Myocardial perfusion as an indicator of graft patency after coronary artery bypass surgery

Thallium scintigraphy in prediction of occlusion of bypass grafts in asymptomatic and symptomatic patients

To evaluate thallium scintigraphy in predicting coronary artery bypass graft patency, exercise thallium scintigraphy and selective graft and native vessel angiograms were performed in 22 asymptomatic and 29 symptomatic consecutive patients three months after coronary artery bypass grafting (CABG). Twelve out of 22 asymptomatic patients (55%) had reversible thallium defects on postoperative images; in 10 patients the postoperative scans were normal. The graft patency was significantly lower in asymptomatic patients with abnormal thallium perfusion compared to those with normal perfusion after CABG (68% vs. 91%, p less than 0.05). The rate of graft patency in symptomatic patients was 66/87 (76%). Thallium scintigraphy was 77% sensitive and 78% specific in detecting one or more stenosed or occluded bypass grafts in patients without angina (accuracy 77%). When data from exercise electrocardiography were combined with scintigraphy, all but one patient with incomplete revascularization could be detected (positive predictive accuracy 92%). In symptomatic patients, thallium scintigraphy accurately predicted the presence or absence of graft occlusion in 24/29 (83%) cases. Thus, abnormal myocardial perfusion due to stenosis or occlusion of bypass grafts is common in both asymptomatic and symptomatic patients after CABG. Thallium scintigraphy together with exercise electrocardiography appear to be useful non-invasive methods in detecting painless myocardial ischemia and in predicting bypass graft occlusion after CABG.

Exercise technetium 99m sestamibi single-photon emission computed tomography late after coronary artery bypass surgery: long-term follow-up

BACKGROUND AND HYPOTHESIS

The prognostic value of exercise technetium 99m sestamibi single-photon emission computed tomography (SPECT) imaging in patients with previous bypass surgery is unknown. The aim of our study was to assess the prognostic information obtained with exercise scintigraphy performed for routine follow-up or reappearance of symptoms.

METHODS

We studied 75 patients referred to our Center at a mean of 38 +/- 53 months from the revascularization procedure and prospectively followed them for 38 +/- 24 months.

RESULTS

Fifteen patients (20%) had events at follow-up: there were 4 cardiac deaths, 3 nonfatal acute myocardial infarctions, 8 late revascularization procedures (4 percutaneous transluminal angioplasty and 4 repeat bypass surgery). Univariate analysis identified a history of typical angina (p = 0.001), a clinically positive ergometric test (p = 0.009), peak exercise heart rate (p = 0.0003), percentage of maximal predicted heart rate (p = 0.0001), peak exercise double product (p = 0.048), therapy during exercise (p = 0.003), scintigraphic summed reversibility score (i.e., the summation of the segmental differences between stress and rest) (p = 0.014), as significant predictors of events. Three multivariate models were built, with clinical variables (Model 1, chi square 15.97), ergometric variables (Model 2, chi square 19.66), and with scintigraphic variables added to clinical/ergometric variables (Model 3, chi square 31.13). The scintigraphic variable selected in the model as significant predictor of events was the summed reversibility score (p = 0.008).

CONCLUSIONS

Exercise sestamibi SPECT scintigraphy provides optimal prognostic information after clinical and ergometric parameters in patients with previous bypass surgery.

Evaluation of coronary artery bypass grafts using helical scan computed tomography

The patency of coronary artery bypass grafts was evaluated by helical computed tomographic (CT) scan. One hundred forty patients who received an enhanced chest CT scan and a coronary angiography after bypass surgery were studied before discharge at the Fukui Cardiovascular Center. Among them was a total of 398 grafts and 436 anastomoses. For the evaluation of 248 vein grafts, the CT scan showed a 99.5% correct positive ratio and an overall accuracy of 96.4%. In 122 internal thoracic arteries tested, an overall accuracy of 64.7% was obtained. In gastroepiploic artery tests the overall accuracy was 33.3% and in radial artery tests it was 45.5%. Thirty-seven venous sequential anastomoses in 34 grafts and 1 internal thoracic artery sequential anastomosis were also evaluated. In the venous sequential anastomosis, the CT scan showed a 73.0% of accuracy overall. This study showed that the helical CT scan was useful to evaluate graft patency following bypass surgery.

