Brevibloc

This study was conducted to compare the coronary flow distributed by single and bilateral internal thoracic artery (ITA) grafts in the setting of the left main coronary occlusion. Ten dogs underwent coronary artery bypass grafting through a left thoracotomy, off pump, using a brief local occlusion to perform the anastomosis. Dogs were randomly assigned to receive either a single left ITA (LITA) graft to the circumflex coronary artery (CFX), or bilateral ITA grafts, with additional placement of the right ITA (RITA) to the left anterior descending artery (LAD). After the grafts were placed, the left main coronary artery was ligated. Electromagnetic flows were obtained in the LAD and the CFX proximally and distally to ITA grafts in both groups before grafting and after grafting. ITA flow in situ was also measured before rotation from the chest wall. Total left ventricular flow requirements were satisfied equally well by either a single LITA graft (116.7 [+ or -] 11.6 mL/min) or bilateral ITA grafts (total, 116.8 [+ or -] 9.6 mL/min divided as LITA, 55.9 [+ or -] 7.4 mL/min; RITA, 60.9 [+ or -] 12.0 mL/min). When two grafts were replaced, competitive flow in the proximal regions of both native vessels was noted, although basal flow requirements were maintained. When an individual graft was occluded in the bilaterally grafted system, the remaining graft immediately recruited the additional flow, demonstrating that either right or left ITA can support flow demands five to six times higher than in situ chest wall flow (RITA, 21.9 [+ or -] 3.1 mL/min; LITA, 22.3 [+ or -] 4.9 mL/min). These data suggest that in this canine model, a single ITA graft can support the entire flow requirements of the left ventricle. Assuming no intervening stenosis is present in native coronary systems, bilateral ITA grafting may provide a margin of safety, but under resting conditions, provides no perfusion advantages over a single ITA graft.

(Chest 1994; 106:1260-63) CFX = circumflex coronary artery; [D.sub.1] and [D.sub.2] = first and second diagnonal branch; ITA = internal thoracic artery; LAD = left anterior descending artery; LITA = left ITA; LM = left main artery; PL = posterolateral; RITA = right ITA

Key words: bilateral ITA grafts; competitive flow; left main coronary artery

In recent years, the internal thoracic artery (ITA) has been used as the conduit of choice for coronary bypass procedures and use of the ITA graft has been extended because of the excellent late patency of the graft. Although the general concept of the ITA patency was reasonably accepted at present, there has been controversy about inadequate flow through the ITA graft.(1)(2)(3) Most surgeons have the notion that use of bilateral ITA grafts or a single ITA with an additional vein graft is a safe procedure, particularly in patients with isolated left main artery (LM) diseases. On the other hand, extended use of a single ITA graft for patients with isolated LM diseases was reported by Yasuura et al(4) and Kitamura et al;(5) also, the translocation of the aortic valve for prosthetic valve endocarditis with coronary bypass is another clinical situation for our hypothesis on single ITA grafting to the entire left ventricle.(6)(7)

The purpose of this experimental study, therefore, was to evaluate the flow capacity and characteristics of a single ITA graft compared with bilateral ITA grafting in the setting of the isolated LM occlusion.

MATERIAL AND METHODS

Ten mongrel dogs, weighing 17 to 24 kg (mean weight, 20.1 [+ or -] 3.1 kg), were anesthetized with pentobarbital sodium (26 mg/kg intravenously), intubated, and ventilated with a volume ventilator. Arterial blood pressure was monitored with high-fidelity catheter micromanometers (Millar instruments, Houston) and the electrocardiogram was continuously monitored. A left fifth interspace thoracotomy was performed and the ITA grafts were harvested bilaterally from the chest wall with a wide pedicle. The dissection was continued proximally to the origin of the ITA and distally to the level of eighth costal interspace. The pericardium was then incised and the left atrial appendage was ligated at its tips and gently retracted with this suture to expose the atrioventricular groove. All dogs in this study had a bifurcation of the LM, and the first septal perforator was branched off from the left anterior descending artery (LAD) system.

The coronary anastomosis was performed using a modification of a technique described by McCarthy and Schaff,(8) without cardiopulmonary bypass. When 1.5 cm of the circumflex coronary artery (CFX) artery was prepared, snares of No. 2 silk were placed around the vessel at each end of the dissected segment. Lidocaine 1.0% (Abbott Laboratories, North Chicago, Ill) at a dose of 1.5 mg/kg and esmolol (Brevibloc, Du Pont Merck Pharma Manati, Puerto Rico) at a dose of 1.0 mg/kg were intravenously administered. The snares were then tightened around the vessel and lifted to reduce the vessel movement with cardiac activity. A coronary arteriotomy about 5 mm long was made, and the distal end of the left ITA (LITA) was anastomosed to the CFX 1.0 cm proximally to the posterolateral branch (PL) with a continuous suture of 7-0 polypropylene (Prolene) (Ethicon, Somerville, NJ).

