Cardiac Scintigraphy

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How do you prepare a patient for stress myocardial perfusion imaging (SMPI)?

Patients should be informed that the test takes approximately one half day and consists of two parts: a resting part and a stress part. The placement of an intravenous catheter (usually inserted at the beginning of the study) is required to administer the radiotracer. The order of the study can be either rest-stress or stress-rest, although the higher dose is given for the second scan, and the stress test is often performed second to provide the best image quality. Single photon emission computed tomography (SPECT) or positron emission tomography (PET) scans are obtained approximately 30 to 60 minutes after radiotracer administration, and each scan takes approximately 15 to 30 minutes. The patient should not eat or drink (except water) from midnight before the examination. Patients are commonly asked to stop taking their medications unless there is some reason (e.g., very high blood pressure or serious dysrhythmias) that requires the stress test to be performed while the patient continues to take medications.

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What are the different types of radiotracers available for stress testing?

There are three basic categories: thallium-201 ( ²⁰¹ Tl) and technetium-99m ( ^99m Tc)-labeled compounds for SPECT, and positron emitting compounds for PET. Table 82-1 compares ²⁰¹ Tl and ^99m Tc-labeled compounds. The most commonly used ^99m Tc-labeled compounds are tetrofosmin and sestamibi, which have similar imaging and chemical characteristics. ^99m Tc-labeled compounds generally provide more flexibility in terms of imaging times. Because of the photon energy and the number of photons emitted, ^99m Tc-labeled compounds are generally easier to use for image acquisition and analysis. PET radiotracers, such as rubidium-82 ( ⁸² Rb), nitrogen-13 ( ¹³ N) ammonia, and fluorine-18 ( ¹⁸ F) fluorodeoxyglucose (FDG), can be used to evaluate myocardial perfusion and glucose metabolism, respectively. ¹³ N ammonia has a short (20 minute) half-life, so there are some logistical limitations in using this for PET cardiac imaging.

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What are the three types of stress tests?

The three basic types are exercise, pharmacologic vasodilatory, and adrenergic agonist stress tests.

• Exercise stress tests are typically performed using a treadmill with several possible protocols that sequentially increase the speed and grade of the treadmill. An alternative exercise test uses a stationary bicycle. The goal is to increase heart rate to 85% of the patient's age-predicted maximum, although others have used a combination of heart rate and blood pressure as a target.

• Pharmacologic vasodilatory stress tests are performed with adenosine itself, dipyrimadole (an inhibitor of adenosine deaminase that results in increased adenosine within the epicardial coronary arteries), or regadenoson (an A _2A adenosine receptor agonist). Upon injection of these agents, normal coronary arteries will dilate, whereas stenosed arteries will not. The relative discrepancy in myocardial blood flow results in a perfusion defect to the myocardial wall supplied by the stenosed vessel.

• Adrenergic agonist stress tests are performed using an adrenergic agonist (dobutamine) with which inotropic and chronotropic stimulation results in increased heart rate and contractility simulating exercise.

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What are contraindications for an exercise stress test?

Any patient with significant musculoskeletal problems that would prevent him or her from achieving a maximal heart rate should undergo pharmacologic stress testing. Patients with certain ongoing dysrhythmias, such as ventricular tachycardia, supraventricular tachycardia, new-onset atrial fibrillation, or heart block, should receive treatment for those dysrhythmias before the stress test is rescheduled. Known atrial fibrillation that is adequately treated is not a contraindication. Patients with severe pulmonary disease, severe hypertension (i.e., systolic blood pressure >210 mm Hg and diastolic blood pressure >110 mm Hg), abdominal aortic aneurysms, symptomatic aortic stenosis, unstable angina, or active myocardial ischemia on electrocardiography (ECG) should also be excluded.

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What are the contraindications for pharmacologic vasodilatory and adrenergic agonist stress tests?

Pharmacologic vasodilatory stress tests are not performed in patients with severe reactive airway disease, particularly when a patient has active wheezing on physical examination. Patients who use inhalers but who otherwise have stable airway disease can often safely undergo these stress tests. Any patient taking dipyrimadole or methylxanthines does not undergo a vasodilatory stress test, unless the medication can be stopped for at least 24 to 48 hours. All patients who are to undergo vasodilatory studies should abstain from caffeine for 24 hours because caffeine blocks the effect of vasodilatory compounds and adenosine.

Adrenergic stress tests have similar contraindications to those of exercise testing except that musculoskeletal problems are not an issue. Furthermore, dobutamine can be used in patients with existing airway disease.

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Should patients taking medications that can affect the heart be given stress tests?

Generally, any patient can undergo a cardiac stress test. In patients taking medications for blood pressure, such as β-blockers or calcium channel blockers, the medication may prevent the patient from achieving the necessary heart rate. In such patients, it is recommended that the medications be stopped for approximately 24 hours before the study, unless the ordering physician wants to determine the presence or absence of inducible ischemia while the patient is taking these medications. If a patient has severe hypertension, the patient may need to be tested while taking the medication for this condition because without the medication, the blood pressure would be too high to proceed safely with the study.

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