Asian Cardiovasc Thorac Ann 2004;12:130-132
© 2004 Asia Publishing EXchange Ltd
Echocardiographic Evaluation of Internal Mammary Artery Graft Patency
Min-Ho Song, MD,
Mamoru Ito, MS1,
Sachie Toki, MD,
Keisuke Tanaka, MD,
Wataru Kato, MD,
Jinichi Iwase, MD,
Kazuyoshi Tajima, MD
Department of Cardiovascular Surgery
1 Department of Sonographic Examination, Nagoya Daini Red Cross Hospital, Nagoya, Japan
For reprint information contact: Min-Ho Song, MD Tel: 81 52 481 5111 Fax: 81 52 482 7733 Email: songmhmd{at}yahoo.co.jp Department of Cardiovascular Surgery, The Japanese Red Cross Nagoya First Hospital, 3-35 Michisita-cho, Nakamura-ku, Nagoya, Aichi, 453-8511, Japan.
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ABSTRACT
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The feasibility of using transthoracic echocardiography to assess internal mammary artery graft patency and function was examined. Coronary angiography and transthoracic echocardiography was performed in 60 consecutive patients undergoing coronary artery bypass from April 2000 to March 2002. Flow velocity, velocity-time integral, and the ratio of the diastolic fraction of the velocity-time integral to total velocity-time integral were measured by doppler transthoracic echocardiography. A stress test was carried out to detect coronary flow changes by echocardiography. The relationship between the coronary angiogram and the echocardiogram was analyzed. The overall graft patency rate was 98%. The mean diastolic velocity-time integral in patients with patent grafts was 0.64, and it increased up to 0.72 in response to physical stress. The occluded grafts showed diastolic velocity-time integral fractions of less than 0.60 in all grafts. There was a statistical correlation between patency 
FitzGibbon grade B and diastolic velocity-time integral > 0.60. The diastolic velocity-time integral recorded by transthoracic echocardiography predicted the patency of internal mammary artery grafts. As echocardiography is noninvasive and accurate, it might be a useful method of verifying arterial graft patency.
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INTRODUCTION
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Confirmation of satisfactory graft function is the cornerstone of successful coronary artery bypass grafting (CABG). Postoperative angiography tends to be avoided for both clinical and economical reasons, if not absolutely indicated. However, the anastomotic status of internal mammary artery (IMA) grafts should be examined to ensure complete myocardial revascularization. Magnetic resonance imaging is not yet sufficiently sensitive to assess the function of coronary grafts.1 Multi-detector row computed tomography has evolved to include cardio-synchronous software and it can now provide images of coronary arteries, but it is not yet widely available.2 Several tools for intraoperative evaluation of grafts are being developed, but their reliability has not been determined. Coronary angiography remains the gold standard for verification of graft patency, regardless of its invasiveness. However, transthoracic echocardiography (TTE) is available in most hospitals and it is far less invasive than other imaging modalities. The well-established accuracy and reproducibility of echocardiography have been improved by the development of high frequency doppler echocardiography.3 Blood flow and the diameter of the IMA graft can be obtained by TTE. Although there is some controversy as to whether the ratio of diastolic to systolic flow reflects the function of the graft, the velocity-time integral (VTI) is accepted as an indication of anastomotic status.4,5 The aim of this study was to assess whether TTE could be used as an alternative to angiography.
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PATIENTS AND METHODS
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From April 2000 to March 2001, 60 consecutive patients undergoing CABG were enrolled in this study. In all patients, both the right and left IMA were used as grafts, so 120 anastomoses were studied. For evaluation of graft status, coronary angiography was performed within one month postoperatively. Anastomotic status was classified according to FitzGibbon and colleagues.6 Grade A was defined as an excellent graft with unimpaired runoff, grade B implied stenosis reducing the caliber of the proximal or distal anastomosis or the trunk to less than 50% of that of the native coronary artery, and grade O indicated occlusion. The TTE examination was undertaken within one month after surgery, usually prior to coronary angiography. The physical stress used during TTE recordings was the Azoulay maneuver which consists of 3 steps: rest (step 1), patient’s legs passively lifted up and actively maintained by the patient (step 2), and active raising of the legs by the patient (step 3).7 A real-time two-dimensional echocardiographic unit incorporating a phased array and a pulsed doppler flowmeter was used with a 7.5 MHz linear probe (Agilent Sonos 2000; Hewlett Packard, Inc., Anaheim, CA, USA). The IMA grafts were examined via the bilateral first intercostal spaces. The VTI was obtained at each step of the Azoulay maneuver. The diastolic fraction of the VTI was calculated to provide an indication of IMA graft function.
