Increase In Mean Flow Velocity Research Articles

Comparison of a thickness-tapered channel in flow field-flow fractionation with a conventional channel with flow rate programming

The thickness-tapered channel structure in flow field-flow fractionation (FlFFF), recently introduced by constructing a channel with a linear decrease in thickness along its length, demonstrated effectiveness in steric/hyperlayer separation of supramicron particles with improvements in separation speed, elution recovery, and an expanded dynamic size range of separation. In this study, we conducted a comparative analysis of the performance between the impact of field (or crossflow rate) programming or outflow rate programming for the separation of polystyrene latex standards (50 ∼ 800 nm) with a conventional channel having uniform thickness and a thickness-tapered channel without programming. Outlet flow rate and crossflow rate conditions were also varied. Although the particle size resolution of the tapered channel does not surpass that of field programming in uniform thickness channel, it achieves higher-speed separation without a significant loss of resolution and without the need for a complex flow controller system even at a low outflow rate condition. Furthermore, it yielded an improved resolution for particles close to the steric transition regime (400 ∼ 600 nm) in the normal mode of separation. Due to the continuous increase in mean flow velocity down the channel, the tapered channel exhibits flexibility in separating submicron-sized particles at high crossflow rate conditions or low outflow rate conditions, of which the latter can be advantageous when coupled with mass spectrometry in a miniaturized setup.

Cold stimulation of the oral cavity redistributes blood towards the brain in healthy volunteers.

The aim of this study was to analyze cold stimulation-induced changes in cerebral and cardiac hemodynamics. Upon ingestion of an ice cube, the changes in resistance index, mean flow velocity and flow index of the middle cerebral arteries (MCA) were assessed using transcranial Doppler sonography. Extracranial duplex sonography was used to measure the mean flow velocity and resistance index of the right internal carotid artery (ICA). The change in mean arterial pressure, heart rate, root mean square of successive differences (RMSSD) and end-tidal carbon dioxide pressure were analyzed additionally. These changes were compared to sham stimulation. Compared with sham stimulation, cooling of the oral cavity resulted in significant changes in cerebral and cardiac hemodynamics. The cold stimulation decreased the resistance index in the MCA (-4.5% ± 5.4%, p < 0.0001) and right ICA (-6.3% ± 15.6%, p = 0.001). This was accompanied by an increase in mean flow velocity (4.1% ± 8.0%, p < 0.0001) and flow index (10.1% ± 43.6%, p = 0.008) in the MCA. The cardiac effects caused an increase in mean arterial pressure (1.8% ± 11.2%, p = 0.017) and RMSSD (55% ± 112%, p = 0.048), while simultaneously decreasing the heart rate (-4.3% ± 9.6%, p = 0.0001). Cooling of the oral cavity resulted in substantial changes in cerebral and cardiac hemodynamics resulting in a blood flow diversion to the brain.

Experimental study on the spark ignition characteristics at elevated temperature and pressure under flow conditions

This study investigates the fundamental spark-ignition characteristics under engine-relevant environments, focusing on spark discharge energy and the shapes of spark channels. The pressure, temperature, and mean flow velocity conditions were independently controlled (up to 20 bar, 567 K, and 18 m/s, respectively) to study the individual effects on spark channels. In terms of analyzing spark discharge energy, the breakdown and arc/glow phases were individually examined. Results showed that the breakdown voltage and energy are predominantly affected by ambient density, which is consistent with Paschen's law. During the arc/glow phases, the total discharge energy increased with an increase in mean flow velocity, whereas relatively minor effects on the total discharge energy were observed with pressure and temperature variances. In terms of the shapes of spark channels, it was found that the shapes vary significantly under different thermodynamic and flow conditions. Specifically, ambient pressure and spark channel stretching by flow had considerable effects on the shapes of spark channels, such as the thickness of spark channels and the extent of corrugation. This study also proposes a mechanism for forming short circuits, which explicates the spark behaviors under flow conditions, based on the prior analyses of spark characteristics. In the process of examining the mechanism, the formulae for the required voltage for short circuits and the resistance of spark channels are additionally suggested.

