Ann Thorac Surg 1997;64:1694-1701
© 1997 The Society of Thoracic Surgeons
Original Articles: Cardiovascular
PET Scan Predicts Recovery of Left Ventricular Function After Coronary Artery Bypass Operation
Willem J. Flameng, MD, PhD,
Bharati Shivalkar, MD,
Bart Spiessens,
Alex Maes, MD,
Johan Nuyts, PhD,
Johan VanHaecke, MD,
Luc Mortelmans, MD
Departments of Cardiac Surgery, Cardiology, Biostatistics, and Nuclear Medicine, Katholieke Universiteit Leuven, Leuven, Belgium
Accepted for publication May 30, 1997.
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Abstract
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Background. Viable but hypocontractile myocardium can show functional improvement after revascularization (hibernation). It is sometimes difficult, however, to predict viability and recovery in patients with severe left ventricular function. This study sought to identify possible predictive factors of recovery of cardiac function after revascularization in patients with three-vessel disease.
Methods. Positron emission tomography (fluoro-18-deoxyglucose uptake for metabolism; nitrogen 13–labeled ammonia for flow) and equilibrium-gated nuclear angiography (for the global ejection fraction) were performed in 59 patients with three-vessel disease before and after undergoing coronary artery bypass grafting. The positron emission tomographic data were expressed as match normal (flow and metabolism normal), mismatch (low flow, high metabolism), match viable (moderate decrease in flow and metabolism), and match necrosis (low flow and metabolism).
Results. Stepwise logistic regression analysis showed that only mismatch regions played a significant role in predicting postoperative improvement in function (p = 0.019). There were 1.7 ± 1.5 mismatch regions in 31 patients who showed an improvement in their ejection fraction (0.47 ± 0.14 versus 0.58 ± 0.11; mean ± standard deviation) versus 0.8 ± 1.0 mismatch regions (p = 0.017) in patients who did not show recovery. There was more pronounced functional improvement with increasing numbers of mismatch regions, and patients with at least one mismatch region had a high likelihood of recovery (p < 0.001). In patients with a very low preoperative ejection fraction and two or more mismatch regions, there was early significant recovery (0.27 ± 0.08 versus 0.46 ± 0.06; p = 0.009).
Conclusions. At least one mismatch region must be present for there to be a postoperative functional benefit. When a low left ventricular ejection fraction is associated with mismatch, early recovery is substantial.
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Introduction
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See also page 1701.
Reports of dramatic improvement in left ventricular (LV) function after coronary artery bypass grafting (CABG) were published as early as the late 1960s [1–3], and there was even suggestion that ischemic noninfarcted myocardium can exist in a state of "functional hibernation" [4].
In most patients undergoing CABG, the global function is within normal limits and CABG relieves the ischemia. In patients with reduced LV function, the goal of coronary revascularization is indeed recovery of function. However, not every patient undergoing surgical revascularization for LV dysfunction necessarily benefits from the procedure. Numerous contributing factors, such as LV function, previous myocardial infarction, the degree of distal disease, the caliber of the diseased vessel, and operative and myocardial protection techniques, also play a role.
Several studies [5–8] have shown that surgical revascularization can be beneficial to patients who even have symptoms of heart failure. Indeed, the LV functional outcome along with early and late survival are positively influenced if there is evidence of an "improvable element" [9].
Therefore it is essential to carefully select patients who could benefit from surgical revascularization. In the assessment of patients as possible candidates for revascularization, many methods have been employed to evaluate the viability of the myocardium (which could benefit from revascularization). These methods include stress-rest redistribution reinjection thallium scintigraphy, positron emission tomography (PET), evolution of the inotropic reserve of dysfunctional myocardium, and testing of the anginal response to nitrates. Information on the effect of patient selection using these tests on early and long-term results after revascularization is scarce [10].
In this study, we retrospectively investigated the functional outcome in terms of global and regional LV function in CABG candidates, most with three-vessel disease and varying degrees of regional and global dysfunction, and tried to identify those patients who could benefit from coronary surgical revascularization. We also wanted to assess the value of PET scan findings in predicting the degree of postrevascularization functional recovery.
