  The basic mechanism driving aneurysm growth is unknown. Currently, clinical
diagnosis of an aneurysm is mainly informed by retrospective tracking of its
size and growth rate. However, aneurysms can rupture before reactive criteria
are met or remain stable when they are exceeded. Here, we identify a
fluid-structure instability that is associated with abnormal aortic dilatation.
Our analysis yields a measurable dimensionless number and its analytically
derived critical threshold. This threshold pinpoints the transition from stable
flow to unstable aortic fluttering as a function of the physiological
properties composing the dimensionless number, like blood pressure and aortic
compliance. A retrospective study was then conducted with 4D-flow MRI data from
117 patients indicated for cardiac imaging and 100 healthy volunteers recruited
prospectively. The difference between the dimensionless number and its critical
threshold was calculated for every subject from their earliest MRI data and
used as an aneurysm physiomarker to forecast future growth. As a binary
predictor for abnormal growth and subsequent surgical intervention reported
from follow-up imaging, the aneurysm physiomarker yielded an AUC of 0.997 in a
receiving operator characteristic analysis. Though validated here for thoracic
ascending aortic aneurysms, this instability mechanism may be used to
understand, predict and inform patient-specific treatment of aneurysms in any
location without fundamental differences.
