Discussion
Post-operative renal artery stenosis and renal dysfunction have attracted more and more attentions.[22] The causes for these complications remain unknown, and studies have been conducted to investigate the relationship between EVAR and the resultant complications. A few studies have focused on the altered hemodynamic environment in the renal artery caused by stent-graft intervention. A few possible factors have been proposed, such as a fenestrated vessel stent placement [13] and the presence of suprarenal stent wires [14]. However, these studies demonstrated that EVAR had a less significant hemodynamic effect on the renal arteries than predicted.
In this study, we investigated lower limb exercise as another potential factor for renal complications after EVAR. This investigation was inspired by previous studies that were evaluating hemodynamic changes in AAA [15,23]. We reconstructed pre-operative and post-operative models of AAA under rest and lower limb exercise conditions. Between these two conditions, there was an apparent difference in the division of blood flow to the renal arteries and the abdominal aorta. A higher blood perfusion rate to the iliac arteries through the abdominal aorta was observed under the lower limb exercise condition. This indicated that the flow rate in the renal arteries significantly decreased under the exercise conditions. The flow rate change directly induced a decrease in the WSS in the two renal arteries. Meanwhile, the OSI increased in the renal arteries, which indicated a more oscillatory flow profile. The RRT also increased in the renal arteries, which increased the contact and adherence times for lipids or emboli to interact with the renal artery wall.
Because low WSSs, high OSIs and high RRTs are all indicators of arterial stenosis and atherosclerosis [24], lower limb exercises would favor a hemodynamic environment in the renal arteries that promoted these conditions. Because the onset of AAA traditionally has strong clinical associations with aging, smoking, and atherosclerosis, [25–27], AAA patients have increased risks for atherosclerotic events. Therefore, the authors speculated that lower limb exercise would increase the risk of renal artery stenosis and atherosclerosis in AAA patients. To valid this speculation, more experiments and clinical evaluations should be conducted in the future.
When the results of the pre-operative and post-operative models were compared, we found that EVAR reduced the WSS in the renal arteries under rest and exercise conditions. EVAR also slightly increased the OSI and the RRT in the post-operative model under rest and exercise conditions. The stent-graft intervention used in EVAR seemed to further deteriorate the hemodynamic environment of the renal artery.
Ga-Young Suh suggested that mild lower limb exercise may be sufficient to reduce the oscillatory and stagnant hemodynamic conditions in AAA [23]. However, the present study illustrated that lower limb exercise could worsen the renal artery hemodynamic environment, especially after EVAR, and increase the risk of renal artery stenosis. This study could help elucidate the mechanism of renal artery complications after EVAR and additionally, reevaluate the recovery care methods for AAA patients.
The morphology of an AAA varies from patient to patient. Thus, the hemodynamic environment also varies. The aorta artery model used in this study has a prominent curve at the neck of the aneurysm, and the two renal arteries are asymmetrical. The asymmetric geometry induced very different hemodynamic characteristics in the two renal arteries. This phenomenon could be directly embodied in the results: the velocity and the WSS of the right renal artery was greater than those in the left renal artery, and the RRT of the right renal artery was lower than that in the left renal artery. More patient-specific model reconstructions are necessary to determine a generalized model. However, for the purpose of clinical use, individualized analyses would be essential.