Asian Cardiovasc Thorac Ann 2003;11:143-146
© 2003 Asia Publishing EXchange Ltd
Intravascular Ultrasound: Potential Tool to Assess Coronary Anastomosis Quality
Piergiorgio Tozzi, MD,
Antonio F Corno, MD,
Ludwig K von Segesser, MD
Department of Cardiovascular Surgery, Centre Hospitalier Universitaire Vaudois, Lausanne, Switzerland
For reprint information contact: Piergiorgio Tozzi, MD Tel: 41 21 3142280 Fax: 41 21 3142278 email: tozzig{at}hotmail.com Service de Chirurgie CardioVasculaire - BH10, Centre Hospitalier Universitaire Vaudois (CHUV), Rue du Bugnon, 46,1011–Lausanne, Switzerland.
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ABSTRACT
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Coronary angiography and Doppler flow measurements are most commonly used to assess the patency of anastomoses in the operating theater. Intravascular ultrasound might be another means of monitoring the surgical procedure during coronary artery bypass. Five sheep underwent off-pump bypass of the left anterior descending coronary artery using the left internal mammary artery. The running suture was evaluated by intraoperative fluoroscopy and a coronary intravascular ultrasound probe inserted into the target artery proximal to the anastomosis. Macroscopic examination of the anastomosis was performed to validate the angiographic and intravascular ultrasound images. The diameter, cross-sectional area, and compliance of each anastomosis were calculated in systole and diastole. All anastomoses were patent without signs of stenosis. In one case, intravascular ultrasound showed an intimal flap, which was confirmed by macroscopic examination. The mean major anastomotic diameter was 4.5 ± 0.5 mm on angiography and 4.0 ± 0.5 mm on intravascular ultrasound. From the ultrasound data, the mean cross-sectional anastomotic area was calculated as 6.21 ± 0.1 mm2 in systole and 5.49 ± 0.1 mm2 in diastole, and these data were used to calculate the cross-sectional anastomosis compliance. Coronary intravascular ultrasound can visualize intima-to-intima apposition and provide reliable calculations of anastomosis compliance.
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INTRODUCTION
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The long-term patency of any vascular reconstruction depends on several parameters, the most important being intimal integrity, blood flow velocity, and anastomotic compliance.1,2 Unfortunately, the surgeon has very few means of evaluating the quality of the anastomosis in the operating theater; these include correct intima-to-intima apposition, regularity of the suture line, and magnitude of the inflow. Coronary angiography is considered the gold standard for morphological assessment of coronary arteries and bypass grafts, but it is less sensitive than intravascular ultrasound (IVUS).3,4 An animal study was designed to examine the feasibility of using IVUS to assess the quality of coronary anastomosis during off-pump coronary artery bypass grafting (CABG). The information gathered by this technique was compared with that acquired by angiography.
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MATERIALS AND METHODS
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Five adult sheep (mean weight, 49 ± 2 kg) were given ketamine 15 mg•kg-1, azaperone 0.5 mg, and atropine 2 mg. General anesthesia was induced and maintained with Fluothane 1.5%. Blood pressure, electrocardiogram, and blood oxygen saturation were continuously monitored. A left lateral thoracotomy was performed in the 4th intercostal space. A 12-cm length of the left internal mammary artery (LIMA) was harvested. A pressure probe was inserted in the descending thoracic aorta. The pericardium was opened, the distal left anterior descending coronary artery (LAD) was identified, and a 4/0 polypropylene suture was placed around the proximal LAD for snaring after injection of 100 IU•kg-1 of heparin. Lidocaine (1 mg•kg-1) was given as a bolus followed by continuous intravenous injection of 0.1 mg•kg-1•min-1 to reduce the risk of ventricular fibrillation. A stabilizer (Guidant, Cupertino, CA, USA) was placed on the LAD, and a 4-mm arteriotomy was carried out. The LIMA-to-LAD anastomosis was constructed with a 7/0 polypropylene running suture. An intracoronary shunt (2.0 mm) was also used (Figure 1
). To acquire fluoroscopic images of the anastomosis, 20 mL of contrast medium was injected into the LIMA through one of its side branches. A 20-MHz IVUS imaging catheter (Sonicath Ultra 3.2; Boston Scientific Scimed, Fremont, CA, USA) was inserted into the proximal LAD using the Seldinger technique. Images of the anastomosis were recorded at a frame rate of 30 sec-1, with a pullback of 0.5 mm•sec-1, and stored on videotape. After the procedure, the anastomosis was carefully harvested and a macroscopic examination was performed to validate the angiographic and IVUS images. All animals received humane care in compliance with the European Convention on Animal Care, and the study was approved by our ethics committee.
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Figure 1. Off-pump coronary artery bypass grafting (left internal mammary artery-to-left anterior descending artery) in an adult sheep, using a stabilizer and an intracoronary shunt.
