Stress Testing and Non Invasive Imaging

Introduction

Ischemic heart disease (IHD), also known as coronary artery disease (CAD), remains one of the leading causes of morbidity and mortality worldwide. Early and accurate diagnosis is critical to prevent complications such as myocardial infarction, arrhythmias, and heart failure. While invasive coronary angiography is considered the gold standard for evaluating coronary artery stenosis, it is not always the first-line choice due to its cost, invasiveness, and associated risks.

Instead, non-invasive stress testing and imaging modalities play a crucial role in both the initial evaluation and long-term management of patients with suspected or established IHD. These tests help clinicians assess not only the presence of ischemia but also its functional significance, myocardial perfusion, and structural impact on the heart.

In this post, we will explore four major diagnostic strategies:

1. Exercise electrocardiography (treadmill test)
2. Pharmacologic stress testing
3. Echocardiography and myocardial perfusion imaging
4. Coronary computed tomography angiography (CCTA)

---

1. Exercise ECG (Treadmill Test)

Principle and Rationale

The exercise treadmill test (ETT) is one of the most widely used initial diagnostic tests for IHD. It evaluates the heart’s response to physical exertion by monitoring ECG changes, heart rate, blood pressure, and clinical symptoms. The test is based on the principle that exercise increases myocardial oxygen demand. If coronary arteries are narrowed, ischemia will be unmasked during exertion, manifesting as ST-segment changes, arrhythmias, or angina.

Procedure

• Conducted on a treadmill or cycle ergometer.
• Follows standardized protocols, such as the Bruce protocol, which gradually increases workload by adjusting speed and incline.
• Continuous ECG monitoring with 12 leads.
• Blood pressure and symptoms assessed at each stage.
• The test is stopped upon reaching target heart rate, significant ECG changes, or intolerable symptoms.

Interpretation

• Positive test: ≥1 mm horizontal or downsloping ST depression/elevation in two contiguous leads.
• Negative test: No ischemic changes with adequate workload achieved.
• Equivocal: Submaximal heart rate achieved or inconclusive results.

Advantages

• Widely available, low cost.
• Provides prognostic information (exercise capacity, hemodynamic response).
• Non-invasive and easy to perform.

Limitations

• Requires patient ability to exercise.
• Sensitivity: ~65–70%; Specificity: ~75–80%.
• False positives (e.g., women, baseline ST-T abnormalities, LVH).
• Does not localize ischemia.

Clinical Applications

• First-line test in low-to-intermediate risk patients with suspected CAD.
• Useful in follow-up of patients with stable angina.
• Prognostic assessment after myocardial infarction.

---

2. Pharmacologic Stress Testing

Need for Pharmacologic Stress

Some patients cannot perform adequate exercise due to comorbidities like arthritis, obesity, or pulmonary disease. For these patients, pharmacologic stress testing is used, where drugs simulate the cardiovascular effects of exercise.

Common Agents

1. Vasodilators:
   • Adenosine, Dipyridamole, Regadenoson
   • Induce coronary vasodilation. Normal vessels dilate maximally, while stenotic arteries show limited response → perfusion mismatch.
2. Inotropes/Chronotropes:
   • Dobutamine
   • Increases heart rate, contractility, and oxygen demand, mimicking exercise.

Monitoring

• Usually combined with imaging modalities (nuclear perfusion imaging or echocardiography).
• ECG, BP, and symptoms are closely monitored.

Advantages

• Suitable for patients unable to exercise.
• Allows combination with imaging for improved sensitivity and specificity.
• Provides prognostic information similar to exercise stress testing.

Limitations

• More expensive than treadmill ECG.
• Contraindications: severe asthma (adenosine), uncontrolled arrhythmias (dobutamine).
• Side effects: flushing, chest pain, headache, hypotension.

Clinical Applications

• Alternative for patients unable to undergo treadmill test.
• Commonly used in nuclear myocardial perfusion imaging and stress echocardiography.

---

3. Echocardiography and Myocardial Perfusion Imaging

Stress Echocardiography

• Combines stress testing (exercise or pharmacologic) with ultrasound imaging of the heart.
• Ischemia leads to regional wall motion abnormalities (RWMA) before ECG changes or symptoms.
• Rest vs stress images compared for assessment.

Advantages

• No radiation exposure.
• Detects ischemia, viability, and LV function.
• Higher sensitivity and specificity compared to treadmill ECG.

Limitations

• Operator-dependent.
• Poor acoustic windows in obese or COPD patients.

Myocardial Perfusion Imaging (MPI)

• Uses nuclear tracers (e.g., Thallium-201, Technetium-99m) to visualize blood flow.
• Performed with SPECT or PET imaging.
• Shows areas of reduced perfusion during stress compared to rest.

Advantages

• Provides perfusion and viability data.
• Quantitative and highly sensitive.
• Can detect multi-vessel disease.

Limitations

• Radiation exposure.
• Expensive and not universally available.

Clinical Applications

• Stress echocardiography: patients with equivocal treadmill ECG.
• MPI: intermediate-to-high-risk patients, pre-revascularization evaluation.

---

4. Coronary CT Angiography (CCTA)

Principle

CCTA uses multidetector CT scanning with contrast injection to obtain high-resolution images of coronary arteries. Unlike stress testing, it provides anatomical rather than functional information.

Capabilities

• Detects coronary artery stenosis and plaque characteristics.
• Evaluates both calcified and non-calcified plaques.
• Useful in ruling out CAD in low-to-intermediate risk patients.

Advantages

• Non-invasive, rapid, and highly sensitive.
• Excellent negative predictive value → rules out CAD.
• Provides additional information: coronary calcium scoring, plaque vulnerability.

Limitations

• Radiation exposure (though reduced with modern scanners).
• Requires iodinated contrast (contraindicated in renal dysfunction, contrast allergy).
• Limited accuracy in severe calcification or high heart rates.

Clinical Applications

• Low-to-intermediate risk patients with chest pain.
• Alternative to invasive angiography when probability of CAD is uncertain.
• Useful in emergency settings for ruling out acute coronary syndrome.

---

Comparative Summary

Modality	Primary Use	Advantages	Limitations
Exercise ECG	First-line for low-to-intermediate risk	Cheap, widely available	Lower sensitivity, requires exercise ability
Pharmacologic Stress	Patients unable to exercise	Can combine with imaging	Side effects, contraindications
Stress Echo	Ischemia detection via wall motion	No radiation, functional info	Operator-dependent
MPI (SPECT/PET)	Perfusion and viability	High sensitivity, quantitative	Radiation, cost
CCTA	Coronary anatomy and plaque	Excellent negative predictive value	Contrast + radiation