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Ann Thorac Surg 1999;68:1619-1622
© 1999 The Society of Thoracic Surgeons
a Department of Cardiothoracic Surgery, Royal Melbourne Hospital, Melbourne, Australia
b Department of Anaesthesia, Royal Melbourne Hospital, Melbourne, Australia
Address reprint requests to Dr Royse, Department of Cardiothoracic Surgery, Royal Melbourne Hospital, PO Box 2135, Parkville, Victoria 3050, Australia
e-mail: alistair.royse{at}nwhcn.org.au
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Abstract |
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 Top Abstract IntroductionMaterial and methodsResultsCommentReferences |
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Methods. The radial artery was anastomosed to the left internal mammary artery as a Y graft in 17 patients and all coronary arteries grafted. Intraoperative blood flow through the composite grafts was evaluated by the transit-time Doppler technique.
Results. Against no resistance, blood flow in the left internal mammary artery alone was 99 ± 9 mL/min and rose to 173 ± 16 mL/min when the radial artery was anastomosed as a Y graft. Composite-graft flow following grafting was 88 ± 9 mL/min, 49 ± 6 mL/min when the aortic clamp was removed and native coronary flow restored and 82 ± 13 mL/min following weaning from cardiopulmonary bypass. The maximal potential flow through the composite graft was 2.3-fold (95% CI 1.6 to 3.2) greater than that after cardiopulmonary bypass.
Conclusions. Total arterial revascularization, using a composite graft, provided a 2.3-fold reserve of blood flow to the coronary vascular bed through the grafts.
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Introduction |
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TopAbstractIntroductionMaterial and methodsResultsCommentReferences |
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Little is known about blood flow in coronary artery bypass grafts other than in the isolated LIMA grafted to the left anterior descending coronary artery. Even less is known of the impact of native coronary flow on graft blood flow. This is because most patients have at least one aortic anastomosis, which cannot conduct blood flow while the aortic clamp is in place. This means that graft flow cannot be evaluated independently of native coronary flow in most patients.
This study is of patients with composite arterial grafts based entirely on the left internal mammary artery. There were no aortic anastomoses, so graft blood flow could be assessed separately from native coronary flow. Blood flow was measured by the transit-time Doppler technique before and after construction of composite grafts that supplied all three coronary territories. The effect of competition between blood flow in the grafts and native coronary blood flow was also assessed, by making measurements before and after removal of the aortic clamp.
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Material and methods |
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TopAbstractIntroductionMaterial and methodsResultsCommentReferences |
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Pilot study
The use of transit-time Doppler flow probes has previously been validated in cardiac surgical patients [6]. However, we performed a further check in 18 patients, some of whom were also part of this series. We measured blood flow in (1) the left internal mammary and radial arteries after the distal end of these vessels was transected and (2) the radial artery after it was anastomosed to the left internal mammary artery as a Y graft. The arteries had been vasodilated (see below), and were allowed to bleed freely against no resistance. Doppler-measured flow was compared with simultaneously timed volumes of blood collected in a measuring beaker. Model II regression analysis [7] of the results in mL/min gave the model (Actual flow) = 5.94 ± 0.89 (Doppler flow). Pearson’s product-moment r = 0.902 (p < 0.001). The 95% confidence interval for the y intercept was -13.00 to 24.88 (no fixed bias), and for the slope 0.752 to 1.032 (no proportional bias) [7].
Protocol for definitive study
In the study proper, blood flow and pressure measurements were made. After Y graft construction (Fig 1), following vasodilatation with papaverine HCl (1 mg/mL) and against no resistance (free-flow). The flow probe was placed on the LIMA proximal to the Y graft (Fig 1) and flow was measured while the distal LIMA and RA were clamped in turn (RA free-flow or LIMA free-flow), then again with no clamps on either conduit (composite-graft free-flow). After all coronary graft anastomoses had been performed, but before removal of the aortic cross-clamp (cross-clamp ON). Thus, all three coronary artery territories were being perfused through the grafts but there was no flow through the native coronary circulation. After unclamping the aorta but with full cardiopulmonary bypass support maintained (cross-clamp OFF). This detected the immediate effect of reintroducing native coronary artery flow from the aortic root. After weaning from cardiopulmonary bypass, when the heart was working against the systemic vascular resistance (postbypass).
Analysis of results
The flow reserve in the arterial graft was calculated as the geometric mean ratio between the flow after both conduits had been joined, but prior to grafting (free composite-graft blood flow), and following weaning from cardiopulmonary bypass (postbypass blood flow) (see Table 1). The geometric mean was used because the arithmetic ratios were strongly positively skewed. The corresponding 95% confidence interval was calculated.
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Results |
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TopAbstractIntroductionMaterial and methodsResultsCommentReferences |
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MAP during the various conditions of the protocol ranged from 60 to 77 mm Hg (p < 0.001) (Table 1). It was lowest after release of the aortic clamp (cross-clamp OFF).
