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DOI: 10.1148/radiol.2283020305
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(Radiology 2003;228:834-841.)
© RSNA, 2003

Cardiac Imaging

Vein Graft Function Improvement after Percutaneous Intervention: Evaluation with MR Flow Mapping1

Susan E. Langerak, MD, Hubert W. Vliegen, MD, J. Wouter Jukema, MD, Aeilko H. Zwinderman, PhD, Hildo J. Lamb, PhD, Albert de Roos, MD and Ernst E. van der Wall, MD

1 From the Departments of Cardiology (S.E.L., H.W.V., J.W.J., E.E.v.d.W.), Radiology (S.E.L., H.J.L., A.d.R.), and Medical Statistics (A.H.Z.), Leiden University Medical Center, Albinusdreef 2, C5-P, 2300 RC Leiden, the Netherlands; and the Interuniversity Cardiology Institute of the Netherlands, Utrecht (S.E.L., A.d.R., E.E.v.d.W.). Received March 29, 2002; revision requested June 11; final revision received December 30; accepted January 2, 2003. S.E.L. supported by grant 97.173 from the Netherlands Heart Foundation. Address correspondence to E.E.v.d.W. (e-mail: e.e.van_der_wall@lumc.nl).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
RESULTS
DISCUSSION
 REFERENCES
 
PURPOSE: To provide functional reference values in single and sequential vein grafts by using magnetic resonance (MR) flow mapping and to examine the effect of percutaneous intervention (PCI) on coronary artery bypass graft function.

MATERIALS AND METHODS: Fast MR flow mapping at baseline and during adenosine-induced stress was performed in 39 nonstenotic single vein grafts and 20 nonstenotic sequential vein grafts, as well as in 15 stenotic vein grafts before and 7.3 weeks ± 1.5 after successful PCI. We evaluated the following parameters (in terms of mean values ± SDs): average peak velocity (APV) at baseline, stress APV, and velocity reserve. Parameters in nonstenotic single and sequential vein grafts were compared by means of unpaired two-tailed Student t testing. To evaluate changes in velocities before and after PCI, a paired two-tailed Student t test was used. P < .05 was considered to indicate a statistically significant difference.

RESULTS: Reference values in single vein grafts for baseline APV, stress APV, and velocity reserve were 8.6 cm/sec ± 3.4, 20.2 cm/sec ± 9.5, and 2.4 ± 0.8, respectively. In sequential vein grafts, significantly higher values for baseline APV (12.2 cm/sec ± 5.0) and stress APV (27.2 cm/sec ± 10.6) but a similar velocity reserve (2.3 ± 0.7) were found. Significant improvements were observed after PCI in baseline APV (before PCI: 9.2 cm/sec ± 6.6; after PCI: 12.9 cm/sec ± 7.9; P = .008) and stress APV (before PCI: 12.9 cm/sec ± 6.3; after PCI: 27.1 cm/sec ± 13.9; P < .001). No improvement in velocity reserve was observed.

CONCLUSION: Significantly higher absolute velocity and flow values were observed in sequential versus single vein grafts, underscoring the need for separate functional reference values for different graft types. Graft function showed significant improvement after PCI to the point that it was restored or nearly restored to reference values.

© RSNA, 2003

Index terms: Coronary vessels, bypass graft, 54.4551 • Coronary vessels, flow dynamics, 54.12144 • Coronary vessels, MR, 54.12144 • Coronary vessels, stenosis or obstruction, 54.76 • Interventional procedures, 54.1269


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
 REFERENCES
 
Magnetic resonance (MR) flow mapping enables noninvasive and accurate quantification of flow and flow reserve in coronary arteries (1–5) and in nonstenotic and stenotic coronary artery bypass grafts (6–9). MR flow mapping is a potential noninvasive diagnostic tool for evaluation of grafts and recipient vessels. Specific MR imaging reference values for velocity and flow in different graft types (eg, single and sequential vein grafts) are essential for accurate interpretation of the results of such MR flow mapping examinations. Most functional values have been obtained in coronary arteries. These values cannot be translated to grafts because reduced values of flow reserve have been observed in grafts as compared with coronary arteries (10–12); this can be explained as a manifestation of microvascular endothelial dysfunction and/or diffuse atherosclerosis. These conditions are the rule rather than the exception in patients undergoing bypass surgery. Moreover, vessel diameter, endothelial function, and the histologic nature of atheroma (13,14) in vein grafts differ from those in native coronary arteries. Data on MR imaging reference values for single and sequential vein graft function are only available at baseline (15).

