Eur J Cardiothorac Surg 1998;13:36-41
© 1998 Elsevier Science NL
Double grafting of the left anterior descending artery: is the distance between the internal mammary artery and supplemental vein graft anastomoses relevant in graft survival?
Sebastian Pagni,
Ellis Salloum,
John Storey,
William Montgomery,
Patricia Cerrito,
Daniel Van Himbergen,
Laman A. Gray, Jr.,
Paul A. Spence
Department of Surgery, Division Thoracic and Cardiovascular Surgery, University of Louisville, 201 Abraham Flexner Way, Suite 1200, Louisville, KY 40202, USA
Received 15 July 1997;
received in revised form 4 November 1997;
accepted 11 November 1997.
Corresponding author. Tel.: +1 502 5612180; fax: +1 502 5612190.
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Abstract
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Introduction: Under certain conditions (small internal mammary artery (IMA) or large runoff), double grafting of the left anterior descending (LAD) artery system is necessary to avoid the ominous consequences of myocardial hypoperfusion. Previous studies have shown that a saphenous vein (SVG) adjacent to an IMA graft leads to failure of the IMA. This study compares IMA flow patterns when adjacent (<1 cm) and separated (3–4 cm) from a SVG placed on a proximally occluded LAD. Methods: A SVG and right IMA (PIMA) to proximal LAD (2.5–3 mm) coronary bypass were performed in 12 mongrel dogs. The left IMA (DIMA) was anastomosed to the distal LAD (1.5 mm). All anastomoses were carried out without cardiopulmonary bypass. The native LAD was occluded proximally to the PIMA anastomosis, and all graft flows were measured in competitive and non-competitive flow conditions. Results: Isolated graft to LAD flows were similar for the three conduits. There was a drop in flow in both the PIMA and DIMA when placed in competition with the SVG (10.1±3.0 vs. 19.1±4.6 ml/min; P<0.05). The total drop in flow was significantly greater in the PIMA (67.6 vs. 39.9%; P<0.05). Diastolic flow was better preserved in the distal IMA graft (19.6±5.6 vs. 10.2±3.0 ml/min; P<0.05). The patterns of flow were much different during competition and there was significant retrograde systolic flow in all PIMA grafts while there was no (n=5) or minimal retrograde flow (n=7) in the DIMA grafts. Conclusion: An IMA graft, when adjacent to a SVG, sustains a significant decrease in both total and diastolic flows and develops an oscillating pattern of flow in early systole (retrograde then antegrade). Placing the IMA more distally on the LAD improves flow and decreases retrograde flow. In clinical situations requiring double grafting on the LAD, distance between grafts may be an important factor in maintaining IMA patency.
Key Words: Left anterior descending artery • Internal mammary artery • Supplemental vein graft anastomoses
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Introduction
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The internal mammary artery (IMA) graft undoubtedly provides superior patency rates to the saphenous vein grafts (SVGs)
[1]. Occasionally, however, the IMA flow may be insufficient to supply large myocardial O2 demands and may predispose to the hypoperfusion syndrome which is characterized by a low perioperative cardiac output state associated with high mortality.
Prevention of this condition is very important in reoperative coronary surgery, where often a dilated or hypertrophied ventricle is supplied by a stenotic vein graft. Coronary revascularization in this setting requires a thoughtful clinical and physiological approach to avoid deleterious effects on both the grafts and the myocardium. Different reoperative strategies have been proposed, as pointed out by Navia et al. from the Cleveland Clinic
[2]. Their retrospective analysis of this problem favored supplementing the IMA graft with an additional vein graft or maintaining the diseased left anterior descending (LAD) graft. Although the follow-up in patients with double grafting of the LAD suggests that both grafts remain patent, there has been little physiologic assessment of the effect of double grafting of the LAD on IMA flow depression.
This study, using a canine model, was designed to evaluate the acute effects of competitive flow on IMA grafts located adjacent or distal to a larger SVG in order to evaluate changes in IMA flow associated with double LAD grafting.
