Influence of external stenting on venous graft flow parameters in coronary artery bypass grafting: a randomized study

Influence of external stenting on venous graft flow parameters in coronary artery bypass... Abstract OBJECTIVES Long-term patency of saphenous vein grafts (SVGs) remains a concern after coronary artery bypass grafting. Interventions to overcome this problem include monitoring intraoperative flow profile and, more recently, external stenting of SVGs. It is not known to what extent external stenting changes the perioperative flow characteristics of SVGs. The aim of this study was to assess whether the presence of an external stent affects perioperative graft flow parameters as evaluated by transit time flowmetry. METHODS Thirty-five patients were included from 1 centre participating in a multicentre, randomized clinical trial of external stenting of SVGs. Patients were eligible if scheduled for on-pump multivessel coronary artery bypass grafting including planned SVGs to both the right and the left coronary territories. Each patient received external stenting of a single SVG randomly allocated intraoperatively to either coronary territory. The primary end-points were mean graft flow, pulsatility index, percentage of diastolic filling and percentage of backward flow in stented versus non-stented SVGs. RESULTS External stenting was performed in 17 SVGs supplying the left territory (20 non-stented SVGs for control) and in 18 SVGs supplying the right territory (18 non-stented SVGs for control). No significant difference was found in flow parameters between stented and non-stented SVGs in the overall group or between pre-defined groups of SVGs supplying the right and left territories, respectively. CONCLUSIONS External stenting of SVGs do not affect intraoperative flow parameters significantly. CLINICAL TRIAL REGISTRATION clinicaltrials.gov identifier: NCT02511834. Coronary artery bypass grafting, Saphenous vein graft, Transit time flowmetry, External stenting, Graft patency INTRODUCTION Coronary artery bypass grafting (CABG) remains the gold standard treatment for multivessel coronary artery disease [1]. Despite the survival benefit of CABG, one of its major challenges is the maintenance of long-term graft patency. Although arterial grafts have demonstrated compelling benefits regarding surgical outcomes including long-term patency [2, 3], autologous saphenous vein grafts (SVGs) remain the most widely used conduits in CABG [4], and their use appears to be on the rise [5, 6]. To improve SVG patency, recent clinical studies have demonstrated the benefits of external stenting on SVGs [5, 7, 8]. Transit time flowmetry (TTFM) is a quality control tool for intraoperative graft assessment in CABG [9] and has been recommended in the most recent European guideline on myocardial revascularization [10]. TTFM measures 4 flow parameters: mean graft flow (MGF), pulsatility index (PI), percentage of diastolic filling (%DF) and percentage of backward flow (%BF) (Table 1). Only 1 previous study has directly compared MGF and PI in stented and non-stented SVGs and found no significant difference between the 2 groups [5]. However, it later became apparent that the initially applied surgical technique of stent deployment was incorrect [11], and no studies exist on flow parameters with the improved technique. Thus, it is not known to what extent external stenting changes perioperative flow characteristics of SVGs, which is of importance for studies on graft patency and future guidelines on cut-off values for such bypass grafts to predict potential anastomotic problems [10]. Table 1: Definition and cut-off values of TTFM parameters TTFM parameters  Definition  Novel values  MGF  MGF expresses the mean forward flow through the graft (Qmean; ml/min)  20 ml/min for arterial grafts and 30–40 ml/min for vein grafts [10]  PI  The PI, expressed as an absolute number, represents an estimate of the resistance to graft flow and the distal target vessel run-off. PI = (Qmax − Qmin)/Qmean  PI <5, ideally <3 [9]  %DF  %DF expresses the proportion of diastolic graft flow during the entire graft flow: DF = Qdiastole/(Qsystole + Qdiastole)  Right-sided grafts: >50%  Left-sided grafts: >60% [9]  %BF  The %BF expresses the percentage of blood flow directed backward across the anastomotic site when compared with the total forward flow during one cardiac cycle.  ≤3% [9]  TTFM parameters  Definition  Novel values  MGF  MGF expresses the mean forward flow through the graft (Qmean; ml/min)  20 ml/min for arterial grafts and 30–40 ml/min for vein grafts [10]  PI  The PI, expressed as an absolute number, represents an estimate of the resistance to graft flow and the distal target vessel run-off. PI = (Qmax − Qmin)/Qmean  PI <5, ideally <3 [9]  %DF  %DF expresses the proportion of diastolic graft flow during the entire graft flow: DF = Qdiastole/(Qsystole + Qdiastole)  Right-sided grafts: >50%  Left-sided grafts: >60% [9]  %BF  The %BF expresses the percentage of blood flow directed backward across the anastomotic site when compared with the total forward flow during one cardiac cycle.  ≤3% [9]  %BF: percentage of backward flow; %DF: percentage of diastolic filling; MGF: mean graft flow; PI: pulsatility index; TTFM: transit time flowmetry parameters. Table 1: Definition and cut-off values of TTFM parameters TTFM parameters  Definition  Novel values  MGF  MGF expresses the mean forward flow through the graft (Qmean; ml/min)  20 ml/min for arterial grafts and 30–40 ml/min for vein grafts [10]  PI  The PI, expressed as an absolute number, represents an estimate of the resistance to graft flow and the distal target vessel run-off. PI = (Qmax − Qmin)/Qmean  PI <5, ideally <3 [9]  %DF  %DF expresses the proportion of diastolic graft flow during the entire graft flow: DF = Qdiastole/(Qsystole + Qdiastole)  Right-sided grafts: >50%  Left-sided grafts: >60% [9]  %BF  The %BF expresses the percentage of blood flow directed backward across the anastomotic site when compared with the total forward flow during one cardiac cycle.  ≤3% [9]  TTFM parameters  Definition  Novel values  MGF  MGF expresses the mean forward flow through the graft (Qmean; ml/min)  20 ml/min for arterial grafts and 30–40 ml/min for vein grafts [10]  PI  The PI, expressed as an absolute number, represents an estimate of the resistance to graft flow and the distal target vessel run-off. PI = (Qmax − Qmin)/Qmean  PI <5, ideally <3 [9]  %DF  %DF expresses the proportion of diastolic graft flow during the entire graft flow: DF = Qdiastole/(Qsystole + Qdiastole)  Right-sided grafts: >50%  Left-sided grafts: >60% [9]  %BF  The %BF expresses the percentage of blood flow directed backward across the anastomotic site when compared with the total forward flow during one cardiac cycle.  ≤3% [9]  %BF: percentage of backward flow; %DF: percentage of diastolic filling; MGF: mean graft flow; PI: pulsatility index; TTFM: transit time flowmetry parameters. The aim of this study was to determine, in a randomized fashion, intraoperative blood flow parameters in stented and non-stented SVGs to the left and the right coronary territories as determined by the TTFM technique. CABG patients from 1 centre who are participating in a multicentre, randomized clinical trial of external stenting of SVGs were included. Patients were eligible if scheduled for at least 1 SVG to each of the 2 coronary territories (the right and the left territories), and each patient received external stenting of a single SVG, randomly allocated intraoperatively, to either the right or the left coronary territories. We hypothesized that external stenting would not significantly alter blood flow in an SVG as determined by TTFM. MATERIALS AND METHODS Study design and population This study was part of a multicentre, randomized clinical study with patients serving as their own controls (the venous external stenting trial (VEST) III trial). The study was approved by the UK Research Ethics Committee, and all subjects gave informed consent. Furthermore, a CONSORT 2010 checklist was used for this study. A total of 35 patients from 1 centre (John Radcliffe Hospital, Oxford, UK) were enrolled from October 2015 to the set deadline on January 2017. Each CABG procedure was specially designed for each patient to compliment both preclinical risk factors and coronary anatomy. Patients were eligible if scheduled for on-pump multivessel CABG including the left internal mammary artery to the left anterior descending artery and SVG to both the right and the left coronary territories. Patients with only left-sided or only right-sided coronary disease were excluded. Eligibility required a target vessel diameter of ≥1.5 mm with a coronary artery stenosis of greater than 75% and with an adequately dimensioned distal vascular bed as assessed from preoperative angiography. Each patient received 1 external stent (VEST stent, Vascular Graft Solutions Ltd, Tel Aviv, Israel), consisting of a cobalt–chrome braid with axial plasticity and radial elasticity, to a single SVG, randomly assigned intraoperatively by the opening of a sealed non-transparent