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Material and Methods
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Fifty-nine consecutive patients (55 male, 4 female) with coronary artery disease (CAD) undergoing CABG and 8 patients with advanced heart failure resulting from ischemic cardiomyopathy were included in the study. The patients ranged in age from 44 to 74 years (median, 60 years). The preoperative electrocardiograms were classified according to the Minnesota code for Q and QS patterns. Nineteen of the 59 patients with CAD showed a Q-wave pattern in the anterior region, and 5 patients showed a Q-wave pattern in the inferior region. All heart failure patients had an anterior wall infarct. There were no diabetics in our CABG population, but 2 of the 8 heart transplant candidates were diabetics. Twenty-eight patients were being treated for hyperlipidemia, 23 of the 59 patients with CAD suffered from hypertension, 11 patients had evidence of peripheral vascular disease, 18 patients suffered from chronic bronchitis, and a smaller number of patients suffered from various maladies (eg, atrial fibrillation, gout, mild kidney insufficiency, kidney stones, gallstones, and varying abdominal problems). All patients were taking 100 to 160 mg of aspirin daily, almost two thirds were also taking a ß-blocker, and a smaller number were also taking other medications (eg, calcium antagonists, angiotensin-converting enzyme inhibitors, nitrates, diuretics, and other specific medications for the treatment of the various medical indications already mentioned). All patients underwent the following investigative procedures.
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Cardiac Catheterization
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One to 3 months preoperatively all patients underwent left heart catheterization with contrast-enhanced angiography and ventriculography. Significant CAD was defined as an 80% or more stenosis of the coronary luminal diameter. The degree of collateralization was interpreted visually. Forty-eight of the 59 patients with CAD had three-vessel disease and 11 had two-vessel disease; the left anterior descending artery was totally occluded in 18 patients. The angiographically determined LV global ejection fraction (EF) ranged from 0.21 to 0.87 (median, 0.48). None of the patients had cardiac catheterization postoperatively, and therefore the functional data obtained were excluded from the data analysis.
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Radionuclide Angiography
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One to 2 days preoperatively, all patients underwent equilibrium-gated nuclear angiography (EGNA). In this study, red blood cells were labeled with 20 mCi of technetium 99m using an in vitro technique. After centrifugation and removal of the supernatant, the labeled red blood cells were injected into the patients. Ten minutes after the injection, EGNA data were acquired for 10 minutes while the patient was positioned under the gamma camera in a left anterior oblique, 45-degree position. The same study was repeated with the patient in a left anterior oblique, 70-degree position and in an anteroposterior position. A small-field gamma camera (PHO/GAMMA V; Siemens, Germany) fitted with a high-resolution parallel collimator was used and connected to a dedicated computer. In the left anterior oblique, 45-degree projection there is minimum overlap of the left and right ventricles and the left ventricle is divided into eight sectors. Global and regional EFs were calculated using standard software (Sopha Medical Benelux), with the normal global EF being 0.67 ± 0.12 (mean ± 2 standard deviations). Therefore the cutoff point for normal global EF was considered to be 0.55. Three months postoperatively all patients had a control EGNA study performed.
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Positron Emission Tomography
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One to 2 days preoperatively all patients underwent PET scanning. A whole-body positron emission tomograph (model 931-08/12; CTI Siemens, Knoxville, TN) with eight detector rings allowing the acquisition of 15 planes with an interplane spacing of 6.75 mm was used. A small cyclotron (Cyclone 10/5; Ion Beam Applications, Louvain La Neuve, Belgium) and auxilliary chemical equipment were used to produce fluoro-18-deoxyglucose (FDG) and nitrogen 13–labeled ammonia (13N-NH3). A 2-minute rectilinear scan, used for positioning the heart within the field of view, was followed by a 15-minute transaxial transmission scan using a germanium 68–labeled ring source for photon attenuation correction.
PET MYOCARDIAL PERFUSION IMAGING.