Assessment of coronary flow reserve as measured with MR flow mapping has recently been used to identify coronary restenosis after successful percutaneous intervention (PCI) in patients with recurrent chest pain (16). MR flow mapping is a potential noninvasive tool for follow-up study of graft function after PCI. However, the feasibility of using MR flow mapping to detect changes in graft function after PCI is unknown.

The aim of the present study was to provide functional reference values for single and sequential vein grafts by using MR flow mapping and to examine the effect of PCI on graft function.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
DISCUSSION
 REFERENCES
 
Patients
From March 1999 to January 2001, we studied 39 consecutive nonstenotic (ie, <50% luminal diameter stenosis) single vein grafts and their recipient vessels and 20 consecutive nonstenotic sequential vein grafts and their recipient vessels in a reference group of 46 patients (41 men and five women; mean age, 67.1 years ± 9.1 [SD]) who underwent coronary angiography because of recurrent chest pain. In addition, 15 vein grafts (10 single and five sequential grafts) that were scheduled to be treated with PCI were studied in 13 consecutive patients (eight men and five women; mean age, 66.9 years ± 6.2) (PCI group). Data from 27 grafts in 23 patients were described earlier for validation purposes (8).

None of the patients had chronic obstructive pulmonary disease, second- or third-degree atrioventricular block (adenosine-related exclusion criteria) (17), implanted metallic devices, unstable angina, atrial fibrillation, rhythm disturbances, claustrophobia, or inability to lie flat (MR imaging–related exclusion criteria). Patients were not allowed to have caffeine-containing beverages on the day of the MR imaging studies. Informed consent was obtained for the study protocol, which was approved by the medical ethics committee of Leiden University Medical Center.

PCI and Quantitative Coronary Analysis
Coronary angiography and PCI were performed by five experienced interventional cardiologists (including J.W.K.). Vascular access was obtained by using the femoral approach with the Seldinger technique and a 6- or 7-F catheter. After a 0.3-mg bolus injection of nitroglycerin selectively into the graft, the stenotic graft was visualized with standard projections (ie, a right anterior oblique and left anterior oblique projection more than 60° apart). PCI, including angioplasty or angioplasty with stent placement, was performed to achieve maximal luminal diameter gain. The interventional procedure consisted of angioplasty with stent placement in 12 grafts and angioplasty without stent placement in three grafts. The culprit lesion was defined as the most severe flow-limiting stenosis over the graft and runoff area that was accessible for an intervention. For quantification of stenosis severity, off-line computer-assisted quantitative coronary analysis of the culprit lesion was performed before and after PCI by an independent core laboratory (Heart Core, Leiden, the Netherlands); this technique has been extensively validated (18,19).

MR Imaging Measurements
All patients were examined by using a 1.5-T MR imaging unit (Gyroscan ACS-NT15; Philips Medical Systems, Best, the Netherlands) equipped with a Powertrack 6000 gradient system (25 mT/m, 100 mT/m/msec) and a CPR-6 cardiac research software package. MR imaging was performed with patients in the supine position after placement of the following: a respiratory belt for monitoring the breath-hold procedure, three electrocardiographic (ECG) leads for cardiac triggering, an ECG patch at the center of the thorax for patient monitoring, and a five-element cardiac synergy coil.

Transverse overcontiguous sections at the level of the ascending aorta were obtained by using conventional gradient-echo MR imaging to visualize the grafts as they coursed from their origin at the ascending aorta to the location of their distal anastomosis. Fast flow mapping was performed in the proximal or middle part of the grafts by using a turbo-field echo-planar MR imaging sequence with temporal resolution of 23 msec, in-plane spatial resolution of 1.6 mm ± 1.6 reconstructed to 0.8 mm ± 0.8, imaging duration of 20 heartbeats, and velocity encoding of 75 cm/sec (7,8). To avoid image blurring due to respiratory motion, all flow imaging was performed during an end-expiratory breath hold of 20 cardiac cycles. After baseline flow mapping was completed, intravenous infusion of 140 µg/kg/min of adenosine was initiated, and, 2 minutes later, when maximal hyperemia was achieved, flow measurement was repeated. Side effects in the group of patients who were to undergo PCI (PCI group) were reported. ECG and blood pressure monitoring was performed with a Millennia 3500 device (Invivo Research, Orlando, Fla). The MR imaging examination was performed again in the patients in the PCI group after intervention (mean time after PCI, 7.3 weeks ± 1.5; range, 5–10 weeks).

