What Is the True Value of Normal Myocardial Perfusion SPECT with Positive Stress Electrocardiogram?
Fernando Fabián Faccio, Bruno Nicolás Strada
Departamento de Imágenes, Sanatorio San Gerónimo.
Santiago del Estero 2750. (3000) Santa Fe, Argentina.
Recibido 07-AGO-13, ACEPTADO 17-AGOSTO-2013.
The authors declare not having conflicts of interest.
Rev Fed Arg Cardiol. 2013; 42(3): 163-165
Ischemic heart disease is considered as one of the most frequent causes of disability and mortality in Western countries, so continuous work is made to improve the identification of patients in high risk of suffering an acute coronary event .
The assessment of risk has become in this era of evidence-based medicine, the great paradigm to be predicted, considering the clinical indicators, adding to this supplementary methods, which the physician should know how to interpret, and mostly consider in the proper scenario to obtain the best diagnostic information.
Historically, the usefulness of different tests for the diagnosis of coronary artery disease (CAD) is precisely based on providing the best diagnostic performance by increasing the sensibility and specificity of every method. The second purpose, and maybe more important, is to try to assess the risk of the disease after establishing its existence. We can solve this challenge based on the severity and extension of ischemia, along with the analysis of predictors or variables that add or define management, as the already known symptoms of ischemia, electrocardiographic changes, level of effort reached, behavior of blood pressure, heart failure, arrhythmias, transient ischemic dilatation of the left ventricular chamber, and pulmonary hyper-uptake.
Mismatch between stress electrocardiogram and myocardial perfusion
Currently, the studies available that add imaging, provide a very good diagnostic and evaluation capacity for patients with CAD, but graded exercise tests (GXT) is still the most widely used procedure for its easy application, reliability, safety and relative low cost. Previous studies showed that the presence of ST segment elevation induced by exercise during GXT is a good indicator of myocardial ischemia and predictor of acute coronary events, as unstable angina, acute myocardial infarction and sudden cardiac death [2-4]. In turn, in patients with normal GXT that reach a level of work of 10 metabolic equivalents, a very low risk of inducible ischemia is observed.
However, the sensibility of this test is known to be close to 68%, so the introduction of the imaging methods achieved increasing the capacity of detecting ill people in 20-25% .
Many studies show the prognostic value of myocardial perfusion (MP), which is added to that of stress tests, but that also has an independent value from them, which shows its better sensibility.
In one end of probabilities we find the patients, as those of the work presented , who have an abnormal electrocardiogram (ECG) during stress test, but with a normal perfusion result. Evidence is known on this contradictory subset in favor of perfusion, and not just assessing the electrocardiographic data, but doing it in a more comprehensive way through the DUKE score .
In spite of the knowledge and ratification with the different works presented of the low probability of events in this subset of patients, guidelines recommend before this mismatch the need to require another study to rule out CAD, with coronary computed tomography angiography emerging as the test of choice . This medical actions are very likely due to the concern of being before a balanced ischemia, which will lead to a false negative of scintigraphy. However, we should remember that MP has validated different parameters with the aim of discriminating the balanced ischemia, as the clinical indicators and predictor variables mentioned previously.
In this paper , although some of these indicators are not mentioned, the prognostic value of a normal MP is ratified, giving value once again to this functional test and generating a question about the need of requiring larger studies, which may lead to an increase in the costs of health and a greater radiation, without obtaining a true change in the medical behavior of our patients.
Normal myocardial perfusion SPECT
MP SPECT of normal stress is associated with nonsignificant coronary lesions, just as determined by different publications [9-10], also offering a favorable prognosis in the long term, both for all-cause mortality or cardiac events, as cardiac death or acute myocardial infarction (AMI) [11-12]. Just as it has been reported, the events in this population of patients occur late .
In some studies, variations in the risk have been proven, according to different variables like gender, age and known CAD . Also, recently Romero-Farina et al, observed which would be the variables influencing to indicate a second MP Gated SPECT after normal Gated SPECT in stress/rest with positive stress ECG, and consider that the presence of three or more cardiovascular risk factors, previous AMI, history of coronary revascularization and positive stress test are the variables with greater prediction .
Besides the ischemic defects in the images of the second study, presented more frequently in the male gender, and in those in whom the first MP had been made just with pharmacological stress.
Previous publications agree exactly with what was presented by Hominal et al , in that the electrocardiographic changes with normal MP SPECT are not that frequent, and even less so in pharmacological stress with vasodilators. This type of results happen essentially in women with advanced ages, associating to a greater number of cardiac events, unlike what was quoted by this paper [15-16]. Other presentations ratify the low prevalence of normal MP with openly positivestressECG, and in comparison to coronary angiography, normal coronary arteries are observed in the female gender, and mild to moderate lesions in the male gender .
As to the theory of balanced ischemia, which states MP SPECT could express as normal and in fact correspond to a trunk lesion of the left coronary artery or multiple vessel disease, it is worth remembering that different papers give value to ST alterations at a low load, symptoms and signs of heart failure, complex arrhythmias, paradoxical behavior of blood pressure, transient ischemic dilatation, pulmonary hyper-uptake, ventricular volumes and ejection fraction, as adding to the set of the study a similar sensibility to that shown for scintigraphy images per se . The Hominal et al  study, by being retrospective, may probably present the limitation of not considering these variables, and maybe associated to the low number of the sample, it may not allow to estimate the real evolution in the follow-up of patients that have as their single alteration positive stress ECG. In the subset of asymptomatic diabetic patients, the presence of positive stress ECG and normal MP was extremely low and when not associated with risk variables as advanced age and family history of ischemic heart disease, they relate to a good prognosis [19-20]. See the paper published.
The Hominal et al paper ratifies the low incidence of positive stress ECG and normal MP. Probably depending on the variables or sub-populations that are considered, this presentation may have a different prognosis.
We consider that the analysis of all the variables that the PM Gated SPECT study in stress/rest yields, beyond the scintilography images, contributes to the sensibility and specificity maintaining the known and accepted values for this practice.
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Publication: September 2013