| 1. |
- Engquist, Björn, et al.
(författare)
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Analysis of HMM for One Dimensional Wave Propagation Problems Over Long Time
- 2011
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Tidskriftsartikel (refereegranskat)abstract
- Multiscale problems are computationally costly to solve by direct simulation because the smallest scales must be represented over a domain determined by the largest scales of the problem. We have developed and analyzed new numerical methods for multiscale wave propagation following the framework of the heterogeneous multiscale method. The numerical methods couple simulations on macro- and microscales for problems with rapidly fluctuating material coefficients. The computational complexity of the new method is significantly lower than that of traditional techniques. We focus on HMM approximation applied to long time integration of one-dimensional wave propagation problems in both periodic and non-periodic medium and show that the dispersive effect that appear after long time is fully captured.
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| 2. |
- Engquist, Björn, et al.
(författare)
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Multi-scale methods for wave propagation in heterogeneous media
- 2011
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Ingår i: Communications in Mathematical Sciences. - 1539-6746 .- 1945-0796. ; 9:1, s. 33-56
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Tidskriftsartikel (refereegranskat)abstract
- Multi-scale wave propagation problems are computationally costly to solve by traditional techniques because the smallest scales must be represented over a domain determined by the largest scales of the problem. We have developed and analyzed new numerical methods for multi-scale wave propagation in the framework of heterogeneous multi-scale method. The numerical methods couple simulations on macro-and micro-scales for problems with rapidly oscillating coefficients. We show that the complexity of the new method is significantly lower than that of traditional techniques with a computational cost that is essentially independent of the micro-scale. A convergence proof is given and numerical results are presented for periodic problems in one, two, and three dimensions. The method is also successfully applied to non-periodic problems and for long time integration where dispersive effects occur.
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| 3. |
- Engquist, Björn, et al.
(författare)
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Multiscale Methods for One Dimensional Wave Propagation with High Frequency Initial Data
- 2011
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- High frequency wave propagation problems are computationally costly to solve by traditional techniques because the short wavelength must be well represented over a domain determined by the largest scales of the problem. We have developed and analyzed a new numerical method for high frequency wave propagation in the framework of heterogeneous multiscale methods, closely related to the analytical method of geometrical optics. The numerical method couples simulations on macro- and micro-scales for problems with highly oscillatory initial data. The method has a computational complexity essentially independent of the wavelength. We give one numerical example with a sharp but regular jump in velocity on the microscopic scale for which geometrical optics fails but our HMM gives correct results. We briefly discuss how the method can be extended to higher dimensional problems.
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| 4. |
- Engquist, Björn, et al.
(författare)
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Multiscale methods for the wave equation
- 2007
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Ingår i: PAMM · Proc. Appl. Math. Mech. 7. - : Wiley. ; , s. 1140903-1140904
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- We consider the wave equation in a medium with a rapidly varying speed of propagation. We construct a multiscale schemebased on the heterogeneous multiscale method, which can compute the correct coarse behavior of wave pulses traveling in themedium, at a computational cost essentially independent of the size of the small scale variations. This is verified by theoreticalresults and numerical examples.
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| 5. |
- Engquist, Björn, et al.
(författare)
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Multiscale Methods for Wave Propagation in Heterogeneous Media Over Long Time
- 2012
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Ingår i: Numerical Analysis of Multiscale Computations. - Berlin, Heidelberg : Springer Verlag. - 9783642219429 ; , s. 167-186
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Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
- Multiscale wave propagation problems are computationally costly to solve by traditional techniques because the smallest scales must be represented over a domain determined by the largest scales of the problem. We have developed and analyzed new numerical methods for multiscale wave propagation in the framework of the heterogeneous multiscale method (HMM). The numerical methods couple simulations on macro- and microscales for problems with rapidly oscillating coefficients. The complexity of the new method is significantly lower than that of traditional techniques with a computational cost that is essentially independent of the smallest scale, when computing solutions at a fixed time and accuracy. We show numerical examples of the HMM applied to long time integration of wave propagation problems in both periodic and non-periodic medium. In both cases our HMM accurately captures the dispersive effects that occur. We also give a stability proof for the HMM, when it is applied to long time wave propagation problems.
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| 6. |
- Engquist, Henrik, et al.
