CLINICAL STUDY: EXERCISE TESTING
When to stress patients after coronary artery bypass surgery?
Risk stratification in patients early and late post-CABG using stress myocardial perfusion SPECT: implications of apppropriate clinical strategies
Michael J. Zellweger, MD*,
Howard C. Lewin, MD*,
Shenghan Lai, MD, PhD ,
Eric A. Dubois, MD, PhD*,
John D. Friedman, MD, FACC*,
Guido Germano, PhD, FACC*,
Xingping Kang, MD*,
Tali Sharir, MD* and
Daniel S. Berman, MD, FACC*
* Department of Imaging (Division of Nuclear Medicine), Medicine (Division of Cardiology), and Artificial Intelligence in Medicine Program, Cedars–Sinai Medical Center; the CSMC Burns and Allen Research Institute, and the Department of Medicine, University of California Los Angeles, School of Medicine, Los Angeles, California, USA
University of Miami, Miami, Florida, USA
Manuscript received July 30, 1999;
revised manuscript received August 14, 2000,
accepted September 27, 2000.
Reprint requests and correspondence: Dr. Daniel S. Berman, Cedars–Sinai Medical Center, 8700 Beverly Boulevard, Room A-041, Los Angeles, California 90048
bermand{at}cshs.org
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Abstract
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OBJECTIVES
The study compared the prognostic significance of myocardial perfusion single-photon emission computed tomography (SPECT) (MPS) in patients early and late after coronary artery bypass graft surgery (CABG).
BACKGROUND
The long-term effectiveness of CABG is limited by graft stenosis. The greatest incidence of graft occlusion occurs between five and eight years after surgery. However, little is known regarding the appropriate time to stress patients post-CABG with respect to risk stratification.
METHODS
We identified 1,765 patients, who underwent MPS 7.1 ± 5.0 years post-CABG. All patients underwent rest Tl-201/stress Tc-99m sestamibi MPS and were followed up  1 year after testing. Patients with early CABG or PTCA (<60 days after MPS) were censored. The prognostic population consisted of 1,544 patients. A semiquantitative visual analysis employing a 20-segment model was used to define summed stress score (SSS), summed rest score (SRS), summed difference score (SDS), and the number of nonreversible segments (NRS).
RESULTS
During follow-up, 53 cardiac deaths (CD) occurred. There was a significant increase in annual CD rates as a function of SSS. A multivariate analysis identified age, ischemia (SDS), and infarct size (NRS) as independent predictors of CD. Nuclear variables added incremental value to prescan information. The annual CD rate was relatively low (1.3%) in patients  5 years post-CABG. In this subgroup only age and infarct size (NRS) were predictive of CD.
CONCLUSION
MPS is strongly predictive of subsequent CD in post-CABG patients and adds incremental value over clinical and treadmill test information. Our data suggest that symptomatic patients 5 years and all patients >5 years post-CABG may benefit from testing.
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Abbreviations and Acronyms
| | CABG | = coronary artery bypass graft surgery | | CAD | = coronary artery disease | | CD | = cardiac death | | LVEF | = left ventricular ejection fraction | | MPS | = myocardial perfusion SPECT | | NRS | = nonreversible segments | | PTCA | = percutaneous transluminal coronary angioplasty | | SDS | = summed difference score | | SPECT | = single-photon emission computed tomography | | SRS | = summed rest score | | SSS | = summed stress score |
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Myocardial perfusion SPECT (single-photon emission computed tomography) (MPS) has been shown to be an effective method for risk-stratification and guide for referral to catheterization (1–3) in general coronary artery disease (CAD) populations as well as in patients late (>5 years) post-CABG (coronary artery bypass graft surgery) (4). However, little is known regarding the value of MPS in risk stratification or management of patients early (5 years) compared to late (>5 years) post-CABG.