envelope, to either the right or the left coronary territory. One or more SVGs remained non-stented and served as control. All SVGs were performed as single grafts. No revisions were needed based on adequate TTFM findings (MGF >20 ml/min and PI <5). The degree of native coronary artery stenosis was measured by an independent observer using quantitative coronary angiography (Horizon Cardiology version 12.2, McKesson, Israel). Data from quantitative coronary angiography included host coronary artery diameter, average lumen diameter, host coronary artery stenosis and lesion length. The primary end-points were MGF, PI, %DF and %BF measured by TTFM in stented as well as in non-stented SVGs. All flow measurements were carried out with the VeriQC device (Medistim ASA, Oslo, Norway). Procedure Routine on-pump CABG was performed by 2 surgeons, D.P.T. (n = 21) and G.K. (n = 14). The study protocol was followed in all patients. SVGs were harvested by endoscopic technique (100%), and sutures (5.0 prolene) were used to ligate their side branches. SVGs were preserved in heparinized blood-buffered saline. After completing distal anastomoses of the SVG, the sealed randomization letter was opened, and an adequately sized VEST device was selected from 12 available models based on the diameter and length of the SVG. The device was threaded on the non-pressurized SVG. After completion of the proximal anastomosis, the device was expanded to cover the entire SVG. Gaps of 5–10 mm were intentionally maintained from the device to the distal and proximal anastomoses. No fixation of the device is required, as it is meant to maintain its shape post-deployment without compression of the vessel lumen [5, 11]. Principle of transit time flowmetry measurement Prior to TTFM measurement, an appropriately sized probe was selected according to the external SVG diameter. Mainly, probe sizes of 4 or 5 mm were used. Ultrasound gel was applied to the probe lumen before positioning it on the SVG, such that the graft occupied a minimum of 75% of the probe lumen. The probe was placed as close as possible to the surgical anastomosis during TTFM measurement in non-stented SVGs, whereas the probe was placed close to the proximal anastomosis in the 5–10-mm gap intentionally maintained. Prior to measurements, a systemic mean arterial blood pressure of 75–85 mmHg was maintained. Traction on the pericardium was released, and the stabilizer was removed from the pericardial surface to allow the heart to return to its natural anatomic position. TTFM parameters and mean arterial blood pressure were determined after reversal with heparin. Statistical analysis Continuous variables were reported as mean ± standard deviation (SD) and were compared using the unpaired t-test for normal distributions and the Mann–Whitney U-test for non-normal distributions. Normality was assessed with the Kolmogorov–Schmirnov test. Categorical variables were expressed as frequencies and percentages and were compared using the χ2–Pearson test. All reported P-values are 2-sided, and a value of P <0.05 was considered statistically significant. The statistical power analysis was based on the assumption that for a flow measurement to become clinically significant, external stenting should change MGF to <40 ml/min or PI to >3 (Table 1). The power analysis was based on previous values for MGF (58 ± 26 ml/min) and PI (2.1 ± 1.0) from our group. We conservatively assumed SD for the difference comparable with the SD of the population. With 31 and 33 patients in each group, such a difference will be detected with a beta of 0.90 and an alpha of 0.05 (double sided). To allow for a dropout/non-analysable rate of at least 5%, we included 35 patients in each group. Statistical analysis was performed in IBM SPSS Statistics Version 22 (SPSS Inc., Chicago, IL). RESULTS Baseline characteristics A total of 35 patients (mean age 68.0 ± 6.9 years, 88.6% men) were enrolled between October 2015 to the set deadline on January 2017. All stent deployments were successful. A total of 115 bypass grafts, including 42 arterial conduits and 73 grafts, were included in the analysis (35 stented SVGs and 38 non-stented SVGs). Of the stented SVGs, 17 (49%) were grafted to the left coronary territory and 18 (51%) were grafted to the right coronary territory. Five patients received bilateral internal mammary artery treatment. The clinical profile of the cohort is presented in Table 2. Table 2: Patient demographics Characteristics (n = 35)  Mean ± SD or n (%)  Age (years)  68.0 ± 6.9  Men  31 (89)  Body mass index  28.5 ± 3.4  Diabetes mellitus     Insulin dependent  4 (11.4)   Non-insulin dependent  5 (14.3)   No history of diabetes  26 (74.3)  Chronic obstructive pulmonary disease  5 (14.3)  New York Heart Association class     I  7 (20)   II  18 (52)   III  8 (23)   IV  2 (6)  Canadian Cardiovascular Society class   I  2 (6)   II  25 (71)   III  3 (9)   IV  5 (14)  Left ventricular ejection fraction (%)  57.4 ± 6.1  Number of conduits  3.3 ± 4.6  Bypass time (min)  99.5 ± 20.6  Cross-clamp time (min)  63.8 ± 16.5  Characteristics (n = 35)  Mean ± SD or n (%)  Age (years)  68.0 ± 6.9  Men  31 (89)  Body mass index  28.5 ± 3.4  Diabetes mellitus     Insulin dependent  4 (11.4)   Non-insulin dependent  5 (14.3)   No history of diabetes  26 (74.3)  Chronic obstructive pulmonary disease  5 (14.3)  New York Heart Association class     I  7 (20)   II  18 (52)   III  8 (23)   IV  2 (6)  Canadian Cardiovascular Society class   I  2 (6)   II  25 (71)   III  3 (9)   IV  5 (14)  Left ventricular ejection fraction (%)  57.4 ± 6.1  Number of conduits  3.3 ± 4.6  Bypass time (min)  99.5 ± 20.6  Cross-clamp time (min)  63.8 ± 16.5  SD: standard deviation. Table 2: Patient demographics Characteristics (n = 35)  Mean ± SD or n (%)  Age (years)  68.0 ± 6.9  Men  31 (89)  Body mass index  28.5 ± 3.4  Diabetes mellitus     Insulin dependent  4 (11.4)   Non-insulin dependent  5 (14.3)   No history of diabetes  26 (74.3)  Chronic obstructive pulmonary disease  5 (14.3)  New York Heart Association class     I  7 (20)   II  18 (52)   III  8 (23)   IV  2 (6)  Canadian Cardiovascular Society class   I  2 (6)   II  25 (71)   III  3 (9)   IV  5 (14)  Left ventricular ejection fraction (%)  57.4 ± 6.1  Number of conduits  3.3 ± 4.6  Bypass time (min)  99.5 ± 20.6  Cross-clamp time (min)  63.8 ± 16.5  Characteristics (n = 35)  Mean ± SD or n (%)  Age (years)  68.0 ± 6.9  Men  31 (89)  Body mass index  28.5 ± 3.4  Diabetes mellitus     Insulin dependent  4 (11.4)   Non-insulin dependent  5 (14.3)   No history of diabetes  26 (74.3)  Chronic obstructive pulmonary disease  5 (14.3)  New York Heart Association class     I  7 (20)   II  18 (52)   III  8 (23)   IV  2 (6)  Canadian Cardiovascular Society class   I  2 (6)   II  25 (71)   III  3 (9)   IV  5 (14)  Left ventricular ejection fraction (%)  57.4 ± 6.1  Number of conduits  3.3 ± 4.6  Bypass time (min)  99.5 ± 20.6  Cross-clamp time (min)  63.8 ± 16.5  SD: standard deviation. Target vessel disease The extent of native vessel disease and the length of the conduit used for CABG are summarized in Table 3. Stented and non-stented SVGs were comparable with respect to host coronary artery stenosis (the left territory, P = 0.2; the right territory, P = 0.09), host coronary artery diameter (the left territory: P = 0.9, the right territory: P = 0.9), average lumen diameter (the left territory, P = 0.3; the right territory, P = 0.4) and lesion length (the left territory, P = 0.5; the right territory, P = 0.9). The mean length of stented and non-stented SVGs was identical in the 2 coronary systems (the left territory, P = 0.7; the right territory, P = 0.3). Table 3: Native vessel disease: quantitative coronary angiography data Variables  Host coronary artery diameter (mm)  Average lumen diameter (mm)  Host coronary artery stenosis (% area)  Lesion length (mm)  Graft length (cm)  Averaged overall (n = 115), mean ± SD   Stented SVG (n = 35)  2.1 ± 0.7  0.6 ± 0.3  93.3 ± 8.8  10.1 ± 5.7  15.3 ± 3.3   Non-stented SVG (n = 38)  2.1 ± 0.8  0.7 ± 0.4  89.4 ± 10.1  9.2 ± 3.7  15.5 ± 2.8   Arterial grafts (n = 42)  2.2 ± 0.7  0.8 ± 0.5  86.8 ± 11.7  10.9 ± 3.6     P-value  0.9  0.9  0.2  0.6  0.8  The left territory (n = 79), mean ± SD   Stented SVG (n = 17)  2.1 ± 0.6  0.5 ± 0.2  94.4 ± 6.3  10.3 ± 6.1  13.8 ± 3.4   Non-stented SVG (n = 20)  2.1 ± 1.0  0.7 ± 0.5  89.1 ± 8.4  8.9 ± 3.5  14.2 ± 2.8   Arterial grafts (n = 42)  2.2 ± 0.7  0.8 ± 0.5  86.8 ± 11.7  10.9 ± 3.6     P-value  0.9  0.3  0.2  0.5  0.7  The right territory (n = 36), mean ± SD   Stented SVG (n = 18)  2.1 ± 0.9  0.7 ± 0.4  92.2 ± 10.8  9.9 ± 5.6  16.5 ± 2.8   Non-stented SVG (n = 18)  2.1 ± 0.6  0.6 ± 0.3  89.7 ± 12.0  9.6 ± 4.0  17.4 ± 1.4   Arterial grafts (n = 0)             P-value  0.9  0.4  0.09  0.9  0.3  Variables  Host coronary artery diameter (mm)  Average lumen diameter (mm)  Host coronary artery stenosis (% area)  Lesion length (mm)  Graft length (cm)  Averaged overall (n = 115), mean ± SD   Stented SVG (n = 35)  2.1 ± 0.7  0.6 ± 0.3  93.3 ± 8.8  10.1 ± 5.7  15.3 ± 3.3   Non-stented SVG (n = 38)  2.1 ± 0.8  0.7 ± 0.4  89.4 ± 10.1  9.2 ± 3.7  15.5 ± 2.8   Arterial grafts (n = 42)  2.2 ± 0.7  0.8 ± 0.5  86.8 ± 11.7  10.9 ± 3.6     P-value  0.9  0.9  0.2  0.6  0.8  The left territory (n = 79), mean ± SD   Stented SVG (n = 17)  2.1 ± 0.6  0.5 ± 0.2  94.4 ± 6.3  10.3 ± 6.1  13.8 ± 3.4   Non-stented SVG (n = 20)  2.1 ± 1.0  0.7 ± 0.5  89.1 ± 8.4  8.9 ± 3.5  14.2 ± 2.8   Arterial grafts (n = 42)  2.2 ± 0.7  0.8 ± 0.5  86.8 ± 11.7  10.9 ± 3.6     P-value  0.9  0.3  0.2  0.5  0.7  The right territory (n = 36), mean ± SD   Stented SVG (n = 18)  2.1 ± 0.9  0.7 ± 0.4  92.2 ± 10.8  9.9 ± 5.6  16.5 ± 2.8   Non-stented SVG (n = 18)  2.1 ± 0.6  0.6 ± 0.3  89.7 ± 12.0  9.6 ± 4.0  17.4 ± 1.4   Arterial grafts (n = 0)             P-value  0.9  0.4  0.09  0.9  0.3  P-value: comparing stented SVGs and non-stented SVGs. SD: standard deviation; SVG: saphenous vein graft. Table 3: Native vessel disease: quantitative coronary angiography data Variables  Host coronary artery diameter (mm)  Average lumen diameter (mm)  Host coronary artery stenosis (% area)  Lesion length (mm)  Graft length (cm)  Averaged overall (n = 115), mean ± SD   Stented SVG (n = 35)  2.1 ± 0.7  0.6 ± 0.3  93.3 ± 8.8  10.1 ± 5.7  15.3 ± 3.3   Non-stented SVG (n = 38)  2.1 ± 0.8  0.7 ± 0.4  89.4 ± 10.1  9.2 ± 3.7  15.5 ± 2.8   Arterial grafts (n = 42)  2.2 ± 0.7  0.8 ± 0.5  86.8 ± 11.7  10.9 ± 3.6     P-value  0.9  0.9  0.2  0.6  0.8  The left territory (n = 79), mean ± SD   Stented SVG (n = 17)  2.1 ± 0.6  0.5 ± 0.2  94.4 ± 6.3  10.3 ± 6.1  13.8 ± 3.4   Non-stented SVG (n = 20)  2.1 ± 1.0  0.7 ± 0.5  89.1 ± 8.4  8.9 ± 3.5  14.2 ± 2.8   Arterial grafts (n = 42)  2.2 ± 0.7  0.8 ± 0.5  86.8 ± 11.7  10.9 ± 3.6     P-value  0.9  0.3  0.2  0.5  0.7  The right territory (n = 36), mean ± SD   Stented SVG (n = 18)  2.1 ± 0.9  0.7 ± 0.4  92.2 ± 10.8  9.9 ± 5.6  16.5 ± 2.8   Non-stented SVG (n = 18)  2.1 ± 0.6  0.6 ± 0.3  89.7 ± 12.0  9.6 ± 4.0  17.4 ± 1.4   Arterial grafts (n = 0)             P-value  0.9  0.4  0.09  0.9  0.3  Variables  Host coronary artery diameter (mm)  Average lumen diameter (mm)  Host coronary artery stenosis (% area)  Lesion length (mm)  Graft length (cm)  Averaged overall (n = 115), mean ± SD   Stented SVG (n = 35)  2.1 ± 0.7  0.6 ± 0.3  93.3 ± 8.8  10.1 ± 5.7  15.3 ± 3.3   Non-stented SVG (n = 38)  2.1 ± 0.8  0.7 ± 0.4  89.4 ± 10.1  9.2 ± 3.7  15.5 ± 2.8   Arterial grafts (n = 42)  2.2 ± 0.7  0.8 ± 0.5  86.8 ± 11.7  10.9 ± 3.6     P-value  0.9  0.9  0.2  0.6  0.8  The left territory (n = 79), mean ± SD   Stented SVG (n = 17)  2.1 ± 0.6  0.5 ± 0.2  94.4 ± 6.3  10.3 ± 6.1  13.8 ± 3.4   Non-stented SVG (n = 20)  2.1 ± 1.0  0.7 ± 0.5  89.1 ± 8.4  8.9 ± 3.5  14.2 ± 2.8   Arterial grafts (n = 42)  2.2 ± 0.7  0.8 ± 0.5  86.8 ± 11.7  10.9 ± 3.6     P-value  0.9  0.3  0.2  0.5  0.7  The right territory (n = 36), mean ± SD   Stented SVG (n = 18)  2.1 ± 0.9  0.7 ± 0.4  92.2 ± 10.8  9.9 ± 5.6  16.5 ± 2.8   Non-stented SVG (n = 18)  2.1 ± 0.6  0.6 ± 0.3  89.7 ± 12.0  9.6 ± 4.0  17.4 ± 1.4   Arterial grafts (n = 0)             P-value  0.9  0.4  0.09  0.9  0.3  P-value: comparing stented SVGs and non-stented SVGs. SD: standard deviation; SVG: saphenous vein graft. Intraoperative transit time flowmetry TTFM measurements and mean arterial pressure at TTFM are presented in Table 4 and Fig. 1. There were no significant differences between stented and non-stented SV grafts in MGF (the left territory, P = 0.9; the right territory, P = 0.3), PI (the left territory, P = 0.2; the right territory, P = 0.1), % DF (the left territory, P = 0.6; the right territory, P = 0.3) and % BF (the left territory, P = 0.4; the right territory, P = 0.6). As intended, mean arterial blood pressure at the time of TTFM measurement was similar in stented and non-stented SVGs (the left territory, P = 0.9; the right territory, P = 0.8). Table 4: Comparison of intraoperative TTFM data Variable  Flow (ml/min)  Pulsatility index  Diastolic filling (%)  Backflow (%)  Mean arterial pressure at TTFM (mmHg)  Averaged overall (n = 115), mean ± SD   Stented SVG (n = 35)  50.6 ± 26.3  2.2 ± 1.0  60.5 ± 13.2  1.2 ± 3.2  81.7 ± 5.7   Non-stented SVG (n = 38)  58.6 ± 34.8  2.2 ± 0.9  60.4 ± 8.9  1.0 ± 2.2  81.6 ± 6.4   Arterial grafts (n = 42)  45.3 ± 34.7  2.3 ± 0.8  69.9 ± 7.2  1.8 ± 2.2  82.1 ± 6.8   P-value  0.5  0.9  0.9  0.9  0.9  The left territory (n = 79), mean ± SD   Stented SVG (n = 17)  52.9 ± 30.7  2.4 ± 1.2  61.1 ± 16.8  1.8 ± 4.5  81.8 ± 6.4   Non-stented SVG (n = 20)  57.2 ± 27.2  2.0 ± 0.7  63.9 ± 6.2  0.8 ± 1.3  82.0 ± 6.2   Arterial grafts (n = 42)  45.3 ± 34.7  2.3 ± 0.8  69.9 ± 7.2  1.8 ± 2.2  82.1 ± 6.8   P-value  0.9  0.2  0.6  0.4  0.9  The right territory (n = 36), mean ± SD   Stented SVG (n = 18)  48.3 ± 22.0  2.0 ± 0.6  60.0 ± 9.0  0.6 ± 1.1  81.7 ± 5.1   Non-stented SVG (n = 18)  60.1 ± 42.5  2.4 ± 1.0  56.5 ± 9.9  1.2 ± 3.0  81.1 ± 6.8   Arterial grafts (n = 0)             P-value  0.3  0.1  0.3  0.6  0.8  Variable  Flow (ml/min)  Pulsatility index  Diastolic filling (%)  Backflow (%)  Mean arterial pressure at TTFM (mmHg)  Averaged overall (n = 115), mean ± SD   Stented SVG (n = 35)  50.6 ± 26.3  2.2 ± 1.0  60.5 ± 13.2  1.2 ± 3.2  81.7 ± 5.7   Non-stented SVG (n = 38)  58.6 ± 34.8  2.2 ± 0.9  60.4 ± 8.9  1.0 ± 2.2  81.6 ± 6.4   Arterial grafts (n = 42)  45.3 ± 34.7  2.3 ± 0.8  69.9 ± 7.2  1.8 ± 2.2  82.1 ± 6.8   P-value  0.5  0.9  0.9  0.9  0.9  The left territory (n = 79), mean ± SD   Stented SVG (n = 17)  52.9 ± 30.7  2.4 ± 1.2  61.1 ± 16.8  1.8 ± 4.5  81.8 ± 6.4   Non-stented SVG (n = 20)  57.2 ± 27.2  2.0 ± 0.7  63.9 ± 6.2  0.8 ± 1.3  82.0 ± 6.2   Arterial grafts (n = 42)  45.3 ± 34.7  2.3 ± 0.8  69.9 ± 7.2  1.8 ± 2.2  82.1 ± 6.8   P-value  0.9  0.2  0.6  0.4  0.9  The right territory (n = 36), mean ± SD   Stented SVG (n = 18)  48.3 ± 22.0  2.0 ± 0.6  60.0 ± 9.0  0.6 ± 1.1  81.7 ± 5.1   Non-stented SVG (n = 18)  60.1 ± 42.5  2.4 ± 1.0  56.5 ± 9.9  1.2 ± 3.0  81.1 ± 6.8   Arterial grafts (n = 0)             P-value  0.3  0.1  0.3  0.6  0.8  P-value: comparing stented SVGs and non-stented SVGs. SD: standard deviation; SVG: saphenous vein graft; TTFM: transit time flowmetry. Table 4: Comparison of intraoperative TTFM data Variable  Flow (ml/min)  Pulsatility index  Diastolic filling (%)  Backflow (%)  Mean arterial pressure at TTFM (mmHg)  Averaged overall (n = 115), mean ± SD   Stented SVG (n = 35)  50.6 ± 26.3  2.2 ± 1.0  60.5 ± 13.2  1.2 ± 3.2  81.7 ± 5.7   Non-stented SVG (n = 38)  58.6 ± 34.8  2.2 ± 0.9  60.4 ± 8.9  1.0 ± 2.2  81.6 ± 6.4   Arterial grafts (n = 42)  45.3 ± 34.7  2.3 ± 0.8  69.9 ± 7.2  1.8 ± 2.2  82.1 ± 6.8   P-value  0.5  0.9  0.9  0.9  0.9  The left territory (n = 79), mean ± SD   Stented SVG (n = 17)  52.9 ± 30.7  2.4 ± 1.2  61.1 ± 16.8  1.8 ± 4.5  81.8 ± 6.4   Non-stented SVG (n = 20)  57.2 ± 27.2  2.0 ± 0.7  63.9 ± 6.2  0.8 ± 1.3  82.0 ± 6.2   Arterial grafts (n = 42)  45.3 ± 34.7  2.3 ± 0.8  69.9 ± 7.2  1.8 ± 2.2  82.1 ± 6.8   P-value  0.9  0.2  0.6  0.4  0.9  The right territory (n = 36), mean ± SD   Stented SVG (n = 18)  48.3 ± 22.0  2.0 ± 0.6  60.0 ± 9.0  0.6 ± 1.1  81.7 ± 5.1   Non-stented SVG (n = 18)  60.1 ± 42.5  2.4 ± 1.0  56.5 ± 9.9  1.2 ± 3.0  81.1 ± 6.8   Arterial grafts (n = 0)             P-value  0.3  0.1  0.3  0.6  0.8  Variable  Flow (ml/min)  Pulsatility index  Diastolic filling (%)  Backflow (%)  Mean arterial pressure at TTFM (mmHg)  Averaged overall (n = 115), mean ± SD   Stented SVG (n = 35)  50.6 ± 26.3  2.2 ± 1.0  60.5 ± 13.2  1.2 ± 3.2  81.7 ± 5.7   Non-stented SVG (n = 38)  58.6 ± 34.8  2.2 ± 0.9  60.4 ± 8.9  1.0 ± 2.2  81.6 ± 6.4   Arterial grafts (n = 42)  45.3 ± 34.7  2.3 ± 0.8  69.9 ± 7.2  1.8 ± 2.2  82.1 ± 6.8   P-value  0.5  0.9  0.9  0.9  0.9  The left territory (n = 79), mean ± SD   Stented SVG (n = 17)  52.9 ± 30.7  2.4 ± 1.2  61.1 ± 16.8  1.8 ± 4.5  81.8 ± 6.4   Non-stented SVG (n = 20)  57.2 ± 27.2  2.0 ± 0.7  63.9 ± 6.2  0.8 ± 1.3  82.0 ± 6.2   Arterial grafts (n = 42)  45.3 ± 34.7  2.3 ± 0.8  69.9 ± 7.2  1.8 ± 2.2  82.1 ± 6.8   P-value  0.9  0.2  0.6  0.4  0.9  The right territory (n = 36), mean ± SD   Stented SVG (n = 18)  48.3 ± 22.0  2.0 ± 0.6  60.0 ± 9.0  0.6 ± 1.1  81.7 ± 5.1   Non-stented SVG (n = 18)  60.1 ± 42.5  2.4 ± 1.0  56.5 ± 9.9  1.2 ± 3.0  81.1 ± 6.8   Arterial grafts (n = 0)             P-value  0.3  0.1  0.3  0.6  0.8  P-value: comparing stented SVGs and non-stented SVGs. SD: standard deviation; SVG: saphenous vein graft; TTFM: transit time flowmetry. Figure 1: View largeDownload slide Boxplots depicting all 4 transit time flowmetry parameters (mean graft flow, pulsatility index, percentage of diastolic filling and percentage of backward flow) in stented versus non-stented SV grafts to the left and the right coronary territories. Dots and stars depict data outliers. SV: saphenous vein. Figure 1: View largeDownload slide Boxplots depicting all 4 transit time flowmetry parameters (mean graft flow, pulsatility index, percentage of diastolic filling and percentage of backward flow) in stented versus non-stented SV grafts to the left and the right coronary territories. Dots and stars depict data outliers. SV: saphenous vein. DISCUSSION As the literature does not address the influence of external stenting