Twenty-millicuries of 13N-NH3 in 5 mL of saline solution was slowly infused at a constant rate of 10 mL/min, followed by a 20-mL flush of saline solution at the same rate. Data acquisition was started simultaneously with the injection of 13N-NH3. Twenty-one frames were recorded in each patient, with a total acquisition time of 20 minutes.
PET MYOCARDIAL METABOLIC IMAGING.
Metabolic studies were done after a 12-hour overnight fast, using the euglycemic hyperinsulinemic clamp technique [11], which results in a postabsorptive steady-state. A constant intravenous infusion of 20% glucose and insulin was started, and the degree of glycemia was determined by withdrawing serial blood samples every 5 minutes. The rate of glucose infusion was adjusted to achieve normoglycemia, and the insulin infusion rate was adjusted according to the patient's weight. The glucose clamp was optimized during myocardial perfusion imaging. After stabilization of the glycemia between 85 and 95 mg/dL, and after allowing time for isotope decay (not earlier than 50 minutes after 13N-NH3 injection), 10 mCi of FDG was injected as a bolus. Data acquisition was started immediately after the injection, with 22 frames recorded in each patient. The total acquisition time was 70 minutes. The insulin infusion was stopped 15 minutes before the end of acquisition.
IMAGE ANALYSIS.
The 19 frames of the flow study were reconstructed using a Hanning filter (cutoff frequency, 0.3). A three-dimensional delineation of the LV wall was used to construct a polar map of every frame of the dynamic study. A correction for spillover and recovery is based on the measured-point spread function and on the epicardial and endocardial boundaries provided by the delineation. Because these same resolution effects preclude accurate estimation of the wall thickness, we have imposed a constant thickness of 13 mm. Using the PET data from the short axis (apex, mid, base) and long axis (vertical and horizontal), we could divide the left ventricle into six anatomic regions: apical, anterior, anterolateral, septal, inferior, and posterior (Fig 1
). The values obtained within a given segment in a given tomographic plane were averaged throughout the entire heart to give an average value per anatomic segment of the heart. In this way a six-segment model of the left ventricle could be constructed. The flow in the different regions was expressed as a percentage of the maximum uptake in the area considered normal (50% or less stenosis of an epicardial coronary artery). Segments with uptake values of more than 80% of flow in the reference segment (flow index of more than 0.8) were considered normal. The 22 frames of the metabolic study were reconstructed with a Hanning filter (cutoff frequency, 0.4). After the creation of polar maps identical to those constructed for the flow studies, regional FDG-uptake values were estimated by applying a Patlak graphical analysis [12]. The flow reference zone was used as a reference region for FDG. The FDG uptake in the various regions was also expressed as a percentage of the FDG uptake in the reference zone. The data are expressed as a combination of the flow and metabolism data for the six regions already described, the four possible combinations being match normal (with normal flow and metabolism), mismatch (with low flow and normal metabolism), match viable (with a moderate decrease in flow and metabolism), and match necrosis (with a severe decrease in flow and metabolism).
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Fig 1. . Long-axis (horizontal and vertical) and short-axis (apical, mid, and basal level) positron emission tomographic images were used to divide the left ventricle into six anatomic regions (see text for further description).
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Coronary Artery Bypass Grafting
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Surgical coronary revascularization was performed using standard cardiopulmonary bypass techniques and intermittent aortic cross-clamping. Primarily the internal mammary arteries were used to revascularize the left anterior descending coronary artery. The right internal mammary artery was used in 16 patients, 40 received the left mammary artery, and 3 patients received a venous graft on the left anterior descending artery. Forty-six patients (78%) received a total of three grafts, 11 patients (19%) received two grafts, and 2 patients (3%) received one and four grafts each. There was thus an average of 2.8 ± 0.5 grafts per patient. There were no intraoperative surgical problems or other cardiac-related complications in the immediate postoperative period. All patients were discharged on the tenth postoperative day on a low-fat and low-salt diet. Aspirin and ß-blockers were prescribed for all patients (if they had not previously been taking them); nitrates were withdrawn, but other medications being taken preoperatively were continued. No clinical events such as recurrence of angina or myocardial infarction were reported at the 3-month follow-up.
