Asian Cardiovasc Thorac Ann 2000;8:325-329
© 2000 Asia Publishing EXchange Pte Ltd
Assessment of Internal Mammary Artery Graft Patency: Angiography or Doppler?
Bharat Dubey, MCh,
Anil Bhan, MCh,
Shiv Kumar Choudhary, MCh,
Sanjeev Sharma, MD,
Rajesh Sharma, MCh,
Balram Airan, MCh,
Panangipalli Venugopal, MCh
Department of Cardiothoracic and Vascular Surgery
Cardiothoracic Sciences Centre
All India Institute of Medical Sciences
New Delhi, India
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For reprint information contact: Anil Bhan, MCh Tel: 91 11 686 4851 Fax: 91 11 686 2663 email: anil_bhan{at}hotmail.com Department of Cardiothoracic and Vascular Surgery, Cardiothoracic Sciences Centre, All India Institute of Medical Sciences, Ansari Nagar, New Delhi 110029, India.
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Abstract
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Angiography is the usual means of assessing the patency of grafted internal mammary artery. The feasibility of assessing patency by Doppler echocardiography was studied prospectively in 49 patients undergoing left internal mammary artery-to-left anterior descending coronary artery anastomosis. Postoperative angiographic findings were normal in 45 patients (92%), 2 had > 70% and 2 had < 50% narrowing of either the arterial pedicle or the graft. The left internal mammary artery could be visualized by Doppler echocardiography in 44 patients, 4 of whom had abnormal angiograms; there was an unobstructed flow pattern in 42 patients and an obstructed flow pattern in 2 (completely blocked internal mammary artery in one and > 70% narrowing of the graft in the other). Thallium scanning in patients with suboptimal angiographic results but unobstructed Doppler flow patterns showed no evidence of myocardial ischemia in the left anterior descending artery territory. Echocardiography is a sensitive noninvasive screening modality to diagnose critical narrowing of internal mammary artery grafts. It is suggested that angiography may be reserved for cases in which Doppler echocardiography fails to visualize the internal mammary artery or reveals an abnormal flow pattern.
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Introduction
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In recent years, the left internal mammary artery (LIMA) has become the conduit of choice for revascularization of ischemic myocardium.1–3 Assessment of LIMA graft patency currently requires an invasive investigation. A noninvasive method of assessment would be useful in patients with recurrence of anginal symptoms after coronary artery bypass grafting (CABG) and for evaluating the efficacy of evolving techniques such as off-pump CABG and minimally invasive CABG. This prospective study was carried out to assess the feasibility of Doppler echocardiography in determining the patency of LIMA grafts to the left anterior descending coronary artery (LAD).
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Patients and Methods
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The study group comprised 49 patients (47 males) with a mean age of 48.2 ± 7.3 years (range, 32 to 74 years). Forty-four patients were studied during routine assessment of the safety of off-pump CABG, and 5 were evaluated for recurrence of symptoms following conventional CABG. The study period was from October 1997 to September 1998. All patients had LIMA-to-LAD anasto-mosis and were assessed for graft patency by angiography and Doppler echocardiography.
Conventional angiography was performed by the standard Judkins technique with a single-plane imaging system. Images were recorded on 35-mm cine film at a frame rate of 25 per minute, and analyzed independently by an expert observer blinded to the Doppler results. The LIMA grafts were examined using multiple projections and stenosis was graded as mild (< 50%), moderate (50% to 70%), severe (> 70%) narrowing of the vessel diameter, or complete blockage of the anastomosis. Patients with a demonstrable obstruction on angiography were subjected to a stress thallium scan and the findings were compared with those of angiography and Doppler echocardiography.
