Coronary artery disease (CAD) is the leading cause of morbidity and mortality in many countries. For multivessel coronary disease, coronary artery bypass grafting (CABG) may be the method of choice for treatment. Surgical myocardial revascularization requires validation of graft patency in the early and late postoperative period, not only to determine the success of the procedure but also to determine the risk-benefit ratio and long-term results. Conventional invasive coronary angiography (CCA) is still considered the standard procedure for the assessment of CAD and graft patency. However, the technique is limited by its invasive character and related complications.1,2 Because of greater patient compliance and greater patient safety, there is growing interest in noninvasive technologies to visualize coronary arteries and bypass grafts.3,4 Currently, 3 techniques are being developed for this purpose: magnetic resonance imaging, electron beam tomography, and multislice spiral computed tomography (MSCT).5
The major difficulty with imaging of the coronary arteries is related to the small caliber of arteries and rapid motion during the cardiac cycle, combined with patient respiration. These movement artifacts produce blurring of the images and difficulty in interpretation.6 With 16-slice MSCT, coronary angiography can be performed with higher specifications. An acquisition collimation of 16 × 0.75 mm provides spatial resolution in the long axis that approaches that of the in-plane resolution. Using acquired data from 2 or more cardiac cycles to reconstruct an image (partial subsegmental reconstruction) may reduce effective temporal resolution to an estimated 92 milliseconds. In addition, a large number of postprocessing techniques are now available for image interpretation.7
The aim of this study was to evaluate and compare the postoperative graft patency at 1 year or longer by MSCT and CCA.
Patients who had received a CABG 1 year or more before the study were included. Patients with known history of iodine dye allergy, atrial fibrillation, or heart rate over 70 were not included. To avoid dye overload, follow-up was done in 2 days. On day 1, MSCT was done, and CCA was done the following day. Sixty-nine patients underwent both MSCT coronary angiography and CCA as outpatients. All patients were already receiving β-blockers with the mean (SD) heart rate of 64 beats/minute. The interval between surgical procedure and angiographic examination was 1.3 ± 0.3 years (mean ± SD).
The institutional review board approved the study, and all patients gave written informed consent.
Cardiac Catheterization and Coronary Angiography
Cardiac catheterization and coronary angiography was carried out according to Seldinger technique through a femoral approach by a consultant cardiologist, including selective catheterization of the grafts or graft stumps. Multiple views of the coronary arteries and the grafts were obtained. All the venous and arterial grafts were evaluated for stenosis. Each graft was declared patent, < 50% stenotic, or ≥ 50% stenotic. Angiographic stenosis was graded as maximum lumen diameter loss for 2 views. In cases of failed cannulation of the proximal site of the vein graft to the ascending aorta, an aortogram was performed to confirm occlusion of the graft.
Multislice CT Coronary Artery Imaging Acquisition
The MSCT examinations were performed using a Somatom sensation 16-detector row CT scanner (Somatom sensation 16, Siemens AG Medical solutions, Forchheim, Germany) with a gantry rotation time of 370 milliseconds (collimation 0.75 milliseconds, table-speed 5.7 mm/gantry rotation, reconstruction increment 0.5 mm, tube current 550 ± 600 mA). To enable both prospective and retrospective electrocardiographic gating, the electrocardiogram was recorded continuously throughout the preparation and image acquisition period. An 18-G antecubital vein catheter was inserted for contrast administration. After attachment of the leads for electrocardiographic recording, patients were examined while in a supine position, using a breath-hold technique. To facilitate the relatively long breath hold (20–30 seconds), a short session of instructed hyperventilation was performed before scanning. Our imaging protocols consisted of a noncontrast scanogram and plain chest sequence. This helped to determine the position of heart and define the scan volume from the origin of the left internal mammary artery to the base of the heart.
Using a dual-head injector,120-130 mL of the iodinated contrast agent (350 mg iodine/mL) was injected at a rate of 4–5 mL/s followed by 60 mL of normal saline at the same speed. Bolus tracking technique was used. The sequence was triggered with a 4-second delay when the contrast in ascending aorta reached 100 Hounsfield Units. The scans were obtained in craniocaudal or caudocranial direction with an average scan time of 28 seconds. The patients were instructed to maintain an inspiratory breath-hold, and data acquisition commenced. Depending on the average heart rate, the images were reconstructed at an appropriate R-R interval separately for each graft and native vessel, depending on preview series, using a 0.75 mm slice thickness and 0.4 mm slice interval.
After reconstruction, CT raw data were transferred to a PC-based workstation (Wizard, Siemens medical solutions, Forchheim, Germany) to permit real-time axial, multiplanar, reconstruction, maximum intensity–projection, and volume-rendering techniques. The bypass graft patency, presence of stenosis, and proximal and distal anastomoses were evaluated.
The sensitivity and specificity of MSCT for the assessment of bypass graft patency were calculated using the following definitions and expressed as percentage:
Sensitivity = [true positive/(true positive + false negative)] × 100
Specificity = [true negative/(true negative+ false positive)] × 100
Predictive accuracy = [(true positive + true negative)/total group] × 100
Sixty-nine patients were fully evaluated. Both MSCT scans and CCA were universally well tolerated and no complications occurred. However, 2 patients moved significantly during MSCT acquisition and were called again after CCA for repeat MSCT scanning. There were 209 grafts. A total of 69 internal mammary artery grafts (IMA) grafts, 8 radial arteries, and 132 saphenous venous grafts were compared. No right IMA was used as bypass graft. Seventy-eight grafts were on the anterior surface (target vessels: left anterior descending and diagonal artery), 83 grafts on the lateral surface (target vessels: obtuse marginal, ramus intermedius, and distal circumflex arteries), and 48 grafts were on the inferior surface (right coronary artery, posterior descending and posterior lateral artery) of the heart. Of the 209 grafts, 11 grafts (5.3%), were found to be occluded or stenosed (anterior surface 0/78, lateral surface 6/83 (7.2%), inferior surface 5/48 (10.4%).
