A nuclear stress test can be performed for many different indications. Some of the common reasons are:
- shortness of breath
- chest pain or tightness
- previous history or family history of cardiac problems
- follow-up on progression of known disease
- establishment of a baseline for newly diagnosed cardiac patients
- arm/jaw/back pain (less likely)
- assessment of damage from myocardial infarction
- cardiac arrhythmias
- congestive heart failure staging
- pre-operative evaluations
Women in particular have unusual or atypical symptoms of heart disease and heart attacks. This particular imaging test is especially important for women for proactive cardiac evaluation that will lead to an early intervention when indicated.
The test has three basic components ‚Äì acquisition of resting images, stress portion of the test, and acquisition of stress images.
The specific imaging agent used for stress testing is a technetium/sestamibi combination. As I explained in a previous blog, technetium is the radioactive component of the imaging dose, which provides the detectable energy. Sestamibi, with the trade name of Cardiolite®, delivers the radioisotope to the target organ or system. In this case, sestamibi is taken up by mitochondria, based on two critical factors, both of which are at the heart of the study (pun intended):
- Blood flow to the region
- Cellular viability
There is another agent available, tetrofosmin with the brand name Myoview®, which has very similar chemical properties to sestamibi, and their imaging protocols are interchangeable. In addition, thallium-201 is the original radiotracer that is not used any longer, due to less favorable timing and energy properties.
For those interested in greater technical detail, here is brief additional information about the mechanism of uptake of both sestamibi and tetrofosmin. Sestamibi uptake is a function of plasma and mitochondrial membrane potentials. Accumulation of tetrofosmin is based on similar mechanism of mitochondrial ability to transduce metabolic energy into electronegative membrane potentials. Mitochondria generate cellular energy and are especially numerous in muscle cells due to their high metabolic needs. This is precisely the reason heart muscle uptake of both imaging agents is much greater than background and is the foundation of nuclear cardiac imaging. Once in mitochondria, neither agent leaves in an appreciable amount of time, thus allowing delayed imaging up to several hours.
Since the uptake is proportionate to blood flow to the heart muscle and viability of cells and their mitochondria, the study addresses both questions. The results provide information on regional blood flow patterns and patency, as well as the health of the heart muscle. This information is particularly important for patients with low risk for heart disease, as it determines their cardiac disease status in a non-invasive manner and allows early intervention if indicated.