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Ann Thorac Surg 2007;83:2093-2097
© 2007 The Society of Thoracic Surgeons

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Original Articles: Cardiovascular

Does Stenosis Severity of Native Vessels Influence Bypass Graft Patency? A Prospective Fractional Flow Reserve–Guided Study

^a Department of Cardiology, Catharina Hospital, Eindhoven, the Netherlands
^b Department of Cardiothoracic Surgery, Catharina Hospital, Eindhoven, the Netherlands
^c Department of Cardiology, CHUV, Lousanne, Switzerland
^d Department of Cardiology, Arizona Heart Institute, Phoenix, Arizona

Accepted for publication January 16, 2007.

^_* Address correspondence to Dr Botman, Department of Cardiology, Catharina Hospital, PO Box 1150, Eindhoven, 5602 ZA, the Netherlands (Email: bot.joost{at}wxs.nl).

	Abstract

Top Abstract Introduction Patients and Methods Results Comment References

 
Background: After coronary bypass surgery, occlusion or narrowing of bypass grafts may occur over time. The present study prospectively evaluated the angiographic patency of bypass grafts after 1 year in relation to the preoperative angiographic and functionally severity of the coronary lesion assessed by fractional flow reserve measurement to test the hypothesis that grafting of less critical stenosis may be a risk factor for early dysfunction of the graft.

Methods: The study comprised 164 patients eligible for coronary artery bypass surgery who were not suitable for percutaneous intervention and with at least one intermediate lesion. Fractional flow reserve was measured in all lesions to be grafted to establish if a lesion was functionally significant. The surgeon was blinded to the results of these measurements. One year after surgery, coronary angiography was performed to establish bypass graft patency.

Results: At coronary angiography after 1 year, 8.9% of the bypass grafts on functionally significant lesions were occluded, and 21.4% of the bypass grafts on functionally nonsignificant lesions were occluded. There was no difference in angina class or repeat interventions between patients with or without occluded bypass grafts.

Conclusions: The patency of bypass grafts on functionally significant lesions is significantly higher than the patency of bypass grafts on nonsignificant lesions; however, this finding has no clinical relevance because patients with patent or occluded bypass grafts on nonsignificant lesions did not experience an excess of angina or repeat interventions.

	Introduction

Top Abstract Introduction Patients and Methods Results Comment References

 
Coronary artery bypass grafting (CABG) is a well-established treatment for patients with obstructive coronary artery disease; however, occlusion or narrowing of these bypass grafts may occur over time. During the first year after intervention, thrombosis or fibrointimal proliferation of the graft predominate, and later on, atherosclerosis appears [1]. Little is known about factors that may contribute to these adverse events, although antiplatelet therapy and correction of cardiovascular risk factors have been shown of value. It has been suggested that grafting of a less critical stenosis, which may imply lower flow rates through the bypass graft, may be a risk factor of early dysfunction of the graft [2].

In normal clinical practice, coronary arteries eligible for surgical revascularization are selected subjectively by visual estimation of stenosis severity, morphologic appearance of the lesion that may be judged as unfavorable for percutaneous coronary intervention (PCI), and the physician’s personal preference.

Fractional flow reserve (FFR) calculated from coronary pressure measurements permits reliable assessment of the functional severity of a stenosis in a coronary artery. FFR of the coronary artery equals the ratio Pd/Pa at maximal hyperemia where Pa is the mean aortic pressure, measured by the guiding catheter, and Pd is the distal coronary pressure, measured by a pressure wire. FFR is the gold standard for physiologic stenosis severity, and a value below 0.75 indicates a functionally significant stenosis [3].

The purpose of the present study was to evaluate prospectively the angiographic patency of bypass grafts after 1 year in relation to the preoperative angiographic and functionally severity of the coronary lesion assessed by FFR measurement in patients who underwent CABG.

	Patients and Methods

Top Abstract Introduction Patients and Methods Results Comment References

 
Between September 2003 and September 2004, 164 patients who were eligible for CABG were included in the present study. The patients were selected for the surgical procedure on the basis of coronary artery disease not suitable for PCI and at least one intermediate lesion with a stenosis severity of 50% to 70% on visual estimation. Before bypass surgery, FFR was measured in all vessels intended to be grafted, and the surgeon was blinded to these measurements. These patients were monitored clinically for 1year, and at the end of that period, graft patency was evaluated by coronary angiography. A flow chart of the study design is presented in Figure 1.

View larger version (9K): [in this window] [in a new window]   Fig 1. A flow chart shows the study design. (CABG = coronary artery bypass grafting; FFR = fractional flow reserve.)