In 1996, an estimated 598,000 CABG procedures were reported in the U.S. (5). The long-term effectiveness of this now common procedure is limited by graft stenosis and by progression of disease in native vessels. Saphenous vein graft occlusion rates have been reported to range from 41% to 50% at 10 years post-CABG (6–8). Fitzgibbon et al. (7) observed in 222 patients that the greatest incidence of graft occlusions and of severe graft atherosclerosis occurred in the time between 5 and 7.5 years post-CABG surgery. Grondin et al. (8) reported an occlusion rate of 12% to 20% in the first year post-CABG, 2% to 4% annual rates for the next four or five years, and approximately 50% at 10 years. The time interval between 5 and 10 years after surgery also corresponds to the period when severe symptoms most often recur. However, the new development or recurrence of chest pain is not useful for detecting graft occlusion, with a reported sensitivity of 60% and a specificity of 20% (9). Based on these findings we have chosen the cutoff of five years for comparing two different groups (early and late) post-CABG. The goals of the present retrospective study were threefold: 1) to define the incremental prognostic value of MPS for the prediction of cardiac death (CD) in patients who underwent CABG, either or >5 years before nuclear testing; 2) to define the ability of nuclear testing to risk-stratify these patients; and 3) to determine the impact of nuclear testing on referral to early catheterization.
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Methods
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Study population.
We identified 1,765 consecutive CABG patients (excluding patients with significant valvular heart disease or nonischemic cardiomyopathies) for rest Tl-201/Tc-99m sestamibi SPECT in our laboratory between 1/17/91 and 2/4/97. Of these, 58 (3.3%) were lost to follow-up. A total of 163 patients who underwent early revascularization (percutaneous transluminal coronary angioplasty, PTCA; or redo-CABG <60 days after nuclear testing) (10,11) were censored from the prognostic analysis of this study. The final prognostic population was 1,544 patients, of whom 1,015 (65.7%) patients underwent exercise and 529 (34.3%) adenosine SPECT.
Rest/stress myocardial perfusion SPECT protocol.
All patients underwent rest Tl-201/stress Tc-99m sestamibi MPS as previously described (12). Initially, Tl-201 (2.5 to 4 mCi) was administered intravenously at rest, with the dose varying by weight. Rest Tl-201 SPECT was performed 10 min after the tracer injection. For the patients undergoing exercise stress, a symptom-limited exercise test was performed, using standard protocols with a 12-lead electrocardiogram (ECG) recording each minute of exercise and continuous monitoring of leads aVF, V1 and V5. Whenever possible, beta-blocking medications, as well as negative chronotropic calcium antagonists, were withheld for 48 h, and long-acting nitrates were withheld for 6 h before the exercise testing. End points of exercise testing were physical exhaustion, severe angina, sustained ventricular arrhythmia or exertional hypotension. At near-maximal exercise, a 20–35-mCi dose of Tc-99m sestamibi was injected (actual dose varied with patient weight) and exercise was continued for an additional minute after injection. For the adenosine stress test, patients were instructed not to consume products containing caffeine for 24 h before the test. After rest Tl-201 SPECT, adenosine was infused (140 µg/kg/min for 6 min) for pharmacologic stress. Tc-99m sestamibi was injected at the end of the 3rd min of infusion, and SPECT was performed approximately 1 h later. Twenty-four hour Tl-201 redistribution imaging was performed when fixed defects were present, whenever possible.
For both types of stress, blood pressure was measured and recorded at rest, at the end of each stress stage, and at peak stress. Maximal degree of ST-segment change at 80 ms after the J-point of the ECG was measured and assessed as horizontal, downsloping or upsloping. The ECG response to testing was categorized as either nonischemic (no significant ECG changes), ischemic (>1 mm horizontal or downsloping or 1.5 mm upsloping ST-segment elevation or depression except in leads with significant Q-waves or in lead aVR), equivocal (borderline ECG changes), or nondiagnostic (exercise-induced ECG changes uninterpretable because of digoxin use, paced rhythm, bundle branch block). In 703 patients with interpretable stress ECG, the Duke treadmill score was calculated and defined as low risk (>4); moderate risk (–10 to +4); or high risk (<–10) (13). In 86% of patients the standard Bruce protocol was utilized, and the Duke treadmill score was defined as:
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In 14% of patients a modified Bruce protocol was used, here when the exercise duration was 4.5 min the exercise duration was modified by subtracting 3 min.