(författare)
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CBF changes and cerebral energy metabolism during hypervolemia, hemodilution, and hypertension therapy in patients with poor-grade subarachnoid hemorrhage
- 2021
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Ingår i: Journal of Neurosurgery. - : Journal of Neurosurgery Publishing Group (JNSPG). - 0022-3085 .- 1933-0693. ; 134:2, s. 555-564
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Tidskriftsartikel (refereegranskat)abstract
- OBJECTIVE: Despite the multifactorial pathogenesis of delayed cerebral ischemia (DCI) after subarachnoid hemorrhage (SAH), augmentation of cerebral blood flow (CBF) is still considered essential in the clinical management of DCI. The aim of this prospective observational study was to investigate cerebral metabolic changes in relation to CBF during therapeutic hypervolemia, hemodilution, and hypertension (HHH) therapy in poor-grade SAH patients with DCI.METHODS: CBF was assessed by bedside xenon-enhanced CT at days 0–3, 4–7, and 8–12, and the cerebral metabolic state by cerebral microdialysis (CMD), analyzing glucose, lactate, pyruvate, and glutamate hourly. At clinical suspicion of DCI, HHH therapy was instituted for 5 days. CBF measurements and CMD data at baseline and during HHH therapy were required for study inclusion. Non-DCI patients with measurements in corresponding time windows were included as a reference group.RESULTS: In DCI patients receiving HHH therapy (n = 12), global cortical CBF increased from 30.4 ml/100 g/min (IQR 25.1–33.8 ml/100 g/min) to 38.4 ml/100 g/min (IQR 34.2–46.1 ml/100 g/min; p = 0.006). The energy metabolic CMD parameters stayed statistically unchanged with a lactate/pyruvate (L/P) ratio of 26.9 (IQR 22.9–48.5) at baseline and 31.6 (IQR 22.4–35.7) during HHH. Categorized by energy metabolic patterns during HHH, no patient had severe ischemia, 8 showed derangement corresponding to mitochondrial dysfunction, and 4 were normal. The reference group of non-DCI patients (n = 11) had higher CBF and lower L/P ratios at baseline with no change over time, and the metabolic pattern was normal in all these patients.CONCLUSIONS: Global and regional CBF improved and the cerebral energy metabolic CMD parameters stayed statistically unchanged during HHH therapy in DCI patients. None of the patients developed metabolic signs of severe ischemia, but a disturbed energy metabolic pattern was a common occurrence, possibly explained by mitochondrial dysfunction despite improved microcirculation.
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| 7. |
- Engquist, Henrik, 1964-
(författare)
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Clinical Bedside Studies of Cerebral Blood Flow in Severe Subarachnoid Hemorrhage Using Xenon CT
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Aneurysmal subarachnoid hemorrhage (SAH) is frequently complicated by delayed cerebral ischemia (DCI), contributing to poor outcome. Particularly for patients in poor neurological state, prediction of the acute clinical course is difficult, as is the early detection of DCI. Repeated measurement of global and regional cerebral blood flow (CBF) could potentially identify patients at risk of deterioration and guide in the clinical management.The studies in this thesis are based on bedside measurements of CBF by xenon-enhanced CT with the aim to assess and characterize global and regional CBF disturbances at different phases in the acute course after severe SAH. Furthermore, the effects of hemodynamic augmentation by hypervolemia, hemodilution and hypertension (HHH-therapy) on CBF and cerebral energy metabolism in patients with DCI are addressed.In Paper I, CBF disturbances at the early phase (day 0–3) after SAH were found common and often heterogeneous with substantial regions of near ischemic CBF. Older age and more severe hemorrhage (graded according to Fisher from CT) were factors associated with more compromised CBF. In Paper II, exploring the temporal dynamics of CBF, low initial CBF was associated with a persistent low level of CBF at day 4–7. The association was more pronounced when patients receiving HHH-therapy were separated, and indicates that patients with low CBF, even without clinical signs of DCI, could benefit from careful surveillance and optimization of circulation. In Paper III, the effects on CBF from HHH-therapy in patients with DCI was assessed. Hematocrit decreased during treatment, while the increase in systemic blood pressure was modest. Global CBF and CBF of the worst perfused regions increased, and the proportion of regions with critically low flow decreased accordingly. In Paper IV, the effects of HHH was further assessed in patients also monitored with cerebral microdialysis (CMD). CBF improved during HHH-therapy, while the cerebral energy metabolic CMD parameters stayed statistically unchanged. None of the patients developed metabolic signs of severe ischemia, but a disturbed energy metabolic pattern was common, possibly explained by mitochondrial dysfunction.
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| 8. |
- Engquist, Henrik, et al.