Are Metallic Markers Necessary for Coronary Artery Bypass Grafts? A Study Using X-ray Computed Tomography and Selective Graft Angiography

Using X-ray computed tomography (CT) and selective graft angiography, the authors studied the necessity of metallic markers in coronary artery bypass grafts on 45 patients (mean age 57.2 years) with 87 saphenous vein grafts. Eight patients had 17 markers. X-ray CT was performed after surgery using an apparatus with a 1-second scanning time. Noncontrast X-ray CT was performed on horizontal sections, at 5-mm intervals, from the lower margin of the aortic arch to the lower left ventricle. A contrast medium was then injected into the antecubital vein (3 ml/second, total 30 ml) in one cross-section at the level of bifurcation of the pulmonary artery. Aortography (60 degrees in the left anterior and oblique positions, 20 ml/second, total 40 ml) was performed concurrently. Selective graft angiography was taken in the same direction, using 4 cm right of the Judkins with reference to the aortographic image and position of five clips on the sternum. Aortography revealed 79 patent and 8 occluded grafts. Selective graft angiography was easily performed even in grafts without markers. A cross-section of the occluded graft could not be seen with X-ray CT. Grafts with markers were often masked by artifacts produced by markers on X-ray CT. The number of observed graft slices (marker-positive grafts) was only 1.2 +/- 1.1 slices, significantly (p < 0.01) lower than marker-negative grafts (4.1 +/- 3.1 slices). In particular, the number of marker-positive right coronary artery grafts was 0.4 +/- 0.9 slices. Four of five right coronary artery grafts were unobservable due to artifacts. In grafts without markers, sensitivity, specificity, positive predictive value, negative predictive value, and accuracy of X-ray CT to graft patency were 100%, 85.7%, 98.4%, 100%, and 98.6%, respectively. This study suggests that metallic markers may not be necessary for coronary artery bypass grafts.

Utility of stress single-photon emission computed tomography (SPECT) perfusion imaging in predicting outcome after coronary artery bypass grafting

Previous studies have examined the predictors of outcome in medically treated patients with coronary artery disease (CAD). There is limited information on predictors of outcome after coronary artery bypass grafting (CABG). This study examined the predictors of outcome of 255 patients with CAD, at a mean time of 5 years after CABG for angina pectoris. The 255 patients underwent coronary angiography and stress single-photon emission computed tomography (SPECT) myocardial perfusion imaging after CABG. During a mean follow-up of 41 +/- 28 months after stress testing, there were 34 hard events (24 cardiac deaths and 10 nonfatal myocardial infarctions). The hemodynamics during stress testing, and age and gender were not predictors of events. The SPECT variables of multivessel perfusion abnormality, perfusion deficit size, and increased lung thallium uptake were predictors of death and total events by uni- and multivariate survival analysis. There were 14 events in 45 patients (31%) with multivessel abnormality and increased lung thallium uptake, 14 events in 101 patients (14%) with either multivessel abnormality or increased lung uptake, and 6 events in 109 patients (6%) with neither of these 2 variables (p = 0.0001). The annual mortality and total event rates were 7.5% and 9.5% with both variables, 3.4% and 4.3% with either variable, and 0.6% and 1.7% with neither of the variables (p = 0.01). Thus, stress SPECT perfusion imaging is useful to stratify patients after CABG into low, intermediate, and high risk groups for future cardiac events.