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Material and method
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A total of 12 mongrel dogs, weighing 30.4–37.3 kg (mean 32.8±2.0) were anesthetized with sodium pentobarbital (25 mg/kg i.v.) and ventilated using a volume-controled ventilator. Arterial blood gases were controled every 30 min, and bicarbonate added to maintain a physiologic pH between 7.35 and 7.45. Aortic blood pressure was monitored using a 5F micromanometer tipped catheter (Millar Instruments, Houston, TX) introduced through the left carotid artery into the ascending aorta. Electrocardiogram was monitored continuously.
A left fifth interspace antero-lateral thoracotomy was performed and both internal mammary arteries were harvested ‘skeletonized’ under direct vision from its origin to the seventh intercostal space and left covered in a papaverine-soaked gauze (1 cc diluted in 10 cc of saline). A 10–12 cm length of left saphenous vein was harvested, and the wound closed. The pericardium was opened and the margins sutured to the edges of the incision for heart support. The LAD was dissected circumferentially within 1 cm of its origin, and encircled with a flow probe (3SB, Transonic Systems, Ithaca, NY). The distal LAD (DIMA graft) anastomotic site was selected just distal to the second diagonal branch (1.0–1.5 mm diameter) and snares were used for control and stabilization. The coronary anastomoses were performed using a modification of the technique described by McCarthy and Schaff
[3] for the circumflex artery, without the use of a distal shunt, and were constructed using a continuous 7.0 monofilament suture on the beating heart. The LIMA was anastomosed to the distal LAD, and after 30 min of reperfusion time, in the presence of hemodynamic stability, the SVG-LAD anastomosis was constructed proximal to the first diagonal (within 2 cm of circumflex take off) in the fashion described previously. The SVG-aorta anastomosis was then placed just distal to the last aortic arch branch using a side clamp. The RIMA was anastomosed to the hood of the vein graft anastomosis ((PIMA graft) within 1.5 cm of the LAD). This operation produced a competitive flow model, with a low operative mortality and cost. Both IMAs and the SVG were encircled distally with flow probes (2-3SB, Transonic Systems, Ithaca, NY) and a No. 3 French micromanometer tipped catheter (Millar Instruments, Houston, TX) was inserted in the vein graft through a lateral branch. Afterwards, the flow probes were calibrated electronically and the pressure catheters were zeroed.
Data were recorded and analyzed using the MacLab 8e data acquisition system (AD Instruments, Milford, MA). Conditions were monitored until hemodynamic stability was reached, and then recorded. Systolic, diastolic and whole cycle flows of the grafts and LAD were measured isolated and in competition. Data was initially recorded from the isolated grafts as the sole inflow to the LAD. To study competitive flow the LAD was occluded and PIMA and DIMA flows and pressures were measured individually when in competition with the SVG. At completion, the animals were sacrificed with an intracardiac injection of 10 cc of saturated potassium and the heart excised. Casts were constructed of the LAD system and anastomosed grafts using silicone rubber (Dow Corning, Midland, MI) catalyzed with Microfil (Flow Tek, Boulder, CO) solution and were grossly examined for anatomic geometric adequacy and diameter.
All data are expressed as the mean and S.E.M. The multivariate method ANOVA was used to compare the differences between graft flows. Statistical significance was given to P-values equal to or less than 0.05. The University of Louisville approved this study, which was conducted under strict guidance of the principles for the Care and Use of Laboratory Animals of the American Physiological Society.
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Results
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Mean total flows were similar in all grafts as the sole inflow to the LAD. Absolute total and phasic PIMA and SVG flows in competition are summarized in Table 1. PIMA flow was decreased more significantly in the diastolic phase. The relationship of flow shift of both grafts in competition is shown in
Fig. 1 . Note that total PIMA flow was greatly reduced when in competition with the adjacent vein graft flow (PIMA flow reduced 67.7% versus SVG flow reduced 26.0%; P=0.0001).
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Fig. 1. Total flow shift of PIMA and SVG during competition. Note the significantly decreased total PIMA flow when compared to the SVG. Total LAD flow is slightly increased during competitive flow conditions.
