Nuclear Cardiac Stress Testing: What and When

Transcription

1 August 2010 Nuclear Cardiac Stress Testing: What and When Coronary artery disease (CAD) remains a major cause of mortality and morbidity in Hong Kong. The objective of cardiac stress testing is to detect significant CAD and to prevent future adverse events. Myocardial perfusion imaging (MPI) is a wellestablished non-invasive imaging technique that provides valuable diagnostic and prognostic information. In this article, there is a brief review of this imaging technique, including the procedure and its safety profile, and its appropriate indications in different clinical scenarios according to an up-to-date guideline. What is MPI? MPI using 201 Tl ( thallium scan ) was introduced as an adjunct to electrocardiogram (ECG) treadmill tests in the mid-1970s. It has flourished in the past 30 years in the field of clinical cardiology, together with the advancements in technology (including the single-photon emission computed tomography [SPECT], pharmacological stress and ECG-gated imaging) and radiopharmacy (use of technetium [Tc]-based tracers). The technique involves intravenous injection of a small amount of radioactive tracer, like 99m Tc-2-methoxy-isobutyl-isonitrile (MIBI), under stress (exercise or pharmacological) and rest conditions. Uptake of MIBI by cardiac myocytes depends on its delivery to the cell surface through the coronary vasculature as well as cellular extraction and retention of the compound. As a result, coronary perfusion and myocardial viability can be assessed. Because decreased perfusion due to narrowed coronary arteries precedes the steps of the ischaemic cascade, MPI has an inherent advantage over ECG or regional wall motion-based techniques for detection of CAD. SPECT imaging, whereby the gamma camera rotates around the patient s chest for around minutes (Figure 1), is required after each injection to obtain raw projection images (with ECG-gating) for reconstruction into 3-dimensional tomographic slices of the myocardium. Patient cooperation for breath holding is not required and, on the contrary, tidal breathing during imaging is encouraged. The resultant tomograms are displayed in a colour scale to illustrate segmental uptake in the myocardium. The data are also compared semi-quantitatively with a normal reference database to facilitate assessment of the presence, severity and extent of perfusion abnormality. The post-stress left ventricular ejection fraction can also be reliably reproduced, and global and regional wall motion assessment can be achieved through robust computer software. Figure 1. Patient lies comfortably on the imaging table with tidal breathing, while the gamma camera rotates around the patient s chest to obtain the projection images after radiotracer injection at stress and rest conditions. 222 Waterloo Road, Kowloon, Hong Kong Tel: Fax: Website: 1