of SVGs on perioperative flow characteristics, we sought to assess perioperative blood flow parameters in stented versus non-stented SVGs in a randomized clinical study of CABG patients during on-pump surgery. As determined by TTFM, our study showed no statistically significant difference in terms of MGF, PI, percentage of %DF or percentage of %BF between stented and non-stented SVGs, in neither the left nor the right coronary territory (Table 4). In keeping with our findings, the VEST trial [5], which enrolled a total of 35 patients with external stenting of SVGs going to either the right coronary artery (RCA) or the circumflex coronary territory, found an MGF of 67 ± 28 ml/min vs 66 ± 33 ml/min (P = 0.89) and a PI of 2.2 ± 1.1 vs 2.2 ± 1.0 (P = 1.0) in stented versus non-stented SVGs. In the VEST trial [5], %DF and %BF were not assessed, and it was later suggested that the initially applied surgical technique of how the stents were deployed was incorrect, which has now been improved [11]. With modern technique of external stenting, we can now confirm that external stenting is of no clinically significant concern for any aspect of haemodynamics of SVGs as assessed by TTFM. In general, one might have hypothesized some improvement of flow dynamics in stented SVGs, since previous studies have demonstrated that external stenting of SVGs improve lumen uniformity and haemodynamic flow patterns, as determined by angiography and intravascular ultrasound at 1-year follow-up [7]. In a report of the VEST trial [7], post hoc computational fluid dynamics analysis of grafts from patients in the VEST trial were used to calculate and compare 3 haemodynamic parameters: time-averaged wall shear stress, oscillatory shear index and relative residence time using a technique established in a previous study by the same group [5, 12]. With this technique, a significantly reduced mean oscillatory shear index was found in the stented group when compared with the non-stented group. Pertinently, it was also found that mean oscillatory shear index correlated with the development of diffuse intimal hyperplasia [12], suggesting that external stenting of SVGs improve graft patency at follow-up. In fact, the key finding of the VEST trial [5] was the potential of mechanical external stents to reduce the intimal hyperplasia area in SVGs 1 year after CABG by approximately 15% (P = 0.04) [5]. Although we did observe a trend towards higher flows in non-stented SVGs, this was statistically insignificant, and the high SD prevents drawing any definite conclusions. Furthermore, PI was higher in stented versus non-stented SVGS going to the left coronary territory but conversely so in stented versus non-stented SVGS going to the right coronary territory, and overall, there was no statistically significant difference. Lack of statistical significance in our study may be due to the fact that stents do not compromise the vessel lumen but simply support the vascular wall [11]. In fact, most stents are deployed loosely at first, as repositioning of the heart back to anatomical place in combination with restoring normal blood pressure following CABG results in an increase of vessel diameter, hereby eliminating any gap between the vessel wall and the stent without compromising the vessel lumen [11]. As such, a change in haemodynamic flows of stented SVGs may potentially arise after CABG when the vein graft has adjusted to the stent. Taken together with our findings, external stenting seems to improve long-term flow characteristics irrespective of initial changes and demonstrate that external support with VEST stents is safe and may improve graft patency, at least in the short term to mid-term with favourable changes in vessel structure. Limitations In this study, we did not assess TTFM parameters (MGF, PI, %DF and %BF) in correlation with graft patency at follow-up, either by conventional angiography or by intravascular ultrasound. As such, a definite conclusion regarding the effect of intraoperative flow dynamics in stented versus non-stented SVGs at follow-up cannot be drawn. Our statistical power analysis was based on the ability to see changes that, based on previously established cut-off points for TTFM parameters, would have been associated with clinically relevant compromised flows, but we may not have been able to see much smaller changes, which may eventually translate into significant changes including the development of hyperplasia. CONCLUSION In conclusion, external stenting of SVGs with the VEST stent does not affect intra-operative flow parameters and does not deteriorate the vessels’ haemodynamics in the perioperative period in coronary artery bypass surgery. Further results from randomized clinical studies are warranted to address whether changes of flow dynamics in stented SVGs arise after CABG. Based on our results, cut-off points for native and externally stented SVGs should be the same. Conflict of interest: David P. Taggart declares a conflict of interest as a shareholder at VGS and lecturing honoraria as an advisor and speaker for VGS. REFERENCES 1 Benedetto U, Gaudino M, Ng C, Biondi-Zoccai G, D’Ascenzo F, Frati G et al.  . Coronary surgery is superior to drug eluting stents in multivessel disease. Systematic review and meta-analysis of contemporary randomized controlled trials. Int J Cardiol  2016; 210: 19– 24. Google Scholar CrossRef Search ADS PubMed  2 Taggart DP, D’Amico R, Altman DG. Effect of arterial revascularisation on survival: a systematic review of studies comparing bilateral and single internal mammary arteries. Lancet  2001; 358: 870– 5. Google Scholar CrossRef Search ADS PubMed  3 Yi G, Shine B, Rehman SM, Altman DG, Taggart DP. Effect of bilateral internal mammary artery grafts on long-term survival: a meta-analysis approach. Circulation  2014; 130: 539– 45. Google Scholar CrossRef Search ADS PubMed  4 Tabata M, Grab JD, Khalpey Z, Edwards FH, O’Brian SM, Cohn LH et al.  . Prevalence and variability of internal mammary artery graft use in contemporary multivessel coronary artery bypass graft surgery: analysis of the Society of Thoracic Surgeons National Cardiac Database. Circulation  2009; 120: 935– 40. Google Scholar CrossRef Search ADS PubMed  5 Taggart DP, Ben Gal Y, Lees B, Patel N, Webb C, Rehman SM et al.  . A randomized trial of external stenting for saphenous vein grafts in coronary artery bypass grafting. Ann Thorac Surg  2015; 99: 2039– 45. Google Scholar CrossRef Search ADS PubMed  6 Schwann TA, Tatoulis J, Puskas JD, Taggart DP, Kurlansky P, Jacobs J et al.  . Worldwide trends in multi-arterial CABG surgery 2004-2014: a tale of two continents. Autumn  2017; 29: 273– 80. 7 Meirson T, Orion E, Di Mario C, Webb C, Patel N, Channon KM et al.  . Flow patterns in externally stented saphenous vein grafts and development of intimal hyperplasia. J Thorac Cardiovasc Surg  2015; 150: 871– 8. Google Scholar CrossRef Search ADS PubMed  8 Webb CM, Orion E, Taggart DP, Channon KM, Di Mario C. OCT imaging of aorto-coronary vein graft pathology modified by external stenting: 1-year post-surgery. Eur Heart J Cardiovasc Imaging  2016; 17: 1290– 5. Google Scholar CrossRef Search ADS PubMed  9 Amin S, Pinho-Gomes AC, Taggart DP. Relationship of intraoperative transit time flowmetry findings to angiographic graft patency at follow-up. Ann Thorac Surg  2016; 101: 1996– 2006. Google Scholar CrossRef Search ADS PubMed  10 Kolh P, Wendecker S, Alfonso F, Collet JP, Cremer J, Falk V et al.  . 2014 ESC/EACTS guidelines on myocardial revascularization: the task force on myocardial revascularization of the European Society of Cardiology (ESC) and the European Association for Cardio-Thoracic Surgery (EACTS). Developed with the special contribution of the European Association of Percutaneous Cardiovascular Interventions (EAPCI). Eur J Cardiothorac Surg  2014; 46: 517– 92. Google Scholar CrossRef Search ADS PubMed  11 Taggart DP, Amin S, Djordjevic J, Oikonomou EK, Thomas S, Kampoli AM et al.  . A prospective study of external stenting of saphenous vein grafts to the right coronary artery: the VEST II study. Eur J Cardiothorac Surg  2017; 51: 952– 8. Google Scholar CrossRef Search ADS PubMed  12 Meirson T, Orion E, Avrahami I. Numerical analysis of venous external scaffolding technology for saphenous vein grafts. J Biomech  2015; 48: 2090– 5. Google Scholar CrossRef Search ADS PubMed  © The Author(s) 2018. Published by Oxford University Press on behalf of the European Association for Cardio-Thoracic Surgery. All rights reserved. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/about_us/legal/notices) http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Interactive CardioVascular and Thoracic Surgery Oxford University Press

Influence of external stenting on venous graft flow parameters in coronary artery bypass grafting: a randomized study

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Oxford University Press
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© The Author(s) 2018. Published by Oxford University Press on behalf of the European Association for Cardio-Thoracic Surgery. All rights reserved.