Preoperative evaluation of myocardial viability by thallium-201 imaging in patients with old myocardial infarction who underwent coronary revascularization

The myocardial uptake and redistribution in thallium scintigraphy and the regional wall motion by echocardiography were evaluated by a semi-quantitative method in 42 patients who previously had myocardial infarction (50 target vessels) and underwent coronary revascularization. The aim of this study was to elucidate the significance of the initial image, delayed image and redistribution on thallium-201 scintigraphy for clinical diagnosis of the myocardial viability. As a semi-quantitative analysis, we used a bull's-eye display for thallium image and centerline method for echocardiographic wall motion, and compared the results before and after revascularization. As a result, the thallium grade improved postoperatively in all 17 areas which preoperative had showed redistribution, and also in 11 of the 32 areas without preoperative redistribution. The sensitivity, specificity and accuracy of preoperative thallium redistribution for predicting myocardial viability were 61%, 100% and 78%, respectively, when the postoperative improvement in the thallium grade was used as the standard. The postoperative probability of improvement in the thallium grade increased in proportion to the preoperative thallium grade (delayed image) (p less than 0.01). There was no correlation between the preoperative thallium delayed image and postoperative improvement in wall motion. Postoperative improvement in thallium image and wall motion could not be predicted from the preoperative wall motion. Thus, postoperative improvement in thallium images can be anticipated if redistribution is present on the preoperative thallium image, and the preoperative thallium delayed image is useful for predicting myocardial viability. Improvement in wall motion could not be predicted preoperatively by these methods.

Value and limitation of biplane rest and exercise radionuclide angiography for assessing individual bypass grafts: a prospective study

This prospective study evaluated the ability of serial biplane rest and exercise radionuclide angiography to predict the status of individual coronary bypass grafts in 20 patients 2 to 6 months after surgery. The preoperative coronary angiogram was used to assign vessels to 10 regions of distribution on the radionuclide angiogram. Predictions of graft adequacy for individual vessels were based on a detailed assessment of rest and exercise wall motion in their regions of supply. Of 59 grafts, 38 were judged adequate (patent with less than 75% stenosis) and 21 inadequate by postoperative catheterization. Radionuclide prediction of graft status was possible for 32 of the 59 grafts, including 19 of 24 left anterior descending, 7 of 19 circumflex and 6 of 16 right coronary artery grafts. The status of the remaining 27 grafts could not be assessed because of normal wall motion in their region of supply both pre- and postoperatively (22 vessels) or because a region of supply was not represented on the biplane radionuclide angiogram (5 vessels). Of the 32 predictions made, 25 (78%) were correct, including 13 (93%) of 14 predictions of graft adequacy and 12 (67%) of 18 predictions of graft inadequacy. The single incorrect prediction of graft adequacy resulted from improved exercise wall motion in a region supplied by a graft judged as having a 75% anastomotic stenosis. Most incorrect predictions of graft inadequacy were due to new septal or other rest wall motion abnormalities postoperatively. The comparison of pre- and postoperative studies was essential to maintain the predictive ability of the test. Thus, a detailed analysis of regional wall motion by rest and exercise radionuclide angiography can be used to predict the status of individual coronary artery bypass grafts. Reliable predictions can be made for most successful anterior descending grafts, and may permit cardiac catheterization to be deferred in certain cases. However, the method is limited by the need to perform preoperative exercise studies, by the low number of right and circumflex coronary artery grafts that can be evaluated and by the poor specificity of predictions of graft failure.

Assessment of aortocoronary bypass grafting using exercise ST segment/heart rate relation

The maximal rate of progression of ST segment depression relative to increases in heart rate (maximal ST/HR slope) has recently been shown to be an accurate index of the presence and the severity of coronary heart disease in patients with angina. The value of this new exercise test was assessed in patients undergoing aortocoronary bypass. The maximal ST/HR slope and the results of coronary angiography were obtained in each of 46 patients before aortocoronary bypass surgery and in 26 of the 46 patients six months after the operation. At each stage of the investigation the maximal ST/HR slope detected without false results the absence and the number of significantly diseased vessels as shown by angiocardiography. As in previous findings the ranges of the maximal ST/HR slope showed no overlap between the four groups of patients: those with no significant disease and those with single, double, or triple vessel disease. In each of the 46 patients in whom the maximal ST/HR slope was determined before operation and three months afterwards the slope was lower after operation than before, indicating improvement. Follow up examinations showed that the maximal ST/HR slopes accurately detected the number of patent grafts used to bypass significantly diseased coronary arteries. Furthermore, the development of a significant narrowing or occlusion in any vein graft caused an increase in the maximal ST/HR slope which was equivalent to the value of single vessel disease. It is suggested that the maximal ST/HR slope may be used reliably in individual patients to indicate restoration of adequate blood supply to the myocardium after successfully aortorcoronary bypass surgery and the to detect in the period of six months after the operation the degree of severity of coronary heart disease whether it is caused by occlusion of the graft of significant disease of the coronary arteries.

