi. We therefore hypothesized that 99m Tc-N-NOET uptake would not be attenuated by dobutamine and that 99m Tc-N-NOET uptake would be comparable to 201 Tl uptake during dobutamine stress. Methods and Results —In 28 open-chest dogs, after placement of a stenosis in the left anterior descending coronary artery that reduced flow reserve by >50%, adenosine (300 μg · kg −1 · min −1 ; n=15) or dobutamine (2.5 to 30 μg · kg −1 · min −1 ; n=13) was infused. During adenosine stress, the stenotic-to-normal activity ratio for 99m Tc-N-NOET was 0.55±0.05. The stenotic-to-normal flow ratio was 0.33±0.04 at the time of 99m Tc-N-NOET injection. During dobutamine stress, the stenotic-to-normal 99m Tc-N-NOET activity ratio was 0.63±0.04, comparable to the 201 Tl activity ratio of 0.59±0.04. The stenotic-to-normal flow ratio was 0.47±0.04 at the time of 99m Tc-N-NOET and 201 Tl injection. The relationship between 99m Tc-N-NOET uptake and blood flow was comparable for adenosine and dobutamine stress, with no evidence of attenuation of 99m Tc-N-NOET extraction by dobutamine. Conclusions —In the presence of coronary stenoses that reduced regional flow reserve, the myocardial uptake of 99m Tc-N-NOET and 201 Tl are closely proportional to blood flow during both adenosine and dobutamine stress, suggesting that the adverse effect of dobutamine on 99m Tc-sestamibi uptake is a tracer-specific phenomenon rather than a generalized effect. The clinical implication of this finding is that 99m Tc-N-NOET might be preferable to 99m Tc-sestamibi when used with dobutamine stress for detection of coronary stenoses.