SPECT acquisition protocol.
The SPECT imaging was performed as previously described (12). The SPECT studies were performed using a circular or elliptical 180° acquisition for 60 to 64 projections at 20 to 25 s per projection. During imaging, two energy windows were used for Tl-201, including a 30% window centered on the 68 to 80 keV peak and a 10% window centered on the 167-keV peak. For Tc-99m sestamibi SPECT, a 15% window centered on the 140-keV peak was used. No attenuation or scatter correction was used. All images were subject to quality control measures, as previously described (12).
Image interpretation.
Semiquantitative visual interpretation was performed using a 20-segment model as previously described (1). Each segment was scored using a 5-point scoring system (0 = normal, 1 = equivocal, 2 = moderate, 3 = severe reduction of radioisotope uptake, and 4 = apparent absence of detectable tracer uptake in a segment). A summed stress score (SSS) was obtained by adding the scores of the 20 segments of the stress sestamibi images (12). A summed rest score (SRS) was obtained similarly by adding the scores of the 20 segments on the rest images. Subsequently, to assess the defect reversibility, a summed difference score (SDS) was calculated by subtracting the SRS from the SSS. As per our previous work, SSS <4 was considered normal; 4 to 8, mildly abnormal; >8 moderately to severely abnormal (2). Similarly, for the degree of reversibility, SDS <2 was considered nonischemic; between 2 to 4 mildly ischemic; and >4 moderately to severely ischemic. The number of nonreversible segments (NRS) (stress/rest score combinations of 4/4, 4/3, 3/3, 3/2, or 2/2) was used as a surrogate marker of infarct size and left ventricular function, (with each one unit of NRS representing approximately 5% of the myocardium) since left ventricular ejection fraction (LVEF) data from gated SPECT was not available in the entire population. We choose to use NRS because it is more closely related to the extent of infarcted myocardium. When available, the late-redistribution Tl-201 scores were used in place of rest scores for calculation of SRS, SDS and NRS.
An annual event rate <1% was considered as low risk, 1% to 5% as intermediate, and >5% as high risk (14).
Patient follow-up.
The follow-up was performed by scripted telephone interviews by individuals blinded to the patient’s test results. All patients were followed for at least one year for the outcome of CD and early catheterization. The mean follow-up time was 686 ± 194 days. Cardiac death was confirmed by review of death certificate and hospital chart or physician’s records. Early catheterization was defined by catheterization performed within 60 days after the nuclear testing (2).
Prescan likelihood of ischemia.
For purposes of analyzing patients in different risk subsets we used analysis of the prescan likelihood of ischemia as an aggregate descriptor of proven prognostic importance based on Bayesian analysis of patient data and calculated by CADENZA (15). For patients undergoing pharmacologic stress, historical information alone was considered (age, gender, presenting symptoms, resting blood pressure, smoking history, cholesterol, glucose intolerance, and resting ECG); whereas for patients undergoing exercise stress, the prescan likelihood of ischemia also included exercise test information (exercise protocol, exercise duration, metabolic equivalents [METs] achieved, maximum heart rate, maximum blood pressure, exertional hypotension, exercise-induced symptoms, ST-segment change and slope). For purposes of a subanalysis, assessing whether symptoms affected outcomes, we used the patient’s symptoms at the time of testing. For this purpose, patients with typical anginal chest pain, atypical anginal chest pain or shortness of breath were considered symptomatic, whereas patients with nonanginal chest pain or no symptoms were considered asymptomatic.
Statistical analysis.
Comparisons between patient groups were performed using the Student t-test for continuous variables and a chi-square test for the categorical variables. All continuous variables are described as a mean ± SD. A p value <0.05 was considered statistically significant.
Because NRS was used as a surrogate marker for LVEF we compared post-stress EF (16) and NRS by linear regression in the 235 patients who underwent gated SPECT.