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Fluoroscopic images were also recorded on videotapes. The best quality fluoroscopic and IVUS photograms were saved in JPG file format using Matrox PC-VCR software. Image-Pro Plus 4 software was used for image analysis. For calculation of anastomosis parameters, it was assumed that the anastomosis had an elliptical shape. The axial length was considered as the major diameter (M) and the circumferential length as the minor diameter (m). On angiographic images, M was measured using the thickness of the hemostatic clip (0.5 mm) as a reference; it was not possible to measure m. On IVUS images, m was measured in systole and diastole (Figure 2), while M was calculated using the pullback technique at a rate of 0.5 mm•sec-1 (Figure 3). The cross-sectional anastomotic area (CSAA) in systole and diastole was calculated as:
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Figure 2. Intraoperative intravascular ultrasound and angiography. (A) Ultrasound image from a probe inserted in the left anterior descending artery proximal to the anastomosis. (B) Angiography of the left internal mammary artery-to-left anterior descending artery graft. (C) Ultrasound image showing an intimal flap at the anastomotic site (arrow).
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Figure 3. Intravascular ultrasound imaging of a coronary artery proximal to an anastomosis: M is the axial anastomotic diameter that is measured by the pullback technique, m is the circumferential anastomotic diameter that is measured directly on the ultrasound image. IVUS = intravascular ultrasound, LIMA = left internal mammary artery.
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Cross-sectional anastomotic compliance (CSAC) was calculated as:
Data were expressed as mean ± standard deviation.
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RESULTS
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Mean pulse pressure was 38 ± 8 mm Hg. All anastomoses were determined to be patent by angiography and IVUS, with no signs of stenosis. In one case, IVUS showed an intimal flap (Figure 2
), and macroscopic examination confirmed the presence of an intimal flap in 1 of the 5 anastomoses. Mean axial anastomotic diameter (M) was 4.5 ± 0.5 mm at angiography and 4.0 ± 0.5 at IVUS. The mean circumferential anastomotic diameter (m) measured by IVUS was 2.0 ± 0.2 mm in systole and 1.8 ± 0.2 mm in diastole. The data gathered by IVUS were used to calculate the mean CSAA in systole and diastole, and the mean CSAC (Table 1).
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DISCUSSION
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The conduit flow measurement and diastolic flow pattern acquired during CABG do not always identify significant lesions in arterial or venous conduits, and cannot predict graft patency.5 Angiography is considered the most specific investigative tool to study vascular anastomosis morphology, and in some institutions, it is performed routinely after CABG to verify the quality of the surgical procedure.3 Intravascular ultrasound has been used to study cross-sections of arterial and venous conduits since 1990, and it is now considered the ideal invasive tool to study the vessel lumen, atherosclerotic lesions, thrombi, intimal flaps, and wall motion.6 Moreover, its sensitivity and specificity are higher than those of other investigations such as angiography and magnetic resonance imaging.3,7 This study has demonstrated that IVUS has at least 4 advantages over angiography in coronary anastomosis quality assessment. There is the possibility of measuring both anastomotic diameters (m and M), whereas the minor anastomotic diameter cannot be measured by angiography because of overlapping of the LIMA on the LAD, which severely reduces the accuracy. There is the capacity to study intima-to-intima apposition, because IVUS images have a higher resolution (0.1 mm), and the anastomosis is studied from the inside; angiography did not detect the intimal flap revealed by IVUS. Intravascular ultrasound also affords the opportunity to calculate anastomotic compliance. Better anastomotic compliance means less suture-line stress and lower flow disturbance, and it may reduce the disposition to develop intimal hyperplasia or thrombosis.1,2,8,9 Computer flow-dynamic simulation demonstrates that a more compliant anastomosis is associated with a lower stagnation point due to flow separation (typically on the heel, toe, and hood of the graft), giving rise to low wall shear stress that is associated with intimal hyperplasia.10 With adequate procedural modifications, IVUS could be used for clinical evaluation during CABG. In the case of suboptimal vascular reconstruction assessed by IVUS, it would be possible to redo the anastomosis immediately, avoiding a period of myocardial ischemia.
We noted that the CSAA was greater in systole even though it is known that maximal flow occurs in diastole.5 This was probably due to the fact that anastomosis shape changes according to the modification of LIMA diameter due to pulse pressure, and the increase in the CSAA was not associated with an increase in blood flow, at least in this acute model. This study has some limitations. It represents a situation that very few surgeons would reproduce in surgical practice. Opening a diseased coronary artery to insert the IVUS probe could have drastic consequences. The alternative involves introducing the IVUS catheter through the coronary ostium under fluoroscopy, as cardiologists do routinely. Unfortunately, this is not possible if there is coronary occlusion proximal to the vascular reconstruction. The insertion of the IVUS catheter into a side branch of the graft could overcome this problem, giving coaxial anastomotic images. This procedure is easier and safer when a venous conduit is used. Another limitation is the pullback procedure that is fundamental for axial anastomotic diameter (M) computation; this technique has a long learning curve because results are easily affected by heart movements and by catheter positioning. Despite these limitations, this preliminary study demonstrated that coronary IVUS can provide information on anastomosis geometry, intima-to-intima apposition, and compliance, which may be helpful in predicting the outcome of the vascular reconstruction. We are now trying to develop an anastomosis quality score system based on operative anastomosis images acquired by IVUS, coupled with specific flow patterns, to quantify the risk of long-term anastomotic stenosis.
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Footnotes
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Presented at the 8th Annual Cardiothoracic Techniques & Technologies, Fort Lauderdale, Florida, USA, January 23–26, 2002.
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REFERENCES
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ABSTRACT
INTRODUCTION
