Residency · Residency · Nuclear Medicine

Myocardial Perfusion Imaging: Stress Protocols

Rationale for Stress Testing

Physiologic Basis

At rest, coronary blood flow is tightly autoregulated, and perfusion to the myocardium may appear entirely normal even in the presence of a significant coronary stenosis. The purpose of stress testing is to expose this hidden limitation. Coronary stenoses restrict the artery's ability to increase flow during periods of high demand -- what is termed a reduced coronary flow reserve. Stress testing unmasks perfusion deficits either by increasing myocardial oxygen demand (exercise, dobutamine) or by inducing direct coronary vasodilation (pharmacologic vasodilators). A hemodynamically significant stenosis, typically defined as greater than 70% luminal narrowing, limits the ability of the artery to augment flow during stress. A perfusion defect seen on stress images that normalizes on rest images represents ischemia, meaning viable but underperfused myocardium. A defect that persists on both stress and rest images represents scar or infarction.

Coronary Flow Reserve

Normal coronary arteries can increase blood flow 3 to 5 times above resting levels during maximal stress. Stenotic arteries have a diminished capacity to augment flow. Pharmacologic vasodilators exploit this difference by maximally dilating normal coronary vessels while stenotic segments cannot respond proportionally, creating relative hypoperfusion in the territory supplied by the diseased vessel -- a phenomenon known as coronary steal.

Exercise Stress

Treadmill Exercise (Bruce Protocol)

Exercise stress on a treadmill is the gold standard when the patient is physically able to exercise adequately. The Bruce protocol involves progressive increases in treadmill speed and incline every 3 minutes. A modified Bruce protocol starts at a lower workload and is used for deconditioned patients. The target is to achieve at least 85% of the age-predicted maximum heart rate, calculated as 220 minus the patient's age. An adequate exercise test is defined by achieving both the target heart rate and exercising for a sufficient duration.

Hemodynamic Response

During a normal exercise test, heart rate, systolic blood pressure, and the rate-pressure product (heart rate multiplied by systolic blood pressure) all rise progressively. The rate-pressure product correlates with myocardial oxygen consumption, and a target of at least 25,000 indicates adequate stress. Several abnormal hemodynamic responses carry important clinical significance. Exercise-induced hypotension -- a drop in systolic blood pressure of more than 10 mmHg during exercise -- is a high-risk finding suggesting severe coronary artery disease, including possible multivessel or left main involvement. Chronotropic incompetence, defined as failure to reach 80% of the age-predicted maximum heart rate, is another adverse prognostic marker. ST-segment depression greater than 1 mm that is horizontal or downsloping, measured at 80 ms after the J-point, is the electrocardiographic hallmark of exercise-induced ischemia.

Radiotracer Injection During Exercise

The radiotracer is injected at peak exercise, at or near the target heart rate, and the patient continues exercising for 1 to 2 minutes afterward to allow first-pass myocardial extraction of the tracer during the hyperemic state. For Tc-99m-based agents, first-pass extraction is approximately 65% at rest but decreases at higher flow rates -- a phenomenon known as extraction roll-off. Exercise imaging provides both perfusion data and prognostically valuable exercise hemodynamic and ECG information.

Contraindications to Exercise Stress

Absolute contraindications include acute myocardial infarction within 2 to 3 days, unstable angina that has not been stabilized, uncontrolled symptomatic heart failure, severe aortic stenosis, uncontrolled hypertension (systolic above 200 or diastolic above 110 mmHg), acute pulmonary embolism or aortic dissection, and severe physical disability precluding adequate exercise.

Indications for Pharmacologic Stress

Pharmacologic stress is chosen when the patient cannot exercise adequately due to orthopedic, neurologic, or pulmonary limitations, when the patient cannot achieve the target heart rate, or when baseline ECG abnormalities -- left bundle branch block (LBBB), Wolff-Parkinson-White pattern, or left ventricular hypertrophy with repolarization abnormalities -- limit the diagnostic value of exercise ECG. Patients with LBBB or ventricular paced rhythms should specifically receive vasodilator stress rather than exercise, because exercise-induced tachycardia can produce a paradoxical septal perfusion artifact that mimics LAD territory ischemia.