Among the 60 patients, 30 underwent off-pump beating heart CABG and the others had CABG under cardiopulmonary bypass with antegrade and retrograde cardioplegia and cardiac arrest. Standard techniques of off-pump and on-pump CABG through a midline sternotomy were employed. The mammary arteries were skeletonized with a Harmonic Scalpel (Ethicon, Sommerville, NJ, USA). Off-pump CABG was performed with the use of an Octopus II tissue stabilization system (Medtronic, Inc., Minneapolis, MN, USA) and intracoronary shunts (Baxter Healthcare, Irvine, CA, USA). The anastomoses were carried out with 8–0 polypropylene suture in the usual manner. The left IMA was always used for grafting on the left anterior descending coronary artery, and the right IMA was used to bypass circumflex lesions.
Data were expressed as mean ± standard deviation. Statistical analyses were performed by the unpaired Student’s t test. Probability values less than 0.05 were considered to be statistically significant.
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RESULTS
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The angiographic results are shown in Table 1
. The patency rate for all 120 grafts was 98.3% (118/120). The 2 occluded grafts were both right IMA grafts to the circumflex artery. Table 2 shows the echocardiographic results. The diastolic VTI fraction was increased during active leg raising. The 2 left IMA grafts classified as grade B did not responded to the Azoulay maneuver and the diastolic VTI fractions remained at 0.63 and 0.62, even in step 3. Two occluded IMA grafts showed diastolic VTI fractions of 0.51 and 0.54 and they did not increase even in step 3.
There was a statistically significant positive correlation between the angiographic and echocardiographic results. Discrimination was possible when the diastolic VTI fraction was more than 0.60. When this criterion was met, the graft was patent and either FitzGibbon grade A or grade B.
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DISCUSSION
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This study indicates that it is feasible to evaluate the functional status of IMA grafts by means of TTE, as noted in previous reports.8,9 In experienced hands, bilateral IMA imaging is always possible with a 7.5 MHz linear probe, and overestimation or underestimation is rare. The technique of TTE is less invasive than angiography and applicable to all patients including those with a contraindication to contrast medium injection. The mammary arteries were skeletonized and harvested up to the first intercostal branch, but they were seen and evaluated via the first intercostal space. Very poor images were obtained in one obese patient, so that a supraclavicular approach was necessary.10
This study also found that VTI during the Azoulay maneuver had significant predictive power for the functional status of IMA grafts. It has been noted previously that a diastolic/systolic peak velocity ratio < 0.6 predicted severe graft stenosis (>75%).10 Flow rates between 50 and 60 mL·min–1 are considered essential for optimal function of IMA grafts.11 Recently, it was proposed that a diastolic VTI fraction < 0.5 predicts > 70% stenosis with a sensitivity and specificity of 100%.9 This agrees with another report that a diastolic VTI fraction < 0.5 predicted severe stenosis with a sensitivity and specificity of 100%.5 The diastolic VTI fraction is thought to be the most powerful predictor because the peak velocity ratio is affected by native vessel stenosis, and the diameter and flow rate are influenced by the same factors.5,12 In our study, the discriminating diastolic VTI fraction was 0.66, and it had a specificity and sensitivity of 100%. Physical stress by the Azoulay maneuver increased the diastolic VTI fraction to 0.71, which is an appropriate degree of increment.9 Pharmacological stress, such as with dipyridamole or papaverine has been noted to be useful, but it may induce palpitations or dyspnea.13,14 The Azoulay maneuver is simple and noninvasive, and all patients in our study tolerated it well. Although this study was limited by the fact that there were very few dysfunctional grafts, TTE with the Azoulay maneuver was very useful in assessing postoperative IMA graft status, and we are moving towards management without coronary angiography.
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ACKNOWLEDGMENTS
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Ms Liya Jeon was acknowledged for her manuscript preparation and secretary contributions.
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REFERENCES
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ABSTRACT
INTRODUCTION