Survival analyses were performed to investigate factors that influence time-to-event (survival time), which was defined from the index nuclear testing to CD. Because the nuclear parameters were not normally distributed, and the link function between the nuclear parameters and the distribution of time-to-event can be very general, nonparametric Cox proportional hazards survival models were used to explore appropriate regression functions or transformations of the nuclear parameters. Then, a conventional Cox proportional hazards survival model was used to identify factors that are independently associated with CD (17). Because the proportionality assumption for the Cox proportional hazards survival model requires that the ratio of hazard rates for different levels of an independent variable must be constant, and patients who underwent exercise stress were different from those who underwent pharmacologic stress with respect to the hazard of an event, a stratified Cox proportional hazards survival model was used. Stress type was treated as the stratification variable. To investigate the prognostic value of nuclear variables (SSS, SDS, NRS), age, gender, time elapsed between CABG and nuclear testing, prior myocardial infarction (MI), prescan likelihood of ischemia, and symptoms were included as covariates in the initial Cox model. Variables that were not significant were eliminated from the initial model in a backward manner, yielding a final model. The final model includes only variables that were independently associated with CD. The incremental value of nuclear testing was evaluated by calculating the global  2 after having added the nuclear variables to the prescan information. All multivariate analyses were done using the SAS 6.12 statistical package.
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Results
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Study population.
The baseline characteristics of patients undergoing exercise and adenosine stress are shown in Table 1. The former were younger, more likely to be men, were less symptomatic, had a lower prescan likelihood of ischemia, and lower SSS, SRS and NRS than the latter.
Mean left ventricular EF in the 235 patients (14%) who underwent gated SPECT was 49.1 ± 15.2. There was an inverse correlation between the EF and the NRS (y = 55.3 to 3.1 x, r = 0.59). Using this equation, an NRS of 4 was roughly equivalent to an EF of 43%. Since Sharir et al. (18) have shown that an LVEF <45% to be the threshold below which there is a significant increase risk of CD, we used this value when developing the follow-up strategy described below.
Outcome events.
During follow-up, 53 CDs occurred. Sixteen CDs (2.5%) occurred in 628 patients studied 5 years post-CABG and 37 (4.0%) CDs in 916 patients studied >5 years post-CABG (p = 0.11). Symptoms contributed no prognostic value (Fig. 1). Patients who suffered CD were significantly older, were more commonly referred for pharmacologic stress, had a more remote surgery and had significantly higher SSS, SRS and NRS (Table 2). Fifty-seven patients had an early (<60 days postnuclear) redo-CABG and 66 patients had late redo-CABG. Their annual CD rate was 2.7% and 2.4%, respectively. Of note, in patients 5 years post-CABG undergoing repeat surgery no CDs occurred. In patients >5 years post-CABG, CDs occurred principally in patients with moderately to severely abnormal scan results.
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Figure 1 Annual cardiac death (CD) rates as a function of time and symptoms (n = 1544). p = NS for all comparisons.
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Relationship between SPECT result and outcomes.
Patients with perfusion defects were more likely to die (2.1% vs. 0.4% per year, p = 0.004). The annual CD rates increased significantly as a function of SSS in patients and >5 years post-CABG surgery (Fig. 2).
Early catheterization rates increased significantly as a function of SSS (Fig. 3), p < 0.0001.
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Figure 3 Early catheterization rates as a function of SSS in patients 5 and >5 years post-CABG (n = 1,707). *Statistically significant increase as a function of SSS (p < 0.001). CABG = coronary artery bypass graft surgery; SSS = summed stress score.
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Multivariate analysis.
Age, ischemia (SDS), and infarct size (NRS) were independent predictors of CD with respect to the whole study population (Table 3). In patients 5 years post-CABG, age and infarct size (NRS) were independent predictors of CD. In patients >5 years post-CABG, infarct size (NRS) and ischemia (SDS) were independent predictors of CD.
Incremental value of nuclear testing.
A significant increase in global chi-square occurred with respect to CD after adding nuclear data (SSS, SDS, NRS) to prescan information and adjusting for the type of stress performed (Fig. 4). For the treadmill group, a significant increases in global chi-square (p < 0.05) occurred, with respect to CD, after first adding the Duke treadmill score in a first step and then nuclear data in a second step (Fig. 5).