Graft patency in patients with coronary artery bypass operation complicated by perioperative myocardial infarction

Coronary artery bypass graft patency was examined by contrast-enhanced computed tomography in 18 patients with perioperative myocardial infarction soon after surgery to determine the role of graft occlusion. Preoperative coronary angiograms were reviewed to assess native coronary disease and visible collateral channels in the distribution of the myocardial infarction. Perioperative myocardial infarction was diagnosed if creatine kinase-MB was elevated, characteristic electrocardiographic changes occurred and, in the majority of cases, the pyrophosphate scan was positive. Fourteen patients (78%) had patent grafts and perioperative myocardial infarction in the distribution of the grafted vessel. Four patients had an occluded graft with infarction in the distribution of the grafted vessel. Among the 14 patients with patent grafts, there was a significant difference (p less than 0.0005) in the degree of the mean (+/- standard deviation) diameter stenosis of 80 +/- 11% in native coronary vessels supplying the perioperatively infarcted myocardium versus a 55 +/- 12% mean diameter stenosis in the 23 bypassed native coronary vessels supplying noninfarcted myocardium. It is concluded that the majority of perioperative myocardial infarcts associated with coronary artery bypass operations are not caused by graft occlusion. The severity of coronary obstruction in the grafted vessel and the lack of collateral vessels to the region of perioperative infarction in patients with patent grafts suggests that an island of jeopardized myocardium exists that is subject to inadequate intraoperative preservation.

Evaluation of early postoperative coronary artery bypass graft patency by contrast-enhanced computed tomography

Fifty patients with 117 coronary bypass grafts were studied by contrast-enhanced computed tomography at an average of 5 +/- 4 days after surgery to determine if this technique was a feasible method for detecting early postoperative graft occlusion. The study was limited in only three patients because of incisional chest pain (one patient) or multiple metal clips attached to the graft (two patients). The distal patency of sequential grafts cannot be determined by current techniques. There was a lower graft patency rate (70%) in the 10 patients with perioperative myocardial infarction than in the 40 (95%) without (p less than 0.025), but most regions of infarcted myocardium were perfused by patent grafts. There were eight graft occlusions in eight patients. The graft occlusion rate (30%) was significantly higher (p less than 0.025) in grafts with intraoperative flows less than 45 ml/min. The postoperative complications of myocardial dysfunction, arrhythmia and coronary artery spasm did not correlate with graft occlusion. Early graft occlusion is uncommon (7%) and usually occurs in grafts with low flows or severe distal disease (seven of eight grafts), or both. Thus, the need for early reoperation is very infrequent. It is concluded that contrast-enhanced computed tomography is feasible for the assessment of coronary bypass graft patency. Because early graft occlusion is unusual the technique may be an ideal noninvasive screening method.

Prospective assessment of regional myocardial perfusion before and after coronary revascularization surgery by quantitative thallium-201 scintigraphy