Pharmacologic Vasodilator Stress

Regadenoson (Lexiscan)

Regadenoson is a selective A2A adenosine receptor agonist and is currently the most widely used pharmacologic stress agent in North America. It is administered as a fixed-dose 0.4 mg IV bolus over 10 to 15 seconds, regardless of patient weight -- a feature that simplifies administration. The onset of vasodilation occurs within 30 seconds, with peak effect at 1 to 4 minutes, and the duration of action is approximately 2 to 3 minutes, though hemodynamic effects may persist somewhat longer. The radiotracer is injected 10 to 20 seconds after the regadenoson bolus. Aminophylline (100 to 200 mg IV) is available as a reversal agent for persistent side effects. Compared to older vasodilator agents, regadenoson is better tolerated and requires a shorter protocol.

Dipyridamole (Persantine)

Dipyridamole is an indirect adenosine receptor agonist that works by inhibiting adenosine reuptake and metabolism, thereby increasing endogenous adenosine levels at the receptor. The dose is 0.56 mg/kg IV administered over 4 minutes, with radiotracer injection 3 to 5 minutes after completion of the dipyridamole infusion. It has a longer duration of action than regadenoson, and aminophylline is available for reversal. Dipyridamole has been largely replaced by regadenoson in clinical practice.

Adenosine

Adenosine is a direct, non-selective adenosine receptor agonist infused at 140 mcg/kg/min for 6 minutes, with the radiotracer injected at the 3-minute mark. Its extremely short half-life of less than 10 seconds means that effects cease rapidly when the infusion is stopped. However, it produces more side effects than regadenoson, including flushing, chest pain, AV block, and bronchospasm, and it is used less frequently now.

Mechanism of Vasodilator Stress

All vasodilator stress agents work through A2A receptor activation, which causes smooth muscle relaxation in coronary arterioles. Normal coronary arteries dilate maximally, increasing flow 3 to 5 times above baseline, while stenotic segments cannot vasodilate proportionally. This creates flow heterogeneity that is detected by the perfusion tracer: the territory supplied by the stenotic vessel appears hypoperfused relative to normally perfused regions. Unlike exercise or dobutamine, vasodilator stress does not depend on increasing myocardial oxygen demand.

Side Effects of Vasodilator Agents

Common side effects include flushing, chest discomfort (which is not necessarily ischemic), headache, dyspnea, and gastrointestinal symptoms. All adenosine-based agents can cause transient AV block, usually first-degree or Wenckebach. Bronchospasm can occur through activation of A2B and A3 receptors. Mild, transient hypotension is also common.

Contraindications to Vasodilator Stress

Absolute contraindications include active bronchospasm or severe reactive airway disease, second- or third-degree AV block without a pacemaker, systolic blood pressure below 90 mmHg, and acute coronary syndrome. Moderate asthma or COPD is a relative contraindication, though regadenoson may be used cautiously in patients with mild to moderate asthma. Caffeine intake is an important consideration: caffeine is a competitive antagonist at adenosine receptors that attenuates the vasodilatory response, and must be withheld for at least 12 hours (some protocols require 24 hours) before vasodilator stress testing.

Dobutamine Stress

Indications

Dobutamine stress is reserved for situations in which both exercise and vasodilator stress are contraindicated -- most commonly when bronchospasm precludes the use of vasodilator agents and the patient cannot exercise. It is used less commonly overall.

Protocol

Dobutamine is infused starting at 10 mcg/kg/min and increased every 3 minutes up to 40 mcg/kg/min, with the target being at least 85% of the age-predicted maximum heart rate. If the target is not reached with dobutamine alone, atropine (0.25 to 1 mg IV) may be added. The radiotracer is injected at peak heart rate. Like exercise, dobutamine increases heart rate, contractility, and myocardial oxygen demand.

Side Effects

Side effects include palpitations, chest pain, and arrhythmias (premature ventricular contractions, ventricular tachycardia), as well as hypertension. Dobutamine is contraindicated in patients with severe hypertension, unstable arrhythmias, or hypertrophic obstructive cardiomyopathy.

