The Summed Stress Score (SSS) is a quantitative measurement derived from a Myocardial Perfusion Imaging (MPI) test, commonly known as a nuclear stress test. This non-invasive procedure uses a radioactive tracer and specialized cameras to visualize blood flow through the heart muscle under peak physical or pharmacological stress. The SSS is generated by computer analysis of these stress images, providing an objective numerical index of the heart’s ability to receive an adequate blood supply when demand is highest.
What is the Summed Stress Score?
The Summed Stress Score is a standardized index that quantifies the extent and severity of reduced blood flow in the heart muscle, known as myocardial perfusion defects, during the stress phase of the test. A higher SSS indicates more heart muscle tissue is affected by temporary blood flow reduction (ischemia) or permanent damage (infarct or scar tissue). The SSS helps physicians assess the overall status of the coronary arteries supplying the heart by converting complex image data into a single number.
The SSS is calculated from images taken immediately following the stress phase, when blood flow abnormalities are most evident. This score is distinct from the Summed Rest Score (SRS), which measures perfusion defects at rest. The difference between these two scores yields the Summed Difference Score (SDS), which isolates the amount of temporary, reversible ischemia. The SSS thus provides a comprehensive measure of all perfusion abnormalities present during peak cardiac demand.
The Scoring Mechanics: How the Number is Calculated
The calculation of the SSS begins by dividing the left ventricle, the heart’s main pumping chamber, into a standardized 17-segment model. This model ensures every area of the heart muscle is accounted for and assessed uniformly. Each of these 17 segments is then individually evaluated by a computer or trained physician based on the amount of radioactive tracer uptake visible in the stress image.
The tracer uptake in each segment is graded on a five-point, semi-quantitative scale ranging from 0 to 4. A score of 0 signifies normal radiotracer uptake, indicating healthy blood flow. Scores 1 through 4 represent progressively reduced uptake: 1 is mildly reduced, 2 is moderate reduction, 3 is severely reduced, and 4 means virtually absent radiotracer uptake.
The final Summed Stress Score is the arithmetic sum of the scores assigned to all 17 segments. Since each segment can score a maximum of 4, the highest possible SSS is 68, indicating a complete lack of blood flow throughout the entire left ventricle. This final number combines the extent (how many segments are affected) and the severity (how poorly each segment is perfused) of the perfusion abnormality into a single, objective metric.
Translating the Score: Understanding Severity Levels
The SSS translates directly into a measure of disease burden used to categorize the severity of test results. A score of 0 to 3 is considered normal or minimally abnormal, suggesting a low likelihood of significant Coronary Artery Disease (CAD). Patients in this range are categorized as low-risk for future cardiac events.
As the score increases, it signifies a greater volume of compromised heart tissue, moving the patient into higher risk categories.
Severity Levels
- A score between 4 and 7 is considered mildly abnormal, representing a low-risk range.
- A score of 8 to 12 is moderately abnormal, placing the patient in an intermediate-risk group.
- A score of 13 or higher is classified as severely abnormal, indicating an extensive perfusion abnormality.
This high score suggests a substantial burden of disease and is associated with the highest risk for future cardiac events. The numerical cutoffs provide a consistent framework for translating the scan’s visual findings into an actionable assessment of the patient’s cardiac health.
Clinical Implications and Prognosis
The SSS is a tool for establishing a patient’s prognosis and guiding subsequent medical decisions. A low SSS, particularly one in the normal to mildly abnormal range, correlates with an excellent long-term outlook and a low annual risk of a major cardiac event, such as a heart attack or cardiac death. This low-risk profile supports a treatment approach focused on medical management and lifestyle adjustments.
Conversely, a high SSS suggests a worse prognosis and an elevated risk of adverse cardiac events. Patients with a severely abnormal SSS often have a substantially higher annual cardiac event rate than those with a normal scan. This heightened risk drives physicians to consider more aggressive management strategies, including potential referral for invasive procedures like coronary angiography.
By providing a quantitative measure of the total amount of myocardium at risk, the SSS helps differentiate patients who can be safely managed with medication from those who may benefit from revascularization procedures, such as angioplasty or bypass surgery. The SSS is integral to risk stratification, serving as a reliable predictor of future cardiac health and the need for intervention.