Abstract
Ischemic cardiomyopathy remains one of the largest subgroups of the growing epidemic of heart failure. Coronary artery bypass grafting remains an important therapeutic option for select patients with ischemic cardiomyopathy. Though the techniques of revascularization are established, the imaging modalities to determine the potential for myocardial recovery remain imprecise. Different imaging modalities to determine myocardial viability and ischemic burden will be discussed. Correlating areas of viability with coronary anatomy suitable for revascularization is currently the recommended strategy.
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Keywords
- Left Ventricular Ejection Fraction
- Coronary Artery Bypass Grafting
- Cardiac Compute Tomography
- Ischemic Cardiomyopathy
- Compute Tomography Perfusion
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Introduction
Coronary artery disease (CAD) is becoming the dominant cause of heart failure [1]. Coronary artery bypass grafting (CABG) has only recently been more broadly utilized to address this population. There are only two studies comparing medical therapy to CABG in patients with ischemic cardiomyopathy, the Coronary Artery Surgery Study (CASS) [2] and the Surgical Treatment of Congestive Heart Failure (STICH) trial Hypothesis I [3]. In the CASS study, only patients with three-vessel disease benefited CABG over medical therapy demonstrated at 7 years of follow-up. This is a study performed in the 1980s with limited practical relevance today. The recently published STICH Hypothesis I data showed there was no significant difference between medical therapy alone and medical therapy plus CABG with respect to the primary end point of death from any cause. However, patients assigned to CABG, as compared with those assigned to medical therapy alone, had lower rates of death from cardiovascular causes and of death from any cause or hospitalization for cardiovascular causes [3].
Other smaller series and single center studies add to the increasing evidence supporting this therapeutic option as beneficial to our patients both for survival benefit and symptom relief for those having chronic CAD and left ventricular (LV) dysfunction with viable myocardium [4]. The surgical techniques whether performed off pump, pump assisted, or with the heart cross clamped can all be performed successfully without clear benefit from one technique over another. Several important patient and anatomic factors must be assessed prior to determining the appropriateness of an individual for this therapy.
“Stunned” Versus “Hibernating” Myocardium
“Stunned” myocardium occurs as a sequela to an acute ischemic insult and the associated regional dysfunction from inflammation despite adequate perfusion (can be remote myocardium) or as part of reperfusion. This is largely a reversible process as the inflammatory process abates and is associated with adequate perfusion leading to a perfusion-contraction mismatch [5]. Revascularization, if needed, can be performed safely with “stunned” myocardium, though patients may benefit from an interval recovery phase prior to surgery pending the size of the infarct [6]. Short-term mechanical circulatory support may be required in some of these patients to maintain end-organ function as well as facilitate some myocardial recovery with ventricular unloading prior to revascularization.
“Hibernating” myocardium is a decrease of myocardial contractility and metabolism due to sustained hypoperfusion of the myocyte. Hibernating myocardium requires the restoration of a normal blood supply for an improvement in contractile function; global increases in left ventricular ejection fraction (LVEF) following CABG may be seen in as many as 40 % of patients with ischemic cardiomyopathy [7]. Chronic hypoperfusion may convert hibernating myocardium into dedifferentiated myocytes as well as fibrosis. Revascularization can be beneficial during the window where myocardial function may be restored. Identifying this window can be problematic. The information below draws largely on the growing body of nonrandomized studies evaluating efficacy of various techniques to quantify myocardial viability pre- and post-revascularization to help develop a strategy for patient selection and treatment.
Anatomic
Determining myocardial viability and functional myocardial recovery corresponding to coronary anatomy that is amenable to revascularization is the key to patient selection. Though conceptually obvious, the tools for this execution have variable sensitivity and specificity with no consensus. A feature of viable myocardium is the presence of inotropic reserve, which may be elicited by catecholamine stimulation. Hence dobutamine or adenosine is used, and many of the subsequently described imaging techniques employ this response to help differentiate attenuated regions from scar [8].
Echocardiography
Stress echocardiography is widely used as the yard stick to determine the potential for myocardial recovery after revascularization. The thickness of the myocardial wall corresponding to anatomic targets is the first-pass assessment of viability. Incremental diastolic wall thickness changes >0.8 cm with dobutamine infusion may improve the sensitivity, though decrease the specificity of this technique in akinetic regions [9].
Scintigraphy
Single-photon emission computed tomography perfusion scintigraphy, whether using thallium-201, Tc-99m sestamibi, or Tc-99m tetrofosmin, in stress and/or rest protocols, has consistently been shown to be an effective modality for identifying myocardial viability and guiding appropriate management. Metabolic imaging with positron emission tomography (PET) radiotracers frequently adds additional information and is a powerful tool for predicting which patients will have an improved outcome from revascularization [10].