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1569-9293
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1569-9285
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10.1093/icvts/ivy007
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Abstract

Abstract OBJECTIVES Long-term patency of saphenous vein grafts (SVGs) remains a concern after coronary artery bypass grafting. Interventions to overcome this problem include monitoring intraoperative flow profile and, more recently, external stenting of SVGs. It is not known to what extent external stenting changes the perioperative flow characteristics of SVGs. The aim of this study was to assess whether the presence of an external stent affects perioperative graft flow parameters as evaluated by transit time flowmetry. METHODS Thirty-five patients were included from 1 centre participating in a multicentre, randomized clinical trial of external stenting of SVGs. Patients were eligible if scheduled for on-pump multivessel coronary artery bypass grafting including planned SVGs to both the right and the left coronary territories. Each patient received external stenting of a single SVG randomly allocated intraoperatively to either coronary territory. The primary end-points were mean graft flow, pulsatility index, percentage of diastolic filling and percentage of backward flow in stented versus non-stented SVGs. RESULTS External stenting was performed in 17 SVGs supplying the left territory (20 non-stented SVGs for control) and in 18 SVGs supplying the right territory (18 non-stented SVGs for control). No significant difference was found in flow parameters between stented and non-stented SVGs in the overall group or between pre-defined groups of SVGs supplying the right and left territories, respectively. CONCLUSIONS External stenting of SVGs do not affect intraoperative flow parameters significantly. CLINICAL TRIAL REGISTRATION clinicaltrials.gov identifier: NCT02511834. Coronary artery bypass grafting, Saphenous vein graft, Transit time flowmetry, External stenting, Graft patency INTRODUCTION Coronary artery bypass grafting (CABG) remains the gold standard treatment for multivessel coronary artery disease [1]. Despite the survival benefit of CABG, one of its major challenges is the maintenance of long-term graft patency. Although arterial grafts have demonstrated compelling benefits regarding surgical outcomes including long-term patency [2, 3], autologous saphenous vein grafts (SVGs) remain the most widely used conduits in CABG [4], and their use appears to be on the rise [5, 6]. To improve SVG patency, recent clinical studies have demonstrated the benefits of external stenting on SVGs [5, 7, 8]. Transit time flowmetry (TTFM) is a quality control tool for intraoperative graft assessment in CABG [9] and has been recommended in the most recent European guideline on myocardial revascularization [10]. TTFM measures 4 flow parameters: mean graft flow (MGF), pulsatility index (PI), percentage of diastolic filling (%DF) and percentage of backward flow (%BF) (Table 1). Only 1 previous study has directly compared MGF and PI in stented and non-stented SVGs and found no significant difference between the 2 groups [5]. However, it later became apparent that the initially applied surgical technique of stent deployment was incorrect [11], and no studies exist on flow parameters with the improved technique. Thus, it is not known to what extent external stenting changes perioperative flow characteristics of SVGs, which is of importance for studies on graft patency and future guidelines on cut-off values for such bypass grafts to predict potential anastomotic problems [10]. Table 1: Definition and cut-off values of TTFM parameters TTFM parameters  Definition  Novel values  MGF  MGF expresses the mean forward flow through the graft (Qmean; ml/min)  20 ml/min for arterial grafts and 30–40 ml/min for vein grafts [10]  PI  The PI, expressed as an absolute number, represents an estimate of the resistance to graft flow and the distal target vessel run-off. PI = (Qmax − Qmin)/Qmean  PI <5, ideally <3 [9]  %DF  %DF expresses the proportion of diastolic graft flow during the entire graft flow: DF = Qdiastole/(Qsystole + Qdiastole)  Right-sided grafts: >50%  Left-sided grafts: >60% [9]  %BF  The %BF expresses the percentage of blood flow directed backward across the anastomotic site when compared with the total forward flow during one cardiac cycle.  ≤3% [9]  TTFM parameters  Definition  Novel values  MGF  MGF expresses the mean forward flow through the graft (Qmean; ml/min)  20 ml/min for arterial grafts and 30–40 ml/min for vein grafts [10]  PI  The PI, expressed as an absolute number, represents an estimate of the resistance to graft flow and the distal target vessel run-off. PI = (Qmax − Qmin)/Qmean  PI <5, ideally <3 [9]  %DF  %DF expresses the proportion of diastolic graft flow during the entire graft flow: DF = Qdiastole/(Qsystole + Qdiastole)  Right-sided grafts: >50%  Left-sided grafts: >60% [9]  %BF  The %BF expresses the percentage of blood flow directed backward across the anastomotic site when compared with the total forward flow during one cardiac cycle.  ≤3% [9]  %BF: percentage of backward flow; %DF: percentage of diastolic filling; MGF: mean graft flow; PI: pulsatility index; TTFM: transit time flowmetry parameters. Table 1: Definition and cut-off values of TTFM parameters TTFM parameters  Definition  Novel values  MGF  MGF expresses the mean forward flow through the graft (Qmean; ml/min)  20 ml/min for arterial grafts and 30–40 ml/min for vein grafts [10]  PI  The PI, expressed as an absolute number, represents an estimate of the resistance to graft flow and the distal target vessel run-off. PI = (Qmax − Qmin)/Qmean  PI <5, ideally <3 [9]  %DF  %DF expresses the proportion of diastolic graft flow during the entire graft flow: DF = Qdiastole/(Qsystole + Qdiastole)  Right-sided grafts: >50%  Left-sided grafts: >60% [9]  %BF  The %BF expresses the percentage of blood flow directed backward across the anastomotic site when compared with the total forward flow during one cardiac cycle.  ≤3% [9]  TTFM parameters  Definition  Novel values  MGF  MGF expresses the mean forward flow through the graft (Qmean; ml/min)  20 ml/min for arterial grafts and 30–40 ml/min for vein grafts [10]  PI  The PI, expressed as an absolute number, represents an estimate of the resistance to graft flow and the distal target vessel run-off. PI = (Qmax − Qmin)/Qmean  PI <5, ideally <3 [9]  %DF  %DF expresses the proportion of diastolic graft flow during the entire graft flow: DF = Qdiastole/(Qsystole + Qdiastole)  Right-sided grafts: >50%  Left-sided grafts: >60% [9]  %BF  The %BF expresses the percentage of blood flow directed backward across the anastomotic site when compared with the total forward flow during one cardiac cycle.  ≤3% [9]  %BF: percentage of backward flow; %DF: percentage of diastolic filling; MGF: mean graft flow; PI: pulsatility index; TTFM: transit time flowmetry parameters. The aim of this study was to determine, in a randomized fashion, intraoperative blood flow parameters in stented and non-stented SVGs to the left and the right coronary territories as determined by the TTFM technique. CABG patients from 1 centre who are participating in a multicentre, randomized clinical trial of external stenting of SVGs were included. Patients were eligible if scheduled for at least 1 SVG to each of the 2 coronary territories (the right and the left territories), and each patient received external stenting of a single SVG, randomly allocated intraoperatively, to either the right or the left coronary territories. We hypothesized that external stenting would not significantly alter blood flow in an SVG as determined by TTFM. MATERIALS AND METHODS Study design and population This study was part of a multicentre, randomized clinical study with patients serving as their own controls (the venous external stenting trial (VEST) III trial). The study was approved by the UK Research Ethics Committee, and all subjects gave informed consent. Furthermore, a CONSORT 2010 checklist was used for this study. A total of 35 patients from 1 centre (John Radcliffe Hospital, Oxford, UK) were enrolled from October 2015 to the set deadline on January 2017. Each CABG procedure was specially designed for each patient to compliment both preclinical risk factors and coronary anatomy. Patients were eligible if scheduled for on-pump multivessel CABG including the left internal mammary artery to the left anterior descending artery and SVG to both the right and the left coronary territories. Patients with only left-sided or only right-sided coronary disease were excluded. Eligibility required a target vessel diameter of ≥1.5 mm with a coronary artery stenosis of greater than 75% and with an adequately dimensioned distal vascular bed as assessed from preoperative angiography. Each patient received 1 external stent (VEST stent, Vascular Graft Solutions Ltd, Tel Aviv, Israel), consisting of a cobalt–chrome braid with axial plasticity