The results of the VIRTUS feasibility study—analysing the performance of the Vici venous stent system (Veniti) in achieving patency of venous lesions up to 12 months—have recently been released, indicating that intravascular ultrasound (IVUS) may be a more accurate method than venography for assessing lesion severity and proper venous stent size. In this article, Patricia Thorpe, Phoenix, USA, explains the relevance of the findings, and what the data could mean for the future practice of venous stenting.
The VIRTUS feasibility cohort represents the first 30 patients of the staged feasibility/pivotal 200-patient trial of the Vici venous stent. The data from the feasibility cohort was studied to assess the diagnostic value of venography and intravascular ultrasound (IVUS). Accurate assessment of a vein is important for two reasons; first, to evaluate the haemodynamic significance of a lesion and second, to precisely size a stent. Overestimation of a venous stenosis will result in treatment of mild lesions that may not be the source of a patient’s symptoms, while underestimation is associated with missed opportunities to treat a significant lesion.
Accurate sizing of a vein or a venous lesion can be difficult. Unlike arteries, veins often assume an elliptical cross-sectional configuration. For this reason, a single or even two venographic obliquities may or may not identify the most severe axis of a stenosis. If the plane of the venogram is parallel to the long axis of an eccentric venous stenosis, the vein diameter may appear entirely normal. This problem attains increased importance with non-thrombotic venous lesions from external compression, where the lesions can be quite eccentric. The same issues can arise in the measurement of the reference vein diameter, a measurement that attains great importance in the proper sizing of a venous stent. Like a venous stenosis, the normal vein segment may also be elliptical or enlarged due to prestenotic dilatation on cross-section. A stented vein assumes a more cylindrical configuration, raising the question of which measurements and which imaging studies are best suited to quantify the reference vein diameter and determine the appropriate stent size. Veins assume a circular cross-section following stent deployment. Since the perimeter of an elliptical vein is constant before and after stent deployment, the appropriate stent diameter is that diameter associated with the measured pre-stent vein perimeter. Unlike venography, the output of IVUS machines can display vein perimeter. It is also important to recognise pre-stenotic dilatation that widens the distal common iliac vein, but narrows the segment in the common iliac vein anterior to posterior, thus causing the external iliac to become an important baseline measurement for what a normal calibre should be.
The VIRTUS feasibility data presented by Stephen Black go a long way toward answering the aforementioned questions. First, IVUS appears to be best suited to quantify the significance of a venous stenosis. While the minimum luminal diameters of the venous lesions were similar as measured with each imaging modality, cross-sectional area IVUS measurements may better assess the severity of venous lesions; especially those that are eccentric in cross-section. Furthermore, venography resulted in an underestimation of the reference vein diameter, resulting in a greater degree of stent oversizing with venography. Lastly, the residual stenosis after stent implantation was underestimated by venogram, suggesting that IVUS may be better suited to guide post-stent angioplasty and other adjunctive procedures.
In summary, Black’s data suggest that venography may underestimate the severity of a lesion and may be less precise than IVUS for stent sizing and for gauging residual stenosis after stent deployment. These observations are not surprising noting the eccentric nature of venous lesions, as well as the elliptical character of the reference veins. While venography will be necessary for stent delivery and deployment, IVUS appears to be necessary for accurate assessment of venous lesions and appropriate sizing of a venous stent.
Patricia Thorpe is at the Abrazo Arizona Heart Hospital, Phoenix, USA