1. Introduction
Coronary artery bypass graft (CABG) operations are one of the most commonly performed surgical procedures, with a worldwide prevalence of over 800,000 annually and more than 350,000 operations being performed in the United States each year [1]. The use of the left internal mammary artery (LIMA) is widely considered to be the gold standard for conventional CABG operations. Its use has been shown to result in a lower incidence of reintervention, fewer myocardial infarctions, a lower incidence of angina, and lower associated mortality rates than with the use of saphenous vein grafts alone. Also when compared to saphenous vein grafts, LIMA use has been shown to have greater long-term patency results [1, 2]. For patients with multivessel coronary disease undergoing what is usually referred to as conventional CABG, the LIMA is typically grafted to the left anterior descending (LAD) artery with saphenous vein grafts often used to bypass the remaining coronary occlusions. However, arterial conduits are now being more frequently used as choices for the second and third conduits in place of saphenous vein grafts to achieve total arterial revascularization (TAR) of the myocardium due to superior patency and long-term survival results. This article provides a review of TAR using the right internal mammary artery (RIMA) and radial artery as additional arterial conduits in conjunction with the LIMA as a first choice conduit. The reported benefits of TAR when compared to conventional CABG procedures using the LIMA and saphenous vein grafts are discussed.
2. LIMA use in CABG
The LIMA is widely considered to be the best conduit for CABG procedures. In a study of the Society of Thoracic Surgeons National Cardiac Database performed by Tabata
While anatomically identical to the LIMA, the RIMA is rarely used in CABG procedures, and is almost always used as part of bilateral internal mammary artery (BIMA) grafts when it is utilized. Despite several studies showing that BIMA use confers significantly improved clinical outcomes [4-6], between 2003 and 2005 the frequency of BIMA use was only 4% [3]. Reasons for not using the RIMA include increased operative time and perceived technical difficulty associated with the harvest, concern for perioperative morbidity and mortality, the possibility of reoperations for bleeding, sternal wound infection, and uncertainty as to whether there is a significant benefit with BIMA grafting [7, 8]
3. Outcomes of BIMA in CABG
Despite its low prevalence of use, many studies have shown that RIMA use in conjunction with the LIMA can confer significantly better clinical outcomes when compared to conventional CABG procedures with the LIMA and saphenous vein grafts.
Several observational, retrospective studies have found that there are significantly greater long-term survival benefits in patients who received BIMA grafting compared to SIMA grafting. Lytle
Nasso
In the longest reported retrospective analysis of CABG procedures, ranging from 6 weeks to 32 years of follow up, Kurlansky
The location of the distal anastomosis of the RIMA graft also does not appear to significantly affect clinical outcomes of patients undergoing BIMA grafting. Kurlansky
Not all studies have found significantly increased survival rates for BIMA use over SIMA use. In a study performed by Dewar
4. Patency of RIMA versus LIMA
Patency is the most important determinant in long-term prognosis [7]. Due to the extremely low prevalence of use for the RIMA, there have been few studies evaluating its patency compared to the LIMA. However, the studies that have been performed suggest that the RIMA has similar early and even long-term patency rates as the LIMA, especially when grafted to similar coronary territories [17].
Fukui
Tatoulis
There are a variety of grafting techniques for BIMA, such as
5. Myocardial infarction, cerebrovascular accidents, freedom from reoperation, and quality of life
Stevens
While Burfeind
In their original retrospective study on BIMA versus SIMA grafting in elective CABG patients, Lytle
As previously mentioned, Nasso
Damgaard
6. Incidence of sternal wound infection, subset of patients benefiting from BIMA, IMA harvesting techniques, and operative time in BIMA CABG
One of the main concerns amongst surgeons regarding the use of BIMA in CABG procedures is the occurrence of sternal wound infections (SWI). When both internal mammary arteries are harvested, blood supply to the sternum may be more severely compromised than in single IMA procedures, thus increasing the risk for developing SWI. Various pre-operative and intra-operative techniques have been used to prevent the incidence of SWI, such as the use of prophylactic antibiotics, double gloving, and skeletonized IMA harvesting [7]. Skeletonized IMA harvesting is thought to preserve the collateral blood supply to the sternum and reduce the risk of infection [22].
Patients who are insulin-dependent diabetics, morbidly obese, or who have severe COPD are at a higher risk of developing SWI (DSWI = deep sternal wound infection, definition varies) and, in general, bilateral harvesting of the IMAs is avoided in these patients [7, 8].
