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|Year : 2016 | Volume
| Issue : 7 | Page : 98-100
Computing the difference between life and death: Prerupture blood flow analysis of a fatal aneurysm bleed
BJ Sudhir1, JB Reddy2, Girish Menon1, T Jayachandran2
1 Department of Neurosurgery, Sree Chitra Tirunal Institute for Medical Sciences and Technology, Trivandrum, Kerala, India
2 Fluid Mechanics and Thermal Analysis Division, Vikram Sarabhai Space Centre, Trivandrum, Kerala, India
|Date of Web Publication||3-Mar-2016|
B J Sudhir
Department of Neurosurgery, Sree Chitra Tirunal Institute for Medical Sciences and Technology, Trivandrum - 695 011, Kerala
Source of Support: None, Conflict of Interest: None
Although hemodynamics plays a key role in the genesis, expansion, and rupture of an aneurysm, quantified hemodynamic parameters for comparison have not been standardized for predicting the risk of rupture of intracranial aneurysms. Computational fluid dynamics is being increasingly used in near-realistic, patient-specific simulation of blood flow in intracranial aneurysms. A simulation was carried out on the computed tomography (CT) angiogram image of a patient harboring a giant internal carotid artery aneurysm. Since the CT angiogram was performed a few hours before the fatal rupture of the aneurysm, the study could give an insight into the hemodynamics of the aneurysm that tipped it to rupture. Wall shear stress, pressure distribution, and flow streamlines were obtained using computational methods. These objective results could form the basis of reference for future studies employing simulation techniques for predicting aneurysmal rupture.
Keywords: Aneurysm; computational fluid dynamics; hemodynamics
|How to cite this article:|
Sudhir B J, Reddy J B, Menon G, Jayachandran T. Computing the difference between life and death: Prerupture blood flow analysis of a fatal aneurysm bleed. Neurol India 2016;64, Suppl S1:98-100
|How to cite this URL:|
Sudhir B J, Reddy J B, Menon G, Jayachandran T. Computing the difference between life and death: Prerupture blood flow analysis of a fatal aneurysm bleed. Neurol India [serial online] 2016 [cited 2019 Jun 27];64, Suppl S1:98-100. Available from: http://www.neurologyindia.com/text.asp?2016/64/7/98/178049
| » Introduction|| |
Hemodynamics is a key factor in the genesis, expansion, and rupture of intracranial aneurysms. A lot of research is now focused on understanding hemodynamic predictors of aneurysmal rupture. Reaching a consensus on the definite predictive determinants using computational fluid dynamics (CFD) is difficult considering the plethora of assumptions, methodologies, and input parameters required in various studies. Through the last decade, effort is being directed to shift the focus from nonspecific demographic and anatomical correlates to patient- and aneurysm-specific criteria for predicting the risk of rupture.  Research is advancing using computational methods to simulate the blood flow pattern within an aneurysm and identify objective parameters that could potentially yield aneurysm-specific criteria for predicting the aneurysm rupture risk. This study analyzes the hemodynamic patterns in a large communicating segment internal carotid artery (ICA) aneurysm, a few hours before its rupture.
| » Case Report|| |
A 65-year-old male was evaluated with a computed tomography (CT) scan for right oculomotor nerve paresis. The scan revealed a hyperdense, enhancing lesion in the right parasellar location. A suspicion of an aneurysm was confirmed by a CT angiogram study. The patient had a large, wide-necked posterior communicating artery aneurysm measuring 28 mm across its maximum dimension [Figure 1]. He was slated for a priority surgery. Three hours after the CT angiogram, the patient became irritable, vomited, and had tonic posturing of his limbs. He was intubated immediately and mechanical ventilation commenced. A CT scan head showed a diffuse subarachnoid bleed with bleed into the temporal lobe as well as intraventricular bleed. The patient expired 6 h later.
|Figure 1: Computed tomography angiogram showing a large aneurysm arising from the posterior communicating segment of internal carotid artery|
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| » Discussion|| |
The CT angiogram image had been acquired using a 256-slice scanner. The images were processed and the region of interest vascular anatomy was extracted and transferred to a workstation for computational modeling and flow simulation. The output from the image processing represented the shape of the aneurysm and the connected blood vessels with vertices connected by edges. This shape was skinned by the patching technique and volume was generated by stitching the surfaces. This method gives an accurate geometric representation of the aneurysm. The blood vessels were trimmed near the ends perpendicular to the flow direction to obtain the inlet and outlets. Further, the inlet was extended by extrusion to obtain sufficient length that ensures a fully developed flow. The volume was meshed with tetrahedral elements with a near-wall resolution of 0.12 mm. This grid size was arrived at with previously conducted grid independence studies. The boundaries were identified as inlet and outlet, and, no-slip wall condition was assumed. The blood was assumed as a Newtonian fluid with a density of 1060 kg/m 3 and a viscosity of 1.326 E-3 kg/m-s, both values being obtained from literature. An inflow of blood at 0.8 m/s was applied at inlet, obtained using trans-cranial Doppler and, with an outlet pressure of 140 mmHg. A three-dimensional, incompressible, finite volume, Navier-Stokes solver with coupled formulation was used for the simulation. The domain was initialized and flow field was solved till steady state was reached. The results obtained included velocity streamlines, pressure distribution, and wall shear stress distribution.
