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Ann Thorac Surg 1998;66:1873-1875
© 1998 The Society of Thoracic Surgeons
a Division of Cardiology, Department of Internal Medicine, St. Louis University Health Sciences Center, St. Louis, Missouri, USA
Address reprint requests to Dr Kern, J.G. Mudd Cardiac Catheterization Laboratory, Saint Louis University Health Sciences Center, 3635 Vista Avenue at Grand Blvd, St. Louis, MO 63110
e-mail: (kernm{at}slu.edu)
In this issue of The Annals, Gaudino and colleagues [1] asked the question, When does the steal phenomenon from mammary side branches occur? To address this important question, internal mammary artery (IMA) graft flow by Doppler echocardiography was obtained at rest and during dipyridamole-induced coronary vasodilatation in 10 patients with mammary side branches and 10 patients who had no such side branches. Thallium scintigraphy was also performed to address the adequacy of flow relative to myocardial demand. The results indicated that there was no difference in IMA flow between the two groups. Patients with IMA side branches had a maintained lower systolic-to-diastolic flow ratio compared with the response after combined coronary and peripheral dilatation, wherein selective muscular relaxation led to an increase in the systolic and reduction in diastolic flow, as might be expected for the flow component of a systemic artery. This study supported the concept that coronary branch steal from a patent IMA with side branches was possible only when there was excessive selective muscular vasodilatation, a situation unlikely to occur in a clinical setting. The potential for flow steal of mammary side branches after cardiac operation is truly minimal despite the anecdotal literature to the contrary.
I recently had the opportunity to address this issue again for invasive cardiologists [2]. We know that left internal mammary artery (LIMA) side branches to the chest wall (pectoralis branch artery) are common, occurring in between 10% and 20% of patients identified both preoperatively and postoperatively [3, 4]. Why would an occasional side branch supplying skeletal muscle in some patients cause myocardial ischemia and in other patients remain entirely asymptomatic? The true internal mammary "steal" (ie, diverting coronary flow from the heart) phenomenon occurs only when a hemodynamic abnormality is present with lowered pressure at or proximal to the origin of the IMA, thus producing preferential flow up the LIMA to the low-pressure source [5, 6]. "Preferential" blood flow to the chest wall, determined angiographically (exactly how I am not certain because LIMA angiographic flow and pectoralis flow are both antegrade), has frequently been reported but rarely objectively measured. The LIMA side branch steal is more accurately defined as a systolic flow diversion and not a true coronary flow steal because the arterial flow to the chest wall is predominantly systolic and out of phase with the predominantly diastolic coronary circulation. Systole cannot steal coronary flow from diastole in normal circumstances. The increase in systolic flow in the setting of muscular vasodilatation also would most likely have minimal physiologic effects because coronary flow remains predominantly diastolic, with a minimal or limited systolic myocardial supply. The precise nature of the noncompetitive physiology has been demonstrated in single case studies [7] and in a large series by Luise and coworkers [8] wherein the LIMA blood flow was similar between groups with and without side branches when directly measured. In addition, Abhyankar and colleagues [9] noted that the occlusion of a large LIMA side branch did not significantly alter antegrade LIMA flow directly to the left anterior descending coronary artery determined by intraarterial Doppler guidewire velocity measurements. Whether a LIMA side branch should be occluded can be justified through such a technique wherein a trial occlusion is demonstrated to increase flow to the target vessel.
From the surgical, radiologic, and cardiologic literature, LIMA side branch occlusion has been variably purported to improve myocardial ischemia (Table 1). Three major concerns arise when reviewing these studies: (1) Objective ischemia is variably present before and usually only subjectively relieved after LIMA branch occlusion, most often attributable to the "preferential distribution" of flow away from the target vessel. These data are usually derived from angiographic information alone. (2) Side branch occlusion is variably successful over time, although symptoms are usually improved despite a return of patency to the offending side branch; and (3) subjective ischemia is always better, but the objective demonstration of myocardial ischemia before and after a side branch to demonstrate its effective contribution to coronary flow is often not reported. Reports of the angiographic diameter of the LIMA or left anterior descending artery, or both, as a surrogate for coronary blood flow determinations before and after intervention are using a nonquantitative method and erroneously presuming that the alteration in diameter is equivalent to the alteration in coronary flow. Although there is a relationship, this is not linear nor should it be accepted as such for quantitative determinations of these events. The multiplicity of technical reports on ways to occlude a LIMA side branch, both surgical and percutaneous, are often highlighted by the operator’s enthusiasm for the new approach of the mechanical aspects of balloon or coil occlusion or the implantation of other thrombogenic material for obstruction of arterial flow. Most single experiences are often supported by a literature review involving subjective descriptions of improved chest pain, exercise tolerance, and feeling of well-being, with documentation of improved myocardial ischemic lacking. The LIMA chest wall branch steal phenomenon has been reported repeatedly in the surgical literature, with rarely objective evidence that symptom alteration after treatment was not merely a placebo effect. Luise and colleagues [8] provide strong evidence that indicates that side branch steal is rare and may not, in fact, exist at all. In examining whether LIMA side branches should be occluded, one should consider objectively determining the physiology of this common anatomic condition. Using intracoronary Doppler velocity, a trial occlusion of the LIMA side branch with a balloon technique can easily demonstrate whether flow through the LIMA would increase after the intervention and warrant the attendant procedural risks.
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References
This article has been cited by other articles:
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  K. Tsakiridis, D. Mikroulis, V. Didilis, and G. Bougioukas Internal Thoracic Artery Side Branch Ligation for Post Coronary Surgery Ischemia Asian Cardiovasc Thorac Ann, August 1, 2007; 15(4): 339 - 341. [Abstract] [Full Text] [PDF]
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 A. M. Calafiore Reply Ann. Thorac. Surg., May 1, 2000; 69(5): 1650 - 1650. [Full Text] [PDF]
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A. M. Calafiore, M. Contini, A. L. Iaco, N. Maddestra, L. Paloscia, T. Iovino, and M. Di Mauro Angiographic anatomy of the grafted left internal mammary artery Ann. Thorac. Surg., November 1, 1999; 68(5): 1636 - 1639. [Abstract] [Full Text] [PDF]
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