MR Image Analysis
Flow images were analyzed at a workstation (Sun Microsystems, Mountain View, Calif) by using an analytic software package (FLOW; Medis, Leiden, the Netherlands). To determine peak velocity values, one author (S.E.L.) placed a region of interest of 2 x 2 pixels in the center of the lumen area (4). The peak velocity (in centimeters per second) was calculated as the average value in these four pixels, and peak velocity–versus-time curves were constructed. The average peak velocity (APV, in centimeters per second) was calculated as the average value of the peak velocity over time. The systolic and diastolic peak velocities (SPV and DPV, respectively; both in centimeters per second) were defined as the maximal peak velocity during systole and diastole, respectively. The diastolic-to-systolic velocity ratio (DSVR) was calculated as the ratio between the DPV and SPV values. The velocity reserve was defined as the ratio between the stress APV and baseline APV values.

To determine flow values, one author (S.E.L.) manually traced graft lumen area, and the position and size of each contour was adjusted according to the heart phase (7). For each heart phase the flow rate (in milliliters per second) was calculated by multiplying the average velocity across the lumen area by the lumen area. Subsequently, flow rate–versus-time curves were reconstructed, and total flow (in milliliters per minute) was calculated as the integrated volumetric flow per minute. Systolic and diastolic peak flow rates (SPF and DPF, respectively; both in milliliters per second) were defined as the maximal flow rates during systole and diastole, respectively. The ratio between the DPF and SPF was termed the diastolic-to-systolic flow ratio (DSFR). Flow reserve was defined as the ratio of total flow during adenosine infusion tototal flow at baseline. These velocity and flow measurements have been validated extensively (7,8).

Statistical Analysis
All data are expressed as means ± SDs. Functional parameters in nonstenotic single vein grafts were compared with functional parameters in nonstenotic sequential vein grafts by means of unpaired two-tailed Student t testing. Results in nonstenotic single vein grafts were stratified into one of two groups according to the velocity reserve of the graft (whether >2.0 or <=2.0) (20). Values in the group with normal velocity reserve were compared with those in the group with low velocity reserve by means of unpaired two-tailed Student t testing.

A paired two-tailed Student t test was used to evaluate changes in mean luminal stenosis of the culprit lesion, MR velocity and flow values, heart rates, and systolic and diastolic blood pressures before and after PCI. Results in nonstenotic single vein grafts after PCI were also stratified into one of two groups according to the velocity reserve of the graft (whether >2.0 or <=2.0). Values in the group with normal velocity reserve were compared with those in the group with low velocity reserve by means of unpaired two-tailed Student t testing. For comparison of velocity and flow parameters in stenotic single vein grafts before and after PCI with reference values in nonstenotic single vein grafts, an unpaired two-tailed Student t test was used. P < .05 was considered to indicate a statistically significant difference.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
DISCUSSION
 REFERENCES
 
Reference Values for Peak Velocity and Flow in Single and Sequential Vein Grafts
Figure 1 shows results of an MR imaging examination of grafts. Table 1 provides an overview of the peak velocity and flow reference values in nonstenotic single and nonstenotic sequential vein grafts. The mean baseline APV (P = .007), stress APV (P = .02), baseline flow (P = .01), and stress flow (P = .01) were significantly higher in nonstenotic sequential vein grafts than in nonstenotic single vein grafts. Similar values for velocity reserve (2.3 ± 0.7 in sequential vein grafts vs 2.4 ± 0.8 in single vein grafts, P = .76) and flow reserve (2.5 ± 0.9 in sequential vein grafts vs 2.9 ± 2.5 in single vein grafts, P = .47) were observed in the two kinds of graft.