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Statistics
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All numerical data are presented as average values ± standard deviation. Multiple logistic regression analysis (CATMODE procedure, maximum likelihood analysis) was used to determine the significance of the variables in predicting success (postrevascularization recovery in global EF of at least 0.05: yes/no). The change in EF was the dependent variable, with the independent variables being those obtained from the preoperative PET data (ie, the six LV regions defined earlier) and the regional status of flow and metabolism, which was expressed as a combination of the two (match normal, mismatch, match viable, and match necrosis). Further, a paired t test, Wilcoxon ranks sums test, and a one-way analysis of variance were used to determine whether there were significant differences between groups. A p value of less than 0.05 denoted a significant difference. All statistical calculations were done with the Statistical Analysis System software [13].
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Results
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Prediction of Postrevascularization Recovery of Global EF From Preoperative PET Data
A stepwise logistic regression analysis was performed using the change in EF as the dependent variable, with the independent variables being those obtained from the preoperative PET data (ie, the six LV regions defined earlier: apical, anterior, anterolateral, septal, inferior, and posterior) and the regional status of flow and metabolism, which was expressed as a combination of the two (match normal, mismatch, match viable, and match necrosis), to determine which variables were important or significant in predicting success (ie, postoperative improvement of at least 0.05 in the EF: yes/no). Only mismatch regions were found to play a significant role in predicting success when all variables were included in the model (p = 0.019). This appeared to be the case also when mismatch regions were considered alone. Furthermore, a combination of any of the given six regions with mismatch was found to not improve the predictive value (eg, the presence of mismatch in the anterior region or any other region), denoting that the presence of mismatch in any of the six regions contributed equally to improvement in the global EF.
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Relationship Between Regional Flow, Metabolism, and Recovery of Segmental Function
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The anterior, apical, and posterior free wall regional EF (REF) can be determined with reasonable accuracy from the various EGNA projections (45- and 70-degree, left anterior oblique; anteroposterior). The anterolateral and inferior portions cannot be evaluated.
ANTERIOR WALL.
Seventeen of the 59 patients had preserved metabolism but decreased flow (mismatch) in the anterior region of the left ventricle. The REF in this segment improved significantly, from 0.40 ± 0.20 to 0.53 ± 0.19 (p = 0.0043; normal REF, 0.58 ± 0.26). Twenty-six patients showed moderately decreased flow and metabolism (match viability) in the anterior wall, with no change in the postoperative REF (0.38 ± 0.18 versus 0.39 ± 0.21; p = not significant [NS]). Seven patients showed a severe decrease in both flow and metabolism (ie, match necrosis) in the anterior region, with very low REFs (preoperative REF, 0.14 ± 0.07; postoperative REF, 0.10 ± 0.10; p = NS), and the remaining 9 patients had preoperative and postoperative data within normal limits. The results are summarized in Figure 2.
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Fig 2. . The regional changes in ejection fraction before (PRE) and after (POST) bypass operations for the various positron emission tomographic combinations of perfusion and metabolism. (A) The changes in the regional ejection fraction in segments that are match normal. (B) Changes in the regional ejection fraction in segments with mismatch. (C) Changes in the regional ejection fraction in segments that are match viable. (D) Changes in the regional ejection fraction in segments that show match necrosis. (A = anterior; AP = anteroposterior; P = posterior; REF (%) = regional ejection fraction [%]; *p < 0.05.)
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APEX.
Fifteen of the 59 patients showed a mismatch pattern. The REF in this segment improved significantly, from 0.37 ± 0.25 to 0.52 ± 0.24 (p = 0.006; normal REF, 0.75 ± 0.22). Twenty-one patients showed match viability, with no change in their postoperative REF (0.54 ± 0.25 versus 0.55 ± 0.28; p = NS). Fourteen patients showed a severe decrease in both flow and metabolism (ie, match necrosis), with very low REFs (preoperative REF, 0.28 ± 0.20; postoperative REF, 0.29 ± 0.26; p = NS). The remaining 9 patients had normal REFs that did not change significantly.
POSTERIOR WALL.
