This study showed that the SV valves were clearly visualized in the standing position utilizing upright CT both in cadaveric legs and in-vivo study of healthy volunteers. In the cadaver study, SV valves and tributaries identified by upright CT were confirmed by anatomical analysis. The minimum dilation ratio of “1.20” was found to be the cut-off point to define the level of the valve to confirm the valve site in the in-vivo study. In the volunteer study, upright CT detected larger number of valves and tributaries and larger size of vessel area comparison with supine CT. We propose that upright CT could have an advantage for SV characterization prior to bypass surgeries.
The SV grafts have surgeries for long due to their easy availability, they are more resistant to iatrogenic injury, less susceptible to vasospasm 13 and provide longer length compared with other kind of grafts. The SV grafts have been most used for non-LAD coronary territories worldwide. It has major disadvantage of high graft failure rate 2,3; approximately 40-50% grafts occlude at 10 years after the CABG surgery, of which 10-25% are occluded within first year post-operatively 4,14,15. In comparison, the occlusion rate of other grafts is lower, for instance the radial artery shows 17-37% failure rate 14,16. One of the several causes of SV failure is the endothelium dysfunction during the harvest and new harvesting techniques including bridge, no-touch, endoscopic resection help reduce the dysfunction and the graft failure. The other underlying cause has been reported as the presence of venous vales. Although the mechanisms of primary stenosis of coronary artery bypass graft and peripheral artery bypass graft have been suggested to be different, they share thrombosis as the basis of luminal loss 17. In SV grafts, valve sites frequently gather clots 18, which cause valve dysfunction, scarring and develop post-thrombotic syndrome 17; the response of local smooth muscle cells to injury is found to be accelerated in-vitro19. The clinical studies have confirmed that the valve sites frequently cause stenotic lesions when SV graft reoperations are performed 5-7,20-22. We assumed that knowing SV valve position and avoiding the valve sites during harvesting may offer another approach to reduce the SV graft failure.
To identify the best portion of the SV as a conduit for bypass surgery, the number and position of valves in SV were reviewed. A Brazilian study characterized 60 veins from 30 adult cadavers and reported that the average number of the valves from the medial epicondyle of the femur to the saphenous hiatus were 4.77 and 4.87 on the left and right side, respectively 23. Similarly a Japanese study of 26 SV from 20 adult cadavers found 111 valves (average: 4.27) between the SF junction and the upper patellar margin compared to 63 (average: 2.42) valves between the upper patellar margin and the medial malleolus 24; they also reported that the valves most valves were observed within 10 cm from the SF junction and between 35-45 cm from the SF junction. Because we identified 4 (IQR: 3-5) valves from between 0 cm to 35 cm from the SF junction and the distribution of valves was mainly observed between 0 to 10 cm, the results of our volunteer study were comparable with these autopsy studies; however, the data beyond 35 cm from the SF junction was not available in this study. Therefore, the upright CT demonstrated an accurate characterization of SV valves not easily possible by the conventional computed tomography.
Another merit of the upright CT was the noninvasive identification of tributaries and vessel area. Vessel size is an important factor to predict graft failure and a luminal diameter of over 2.0 mm is preferable for SV graft patency and longevity 3,14,25. New harvesting techniques less touch SV may reduce the damage compared with conventional open harvesting technique, however; it is difficult to evaluate the valve sites and the vessel size in the whole SV. In addition, it is important to be able to ligate all side tributaries 26. The upright CT allowed to locate valves, identify tributaries and accurately define vessel size noninvasively and should assist the strategy for harvesting.
Recently upright (or weight-bearing) CT 27 and MRI 28 have been proposed to evaluate the effect of gravity. However, the previous upright CT was equipped with a cone beam CT and scan range was limited. The upright MRI has lower spatial resolution compared with CT 29. Compared to these machines, the upright CT used in the current study carries a wide-scan range with quick motion and high spatial resolution, similar to a high-end supine CT at standing position. It has an ability to evaluate blood distribution of whole body 9, and may contribute to the development of the Phlebology, which has room for the development compared with the Arteriology.
Limitations of the study. Although a novel attempt, this study has several limitations. First, since the upright CT is a prototype machine, this is a single center study. Second, the protocol of this study was designed to obtain body-trunk images and the scan range of this prototype machine could not detect beyond 37 cm height, not allowing the images of entire saphenous vein. However, we could acquire the whole SV with pedestal with 42 cm height, which we utilized for the evaluation of ankles 30. Third, the valve insertion sites were identified by defining venous (expansive) dilation ratio compared to the lumen and a cut-off point of 1.20 was based on the anatomic observation in the cadaveric limb. An application of workstation is being developed to enable detecting the valve automatically in future clinical studies. Fourth, we could not identify the valve leaflet directly, because the upright CT scans were performed without contrast medium and the limit of the spatial resolution of CT is 0.4 to 0.6 mm. The invasive OCT with 15 micro-meter spatial resolution 29 have allowed to recognize the culprit valve lesions after CABG 31. However, it is difficult to identify the valves of SV in supine CT even with contrast materials. Thus, this approach will be practically useful to detect valves positions.