Coronary Angiography and Fractional Flow Reserve Measurements
After administration of 5000 U of heparin, a left or right coronary guiding catheter was advanced in the left or right coronary ostium, 200 µg of nitroglycerin was administered intracoronarily, and angiograms were performed of the left and right coronary arteries in at least two orthogonal views. A 0.014-inch sensor-tipped pressure guidewire (Pressure Wire, Radi Medical Systems, Uppsala, Sweden) was advanced to the tip of the guiding catheter. After equal pressures were confirmed at that location, the wire was advanced into a coronary artery. Intravenous adenosine at 140 µg/(kg · m) was administered through the femoral vein to induce maximum coronary hyperemia.

FFR was calculated by the ratio Pd/Pa at steady-state maximum hyperemia, where Pd is the mean coronary pressure distal in the coronary artery (recorded by the pressure wire) and Pa is the mean aortic pressure (recorded by the guiding catheter), as described before [4]. All measurements were performed twice. After the second measurement, a pullback curve was performed at sustained hyperemia for precise localization of a pressure gradient and precise determination of the functional severity of a localized stenosis. At present, FFR is considered the gold standard for physiologic assessment of the coronary artery [3].

Surgical Technique
The surgical technique used was decided by the surgeon’s personal preference. The internal mammary artery (IMA), the radial artery, and the saphenous vein were used as bypass grafts. The surgeon was blinded to the results of the FFR measurements, and those vessels were grafted as judged relevant by the surgeon and the cardiologist from the angiographic assessment when the patient was accepted for CABG. In this way, all patients received optimal clinical treatment according to the present standard for CABG as determined by the angiographic stenosis assessment. The five classifications of stenosis severity were total occlusion (100% stenosis), subtotal (>90% stenosis), significant (70% to 90% stenosis), intermediate (50% to 70% stenosis), and nonsignificant (<50% stenosis).

Follow-Up
All patients were followed up at the outpatient clinic at 3 months, 6 months, and 1 year. After 1 year, coronary angiography was performed.

Data Analysis
All FFR measurements were stored digitally and analyzed off-line. FFR is expressed by a number between 0 and 1, representing the achieved maximum blood flow as a fraction of the normal blood flow if no coronary artery disease were present at all. All data are reported as mean ± standard deviation. Differences between proportional (discrete or categoric) data were tested by Fisher’s exact test and analysis of variance. A value of p < 0.05 was considered significant.

	Results

Top Abstract Introduction Patients and Methods Results Comment References

 
Procedural Results
The study included 164 patients (128 men, 36 women) with a mean age of 62 years (range, 33 to 84 years). Two patients who were scheduled for CABG refused the operation, and 1 patient died the day before operation. Another patient died postoperatively owing to left ventricular failure. The baseline characteristics of these patients are summarized in Table 1.

Graft Patency in Relation to Angiographic and Functional Stenosis Severity
FFR measurements were recorded for all 164 of the study patients. Repeat angiography was performed in 153 patients after 1 year of follow-up to establish patency of the bypass grafts. Two patients refused operation, 2 patients died, and 7 patients refused repeat angiography. In these 153 patients, FFR was used to analyze 525 lesions and all these lesions were grafted. With FFR measurements, 357 lesions were considered as significant (FFR < 0.75) and 168 as nonsignificant (FFR > 0.75).

At catheterization after 1 year of follow-up, 8.9% of the bypass grafts on functionally significant lesions were occluded, and 21.4% of the bypass grafts on functionally nonsignificant lesions were occluded. There was no difference in angina class between patients with or without occluded bypass grafts after 1year of follow-up.

In functionally significant lesions, 13.7% of the arterial and 5.9% of the venous conduits were occluded after 1 year, and in functionally nonsignificant lesions, respectively, 21.9% and 20.0% were occluded. The patency of the left IMA (LIMA) grafts was 93.8%, and the patency of the radial conduits was 71%.

In the group of visually intermediate lesions, graft occlusion occurred in 9.8% of the functional significant lesions and in 20.2% of the functionally nonsignificant lesions. When the vessel diameter was considered, the graft patency was 79% with a vessel diameter of less than 2.0 mm, and 96.1% with a vessel diameter exceeding 2.0 mm. The results are presented in Table 2.

View larger version (14K): [in this window] [in a new window]   Fig 2. The relation between angiographic stenosis severity and graft failure after angiographic follow-up at 1 year.

View larger version (15K): [in this window] [in a new window]   Fig 3. The relation between functional stenosis severity established by fractional flow reserve (FFR) measurements and graft failure at angiographic follow-up after 1 year.

	Comment

Top Abstract Introduction Patients and Methods Results Comment References

The data known about bypass graft dysfunction after CABG are mostly retrospective [5]. IMA patency would decrease as coronary competitive flow increases owing to a less severe stenosis rate. This was concluded retrospectively, however, and only a part of the study population underwent repeated coronary angiography [6, 7]. Competitive flow also has a detrimental effect on the patency of IMA-anastomosed radial artery grafts, and this technique should be used only for a target vessel with subocclusive stenosis [8].

