Partial renal coverage in EVAR causes unfavorable renal flow patterns in an infrarenal aneurysm model

TL;DR

This study demonstrates that partial renal artery coverage during EVAR alters renal flow patterns, increasing oscillating shear stress and backflow, which may promote atherosclerosis and stenosis, emphasizing the importance of precise device placement.

Contribution

It provides the first in-vitro analysis of how partial renal coverage affects flow dynamics and shear stress in an aneurysm model, highlighting clinical implications.

Findings

Partial coverage causes low and oscillating wall shear stress.

Coverage increases renal backflow area percentage.

Flow alterations may promote renal artery stenosis.

Abstract

Objective: To achieve an optimal sealing zone during EVAR, the intended positioning of the proximal end of the endograft fabric should be as close as possible to the most caudal edge of the renal arteries. Some endografts exhibit a small offset between the radiopaque markers and the proximal fabric edge. Unintended partial renal artery coverage may thus occur. This study investigates the consequences of partial coverage on renal flow patterns and wall shear stress. Methods: In-vitro models of an abdominal aortic aneurysm were used to visualize pulsatile flow using 2D particle image velocimetry under physiologic resting conditions. One model served as control and two models were stented with an Endurant endograft, one without and one with partial renal artery coverage with 1.3 mm of stent fabric extending beyond the marker (16\% area coverage). The magnitude and oscillation of wall shear stress, relative residence time and backflow in the renal artery were analyzed. Results: In both stented models, a region along the caudal renal artery wall presented with low and oscillating wall shear stress, not present in the control model. A region with very low wall shear stress (<0.1 Pa) was present in the model with partial coverage over a length of 7 mm, compared to a length of 2 mm in the model without renal coverage. Average renal backflow area percentage in the renal artery incrementally increased from control (0.9%) to the stented model without (6.4%) and with renal coverage (18.8%). Conclusion: In this flow model partial renal coverage after EVAR causes low and marked oscillations in wall shear stress, potentially promoting atherosclerosis and subsequent renal artery stenosis. Awareness of the device-dependent offset between the fabric edge and the radiopaque markers is therefore important in endovascular practice.

Tables

Table 1. Table 1 : Distance between proximal marker center and edge of stentgraft

o@ X[1] X[-1] X[2 c] @   Device	Manufacturer	Marker distance (mm)
Endurant	Medtronic	$1\text{–}1.5$
Talent	Medtronic	0
Excluder	Gore	$1.2\text{–}1.7$
Zenith	Cook
Anaconda	Vascutek	0

Table 2. Table 2 : Calculation of hemodynamic parameters that are associated with atherosclerosis

o@ X[-1] X[2] X[1$c]@$	Description	Equation
${\tau }_w$	WSS is the product of dynamic viscosity times the derivative of the parallel wall velocity in the direction perpendicular to the wall	μ∂vi∂xj
TAWSS	Average WSS magnitude over one heart cycle	1T∫_0^T — τ_w — d t
OSI	Index WSS alternation between negative and positive shear (0 for no oscillation up to 0.5 for maximal oscillation)	12 ( 1- ∫0T— τw— d t —∫0Tτwd t — )
RRT	Theoretical relative residence time of blood solutes and particles near the wall	11-2 OSI) — TAWSS —

Full text

Methods In-vitro models of an abdominal aortic aneurysm were used to visualize pulsatile flow using 2D particle image velocimetry under physiologic resting conditions. One model served as control and two models were stented with an Endurant endograft, one without and one with partial renal artery coverage with 1.3 mm of stent fabric extending beyond the marker (16% area coverage). The magnitude and oscillation of wall shear stress, relative residence time and backflow in the renal artery were analyzed.

Results In both stented models, a region along the caudal renal artery wall presented with low and oscillating wall shear stress, not present in the control model. A region with very low wall shear stress ($<0.1\text{\,}\mathrm{P}\mathrm{a}$) was present in the model with partial coverage over a length of 7 mm, compared to a length of 2 mm in the model without renal coverage. Average renal backflow area percentage in the renal artery incrementally increased from control (0.9%) to the stented model without (6.4%) and with renal coverage (18.8%).

Conclusion In this flow model partial renal coverage after EVAR causes low and marked oscillations in wall shear stress, potentially promoting atherosclerosis and subsequent renal artery stenosis. Awareness of the device-dependent offset between the fabric edge and the radiopaque markers is therefore important in endovascular practice. (J Vasc Surg 2018;67:1585–1594)

Clinical Relevance The location of the proximal markers of EVAR devices is not always at the most proximal site of the graft material and as such part of the coverage material may be placed over the orifice of the renal artery. A recent analysis showed an incidence of inadvertent partial coverage of 28%. The association of a slight renal artery coverage with adverse flow patterns in this study stresses the importance of avoiding any renal artery coverage during deployment of the main body. The results further act to emphasize the importance of optimal C-arm adjustment to obtain a perpendicular projection of the lowermost renal artery.

††journal: Journal of Vascular Surgery

1 Introduction

Advantages of endovascular aneurysm repair (EVAR) over conventional open surgical abdominal aortic aneurysm (AAA) repair include a lower 30-day mortality rate (1.7% vs. 4.2%) and a shorter recovery time (3-10 vs. 7-16 days). A higher re-interventation rate is the most important drawback of EVAR.1 Although EVAR is associated with a lower risk of acute renal failure compared to open surgery (adjusted odds ratio 0.42, 95% confidence interval [0.33 to 0.53] ),2 both treatments have similar renal complication rates in the mid-term.1 Acute kidney injury or hemodialysis requirement occurred in 3.3% patients after EVAR in a retrospective analysis,3 associated with a more than 20% decrease in 5-year survival, also when adjusted for differences in pre-operative eGFR. The mechanisms by which EVAR influences renal function are probably multifactorial. Thromboembolization, MRI and CT contrast media usage, stent fixation4, 5 and ischemic reperfusion injury are potential factors.6 In addition, unintended partial coverage of a renal ostium with covered stent material may occur,7 both as a result of an unintended overly cranial release of the endograft and also as a result of inaccurate positioning of the radiopaque marker on the endograft. In many endografts the covered stent material extends about 1 mm beyond their proximal radiopaque markers and positioning the marker flush below the renal artery will lead to partial coverage by the graft material above the marker (Fig. 1). The position of the radiopaque marker is not mentioned by some manufacturers in their instructions for use (IFU) and the exact distance to covered material may therefore be unknown to interventionalists. The extent to which the renal orifice is blocked varies in a range of 0–2 mm for the different endografts (Table I) and may not always be appreciated on completion angiography.7 In a recent postoperative CT analysis of EVAR procedures even 28% of all endograft placements were reported to partially cover the renal artery.8 Partial coverage of the renal orifice may induce unfavorable hemodynamics such as regions of low and oscillatory flow. Low levels of wall shear stress (WSS $\ll 1\text{\,}\mathrm{P}\mathrm{a}$), in addition to alternating flow directions throughout the cardiac cycle quantified by an oscillatory shear index (OSI),9 have been associated with early atherosclerosis.10, 11, 12 Based on atherosclerotic pathophysiology, WSS and OSI can be combined into a single risk factor, the relative residence time (RRT).13, 14 Furthermore, flow separation and recirculating flow with an associated region of reversed flow have been suggested as pro-atherogenic12, 15 and can impede inflow at arterial branches.16 The present in-vitro study investigates the effect of partial renal artery coverage by graft material on renal flow patterns and WSS in the renal artery.