Doppler echocardiography was performed with a Sonos 2000 imaging system (Hewlett Packard, Inc., Andover, MA, USA) using a 7.5-MHz transducer. The LIMA was located in the second to fifth left intercostal space parasternally, in the left lateral position, by 2-dimensional echocardiography using a modified left parasternal window. Long-axis images of the left ventricle were obtained, and the area anterior to the right ventricular outflow tract and the anterior interventricular sulcus was carefully examined by combined imaging and color-flow mapping. After confirming the location of the LIMA by color Doppler intraluminal flow signals, flow patterns were obtained by pulsed Doppler. The vessel diameter was calculated using internal calipers on frozen-frame images from 2-dimensional recordings. A technique of leading-edge to edge was used. The long-axis sections were carefully adjusted to minimize the angle between the Doppler beam and the long axis of the artery. Parameters measured were: peak systolic velocity (PSV), end-diastolic velocity (EDV), peak diastolic velocity (PDV), ratio of peak diastolic velocity to peak systolic velocity (PDV/PSV), vessel diameter, and pulsatility index. The Pourcelot resistance index (RI) was calculated from PSV–EDV/PSV. Values for each parameter were obtained in the second, third, and fourth intercostal spaces. The means of values from 2 specialist observers, blinded to each other's findings and to the angiographic results, were used to reduce observer bias.
In ungrafted LIMA, the blood flow faces significant diastolic resistance offered by small branches of the LIMA, which supply blood to the pectoral area and breast tissue, and henceforth it gives a triphasic flow pattern on Doppler echocardiography, similar to that of the external carotid artery (Figure 1
). There is a high PSV, a very low EDV, a high RI of 0.9, 1.0, or even greater, and PDV/PSV less than 0.6. When this vessel is grafted to the LAD, presuming a good surgical anastomosis and normal distal LAD, the flow pattern on Doppler scanning undergoes a change from the typical triphasic pattern to a biphasic pattern similar to that of the internal carotid artery (Figure 2), with increased diastolic flow velocity due to the low resistance offered by the coronary circulation. The PSV remains the same or decreases slightly, EDV increases in amplitude, PDV/PSV rises above 0.6, and RI decreases to 0.5 or less. The PDV/PSV is used because this ratio is independent of the ultrasound beam angle of incidence that effects Doppler calculations.4 The flow pattern of an obstructed LIMA shows a triphasic pattern similar to that of the external carotid artery, with high PSV, low EDV, PDV/PSV less than 0.6, and RI nearly 0.9 or greater. Obstruction can occur between the origin of the LIMA and the distal coronary bed. Proximally occluded grafts have an absence of flow and distally occluded grafts have a systolic-dominant pattern.4
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Figure 1. Doppler flow pattern (triphasic) of ungrafted internal mammary artery, similar to that of the external carotid artery.
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Figure 2. Doppler flow pattern (biphasic) of a left internal mammary artery-to-left anterior descending coronary artery graft with good surgical anastomosis, similar to that of the internal carotid artery.
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Results
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The LIMA grafts could be visualized angiographically in all cases. In 45 patients, the graft was patent and the anastomosis was satisfactory; suboptimal results were obtained in 4 patients. Patient no. 8 had a mid-segment LIMA block (Figure 3). Patient no. 29 had critical narrowing (> 70%) of the LIMA-to-LAD anastomosis. In patient no. 13, there was < 50% narrowing of the LIMA-to-LAD graft, and patient no. 47 had < 50% narrowing of the LIMA pedicle, possibly due to a pedicle hematoma (Figure 4).
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Figure 4. Angiogram of a patient with < 50% narrowing of the left internal mammary artery pedicle, possibly due to a pedicle hematoma.
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On Doppler echocardiography, the LIMA could not be visualized in 5 patients (10%). In the other 44 patients (40 angiographically normal and 4 abnormal), the LIMA flow pattern could be assessed by Doppler; the data indicated unobstructed flow in the graft in 42. However, the Doppler data suggested an obstructed flow pattern (Figure 5) of low PDV, PSV, RI, and PDV/PSV in 2 cases (patient nos. 8 and 29). All Doppler echocardiographic measurements are listed in Table 1.
