["The Journal of Physiology, Volume 604, Issue 13, Page 5342-5354, 1 July 2026. ", "\nAbstract figure legend Stasis before and after left atrial appendage occlusion (LAAO) in participants with high stasis and low stasis. Stasis is predominantly located in LAA and reduced after LAAO. But in the participant with high stasis, stasis remains close to the occlusion site, indicating a potential risk for device related thrombi.\n\n\n\n\n\n\n\n\n\nAbstract\nLeft atrial appendage occlusion (LAAO) is an emerging treatment option for cardioembolic stroke risk reduction in patients with sustained or paroxysmal atrial fibrillation (AF). How LAAO affects the atrial blood flow field during sinus rhythm has not yet been defined. Here, we simulated virtual LAAO in 21 paroxysmal AF patients and eight controls in normal sinus rhythm using computational fluid dynamics (CFD). We found that in AF patients, LAAO reduces the amount of blood that remains in the LA for more than three cardiac cycles to levels similar to the control group with intact LAA. However, there is a large variation among the AF group and even after LAAO the patients with highest stasis still had a relatively large amount of blood remaining in the LA over multiple cycles. This remaining blood is predominately located close to the site of LAA occlusion, a region where occlusion device related thrombi are known to occur. Accurate patient stratification is important to identify the impacts of LAAO on the patient specific haemodynamics.\n\n\n\n\n\n\n\n\n\nKey points\n\nPatients with atrial fibrillation (AF) have an increased risk for stroke. One underlying mechanism for this is that thrombi form in the left atrial appendage (LAA).\nTo reduce the risk of thrombi forming in the LAA, it can be closed with an occlusion device.\nIn this study, we simulated how the blood flows in the left atrium of AF patients before and after a virtual LAA occlusion and compared it to a control group.\nWe found that LAA occlusion could reduce the time blood stays in the left atrium in most patients with AF to similar levels as the control group. But in some patients, blood stagnated for a long time even after LAA occlusion.\nOur results help us understand why thrombi can form even after LAA occlusion and might help to predict which patients could benefit most from LAA occlusion.\n\n\n"]