Because thallium-201 uptake relates directly to the amount of viable myocardium and nutrient blood flow, the potential for exercise scintigraphy to predict response to coronary revascularization surgery was investigated in 47 consecutive patients. All patients underwent thallium-201 scintigraphy and coronary angiography at a mean (+/- standard deviation) of 4.3 +/- 3.1 weeks before and 7.5 +/- 1.6 weeks after surgery. Thallium uptake and washout were computer-quantified and each of six segments was defined as normal, showing total or partial redistribution or a persistent defect. Persistent defects were further classified according to the percent reduction in regional thallium activity; PD25-50 denoted a 25 to 50% constant reduction in relative thallium activity and PD greater than 50 denoted a greater than 50% reduction. Of 82 segments with total redistribution before surgery, 76 (93%) showed normal thallium uptake and washout postoperatively, versus only 16 (73%) of 22 with partial redistribution (probability [p] = 0.01). Preoperative ventriculography revealed that 95% of the segments with total redistribution had preserved wall motion, versus only 74% of those with partial redistribution (p = 0.01). Of 42 persistent defects thought to represent myocardial scar before surgery, 19 (45%) demonstrated normal perfusion postoperatively. Of the persistent defects that showed improved thallium perfusion postoperatively, 75% had normal or hypokinetic wall motion before surgery, versus only 14% of those without improvement (p less than 0.001). Whereas 57% of the persistent defects that showed a 25 to 50% decrease in myocardial activity demonstrated normal thallium uptake and washout postoperatively, only 21% of the persistent defects with a decrease in myocardial activity greater than 50% demonstrated improved perfusion after surgery (p = 0.02). Thus, preoperative quantitative thallium-201 scintigraphy appears useful in predicting response to revascularization surgery, and some persistent defects may revert to normal thallium uptake after surgery. Importantly, the preoperative distinction between viable and nonviable myocardium can be reasonably established by quantitating the amount of persistent reduction in thallium uptake and correlating this with preoperative wall motion.

Spontaneous changes in thallium-201 myocardial perfusion imaging after myocardial infarction

To examine regional myocardial perfusion after myocardial infarction, 26 patients underwent exercise electrocardiographic testing with thallium-201 myocardial perfusion imaging 3 weeks and 3 months after infarction. At 3 weeks, 9 of 26 patients (35%) had myocardial ischemia by exercise electrocardiographic testing, whereas 18 of 26 (69%) had ischemia by thallium-201 imaging. The thallium-201 scintigrams were scored by dividing each image, in 3 views, into 5 segments, using a 5-point scoring scheme. The exercise thallium-201 score was 44.3 +/- 1.2 and increased to 47.3 +/- 1.2 in the redistribution study (p less than 0.001). Three months after infarction, although there was a significantly greater rate-pressure product which would predict a larger ischemic defect and a decrease in the stress thallium-201 score, the stress score was improved (48.3 +/- 1.1, p less than 0.001). The redistribution score was similar, that is, 48.9 +/- 1.0. The improvement in thallium-201 myocardial perfusion was associated with a loss of stress-induced ischemia in 8 patients (30%). These results indicate that spontaneous improvements in thallium-201 myocardial perfusion imaging may occur after myocardial infarction.

Use of exercise tests in assessment of the functional result of aortocoronary bypass surgery

The value of an objective exercise test for the assessment of the functional results of aortocoronary bypass was investigated in 19 patients who were studied before and six months after the operation. For positive tests the end point was defined as a net ST segment depression of 0·1 mv 80 ms after the J point of the ECG. For negative tests the end point was 85% of the age-predicted maximal heart rate response. One patient who was not able to attain either of these points after the operation was excluded. In the remaining 18 patients three indices were used in the analysis. First, the heart rate (HR) and the product of heart rate and systolic blood pressure (RPP) were measured at the defined level of ST segment depression during positive exercise tests to yield HR/ST and RPP/ST threshold respectively. Second, the HR and RPP were measured at the end point of the negative tests. Third, the duration of exercise till the end point of the tests was measured. In each patient the duration of the postoperative test was longer than that of the preoperative test. While all the patients had a positive exercise test before the operation, the test was negative in 11 after it. In 10 of these 11 patients the HR and RPP attained at the end point of the postoperative test had increased; the HR and RPP remained unchanged in one patient. Positive tests were still present in seven of the 18 patients. In five of these the HR/ST threshold and RPP/ST threshold were greater after than before operation, and they remained unchanged in two. An improvement in myocardial blood supply after aortocoronary bypass was suggested indirectly by the ability to attain, during exercise, a higher HR and RPP at the end point of the test. The test proved especially valuable in patients who retained a positive exercise test after the operation.