MSCT and Coronary Angiographic Findings
On MSCT, all IMAs and venous grafts were well visualized along their paths, even on the anastomotic site, and 3-dimensional reconstruction was possible. The anatomic relationship between cardiac cavities and bypasses was well demonstrated by 3-dimensional reconstruction.
Arterial Bypass Grafts
All the IMAs were found to be patent by both evaluation methods (MSCT and CCA). There were 8 radial artery grafts; 2 radial artery grafts were occluded, and both were correctly assessed by MSCT when compared with CCA. Good-quality images of all grafts were obtained.
Venous Bypass Grafts
Among the 132 venous grafts, 9 (6.8%) were found to be significantly obstructed (either > 50% or total occlusion). All the patent grafts were correctly evaluated by MSCT. However, 2 venous grafts (both on the lateral wall) that were depicted as patent by MSCT were found to be blocked on CCA (sensitivity 81.8%, specificity 100%, predictive accuracy 99.0%) (Table 1).
Direct control of bypass grafts is required after CABG surgery. Doppler remains the method of choice intraoperatively.8 Selective coronary angiography is the standard procedure in the postoperative period. However, this invasive procedure presents a significant risk of morbidity and has poor patient compliance, especially in asymptomatic postoperative patients. Thus, there is a place for a noninvasive examination of the coronary bypasses during the early and late postoperative period. Initial studies using the angioscanner suggested successful evaluation of the native coronary network.9 After the preliminary work of Achenback et al,10 the MSCT scan showed that it could precisely evaluate the patency of an arterial or venous coronary bypass graft.11,12 Our results are consistent with this data, with precise images demonstrating the patency of coronary bypass grafts and detecting stenosis or occlusion with sensitivity of 81.8% and specificity of 100%. We examined our patients 1 year or more after CABG, the optimal time for follow-up and validation of graft patency by MSCT and conventional angiography. In this study, both procedures were well tolerated by the patients. Because MSCT scan is a noninvasive procedure, it makes follow-up of CABG patients easier, with improved patient compliance in postoperative period.
MSCT has multiple advantages. The current MSCT scans can be performed in a very short period, so is well tolerated by patients. The lumen of the vessels is clearly shown in the images, and the anastomotic sites are correctly demonstrated. Very useful information is provided in the event of redo surgery, as 3-dimensional images show anatomic relationships of bypasses with cardiac structures and the chest wall and this helps to plan the operation so as to avoid injury to the grafts. The disadvantages are those of any iodized examination (eg, allergic reactions, impaired renal function).
The current limitations of MSCT include inappropriateness for patients with cardiac rhythm disturbances and patients with heart rates > 70, as it becomes difficult to rebuild the images. Vessels with diameters < 1.5mm are poorly visualized, and stenoses located in calcified coronary arteries are more difficult to quantify. The surgical clips also can produce artifacts. This is particularly important with arterial grafts of IMAs, which are smaller and have metal clips along their course.
Previous studies have raised concerns about the visualization of arterial bypass grafts through MSCT.19 However, in our study we could visualize IMA and venous grafts very well through out their course and at the anastomotic site (Fig. 1). Despite these technical limitations, our study shows that MSCT allows very accurate assessment of graft patency and, in addition, it provides relevant information concerning the presence of substantial obstructive disease in the bypass graft. Also, the images obtained by MSCT are comparable to the images obtained by CCA (Figs. 1 and 2). Our study results are consistent with those of other authors who used both MSCT and CCA to evaluate graft patency and reported a sensitivity varying from 71% to 100% and specificity of 97% to 100%.7,13–19
In the postoperative period, many patients report atypical chest pain after CABG, which can be confused with angina. MSCT can reliably check the patency of coronary bypass grafts and the quality of the anastomosis by a noninvasive method, enabling one to better determine if the pain is cardiac.
MSCT seems to be a promising technique to noninvasively evaluate coronary artery bypass grafts in the postoperative period. Patency of bypass grafts, grade of stenosis, and quality of anastomoses can be evaluated. Scanners of increasing performance (64-slice CT scanner) can overcome the current limitations of 16-slice MSCT. The 64-slice CT system provides better temporal resolution and better image quality. In terms of the patient comfort, higher-slice CT systems allow a more comfortable, shorter breath hold and a reduced amount of contrast.20
Increasing clinical practice and more validation studies comparing MSCT scan with CCA will help to demonstrate the full application spectrum available with the new generation of MSCT systems. Hopefully, in the near future there will be successful integration of CT cardiac imaging in the routine postoperative evaluation of grafts in CABG patients.
The authors thank Mr. Sudhir Shekhawat, Mr. Amit De, Mrs. Preeti Saxena, and Mr. Mohan Singh Mehra for their secretarial assistance, data collection, and evaluation.
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Keywords:© 2005 Lippincott Williams & Wilkins, Inc.
Multislice spiral computer tomography angiography; Coronary angiography