Dogs were randomly assigned to receive either a single LITA graft to the CFX (single LITA), or bilateral ITA grafts (bilateral ITA), with additional placement of the right ITA (RITA) to the LAD in the segment between the first diagonal ([D.sub.1]) and second diagonal branch ([D.sub.2]). After the grafts were placed, the left main coronary artery was ligated. Electromagnetic flows were obtained in the distal LAD (just distally to the [D.sub.2]), the distal CFX (just distally to the PL), and both ITA grafts before grafting (control), and then in these arteries again as well as the proximal LAD (just proximally to the RITA graft) and the proximal CFX (just proximally to the LITA graft) after grafting with LM occlusion.

Regarding the definition of "distal" and "proximal" in the LAD and the CFX in this series, we defined the proximal LAD, the distal LAD, the proximal CFX, and the distal CFX as the LAD proximal to the RITA graft, the LAD distal to the RITA graft, the CFX proximal to the LITA graft, and the CFX distal to the LITA graft, respectively.

This experiment was conducted according to the Guiding Principles for the Care and Use of Laboratory Animals of the American Physiological Society. All data are expressed as mean [+ or -] SD. The Student's t test was used for the statistical analysis of the data, and p values of less than 0.05 were regarded as significant.

RESULTS

Table 1 summarizes the various flow data in each situation. When the LM was ligated, total left ventricular flow requirements were satisfied equally well by either single LITA (116.7 [+ or -] 11.6 mL/min) or bilateral ITA (116.9 [+ or -] 9.6 mL/min divided as LITA, 55.9 [+ or -] 7.4 mL/min; RITA, 60.9 [+ or -] 12.0 mL/min) grafting (Fig 1). Grafting produced bilateral flow in the native artery from the point of graft insertion. When two grafts were placed, competitive flow in the proximal regions of both native vessels was noted. Total flow combined the proximal LAD and the proximal CFX with LM ligation was 31.8 [+ or -] 11.6 mL/min in single LITA, and 31.2 [+ or -] 7.8 mL/min in bilateral ITA (Fig 2). There was no significant difference between single LITA and bilateral ITA. The distal LAD perfusion with LM occlusion was maintained at 30.2 [+ or -] 6.0 mL/min in single LITA and 32.1 [+ or -] 7.6 mL/min in bilateral ITA, as compared with LAD flow in control (31.7 [+ or -] 6.7 mL/min) (Fig 3). These differences were not statistically significant. The distal CFX flow with LM ligation was also maintained at 38.4 [+ or -] 8.9 mL/min in single LITA as well as 36.7 [+ or -] 7.5 mL/min in bilateral ITA, compared with CFX flow in control (38.6 [+ or -] 8.1 mL/min) (Fig 4). These differences were not significant. When an individual graft in the bilaterally graft system was occluded transiently in the setting of LM ligation, the remaining graft immediately recruited the additional flow, demonstrating that either RITA or LITA can support flow demands five to six times higher than the ITA in situ chest wall flow (RITA in situ, 21.9 [+ or -] 3.1 mL/min; LITA in situ, 22.3 [+ or -] 4.9 mL/min). After the LM was ligated in bilateral ITA, both flows through the proximal LAD (15.3 [+ or -] 5.8 mL/min) and the proximal CFX (15.9 [+ or -] 10.9 mL/min) were strictly limited as a consequence of competitive flow by LITA and RITA, although basal flow requirement was maintained.

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DISCUSSION

Superior long-term patency has been demonstrated with ITA grafts, though concern exists over the flow limitation or the ability of these diminutive vessels to provide adequate flow to myocardium.(1)(2)(3)(9)(10)(11) Moreover, the flow capacity of the ITA graft remains to be demonstrated, as are the consequences of grafts perfusing in competition with an additional ITA graft.

Questions regarding the relative safety and efficacy of a single ITA graft vs bilateral ITA grafts in patients with isolated LM disease are important but have not been specifically addressed in the literature (to our knowledge). Therefore, it seems beneficial to evaluate the flow limitation of a single ITA graft and then to compare the flow characteristics between a single ITA graft and bilateral ITA grafts in the setting of the LM occlusion.