Eighteen of the 59 patients showed a mismatch pattern. The REF in this segment improved significantly, from 0.69 ± 0.22 to 0.75 ± 0.23 (p = 0.002; normal REF, 0.85 ± 0.14). Only 4 patients showed match viability, with no change in their postoperative REF (0.32 ± 0.13 versus 0.42 ± 0.10; p = NS). Further, only 4 patients showed a severe decrease in flow and metabolism (ie, match necrosis), with low REFs (preoperative REF, 0.42 ± 0.22; postoperative REF, 0.53 ± 0.22; p = NS). Thirty-three patients had regional posterior wall function within normal limits, with no significant change postoperatively.
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Significance of Increasing Number of Mismatch Regions
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Thirty-nine of the 59 patients showed an improvement in their global EF of at least 0.05 3 months postoperatively (0.47 ± 0.14 versus 0.58 ± 0.12; p = 0.042). These patients had 1.7 ± 1.5 mismatch regions, versus 0.8 ± 1.0 mismatch regions (p = 0.017) in the 20 patients without recovery. Further analysis of the data revealed that functional improvement became more pronounced with an increasing number of mismatch regions. We compared patients with two or more mismatch regions with those with one or no mismatch regions and found that there was a significant difference (p = 0.031) between the groups. In patients with two or more mismatch regions (n = 23), the EF improved from 0.47 ± 0.15 to 0.56 ± 0.15 (p = 0.0043), as compared with patients with only one mismatch region (n = 16), who showed significant but less pronounced improvement in their EF (0.52 ± 0.16 versus 0.58 ± 0.14; p = 0.012). On the other hand, patients with no mismatch regions (n = 20) showed no improvement in their LV function 3 months after CABG (EF, 0.49 ± 0.15 versus 0.50 ± 0.16; p = 0.24). The likelihood of postoperative improvement was high in patients with at least one mismatch region (p < 0.001).
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Extent of Functional Recovery in Relation to the Degree of Preoperative Global Dysfunction and Viability
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The preoperative global EF ranged from 0.17 to 0.79 (normal, 0.67 ± 0.12; mean ± 2 standard deviations). To analyze the postoperative outcome in patients with differing degrees of preoperative dysfunction and varying numbers of mismatch regions, the patients were divided into groups of patients with normal LV function (group 1: LVEF, 0.55 or more), with mildly depressed function (group 2: LVEF, 0.55 to 0.40), or with moderately to severely depressed LV function (group 3: LVEF, less than 0.40).
- Group 1 (n = 26). These patients had a normal preoperative global EF of 0.65 ± 0.07. The group was further divided as follows:
- Group 1a (n = 20): These patients had at least one mismatch region and showed no change in their EF (0.64 ± 0.07 versus 0.66 ± 0.07; p = NS).
- Group 1b (n = 6): These patients had no mismatch regions and showed no change in their postoperative EF (0.68 ± 0.07 versus 0.69 ± 0.09; p = NS).
Group 2 (n = 17): These patients had a mildly depressed preoperative global EF of 0.47 ± 0.08. The group was further divided as follows:
- Group 2a (n = 10): These patients had at least one mismatch region and showed a significant improvement in their EF (0.48 ± 0.05 versus 0.56 ± 0.07; p = 0.0002).
- Group 2b (n = 7): These patients had no mismatch regions and showed no change in their postoperative EF (0.46 ± 0.05 versus 0.46 ± 0.07; p = NS).
Group 3 (n = 16): These patients had a moderately to severely depressed preoperative global EF of 0.35 ± 0.2. The group was further divided as follows:
- Group 3a (n = 9): These patients had at least one mismatch region and showed a significant improvement in their EF (0.30 ± 0.07 versus 0.44 ± 0.12; p = 0.009).
- Group 3b (n = 7): These patients had no mismatch regions and showed a slight or nonsignificant improvement in their postoperative EF (0.35 ± 0.04 versus 0.37 ± 0.12; p = NS).
The functional data in the different groups are summarized in Table 1, and Figure 3 shows the postoperative outcome of patients with mismatch.
