An online search for the term “myocardial viability” yields 1,090,000 hits with Google and 4,419 (532 in 2010 and 2011 alone) citations with PubMed (both accessed on 12/24/2011). During the past 3-4 decades, viability assessment has occupied the soul and mind not only of the imaging community but also of the clinical cardiology, cardiac surgery, and basic science. It is fair to say that until the Surgical Treatment of Ischemic Heart Failure (STICH) trial1 viability testing was published, the debate in almost all the major national and international cardiology meetings has not been on the relevance of viability testing but rather on the comparison of one method versus another; a beauty contest of some sort! The debates between the experts in interventional cardiology, electrophysiology, nuclear cardiology, echocardiography, and magnetic resonance imaging are often heated and at times entertaining with the audience cheering one side or the other depending on the venue. Frankly this is to be expected as these dedicated individuals have spent years improving their beloved methods; anything less is hypocrisy! This editorial viewpoint is not going to discuss the attributes and follies of the various methods used to determine myocardial viability but rather to try to understand the “Achilles heel” of viability testing. We hope that in doing so we may shed some light on the surprising discrepancy of the results from STICH from prior reports which were almost entirely based on single-center studies. This is not intended to detract from the tremendous importance of STICH, which was nicely articulated by Bonow and Holly2 on the pages of this Journal. We will use nuclear imaging to make few relevant points with regards to the utilization of viability testing in patient care; these can easily be translated to other imaging methods (Table 1). Details of the method used in our laboratory and the definitions could be found elsewhere3:
Table 1
Issues specific to viability assessment
1
Communicating the results: the report versus verbal communication
2
The method of coronary revascularization: percutaneous coronary intervention versus coronary artery bypass grafting
3
The status of target vessels
4
The endpoint(s) of interest and the timing of endpoint in relation to coronary revascularization
5
The severity of co-morbid conditions
6
The degree of left ventricular remodeling
(1)
The report “Does Mr. (Mrs.) XXY have viable myocardium”? Is not an unusual question that triggers testing but sadly the wrong question as all hearts must have some viable myocardium for the patient to be alive. A more relevant question that should be answered is: “Does Mr. (Mrs.) XXY have sufficient viable but inadequately perfused myocardium to assure good results with coronary revascularization (CR)?” One would expect that a large area of viable myocardium would favor CR whereas with an extensive area of non-viable myocardium CR may be detrimental.4 The challenge is how to put the response in a report to be useful in patient management. Wackers’ recent editorial “The Art of Communicating: The Nuclear Cardiology Report” is a great guide in this regards.5 The report is art and science, bundled in one, and it is especially more demanding when it comes to viability assessment. The reader should always think of what kind of information he or she would like to have if they were taking care of the patient. This should be communicated in simple and clear language without unnecessary technical jargon. Reports have come a long way since the American Society of Nuclear Cardiology (ASNC) became involved, but they are far from being perfect in viability assessment. The average report may be satisfactory in some patients, usually those on the far ends of the spectrum, but we still have ways to go with regards to improving the report of viability testing for the majority of patients.
The solution to the problem is simple; verbal communication. In our laboratory, we often review the coronary angiograms and clinical presentation in addition to the viability images (based on physicians’ request) to have a better appreciation of the overall clinical picture before a decision is made for or against CR (see below). As the reader may surmise, this decision goes beyond categorical interpretation of the images as “viable” versus “non-viable.” This interaction between the imaging experts, especially those with additional clinical and catheterization experience and expertise, with the general cardiologists, the interventional cardiologists, and the cardiac surgeons may at least in part explain why single-center studies might be more relevant to clinical care than a multicenter study where the viability information is collected for “research use” and does not allow for that interaction. This may also explain why one imaging modality is favored over others depending on the institution.
(2)
Type of CR procedure Clinical practice guidelines are available and have recently been updated for percutaneous coronary interventions (PCI) and coronary artery bypass grafting (CABG),6,7 but should the findings on imaging influence the choice to perform one versus the other in a particular patient? To illustrate this point, let us consider a patient with non-viable myocardium in distal ½ of anterior wall and septum plus the apex but viable myocardium in proximal ½ of anterior wall and septum (a rather common scenario). Let us assume the coronary angiogram shows lesions amenable to either PCI or CABG. We argue that the location of the lesion(s) may help guide our choice (Figure 1).