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  		Figure 1. A, Conventional coronary angiogram (right anterior oblique view) of a sequential vein graft to the left circumflex coronary artery in a 61-year-old man shows a 74% origin stenosis (arrow) in the graft. Cath = catheter, sw = sternal wire, vc = vascular clip. B, Multiplanar reconstruction of navigator-gated three-dimensional MR angiographic data (repetition time msec/echo time msec, 7.1/1.9; {alpha} pulses per shot, 10; field of view, 360 mm; rectangular field of view, 75%; matrix, 512 x 512; scan percentage, 70%; flip angle, 30°; section thickness, 3 mm reconstructed to 1.5 mm; imaging time, 15-20 minutes) confirms the origin stenosis (arrow). Ao = ascending aorta, LV = left ventricle, PA = pulmonary artery. C, D, Oblique sagittal flow MR images (11.0/4.6; field of view, 200 x 100 mm; matrix, 128 x 60; flip angle, 20°; temporal resolution, 23 msec; velocity encoding, 75 cm/sec; imaging time, 20 heartbeats) obtained, C, at baseline and, D, during stress. Images on the left are modulus images with anatomic information; images on the right are phase images with velocity information. A detail of the graft cross section is depicted in the upper left corner of each image. Arrows = cross section of the graft depicted in A and B. In addition and more anterior to a cross section of the stenotic vein graft, the cross section of a left internal mammary artery graft to the left anterior descending coronary artery can be seen.

 

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  		TABLE 1. Reference Values for Velocity and Flow Parameters in Nonstenotic Single and Sequential Vein Grafts

 
Stratification of the nonstenotic single vein grafts according to velocity reserve values resulted in 22 grafts with a velocity reserve greater than 2.0 (mean velocity reserve, 2.8 ± 0.6) and 13 with a velocity reserve of 2.0 or lower (mean velocity reserve, 1.5 ± 0.3) (stress measurements were available for 35 single vein grafts). There were no significant angiographic differences between the two velocity reserve groups. The reason for the impaired velocity reserve in the 13 nonstenotic single vein grafts was a reduced mean stress APV (14.7 cm/sec ± 6.0) as compared with the mean stress APV in the 22 grafts with a normal velocity reserve (23.4 cm/sec ± 9.8, P = .007). Baseline APV values were similar between nonstenotic single vein grafts with impaired velocity reserve (9.5 cm/sec ± 3.2) and those with normal velocity reserve (8.3 cm/sec ± 3.7, P = .34).

Stenotic Grafts Treated with PCI
Quantitative coronary analysis of the culprit lesion showed a mean luminal stenosis of 67.5% ± 14.9 (range, 45.0%–100.0%) before PCI. There were no grafts with multiple or long significant stenoses. Two grafts—one single vein graft and one sequential vein graft—were totally occluded before PCI. After successful PCI, the mean luminal stenosis of the culprit lesion was reduced to 13.2% ± 12.2 (range, 0.0%–33.5%; P < .001).

Graft Function before and after PCI
Figure 2 shows a typical example of MR imaging results before and after PCI. Because two grafts in two different patients were totally occluded, these grafts were not visualized with gradient-echo MR imaging, and, therefore, no flow mapping was performed before PCI in these grafts. After PCI, successful MR flow mapping was performed in these previously occluded grafts.



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  		Figure 2. Images obtained in the same patient as in Figure 1. A, Conventional coronary angiogram (right anterior oblique view) obtained during PCI shows the inflated balloon (b) and stent. Cath = catheter. B, Conventional coronary angiogram (right anterior oblique view) obtained immediately after PCI shows that the graft lumen has normalized. Cath = catheter. C1, C2, C3, Transverse MR angiographic sections (7.1/1.9; {alpha} pulses per shot, 10; field of view, 360 mm; rectangular field of view, 75%; matrix, 512 x 512; scan percentage, 70%; flip angle, 30°; section thickness, 3 mm reconstructed to 1.5 mm; imaging time, 15-20 minutes) obtained after PCI show the course of the vein graft to the left circumflex coronary artery (arrowhead) and the left internal mammary artery graft (arrow). Ao = ascending aorta, PA = pulmonary artery, sw = artifact from sternal wire, * = artifact from stent. D, E, Oblique sagittal flow MR images (11.0/4.6; field of view, 200 x 100 mm; matrix, 128 x 60; flip angle, 20°; temporal resolution, 23 msec; velocity encoding, 75 cm/sec; imaging time, 20 heartbeats) obtained after PCI, D, at baseline and, E, during stress show increased signal intensity reflecting improved flow. Images on the left are modulus images with anatomic information; images on the right are phase images with velocity information. A detail of the graft cross section is depicted in the upper left corner of each image. Arrowheads = cross section of the graft depicted in A and B. In addition and more anterior to a cross section of the stenotic vein graft, the cross section of a left internal mammary artery graft to the left anterior descending coronary artery can be seen.