and radial elasticity, to a single SVG, randomly assigned intraoperatively by the opening of a sealed non-transparent envelope, to either the right or the left coronary territory. One or more SVGs remained non-stented and served as control. All SVGs were performed as single grafts. No revisions were needed based on adequate TTFM findings (MGF >20 ml/min and PI <5). The degree of native coronary artery stenosis was measured by an independent observer using quantitative coronary angiography (Horizon Cardiology version 12.2, McKesson, Israel). Data from quantitative coronary angiography included host coronary artery diameter, average lumen diameter, host coronary artery stenosis and lesion length. The primary end-points were MGF, PI, %DF and %BF measured by TTFM in stented as well as in non-stented SVGs. All flow measurements were carried out with the VeriQC device (Medistim ASA, Oslo, Norway). Procedure Routine on-pump CABG was performed by 2 surgeons, D.P.T. (n = 21) and G.K. (n = 14). The study protocol was followed in all patients. SVGs were harvested by endoscopic technique (100%), and sutures (5.0 prolene) were used to ligate their side branches. SVGs were preserved in heparinized blood-buffered saline. After completing distal anastomoses of the SVG, the sealed randomization letter was opened, and an adequately sized VEST device was selected from 12 available models based on the diameter and length of the SVG. The device was threaded on the non-pressurized SVG. After completion of the proximal anastomosis, the device was expanded to cover the entire SVG. Gaps of 5–10 mm were intentionally maintained from the device to the distal and proximal anastomoses. No fixation of the device is required, as it is meant to maintain its shape post-deployment without compression of the vessel lumen [5, 11]. Principle of transit time flowmetry measurement Prior to TTFM measurement, an appropriately sized probe was selected according to the external SVG diameter. Mainly, probe sizes of 4 or 5 mm were used. Ultrasound gel was applied to the probe lumen before positioning it on the SVG, such that the graft occupied a minimum of 75% of the probe lumen. The probe was placed as close as possible to the surgical anastomosis during TTFM measurement in non-stented SVGs, whereas the probe was placed close to the proximal anastomosis in the 5–10-mm gap intentionally maintained. Prior to measurements, a systemic mean arterial blood pressure of 75–85 mmHg was maintained. Traction on the pericardium was released, and the stabilizer was removed from the pericardial surface to allow the heart to return to its natural anatomic position. TTFM parameters and mean arterial blood pressure were determined after reversal with heparin. Statistical analysis Continuous variables were reported as mean ± standard deviation (SD) and were compared using the unpaired t-test for normal distributions and the Mann–Whitney U-test for non-normal distributions. Normality was assessed with the Kolmogorov–Schmirnov test. Categorical variables were expressed as frequencies and percentages and were compared using the χ2–Pearson test. All reported P-values are 2-sided, and a value of P <0.05 was considered statistically significant. The statistical power analysis was based on the assumption that for a flow measurement to become clinically significant, external stenting should change MGF to <40 ml/min or PI to >3 (Table 1). The power analysis was based on previous values for MGF (58 ± 26 ml/min) and PI (2.1 ± 1.0) from our group. We conservatively assumed SD for the difference comparable with the SD of the population. With 31 and 33 patients in each group, such a difference will be detected with a beta of 0.90 and an alpha of 0.05 (double sided). To allow for a dropout/non-analysable rate of at least 5%, we included 35 patients in each group. Statistical analysis was performed in IBM SPSS Statistics Version 22 (SPSS Inc., Chicago, IL). RESULTS Baseline characteristics A total of 35 patients (mean age 68.0 ± 6.9 years, 88.6% men) were enrolled between October 2015 to the set deadline on January 2017. All stent deployments were successful. A total of 115 bypass grafts, including 42 arterial conduits and 73 grafts, were included in the analysis (35 stented SVGs and 38 non-stented SVGs). Of the stented SVGs, 17 (49%) were grafted to the left coronary territory and 18 (51%) were grafted to the right coronary territory. Five patients received bilateral internal mammary artery treatment. The clinical profile of the cohort is presented in Table 2. Table 2: Patient demographics Characteristics (n = 35)  Mean ± SD or n (%)  Age (years)  68.0 ± 6.9  Men  31 (89)  Body mass index  28.5 ± 3.4  Diabetes mellitus     Insulin dependent  4 (11.4)   Non-insulin dependent  5 (14.3)   No history of diabetes  26 (74.3)  Chronic obstructive pulmonary disease  5 (14.3)  New York Heart Association class     I  7 (20)   II  18 (52)   III  8 (23)   IV  2 (6)  Canadian Cardiovascular Society class   I  2 (6)   II  25 (71)   III  3 (9)   IV  5 (14)  Left ventricular ejection fraction (%)  57.4 ± 6.1  Number of conduits  3.3 ± 4.6  Bypass time (min)  99.5 ± 20.6  Cross-clamp time (min)  63.8 ± 16.5  Characteristics (n = 35)  Mean ± SD or n (%)  Age (years)  68.0 ± 6.9  Men  31 (89)  Body mass index  28.5 ± 3.4  Diabetes mellitus     Insulin dependent  4 (11.4)   Non-insulin dependent  5 (14.3)   No history of diabetes  26 (74.3)  Chronic obstructive pulmonary disease  5 (14.3)  New York Heart Association class     I  7 (20)   II  18 (52)   III  8 (23)   IV  2 (6)  Canadian Cardiovascular Society class   I  2 (6)   II  25 (71)   III  3 (9)   IV  5 (14)  Left ventricular ejection fraction (%)  57.4 ± 6.1  Number of conduits  3.3 ± 4.6  Bypass time (min)  99.5 ± 20.6  Cross-clamp time (min)  63.8 ± 16.5  SD: standard deviation. Table 2: Patient demographics Characteristics (n = 35)  Mean ± SD or n (%)  Age (years)  68.0 ± 6.9  Men  31 (89)  Body mass index  28.5 ± 3.4  Diabetes mellitus     Insulin dependent  4 (11.4)   Non-insulin dependent  5 (14.3)   No history of diabetes  26 (74.3)  Chronic obstructive pulmonary disease  5 (14.3)  New York Heart Association class     I  7 (20)   II  18 (52)   III  8 (23)   IV  2 (6)  Canadian Cardiovascular Society class   I  2 (6)   II  25 (71)   III  3 (9)   IV  5 (14)  Left ventricular ejection fraction (%)  57.4 ± 6.1  Number of conduits  3.3 ± 4.6  Bypass time (min)  99.5 ± 20.6  Cross-clamp time (min)  63.8 ± 16.5  Characteristics (n = 35)  Mean ± SD or n (%)  Age (years)  68.0 ± 6.9  Men  31 (89)  Body mass index  28.5 ± 3.4  Diabetes mellitus     Insulin dependent  4 (11.4)   Non-insulin dependent  5 (14.3)   No history of diabetes  26 (74.3)  Chronic obstructive pulmonary disease  5 (14.3)  New York Heart Association class     I  7 (20)   II  18 (52)   III  8 (23)   IV  2 (6)  Canadian Cardiovascular Society class   I  2 (6)   II  25 (71)   III  3 (9)   IV  5 (14)  Left ventricular ejection fraction (%)  57.4 ± 6.1  Number of conduits  3.3 ± 4.6  Bypass time (min)  99.5 ± 20.6  Cross-clamp time (min)  63.8 ± 16.5  SD: standard deviation. Target vessel disease The extent of native vessel disease and the length of the conduit used for CABG are summarized in Table 3. Stented and non-stented SVGs were comparable with respect to host coronary artery stenosis (the left territory, P = 0.2; the right territory, P = 0.09), host coronary artery diameter (the left territory: P = 0.9, the right territory: P = 0.9), average lumen diameter (the left territory, P = 0.3; the right territory, P = 0.4) and lesion length (the left territory, P = 0.5; the right territory, P = 0.9). The mean length of stented and non-stented SVGs was identical in the 2 coronary systems (the left territory, P = 0.7; the right territory, P = 0.3). Table 3: Native vessel disease: quantitative coronary angiography data Variables  Host coronary artery diameter (mm)  Average lumen diameter (mm)  Host coronary artery stenosis (% area)  Lesion length (mm)  Graft length (cm)  Averaged overall (n = 115), mean ± SD   Stented SVG (n = 35)  2.1 ± 0.7  0.6 ± 0.3  93.3 ± 8.8  10.1 ± 5.7  15.3 ± 3.3   Non-stented SVG (n = 38)  2.1 ± 0.8  0.7 ± 0.4  89.4 ± 10.1  9.2 ± 3.7  15.5 ± 2.8   Arterial grafts (n = 42)  2.2 ± 0.7  0.8 ± 0.5  86.8 ± 11.7  10.9 ± 3.6     P-value  0.9  0.9  0.2  0.6  0.8  The left territory (n = 79), mean ± SD   Stented SVG (n = 17)  2.1 ± 0.6  0.5 ± 0.2  94.4 ± 6.3  10.3 ± 6.1  13.8 ± 3.4   Non-stented SVG (n = 20)  2.1 ± 1.0  0.7 ± 0.5  89.1 ± 8.4  8.9 ± 3.5  14.2 ± 2.8   Arterial grafts (n = 42)  2.2 ± 0.7  0.8 ± 0.5  86.8 ± 11.7  10.9 ± 3.6     P-value  0.9  0.3  0.2  0.5  0.7  The right territory (n = 36), mean ± SD   Stented SVG (n = 18)  2.1 ± 0.9  0.7 ± 0.4  92.2 ± 10.8  9.9 ± 5.6  16.5 ± 2.8   Non-stented SVG (n = 18)  2.1 ± 0.6  0.6 ± 0.3  89.7 ± 12.0  9.6 ± 4.0  17.4 ± 1.4   Arterial grafts (n = 0)             P-value  0.9  0.4  0.09  0.9  0.3  Variables  Host coronary artery diameter (mm)  Average lumen diameter (mm)  Host coronary artery stenosis (% area)  Lesion length (mm)  Graft