The accuracy of viability imaging can be improved by combining approaches. In particular, the likelihood of functional recovery is uncertain in patients showing dysfunctional myocardium with preserved EDWT and in those presenting with intermediate grades of scar transmurality (ie, 25% to 75%). Additional low-dose dobutamine stress imaging may provide valuable information with regard to viability in these dysfunctional segments.54–56 Kühl et al57 assessed the thickness of the nonenhanced rim of myocardial segments showing LGE. A cutoff value of 3 mm was superior to a cutoff value of 5.4 mm for EDWT to assess viability. As PET and CMR assess dysfunctional myocardium from a different perspective—that is, metabolic versus scar imaging±contractile reserve, respectively—they can be considered complementary. As such, the emergence of hybrid PET-MR scanners raises new expectations with regard to improved viability detection—for example, to study myocardial metabolism in the nonenhanced rim on LGE-CMR.58 Although initial results are promising, several hurdles need to be overcome to integrate this novel hybrid technique into clinical routine59,60 (Table 1).
If deemed necessary, viability imaging can be combined with stress imaging to exclude concomitant myocardial ischemia. For these combined purposes, stress-rest SPECT/PET imaging, high-dose DSE (±atropine), high-dose dobutamine CMR (±atropine), and/or stress perfusion CMR can be performed.61,62
Finally, with the advent of multidetector CT technology more than a decade ago, reliable cardiac imaging became a reality. Although the focus has mainly been on coronary imaging, this technique has the intrinsic potential to offer a comprehensive approach in evaluating a heart failure patient merging coronary artery imaging, myocardial perfusion, and myocardial viability imaging into a single examination.63–65 However, it should be emphasized that many patients with IHF have advanced CAD with high coronary calcium values hampering an accurate evaluation of coronary artery lumen. Radiation doses for a CT-based myocardial perfusion/viability study remain high (≈15 mSv),65 and in patients with comorbidities such as renal failure, administration of iodinated contrast agents may be contraindicated. Nevertheless, a greater role for CCT in viability imaging might be expected in the coming years.
From a clinical perspective, recovery of global ventricular function and ultimately improved patient survival are more important than regional improvement (Fig. 8).7 Revascularization of the myocardium deemed viable is only indicated if LV ejection fraction is assumed to increase significantly (ie, ≥5%) after revascularization, thereby improving patient survival, stressing the importance to define an optimal cutoff of viability extent. This can be achieved by expressing viable myocardium as a percentage of LV mass or by the number of viable segments, as determined using the standardized myocardial segmentation approach as proposed by the American Heart Association.66 There is substantial evidence that patients with significant myocardial viability treated medically had a significantly worse prognosis compared with patients treated with revascularization.67,68 Importantly, those without proof of viability had intermediate rates of mortality regardless of treatment option.68 The minimal extent of myocardial viability to predict improvement of survival varies among techniques. PET requires the least amount of viable myocardium (25.8%) in comparison with DSE (35.9%) and SPECT (38.7%).69 On CMR, a patient is typically considered to have a viable myocardium when ≥4 dysfunctional segments are considered viable, showing an LGE transmurality ≤50% or improved contractility during low-dose dobutamine stress.68
Since the initial observations of improved contractility in severely dysfunctional myocardium after CABG 4 decades ago, it has become clear that predicting outcome in patients with IHF is difficult, multifactorially determined, and definitely not driven by the presence and extent of myocardial viability alone (Table 3). It is a misunderstanding to consider viability as binomially distributed (ie, viable vs. nonviable); rather than this dichotomy, many shades of gray are present that influence outcome. As preservation of myocyte fraction is an important determinant of functional recovery after revascularization, it is obvious that a higher myocyte fraction is required to maintain contractile reserve than to achieve significant tracer uptake. This may explain, for example, the higher sensitivity of SPECT imaging compared with DSE in the identification of myocardial hibernation.70 Moreover, both myocardial viability and adverse ventricular remodeling (ie, increase in end-systolic volume) provide independent, incremental prognostic value, and thus viability should not be assessed in isolation.71 Although, as discussed above, increasing extent of viability