LOS ANGELES — Nuclear cardiology scans performed in the emergency department can not only reduce admission rates by ruling out acute coronary syndromes, but also sound the alert on patients who warrant further testing despite normal laboratory tests, ECGs, and even coronary angioplasty.
“You need to detect those unstable anginas. Otherwise, you'll have a lot of potential events walking out your door,” Jack A. Ziffer, M.D., medical director of nuclear cardiology for Baptist Cardiac and Vascular Institute, Miami, and Baptist Hospital of Miami, said at a meeting sponsored by the American College of Cardiology.
Other acute coronary syndromes can be missed as well, even by angiography, which misses 3% of MIs and fails to diagnose significant pathology in a third of patients with elevated troponin levels, Dr. Ziffer said at the meeting, cosponsored by the American Society of Nuclear Cardiology and Cedars-Sinai Medical Center.
Spontaneous thrombolysis, prolonged vasospasm, injury location and patient anatomy, and misinterpretation may lead to false-negative test outcomes that might result in a patient being inappropriately discharged. Nuclear cardiology offers a quick, comprehensive look at perfusion and function that can be very helpful in clarifying diagnoses.
Dr. Ziffer described his emergency department (ED) protocol, which has been adopted by Cedars-Sinai and other medical centers. Any patient who presents with symptoms suggestive of a suspected acute coronary syndrome is injected with radionuclide in the ED in preparation for a nuclear scan. A thorough history is taken, laboratory values are assessed, and an ECG and angiogram are performed. Treatment is started based on severity of symptoms and test results. Meanwhile, nuclear imaging is performed after about 30-45 minutes, once some hepatic clearance has been achieved.
Two paradigms direct the myocardial perfusion and function studies ordered for the patient:
▸ In a patient with ongoing chest pain: A rest MIBI (99mTc sestamibi) with gated single-photon emission computed tomography (gSPECT) scan is performed. If this test is normal, along with all other testing, the patient is sent home. If questions are raised, a stress MIBI test is ordered and/or the patient is admitted to the coronary care unit.
▸ In a patient whose pain has resolved: A rest thallium SPECT test is performed, and if questions arise, a stress MIBI gSPECT test follows, with results determining whether a patient will be admitted or can go home.
MRI can be helpful, but it cannot determine the age of an infarct and poses a practical challenge, since few MRI units operate in the middle of the night. Nuclear imaging “is straightforward and can be delivered 24/7,” Dr. Ziffer said.
Even an angiogram, which details anatomy, “doesn't necessarily tell you what you need to know,” he said. “Coronary arteries can be patent and provide blood to nonviable myocardium. They can be occluded and have perfectly normal myocardium.”
Perfusion, ejection fraction, wall motion, and wall thickening are all clues to underlying abnormalities that can be assessed with nuclear studies. Polar maps, for example, quantify wall thickness throughout the heart, which can be helpful when wall motion is not clear. “These are really very powerful tools. When you see 8% thickening and 19%, that's less than normal. You ought to see 30% thickening in normal myocardium, or 40%,” he said.
Wall motion is another adjunctive clue within scans performed in the acute setting. “If a defect is present, we ask the question, 'Is wall motion normal or abnormal?' Normal wall motion does not mean that it's an artifact,” he said. It may mean the patient has abnormal perfusion, but that the treatment initiated in the ED was effective in treating unstable angina, or that the patient has suffered a small infarct in an area where wall motion cannot be seen.
When wall motion is abnormal, the patient is more likely to have unstable angina and persistent stunning, or an infarct “age unknown; it could be 30 years ago, could be 2 minutes ago, or impending,” he said.
Dr. Ziffer stressed that interpretation of nuclear studies requires an understanding of the impact of the delay between injecting and imaging. Perfusion parameters reflect the situation at the time the patient was injected, while cardiac function parameters are assessed in real time, while the patient is under the camera. “Sometimes the perfusion abnormalities we see may not reflect, in patients with resolved pain, for example, the entire jeopardized area.”
He described the case of an obese 63-year-old woman with hypertension, with now-resolved chest pain, whose resting scan was normal. Her ejection fraction was 78% and her wall motion parameters were in the normal range. Interpretation of her stress test was complicated by “tremendous breast attenuation” that obscured a significant portion of the heart. The scan should have been repeated with the patient in the prone position, but the patient was discharged. She arrived back in the ER 3 days later with a very large infarct.