Interplay between cross-clamping and cannular flow during cardiopulmonary bypass assessed by computational fluid-structure interaction

Objectives: The risk of peri-operative neurological complications due to cardiopulmonary bypass surgery varies between 2–13% [1]. The prevalence of emboli, as major contributor to these complications, are especially high during aortic manipulation [2], e.g. cannulation, cross-clamping, cannular flow etc. By simulating the fluid-structure interaction (FSI) using a patient-specific Finite Element (FE) model, we investigated the increased danger of emboli dislodgement caused by cannular flow during the release of the cross-clamps.

Methods: One patient-specific ascending aorta geometry (male, 62yrs.) was segmented from CT images, to which a cross-clamp and straight-tipped cannula (24Fr, Maquet, DE) model were added. Since the in vivo geometry obtained during CT acquisition was not stress free, the wall stress-state present at time of imaging was computationally recovered [3].

The sequence for the FSI simulation was: recovering of in vivo wall stresses (prestressing), ramping up cannular flow and cross-clamping the artery to a 60% closure (semi-occluded). Finally, the cross-clamps were rotated around the arterial centerline axis of the artery by 90°.

Results: Considering a wall shear stress (WSS) threshold of 40Pa, above which endothelial erosion occurs [4], WSS of up to 10x this threshold was experienced by the arterial wall opposite the cannulation site, and 2x this threshold at the clamping site. Furthermore, when the clamps and cannula were parallel to each other, the semi-occluded vessel geometry directed more high velocity cannular flow over the pinching site (due to clamping) compared to a 90° difference in relative orientation.

Conclusions: The site where the cannula jet first hits the arterial wall for the considered cannula has a very high obvious risk for embologenesis. Furthermore, the orientation and distance between the cross-clamps and the cannula influences the risk of embologenesis, especially at region most likely injured during clamping: the pinching site. These results could help to improve clinical protocol in reducing neurological complications.

References:

[1] Kellermann K, Jungwirth B, Semin Cardiothorac Vasc Anesth, 14(2):95–101, 2010

[2] Stump DA, Jones TJJ, Rorie KD, J Cardiothorac Vasc Anesth, 13(5):600–613, 1999

[3] Gee MW, Förster C, Wall WA, Int J Numer Methods Biomed Eng, 26(1):52–72, 2010

[4] Fry DL, Ciba Foundation Symposium 12– Atherogenesis: Initiating Factors, pp.93–125, John Wiley&Sons Ltd, 2008