2 Simply put, homogeneous tracer uptake indicates normal myocardium and perfusion, and hence the absence of clinically significant infarction or CAD. It provides a strong prognostic value for identifying low-risk patients. In a meta-analysis of 14 trials with more than 12,000 patients with chest pain symptoms, normal 99m Tc -MIBI imaging results were associated with a cardiac event rate of 0.6% per year. 1 This is similar to the risk in the normal population. A defect on stress images that normalises on the rest study indicates a stress-induced perfusion abnormality, which in most instances corresponds to haemodynamically significant coronary stenosis. A defect in both stress and rest images suggests loss of viable myocardium like infarction. Hence, this technique is valuable for diagnosing CAD, its severity, and the culprit coronary artery, and for assessing global and regional myocardial function. Safety and Radiation Risk MPI, like other nuclear medicine studies, is non-invasive and safe. The risk of allergy to radioactive tracer is minimal compared with the iodinated contrast used in radiological procedures (~1,000 times less risk). The risks of MPI are mainly in terms of the cardiac stress procedure, and the imposed radiation risk. Treadmill exercise, the most common form of stress used in MPI, has been used for decades and is proven to be a diagnostic test with a low complication rate. It carries, at most, a 0.01% risk of death and 0.02% risk of morbidity, 2 and similar rates have been observed with pharmacological stress. Pharmacological stress is used in patients with functional limitations from pulmonary, orthopaedic, peripheral vascular or neurological conditions that prevent adequate exercise stress, or in patients with left bundle branch block (which may cause artifact with exercise stress). Stress agents include vasodilators (dipyridamole or adenosine), or a beta sympathetic agonist (dobutamine). The radiation exposure to an adult patient is 9 msv for a MIBI study. The risk is small compared with the lifetime risk of cancer in the general population (1 in 3). It compares favourably with CT coronary angiography (mean dose 16 msv). 3 This is particularly important in female patients where radiation dose to breasts is of increasing concern. When would MPI be Appropriate and Beneficial to Your Patients? The American College of Cardiology Foundation (ACCF), in conjunction with the American Society of Nuclear Cardiology and other imaging societies, updated the Appropriate Use Criteria for Cardiac Radionuclide Imaging in In 33 of 67 clinical scenarios, MPI is considered acceptable care and a reasonable approach to the indications, and the benefits of the resultant incremental information exceed the possible risks by a sufficiently wide margin. The more common clinical scenarios are described in the following four categories: A. Asymptomatic patients B. Symptomatic patients C. Patients with prior cardiac test result D. Post-revascularisation patients A. Asymptomatic Patients To consider whether screening for CAD by MPI in asymptomatic patients is appropriate, patients are stratified according to the 10-year absolute risk of developing a cardiac event as assessed using the Framingham risk score. 5 CAD risk 10-year risk of Framingham Risk Score cardiac event Men Women Low <10% Intermediate 10 20% High >20% Figure 2. A 77-year-old asymptomatic male had a history of diabetes mellitus and hypertension (high CAD risk). Myocardial perfusion imaging with dipyridamole stress showed reversible marked perfusion defect at the anterior and anterolateral walls in the reconstructed short-axis tomogram of the left ventricle. Subsequent coronary angiography showed total/subtotal occlusion of LMCA; LAD & LCx filled by collaterals from RCA; and critical stenosis of mlad. Patient was then referred for CABG. Appropriate Indications in Asymptomatic Patients: 1. High CAD risk. 2. Intermediate or high CAD risk with syncope. 3. Patients with diabetes mellitus (age >40) and peripheral vascular disease (factors not considered in the Framingham risk score) (Figure 2). 4. Patients with ventricular tachycardia. 5. Patients with elevated troponin without other evidence of acute coronary syndrome. 6. Patients with new onset or newly diagnosed heart failure with left ventricular dysfunction, without prior CAD evaluation and no planned coronary angiography. 2

3 B. Symptomatic Patients Patients are stratified into very low (<5%), low (5 10%), intermediate (10 90%) and high (>90%) pre-test probabilities of CAD by age, gender and symptoms: Age (years) < >60 Gender Typical/Definite Angina Atypical/Probable Angina Nonanginal Chest Pain Men Intermediate Intermediate Low Women Intermediate Very low Very low Men High Intermediate Intermediate Women Intermediate Low Very low Men High Intermediate Intermediate Women Intermediate Intermediate Low Men High Intermediate Intermediate Women High Intermediate Intermediate Characterisation of chest pain/angina is defined by the ACC/AHA Guidelines on Exercise Testing 6 : Typical/definite angina: 1. Substernal chest pain or discomfort that is 2. provoked by exertion or emotional stress and 3. relieved by rest and/or nitroglycerin. Atypical/probable angina: Chest Pain or discomfort that lacks one of the characteristics of typical/definite angina. Nonanginal chest pain: Chest pain or discomfort that meets one or none of the typical/definite angina characteristics. Appropriate Indications in Symptomatic Patients: 1. All patients with chest pain symptoms with intermediate or high pre-test probability of CAD (Figure 3). 2. Patients with low pre-test probability who have an uninterpretable ECG or who are unable to exercise. 3. Patients with acute chest pain/possible acute coronary syndrome, without acute ischaemic ECG changes (provided the peak troponin levels are normal or only minimally elevated). C. Patients with Prior Cardiac Test Results Appropriate Indications in Patients with Prior Cardiac Tests: 1. Treadmill: Duke treadmill score of intermediate (-10 to 4) or high-risk (<-10) on routine exercise ECG test. 2. CT coronary calcium score (Agatston score): Any patient with a score >400, or high CAD risk asymptomatic patient (Framingham risk score) with Agatston score between 100 and CT or conventional coronary angiography: A coronary stenosis of uncertain significance. 4. Patients with new or worsening symptoms and a prior abnormal coronary angiogram or stress imaging study. 5. Equivocal, borderline or discordant stress testing results when obstructive CAD remains a concern. Figure 3. A 53-year-old male policeman had atypical angina. He is not a smoker and does not have diabetes mellitus, hypertension or hyperlipidaemia. Myocardial perfusion imaging with tomographic slices at three axes showed reversible marked exercise-induced perfusion defects at the anterior, septal, apical and inferior walls of the left ventricle. The patient had coronary angiography showing 90% stenoses at mlad and OM1, and total occlusion at RCA with PCIs performed. 3

