A Nuclear Stress Test (NST) is an imaging procedure designed to evaluate how effectively blood flows through the coronary arteries into the heart muscle. The direct answer to whether this test shows blocked arteries is that it shows the functional effect of those blockages on blood delivery to the heart. It is not an anatomical image that visualizes the plaque or narrowing within the artery itself. Instead, the test highlights areas of the heart muscle that are not receiving adequate blood supply, which is typically caused by Coronary Artery Disease (CAD).
The Role of Radioisotopes in Measuring Blood Flow
The core mechanism of the Nuclear Stress Test relies on the use of a small, safe dose of a radioactive tracer, known as a radioisotope, often a technetium- or thallium-based compound. This tracer is injected into a vein and travels through the bloodstream, where it is absorbed by the healthy, working heart muscle cells. A specialized camera, typically a SPECT (Single-Photon Emission Computed Tomography) or PET scanner, then detects the gamma rays emitted by the radioisotope to create detailed images of the heart.
The procedure is conducted in two distinct phases: a “Rest” phase and a “Stress” phase, with images taken after each tracer injection. The initial images establish the baseline blood flow when the heart is relaxed and has minimal demand for oxygen and nutrients. For the second phase, the heart is subjected to stress, either by having the patient exercise on a treadmill or by administering a pharmacological agent. Pharmacological stress agents, such as regadenoson or dobutamine, temporarily cause the heart’s blood vessels to dilate, mimicking the increased blood flow that occurs during physical exertion. A second dose of the radioisotope is injected at the peak of this stress, and a second set of images is acquired shortly after. Comparing the pattern of radioisotope uptake in the heart muscle between the rest and stress images reveals any differences in blood delivery. In areas where an artery is significantly narrowed, the blood flow cannot increase sufficiently to meet the heart’s elevated demand during stress, resulting in less tracer uptake in that region.
Interpreting Test Results: Identifying Reduced Perfusion
The images produced by the SPECT or PET scanner are analyzed for patterns of reduced perfusion, which appear as areas of decreased radioisotope concentration known as “cold spots” or “defects”. The interpretation focuses on whether these defects are reversible or fixed, which directly translates into a functional diagnosis of Coronary Artery Disease. A reversible defect is a region that shows poor tracer uptake during the stress image but demonstrates normal uptake during the rest image. This pattern strongly suggests stress-induced ischemia, meaning the heart muscle is viable but experiences temporary blood flow inadequacy only when its oxygen demand increases.
Conversely, a fixed defect appears as a cold spot on both the stress and rest images. This result usually signifies an area of scarred or damaged tissue, most commonly due to a prior heart attack or myocardial infarction. By mapping the location and size of these perfusion defects, the cardiologist can infer which specific coronary artery is compromised and estimate the extent of the heart muscle at risk.
Moving Beyond Perfusion: Confirmatory Testing
While the Nuclear Stress Test is highly effective at determining the functional significance of a potential blockage, it does not provide a direct picture of the arterial anatomy. The test indicates that blood flow is impaired, but it cannot show the exact percentage of narrowing or the specific composition of the plaque causing the problem. If the NST reveals significant reversible ischemia, suggesting a severe flow restriction, further anatomical testing is typically recommended.
The definitive next step is often a Coronary Angiography, an invasive procedure where a catheter is threaded to the coronary arteries and a contrast dye is injected. This allows for real-time X-ray imaging, which precisely visualizes the interior of the arteries, revealing the exact location and severity of the blockage. Alternatively, a non-invasive Cardiac CT Angiography may be used to create detailed cross-sectional images of the arteries. These follow-up tests confirm the structural problem underlying the functional defect and determine if intervention, such as stent placement or bypass surgery, is required.