MBF during the various conditions of the protocol ranged from 49 to 173 mL/min (p < 0.001) (Table 1). Outcomes of paired analyses are listed in Table 2. Following construction of the Y graft, with maximal vasodilatation and against no resistance (free-flow), MBF in the LIMA and RA were not consistently different (Table 2), because they share a common inflow through the proximal one third of LIMA (Fig 1). Construction of the Y graft led to a 175% increase of total flow through the LIMA pedicle (Table 1).
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The flow reserve of the composite conduit was 2.29 (geometric mean) and the 95% confidence interval was 1.64 to 3.19. This refers to the potential maximum flow of the conduit against no resistance compared to the actual conduit flow recorded following weaning from cardiopulmonary bypass (ie, the extra capacity for flow of the conduit).
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Comment |
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TopAbstractIntroductionMaterial and methodsResultsCommentReferences |
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Our data show that construction of a Y graft using RA led to an increase of 74 mL/min in total LIMA pedicle flow (Table 2). The resultant maximum potential flow through this composite graft was 173 mL/min when both conduits were unclamped and bleeding freely against no resistance (Table 1). This finding is not surprising because the combined resistance of both conduits would be lower than for each conduit alone. This flow represents the maximum potential flow of the LIMA composite graft.
The RA flow was similar to the LIMA flow even though it is a larger vessel with lower resistance (Tables 1, 2). This is because the LIMA proximal to the Y graft has a smaller diameter than the RA and so represents the flow-limiting segment of the conduit. The combined flow with both conduits unclamped is, however, less than the sum of the LIMA and RA flows. This must be because some flow limitation occurs in the proximal LIMA segment despite a lowering of conduit resistance distal to the Y graft.
Conduit flow to all coronary territories was measured without the presence of native coronary flow and was 88 mL/min (Table 1). This measurement was achieved by removing the conduit clamp but not the aortic clamp. This reduction in flow from the maximum potential recorded earlier is due to the higher resistance of the coronary vascular bed.
When the aortic clamp was removed, a further reduction in conduit flow of 38 mL/min occurred due to the reintroduction of native coronary flow. This reduction represents the proportion of flow to the heart occurring by the native coronary circulation. This proportion varies with each patient depending on the extent of coronary artery disease.
A rise of 29 mL/min in conduit flow occurred following weaning from cardiopulmonary bypass. It is expected that the blood flow requirements of the heart would increase due to the increased work of ejection compared to the beating but empty heart fully supported by cardiopulmonary bypass. However, after correction for multiple-hypothesis testing, this increase was not significant (p' = 0.072) (Table 2). This may reflect a relatively small sample size. It is possible that the flow requirements of the beating heart fully supported by cardiopulmonary bypass may be greater than previously thought. Because the heart is ischemic at the time of clamp removal, it is also possible that reactive hyperemia of the myocardium may be present, leading to greater conduit flow while the oxygen debt is repaid. The blood flow to the myocardium increases with work of ejection; but only the increase in the conduit flow was measured and not the coexisting increase in the native coronary flow. This measurement was most useful in determining the actual conduit flow required to revascularize all coronary territories in the beating and ejecting heart.
The conduit flow reserve was therefore calculated by dividing the maximum potential flow by the actual flow following weaning from cardiopulmonary bypass. This was found to be 2.3. This indicates that there is considerable flow reserve in the composite conduit, which would allow for increases in flow during times of increased requirement, such as exercise. It addresses the key concern by surgeons that composite grafts may not be capable of sufficient flow to all three coronary territories (the "hypoperfusion syndrome").
The LIMA-to-left-anterior-descending-artery graft has flows of 33 to 42 mL/min assessed by the same transit-time Doppler technique [6, 11, 12], or 36 mL/min using an electromagnetic technique [13]. It may be deduced, therefore, that probably half of the conduit flow to the heart is likely to be delivered to the left anterior descending artery alone; and that the remainder of the circumflex and right coronary territories may receive approximately 30 to 45 mL/min. Using this, and data from this series for LIMA potential maximum flow, the flow reserve of the isolated LIMA grafted to the left anterior descending artery in this series would be approximately 2.5 to 3.0 (Table 1). The flow requirements of complete revascularization were roughly double that of isolated left anterior descending artery grafts; but this was compensated for by a roughly doubled flow capability of the composite arterial graft arising from the proximal LIMA (Table 1).
This study is limited by a relatively small number of patients who are heterogenous. The transit-time Doppler probes are accurate but sensitive to differences in angle, which is sometimes difficult to control when the heart and the lungs are moving and the probe itself may be within the pleural cavity and unseen. Thus, operator-dependent measurement errors are possible, but minimized by only analyzing "good-quality" segments of the flow curves. This data cannot be used to judge the adequacy or patency of individual conduits.
Construction of composite arterial grafts results in a significant increase in flow through the LIMA pedicle. The flow reserve of these grafts was 2.3-fold greater than actual conduit flow after weaning from cardiopulmonary bypass. Conduit flow reduced by approximately half due to native coronary artery flow.
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Acknowledgments |
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References |
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TopAbstractIntroductionMaterial and methodsResultsCommentReferences |
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A. G. Royse, C. F. Royse, K. L. Groves, and G. Yu Reply Ann. Thorac. Surg., July 1, 2000; 70(1): 341 - 342. [Full Text] [PDF]
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