4 D. Post-revascularisation Patients (PCI or CABG) Appropriate Indications in Post-revascularisation Patients: 1. Patients post-revascularisation with new chest pain symptoms or ischaemic ECG abnormalities. 2. Patients with prior incomplete revascularisation in whom additional revascularisation is feasible. 3. Patients 5 years or more after coronary artery bypass graft (CABG). Summary Box Most common indications for MPI: Screening of asymptomatic patients with high CAD risk, including DM (>40 years) and PVD patients. Intermediate to high pre-test probability of CAD in symptomatic patients. CT or conventional coronary angiography detected coronary stenosis of uncertain significance. New chest pain symptoms or ischaemic ECG abnormalities in post-revascularisation (PCI or CABG) patients. Conclusion Despite the ever-evolving imaging technology in the management of CAD, the role of nuclear cardiology, including the application of MPI, will continue, based on its inherent advantage in providing clinically useful information at functional, physiological, cellular and molecular levels. The most up-to-date appropriateness criteria issued by the ACCF will help clinicians decide when to order an MPI in a reasonable and cost-effective manner. The appropriate use of MPI is likely to improve patients clinical outcomes. Dr Wai Tat NGAI Consultant in Nuclear Medicine Nuclear Medicine Centre References 1. Iskander S, Iskandrian AE. Risk assessment using single-photon emission computed tomographic technetium- 99m sestamibi imaging. J Am Coll Cardiol 1998;32: Rochmis P, Blackburn H. Exercise tests. A survey of procedures, safety and litigation experience in approximately 170,000 tests. JAMA 1971;217: Mettler FA, Huda W, Yoshizumi TY, Mahesh M. Effective doses in radiology and diagnostic nuclear medicine: A catalog. Radiology 2008;248: Hendel RC, Berman DS, Di Carli MF, et al. ACCF/ASNC/ACR/AHA/ASE/SCCT/SCMR/SNM 2009 appropriate use criteria for cardiac radionuclide imaging: a report of the American College of Cardiology Foundation Appropriate Use Criteria Task Force, the American Society of Nuclear Cardiology, the American College of Radiology, the American Heart Association, the American Society of Echocardiography, the Society of Cardiovascular Computed Tomography, the Society for Cardiovascular Magnetic Resonance, and the Society of Nuclear Medicine. J Am Coll Cardiol 2009;53: National Institutes of Health: National Heart, Lung, and Blood Institute. Third Report of the National Cholesterol Education Program (NCEP) Expert Panel on Detection, Evaluation, and Treatment of High Blood Cholesterol in Adults (Adult Treatment Panel III).NIH Publication No September 2002, page III-4-5. Available at: Accessed 12 July Gibbons RJ, Balady GJ, Bricker JT, et al. ACC/AHA 2002 guideline update for exercise testing: summary article: a report of the American College of Cardiology/American Heart Association Task Force on Practice Guidelines (Committee to Update the 1997 Exercise Testing Guidelines). J Am Coll Cardiol 2002;40:

