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Eur J Cardiothorac Surg 2007;31:376-381. doi:10.1016/j.ejcts.2006.11.023
Copyright © 2007, European Association for Cardio-Thoracic Surgery. Published by Elsevier B.V. All rights reserved
a University Hospital Saint-Luc, Department of Cardiovascular Medicine and Surgery, Brussels, Belgium
b University Hospital Saint-Luc, Department of Internal Medicine, Brussels, Belgium
c School of Public Health, Unit EPID of the Catholic University of Louvain Medical School, Brussels, Belgium
Received 28 September 2006; received in revised form 27 October 2006; accepted 16 November 2006.
* Corresponding author. Address: Service de Chirurgie cardiovasculaire et thoracique, Cliniques Universitaires Saint-Luc U.C.L. 90, Avenue Hippocrate 10/6107, 1200 Bruxelles, Belgium. Tel.: +32 2 7646106; fax: +32 2 7648960. (Email: david.glineur{at}clin.ucl.ac.be).
| Abstract |
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Abbreviations: FFR = fractional flow reserve ITA = internal thoracic artery LAD = left anterior descending coronary artery SV = saphenous vein LCX = left circumflex RCA = right coronary artery
Key Words: Coronary artery bypass Internal mammary artery Saphenous vein graft Fractional flow reserve
| 1. Introduction |
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Left ITA is most commonly grafted on the LAD whereas the right ITA is preferred for the marginal branches, either used pedicled or used within a Y-graft configuration. The long-term patency of the left ITA has always been found superior to that of the right ITA, even when implanted on the LAD [6,7]. Since the two ITA's are supposed to be identical in terms of anatomy and physiology, this difference has been mainly explained by differences in the quality of the run off of the targeted vessel [8].
The concept of pressure-derived fractional flow reserve (FFR), calculated as the ratio of distal coronary pressure (P d) divided by aortic pressure (P p) during maximal hyperemia (FFR = P d/P p) [9], has been developed to assess the physiological significance of coronary stenosis [10]. The larger the resistance to blood flow, the larger is the decline in pressure and the smaller FFR value. In the absence of resistance along a coronary vessel or by analogy, along a graft conduit, there is no pressure decline and FFR equals one. Previous studies have shown that a FFR value <0.75 discriminates functionally significant coronary lesions susceptible to justify revascularization.
The present study was designed to evaluate more specifically the difference of resistance between both ITA and venous grafts, assuming that venous grafts are the reference conduits that offers the lowest resistance to blood flow. For that purpose, the pressure drop across these different type of grafts was measured in basal conditions and during maximal hyperemia induced by a bolus injection of adenosine. From pressure measurements, the FFR, calculated as the ratio of distal intragraft pressure divided by ostial graft pressure, was determined as an index of the resistance along the grafts.
| 2. Methods |
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In each patient, one or several grafts were selected for the study based on the characteristics of the graft and on the selectivity of the ostial cannulation by the angiographic catheter in order to allow a safe progression of the pressure wire within the graft. Criteria for inclusion were as follow: (a) the absence of major tortuosities of the graft, (b) perfect patency of all anastomoses with a good run off flow to the grafted arterial segments and (c) absence of severe wall motion abnormality in the reperfused area of myocardium.
The study protocol was approved by the Ethics Committee of our institution. All patients gave informed consent at the time of bypass surgery and before the angiographic investigations.
2.2 Study protocol
Patients underwent cardiac catheterization by a standard femoral approach 24 h after discontinuation of all vasoactive medications. Selective catheterization of the ostium of both ITAs or SV was achieved by using 5F guiding catheters without side holes. This catheter was connected to a fluid-filled pressure transducer zeroed at mid-chest level. After intragraft administration of 1 mg of isosorbide dinitrate, a biplane angiogram of the graft was obtained. A 0.014-in. electronic sensor tipped wire (PressureWire 5, Radi Medical Systems, Uppsala, Sweden) was advanced to the tip of the guiding catheter to ensure that the pressures recorded in the ostium of the graft through the fluid-filled catheter and the pressure wire were identical. The wire was then advanced in the distal part of the graft, the pressure sensor being carefully positioned within the last centimeter before the first distal anastomosis on the coronary vessel. The pressures were simultaneously measured at baseline, in the ostium of the graft through the fluid-filled catheter and in the distal part of the graft using the electronic pressure sensor. Coronary arteriolar vasodilation was then induced by a bolus of 40 µg of adenosine inside the graft to produce a transient maximal hyperemia. Pressure signals were recorded continuously until distal pressure returned to baseline values. An example of pressure recording is illustrated in Fig. 1
. When the pressure sensor was pulled back in the guiding catheter, pressures were checked again to exclude any drift of the transducers.
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FFR was considered in relation with the following variables: type of the graft (SV or right ITA or left ITA), revascularized coronary vascular bed, number of distal anastomoses per graft, quality of the run off, number of major coronary branches opacified per graft, severity of the stenosis of the grafted vessel, luminal diameter of the graft, of the coronary artery immediately distal to the graft and minimal luminal diameter of the anastomosis (Table 2 ).
