LETTER TO THE EDITOR
Patient management guided by viability imaging
Rob S. Beanlands, MDa,
Terrence D. Ruddy, MDa,
Michael Freeman, MDa and
Graham Nichol, MDa
a University of Ottawa Heart Institute, H149, 40 Ruskin Street, Ottawa, Ontario, Canada K1Y 4W7
rbeanlands{at}ottawaheart.ca
We read with interest the study by Siebelink et al. (1) comparing positron emission tomography (PET) and single-photon emission computed tomographic imaging (SPECT)-guided patient management. The investigators are to be commended for undertaking this difficult study. However, there are important limitations that may limit the conclusions drawn.
Patient population. The greatest impact of viability imaging is in patients with severe left ventricular (LV) dysfunction (2,3). Such patients are at high risk for events but have the most to gain from revascularization when significant viable myocardium is detected (2–4). In the Siebelink et al. study (1), the majority of the patients (65%) had only mild to moderate LV dysfunction. Thus, it is not surprising that the overall event rate was low. This low event rate with the small sample size makes it unlikely that any difference between the two strategies would be identified.
There was no information regarding angina or prior stress perfusion imaging, both of which could lead to preselection bias and could influence decisions for revascularization (even after viability data are available). All of the patients had preceding angiograms, which likely biases therapy toward revascularization.
The time from randomization to revascularization was lengthy (estimated at 115 vs. 132 days). Previous studies have shown that long waits to revascularization can lead to high mortality rates in patients with severe LV dysfunction (4). In the Siebelink et al. (1) study, there were two deaths (2%) awaiting revascularization, further suggesting that the study population represented a relatively lower-risk group of patients. Furthermore, some of the benefits of revascularization may have been lost because of the revascularization delay (4).
Viability imaging. Severe 2-methoxy isobutyl isonitrile (MIBI) perfusion defects can have viable myocardium by fluorodeoxyglucose (FDG) (5–7). The investigators observed 20% nonviable myocardium by MIBI and 16% by FDG. A focus on more severe LV dysfunction would have increased the number of severe defects and the discrepancies with FDG as observed by others (5–7).
Defects with greater than 50% MIBI uptake were called "jeopardized." For ammonia/FDG, only mismatch was considered "jeopardized." Recent data indicate that the degree of FDG uptake relates to the degree of recovery of function (8,9). The use of 50% cutoff for FDG has been beneficial in directing revascularization (10). In the current study, it would appear that a fixed defect with 65% MIBI uptake would be considered jeopardized, but 65% ammonia uptake matched with FDG may have been called "scar." In some patients, this approach could underestimate viable myocardium defined by FDG imaging and bias against directing patients to surgery after FDG PET.
Sample size (n = 103). A 20% overall event rate was proposed based on a previous study (11). The selection of 20% difference (presumably SPECT 30% and PET 10%) appears arbitrary. This would represent an overly optimistic 66% relative reduction in event rate for this relatively low-risk patient population. Smaller differences would still be clinically relevant particularly for patients with severe LV dysfunction. At least one ongoing randomized controlled study will recruit 412 patients with severe LV dysfunction to show a 30% relative reduction in event rate, based on previous outcome studies in a similar population (2); the impact of FDG PET-guided therapy (9) on outcomes and costs will be compared to standard care in this study.
In summary, the interpretation of the results of the study by Siebelink et al. (1) is hindered by the limitations common to many clinical trials, namely those of appropriate patient selection and sample size. Therefore, it is difficult to draw conclusions regarding the role of either technique in the most appropriate group of patients for viability testing—those with severe LV dysfunction. The study, however, does provoke the need for larger randomized controlled trials evaluating this problem, some of which are under way. The researchers should be congratulated on being the first to take this initiative.
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References
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1. Siebelink H-MJ, Blanksma PK, Crijns HJGM, et al. No difference in cardiac event-free survival between positron emission tomography–guided and single-photon emission computed tomography–guided patient management. J Am Coll Cardiol. 2001;37:81–88[Abstract/Free Full Text]
2. Di Carli MF, Davidson M, Little R, et al. Value of metabolic imaging with positron emission tomography for evaluating prognosis in patients with coronary artery disease and left ventricular dysfunction. Am J Cardiol. 1994;73:527–533[CrossRef][Medline]
3. Pagano D, Lewis ME, Townend JN, et al. Coronary revascularisation for postischaemic heart failure: how myocardial viability affects survival. Heart. 1999;82:684–688[Abstract/Free Full Text]
4. Beanlands RSB, Hendry PJ, Masters RG, et al. Delay in revascularization is associated with increased mortality rate in patients with severe left ventricular dysfunction and viable myocardium on fluorine 18-fluorodeoxyglucose positron emission tomography imaging. Circulation 1998;98:II51–6.
5. Sawada SG, Allman KC, Muzik O, et al. Positron emission tomography detects evidence of viability in rest technetium-99m sestamibi defects. J Am Coll Cardiol. 1994;23:92–98[Abstract]
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7. Dilsizian V, Arrighi JA, Diodati JG, et al. Myocardial viability in patients with chronic coronary artery disease. Comparison of 99mTc-sestamibi with thallium reinjection and [18F]fluorodeoxyglucose. Circulation. 1994;89:578–587[Abstract/Free Full Text]
8. Pagano D, Townend J, Littler WA, et al. Coronary artery bypass surgery as treatment for ischemic heart failure: the predictive value of viability assessment with quantitative positron emission tomography for symptomatic and functional outcome. J Thorac Cardiovasc Surg. 1998;115:791–799[Abstract/Free Full Text]
9. Beanlands RS, de Kemp RA, Ruddy TD, et al. A quantitative method for defining scar with F-18–FDG PET predicts the degree of recovery of LV function post-revascularization (abstr). Circulation 2000;102 Suppl II:II724.
10. Haas F, Haehnel CJ, Picker W, et al. Preoperative positron emission tomographic viability assessment and perioperative and postoperative risk in patients with advanced ischemic heart disease. J Am Coll Cardiol. 1997;30:1693–1700[Abstract]
11. Tamaki N, Kawamoto M, Takahashi N, et al. Prognostic value of an increase in fluorine-18 deoxyglucose uptake in patients with myocardial infarction: comparison with stress thallium imaging. J Am Coll Cardiol. 1993;22:1621–1627[Abstract]
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R. S. B. Beanlands, T. D. Ruddy, R. A. deKemp, R. M. Iwanochko, G. Coates, M. Freeman, C. Nahmias, P. Hendry, R. J. Burns, A. Lamy, et al.
Positron emission tomography and recovery following revascularization (PARR-1): the importance of scar and the development of a prediction rule for the degree of recovery of left ventricular function
J. Am. Coll. Cardiol.,
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40(10):
1735 - 1743.
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