The number of viable segments per patient may be related to the improvement in LVEF after revascularization. In a recent study using TC-99m sestamibi, patients with more than four viable segments representing 24 % of the left ventricle yielded a sensitivity of 83 % and specificity of 79 %, respectively, for predicting improvement in LVEF. Furthermore, the presence of four or more viable segments predicted improvement in heart failure symptoms and quality of life after surgical revascularization [11].
PET using rubidium 82 (Rb 82) or ammonia N-13 can be used in lieu of a single-photon emission computer tomography (SPECT) scan or when a SPECT scan is inconclusive [12].
Cardiac Magnetic Resonance
This noninvasive diagnostic tool is evolving into a one-stop shop for evaluation of myocardial dysfunction. Myocardial wall thickness can be accurately measured as can regional wall motion abnormalities. In addition, delayed contrast enhancement after the intravenous (IV) administration of gadolinium-based contrast material is a very reliable indicator of acute myocardial infarction. Hyperenhancement is not seen in areas of ischemia whether by stunning or hibernation. In addition, the degree of hyperenhancement can correlate with transmural versus subendocardial infarction and may predict improvements in myocardial function after revascularization [13].
Cardiac CT
In ischemic cardiomyopathy, there is limited added benefit with a cardiac computed tomography (CT). Its role in cardiovascular imaging is important in anatomic variants of the coronary anatomy. However its use in determining ischemia or viability as a stand-alone diagnostic modality is very limited. The use of PET with cardiac CT may make this a useful tool for ischemic myopathy.
Results
The recently published STICH trial Hypothesis I is a prospective multicenter, nonblinded, randomized study at 99 clinical sites in 22 countries trial comparing a strategy of medical therapy alone versus medical therapy and surgical revascularization for qualified patients with depressed ejection fractions. There were 1,212 patients randomly assigned to receive medical therapy alone (602 patients) or medical therapy plus CABG (610 patients). There was no significant difference between the two study groups with respect to the primary end point of the rate of death from any cause. The rates of death from cardiovascular causes and of death from any cause or hospitalization for cardiac causes were lower among patients assigned to CABG than among those assigned to medical therapy [3]. This landmark study is clearly important in comparing these two strategies; however, it did not have the fidelity to correlate ischemic regions with coronary targets.
Several nonrandomized studies have retrospectively assessed outcomes in patients with CAD and low left ventricular function. Nardi et al. published a series of 302 consecutive patients with ejection fraction (EF) <35 % who underwent CABG with 298 patients 292 patients receiving complete revascularization subsequently resulting in a 5 % operative mortality and an 87 % freedom from myocardial infarction at 10 years [14]. Shapira et al. published a series of 115 consecutive patients with EF < 30 % operative mortality was a very low 2.6 %. Three- and five-year survival rates were 91 ± 3 % and 76 ± 6 %, respectively, for this group of patients [15]. Filsoufi et al. also published his series of 2,725 consecutive patients undergoing isolated CABG, of whom 495 patients had EF < 30 %. Postoperative mortality was higher in the low ejection fraction group (3.6 % vs. 1.4 %). Long-term survival was significantly decreased in patients with EF of 0.30 or less: 1-year and 5-year survival 88 ± 1.5 % and 75 ± 2.2 % versus 96 ± 0.4 % and 81 ± 1.2 %, respectively (p = 0.001) [16].
Successful coronary revascularization can be successfully performed in patients with low ejection fractions and ischemic cardiomyopathies. Long-term survival is worse in this subpopulation than patients with more normal cardiac function. Nevertheless, long-term survival appears to be robust. There are many different imaging modalities that can be utilized to identify viable myocardium. The STICH trial Hypothesis I is the only contemporary randomized prospective study comparing surgical revascularization to medical therapy in a group of patients with low ejection fraction and CAD. Though the primary end point of all cause mortality did not demonstrate a benefit from the surgical arm, death from cardiovascular causes was lower in the group treated with CABG.
Correlating the quality and the size of the coronary targets to these viable myocardial segments has not been well studied. One would intuitively believe that a complete revascularization would address this limitation. Nevertheless, we currently do not have the tools to accurately answer the question most commonly posed to us from this patient group, “How much better will my heart be when you are finished with the surgery?”
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Sun, B.C. (2013). Surgical Coronary Artery Revascularization in Patients with Advanced Ischemic Cardiomyopathy. In: Morgan, J., Naka, Y. (eds) Surgical Treatment for Advanced Heart Failure. Springer, New York, NY. https://doi.org/10.1007/978-1-4614-6919-3_3
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