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Figure 4 Global chi-square values with respect to prescan information and nuclear variables (n = 1,544). *Significant increase of chi-square (p < 0.001).
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Figure 5 Global chi-square values with respect to clinical, treadmill, and nuclear variables (n = 703). *,# Significant increase of chi-square (p < 0.05).
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Optimized strategy for post-CABG risk assessment.
An optimized strategy for risk stratification based on this population was developed, in which only patients with an annual CD rate of 1% would be referred for catheterization (Fig. 6).
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Figure 6 Outcomes (annual cardiac death rates) with optimized nuclear strategy. CABG: coronary artery bypass grafting; CD = cardiac death; EF = ejection fraction; NRS = number of non-reversible segments; SDS = summed difference score; SSS = summed stress score. *If non-viable benefit of angiography less clear.
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Discussion
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Patients with previous CABG represent a substantial CAD population. Risk stratification for subsequent CD in these patients has clinical significance, because repeat CABG could reduce the risk of death in high-risk cohorts. Our results show a strong relationship between nuclear variables and CD in patients, both and >5 years post-CABG. The number of nonreversible segments (NRS, a surrogate marker of infarct size and LVEF) was the strongest predictor of CD. By NRS, patients with CD had, on average, twice the amount of infarcted myocardium compared to patients without CD. In general, early catheterization rates paralleled the risk of CD.
Risk stratification post-CABG.
Patients undergoing pharmacologic stress were at higher risk for CD. Therefore, type of stress was used as stratification variable.
Nuclear testing added incremental prognostic value to prescan information. The MPS variables for infarct size and ischemia were predictors of CD in the whole patient population and in patients >5 years post-CABG. In patients 5 years post-CABG and in treadmill patients, infarct size was the only nuclear variable predictive of CD.
Although exercise testing without imaging has been evaluated (19–21), these studies have generally shown little prognostic value in post-CABG patients. In contrast, several studies have shown that after evaluation of treadmill and exercise data, MPS provides incremental prognostic information for patients post-CABG (22,23). Palmas et al. (4) demonstrated in 294 patients 5 years post-CABG that the Tl-201 summed reversibility score and the presence of increased lung uptake of Tl-201 added significant prognostic information to a clinical model. Nallamothu et al. (24) reported similar incremental information, even when coronary angiographic variables were taken into account. In 250 patients, the extent of the Tl-201 perfusion abnormality, multivessel perfusion defects, and increased lung thallium uptake were significant independent predictors of events.
Lauer et al. (23) demonstrated that even in asymptomatic patients on average seven years post-CABG, thallium-perfusion defects were independent predictors of subsequent death or MI. Their calculated annual all-cause mortality rates of 3% versus 1% in patients with and without thallium-perfusion defect, respectively, are discretely higher than our reported CD rate. In the only prognostic study to date regarding MPS early (2 years) post-CABG, SSS was the strongest independent predictor for total mortality (24a). Their calculated annual event rate of 1.1% is comparable to ours of 1.3% for patients 5 years post-CABG.
Implications on follow-up strategies in patients post-CABG.
As shown in Figure 6, based on our results, asymptomatic patients 5 years post-CABG have a low CD rate and would not benefit from routine nuclear testing. Symptomatic patients 5 years post-CABG benefit from nuclear testing because the assessment of the extent and severity of ischemia provided can help guide appropriate therapy. Our findings suggest, however, that patients >5 years post-CABG would benefit from MPS irrespective of symptoms (i.e., even when asymptomatic). Patients who have normal scans have a low risk of CD and would, in general, not need catheterization. Patients with an abnormal scan can be further stratified by infarct size (NRS) or LVEF. If >20% of the myocardium is infarcted (NRS >4 or an estimated EF <45%), patients are at high risk for CD (18) regardless of the presence of ischemia. These patients may benefit from viability assessment. If there is evidence of a small infarct (NRS 4 or an estimated LVEF 45%), patients can be further stratified by the extent of ischemia (SDS). If there is no ischemia or mild ischemia (SDS 6) the annual CD rate is low; such patients could be followed conservatively. If there is moderate to severe ischemia (SDS >6), patients have at least a moderate risk of CD and may benefit from catheterization. Current AHA/ACC clinical guidelines argue against routine testing of asymptomatic patients but do allow for "assessment of selected symptom-free patients" (25).