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Table 1. . Summary of Preoperative and Postoperative Functional Data in Patients With and Without Mismatch Regions
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Fig 3. . The postoperative changes in global ejection fraction in patients with mismatch in the three groups. (EF = global ejection fraction; pre = preoperative; post = postoperative. **p = 0.0002; ***p = 0.009.)
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Finally, we considered patients with a very low EF of less than 0.30 and found that only 5 patients in the entire patient population could have theoretically qualified as heart transplant candidates on the basis of EF alone. The EF ranged from 0.17 to 0.29 (0.25 ± 0.06), and 3 months after revascularization there was an improvement to 0.42 ± 0.15 (p = 0.027). In these patients, 2.4 ± 1.5 of the six regions (ie, almost 40% of the left ventricle) showed mismatch. We compared these findings to the pretransplantation PET scan and EGNA data obtained from 8 consecutive heart transplant patients with ischemic cardiomyopathy and found that the EF was not significantly different at 0.24 ± 0.04. However, there were significantly fewer regions with mismatch compared with the number in the matched (age- and EF-matched) group just described that underwent CABG (0.4 ± 0.7; p < 0.02). Furthermore, 50% of the left ventricle showed evidence of scar tissue (match necrosis, 3.0 ± 1.5 versus 0.6 ± 0.9; p < 0.02). Almost 30% (1.8 ± 1.7 and 1.8 ± 1.1 of the six regions) of the left ventricle was coded as match viable in the transplant and CABG patients. The data are summarized in Table 2.
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Comment
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To appropriately select patients for CABG, it is important to understand the pathophysiology of LV dysfunction. Scarred tissue and stunned and hibernating myocardium may coexist in the same patient and may contribute in different proportions to the LV dysfunction. The decision is difficult in patients with very low LV function, and therefore reliable diagnostic procedures are required for the identification of viable myocardium, which can predict the degree of improvement after operation [14].
Numerous studies have shown that longevity is increased in patients who undergo CABG as opposed to those who receive medical therapy. This was most impressive in those with poor LV function (EF of less than 0.30) [15, 16]. When function is assessed 3 months after CABG, we find that these patients appear to benefit the most functionally. This early benefit could indeed have positive implications in terms of longevity.
In a recent study [17] that included 50 CABG patients for whom we had preoperative and postoperative PET data (flow and metabolism), EGNA data (regional anterior wall and global EF), and histologic data (intraoperatively obtained transmural needle biopsy specimens of the anterior myocardial wall), we were able to identify factors that may predict functional recovery. Using a multivariate approach (with the following regional variables: flow, metabolism, regional anterior wall EF, transmural fibrosis, myocyte glycogen content, and the percentage of myolytic myocytes), we were able to identify four different clusters that defined different patient populations with differing postoperative outcomes. From this study we learned that only patients showing a mismatch between function and histology (low function but normal myocardium) on the one hand and a mismatch between flow and metabolism (low flow and high metabolism) on the other hand can invariably be expected to show early functional recovery. Patients with an equal degree of dysfunction, viable but altered myocardium, and a moderate decrease in flow and metabolism (match viability) do not show improvement. Thus the functionally down-regulated viable myocardium was found to have some or no morphologic adaptations. The absence of early functional improvement after revascularization is thought to be closely linked with the degree of structural adaptation in chronically ischemic segments.
In the present study a regional as well as a global approach was used to try to define variables that might be useful in predicting functional outcome after CABG. Interestingly, only mismatch regions (with flow–metabolism and with function–metabolism mismatch) play a significant role in predicting improvement, thereby confirming our earlier observations [17], which included only anterior wall characteristics. Thus the factors that appear to be important regionally also play a role globally and the presence of mismatch in any of the six regions appears to contribute equally to improvement in the global EF.