5 From the Desk of the DMS: ACHS Accreditation Progress Update I would like to take the opportunity to update you on the progress of the Australian Council on Healthcare Standards (ACHS) accreditation of Hong Kong Baptist Hospital (HKBH). The date of the Organizational Wide Survey (OWS) is now fixed, and it will take place from the 18 th to 22 nd October Meanwhile, HKBH is working in full force to address the various priority areas highlighted in the report of the Consultancy Survey conducted in March Dr Hoi Che LEE Needless to say, this is a mammoth task considering the little time we have left, and in this regard I have to express my gratitude to our colleagues for the great effort spent in the preparatory work over the past months and urge you all to sustain this impetus in the months to come. Be that as it may, we are confident that the critical changes will be successfully implemented and our services will be of a higher level of quality and safer for our patients. I would like to highlight and share with you a few important initiatives: 1. The hospital is now piloting a new Nursing Assessment Form that aims to provide a more comprehensive assessment of the patient with inputs from the para-medical departments. This will be rolled out to all wards. 2. A Discharge Summary is now given to patients on discharge and as an interim measure a new discharge summary form has been introduced to eliminate the need to write the prescription twice. It is envisaged that the whole process will be much simpler with the live run of the new hospital information system (Clinical Enterprise System) early next year, when most of the required information can be automatically extracted from the system. 3. Time out for surgical procedures was introduced in April this year to enhance patient safety, and to date no major obstacle has been encountered. We are going to extend its application to other high-risk procedures like interventional radiology and radiotherapy. 4. A new Consent Form for surgery/invasive procedures will be used in August. This form is a slight modification of the agreed version of the Private Hospital Association. Simultaneously, a separate Consent Form for anaesthesia will be used together with a list of conditions under which a patient should be admitted for assessment at least a day before the operation/procedure. We understand that very little adjustment is required, as it merely reflects current practice. 5. Informed Consent for Transfusion of Blood Products is also required and this will be implemented in the coming months. 6. Currently, various clinical audit programmes are being conducted under the leadership of the various Clinical Audit Coordinating Committees. It is expected that both the scope and the depth of the audits will intensify as a natural development of the audit activities. Furthermore, nursing and paramedical record audits will also be conducted in parallel. 7. On Credentialing and defining the Scope of Work of professional staff, HKBH will re-grant admission privileges periodically, which is initially planned to be every 3 years, but the exact process has not been worked out yet. An interview with the doctor is probably beneficial and necessary for better communication and understanding. 8. Finally, you may be aware of the Hand-washing Campaign that HKBH is currently launching to improve the awareness and practice of hand hygiene. Your participation is highly appreciated. I can of course go on with this list of highlights, but I shall stop here and leave the rest for another day. I understand that these changes may engender much inconvenience for our doctors. Therefore, I would like to seek your cooperation, support and feedback so that the changes can be carried out smoothly in the months to come. I am hopeful that I can secure your understanding and tolerance as HKBH strives to improve patient safety and choice, although the effort in this particular period is undeniably intense. Dr Hoi Che LEE Director Medical Services 5

6 CME Surgeons Surgical Management for Difficult Biliary Strictures Director of Programme: Dr Leung Tung YUNG Chairman: Dr On Shing POON Speaker: Dr Chi Leung LIU Date: September 3, :00 9:30 am (Ms Connie LOK) Coming Meetings: October 8, 2010 November 5, 2010 December 3, 2010 Physicians Session A: The Role of Targeted Therapy in Colorectal Cancer Management Session B: Latest Update on Giloblastoma Management Director of Programme: Dr Peter CY WONG Chairman: Dr Peter CY WONG Speakers: Dr Kwok Wing CHIU (Session A) Dr Raymond CHAN (Session B) Date: September 6, :00 10:00 pm (Ms Polly TAM) Coming Meetings: October 4, 2010 November 1, 2010 December 6, 2010 Surgical Pathology Joint Surgical Pathology Meeting Date: September 17, :00 9:00 am (Ms Connie LOK) Coming Meetings: October 15, 2010 November 19, 2010 December 17, 2010 Obstetricians & Gynaecologists Fetal Therapy Speaker: Dr Ben Chong Pun CHAN Date: September 28, :30 8:30 pm (Ms Polly TAM) Coming Meetings: October 12, 2010 November 9, 2010 December 14, 2010 What s ON Expansion of Renal Centre As the demand of haemodialysis service is increasing and with the participation of the Shared-Care Programme of the Hospital Authority in 2010, the Renal Centre will extend its service by increasing the number of beds from 12 to 16 in August All haemodialysis bookings should be done by the attending private nephrologists or our Resident Consultant in Nephrology. For more information, please call our Renal Centre at Patient Support Group (For Breast Cancer Patients) The series of talks listed below will be held from 3:00 to 5:00 pm at HKBH to facilitate rehabilitation of breast cancer patients. All talks are conducted in Cantonese. You are most welcome to encourage your patients to join. For enrolment or enquiries, please call Date August 14 September 11 October 9 Novmeber 13 September to December Topic Eating Properly and Getting Better More Easily (By Dietitian) What if I Have Cancer? (By Surgeon) Dealing with Stress in Illness and Treatment (By Social Worker) Rehabilitation Exercise + Prosthesis (By Physiotherapist and representative from a health care company) A series of emotional-spiritual care groups will also be held monthly Editorial Enquiry 6 We would like to hear from you! Any questions, comments or suggestions are always welcome. Please us at pr@hkbh.org.hk Blessed are they who maintain justice, who constantly do what is right. (Psalm 106:3)