Pharmacologic stress, induced by using either dipyridamole or adenosine, is primarily reserved for patients who cannot complete standard exercise treadmill testing, usually due to some physical limitation. Pharmacologic stress methods may also be important in patients who are deconditioned due to prolonged hospital stays or other chronic diseases.
Why is it important to characterize which stress-induced perfusion defects are reversible and whether myocardial viability is present?
When territories are supplied by a subtotal or critical coronary stenosis, these severely underperfused but viable tissues switch to primarily glycolytic metabolism from normal fatty acid metabolism. Such altered cellular biochemistry may be imaged using the glucose analogue 2-deoxy-glucose labeled with l8F. Positron emission tomographic (PET) imaging with FDG identified glycolytically active tissue that is viable. This phenomenon, severe resting ischema, occurs in up to 40% of fixed thallium-201 defects classified as dead or scarred myocardium. When normal coronary blood flow is resupplied to these regions, otherwise ischemic myocardium normalizes thallium-201 perfusion and regional wall motion recovers, demonstrating the importance of detecting such severely ischemic tissue.
Is it possible to assess both myocardial perfusion and left ventricular (LV) systolic performance from the same myocardium perfusion study?
The new myocardial perfusion agents, such as Tc-MIBI, allow greater doses of the radiotracer to be administered as well as provide improved inherent imaging characteristics. Therefore, a bolus injection of the radiotracer can be used to assess LV performance at peak stress using the first-pass‚ LV imaging techniques. Second, gated perfusion imaging using single-photon emission computed tomography (SPECT) 3-D analysis may quantify LV end-diastolic and end-systolic relative volumes and LV ejection fraction (LVEF). Gating or triggering of the imaging camera at the onset of the R-wave over multiple cardiac cycles provides a 16-frame average of multiple cardiac beats necessary for assessing cardiac motion and capturing end-systole and end-diastole, both of which are required for EF determinations.