In a study performed by Pevni
In a meta-analysis of 13 studies regarding BIMA CABG procedures and the harvesting technique for the IMAs, Saso
Kurlansky
One of the probable factors contributing to the low prevalence of BIMA use is the perceived increased operative time required to harvest both IMAs [7]. However, few studies have actually included operative time in their statistical analyses, most simply report aortic cross-clamp and cardiopulmonary bypass times. Gansera
7. Radial artery grafts as a second arterial conduit
The success of the LIMA in CABG procedures has lead surgeons to search for other arterial conduits. The radial artery has become a popular choice as an additional arterial conduit in attempts to achieve total arterial revascularization of the myocardium. There are numerous advantages to using the radial artery, including its long length, exposure to systemic blood pressures, and the fact that it is seldomly affected by atherosclerosis. However, the radial artery has a thicker tunica media, which is thought to contribute to its greater vasoconstrictor response than the IMA and could possibly lead to vessel occlusion. Thus, care must be taken during operative harvesting and the use of calcium-channel blockers may ameliorate a vasospastic response [24].
Like the LIMA, the radial artery has been shown to have significantly better short and long-term patency results and outcomes than vein grafts. In the radial artery patency study (RAPS), Desai
In a follow-up to the original RAPS study, Deb
Zacharias
Collins
A smaller study by Cameron
Not all studies of radial artery use have been favorable. In a review of 310 patients receiving radial artery grafts between 1996 and 2001, Khot
8. RIMA versus radial artery as a second choice arterial conduit
With favorable clinical results for both RIMA and radial artery use, it is then necessary to decide which is the better choice as a second arterial conduit when attempting to achieve multiple arterial revascularization.
Ruttman
In a 10-year prospective, randomized trial, Hayward
9. Total Arterial Revascularization (TAR)
The clinical benefits of RIMA and radial artery use have been established, and many studies have indirectly examined the results of TAR in patients receiving BIMA or radial artery grafts without the need of concomitant saphenous vein grafts. However, few studies have specifically compared the clinical outcomes of TAR to conventional CABG procedures.
In a prospective study by Muneretto
In a more recent, long-term study with a mean follow-up of 6 years, Chung
Zacharias
10. Summary
Poor long-term patencies of saphenous vein grafts coupled with the greater long term patency results of the LIMA as the gold standard conduit for CABG has prompted surgeons to seek out additional arterial conduits [1,2]. Achieving total arterial revascularization of the myocardium would then be a natural progression for the procedure.
Since it is anatomically identical to the LIMA, the RIMA would be the next logical choice in arterial conduits, yet is rarely used in CABG operations due to the perceived technical difficulty of harvest and increased operating times, a higher risk of developing SWIs, and previous lack of long-term studies of clinical outcomes [7,8]. However, several studies have demonstrated significantly increased long-term survival rates for patients receiving BIMA grafting compared to SIMA grafting [9-12]. BIMA patients also have significantly improved cardiac event-free survival than SIMA patients [4, 6, 9]. Patency rates for RIMA grafts have also been shown to be similar to those of the LIMA, even when considering the sites of distal anastomoses and the proximal anastomosing techniques [16, 17, 18, 19, 20]. Further studies are needed to determine if there is any significant effect on operative length in BIMA grafting versus conventional CABG.
The incidence of SWI has been a significant concern for surgeons, especially among high-risk patients such as the morbidly obese, insulin-dependent diabetics, and those with COPD. BIMA harvesting is generally avoided in these patients [7, 8], however studies have shown that BIMA harvesting in general does not significantly affect the incidence of SWIs [12, 23]. The risk of SWI can be even further reduced with the use of skeletonized BIMA harvesting rather than pedicled harvesting [22, 23].
Studies have shown that the radial artery is also a good choice for an arterial conduit after the LIMA. Studies examining clinical outcomes and patency rates of the radial artery have been mixed, with some studies showing better short-term patency rates than saphenous vein grafts [25-28], while other studies have shown that radial artery outcomes are at least similar to those for the RIMA and saphenous vein [11, 32, 33].
While not all studies have been favorable with regards to BIMA and radial artery use [11, 15, 32, 33], studies generally find patency rates and clinical outcomes of these two arterial conduits are at least as good as the currently accepted standards of care, which should give surgeons flexibility in their choice of conduits, ultimately leading to total arterial revascularization.
Studies in general have provided favorable results for TAR, with TAR at least being similar in outcomes to conventional CABG [35]. Several studies have demonstrated that TAR, and the use of arterial conduits in general, provides significantly better late survival (especially in patients with three vessel coronary disease), cardiac event-free survival, and improved health-related quality of life when compared to conventional CABG [11, 21, 36].
11. Conclusion
With favorable results for the use of arterial conduits and results that are at least as good as those seen in conventional CABG, these results should allow surgeons flexibility in their choice of conduits. Due to the significantly increased long-term survival advantages over saphenous vein grafts, BIMA use should be particularly indicated for younger patients, with special attempts to achieve TAR in patients with three vessel disease. Especially with skeletonized harvesting, BIMA may be safe to use in high-risk patients for SWI, such as insulin-dependent diabetics. BIMA use may also decrease the incidence of postoperative cerebrovascular events due to the decreased manipulation of the ascending aorta if both IMAs are used
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