Stress thallium scans were performed in 3 patients (patient nos. 13, 29, and 47). A thallium scan was not carried out in patient no. 8 who developed unstable angina following angiography and required emergency reoperation. Thallium scanning showed evidence of reversible ischemia in the LAD territory in patient no. 29 with > 70% narrowing of the LIMA-to-LAD anastomosis. However, in patients with < 50% narrowing of the LIMA (patient nos. 13 and 47), the thallium scans did not show any evidence of reversible ischemia in the LAD territory. Findings of cineangiography and Doppler echocardio-graphy, and their correlation with thallium scans are shown in Table 2.
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Table 2. Results of Doppler Echocardiography and Stress Thallium Scan in Patients With Abnormal Angiographic Findings
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Discussion
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The use of a LIMA graft to revascularize the LAD has significantly improved the long-term results of CABG.2,3 Nevertheless, 2 decades after the introduction of mammary artery grafting, a large number of patients have an aging graft, and evaluation of these grafts might become mandatory for assessing symptoms in these patients.5 Until recently, conventional angiography was the only precise investigation available to achieve this. Doppler could be a viable option with the added advantage of being noninvasive, sensitive, and easily reproducible.6
Duplex ultrasound has been used extensively to measure blood flow in peripheral and cerebral vessels, but since Gould and colleagues introduced the use of Doppler echocardiography for assessing LIMA graft function, a number of other investigators have enthusiastically carried out this noninvasive detection of graft flow, using the pulsed Doppler technique from the chest wall.7,8 Active straight leg raising (the Azoulay maneuver) is a useful method of assessing the patency of a LIMA graft.4 In this prospective study, 2 distinct patterns of flow velocity were observed. In patients with normal grafts, the blood flow velocity patterns were similar to those seen in normal coronary arteries (42 patients). However, with significant stenosis, the blood flow velocity pattern reverted to that observed in ungrafted LIMA (2 patients), which corresponded to a completely blocked LIMA in one, and > 70% narrowing of the LIMA-to-LAD anastomosis in the other.
Imaging of the coronary arteries has proven difficult during echocardiography partly because of unfavorable chest wall configuration, coexistent chronic obstructive airway disease, and the small size of these vessels. The coronary arteries are tortuous and mobile so it is difficult to obtain accurate velocity information by Doppler throughout the entire cardiac cycle. The LIMA is less mobile than the coronary arteries, and closer to the surface of the chest wall, making it accessible to imaging by high-frequency echocardiography. Noninvasive visual-ization of LIMA grafts has been attempted in previous studies.6,9 Imaging of the proximal and mid LIMA using the parasternal and supraclavicular approaches as reported, have a low detection rate of 55%. However, the detection rate in our study was 89.8%. Possible reasons, besides using a transducer of higher frequency (7.5 MHz), could be variations in the shape of the chest wall or the position of the grafted LIMA. It is also not clear whether a normal flow pattern during resting conditions necessarily excludes the presence of a critical stenosis.7–9 The functional impairment of a vessel is probably better assessed by measuring its flow reserve. Measurement of blood flow after the use of a coronary vasodilator would allow noninvasive assessment of flow reserve in the LIMA and may be useful in the evaluation of graft stenosis of moderate severity. However, some restrictions apply in measuring the flow reserve accurately. The method is less reliable in patients with < 80% stenosis of the proximal LAD, due to the possible later interference of competitive flow from the native vessel. Furthermore, patients with a large anterior myocardial infarction or left ventricular hypertrophy may have an impaired flow response to coronary vasodilators, thus rendering interpretation of the test more ambiguous as a result of the greater overlap between normal and critically stenosed grafts.
In this experience, Doppler echocardiographic findings had a better correlation with the presence of myocardial ischemia, as detected by thallium scan, than angio-graphically demonstrable lesions. Thus, we feel that Doppler echocardiographic evaluation is a useful screening modality to identify critical obstruction in the LIMA-to-LAD circuit. It is suggested that angiography may be reserved for patients in whom the internal mammary artery is either not visualized by Doppler echocardiography or shows abnormal flow patterns.
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References
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Abstract
Introduction