length (cm)  Averaged overall (n = 115), mean ± SD   Stented SVG (n = 35)  2.1 ± 0.7  0.6 ± 0.3  93.3 ± 8.8  10.1 ± 5.7  15.3 ± 3.3   Non-stented SVG (n = 38)  2.1 ± 0.8  0.7 ± 0.4  89.4 ± 10.1  9.2 ± 3.7  15.5 ± 2.8   Arterial grafts (n = 42)  2.2 ± 0.7  0.8 ± 0.5  86.8 ± 11.7  10.9 ± 3.6     P-value  0.9  0.9  0.2  0.6  0.8  The left territory (n = 79), mean ± SD   Stented SVG (n = 17)  2.1 ± 0.6  0.5 ± 0.2  94.4 ± 6.3  10.3 ± 6.1  13.8 ± 3.4   Non-stented SVG (n = 20)  2.1 ± 1.0  0.7 ± 0.5  89.1 ± 8.4  8.9 ± 3.5  14.2 ± 2.8   Arterial grafts (n = 42)  2.2 ± 0.7  0.8 ± 0.5  86.8 ± 11.7  10.9 ± 3.6     P-value  0.9  0.3  0.2  0.5  0.7  The right territory (n = 36), mean ± SD   Stented SVG (n = 18)  2.1 ± 0.9  0.7 ± 0.4  92.2 ± 10.8  9.9 ± 5.6  16.5 ± 2.8   Non-stented SVG (n = 18)  2.1 ± 0.6  0.6 ± 0.3  89.7 ± 12.0  9.6 ± 4.0  17.4 ± 1.4   Arterial grafts (n = 0)             P-value  0.9  0.4  0.09  0.9  0.3  P-value: comparing stented SVGs and non-stented SVGs. SD: standard deviation; SVG: saphenous vein graft. Table 3: Native vessel disease: quantitative coronary angiography data Variables  Host coronary artery diameter (mm)  Average lumen diameter (mm)  Host coronary artery stenosis (% area)  Lesion length (mm)  Graft length (cm)  Averaged overall (n = 115), mean ± SD   Stented SVG (n = 35)  2.1 ± 0.7  0.6 ± 0.3  93.3 ± 8.8  10.1 ± 5.7  15.3 ± 3.3   Non-stented SVG (n = 38)  2.1 ± 0.8  0.7 ± 0.4  89.4 ± 10.1  9.2 ± 3.7  15.5 ± 2.8   Arterial grafts (n = 42)  2.2 ± 0.7  0.8 ± 0.5  86.8 ± 11.7  10.9 ± 3.6     P-value  0.9  0.9  0.2  0.6  0.8  The left territory (n = 79), mean ± SD   Stented SVG (n = 17)  2.1 ± 0.6  0.5 ± 0.2  94.4 ± 6.3  10.3 ± 6.1  13.8 ± 3.4   Non-stented SVG (n = 20)  2.1 ± 1.0  0.7 ± 0.5  89.1 ± 8.4  8.9 ± 3.5  14.2 ± 2.8   Arterial grafts (n = 42)  2.2 ± 0.7  0.8 ± 0.5  86.8 ± 11.7  10.9 ± 3.6     P-value  0.9  0.3  0.2  0.5  0.7  The right territory (n = 36), mean ± SD   Stented SVG (n = 18)  2.1 ± 0.9  0.7 ± 0.4  92.2 ± 10.8  9.9 ± 5.6  16.5 ± 2.8   Non-stented SVG (n = 18)  2.1 ± 0.6  0.6 ± 0.3  89.7 ± 12.0  9.6 ± 4.0  17.4 ± 1.4   Arterial grafts (n = 0)             P-value  0.9  0.4  0.09  0.9  0.3  Variables  Host coronary artery diameter (mm)  Average lumen diameter (mm)  Host coronary artery stenosis (% area)  Lesion length (mm)  Graft length (cm)  Averaged overall (n = 115), mean ± SD   Stented SVG (n = 35)  2.1 ± 0.7  0.6 ± 0.3  93.3 ± 8.8  10.1 ± 5.7  15.3 ± 3.3   Non-stented SVG (n = 38)  2.1 ± 0.8  0.7 ± 0.4  89.4 ± 10.1  9.2 ± 3.7  15.5 ± 2.8   Arterial grafts (n = 42)  2.2 ± 0.7  0.8 ± 0.5  86.8 ± 11.7  10.9 ± 3.6     P-value  0.9  0.9  0.2  0.6  0.8  The left territory (n = 79), mean ± SD   Stented SVG (n = 17)  2.1 ± 0.6  0.5 ± 0.2  94.4 ± 6.3  10.3 ± 6.1  13.8 ± 3.4   Non-stented SVG (n = 20)  2.1 ± 1.0  0.7 ± 0.5  89.1 ± 8.4  8.9 ± 3.5  14.2 ± 2.8   Arterial grafts (n = 42)  2.2 ± 0.7  0.8 ± 0.5  86.8 ± 11.7  10.9 ± 3.6     P-value  0.9  0.3  0.2  0.5  0.7  The right territory (n = 36), mean ± SD   Stented SVG (n = 18)  2.1 ± 0.9  0.7 ± 0.4  92.2 ± 10.8  9.9 ± 5.6  16.5 ± 2.8   Non-stented SVG (n = 18)  2.1 ± 0.6  0.6 ± 0.3  89.7 ± 12.0  9.6 ± 4.0  17.4 ± 1.4   Arterial grafts (n = 0)             P-value  0.9  0.4  0.09  0.9  0.3  P-value: comparing stented SVGs and non-stented SVGs. SD: standard deviation; SVG: saphenous vein graft. Intraoperative transit time flowmetry TTFM measurements and mean arterial pressure at TTFM are presented in Table 4 and Fig. 1. There were no significant differences between stented and non-stented SV grafts in MGF (the left territory, P = 0.9; the right territory, P = 0.3), PI (the left territory, P = 0.2; the right territory, P = 0.1), % DF (the left territory, P = 0.6; the right territory, P = 0.3) and % BF (the left territory, P = 0.4; the right territory, P = 0.6). As intended, mean arterial blood pressure at the time of TTFM measurement was similar in stented and non-stented SVGs (the left territory, P = 0.9; the right territory, P = 0.8). Table 4: Comparison of intraoperative TTFM data Variable  Flow (ml/min)  Pulsatility index  Diastolic filling (%)  Backflow (%)  Mean arterial pressure at TTFM (mmHg)  Averaged overall (n = 115), mean ± SD   Stented SVG (n = 35)  50.6 ± 26.3  2.2 ± 1.0  60.5 ± 13.2  1.2 ± 3.2  81.7 ± 5.7   Non-stented SVG (n = 38)  58.6 ± 34.8  2.2 ± 0.9  60.4 ± 8.9  1.0 ± 2.2  81.6 ± 6.4   Arterial grafts (n = 42)  45.3 ± 34.7  2.3 ± 0.8  69.9 ± 7.2  1.8 ± 2.2  82.1 ± 6.8   P-value  0.5  0.9  0.9  0.9  0.9  The left territory (n = 79), mean ± SD   Stented SVG (n = 17)  52.9 ± 30.7  2.4 ± 1.2  61.1 ± 16.8  1.8 ± 4.5  81.8 ± 6.4   Non-stented SVG (n = 20)  57.2 ± 27.2  2.0 ± 0.7  63.9 ± 6.2  0.8 ± 1.3  82.0 ± 6.2   Arterial grafts (n = 42)  45.3 ± 34.7  2.3 ± 0.8  69.9 ± 7.2  1.8 ± 2.2  82.1 ± 6.8   P-value  0.9  0.2  0.6  0.4  0.9  The right territory (n = 36), mean ± SD   Stented SVG (n = 18)  48.3 ± 22.0  2.0 ± 0.6  60.0 ± 9.0  0.6 ± 1.1  81.7 ± 5.1   Non-stented SVG (n = 18)  60.1 ± 42.5  2.4 ± 1.0  56.5 ± 9.9  1.2 ± 3.0  81.1 ± 6.8   Arterial grafts (n = 0)             P-value  0.3  0.1  0.3  0.6  0.8  Variable  Flow (ml/min)  Pulsatility index  Diastolic filling (%)  Backflow (%)  Mean arterial pressure at TTFM (mmHg)  Averaged overall (n = 115), mean ± SD   Stented SVG (n = 35)  50.6 ± 26.3  2.2 ± 1.0  60.5 ± 13.2  1.2 ± 3.2  81.7 ± 5.7   Non-stented SVG (n = 38)  58.6 ± 34.8  2.2 ± 0.9  60.4 ± 8.9  1.0 ± 2.2  81.6 ± 6.4   Arterial grafts (n = 42)  45.3 ± 34.7  2.3 ± 0.8  69.9 ± 7.2  1.8 ± 2.2  82.1 ± 6.8   P-value  0.5  0.9  0.9  0.9  0.9  The left territory (n = 79), mean ± SD   Stented SVG (n = 17)  52.9 ± 30.7  2.4 ± 1.2  61.1 ± 16.8  1.8 ± 4.5  81.8 ± 6.4   Non-stented SVG (n = 20)  57.2 ± 27.2  2.0 ± 0.7  63.9 ± 6.2  0.8 ± 1.3  82.0 ± 6.2   Arterial grafts (n = 42)  45.3 ± 34.7  2.3 ± 0.8  69.9 ± 7.2  1.8 ± 2.2  82.1 ± 6.8   P-value  0.9  0.2  0.6  0.4  0.9  The right territory (n = 36), mean ± SD   Stented SVG (n = 18)  48.3 ± 22.0  2.0 ± 0.6  60.0 ± 9.0  0.6 ± 1.1  81.7 ± 5.1   Non-stented SVG (n = 18)  60.1 ± 42.5  2.4 ± 1.0  56.5 ± 9.9  1.2 ± 3.0  81.1 ± 6.8   Arterial grafts (n = 0)             P-value  0.3  0.1  0.3  0.6  0.8  P-value: comparing stented SVGs and non-stented SVGs. SD: standard deviation; SVG: saphenous vein graft; TTFM: transit time flowmetry. Table 4: Comparison of intraoperative TTFM data Variable  Flow (ml/min)  Pulsatility index  Diastolic filling (%)  Backflow (%)  Mean arterial pressure at TTFM (mmHg)  Averaged overall (n = 115), mean ± SD   Stented SVG (n = 35)  50.6 ± 26.3  2.2 ± 1.0  60.5 ± 13.2  1.2 ± 3.2  81.7 ± 5.7   Non-stented SVG (n = 38)  58.6 ± 34.8  2.2 ± 0.9  60.4 ± 8.9  1.0 ± 2.2  81.6 ± 6.4   Arterial grafts (n = 42)  45.3 ± 34.7  2.3 ± 0.8  69.9 ± 7.2  1.8 ± 2.2  82.1 ± 6.8   P-value  0.5  0.9  0.9  0.9  0.9  The left territory (n = 79), mean ± SD   Stented SVG (n = 17)  52.9 ± 30.7  2.4 ± 1.2  61.1 ± 16.8  1.8 ± 4.5  81.8 ± 6.4   Non-stented SVG (n = 20)  57.2 ± 27.2  2.0 ± 0.7  63.9 ± 6.2  0.8 ± 1.3  82.0 ± 6.2   Arterial grafts (n = 42)  45.3 ± 34.7  2.3 ± 0.8  69.9 ± 7.2  1.8 ± 2.2  82.1 ± 6.8   P-value  0.9  0.2  0.6  0.4  0.9  The right territory (n = 36), mean ± SD   Stented SVG (n = 18)  48.3 ± 22.0  2.0 ± 0.6  60.0 ± 9.0  0.6 ± 1.1  81.7 ± 5.1   Non-stented SVG (n = 18)  60.1 ± 42.5  2.4 ± 1.0  56.5 ± 9.9  1.2 ± 3.0  81.1 ± 6.8   Arterial grafts (n = 0)             P-value  0.3  0.1  0.3  0.6  0.8  Variable  Flow (ml/min)  Pulsatility index  Diastolic filling (%)  Backflow (%)  Mean arterial pressure at TTFM (mmHg)  Averaged overall (n = 115), mean ± SD   Stented SVG (n = 35)  50.6 ± 26.3  2.2 ± 1.0  60.5 ± 13.2  1.2 ± 3.2  81.7 ± 5.7   Non-stented SVG (n = 38)  58.6 ± 34.8  2.2 ± 0.9  60.4 ± 8.9  1.0 ± 2.2  81.6 ± 6.4   Arterial grafts (n = 42)  45.3 ± 34.7  2.3 ± 0.8  69.9 ± 7.2  1.8 ± 2.2  82.1 ± 6.8   P-value  0.5  0.9  0.9  0.9  0.9  The left territory (n = 79), mean ± SD   Stented SVG (n = 17)  52.9 ± 30.7  2.4 ± 1.2  61.1 ± 16.8  1.8 ± 4.5  81.8 ± 6.4   Non-stented SVG (n = 20)  57.2 ± 27.2  2.0 ± 0.7  63.9 ± 6.2  0.8 ± 1.3  82.0 ± 6.2   Arterial grafts (n = 42)  45.3 ± 34.7  2.3 ± 0.8  69.9 ± 7.2  1.8 ± 2.2  82.1 ± 6.8   P-value  0.9  0.2  0.6  0.4  0.9  The right territory (n = 36), mean ± SD   Stented SVG (n = 18)  48.3 ± 22.0  2.0 ± 0.6  60.0 ± 9.0  0.6 ± 1.1  81.7 ± 5.1   Non-stented SVG (n = 18)  60.1 ± 42.5  2.4 ± 1.0  56.5 ± 9.9  1.2 ± 3.0  81.1 ± 6.8   Arterial grafts (n = 0)             P-value  0.3  0.1  0.3  0.6  0.8  P-value: comparing stented SVGs and non-stented SVGs. SD: standard deviation; SVG: saphenous vein graft; TTFM: transit time flowmetry. Figure 1: View largeDownload slide Boxplots depicting all 4 transit time flowmetry parameters (mean graft flow, pulsatility index, percentage of diastolic filling and percentage of backward flow) in stented versus non-stented SV grafts to the left and the right coronary territories. Dots and stars depict data outliers. SV: saphenous vein. Figure 1: View largeDownload slide Boxplots depicting all 4 transit time flowmetry parameters (mean graft flow, pulsatility index, percentage of diastolic filling and