Vessel diameter also has an important impact on graft patency: with smaller vessel diameters, lower graft patency was shown, and also retrospectively, in saphenous vein grafts [9]. This phenomenon has also been described for arterial conduits: patency was 65% with stenosis of less than 60% and 90.9% with stenosis exceeding 60%; however, angiography was only performed if a patient was symptomatic [10]. Several studies report that saphenous vein grafts have a higher occlusion rate than arterial conduits, but the superiority of arterial conduits was documented in a negatively selected symptomatic population [10–13].

Other factors have been evaluated in relation to graft patency. The patency rates of sequential saphenous vein grafts are generally superior than individual saphenous vein grafts (86.6% versus 69.6%), especially for poor runoff coronary vessels, provided that the most distal located anastomosis is performed on a coronary vessel with a good diameter [14]. Diabetes mellitus did not made any difference in graft patency, as was concluded in the Bypass Angioplasty Revascularization Investigation (BARI) trial; however, angiography was not regularly performed but was symptom-guided [15].

The outcome of graft patency after off-pump surgery is not yet conclusive. A lower patency rate has been reported, but also equal patency rates compared with conventional on-pump surgery [16–18]. Harvesting techniques of conduits may also influence the outcome because of mechanical damage inflicted during harvesting [19].

We conducted our prospective study to show the relation between functional stenosis severity and angiographic assessment of graft occlusion after 1 year of follow-up.

In this cohort, 525 lesions in 153 patients were analyzed preoperatively with FFR, and after 1 year, the patency of the bypass graft was evaluated by coronary angiography. The primary end point of the present study (graft patency in relation to functional stenosis severity) showed a patency rate of 91.1% of grafts on functionally significant lesions and 78.6% of grafts on nonsignificant lesions (p < 0.0001). Although the finding of a significant difference between patency rates of bypass grafts on functionally significant and functionally nonsignificant lesions was to be expected, the patency rate of bypass grafts on nonsignificant lesions is higher than mentioned in the literature [10]. An explanation for this finding might be the negative selection of patients in whom angiography was done guided by coronary incidents, or angina, or both [10].

Also consistent with what was to be expected was that a smaller diameter of the coronary artery gave a significant lower patency rate of 79% in vessels with a diameter of less than 2.0 mm, and 96.1% on vessels with a diameter exceeding 2.0 mm (p < 0.0001).

Bypass grafts on functionally significant lesions showed a patency rate of 86.3% of arterial conduits and a patency rate of 94.1% of saphenous vein grafts (p = 0.02). This finding seems surprising, but the usual smaller diameter and bigger length of arterial conduits than that of venous conduits may lead to an initial higher resistance and lower flow through these conduits, accompanied by a lower patency rate after 1 year of these arterial conduits (our own experimental, nonpublished data). The disappointing results of arterial revascularization are attributed to the low patency rates of the RA conduits (71%) The patency rate of the LIMA conduits was 93.8%, which is concordant with the literature [20, 21].

Bypass grafts on functionally nonsignificant lesions showed no difference in patency rate between arterial and saphenous vein conduits after 1 year, respectively 78.1% and 80% (p = 1.000). These are higher patency rates of bypass grafts on functionally nonsignificant lesions than reported in previous studies, these studies were not prospective, and the performed follow-up catheterizations were symptom-guided.

A common controversy between cardiothoracic surgeons and interventional cardiologists is how to treat intermediate lesions with a stenosis severity of 50% and 70%. It has already been shown that the FFR measurement is the tool of choice to separate this group into functionally significant or nonsignificant stenosis so that patients with multivessel disease with one or two intermediate lesions can be divided into two groups, one with functional three-vessel disease who underwent CABG and one with functional one-vessel or two-vessel disease who underwent PCI. After 2-year follow-up, the groups had similar outcomes [22]. Of the 252 intermediate lesions analyzed, and 133 were functionally significant and 119 were functionally nonsignificant. The patency rates were, respectively, 91.2% and 79.8%. This difference is significant, but there was no difference in angina class between patients with occluded or patent grafts. Occlusion of a bypass graft on a functionally significant stenosis occurred in the 8 patients who underwent repeat intervention.

With increasing visual stenosis severity the graft patency rate increases, the same phenomenon is seen with assessment of functional stenosis severity. As described here, this finding has no clinical relevance.

The size of the cohort in relation to the number of occluded bypass grafts, which is an outcome variable, and the follow-up of one year are recognized as limitations of this study. It may also not be safe to extrapolate our findings to a longer period of follow-up.

In this prospective blinded study patency of bypass grafts on functional significant lesions is significantly higher than the patency rate of bypass grafts on non-significant lesions, however this finding has no clinical relevance as patients with patent or occluded bypass graft on non-significant lesions did not suffer from a excess of angina or coronary repeat interventions.

	References

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