Learn · Part 1. Foundations · 4 min read
How stress testing works
Supply and demand, flow reserve, the ischemic cascade, and what positive and negative results do and do not mean.
60-second take
- Stress tests create or reveal a mismatch between what the heart needs and what the coronary arteries can deliver. Flow reserve is the margin the arteries have in hand.
- In the ischemic cascade, perfusion and diastolic changes come before wall motion changes. ECG changes and symptoms classically come later still.
- Imaging adds to the ECG alone: it is more accurate, shows which wall is affected, and still works when the resting ECG is hard to read.
- A positive result is not always a blocked artery, and a negative result only counts if the stress was adequate.
- Balanced ischemia across all vessels can be missed by relative perfusion imaging.
The basic logic: supply and demand
A narrowed coronary artery can often deliver enough blood at rest. The trouble starts when the heart has to work harder. During exercise the heart rate and stroke volume rise, and the muscle’s own energy needs rise with them. If coronary flow cannot rise enough to keep up, demand outruns what the arteries can supply. That supply-demand mismatch is ischemia . Every stress test is a controlled way of creating that mismatch, or of revealing that the arteries have little room to spare, while you watch for the result.
Coronary flow reserve in plain words
Coronary flow reserve is the margin the circulation keeps in hand: how much more blood it can send than it does at rest. PET measures this directly as the ratio of peak (hyperemic) myocardial blood flow to resting flow . The ASE guideline gives about 3 to 5 times the resting level as the flow reached with dipyridamole or adenosine . A territory behind a significant narrowing, or with diseased small vessels, has already used up part of that margin, so it cannot keep pace when demand climbs or when the vessels are opened. That is the difference a stress test looks for.
The ischemic cascade
Ischemia is not one event but a sequence. The stress echo guideline links the benefit of adding perfusion imaging to this sequence: perfusion abnormalities occur before wall motion abnormalities . Abnormal diastolic function may also appear before systolic wall motion abnormalities .
Earlier
Later
Wall motion changes arise through the inner layer of the muscle (the subendocardium), which contributes proportionately more to systolic thickening. Faster heart rates lower its flow relative to the outer layer, even without a narrowing. With a narrowing as well, subendocardial flow falls further and the segment thickens and moves less . In the classic order, ECG changes and chest discomfort come later still. The ASNC guideline simply notes that exercise-induced ischemia can show up as symptoms, ECG changes, or imaging findings .
Why imaging adds to the ECG alone
- Accuracy. Stress testing with imaging is more accurate than the exercise ECG . The chest pain guideline reports exercise ECG sensitivity and specificity in the range of 60% to 77%, lower than stress imaging, while noting that it still gives useful prognostic information .
- Location. ST depression does not localize ischemia or the affected artery . Imaging shows which wall is affected, because segments supplied by an obstructed artery move less than segments with normal supply .
- Interpretability. The stress ECG is nondiagnostic when baseline changes mask the ST segment, for example left ventricular hypertrophy with repolarization changes, baseline ST-T changes of 1 mm or more, left bundle branch block, ventricular pacing, or digoxin .
Example from the tool
Abnormal ET
See how the 2023 criteria handle the next step after an abnormal exercise treadmill test.
- Exercise ECGNot applicable
- Exercise SPECTAppropriate
- Pharmacologic SPECTAppropriate
- Pharmacologic PETAppropriate
- Exercise echoAppropriate
- Dobutamine echoAppropriate
- Stress CMRAppropriate
- CAC scoreMay Be Appropriate
- CCTAAppropriate
- Invasive angiographyAppropriate
- No testMay Be Appropriate
What “positive” and “negative” mean, and do not mean
| Test | Positive or ischemic response | Negative or normal response |
|---|---|---|
| Exercise ECG | Horizontal or downsloping ST depression of 1 mm or more, in three consecutive beats | ST depression under 1 mm, with adequate effort and no symptoms |
| Stress echo | A new wall motion abnormality in a segment that was normal at rest, or worsening in a segment already abnormal | Healthy segments become more vigorous than at rest |
Both words need care. The criteria use graded terms for imaging results, such as normal, mild ischemia and moderate to severe ischemia, and also an inconclusive category.
- Positive does not always mean a blocked artery. Wall motion abnormalities can occur without significant stenosis, from microvascular disease, endothelial dysfunction, vasospasm, small vessel disease, amyloid deposits or apical ballooning. Patients with false-positive stress echoes had outcomes similar to those with true-positive results, so they still deserve risk factor management and follow-up .
- Negative only counts if the stress was adequate. Failing to reach at least 80% of the predicted workload may reduce sensitivity , and a shortfall in heart rate is one way a stress study becomes inconclusive . The next lesson covers adequacy.
Balanced ischemia
If all the main arteries are narrowed to a similar degree, flow may fall evenly across the heart. Nuclear perfusion imaging compares relative differences between regions and can miss ischemia that is balanced or global, while stress echo has greater sensitivity for left main or multivessel disease . The ASNC PET statement notes that quantifying myocardial blood flow addresses the concern of false-negative perfusion in balanced ischemia .
With continuous imaging during bicycle or pharmacologic stress, the heart rate at which wall motion abnormalities first appear is called the ischemic threshold. An abnormality at a low heart rate or low rate-pressure product usually indicates severe stenosis or multivessel disease .
In a moderate narrowing with some flow reserve left, function can improve at low-level stress and then worsen at high-level stress as tachycardia lowers subendocardial flow. Spotting this biphasic response improves sensitivity .
In the echo lab, coronary flow reserve can be estimated with Doppler velocity in the mid-distal left anterior descending artery during pharmacologic stress, most often with a vasodilator . With PET, the same idea is reported as myocardial blood flow reserve .
See how the criteria treat an abnormal exercise test →, or build your own patient scenario →.
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