A large body of evidence supports the use of perfusion, metabolism, and contractile reserve markers for detecting viable myocardium. In separate studies, each has proved similarly effective in predicting regional functional recovery, although head-to-head comparisons suggest that contractile reserve is the most specific predictor. Revascularization of viable myocardial segments improves prognosis and functional status. However, given the risks of bypass surgery in left ventricular dysfunction, it is essential to select the patients most likely to benefit. Several recent studies have sought to identify the most useful parameters in this regard.
Several studies have shown that postrevascularization functional improvement depends on the number of viable dysfunctional segments at baseline. This proves true irrespective of the preoperative technique employed to assess viability (nuclear medicine, stress echocardiography, or positron emission tomography). The greater the number of segments or extent of viable myocardium in the preoperative setting, the greater the improvement after surgery in terms of left ventricular ejection fraction and functional class. Di Carli et al showed that the magnitude of viable myocardium before coronary artery bypass surgery (CABG) also predicted the postoperative improvement in clinical heart failure. They found a linear relationship between change in functional status after CABG and the scintigraphic extent of flow-metabo-lism mismatch on [18F]fluorodeoxyglucose (FDG) imaging, suggesting improvement in left ventricular function as a possible mechanism.
Functional outcome after CABG is also related to the baseline severity of left ventricular dysfunction. Improvement in the ejection fraction depends on the number of viable dysfunctional segments that are revascularized. Benefit is greatest with an ejection fraction <30% and the greatest number of dysfunctional, but viable myocardial segments. This could reflect the fact that patients with an ejection fraction <30% have the most recoverable tissue within segments and therefore the greatest potential for improvement. Neither the number of segments with echocardiographic scarring nor the preoperative left ventricular end-diastolic volume predict functional recovery after revascularization.
A recent meta-analysis showed a direct relationship in patients with viability between severity of left ventricular dysfunction and magnitude of benefit with revascularization. In other words, the lower the preoperative ejection fraction, the greater the postprocedure prognostic benefit. Patients without viability derived no benefit from revascularization at any level of ejection fraction.
The concept that the magnitude of viable myocardium best predicts postrevascularization improvement in ischemic left ventricular dysfunction has recently been extended beyond a simple quantification of the number of dysfunctional, but viable left ventricular segments. Pasquet et al found that the change in ejection fraction with low-dose dobutamine is the best discriminator of functional improvement. In a multivariate analysis that included evidence of segment viability by positron emission tomography and dobutamine echocardiography, and change in global left ventricular ejection fraction with low-dose dobutamine, only the latter predicted postoperative functional improvement. Rocchi et al reported similar findings (Figure). The increase in global function with low-dose dobutamine reflects not only the response of dysfunctional segments, but also the ability of normal or less dysfunctional myocardium to improve contractility under inotropic stimulation. In addition, the evaluation of wall motion improvement during stress tests is subjective, with only moderate interinstitutional agreement (70%), and it often overestimates functional improvement after CABG. Calculation of the ejection fraction, on the other hand, is less subjective and provides a
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Fatfi-Ordoubadi F, Pagano D, Marinho NV, Keogh BE, Bonser RS, Camici PG. Coronary revascularization in the treatment of moderate and severe postischemic left ventricular dysfunction. Am J Cardiol. 1998;82:26-31. more accurate quantification of contractile reserve. Thus, global rather than segmental contractility reserve appears the best predictor of postoperative functional improvement.
Other studies have attempted to determine whether the viable but dysfunctional myocardium underlying a specific coronary artery is a better predictor of post-CABG improvement. Di Carli et al found the highest correlations with change in functional status in the magnitude of mismatch on positron emission tomography in the territories of the left anterior descending and right coronary arteries, but more recent studies have failed to confirm their data.
Figure. Correlation between postoperative improvement in left ventricular ejection fraction and change in ejection fraction induced by low-dose dobutamine. Postoperative improvement is predicted by the following equation: 0.5101+0.806 x change in ejection fraction induced by low-dose dobutamine (r2=0.44, P=0.0001).
After: Rocchi G, Poidermans D, Bax Jf, et al. Usefulness of the ejection fraction response to dobutamine infusion in predicting functional recovery after coronary artery bypass grafting in patients with left ventricular dysfunction. Am J Cardiol. 2000;85:1440-1444. Copyright Â© 2000, Elsevier Science, Ltd.
Pasquet A, Lauer MS, Williams MJ, Secknus MA, Lytle B, Marwick TH. Prediction of global left ventricular function after bypass surgery in patients with severe left ventricular dysfunction. Impact of pre-operative myocardial function, perfusion, and metabolism. Eur Heart J. 2000;21:125-136. Qureshi U, Nagueh SF, Afridi I, et al. Dobutamine echocardiography and quantitative rest-redistribution 201T1 tomography in myocardial hibernation. Relation of contractile reverse to 201 Tl uptake and comparative prediction of recovery of function. Circulation. 1997;95:626-635.
Rocchi G, Poidermans D, Bax JJ, et al. Usefulness of the ejection fraction response to dobutamine infusion in predicting functional recovery after coronary artery bypass grafting in patients with left ventricular dysfunction. Am J Cardiol. 2000;85:1440-1444.
management; left ventricular dysfunction; coronary artery bypass grafting; surgical revascularization; myocardial viability; prognosis; functional recovery