 
PCI resulted in significant increases in both baseline velocity and flow values and hyperemic velocity and flow values (Table 2). No increase in graft velocity reserve or flow reserve was observed after PCI. Figure 3 depicts an example of the typical improvement in peak velocity–versus–time curves after PCI at baseline and during stress. Figure 4 shows individual graft changes in baseline APV and stress APV after PCI.


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  		TABLE 2. Peak Velocity and Flow Values in Vein Grafts before and after PCI

 


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  		Figure 3. Graph shows four peak velocity-versus-time curves for the same vein graft as in Figures 1 and 2. The thin line with black circles ({bullet}) represents the baseline measurement before PCI; the thick line with black circles represents the stress measurement before PCI. The thin line with open circles ({circ}) represents the baseline measurement after PCI; the thick line with open circles represents the stress measurement after PCI.

 


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  		Figure 4. Graph depicts changes in baseline APV (APVbase) and stress APV (APVstress) after PCI in each individual graft. The dots above number 1 represent data before PCI; the dots above number 2, data after PCI. MR flow mapping enables the detection of improved graft and/or recipient coronary artery function after PCI.

 
Stratification of post-PCI data in single vein grafts according to velocity reserve (whether >2.0 or <=2.0) yielded six single vein grafts with a velocity reserve greater than 2.0 (mean velocity reserve, 3.1 ± 0.5) and three grafts with a velocity reserve of 2.0 or lower (mean velocity reserve, 1.6 ± 0.4). In the group of grafts with a normal velocity reserve after PCI, almost supernormal values for baseline APV (10.2 cm/sec ± 3.4 vs 8.3 cm/sec ± 3.7) and stress APV (30.9 cm/sec ± 10.0 vs 23.4 cm/sec ± 9.8) were observed when these grafts were compared with nonstenotic single vein grafts that had a velocity reserve greater than 2.0. As in the nonstenotic single vein grafts, the impaired velocity reserve in the post-PCI graft group was the result of a nearly significant reduction in stress APV (mean stress APV after PCI in the grafts with velocity reserve of 2.0 or lower: 15.8 cm/sec ± 7.7; mean stress APV after PCI in the grafts with velocity reserve greater than 2.0: 30.9 cm/sec ± 10.0; P = .06).

Normalization of Graft Peak Velocity and Flow after PCI
Stenotic single vein grafts had significantly lower values during adenosine-induced hyperemia compared with reference values in nonstenotic single vein grafts for mean APV (12.5 cm/sec ± 5.9 vs 20.2 cm/sec ± 9.5, P = .03), mean DPV (20.6 cm/sec ± 9.8 vs 35.0 cm/sec ± 14.5, P = .008), mean flow (55.9 mL/min ± 30.4 vs 89.5 mL/min ± 47.9, P = .05), and mean DPF (2.1 mL/sec ± 1.3 vs 3.5 mL/sec ± 1.7, P = .03). In single vein grafts after PCI, no significant difference was observed in any velocity or flow parameter as compared with the parameters in nonstenotic single vein grafts (Fig 5).



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  		Figure 5. Graphs reflect changes in mean baseline APV (APVbase), mean stress APV (APVstress), and mean velocity reserve (CFVR) in stenotic single vein grafts after PCI in comparison with pre-PCI values in these grafts. For comparison, reference values for nonstenotic single vein grafts are shown.

 
Adenosine Side Effects in PCI Group
Ten of 12 patients (one patient had only an occluded graft) in whom MR flow mapping during adenosine-induced stress was performed before PCI reported mild transient side effects during adenosine infusion. These side effects included chest pain (seven patients), dyspnea (six patients), flush (seven patients), and headache (three patients). Side effects of adenosine infusion at MR imaging performed after PCI were reported in 11 of 13 patients and included chest pain (four patients), dyspnea (five patients), flush (nine patients), headache (one patient), and dizziness (one patient). In one patient before PCI and two patients after PCI, stress flow imaging was not completed owing to the side effects. The side effects disappeared in all patients within 2 minutes after discontinuation of adenosine infusion.

