Low shear stress at baseline predicts expansion and aneurysm-related events in patients with abdominal aortic aneurysm

Background
Low shear stress has been implicated in abdominal aortic aneurysm (AAA) expansion and clinical events. We tested the hypothesis that low shear stress in AAA at baseline is a marker of expansion rate and future aneurysm-related events.
Methods
Patients were imaged with computed tomography angiography (CTA) at baseline and followed up every six months >24 months with ultrasound measurements of maximum diameter. From baseline CTA, we reconstructed three dimensional models for automated computational fluid dynamics simulations and computed luminal shear stress. The primary composite endpoint was aneurysm repair and/or rupture, and the secondary endpoint was aneurysm expansion rate.
Results
We included 295 patients with median AAA diameter of 49mm (IQR 43-54mm) and median follow-up of 914 (IQR 670-1112) days. There were 114 (39%) aneurysm-related events, with 13 AAA ruptures and 98 repairs (one rupture was repaired). Patients with low shear stress (<0.4 Pa) experienced a higher number of aneurysm-related events (44%) compared to medium (0.4-0.6 Pa; 27%) and high (>0.6 Pa; 29%) shear stress groups (p=0.010). This association was independent of known risk factors (adjusted HR 1.72; 95% CI [1.08, 2.73]; p=0.023). Low shear stress was also independently associated with AAA expansion rate (β=+0.28mm/y; 95% CI [0.02, 0.53]; p=0.037).
Conclusions
We show for the first time that low shear stress (<0.4 Pa) at baseline is associated with both AAA expansion and future aneurysm-related events. Aneurysms within the lowest tertile of shear stress, versus those with higher shear stress, were more likely to rupture or reach thresholds for elective repair. Larger prospective validation trials are needed to confirm these findings and translate them into clinical management.