percentage of backward flow) in stented versus non-stented SV grafts to the left and the right coronary territories. Dots and stars depict data outliers. SV: saphenous vein. DISCUSSION As the literature does not address the influence of external stenting of SVGs on perioperative flow characteristics, we sought to assess perioperative blood flow parameters in stented versus non-stented SVGs in a randomized clinical study of CABG patients during on-pump surgery. As determined by TTFM, our study showed no statistically significant difference in terms of MGF, PI, percentage of %DF or percentage of %BF between stented and non-stented SVGs, in neither the left nor the right coronary territory (Table 4). In keeping with our findings, the VEST trial [5], which enrolled a total of 35 patients with external stenting of SVGs going to either the right coronary artery (RCA) or the circumflex coronary territory, found an MGF of 67 ± 28 ml/min vs 66 ± 33 ml/min (P = 0.89) and a PI of 2.2 ± 1.1 vs 2.2 ± 1.0 (P = 1.0) in stented versus non-stented SVGs. In the VEST trial [5], %DF and %BF were not assessed, and it was later suggested that the initially applied surgical technique of how the stents were deployed was incorrect, which has now been improved [11]. With modern technique of external stenting, we can now confirm that external stenting is of no clinically significant concern for any aspect of haemodynamics of SVGs as assessed by TTFM. In general, one might have hypothesized some improvement of flow dynamics in stented SVGs, since previous studies have demonstrated that external stenting of SVGs improve lumen uniformity and haemodynamic flow patterns, as determined by angiography and intravascular ultrasound at 1-year follow-up [7]. In a report of the VEST trial [7], post hoc computational fluid dynamics analysis of grafts from patients in the VEST trial were used to calculate and compare 3 haemodynamic parameters: time-averaged wall shear stress, oscillatory shear index and relative residence time using a technique established in a previous study by the same group [5, 12]. With this technique, a significantly reduced mean oscillatory shear index was found in the stented group when compared with the non-stented group. Pertinently, it was also found that mean oscillatory shear index correlated with the development of diffuse intimal hyperplasia [12], suggesting that external stenting of SVGs improve graft patency at follow-up. In fact, the key finding of the VEST trial [5] was the potential of mechanical external stents to reduce the intimal hyperplasia area in SVGs 1 year after CABG by approximately 15% (P = 0.04) [5]. Although we did observe a trend towards higher flows in non-stented SVGs, this was statistically insignificant, and the high SD prevents drawing any definite conclusions. Furthermore, PI was higher in stented versus non-stented SVGS going to the left coronary territory but conversely so in stented versus non-stented SVGS going to the right coronary territory, and overall, there was no statistically significant difference. Lack of statistical significance in our study may be due to the fact that stents do not compromise the vessel lumen but simply support the vascular wall [11]. In fact, most stents are deployed loosely at first, as repositioning of the heart back to anatomical place in combination with restoring normal blood pressure following CABG results in an increase of vessel diameter, hereby eliminating any gap between the vessel wall and the stent without compromising the vessel lumen [11]. As such, a change in haemodynamic flows of stented SVGs may potentially arise after CABG when the vein graft has adjusted to the stent. Taken together with our findings, external stenting seems to improve long-term flow characteristics irrespective of initial changes and demonstrate that external support with VEST stents is safe and may improve graft patency, at least in the short term to mid-term with favourable changes in vessel structure. Limitations In this study, we did not assess TTFM parameters (MGF, PI, %DF and %BF) in correlation with graft patency at follow-up, either by conventional angiography or by intravascular ultrasound. As such, a definite conclusion regarding the effect of intraoperative flow dynamics in stented versus non-stented SVGs at follow-up cannot be drawn. Our statistical power analysis was based on the ability to see changes that, based on previously established cut-off points for TTFM parameters, would have been associated with clinically relevant compromised flows, but we may not have been able to see much smaller changes, which may eventually translate into significant changes including the development of hyperplasia. CONCLUSION In conclusion, external stenting of SVGs with the VEST stent does not affect intra-operative flow parameters and does not deteriorate the vessels’ haemodynamics in the perioperative period in coronary artery bypass surgery. Further results from randomized clinical studies are warranted to address whether changes of flow dynamics in stented SVGs arise after CABG. Based on our results, cut-off points for native and externally stented SVGs should be the same. Conflict of interest: David P. Taggart declares a conflict of interest as a shareholder at VGS and lecturing honoraria as an advisor and speaker for VGS. REFERENCES 1 Benedetto U, Gaudino M, Ng C, Biondi-Zoccai G, D’Ascenzo F, Frati G et al.  . Coronary surgery is superior to drug eluting stents in multivessel disease. Systematic review and meta-analysis of contemporary randomized controlled trials. Int J Cardiol  2016; 210: 19– 24. Google Scholar CrossRef Search ADS PubMed  2 Taggart DP, D’Amico R, Altman DG. Effect of arterial revascularisation on survival: a systematic review of studies comparing bilateral and single internal mammary arteries. Lancet  2001; 358: 870– 5. Google Scholar CrossRef Search ADS PubMed  3 Yi G, Shine B, Rehman SM, Altman DG, Taggart DP. Effect of bilateral internal mammary artery grafts on long-term survival: a meta-analysis approach. Circulation  2014; 130: 539– 45. Google Scholar CrossRef Search ADS PubMed  4 Tabata M, Grab JD, Khalpey Z, Edwards FH, O’Brian SM, Cohn LH et al.  . Prevalence and variability of internal mammary artery graft use in contemporary multivessel coronary artery bypass graft surgery: analysis of the Society of Thoracic Surgeons National Cardiac Database. Circulation  2009; 120: 935– 40. Google Scholar CrossRef Search ADS PubMed  5 Taggart DP, Ben Gal Y, Lees B, Patel N, Webb C, Rehman SM et al.  . A randomized trial of external stenting for saphenous vein grafts in coronary artery bypass grafting. Ann Thorac Surg  2015; 99: 2039– 45. Google Scholar CrossRef Search ADS PubMed  6 Schwann TA, Tatoulis J, Puskas JD, Taggart DP, Kurlansky P, Jacobs J et al.  . Worldwide trends in multi-arterial CABG surgery 2004-2014: a tale of two continents. Autumn  2017; 29: 273– 80. 7 Meirson T, Orion E, Di Mario C, Webb C, Patel N, Channon KM et al.  . Flow patterns in externally stented saphenous vein grafts and development of intimal hyperplasia. J Thorac Cardiovasc Surg  2015; 150: 871– 8. Google Scholar CrossRef Search ADS PubMed  8 Webb CM, Orion E, Taggart DP, Channon KM, Di Mario C. OCT imaging of aorto-coronary vein graft pathology modified by external stenting: 1-year post-surgery. Eur Heart J Cardiovasc Imaging  2016; 17: 1290– 5. Google Scholar CrossRef Search ADS PubMed  9 Amin S, Pinho-Gomes AC, Taggart DP. Relationship of intraoperative transit time flowmetry findings to angiographic graft patency at follow-up. Ann Thorac Surg  2016; 101: 1996– 2006. Google Scholar CrossRef Search ADS PubMed  10 Kolh P, Wendecker S, Alfonso F, Collet JP, Cremer J, Falk V et al.  . 2014 ESC/EACTS guidelines on myocardial revascularization: the task force on myocardial revascularization of the European Society of Cardiology (ESC) and the European Association for Cardio-Thoracic Surgery (EACTS). Developed with the special contribution of the European Association of Percutaneous Cardiovascular Interventions (EAPCI). Eur J Cardiothorac Surg  2014; 46: 517– 92. Google Scholar CrossRef Search ADS PubMed  11 Taggart DP, Amin S, Djordjevic J, Oikonomou EK, Thomas S, Kampoli AM et al.  . A prospective study of external stenting of saphenous vein grafts to the right coronary artery: the VEST II study. Eur J Cardiothorac Surg  2017; 51: 952– 8. Google Scholar CrossRef Search ADS PubMed  12 Meirson T, Orion E, Avrahami I. Numerical analysis of venous external scaffolding technology for saphenous vein grafts. J Biomech  2015; 48: 2090– 5. Google Scholar CrossRef Search ADS PubMed  © The Author(s) 2018. Published by Oxford University Press on behalf of the European Association for Cardio-Thoracic Surgery. All rights reserved. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/about_us/legal/notices)

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Interactive CardioVascular and Thoracic SurgeryOxford University Press

Published: Jan 24, 2018

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