The question of when to retest patients who have been tested and subsequently treated medically is a much more complex issue that would require a more robust patient population than the one presently being examined to address adequately this important aspect of patient management.
Study limitations.
Because patients tested in our laboratory underwent CABG at various hospitals, we are not able to provide data about the surgical procedure (i.e., how many bypasses per patient were implanted; how many venous and/or arterial conduits were used).
Because gated LVEF SPECT was not available in all patients in this study, we used NRS to substitute for LVEF. Substituting gated LVEF for NRS would probably add incremental prognostic value in patients post-CABG and allow a risk-stratification management based on the interplay of LVEF and ischemia (SDS) as it has been shown for a general population by Lewin et al. (26). This assumption is supported by a study by Borges-Neto et al. (27), who have reported the long-term follow-up of 182 patients undergoing exercise radionuclide angiography and coronary angiography <3 years post-CABG. In their study, which did not include perfusion data, the exercise LVEF was the strongest predictor of CD, contributing beyond clinical and catheterization data.
Quantitative analysis was not performed in the current patient group because it was not in operation on all of our computer systems at the time of the SPECT studies. A semiquantitative visual analysis was utilized, similar to that which was the basis for the quantitative analysis program developed by Cedars-Sinai Medical Center and Emory University. These prognostic results correlate strongly with those of quantitative analysis (28).
Conclusions.
Dual isotope MPS in patients post-CABG is strongly predictive of subsequent CD and adds incremental value over clinical and treadmill test information. Our data suggest that symptomatic patients 5 years and all patients >5 years post-CABG may benefit from testing. However, this may not be true for those patients who are asymptomatic within the first 5 years post-CABG.
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Acknowledgments
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The authors wish to acknowledge the expert technical assistance of Romalisa Miranda, Raluca Dane Agafitei and Ishac Cohen.
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Footnotes
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Michael J. Zellweger, MD, is the recipient of a fellowship granted by Adumed and Novartis Foundation, Basel, Switzerland.
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References
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G. Gatti, C. Bentini, G. Maffei, F. Ferrari, M. Dondi, P. Pacilli, and P. Pugliese
Noninvasive Dynamic Assessment With Transthoracic Echocardiography of a Composite Arterial Y-Graft Achieving Complete Myocardial Revascularization
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T M Bateman and E Prvulovich
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M. J Zellweger, R. Hachamovitch, X. Kang, S. W Hayes, J. D Friedman, G. Germano, M. E Pfisterer, and D. S Berman
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C Anagnostopoulos, M Harbinson, A Kelion, K Kundley, C Y Loong, A Notghi, E Reyes, W Tindale, and S R Underwood
Procedure guidelines for radionuclide myocardial perfusion imaging
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J. Gurevitch, T. Gaspar, B. Orlov, R. Amar, D. Dvir, N. Peled, and D. J. Aravot
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Committee Members, F. J. Klocke, M. G. Baird, B. H. Lorell, T. M. Bateman, J. V. Messer, D. S. Berman, P. T. O'Gara, B. A. Carabello, R. O. Russell Jr, et al.
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F. J. Klocke, M. G. Baird, B. H. Lorell, T. M. Bateman, J. V. Messer, D. S. Berman, P. T. O'Gara, B. A. Carabello, R. O. Russell Jr, M. D. Cerqueira, et al.
ACC/AHA/ASNC Guidelines for the Clinical Use of Cardiac Radionuclide Imaging--Executive Summary: A Report of the American College of Cardiology/American Heart Association Task Force on Practice Guidelines (ACC/AHA/ASNC Committee to Revise the 1995 Guidelines for the Clinical Use of Cardiac Radionuclide Imaging)
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M. J. Zellweger, M. Weinbacher, A. W. Zutter, R. V. Jeger, J. Mueller-Brand, C. Kaiser, P. T. Buser, and M. E. Pfisterer
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F. M. Jara, J. Kalush, and M. L. Kahn
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Abstract
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