Dr. RAVISHWAR NARAYANROLE OF NUCLEAR MEDICINE IMAGING INACUTE CHEST PAIN SYNDROMES
INTRODUCTION Chest pain is one of the most common presenting symptoms in the emergency department (ED) patients encompass a wide spectrum of underlying diagnosis ranging from (1) Acute Coronary Syndrome (2) Non-ACS cardiovascular conditions: myocarditis/myo-pericarditis stress-related cardiomyopathy aortic dissection pulmonary embolism (3) non-cardiac cause of chest pain: gastroesophageal reflux sepsis
ACS Acute coronary syndrome (ACS) is a unifying term representing a common end result, acute myocardial ischemia. It describes a spectrum of clinical syndromes ranging from unstable angina to NSTEMI and STEMI. Patients presenting with ACS are divided into those with ST elevation (lasting ≥20 minutes) or new left bundle branch block, and those with NSTEACS which includes transient ST elevation (lasting <20
Recognizing a patient with ACS is important because of the life-threatening nature of an ACS. It is prudent to have a low threshold in suspecting a patient with acute chest pain as potentially having an ACS.
PATHOPHYSIOLOGY: major causes of ACS are 1. Thrombus 2. mechanical obstruction 3. dynamic obstruction 4. Inflammation 5. increased oxygen demand. The major pathophysiologic mechanism is rupture or fissuring of an atheromatous plaque with superimposed thrombus
ECG: cost-effective tool for the initial diagnosis in patients with chest pain and possible ACS Limitations: up to 40%-65% patients with evolving ACS have a normal or non-diagnostic presenting ECG baseline ECG changes may be present secondary to underlying pericarditis, left ventricular hypertrophy, left bundle branch block, and early repolarization leading to over-diagnosis of ACS
reasons for non diagnostic ecg: 1. small infarct 2. location of infarct - LCX territory 3. collaterals 4. timing of ecg in relation to onset of infarct patientswith acute myocardial infarction (AMI) and normal ECG admitted have adverse event rates of up to 19%
CARDIAC BIOMARKERS: detectable blood levels of these cardiac specific troponins in patients with ACS are associated with unfavorable outcomes. The sensitivity and specificity for standard troponin assays are as high as 99% and 86% respectively for detection of AMI within 24 hours of acute chest pain
Limitations: sensitivity of cardiac troponins is poor within the first 3 hours of onset of symptoms for the detection of myocardial infarction and predicting subsequent major cardiac events. negative troponin T/ CK-MB does not necessarily confer a low risk of complication in patients presenting with acute chest pain. Reason for –ve cardiac markers: Unstable Angina
Highly sensitive troponin assays: negative predictive value 3 hours after admission with chest pain was reported to be 99.6 % the positive predictive value was reported to be 96.5%, making it a powerful tool for both ruling out or confirming MI, respectively at 3 hours
ACUTE REST MYOCARDIALPERFUSION IMAGING (MPI) first documented use of ARMPI in patient dates back to 1970s, using planar Tl-201 imaging
Sensitivity: 90%-100% negative predictive value: 99%-100%. There is a 10% absolute reduction and 20% relative reduction in the rates of unnecessary admissions among patients who undergo MPI in the ED in addition to the usual ED care as compared with those who only receive standard care.
Thus, ARMPI significantly reduces the overall cost involved in managing patients with acute chest pain. This cost saving is primarily due to reduction of unnecessary hospitalization and limiting further diagnostic procedures including coronary angiography
Limitations: to detect ischemia on rest MPI at least 3%-5%of the myocardium must be involved. In patients with coronary artery spasm the diagnostic accuracy of ARMPI is limited because once the vasospasm has resolved there might be normal or super-normal coronary flow secondary to reactive hyperemia.
3 hours from cessation of symptoms is considered the cut off for injection of radiotracers, as later injections may significantly underestimate the extent of at risk myocardium and limit the prognostic ability of ARMPI.
