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Nuclear Cardiology Stress Tests for Coronary Artery Disease
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- Exercise ECG without imaging should be the first choice for
evaluation of coronary artery disease (CAD) when patient can exercise
on a treadmill to 85% of maximum predicted heart rate and
- resting ECG is normal or has minor abnormalities
- unstable myocardial infarction (MI) has been ruled out
- risk of adverse effects from exercise is low to intermediate
- Cardiac stress myocardial perfusion SPECT is indicated:
- when baseline ECG suggests that exercise ECG would be nondiagnostic or uninterpretable for ischemia
- when clinical presentation and exercise ECG indicates at least an intermediate likelihood of CAD
- for preoperative evaluation of those with known high coronary risk factors
- for management of known cardiac disease
- Physical exercise stress myocardial perfusion SPECT is
recommended if patient can exercise to 85% of maximum predicted heart
rate; otherwise pharmacological stress myocardial perfusion SPECT
should be ordered
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When
patients present with chest pain, clinical examination and patient
history are fundamental to determine the probable cause of pain and the
selection of appropriate tests to confirm clinical suspicions. In the
case of myocardial ischemia due to coronary artery disease (CAD), the
patient often but not always presents with the classical symptoms,
chest tightness and left arm pain. However, cardiac symptoms may be
similar to indigestion, muscle spasm, or other non-specific complaints.
Nevertheless, other causes of chest pain that do not have cardiac
origin, including pulmonary embolism, pneumothorax and aortic
dissection must always be considered.
If CAD is
suspected after these initial examinations and MI or severe unstable
angina has been ruled out along with other dangerous but non cardiac
causes of chest pain, the next step is exercise ECG (exercise treadmill
test, ETT), provided that the patient is able to exercise well enough
to reach 85% of maximum predicted heart rate (HR; 200-age is 100% of
the patient maximum predicted HR) and has a normal resting ECG or has
only minor ECG abnormalities that will not interfere with
interpretation of the exercise ECG (See Box 2).
Myocardial Perfusion Imaging SPECT
(MPI SPECT)
Although nuclear cardiology is both time consuming and expensive, MPI
SPECT is valuable for the clarification of a suspected false positive
(or negative) exercise ECG. It is also a valuable test for patients
with an intermediate likelihood of coronary artery disease if the
patient is unable or unwilling to perform an exercise ECG. MPI SPECT
has major utility to define the extent and location of disease,
determine the physiological significance of known coronary stenoses,
evaluate a particular vascular territory, and to risk stratify a
patient with known or suspected CAD once an unstable acute coronary
syndrome has been excluded. Therefore, MPI SPECT is well established
for prognostic evaluation of these patients.
MPI SPECT is also valuable for the management of patients with cardiac
disease, for example, to assess viability and to guide the selection of
by-pass graft or |
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(Box 1) Contraindications for Nuclear Cardiology
Stress Test
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Absolute |
| Acute myocardial infarction |
| Severe aortic stenosis |
| Severe reaction to stress agent |
| Severe pulmonary hypertension |
| Obstructive hypertrophic cardiomyopathy |
Combination of low EF (20%) and
--documented recent VF/VT |
| Cocaine within 24 hours |
| Pregnancy |
| Unstable angina |
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Relative |
| Severe Mitral stenosis |
| Hypertension (>180/100 mm Hg) |
| Hypotension (<90 mm Hg, systolic) |
| Tachycardia (>120/min) |
| Wheezing, bronchospasm |
| Unable to communicate |
Acute illness such as pericarditis, pulmonary
--embolus, infection, fever |
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PCI (percutaneous coronary intervention) region. In addition, the left
ventricular ejection fraction, which is obtained as an integral part of
the MPI SPECT study, is an important determinant of long-term prognosis
and can be used to evaluate symptoms such as shortness-of-breath,
fatigue, and poor exercise tolerance.
Nuclear cardiology images show the distribution of a radioactive agent, such as 99mTc-Sestamibi.
Since the amount of radioactivity is roughly proportional to regional
blood flow, nuclear cardiology images show variations in blood flow in
the myocardial regions that are under-perfused relative to other
regions and may be the cause of myocardial ischemia. Additional
information on cardiac function, such as ventricular motion and
ejection fraction, can be obtained when cardiac gating is used to
synchronize image acquisition to the cardiac cycle.
