Interpreting Stress Test and Myocardial Perfusion Imaging Results

1. The Exercise Stress Test (EST / TMT)

The exercise stress test is often the first-line investigation for patients with suspected CAD who are able to exercise and have a normal, interpretable baseline ECG. It's a non-invasive, widely available, and cost-effective tool.

The Principle and Procedure

The goal of an EST is to increase myocardial oxygen demand through exercise and observe for evidence of ischemia, which manifests as ECG changes, symptoms, or abnormal hemodynamic responses.

A key objective during the test is for the patient to reach at least 85% of their age-predicted maximum heart rate (MPHR), calculated as $220-\text{age}$. If a test is stopped before reaching this target and no ischemic changes are seen, it's considered nondiagnostic because the heart wasn't stressed enough to reliably provoke ischemia.

Interpreting the Results

A comprehensive interpretation of an EST involves more than just looking at the ECG. You must synthesize four key pieces of information.

1. ECG Changes: The Core of Diagnosis

The most important diagnostic finding is ST-segment depression.

• Positive for Ischemia: ≥ 1 mm of horizontal or downsloping ST-segment depression, measured 60-80 milliseconds after the J-point.
• Non-specific: Upsloping ST-segment depression is less specific for ischemia.
• ST-segment elevation (in leads without pre-existing Q waves) is a rare but ominous sign, suggesting severe, transmural ischemia.

2. Functional Capacity: A Powerful Prognostic Indicator

How much work a patient can do is one of the most powerful predictors of their long-term survival. This is measured in Metabolic Equivalents (METs).

• 1 MET = Oxygen consumption at rest.
• < 5 METs = Poor prognosis.

• 10 METs = Excellent prognosis, regardless of ECG changes.

3. Hemodynamic Response

• Blood Pressure: Systolic blood pressure (SBP) should rise with exercise. A drop in SBP > 10 mmHg (exercise-induced hypotension) is a high-risk feature, suggesting severe multivessel disease or left ventricular dysfunction.
• Heart Rate: Failure to achieve 85% of MPHR (chronotropic incompetence) or slow heart rate recovery after exercise are also poor prognostic signs.

4. Symptoms

The development of typical angina during the test, especially when accompanied by ECG changes, increases the likelihood of significant CAD.

Indications, Contraindications, and Limitations

It's crucial to know when to order an EST and when to choose a different test.

Key limitations to remember:

• The test is uninterpretable in patients with baseline ECG abnormalities like LBBB, ventricular paced rhythm, or significant resting ST depression.
• The patient must be able to exercise adequately.
• It has lower diagnostic accuracy compared to imaging tests, with a notable rate of false positives, especially in younger women.

2. Myocardial Perfusion Imaging (MPI)

When a standard EST is not an option or when greater diagnostic certainty is needed, we turn to MPI. This test adds a layer of imaging to the stress test, allowing us to directly see blood flow to the heart muscle.

The Principle: The Ischemic Cascade

MPI is more sensitive than EST because it detects an earlier step in the process of ischemia.

As the video explains, the ischemic cascade follows this order:

1. Perfusion Abnormality (detected by MPI)
2. Diastolic Dysfunction
3. Systolic Dysfunction / Wall Motion Abnormality (detected by Stress Echo)
4. ECG Changes (detected by EST)
5. Chest Pain (Angina)

How MPI is Performed

An MPI study has two key components: the stressor and the radiotracer.

1. The Stressor: Exercise vs. Pharmacologic

Just like with EST, exercise on a treadmill is the preferred stressor because it provides valuable prognostic information about functional capacity.

For patients who cannot exercise, we use pharmacologic stress agents:

• Vasodilators (Regadenoson, Adenosine): These are the most common. They dramatically increase blood flow in normal coronary arteries but have little effect on arteries narrowed by stenosis. This creates a flow disparity ("coronary steal") that the scan can detect.
• Inotropes (Dobutamine): This agent increases heart rate and contractility, mimicking the oxygen demand of exercise.