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The flow in the distal IMA (DIMA) were also evaluated in competition with the open SVG (
Fig. 2
). During competitive flow, total DIMA flow was reduced less than in the PIMA graft (40.8% reduction in DIMA versus 34.8% in SVG flow; P=0.0001). There was greater preservation of DIMA flow in the diastolic phase (Table 2).
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Fig. 2. Total flow shift of DIMA and SVG during competition. Note the similarity in flow reduction seen in both grafts and the significant increase in total LAD flow.
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Reversal of flow in early systole occurred in all PIMA grafts placed in competition with the SVG. The oscillating pattern of flow was notable in all dogs, and is described in
Fig. 3
. This pattern of flow, with a distinct retrograde systolic component, was markedly decreased or abolished when the IMA grafts were placed more distally (3–4 cm) on the LAD. Five DIMA grafts retained antegrade flow during competition and the seven other grafts had minimal negative flow in early systole (
Fig. 4
).
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Fig. 3. Distinct phasic flow pattern (oscillating) found in all dogs with the IMA placed in close proximity to the SVG (PIMA). Note the retrograde flow in early systole and the decreased diastolic phase.
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Fig. 4. Decreased negative flow in early systole and increased diastolic flow in a dog with an IMA graft placed distally (beyond the large diagonal) on the LAD.
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Discussion
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The results obtained in this acute animal model demonstrate that both the saphenous vein and IMA grafts flows contribute to the distal LAD coronary artery perfusion. However, IMA flows were much more limited when anastomosed adjacent to a vein graft. Flow in the distal IMA (DIMA) was greater, especially in the diastolic phase, and the oscillating pattern of flow in early systole was minimized. We speculate that the avoidance of retrograde flow in the more distally-placed IMA graft, and the preservation of diastolic flow may provide the IMA of better patency rates.
This study provides a model of double grafting that in clinical practice is often required to avoid the hypoperfusion syndrome due to insufficient flow to the myocardium. In this situation, the short-term benefit of the IMA may be limited due to the higher flow provided by the nearby supplemental vein graft. The amount of IMA flow, change in the relationship of phasic flow perfusing the myocardium, and the presence of retrograde flow are all factors implicated in failure of arterial grafts. Moreover, this mechanism of altered IMA graft flow observed becomes more relevant as reoperative coronary surgery and the use of multiple arterial revascularization expands. The use of T and Y composite arterial conduits are subjected to altered flow dynamics and may be more exposed to the deleterious effects of competitive flow, especially on the small and reactive alternative arterial conduits. We previously showed
[4]
[5], in a similar animal model, the effects of competitive flow in arterial and vein conduits, with distinct changes in patterns of flow and flow dynamics. When subjected to extreme competitive flow conditions, the arterial grafts sustained a significant decrease in total and diastolic flows and the distal IMA developed early systolic retrograde flow due to a prolonged transit time in the longer arterial conduits. Aortic-originated grafts (shorter transit time) did not show this pattern. These altered flow patterns, triggering a vasomotor and endothelial reaction in the graft, may be involved in graft failure.
Carrel et al.
[6] showed that, in 45 patients with perioperative signs of myocardial hypoperfusion, the distal placement of a vein graft significantly improved the clinical picture in all but 2 of the patients. In their study, coronary angiography was performed in all the survivors within a 24 month postoperatively. Together with a patent vein graft, wide patent IMA was found in 56% (24/43) of the patients, narrowed IMA in 35% (15/43), and totally occluded IMA in 7% (3/43). These data suggest that competitive flow from a vein in double LAD grafting has a role in IMA failure, but other factors are probably involved. Dion et al.
[7] reported a low incidence of hypoperfusion syndrome (1.5%) in 400 patients with predominant arterial revascularization. The author attributed this low incidence to a more liberal use of prophylactic double grafting in situations where ongoing myocardial ischemia was suspected (29 cases, 3.6% of all IMA grafts). Although the IMA grafts were noted to be opened at repeat angiography, no objective data were reported. Distance between grafts was not detailed in this study nor in the others, and whether technical error or distal IMA spasm were involved in graft failure and the development of a hypoperfusion state is uncertain
[8].