The wall shear stress peaked at 280 dynes/cm 2 at the vicinity of the impact zone of the inflow jet. Another region of high wall shear stress to the extent of 220 dynes/cm 2 was noted in the region of the neck of the aneurysm. The rest of the aneurysmal fundus experienced a low wall shear stress. The wall shear map is shown in [Figure 2]a. Pressure peaked at 19,700 Pa at points on the aneurysm fundus, immediately upstream to the point of impact of the inflow jet. This was approximately 900 Pa higher than the outlet pressure. The pressure distribution is represented on the aneurysm geometry in [Figure 2]b. The flow streamlines plotted proportional to the flow velocity [Figure 2]c showed that the zone depicting high wall shear stress and pressure was under considerable impact of the inflow jet. There was minimal splitting of the inflow jet and insignificant direct flow was noted from the ICA into the posterior communicating artery and the distal ICA. This pounding of the aneurysm fundus by the entire inflow jet was the probable causative factor for rupture of the aneurysm. The flow, after impact swirled into a turn, reduced velocity and reverted to flow into the posterior communicating artery in addition to the onward flow into the distal ICA. Of particular interest is the significant difference in pressure between the aneurysm fundus and the outlet suggesting an impending rupture.
|Figure 2: (a) Wall shear stress map, (b) pressure distribution, and (c) flow vector plot depicted on the aneurysm. The adjoining scales indicate the maximum and minimum values obtained for the parameter, the units of measurement being dynes/cm2 for pressure and m/s for velocity|
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The maximum wall shear stress noted in this case was 288 dynes/cm 2 , which is much higher than values reported for unruptured aneurysms and surpasses values reported on ruptured aneurysms as well.  This suggests that the aneurysm was in a state of impending rupture at the time of imaging. The pressure recordings also showed a similar pattern and the two parameters could well be compounding factors. The flow pattern of blood into the aneurysm was another factor that could have tilted the balance of this aneurysm. The inflow jet into the aneurysm was such that almost the entire blood flow coming in through the ICA was directed to the fundus of the aneurysm. Sustained force acting on the fundus of the aneurysm with a "tethering" at a point just below the region of high wall shear stress could have caused the aneurysm wall to give way, resulting in rupture.
Simulation studies on intracranial aneurysms have not been widely accepted into neurointervention practice.  Apart from the various assumptions considered in the study involving computational methods, fallacies of studying blood flow in ruptured and unruptured aneurysms elicit doubts on the clinical validity of simulation studies.  Blood flow simulation studies performed on unruptured aneurysms propose several factors that could possibly determine the risk of rupture. , However, validation of these parameters is impossible in most situations as the aneurysms are secured before rupture occurs. Studies on ruptured aneurysms assume that the event of rupture has caused minimal change in the aneurysm morphology. This assumption could often be far from reality. A noncatastrophic bleed may remodel the aneurysm whereby the shape and size of the aneurysm could be different from the prerupture characteristics.
This case along with similar case reports provides a unique opportunity to validate predictive factors that have been proposed in other numerical studies on aneurysms. Cebral et al., and Sforza et al., have reported on two similar cases with CFD analysis on angiograms performed hours before fatal rupture of basilar tip aneurysms. , The findings are similar to the case reported here. Although the two cases reported were terminal aneurysms, this case study of a side-wall aneurysm reinforces the notion that the geometrical relationship of the inlet to the fundus of the aneurysm determines how much the aneurysm is affected by the force of the inlet jet. Although the parameters studied will help in understanding the hemodynamic environment in the aneurysm in moments preceding rupture, the actual dynamics that tilted the balance at the moment of rupture have to be extrapolated.
Although hemodynamics is the linchpin of aneurysm pathophysiology, it is the factor that has been the most difficult to elucidate and quantify. Albeit with assumptions, simulation studies throw light on the hemodynamic environment prevailing in the aneurysm interior. This case provided a unique opportunity to understand the blood flow characteristics of an aneurysm just before a fatal rupture. Assimilation of data from such case reports will prove to be the best method to validate the hemodynamic parameters derived using CFD to the clinical end-point of aneurysm rupture.
The authors acknowledge the contribution of Mr. H. K. Jha, Engineer, Fluid Mechanics and Thermal Analysis Division, Vikram Sarabhai Space Centre, and Dr. C. Kesavadas, Professor, Imaging Sciences and Interventional Radiology, Sree Chitra Tirunal Institute for Medical Sciences and Technology, Trivandrum, in the study.
Financial support and sponsorship
A grant provided by the Kerala State Council for Science, Technology and Environment for the project titled "Haemodynamic Imaging of Intracranial Aneurysms;" no. 017/SRSHS/2011/CSTE, is gratefully acknowledged.
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2]