In the clinical situation, our physiologic hypothesis will be applicable to left main coronary stenosis or left coronary ostial stenosis without secondary coronary lesions, but the incidence of these clinical situations was not so high.(4)(12) In addition to these, the translocation of the aortic valve for prosthetic valve endocarditis with supravalvular coronary artery bypass graft is also another clinical situation in which our experiment would be valid. In such a complicated patient with infection, one graft has been commonly placed to the LAD system for the entire left coronary system because severe adhesion around a heart makes it difficult to place another graft to the CFX region.(6)(7)

Most surgeons have the notion that the combination of ITA and vein grafting or bilateral ITA grafting is supposed to be a better choice for isolated LM disease rather than the single ITA graft.(3)(12)(13) Barner et al.(13) reported that the use of bilateral ITA grafts for revascularization of the LM disease is a safe and efficacious practice. However, there is the potential for the risks to limit ITA flow and to generate diffuse thinning of the ITA graft when revasculization will be performed by bilateral ITA grafts or a single ITA with an additional vein graft, as a consequence of competitive flow,(14) if assuming no intervening stenosis is present in the native coronary artery.

On the contrary, Yasuura et al(4) reported that the clinical use of a single ITA graft in the setting of the isolated LM disease has been a safe and adequate procedure to perfuse the entire left ventricular myocardium supplied by the left coronary artery, unless a heart is hypertrophied.(4) It has been also demonstrated experimentally that a single ITA graft can adequately supply the myocardium of a canine heart in the face of the acute closure of the left main coronary artery ostia.(15)

On the basis of our experimental study, when two ITA grafts were placed, although basal flow requirements were maintained in the proximal LAD and the proximal CFX, flow competition in these proximal regions was demonstrated. When an individual graft was occluded transiently in the bilaterally grafted system, the remaining graft immediately recruited the additional flow, demonstrating that either right or left ITA can support flow demands five or six times higher than in situ chest wall flow.

One of the disadvantages related to competitive flow is reported to be diffuse thinning of the ITA graft.(14)(16)(17) This has been referred to as the "diffuse atrophy,"(14) the "string sign,"(16) and the "distal thread sign."(17) Various causes for this phenomenon have been considered, but competitive flow from an unimpeded native coronary artery or another graft seems to be the primary factor. In the present study, flow competition in the proximal coronary artery regions has not provided perfusion disadvantages regarding basal coronary flow requirements at least in short-term study; however, bilateral ITA grafts will yield the potential for two major risks in the coronary circulation as a consequence of competitive flow over a long-term follow-up. First, the ITA graft will decrease in size to match the demand in the presence of flow competition, which will subsequently cause generalized narrowing or diffuse atrophy of the graft. When given a situation of increased myocardial oxygen demand or development of a new intervening stenosis, generalized atrophy of the ITA graft will be attributed to a flow deficit and the subsequent hypoperfusion syndrome. Second, the counter-directional coronary flow distributed by bilateral ITA grafts will provide turbulence in coronary circulation and decrease the flow velocity in the proximal regions of the LAD and the CFX. Under these situations, it remains the concern that an intracoronary turbulence will result in the progression of atherosclerosis and the formation of thrombi in proximal coronary arteries in a long-term clinical result.

Regarding the canine ITA model, the canine ITA is relatively large in relation to the size of the coronary arteries, so the recruitable flow capacity in canine ITA grafts is likely much higher than in human. Thus, flow studies involving ITA coronary grafts should be extrapolated to clinical settings with great caution. However, the present studies do offer some advantages not readily obtained in clinical settings, because a clinical investigation concerning competitive flow perfused by bilateral ITA grafts or the combination of the ITA and vein grafts in native coronary arteries may be virtually impossible. Even if possible in the limited patients, it needs a time-consuming process not easily accomplished in the operating room.

In conclusion, these data suggest that in this canine model, a single ITA graft can support the entire flow requirements of left ventricle in isolated left main coronary artery occlusion. In the absence of intervening stenosis, both the proximal and distal myocardial zone demonstrate coronary flow reverse in native coronary circulation similar to that achieved after either a single ITA graft or bilateral ITA grafts. Thus, bilateral ITA grafting may provide a margin of safety, but under resting conditions, it provides no perfusion advantages over a single ITA graft. This study was limited to the condition of resting blood flow to the entire left main coronary bed, but further investigations in our laboratories have also demonstrated the adequate flow reverse for physiologically and pharmacologically stimulated conditions.(18)

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(13)Barner HB, Naunheim KS, Willman VL, Fiore AC. Revascularization with bilateral internal thoracic artery grafts in patients with left main coronary stenosis. Eur J Cardiothorac Surg 1992; 6:66-71

(14)Barner HB. Double internal mammary coronary artery bypass. Arch Surg 1971; 109:627-30

(15)Lee CN, Orszulak TA, Schaff HV, Kaye MP. Flow capacity of the canine internal mammary artery. J Thorac Cardiovasc Surg 1986; 91:405-10

(16)Mills NL, Ochsner JL. Technique of internal mammary-to-coronary artery bypass grafts. Ann Thorac Surg 1974; 17:237-46

(17)Geha AS, Baue AE. Early and late results of coronary revascularization with saphenous vein and internal mammary artery grafts. Am J Surg 1979; 137:456-63

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