Figure 1
Three examples of lesions in the left anterior descending artery (marked as a circle inside the vessel): A at mid level, B at proximal level, and C at both (see text for explanation)
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The assumptions below are not based on actual data and regrettably have not been tested prospectively but the principles are intuitive and we have used them routinely for many years. In panel A, neither PCI nor CABG is expected to improve outcome, as the lesion in the left anterior descending artery (LAD) is at the mid level; the territory beyond the stenosis is not viable while the viable proximal ½ of the LAD territory is nourished via antegrade flow from the proximal non-diseased segment of the artery. In panel B, both PCI and CABG are expected to produce similar results as the LAD lesion is proximal and hence CR by either modality would improve flow to the viable myocardium in the proximal LAD territory. The distal ½ of the LAD territory is again not expected to improve just as in panel A. In panel C, the situation is different and favors PCI over CABG because with PCI of the sequential lesions, flow could improve to the viable myocardium in the proximal ½ of LAD territory. With CABG, the conduit graft is inserted beyond the most distal lesion and therefore the viable zone in the proximal ½ of the LAD territory will continue to have insufficient antegrade and retrograde flow producing results that are expected to be inferior to those obtained by PCI. Therefore, matching the coronary anatomy with the viability images is crucial not only for recommending for or against CR but also the choice of the CR procedure.
Another example is that of a patient with non-viable myocardium in the territory of the left circumflex artery (LCX), and viable myocardium in the right coronary artery (RCA). Angiographically, the LCX lesion is amenable to CR but not the RCA lesion. The available CR in this patient does not match the image results and is not expected to benefit the patient (or the images). We believe these and other examples are common and are better addressed in single-center studies where the imaging experts take an active role in communicating with the other team members.
(3)
The status of target vessels We have already alluded to this above, but in general CR by any means to diseased coronary vessels with poor run-offs is not expected to be beneficial. It goes without saying that CR of vessels with non-obstructive disease is also not expected to produce favorable results. The status of the target vessels may impact the completeness of CR, which is especially important in patients with extensive coronary artery disease. The status of target vessels is a variable that is important in the evaluation of endpoints. It is conceivable that CR may confer benefit early on, which is then lost because of occlusion of the conduit graft or stent. It is amazing how infrequent the status of CR is known at the time of evaluating endpoints, both in single-center studies and also in the STICH trial.
(4)
The endpoints of interest and timing The endpoints used in viability studies and their timing are as varied as those used for other imaging studies. All-cause mortality, cardiac mortality, myocardial infarction, heart failure symptoms, and hospitalization, changes in left ventricular (LV) ejection fraction (EF) and volumes and wall motion abnormalities have been most commonly used. In the ideal world, one would like to see improvement in all these endpoints immediately after CR and expect that these improvements will persist long-term! Neither the timing nor the constellation of these endpoints should be expected to develop in such an ideal manner. In addition to the variability in the status of CR (as discussed above), the process of reorganization of the metabolic and signaling pathways after prolonged periods of hibernation and repetitive stunning is variable and may be delayed for many months after successful restoration of flow.
Traditionally, hard cardiovascular outcomes are held in the highest regards but it is important to appreciate that different patients place different values on different outcomes. One would think that in a 40-year-old patient, mortality improvement is the most desirable endpoint but quality of life would be more important in an 80-year-old patient. Improvement in one outcome may not automatically translate to improvement in others. We all know of patients with LVEF of 20% who are in advanced heart failure or cardiogenic shock while others enjoy playing single tennis without any problems. Our expectations from CR should be tailored to the individual patient as well.
(5)
Co-morbid conditions The reality is that many patients in whom viability assessment is done have severe and multiple co-morbid conditions. These include advanced age, prior CABG or valve surgery, chronic obstructive lung disease, pulmonary hypertension, associated mitral or aortic valve disease, diabetes, chronic kidney disease, peripheral vascular disease, prior stroke or transient ischemic attacks, and atrial fibrillation. These factors may have as much bearing on subsequent patient management and outcome if not more than the viability studies. This may explain why not all the patients with “sufficient viable myocardium” undergo CR. We argue that the viability studies should not have been requested in the first place! These same factors may affect post-CR endpoints independent of viability status. Again we believe these sensitive issues are more closely monitored in individual institutions.
(6)
The degree of LV remodeling marked LV dilatation and wall thinning often by itself is a marker of poor outcome early or late after CR. The so-called wall-to-wall heart on chest x-rays is a point of no return and in our experience these patients should not undergo CR regardless of what the viability imaging show. The unknown in all this is at what degree of LV dilatation, the process becomes irreversible (is there a cut-off for end-diastolic volume or diameter by 2-dimensional echocardiography beyond which CR is not useful?). Again here we use clinical judgment and we believe this favors single center studies where the imaging experts have a consultative role that surpasses reading the images.
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