Technetium 99m-Q12 kinetics in perfused rat myocardium: effects of hypoxia and low flow.

Technetium 99m-Q12 is a new cationic myocardial perfusion imaging agent that produces excellent images in human beings. The purpose of this study was to examine the separate effects of hypoxia and low flow on myocardial clearance kinetics. After a 1 mCi bolus injection, myocardial 99mTc-Q12 clearance was monitored for 1 hour by using an Nal detector in 24 isolated perfused rat hearts. In 6 control hearts, flow was 12 ml/min, and oxygenation was normal. In 6 hypoxic hearts, flow was normal, but oxygenation was reduced (<5% 02). In 6 low-flow hearts, flow was 3 ml/min, and oxygenation was normal. In 6 very low flow hearts, flow was 1 ml/min, and oxygenation was normal. 99mTc-Q12 myocardial clearance was biphasic in all four groups, consisting of a rapid early phase and a second slow phase that began after 10 minutes. Myocardial retention between 1 and 10 minutes was 56.8% +/- 1.8% for control, 49.2% +/- 2.2% (p < 0.05 compared with control) for hypoxic, 56.8% +/- 2.6% (p = NS compared with control) for low flow (3 ml/min), and 63.7% +/- 2.1 % (p < 0.05 compared with control) for very low flow hearts (1 ml/min). Myocardial retention between 10 and 60 minutes was 90.5% +/- 0.2% for control, 90.2% +/- 1.6% for hypoxic, 90.0% +/- 0.8% for low-flow hearts (3 ml/min), and 87.6% +/- 0.3% (p < 0.05 compared with other groups) for very low flow hearts (1 ml/min). In conclusion, 99mTc-012 demonstrates biphasic clearance from normal, hypoxic, low-flow, and very low flow ischemic myocardium. Early-phase myocardial retention is decreased by hypoxia and increased by very low flow.

Effects of no flow and reperfusion on technetium-99m-Q12 kinetics.

UNLABELLED: The purpose of this study were to determine if 99mTc-Q12 tracer kinetics could be affected by the insult of total no flow followed by reflow and the effects of viability on clearance. METHODS: In six control buffer perfused rat hearts, flow was maintained at 12 ml/min throughout uptake and clearance. In six hearts (IR30), 30 min of no flow was followed by 60 min of reflow. In six hearts (IR60), 60 min of no flow was followed by 60 min of reflow. One millicurie 99mTc-Q12 was injected in control hearts or during reflow in the IR30 and IR60 hearts and myocardial clearance was monitored for 1 hr using a Nal detector. RESULTS: Control hearts demonstrated biphasic 99mTc-Q12 myocardial clearance with an early rapid clearance phase ending 5-10 min after injection (73.5% +/- 1.5% retention) followed by a late slow clearance phase (90.5% +/- 0.2% retention). IR30 hearts demonstrated a near identical clearance curve (74.3% +/- 0.9% early retention, 90.9% +/- 0.6% late retention). IR30 heart electron micrographs demonstrated predominantly ischemia insulted but viable cells. IR60 hearts also demonstrated a biphasic myocardial clearance, with a late slow phase similar to controls (91.9% +/- 0.6% retention). The early rapid phase was significantly faster than controls (61.1% +/- 3.4%). IR60 heart electron micrographs demonstrated predominantly injured nonviable cells. Well counting confirmed decreased retention in the IR60 rats compared to controls and IR30 rats. CONCLUSION: Technetium-99m-Q12 myocardial clearance is normally biphasic, with an early rapid phase ending after 5-10 min and a late slow phase. Ischemicaly insulted but viable myocardium created by 30 min of no flow followed by reflow has no effect on either clearance phase. This tracer warrants further study to determine its potential utility in assessing myocardial viability.