(författare)
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Effect of HHH-Therapy on Regional CBF after Severe Subarachnoid Hemorrhage Studied by Bedside Xenon-Enhanced CT
- 2018
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Ingår i: Neurocritical Care. - : Springer Science and Business Media LLC. - 1541-6933 .- 1556-0961. ; 28:2, s. 143-151
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Tidskriftsartikel (refereegranskat)abstract
- BACKGROUND:Management of delayed cerebral ischemia (DCI) following subarachnoid hemorrhage (SAH) is difficult and still carries controversies. In this study, the effect of therapeutic hypervolemia, hemodilution, and hypertension (HHH-therapy) on cerebral blood flow (CBF) was assessed by xenon-enhanced computerized tomography (XeCT) hypothesizing an increase in CBF in poorly perfused regions.METHODS:Bedside XeCT measurements of regional CBF in mechanically ventilated SAH patients were routinely scheduled for day 0-3, 4-7, and 8-12. At clinical suspicion of DCI, patients received 5-day HHH-therapy. For inclusion, XeCT was required at 0-48 h before start of HHH (baseline) and during therapy. Data from corresponding time-windows were also collected for non-DCI patients.RESULTS:Twenty patients who later developed DCI were included, and twenty-eight patients without DCI were identified for comparison. During HHH, there was a slight nonsignificant increase in systolic blood pressure (SBP) and a significant reduction in hematocrit. Median global cortical CBF for the DCI group increased from 29.5 (IQR 24.6-33.9) to 38.4 (IQR 27.0-41.2) ml/100 g/min (P = 0.001). There was a concomitant increase in regional CBF of the worst vascular territories, and the proportion of area with blood flow below 20 ml/100 g/min was significantly reduced. Non-DCI patients showed higher CBF at baseline, and no significant change over time.CONCLUSIONS:HHH-therapy appeared to increase global and regional CBF in DCI patients. The increase in SBP was small, while the decrease in hematocrit was more pronounced, which may suggest that intravascular volume status and rheological effects are of importance. XeCT may be potentially helpful in managing poor-grade SAH patients.
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| 9. |
- Engquist, Henrik, et al.
(författare)
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Hemodynamic Disturbances in the Early Phase After Subarachnoid Hemorrhage : Regional Cerebral Blood Flow Studied by Bedside Xenon-enhanced CT.
- 2018
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Ingår i: Journal of Neurosurgical Anesthesiology. - 0898-4921 .- 1537-1921. ; 30:1, s. 49-58
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Tidskriftsartikel (refereegranskat)abstract
- BACKGROUND: The mechanisms leading to neurological deterioration and the devastating course of delayed cerebral ischemia (DCI) after subarachnoid hemorrhage (SAH) are still not well understood. Bedside xenon-enhanced computerized tomography (XeCT) enables measurements of regional cerebral blood flow (rCBF) during neurosurgical intensive care. In the present study, CBF characteristics in the early phase after severe SAH were explored and related to clinical characteristics and early clinical course outcome.MATERIALS AND METHODS: Patients diagnosed with SAH and requiring mechanical ventilation were prospectively enrolled in the study. Bedside XeCT was performed within day 0 to 3.RESULTS: Data from 64 patients were obtained. Median global CBF was 34.9 mL/100 g/min (interquartile range [IQR], 26.7 to 41.6). There was a difference in CBF related to age with higher global CBF in the younger patients (30 to 49 y). CBF was also related to the severity of SAH with lower CBF in Fisher grade 4 compared with grade 3. rCBF disturbances and hypoperfusion were common; in 43 of the 64 patients rCBF<20 mL/100 g/min was detected in more than 10% of the region-of-interest (ROI) area and in 17 patients such low-flow area exceeded 30%. rCBF was not related to the localization of the aneurysm; there was no difference in rCBF of ipsilateral compared with contralateral vascular territories. In patients who initially were in Hunt & Hess grade I to III, median global CBF day 0 to 3 was significantly lower for patients who were in poor neurological state at discharge compared with patients in good neurological state, 25.5 mL/100 g/min (IQR, 21.3 to 28.3) versus 37.8 mL/100 g/min (IQR, 30.5 to 47.6).CONCLUSIONS: CBF disturbances are common in the early phase after SAH. In many patients, CBF was heterogenic and substantial areas with low rCBF were detected. Age and CT Fisher grade were factors influencing global cortical CBF. Bedside XeCT may be a tool to identify patients at risk of deteriorating so they can receive intensified management, but this needs further exploration.
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| 10. |
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