Another interesting incidental finding in this study involved a direct comparison of two age-matched groups of patients with very low function: one group consisted of patients who underwent cardiac transplantation and the other group consisted of patients who underwent coronary revascularization. In our age-matched groups with the so-called bad ventricles, the PET scan showed that almost 40% of the left ventricle in one group (n = 5) showed mismatch with a potential to recover after revascularization. The other group (n = 8) had 50% scar tissue (match necrosis), with almost 30% of the remainder of the LV regions showing match viability (ie, probably without any early potential to recover). This latter group of patients then underwent an extensive gamut of investigations before being placed on the heart transplantation waiting list, and they eventually underwent heart transplantation. Positron emission tomography data can thus play an important contributory role in the treatment decision-making process.
It should be appreciated that a regional flow reduction could involve the entire myocardial wall or it may result from the coexistence of scar tissue in the subendocardium and normal myocardium in the subepicardium. The restoration of blood flow will improve wall motion in the first instance but not in the second. A variable functional outcome after revascularization may therefore be expected [18]. Sometimes early recovery is not possible in dysfunctional segments with normal PET flow and metabolism patterns. This could stem from the existence of repeatedly stunned myocardium, which probably requires a long time to recover. Such a pattern was indeed observed in some of our patients.
Positron emission tomography has thus by far proved to be an outstanding investigation with a very high degree of sensitivity and specificity in the identification of patients with jeopardized myocardium on the verge of experiencing cardiac-related events. If revascularized completely, such patients can show spectacular improvement in terms of morbidity and mortality. Although the role of CABG remains controversial in the setting of a very low LVEF, there is sufficient evidence that its application is possible in selected patients. The presence of angina or documented ischemia in a patient with demonstrated viable, dysfunctional myocardium provides an argument in favor of CABG. When areas at risk are fewer or absent and LV dysfunction is exclusively the result of irreversible myocardial damage, CABG is not indicated and heart transplantation should be considered as a surgical option.
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Limitations of the Study and Cost-Benefit Ratio of Available Techniques
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The major limitation of this study is indeed that only a few patients with low LV function are included as a result of the mixed patient population studied. This, however, allowed us to show that, regardless of the preoperative function, the presence of mismatch regions predicts recovery. Furthermore, because of such a mixed population of patients, we were able to show that the greater the number of mismatch regions and the lower the preoperative function, the greater the postoperative functional benefit. Second, PET, despite being very promising in terms of detecting reversible ischemia, nevertheless remains a very expensive investigation that is not readily available in most centers where cardiac surgical procedures are performed. A thallium stress test and single photon emission computed tomography, which are less expensive and less time consuming, might serve as possible alternatives but are now regarded as being less interesting from the viewpoint of their predictive capability. The average sensitivity and specificity of thallium stress testing in almost 3,000 cases described in the literature since 1983, weighted for and compared with the weighted averages for PET studies, is as follows: sensitivity, 83% for thallium stress test versus 96% for PET; specificity, 53% for thallium stress test versus 96% for PET [19]. Further, direct comparison of PET and single photon emission computed tomography in appropriate populations has shown that the diagnostic accuracy of PET appears to be significantly greater. An inexpensive and readily available technique for assessing myocardial viability is the infusion of low and high doses of dobutamine during echocardiography. This technique should, however, be used with great care, because high doses of dobutamine can also have a detrimental effect on the myocardium. At present, single photon emission computed tomography with thallium 201 appears to have great potential in terms of cost-benefit ratio. This technique can be made even more sensitive in the identification of viable myocardium by being combined with dobutamine stress echocardiography [20, 21]. Finally, magnetic resonance imaging techniques also promise to constitute a breakthrough in this field. There are thus a wide variety of less expensive techniques available at present (albeit with a sensitivity and specificity somewhat less than those of PET) that could be used to detect viable myocardium.
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Conclusions
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This study shows that it is possible to predict LV functional recovery after CABG using noninvasive techniques. In conditions involving depressed function, mismatch regions (mismatch between flow and metabolism) must be present for there to be a functional benefit. Coronary artery bypass grafting appears to be beneficial in patients with severely impaired LV function if the ischemia is reversible (salvageable myocardium) and the vessels are operable.
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Footnotes
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Address reprint requests to Dr Flameng, Department of Cardiac Surgery, University Hospital Gasthuisberg, Herestraat 49, B-3000 Leuven, Belgium.
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Abstract
Introduction
