Radiologia Brasileira - Publicação Científica Oficial do Colégio Brasileiro de Radiologia

The assessment of myocardial viability in patients with left ventricular dysfunction has a great impact on the decision making process regarding therapy for such patients, as it allows the identification of those who will benefit the most from a revascularization procedure, be it through surgical approach or through percutaneous angioplasty. Additionally, it also has an economic impact, as it selects those patients for whom the revascularization will not improve their prognosis, as demonstrated by the metaanalysis developed by Allman et al.(1).

The identification of the viable muscle can be achieved by resourcing to different technologies, the most important ones being myocardial scintigraphy, stress echocardiography, positron emission tomography and magnetic resonance imaging.

As a potassium analog, thallium-201 uptake by the myocyte is dependent on the presence of cellular membrane integrity. For this reason, this radiotracer allows the identification of myocardial viability even with small amounts of living muscle. Such fact possibly explains the high sensitivity of myocardial scintigraphy, particularly when utilizing Thallium-201 in the identification of myocardial contractility recovery after revascularization. However, its specificity is lower than that of dobutamine stress echocardiography, particularly in cases where a biphasic response is observed, i.e., when there is an initial improvement in contractility of hypokinetic or akinetic segments with a low dobutamine dose, but worsens again under higher doses of this drug.

With the introduction of gated SPECT, myocardial scintigraphy, particularly when utilizing ^99mTc radiopharmaceuticals, may simultaneously evaluate perfusion, global and segmental contractility and systolic and diastolic left ventricle volumes. Thus, the opportunity to associate the greater sensitivity to identify myocardial viability through the cellular integrity detected by perfusion scintigraphy with the higher specificity of low-dose dobutamine gated SPECT in the evaluation of the contractile reserve was identified.

Based on the results reported by Kumita et al.(2), several studies have demonstrated the feasibility, safety and diagnostic usefulness of this protocol. Yoshibaga et al.(3) have demonstrated that the acquisition with low-dose dobutamine increased specificity of the method as compared with echocardiography and positron emission tomography. Leoncini et al.(4) have reported similar findings in revascularized patients.

In the present issue of Radiologia Brasileira, Moraes et al.(5) also demonstrate similar results, achieving a specificity of 84% in the prediction of contractile reserve recovery after revascularization. This is a very relevant aspect, considering that 58 segments evaluated as viable by perfusion scintigraphy did not present improvement in contractility and, by a large majority, there was no contractile reserve indicated by low-dose dobutamine infusion. In this study the utilized dobutamine dose was greater than that in previous studies(3,4,6), starting at 5 µg/kg/min, with image acquisition at 15 µg/kg/min. In patients with severe coronary disease, such a dose may induce ischemia and worsen the function in certain segments, as described in the biphasic response at echocardiography(7). For that reason, although less frequently, the contractility in 34 segments (13%) worsened with the stimulus of dobutamine.

In some previous studies(3,4) the adopted protocol utilized gated ^99mTc SPECT, both at rest and under stress. Thus, the evaluation of the ventricular function was made at rest and with low-dose dobutamine in the post-stress acquisition. In the study developed by Moraes et al.(5), as the rest images were acquired with thallium-201 and the evaluation of the function was performed after stress, two acquisitions were necessary: one basal acquisition, and another with dobutamine. As demonstrated by Johnson et al.(8), even after one hour, particularly in patients presenting with severe coronariopathies, the contractile alterations may persist because of ischemic stunning, which may impair the basal segmental contractility analysis and, consequently, the response to the dobutamine stimulus.

The findings of the relevant study developed by Moraes et al.(5) confirm that the addition of data regarding the contractile reserve assessment by gated SPECT with low-dose dobutamine was relatively safe and allowed the identification with greater accuracy of viable muscle areas with a potential for functional recovery.


REFERENCES

1. Allman KC, Shaw LJ, Hachamovitch R, et al. Myocardial viability testing and impact of revascularization on prognosis in patients with coronary artery disease and left ventricular dysfunction: a meta-analysis. J Am Coll Cardiol. 2002;39:1151–8.

2. Kumita S, Mizumura S, Kijima T, et al. Low-dose dobutamine stress test for the evaluation of cardiac function using ECG-gated SPECT scintigraphy with 99mTc-MIBI. Kaku Igaku. 1995;32:75–9.

3. Yoshinaga K, Morita K, Yamada S, et al. Low-dose dobutamine electrocardiograph- gated myocardial SPECT for identifying viable myocardium: comparison with dobutamine stress echocardiography and PET. J Nucl Med. 2001;42:838–44.

4. Leoncini M, Sciagrà R, Maioli M, et al. Usefulness of dobutamine Tc-99m sestamibi-gated single-photon emission computed tomography for prediction of left ventricular ejection fraction outcome after coronary revascularization for ischemic cardiomyopathy. Am J Cardiol. 2002;89:817–21.

5. Moraes RF, Meneghetti JC, Barroso AA. Cintilografia de perfusão miocárdica sob baixa dose de dobutamina na identificação do miocárdio viável. Radiol Bras. 2010;43:281–7.

6. Simões MV, de Almeida-Filho OC, Pintya AO, et al. Prediction of left ventricular wall motion recovery after acute myocardial infarction by Tl-201 gated SPECT: incremental value of integrated contractile reserve assessment. J Nucl Cardiol. 2002;9:294–303.

7. Cornel JH, Bax JJ, Elhendy A, et al. Biphasic response to dobutamine predicts improvement of global left ventricular function after surgical revascularization in patients with stable coronary artery disease: implications of time course of recovery on diagnostic accuracy. J Am Coll Cardiol. 1998;31:1002–10.

8. Johnson LL, Verdesca SA, Aude WY, et al. Postischemic stunning can affect left ventricular ejection fraction and regional wall motion on post-stress gated sestamibi tomograms. J Am Coll Cardiol. 1997;30: 1641–8.










1. Associate Professor of Cardiology at the School of Medicine of Universidade Federal do Rio de Janeiro (UFRJ), Rio de Janeiro, RJ, Brazil.
2. Full Professor of Nuclear Medicine at the School of Medicine of Universidade Federal do Rio de Janeiro (UFRJ), Rio de Janeiro, RJ, Brazil. E-mail: leamirian@gmail.com