Comparison of 99mTc-teboroxime with thallium for myocardial imaging in the presence of a coronary artery stenosis

Abstract

Background. This study tested the hypotheses in the setting of a coronary artery stenosis that 1) planar 99mTc-teboroxime myocardial scans are capable of providing a good estimate of relative coronary flow reserve, and 2) delayed washout of the tracer from the myocardium is a marker of reduced myocardial blood flow and, in certain cases, myocardial ischemia. Methods and Results. Experiments were conducted in eight closed-chest domestic swine prepared with an artificial stenosis that reduced diameter of the left anterior descending coronary artery by 80%. Measurements of hemodynamics, regional myocardial blood flow, oxygen, and lactate metabolism were made 1) at baseline, 2) after 5 minutes of intravenous infusion of adenosine and neosynephrine ("stress"), and 3) at recovery 2 hours after discontinuing the adenosine/neosynephrine infusion. Simultaneous intravenous injection of teboroxime (∼9 mCi) and thallium (∼3.5 mCi) was made at peak stress, and serial planar teboroxime imaging began 1-2 minutes later. Scans were made in dynamic mode for 30 seconds each for 7 minutes after which a stress thallium scan (7 minutes acquisition) was obtained. A redistribution thallium scan was made 2 hours later after which a repeat teboroxime injection followed by serial imaging for 7 minutes was performed. The animal was then killed, and the heart removed for determination of microsphere activity. Under baseline conditions, transmural myocardial blood flow (ml/min/g) distal to the stenosis (1.06±0.17) was reduced (p<0.01) compared with the normally perfused circumflex zone (1.50±0.31). In response to intravenous infusion of adenosine/neosynephrine, flow increased (p<0.01) compared with baseline in both distal (2.00±0.84) and circumflex (4.67±1.55) zones. However, the distal:circumflex flow declined (0.45±0.17) compared with baseline (0.73±0.17; p<0.01). Two hours later flow had returned to baseline levels in both zones, and lactate production during stress (-41.7±37.5 μmol/min/100 g) had reverted to consumption (13.6±7.7; p<0.05). Analysis of stress teboroxime scans demonstrated 1) an increase (p<0.01) in the ischemic:normal zone (IZ:NZ) count between 30-second (0.50±0.14) and 7-minute scans (0.61±0.11); 2) a good correlation between the 30-second scan IZ:NZ count and the stress distal:circumflex flow (0.45±0.17; r=0.74; p<0.05; slope=0.90; intercept=0); and 3) a close correlation between the IZ:NZ count of the 7-minute scan (0.61±0.11) and the recovery distal:circumflex flow (0.69±0.21; r=0.89; p<0.01). The IZ:NZ count also increased (p<0.01) between 30-second (0.65±0.15) and 7-minute (0.72+0.14) scans following rest injection of teboroxime. As anticipated, serial thallium scans demonstrated evidence of redistribution between stress (IZ:NZ count =0.62±0.08) and recovery (IZ:NZ count =0.75±0.06; p<0.01) time points. The stress thallium scan IZ:NZ, however, was greater than that of the 30-second teboroxime scan as well as that of the stress distal:circumflex flow. Conclusions. Accordingly, the data indicate that 1) myocardial imaging with 99mTc-teboroxime is valuable in the noninvasive assessment of relative coronary flow reserve and that 2) delayed washout of the tracer from the myocardium reflects reduced myocardial blood flow and, under conditions comparable to those of the present study, may be a marker of myocardial ischemia.