COMPARISON OF ACUTE REST MPI WITHCLINICAL AND ECG DATA ARMPI has higher diagnostic accuracy than clinical and ECG changes in patients presenting with chest pain and non diagnostic ECG SENSITIVIT SPECIFICITY PPV for adverse cardiac Y eventsARMPI 94% 83% 85%CLINICAL DATA + 88% 37% 45%ECGECG 35% 74%
COMPARISON OF ACUTE REST MPI WITHCARDIAC BIOMARKERS Sensitivity Specificity ARMPI 73% 93% c Tn T 17% 100% CK MB 4% 93% Ann Emerg Med 1999;33:639-645
Advent of high sensitive troponins has significantly improved the diagnostic accuracy of cardiac biomarkers and offers the opportunity for very early diagnosis of an ACS Currently, there are limited data on direct comparison of hs-troponins and ARMPI
ACC/AHA/ASNC GUIDELINES FOR ARMPI FORCHEST PAIN EVALUATION Current guidelines recommend use of ARMPI as a Class 1, Level of evidence A indication for assessment of myocardial risk in patients with suspected ACS
STRESS OR REST MPI FOR ACUTE CHEST PAIN 29 mths. follow up of patients, who had Stress SPECT within 24 hours of admission in ED, showed that patients with an abnormal SPECT finding had a significantly higher rate of cardiac events compared to those with normal imaging (40%vs 1.6%) Nabi F, Chang SM, Xu J, et al: Assessing risk inacute chest pain: The value of stress myocardialperfusion imaging in patients admitted through theemergency department. J Nucl Cardiol 2012;19: 233-243
Pharmacologic stress allows for the very early assessment of patients after ACS and is so indicated in the appropriate use criteria.
Score 7–9 : Appropriate test for specific indication (test is generally acceptable and is a reasonable approach for the indication). Score 4 – 6 : Uncertain for specific indication (test may be generally acceptable and maybe a reasonable approach for the indication). Score 1–3 : Inappropriate test for that indication (testis not generally acceptable and is not a reasonable approach for the indication)JACC Vol. 53, No. 23, 2009
OTHER NUCLEAR CARDIOLOGYMETHODS PET: ina retrospective study done on more than 7000 patients presenting to the ED with chest pain, 92.5%of patients with a positive rest or stress PET scan were eventually diagnosed with ACS Currently, there are no data available on the utility of rest only PET scans in acute chest pain patients.
Fatty Acid Imaging: free fatty acids are preferentially utilized to produce ATP, however ischemia causes a shift from fatty acid metabolism to glucose utilization. This metabolic derangement may persist long after the resolution of ischemia, a phenomenon described as ISCHEMIC MEMORY 15-(p-Iodophenyl)-3-R,S methyl pentadecanoic acid (BMIPP) is a iodinated branch-chain fatty acid, with high uptake and long retention in the myocardium
sensitivityand specificity of BMIPP SPECT imaging performed within 48 hours of chest pain, towards diagnosing obstructive CAD was reported to be 74%and 92%respectively In addition, in patients with CAD detected both on BMIPP and tetrofosmin SPECT, the extent and severity score was higher with BMIPP as compared with the tetrofosmin MPI.
BMIPP is beneficial for detecting significant CAD in patients who have negative initial tetrofosmin SPECT and are unable to undergo provocative test due to age or unstable symptoms for detection of ischemia BMIPP sensitivity is maintained up to 30 hours after symptoms resolution. This extended time window for detection of ischemia has a potentially important clinical advantage for evaluation of patients with suspected ACS, who present long after their symptoms have resolved.
Immunoscintigraphy: Indium-111-labeled antimyosin has been shown to be highly sensitive in detecting Q-wave and non–Q-wave infarcts. However, slow clearance of this agent from the blood pool does not permit imaging earlier than 18 hours after administration, thereby limiting the usefulness of this agent in evaluating acute chest pain
Tc-99m-labeled annexin-V binds to the plasma membrane of myocytes undergoing apoptosis due to infarction or repetitive ischemia excellent co-localization with sestamibi in areas of acute injury, and interpretable images can be acquired within 4 hours of injection
Tc-99m–labeled glucarate binds to nuclear histones exposed in recently damaged myocytes There is early visualization of both reperfused and nonreperfused infarcts with in vivo imaging with lack of accumulation in ischemia or chronic infarction. These characteristics, along with a short biologic half- life, favorable target-to-background ratio, and rapid blood pool clearance, make this imaging agent ideal for detecting acute infarction in ED patients who have chest pain
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