Routine Visualization of the Left Internal Mammary Artery before Bypass Surgery: Is It Necessary?

The use of the left internal mammary artery (LIMA) to bypass the left anterior descending artery (LAD) is the “gold standard” of coronary artery revascularization. Studies have shown improved patency^1–4 and survival^3,5–9 with LIMA use. It is unclear whether routine visualization of the LIMA prior to CABG is necessary.^10–12 Although infrequent, dissection, embolization with subsequent cerebrovascular ischemia, as well as higher contrast and radiation exposure may occur with routine LIMA visualization. On the other hand, the functional patency of the grafted LIMA may be dependent upon the absence of: 1) significant proximal subclavian artery stenosis; 2) large proximal lateral costal branches; 3) a narrow lumen LIMA diameter; or 4) an obstructive stenosis in the LIMA. In this retrospective study, we evaluated the frequency of significant disease in the LIMA, subclavian and vertebral arteries, as well as the presence of large costal branches arising from the LIMA. Furthermore, we correlated these pre-operative findings with persistent postoperative ischemia on nuclear stress imaging in the bypassed LAD territory. Materials and Methods In 115 consecutive patients referred to CABG from the practice of one cardiologist at our institution between February 1998 and April 2002, 101 patients met the inclusion criteria. This included patients who underwent selective subclavian and internal mammary artery angiography at the time of coronary angiography and had optimal angiographic images and coronary bypass within 6 months of the angiogram. The Institutional Review Board of the Genesis Health System approved the study. Data were collected on the following variables: age, gender, height, weight, ejection fraction, pre-operative creatinine, syncope, hyperlipidemia, stroke or transient ischemic attacks, chronic obstructive pulmonary disease, number of diseased coronary arteries, usage of the LIMA as graft, number of bypass grafts used, type of vessel used to graft the LAD, off- or on-pump CABG, proximal subclavian artery disease (0, 50%), lateral costal branch (LCB) presence and size ( 1.5 mm), location of the LCB from the origin of the LIMA ( 2 cm), vertebral artery disease ( 50%), postoperative ischemia, priority of surgery, postoperative Cardiolite® (Bristol-Myers Squibb Medical Imaging, Inc., North Billerica, Massachusetts) stress test and postoperative follow-up angiography. All vessels were reviewed and quantitatively analyzed by an independent reader using the caliper and ruler method with the diagnostic catheter as a reference, and were blinded to the patients’ clinical history. Disease in the LIMA, left subclavian, and vertebral arteries was considered significant if lesions were > 50%. Postoperatively, patients who underwent noninvasive nuclear stress testing using technetium 99m (Tc99m) pyrophosphate-sestamibi were identified, and the presence of ischemia in the anterior wall was correlated with significant pre-operative disease in the LIMA and subclavian vessels, and the presence of large (> 1.5 mm) lateral costal branches. Statistical methods. Descriptive analysis was performed on all variables included in this study. A logistic regression model was utilized to predict the occurrence of ischemia on Cardiolite testing postoperatively. Results A total of 101 patients who met the inclusion criteria were studied (Figure 1). The patient characteristics are shown in Table 1. All 101 patients underwent CABG within 6 months of the index angiography. Of the 101 patients, the LIMA was used as a conduit in 93 (92.1%) cases. The LAD was bypassed in 90 patients, with the LIMA utilized in 83 patients (92.2%) and a vein graft in 7 patients (8.4%). In 10 patients, the LIMA was utilized to bypass a vessel other than the LAD. An average of 3.37 grafts were used to bypass an average of 3.7 coronary arteries. Off-pump bypass was performed in 7 patients (6.9%). Table 2 illustrates disease severity identified in the subclavian, and vertebral arteries, as well as the presence and size of lateral costal branches arising from the LIMA. Of the 101 patients, 87.1% of the LIMAs prior to surgery were without any disease. One LIMA had a 25–50% narrowing, and the remaining LIMAs showed between 0–25% disease. Postoperatively and during the study period, 10 patients had died, and 