In most cases, blood flow is adequate in the resting state but not
during exercise or physiological stress because diseased blood vessels
are unable to dilate further in response to increased energy demand.
Therefore, nuclear cardiology images are acquired both under stress and
rest conditions to unmask the reserve capacity of the coronary blood
vessels. However, resting SPECT MPI may be ordered as a separate
stand-alone study (i.e. rest gated MIBI) to assess left ventricular
ejection fraction, regional contraction and myocardial viability (not
for ischemia).
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| MPI SPECT findings consistent with inferior and inferolateral ischemia with an incomplete infarction in these areas. |
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(Box 2) Recommended Tests for Intermediate or High Likelihood of CAD
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I. Patient Able to Exercise (to at least 85% maximum predicted heart rate),
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| Normal or mildly abnormal resting ECG |
| Not taking digoxin |
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Stress ECG |
Abnormal ECG that interferes with interpretation of
--stress ECG: |
| Ventricular pre-excitation |
| >1 mm ST depression |
| Left ventricular hypertrophy |
To evaluate physiological significance of coronary
--stenoses (25-75%) |
| Intermediate Duke treadmill score |
| High risk patients (>20% of 10 yr risk of cardiac event) |
Selected high risk asymptomatic patients, 3-5 yrs after
--revascularization |
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Resting and exercise myocardial perfusion SPECT
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| Cardiac pacemaker |
| Left bundle branch block |
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Resting and adenosine or dipyridamole stress myocardial
--perfusion SPECT
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II. Patient Unable to Exercise
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| To evaluate extent, location, and severity of ischemia |
To evaluate physiological significance of coronary
--stenoses (25-75%) |
Known CAD, change in symptoms suggest increased
--likelihood of cardiac event |
| High risk patients (>20% 10 yr risk of cardiac event) |
Selected high risk asymptomatic patients, 3-5 yrs after
--revascularization |
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Resting and adenosine or dipyridamole stress myocardial
--perfusion SPECT |
| Severe COPD, recent (<1 month) asthma attack |
| 2nd and 3rd degree of AV block |
| Severe bradycardia <40/min |
| Adenosine and dipyridamole contraindicated |
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Resting and dobutamine stress myocardial perfusion SPECT |
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III. Indications for PET
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| SPECT study equivocal |
| Obesity (>400 lbs) |
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Resting and adenosine or dipyridamole or dobutamine
--stress myocardial perfusion PET
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Stress Myocardial Perfusion Imaging SPECT
If
the patient is able to exercise to at least 85% predicted maximum heart
rate, an exercise stress MPI SPECT should be performed.
If the patient is unable to exercise, a pharmacological stress MPI
SPECT can be performed by the injection of agents (adenosine,
dipyridamole) that cause vasodilatation or mimic exercise
pharmacologically (dobutamine). Both exercise and pharmacological
stress tests have equal diagnostic accuracy and physiological
pharmacological stress is fast, safe, and reproducible.
Limitations of MPI SPECT
SPECT
myocardial imaging is subject to potentially misleading soft tissue
attenuation artifacts in people who are obese, women with large
breasts, and men with increased abdominal girth as in the Metabolic
Syndrome. Patient specific attenuation correction can be utilized but
may lower sensitivity. In addition, SPECT images only show relative
blood flow, not absolute flow, from the intensities of the regional
radioactivity. Therefore, it is possible to miss CAD if all three
branches of the coronary arteries are occluded and reduce the flow to a
similar extent. Overall, the test sensitivity is 87% and the
specificity 78%. |
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Patient Preparation
Since
ß-blockers slow the heart rate and the stress test is designed to
increase heart rate, it is preferable that ß-blockers be withdrawn
prior to cardiac stress myocardial perfusion SPECT to ensure that
arterial pressure increases appropriately with exercise if the test is
ordered for diagnostic purposes. The patient should be warned to reduce
his/her level of exertion and his/her physician must, of course,
determine that it is safe and appropriate to do so. Unfortunately,
sudden withdrawal of ß-blockers may cause a surge in blood pressure,
which could result in test cancellation. Therefore physicians should
withdraw them slowly and monitor blood pressure to be certain that the
patient can complete a stress test. Other anti-hypertensive medications
may be continued to avoid excess hypertension at the time of the test.