Exercise vs. Pharmacologic Stress: Key Differences

Exercise stress increases myocardial oxygen demand and provides diagnostically useful ECG data and functional capacity assessment. Vasodilator stress works through coronary vasodilation rather than increased demand, provides limited ECG diagnostic information, and requires caffeine restriction. Dobutamine, like exercise, works through increased oxygen demand but does not assess functional capacity. For patients with LBBB or ventricular paced rhythms, vasodilator stress is specifically preferred because exercise-induced tachycardia can produce septal artifacts that mimic LAD disease.

<image>A flowchart for selecting the appropriate stress modality for myocardial perfusion imaging. Start with the question: Can the patient exercise adequately? If yes, use exercise stress (Bruce or modified Bruce protocol). If no, evaluate for vasodilator contraindications (bronchospasm, high-grade AV block, caffeine intake). If no contraindications, use vasodilator stress (regadenoson preferred). If vasodilator contraindicated, use dobutamine stress. Include special considerations: LBBB or paced rhythm pointing to vasodilator stress regardless of exercise ability.</image>

<image>A physiologic diagram illustrating the coronary steal phenomenon during vasodilator stress. Show two coronary arteries: one normal and one with a significant stenosis. At rest, both territories have adequate perfusion. During vasodilator-induced hyperemia, the normal artery dilates maximally with 3-5x flow increase while the stenotic artery cannot augment flow. Show the resulting flow heterogeneity and how the perfusion tracer distributes preferentially to the normal territory, creating a relative perfusion defect in the stenotic territory on imaging.</image>

<image>An ECG and hemodynamic response diagram during treadmill exercise stress. Show progressive increase in heart rate, systolic blood pressure, and rate-pressure product across Bruce protocol stages. Illustrate normal and abnormal responses: normal progressive rise in SBP, versus exercise-induced hypotension (drop in SBP), and ST-segment depression patterns (horizontal and downsloping depression at 80 ms post-J point).</image>

Clinical Pearls

Exercise stress is always preferred when the patient can exercise adequately, because it provides perfusion data along with prognostically valuable functional capacity and ECG information that pharmacologic stress cannot offer.

Patients with LBBB or ventricular pacemakers should receive vasodilator stress, not exercise or dobutamine. Exercise-induced tachycardia causes paradoxical septal perfusion abnormalities in LBBB that mimic LAD disease, leading to false-positive results.

Caffeine must be withheld for at least 12 hours (ideally 24 hours) before vasodilator stress. Caffeine blocks adenosine receptors and can produce a false-negative study by attenuating the vasodilatory response.

Regadenoson is a fixed-dose agent (0.4 mg IV bolus) regardless of patient weight. This is unique among stress agents and simplifies administration, but practitioners must recognize that the same dose is given to a 50 kg patient and a 120 kg patient.

Exercise-induced hypotension -- a drop in systolic blood pressure during exercise -- is a high-risk finding suggesting severe multivessel or left main coronary artery disease. The test should be stopped and the patient managed appropriately.

Aminophylline is the reversal agent for all adenosine-based vasodilator stress agents and should be readily available during any vasodilator stress test.

The radiotracer must be injected at peak stress -- at peak exercise or immediately after the regadenoson bolus -- and stress should continue for 1 to 2 minutes afterward to ensure adequate myocardial extraction during the hyperemic state.

References

• Henzlova MJ, et al. ASNC imaging guidelines for SPECT nuclear cardiology procedures. J Nucl Cardiol. 2016;23(3):606-639.
• Dilsizian V, et al. ASNC imaging guidelines/SNMMI procedure standard for positron emission tomography nuclear cardiology procedures. J Nucl Cardiol. 2016;23(5):1187-1226.
• Iskandrian AE, Garcia EV, eds. Nuclear Cardiac Imaging: Principles and Applications. 5th ed. Oxford University Press; 2016.
• Dorbala S, et al. Single photon emission computed tomography myocardial perfusion imaging guidelines. J Nucl Cardiol. 2018;25(5):1784-1846.

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