17 patients were either lost to follow-up or did not undergo a stress test. Noninvasive nuclear stress testing using technetium 99m (Tc 99m) pyrophosphate-sestamibi was performed in 74 patients (73.3%) as part of a routine follow-up post-CABG or because of a recurrence of symptoms. Results of the stress tests are shown in Table 3. Anterior wall defect was noted in 21 patients (28.4%), with a reversible defect noted in 9 of the 21 patients (42.9%). Of the 74 patients who had a nuclear stress test, 50 patients underwent angiography because of reversible ischemia. In 39 (78%) patients, the LIMA was used and was normal, and in 7 (14%) patients, the LIMA was occluded (n = 6), or had > 50% disease (n = 1). The LIMA was not utilized in 4 patients who underwent a follow-up angiogram. Logistic regression analysis showed that the presence of postoperative anterior ischemia on Cardiolite stress testing does not correlate with the presence of pre-operative asymptomatic subclavian and/or LIMA disease or the presence of large lateral costal branches (> 1.5 mm). Also, among the 7 patients with diseased LIMAs on follow-up angiography (all with no significant disease pre-operatively), 2 patients had ischemia in the anterior wall. Discussion In this study, we demonstrated that LIMA disease is very infrequent on routine evaluation during coronary angiography prior to CABG. On the other hand, the occurrence of asymptomatic, significant subclavian artery disease proximal to the origin of the LIMA was approximately 5%. Both these findings are consistent with previous reports of the occurrence of disease in these vessels.^13–18 To our knowledge, the presence of asymptomatic pre-operative subclavian disease, LIMA disease, and the presence of LCB has not been correlated with the presence of persistent ischemia on nuclear stress testing post-CABG. Our study suggests that these pre-operative findings do not correlate with postoperative ischemia in the LAD territory, and therefore obviates the need to routinely search for these lesions. Our data do not apply, however, to symptomatic subclavian disease, leading to a large difference in both arm pressures, vertebral steal symptoms, or left arm weakness.^17,19 These findings are, however, easily screened for pre-surgery. Ochi et al.²⁰ have also shown that routine visualization of the LIMA is not necessary except if preceded by a clear indication, such as cervical or supraclavicular bruit, > 20 mmHg difference in an upper extremity blood pressure, an extensive aortoiliac occlusion, or inflammatory disorders such as Takayasu’s arteritis or Kawasaki’s disease. A study by English et al.^18M indicated that the presence of severe peripheral vascular disease and upper arm pressure difference > 10 mmHg was also an indication for routine subclavian angiography. Of interest, 68% of the patients screened with nuclear stress testing had reversible ischemia. Although selection bias of these patients could have increased this incidence, Jegaden et al.²¹ have also reported a high frequency of abnormal nuclear scans (26%) postoperatively despite complete revascularization. Though their study speculated that suboptimal flow in the LIMA was the culprit, the true mechanism of this persistent ischemia is unclear. In our study, we encountered no complications in performing catheterization of the subclavian and internal mammary artery. Therefore, we do not believe that the safety of this procedure is a significant issue but its clinical usefulness is questionable. Based on these observations, we are currently not performing routine visualization of the LIMA pre-operatively unless signs or symptoms of severe subclavian disease exist. Limitation of the study. This is retrospective study with inherent biases. However, all patients from the practice of one cardiologist were evaluated, and all who met the study inclusion criteria (n = 110) were analyzed. Also, a blinded reader to the patients’ history quantitatively interpreted the angiographic findings. Our data included asymptomatic subclavian disease and therefore should not be extrapolated to the symptomatic patients. The routine visualization of LIMAs needs to be discouraged pre-operatively unless clinically justifiable such as in severe symptomatic subclavian narrowing and radiation to the chest where the LIMA could be adversely affected.

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