2. The Radiotracer: Technetium vs. Thallium

A radioactive tracer is injected at peak stress and again at rest. A gamma camera then creates images of the heart. The tracer is taken up by viable heart muscle cells in proportion to blood flow.

• Technetium-99m (sestamibi): The workhorse agent. It's a higher-energy photon that provides clearer images with less radiation dose. It gets "fixed" in the myocardium.
• Thallium-201: An older agent. It provides lower-quality images but has a unique property called redistribution. Over time, it will move from its initial location to any living myocardium, making it useful for assessing viability (distinguishing scar from hibernating myocardium).

Interpreting MPI Results

The interpretation hinges on comparing the stress images to the rest images.

SPECT MPI showing a moderate-sized, reversible perfusion defect in the apex and septum (arrows in A, C, E), consistent with ischemia in the LAD territory. The rest images (B, D, F) show normal perfusion, confirming reversibility.

• Normal Scan: Homogeneous tracer uptake at both rest and stress.
• Reversible Defect (Ischemia): A region with reduced tracer uptake on the stress images that "fills in" and looks normal on the rest images. This indicates a lack of blood flow under stress that is restored at rest.
• Fixed Defect (Infarct/Scar): A region with reduced tracer uptake that is present on both the stress and rest images. This indicates non-viable heart muscle.
• Transient Ischemic Dilation (TID): The left ventricle appears larger on the stress images than at rest. This is a sign of severe, extensive ischemia.

3. Clinical Implications and Test Selection

The results of stress testing have profound implications for patient prognosis and management.

Prognostic Power:

• A normal MPI scan is associated with a very low risk (<1% per year) of major cardiac events (death or MI). This is often referred to as a "warranty period" of 1-2 years.
• The risk of future events increases directly with the extent and severity of ischemia found on the scan. A large, severe reversible defect identifies a high-risk patient who may benefit from revascularization.

Which Test Should You Choose?

The selection of the right test depends on the patient's clinical characteristics and the pre-test probability of disease.

• Low-to-Intermediate Risk Patient, Able to Exercise, Interpretable ECG:

  • EST is an excellent and cost-effective first choice.

• Patient Unable to Exercise OR has an Uninterpretable Baseline ECG (e.g., LBBB, paced rhythm):

  • Pharmacologic MPI is the test of choice. (Stress echocardiography is another valid option here).

• Intermediate-to-High Risk Patient or Inconclusive Previous Test:

  • MPI (exercise or pharmacologic) is often preferred for its higher diagnostic accuracy and powerful prognostic information.

• Patient with Obesity:

  • PET-MPI is superior to SPECT-MPI due to better image quality and less attenuation artifact, though its availability is limited.

Conclusion

You have now learned how to interpret and apply two of the most important functional tests in cardiology. This knowledge allows you to move beyond anatomy and biomarkers to understand the physiological impact of coronary artery disease.

Key Takeaways:

• Exercise Stress Testing (EST) evaluates for ischemia based on ECG changes, functional capacity (METs), and hemodynamic response. Its main limitations are baseline ECG abnormalities and the patient's ability to exercise.
• Myocardial Perfusion Imaging (MPI) uses radiotracers to directly visualize blood flow. It is more sensitive than EST and can be performed with either exercise or pharmacologic stress.
• The key interpretation in MPI is differentiating a reversible defect (ischemia) from a fixed defect (infarct/scar).
• Both tests provide powerful prognostic information. A normal stress test (especially MPI) confers an excellent short-to-medium-term prognosis. The degree of abnormality directly correlates with future risk.
• Test selection is guided by the patient's ability to exercise and their baseline ECG.

In our last few lessons, we've built a robust diagnostic toolkit. You can interpret clinical findings, ECGs, chest X-rays, echos, biomarkers, and now, functional stress tests.

This lesson concludes the "Foundations in Clinical Cardiology and Diagnostics" module. You are now well-prepared to diagnose and risk-stratify patients. In our next lesson, we will begin the "Ischemic Heart Disease and Prevention" module. We will start at the very beginning by exploring the pathophysiology of atherosclerosis, the underlying disease process that these stress tests are designed to detect.