Hemodynamic Response to Adenosine in PCI Group
Heart rate during MR flow mapping before PCI was 66.3 beats per minute ± 12.4 and increased to 80.8 beats per minute ± 10.9 (P < .001) during adenosine-induced stress. A similar increase in heart rate during stress was demonstrated after PCI (baseline, 64.5 beats per minute ± 9.7; stress, 79.2 beats per minute ± 9.7), and identical heart rates were observed after PCI as compared with pre-PCI values.

No noteworthy change in systolic blood pressure was observed during adenosine-induced stress both before (baseline, 125.5 mm Hg ± 12.5; stress, 123.2 mm Hg ± 15.5) and after PCI (baseline, 127.2 mm Hg ± 23.3; stress, 132.0 mm Hg ± 25.8). Also, no noteworthy change in diastolic blood pressure was observed during stress before PCI (baseline, 72.8 mm Hg ± 11.6; stress, 70.5 mm Hg ± 10.2) and after PCI (baseline, 72.3 mm Hg ± 12.2; stress, 75.7 mm Hg ± 10.3). The absolute systolic and diastolic blood pressures during MR imaging were identical before and after PCI.


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
 
The present study results provide baseline and stress MR flow mapping reference values for velocity and flow parameters in nonstenotic single and sequential vein grafts. These values are essential for accurate interpretation of future studies with MR flow mapping. Significantly higher absolute velocity and flow values were observed in sequential as compared with single vein grafts, underscoring the need for separate functional reference values for different graft types. Graft function showed significant improvement after PCI to the point that it was restored or nearly restored to reference values. Thus, MR flow mapping, which provides absolute baseline and stress functional parameters in grafts, is a useful approach for monitoring graft function noninvasively over time and can be used to evaluate the effect of an intervention in a serial fashion.

Functional Measurements in Nonstenotic Grafts
Results of an early study of digital coronary radiography already demonstrated that the velocity reserve in graft regions is lower than in normal coronary artery regions (10). More recently, Campisi et al (11) demonstrated that the normal myocardium supplied by grafts exhibited a lower mean myocardial flow reserve than myocardial territories supplied by coronary arteries in healthy volunteers (2.5 ± 0.5 vs 3.2 ± 0.9, P < .02). The lower flow reserve in normal myocardium supplied by grafts was due to reduced hyperemic response. This underlines the need for separate reference values for grafts and for coronary arteries.

In the present study, reference values for graft velocity reserve and flow reserve, respectively, were 2.4 ± 0.8 and 2.9 ± 2.5 in nonstenotic single vein grafts and 2.3 ± 0.7 and 2.5 ± 0.9 in sequential vein grafts. These findings are in agreement with those of Campisi et al (11). However, in the present study, an impaired velocity reserve—despite the presence of no significant stenosis in the graft and recipient vessels—was demonstrated in a subgroup of nonstenotic single vein grafts; this impaired reserve was due to a reduced hyperemic flow. An impaired coronary vasodilator reserve—even in regions subtended by nonstenotic epicardial coronary vessels—has also been demonstrated in patients with native coronary artery disease (21–23).

The precise mechanism underlying this phenomenon remains unknown. It has been explained as a manifestation of global microvascular endothelial dysfunction (eg, in patients with diabetes mellitus, left ventricular hypertrophy, or myocardial ischemia or infarction). This explanation was confirmed by Hartmann et al (12), who noticed a selective decrease in endothelium-dependent flow reserve in myocardial regions supplied by arterial grafts that was possibly due to microvascular changes. Another mechanism could be diffuse epicardial atherosclerotic changes such as nonobstructive proliferative fibrointimal changes of graft conduits, which are undetectable with conventional angiographic methods (11,24). Both conditions may result in reduced hyperemic flow and impaired velocity reserve in a major portion of the grafts. These phenomena should be taken into account when one is interpreting functional graft parameters.

Functional Changes after PCI
The goal of balloon angioplasty and stent placement is to enlarge luminal cross-sectional area and to improve blood flow (25–27). The increase in luminal cross sectional area is the main determinant for normalization of coronary flow velocity reserve (26). However, heterogeneous results for evaluation of the velocity reserve in coronary arteries after balloon angioplasty with or without stent placement have been reported (25,28–35). Several researchers observed a normalized velocity or flow reserve after PCI by using intracoronary Doppler (25,28), transthoracic Doppler flow measurements (29), or positron emission tomography (30). The improved flow reserve resulted from an increased stress flow at a similar baseline blood flow. Other researchers observed a delayed recovery of the coronary flow reserve resulting from an early increase in the baseline flow after angioplasty (31–34) that normalized again in some patients after subsequent stent placement (35). Potential mechanisms for an initial impairment of flow reserve are (a) the slow recovery of the autoregulation of the microcirculatory tonus, (b) release of vasoactive agents after transient ischemia, (c) vasoconstriction at the site of the Doppler flow wire, and (d) the effects of pharmacotherapy (27).