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The SPSS software (release 11.5) was used in the statistical analysis.
| 3. Results |
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In basal conditions, a small pressure gradient between the ostium and the distal part of the graft was recorded in four SV (28%), in six right ITA (60%) and 10 left ITA (83%) (Fig. 2 ). The pressure gradient through the grafts averaged 2.9 ± 2.2 mmHg for left ITA, 1.2 ± 1.2 mmHg for right ITA (p < 0.05 vs left ITA) and 0.4 ± 0.7 mmHg (p < 0.005 vs left ITA; NS vs right ITA) for SV. Adenosine injection resulted in a consistent increase in pressure drop, the maximal pressure gradient during maximal adenosine-induced hyperemia being higher for left ITA (9.6 ± 3.2 mmHg) than for both right ITA (4.5 ± 2.0 mmHg; p < 0.001 vs left ITA) and SV (3.3 ± 2.7 mmHg; p < 0.001 vs left ITA and NS vs right ITA) (Fig. 3 ).
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In univariate analyses, maximal pressure gradient was significantly correlated with the variables: left ITA graft, LAD territory, LCX territory with a positive relationship and with the variables: run off and SV graft with a negative relationship (Table 3 ). Similarly FFR was significantly correlated with the variables: left ITA graft, LAD territory, LCX territory with a negative relationship and with the variables: run off and SV graft with a positive relationship (Table 3).
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| 4. Discussion |
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In the present study, we evaluated the resistances of different graft configurations by measuring the pressure drop during maximal hyperemia and the pressure-derived FFR. In coronary arteries without focal stenosis at angiography, De Bruyne et al. [9] reported FFR values of 0.97 in patients without atherosclerosis and 0.89 in patients with a remote focal angiographic stenosis. We recently reported a FFR of 0.91 in arterial Y-grafts revascularizing the whole left coronary system in a 6-months follow-up study after bypass surgery [11]. The values of FFR observed in left ITA in the present study are close to those previously reported in these Y-graft configurations. As it was the case in our previous study, FFR was superior to 0.75, the cutoff for inducible ischemia in all individual cases.
The main message of the present study is that the resistance of the left ITA appears significantly higher than that of the right ITA or of the SV. This is reflected by a smaller FFR and by a higher drop in perfusion pressure through the left ITA than through the right ITA or a SV graft during maximal hyperemia. Several factors could contribute to this finding. First, the territory of the LAD on which the left ITA is preferentially implanted is likely to require a larger blood flow supply than that of the other territories for which SV or right ITA are more often used. Second, differences in the three-dimensional configuration of various types of grafts could affect the transmission of the pressure wave; curvatures of the proximal left ITA could result in some damping of the pressure waveform that could explain a resistance to phasic pulsatile flow superior to that of the right ITA whose configuration is more rectilinear because of its anatomic course into the tranverse sinus. Third, differences in length and lumen diameter of graft conduits are parameters directly affecting their resistance to flow.
The relative contribution of these factors cannot be directly extrapolated from our data since clinical practice has associated specific graft conduits with specific territories with different anatomic and physiologic characteristics. In our institution, SV grafts and right ITA are preferentially used for distal RCA and LCX territories respectively. Conversely, given the well demonstrated superiority of the left ITA over SV grafts in long-term patency and its impact on survival [13], the left ITA is the conduit of choice for the left anterior descending territory. This preferential association of one type of graft with one coronary vascular bed precludes any definitive conclusion about the influence of the conduit or the reperfused territory. Although left ITA was the variable most strongly associated with pressure drop during hyperemia and a lower FFR, the higher resistance of left ITA demonstrated in the present study must be interpreted only considering its preferential use for the LAD territory. However, although the preferential use of either ITA or SV grafts to revascularize different territories may be viewed as a theoretical limitation of our study, only the type of graft left ITA versus other grafts significantly predicted FFR and pressure drop when all parameters were considered in the multivariate analysis.
Since all individual values of FFR were higher than 0.75, the resistance of the left ITA graft appears low enough to allow an adequate reperfusion of the LAD territory in all cases. The small differences in resistance between grafts are thus unlikely to have clinical consequences on the capacity of these grafts to prevent ischemia. They could however reflect differences in the adaptative mechanisms to chronic changes in blood flow. The pressure drop across a vessel is the direct consequence of the interaction between the blood flow and the vessel wall. Frictional forces on the endothelial surface created by flowing blood contribute to dissipate the mechanical driving force; they also stimulate biological adaptative mechanism controlling vasomotion and vessel remodeling. Nitric oxide and prostacyclin are released from endothelial cells in response to shear stress; these vasodilator agents contribute to flow-mediated vasodilation but are also known to inhibit platelet adhesion and aggregation which are key factors in atherothrombosis. In addition, chronic flow increase induce a remodeling of the vessel resulting in an anatomical enlargement of the lumen. Both flow-mediated vasodilation and chronic vessel remodeling are dependent on the integrity of the endothelium and contribute to normalize longitudinal shear stress [1215].
When compared to other types of grafts, the higher-pressure drop through the left ITA reflects larger interactions between flow and vessel wall and larger frictional forces likely to induce a more effective release of endothelial factors. A more effective stimulation of these endothelial mechanisms in left ITA graft, as a combined result of superior endothelial cell function and favorable hemodynamic conditions, could contribute to the better long-term results of this conduit over the right ITA in coronary bypass surgery.
| Acknowledgments |
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| References |
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