Anti-anginal agents may be continued with the referring physicians
approval. If physicians want to establish the effectiveness of
medication, they may continue the medication to evaluate the symptoms
and ischemic territory and size with stress test MPI SPECT.Patients
should not consume any caffeine or chocolate 24 hours prior to the test
and should not eat, drink, or smoke for 12 hours prior to the test.
Therefore, diabetics, in consultation with their PCP or
endocrinologist, should adjust their insulin and other anti-diabetic
medication accordingly.
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Test Procedure
The
procedure begins with an intravenous injection of radionuclide contrast
agent with the patient at rest. Twenty to thirty minutes later, when
the radionuclide is distributed throughout the body, the patient is
imaged with a gamma camera for a period of 20-30 minutes to obtain
baseline cardiac images. If an exercise stress test is to be done, the
patient will be taken to a treadmill and asked to exercise while their
ECG is monitored. One minute before the patient has reached his or her
limit or HR has reached 85% of the predicted maximum, a second dose of
radionuclide contrast is injected and exercise continued for one more
minute. After another 30 minute wait for the radionuclide to
equilibrate, the patient is imaged for a second time to obtain stress
myocardial images.
If a pharmacological agent is used
to induce coronary dilation, the second injection of radionuclide
contrast is injected at 10 minutes after injection of dipyridamole or 2
minutes into the infusion of intravenous adenosine. The stress images
are obtained after radionuclide equilibration, as above. The
total time for the procedure is 3 – 3.5 hours.
A radiologist and cardiologist will examine the series of SPECT images
in the short axis, the horizontal long axis, and the vertical long axis
to assess perfusion defects. In addition, the cardiac-gated SPECT
images may be reconstructed into cine loops, which are used to assess
regional ventricular wall motion, ejection fraction, and, indirectly,
myocardial viability.
Myocardial Perfusion Imaging PET
(MPI PET)
MPI PET can be performed when MPI SPECT is equivocal or likely to be
uninterpretable because of obesity. PET is a quantitative modality
because tracer kinetic models can be applied to the data and
attenuation artifacts largely avoided. It can therefore help detect
false negative or positive MPI SPECT results. MPI PET should be ordered
with prior consultation of a nuclear cardiologist.
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| (Box 3) Possible Complications of Stress Test |
| Exercise |
| Chest pain, 20% |
| Cardiac event, 0.01-0.02% |
| Death, 0.002% or 1/50,000 |
| Accidental fall |
| Pulmonary edema |
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Pharmacological (adenosine)
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| Chest pain, 20-40% |
| Some degree of AV block, 3-18% |
| Bradycardia, 12% |
| Nausea, 5% |
| Drop in BP, 3-5% |
| Bronchospasm, 0.2% |
| Cardiac event, death, <1% |
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Scheduling
Appointment for nuclear cardiology examinations may be scheduled by
calling 617-643-1552 or through the Radiology Order Entry system,
http://mghroe
. Weekend scheduling should be made directly by calling the
on call nuclear cardiologist at 617-726-9292.
Further Information
Further information may be found on the Nuclear Cardiology website. For questions, please contact James Scott, M.D.
, Associate Professor of Radiology.
We would like to thank Henry Gewirtz, M.D., Associate Professor of
Medicine and Director of Nuclear Cardiology, and Hiro Yasuda, M.D.,
nuclear cardiologist, for their assistance and advice for this issue.
This article provided useful information about the appropriate use of imaging studies:
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References
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Crean, A, Dutka, D and Coulden, R. (2004) Cardiac imaging using nuclear medicine and positron emission tomography. Radiol Clin North Am 42: 619-34
Expert Panel on Cardiovascular Imaging: Stanford W, Bettmann, MA, Boxt, LM, et al. American College Of Radiology ACR Appropriateness Criteria™; Acute Chest Pain—Suspected Myocardial Ischemia. Available at http://www.acr.org/ac_pda. Accessed: 6/2005.
Klocke, FJ, Baird, MG, Lorell, BH, Bateman, TM, et al. (2003) ACC/AHA/ASNC
guidelines for the clinical use of cardiac radionuclide
imaging--executive summary: a report of the American College of
Cardiology/American Heart Association Task Force on Practice Guidelines
(ACC/AHA/ASNC Committee to Revise the 1995 Guidelines for the Clinical
Use of Cardiac Radionuclide Imaging). Circulation 108: 1404-18 |
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