In the present study, PCI resulted (approximately 7 weeks later) in a significant increase in both baseline and stress peak velocities and no improvement in velocity reserve. In another study, transthoracic Doppler echocardiography in a left internal mammary artery graft after PCI also revealed an increase in both the baseline and stress mean diastolic flow velocities 48 hours after intervention (36). In our study, stratification of post-PCI data according to whether a graft had a velocity reserve of 2.0 or lower or of greater than 2.0 revealed that a reduced stress APV—not an elevated baseline APV—after PCI was the reason for the impaired velocity reserve; this was also observed with nonstenotic single vein grafts. When an impaired velocity reserve is caused by chronic microvascular disturbances in the graft region, it is very unlikely that velocity reserve will improve over time. However, when the underlying mechanism is a temporary phenomenon, velocity reserve might improve. MR flow mapping can be used as a noninvasive instrument to study changes in graft function after PCI. Absolute baseline and stress functional parameters allow the best understanding of changes in graft physiology after balloon angioplasty and stenting.

Study Limitations
Although we studied graft function several weeks after PCI, serial noninvasive measurements over time with MR flow mapping would have provided more insight into the type of impairment of stress APV and thus velocity reserve—that is, whether such impairment had a chronic or temporary cause. Such serial measurements might also provide information on the relationships between MR imaging flow parameters over time and clinical findings such as recurrent chest pain and patient exercise level.

No control coronary angiography was performed at the time of the post-PCI MR imaging study, so it is therefore uncertain whether restenosis had occurred in the grafts. However, all patients had clear relief of angina, without recurrence of anginal complaints, and none of the patients showed other evidence of restenosis, such as ECG changes or abnormal exercise test results.

Clinical Implications and Conclusions
The restoration of blood flow to the myocardium after PCI in grafts is important for relieving patients’ symptoms. The results of the present study show that MR flow mapping, which enables noninvasive and accurate quantification of graft function (6–9), can also be used to noninvasively measure improvements in graft function after PCI. Graft function showed significant improvement after PCI to the point that it was restored or nearly restored to reference values. Future studies in which graft function after PCI is monitored over time and related to clinical events are required. Nevertheless, we can conclude that MR flow mapping, which provides absolute baseline and stress functional parameters in grafts, is a promising noninvasive follow-up tool for evaluating (a) graft function after intervention, (b) progression of disease, and (c) the effect of different treatment strategies for preventing restenosis.


   FOOTNOTES
 
Abbreviations: APV = average peak velocity, DPF = diastolic peak flow rate, DPV = diastolic peak velocity, DSFR = diastolic-to-systolic flow ratio, DSVR = diastolic-to-systolic velocity ratio, ECG = electrocardiographic, PCI = percutaneous intervention, SPF = systolic peak flow rate, SPV = systolic peak velocity

Author contributions: Guarantors of integrity of entire study, H.W.V., J.W.J., A.d.R., E.E.v.d.W.; study concepts, S.E.L., H.W.V., J.W.J., A.d.R., E.E.v.d.W.; study design, S.E.L., H.W.V., H.J.L.; literature research, S.E.L., H.W.V., H.J.L.; clinical studies, S.E.L., H.W.V., J.W.J.; experimental studies and data acquisition, S.E.L., H.W.V., H.J.L., J.W.J.; data analysis/interpretation, S.E.L., A.H.Z., H.W.V., J.W.J., E.E.v.d.W.; statistical analysis, S.E.L., A.H.Z.; manuscript preparation, S.E.L., H.W.V., J.W.J., E.E.v.d.W.; manuscript definition of intellectual content, S.E.L., H.W.V., A.d.R., E.E.v.d.W.; manuscript editing, S.E.L., H.W.V., J.W.J., E.E.v.d.W.; manuscript revision/review and final version approval, all authors


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
DISCUSSION
 REFERENCES
 

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