In recent years the need for repeat coronary surgery has dramatically increased as overall longevity increased and the vein grafts reach their limited life-span. Experimental and physiologic clinical data on the type of grafts (different arterial or vein conduits) and its interaction with the diseased coronary vessel and/or residual grafts is limited, with only a few retrospective studies pointing to the occurrence of the hypoperfusion syndrome and the varied and controversial strategies used to avoid it
[2]
[9]
[10]
[11].
Navia et al.
[2] pointed out the value of the different strategies in reoperative coronary surgery. Important factors involved in the choice of the type of grafting were: handling of the stenotic vein with the risk of distal atheroembolism in the coronary vessel, single or double grafting with IMA and/or vein graft, hypoperfusion syndrome and the risk of the IMA graft failure due to new or residual competitive flow. In their study, division of the stenotic vein graft with single IMA grafting to the LAD system resulted in 18.9% of hypoperfusion syndrome due to the inadequate flow capacity of the IMA graft to supply an often dilated or hypertrophied ventricle. Controversy exists, however, with other surgeons supporting single IMA grafting to the LAD in most situations, pointing out the lower incidence of hypoperfusion in their experience
[12]. This author favored the operative strategy of ligation of the old vein graft and double grafting with a SVG and an IMA placed as a more distal graft, especially in situations were there is no severe stenosis in the diagonal system. In contrast, Jones et al.
[13] commented on the low incidence of hypoperfusion syndrome but its catastrophic consequences as occurred in his 5 patients with inadequate flow from single IMA grafts to the LAD. All patients had ventricles with an increased O2 demand secondary to hypertrophy and required immediate supplemental grafting of the LAD, with perioperative death in three and transplantation performed in other patient.
Turner et al.
[14], from the Cleveland Clinic, proposed the alternative strategy of leaving a stenotic vein graft in place and adding an IMA distally on the LAD. This alternative although simpler, may increase the risk of atheroembolism from the diseased graft during the operation. In their study, 2 patients with less than 50% stenosis in the vein graft had IMA failure on repeat angiography for recurrent angina, and repeat studies were not performed or detailed in the other 11 patients with minimally diseased vein grafts. These findings raise the question of IMA graft viability in double grafting with an additional vein graft, suggesting that the real incidence of IMA failure may be higher than expected and that many patients may remain asymptomatic due to adequate flow provided by the patent vein graft.
Although evaluated in animal models, the mechanisms triggering the failure of arterial grafts, other than mechanical, are still unclear and probably multifactorial. Spence et al.
[15] demonstrated, in the acute dog model of competitive flow, that IMA graft flow is maintained above in situ levels when grafted to a fully patent and competing LAD. Using a similar model, Lust et al.
[16] showed that in a chronic phase (2 months) all grafts had decreased their flow significantly although maintained above in situ levels. Upon closing the native coronary artery, all IMA grafts retained their capability to recruit patency and supply adequate flow distally. In contrast, Otaki et al.
[17] demonstrated that early IMA failure occurred when the IMA was put in competition with a large, nearby vein graft anastomosed to the same coronary system (left circumflex artery) with a complete proximal coronary occlusion. All grafts (six dogs) had angiographically demonstrated occlusion of the IMA at 3 weeks. Their data suggested that the necessary acute flow benefit obtained from the supplemental grafting may eliminate the prolonged patency advantages of the IMA grafting. Nevertheless, the follow-up was limited in this study and controversy on the subject persists.
In summary, these acute flow data suggest that in situations requiring tandem grafting to the LAD, the distance between the IMA and supplemental vein graft is relevant to IMA graft survival. The oscillating pattern of flow observed in the distal IMA may be implicated in graft failure and this likelihood can be decreased by a more distal placement of the IMA on the LAD. We speculate that there is an important role of competitive flow in arterial graft failure and therefore a better understanding of the mechanisms involved in graft failure will be important to improve the already superior results achieved with their use.
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Acknowledgments
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Supported by a Grant from the Jewish Hospital Health Care System.
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References
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Abstract
Introduction