generally implies increased potential for recovery, this coincides with an increased risk for major adverse events.14 Moreover, progression of LV remodeling adversely affects outcome, and once the LV has become too dilated, the ability for functional improvement is lost, and revascularization will not improve patient outcome despite the presence of viable myocardium. In addition, ancillary morbidities related to severe LV dilatation such as mitral regurgitation and thrombus formation also negatively influence patient outcome. Several other issues come into play when considering functional recovery after revascularization. Time between viability assessment and intervention is important with improved survival if an early intervention is performed.72 Moreover, the time frame of recovery is highly variable within patients; in some of them it may take longer than 1 year depending on the severity of myocardial hibernation.16,17 In these patients, it may be difficult to differentiate between an unsuccessful revascularization procedure, evolving CAD, or a false-positive myocardial viability test. Moreover, an increasing number of patients are elderly patients with comorbidities such as renal failure or diabetes mellitus that may substantially contribute to mortality in the follow-up period. Conversely, lack of functional recovery not necessarily implies lack of improved patient outcome, suggesting that revascularization may have a beneficial impact on other factors such as protection against future infarction and death possibly by improving myocardial electrical stability.73 Another potentially confounding issue, not yet highlighted, is the periprocedural necrosis after percutaneous coronary intervention or CABG evidenced by elevation of cardiac enzymes, altered myocardial perfusion, and new myocardial enhancement on LGE-CMR, which is associated with increased long-term mortality.74,75 These and other confounding factors explain to a large extent the controversies on predicting patient outcome in literature, and hopefully may help better design future viability studies (Table 3).12,76
Although current standard care of treatment in IHF patients is largely focused on improving heart failure symptoms and minimizing the risk for premature death using optimal medical treatment with/without coronary revascularization, an increasing number of patients receives cardiac resynchronization therapy and/or an implantable cardioverter-defibrillator (Fig. 9).77 Although beyond the scope of this paper, noninvasive imaging may provide valuable information with regard to the presence and severity of ventricular dyssynchrony, to visualize coronary venous anatomy, and to determine the presence, location, and extent of myocardial scarring (using LGE-CMR or PET). This information is helpful to select the optimal access (eg, intravascular vs. pericardial), to locate the cardiac resynchronization therapy leads, and to estimate the risk for future adverse cardiac events. Information with regard to LV geometry may be of interest in patients scheduled for surgical ventricular reconstruction. Although the STICH trial did not show an added benefit of this procedure to bypass surgery, the trial’s inclusion criteria did not contain shape or viability parameters.78 CMR may contribute to a better patient selection, providing information on 2-dimensional or 3-dimensional LV geometry (eg, sphericity index, apical conicity index), scar tissue assessment and the consequences on regional and global function, and evaluate the reshaping and functional recovery after surgery.79 In patients with chronic total occlusions, CCT provides important preprocedural data regarding the occlusion length, degree of calcification, vessel tortuosity, and bridging collaterals, whereas LGE-CMR may show evidence of chronic MI in territories subtended by the occluded coronary artery.80,81
In the appropriate clinical setting, viability imaging has been recommended for patients with CAD and severe LV dysfunction by both the American College of Cardiology/American Heart Association (ie, class IIa recommendation) and by the European Society of Cardiology and European Association for Cardio-Thoracic Surgery.14 Although these recommendations date from before publication of the STICH trial results, a recent conjoint appropriate use document (2013 ACCF/ACR/ASE/ASNC/SCCT/SCMR) has recommended CMR and PET for investigating viability in patients with severe LV dysfunction, while at the same time indicating the possibility for the use of stress imaging with SPECT and echocardiography.4,14
Detection of potentially reversible contractile dysfunction in patients with ischemic heart disease has become a valuable clinical strategy for determining the need for revascularization. Further combined-modality research is needed, however, for better comprehension, patient stratification, and treatment guidance.
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