Modelling blood flow in the circle of Willis in continuous flow left ventricular assist devices: possible relevance to strokes.

Purpose: Despite significant improvements in the design and performance of continuous flow left ventricular assist devices (CFLVADs), one of the most important reasons hampering further penetration of this technology is the occurrence of adverse events, especially strokes. One of the well-known risk factors for strokes is hypertension which is particularly common in patients undergoing a CFLVAD implant. While the device is implanted in the heart, strokes happen due to pathology in the brain and we hypothesised that modelling the blood flow in the circle of Willis might shed light on the causation of strokes in this situation.The aim of the study was two-fold:1. What is the reason for hypertension in CFLVADs? Are there physical factors at play, besides neurohumoral mechanisms?2. Do anatomical factors in the circle of Willis play a role in the causation of strokes in these patients?

Methods: The circle of Willis is often incomplete and has a number of anatomical variations, the commonest being the absence of the posterior communicating artery. Hypertension is common after CFLVAD implantation and is also a well-known risk factor for strokes. We examined the blood pressure in the cerebral circulation with pulsatile and non-pulsatile flow for identical conditions and the effect of the absence of the posterior communicating artery on regional cerebral blood flow and pressure. One-dimensional blood flow model was used, taking into account wave propagation and reflections and physiological data obtained from anatomically detailed arterial network (ADAN86) which has data from 86 arteries including detailed cerebral network.

Results: The mean arterial pressure was significantly higher in the non-pulsatile blood flow of CFLVADs compared to pulsatile flow, for identical conditions, across all arteries. With increasing imparted pulsatility to CFLVAD flow, the mean arterial pressure progressively decreased. Isolated absence of the posterior communicating artery had no effect on the flow as well as pressure in the middle cerebral artery. However, when combined with the absence of flow in the ipsilateral carotid artery, the flow as well as the pressure decreased very significantly in both continuous and pulsatile flow situations.

Conclusions: Physiologically significant pulsatility in CFLVADs can have important clinical advantages in lowering of blood pressure which can lead to lower incidence of strokes, pump thrombosis, gastrointestinal (GI) bleeds, and aortic incompetence. Patient-specific anatomical variations in the circle of Willis, especially the absence of the posterior communicating artery, can have important consequences in regional cerebral perfusion under some circumstances.

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