Transcranial Doppler ultrasonography in tuberculous meningitis: A systematic review.

Tuberculous (TB) meningitis is a common type of central nervous system infection, and may cause multifocal cerebral infarctions due to the involvement of cerebral vasculature. This systematic review aims to review and synthesize the utility of transcranial Doppler (TCD) in TB meningitis. This is a systematic review of observational studies on the use of TCD in patients diagnosed to have TB meningitis. Study outcomes included changes in TCD parameters during stages of TB meningitis, frequency of neurologic complications (such as hydrocephalus, vasculopathy, and cerebral infarction), correlation of TCD findings with neuroimaging, and functional outcomes. Five studies were included with a total of 141 participants, with ages from 4 months to 75 years. The most common neurologic complication was hydrocephalus (87.1%), cerebral infarction (29.2%), and arterial stenosis (26.3%). There was increased mean flow velocity (MFV) most commonly in the middle cerebral artery in the early stage of TB meningitis, and decreased MFV in the advanced disease stage. TCD findings of stenosis were well correlated with CT or MR angiogram. Among patients with hydrocephalus, pulsatility indices were significantly decreased after ventriculoperitoneal shunting. A considerable proportion of patients with TB meningitis develop cerebral hemodynamic disturbances which lead to cerebral ischemia and affect clinical outcomes. TCD is a reliable tool for the diagnosis of vasculopathy and increased intracranial pressure and can thus help monitor disease progression and treatment response. Future studies with larger populations and longer follow-ups are recommended to determine the association of TCD findings with functional outcomes.

Multiscale investigation for CO2 capture using membrane with AEEA: Significance of fluid flow and AEEA content to CO2 permeance

Since it is energy efficient, CO2 capture by a membrane is a promising technique for mitigating an increase in CO2 in the atmosphere. This study considers a membrane containing 2-(2-aminoethylamino) ethanol (AEEA) because of its higher interaction performance than that of the other amines. It was assumed that the feed gas which is a mixture of CO2/He flows in a rectangular channel, and the membrane is placed at the bottom of the channel. Diffusional mass transfer of CO2 from feed gas flowing in the channel to the surface of the membrane was investigated. Incorporating the results for CO2 interaction with AEEA by density functional theory (DFT) calculation, the presented model can describe the effects of fluid (CO2/He mixed gas) flow over the surface of the membrane and the AEEA content of the membrane on the gas transmission rate, i.e. the CO2 permeance. Theoretical results reveal that the CO2 permeance increases as the mean flow velocity increases. The CO2 permeance also depends on the AEEA content of the membrane, i.e. the CO2 permeance increases with increased AEEA content. It was also found that predicted CO2 permeance has a range from 10−8 to 10−7 m3(STP)m−2s−1Pa−1 which is much larger than those obtained in the experiments using the polyamidoamine (PAMAM) dendrimer/polymer hybrid membrane, i.e. 10−10 to 10−9 m3(STP)m−2s−1Pa−1, indicating that AEEA can capture CO2 from the CO2/He mixed gas much more efficiently than the other amines.

Cerebral Augmentation Effect Induced by External Counterpulsation Is Not Related to Impaired Dynamic Cerebral Autoregulation in Ischemic Stroke.

Background and PurposeDynamic cerebral autoregulation is impaired after ischemic stroke. External counterpulsation (ECP) augments the cerebral blood flow of patients with ischemic stroke by elevation of blood pressure (BP). We aimed to investigate if cerebral augmentation effects during ECP were associated with impaired dynamic cerebral autoregulation in patients after acute ischemic stroke.MethodsForty patients with unilateral ischemic stroke and large artery atherosclerosis in the anterior circulation territory within 7 days from symptom onset and eighteen healthy controls were recruited. We monitored changes in mean flow velocity over both middle cerebral arteries (MCA) by transcranial Doppler (TCD) before, during, and immediately after ECP. Cerebral augmentation index was MCA mean flow velocity increase in percentage during ECP compared with baseline to evaluate the augmentation effects of ECP. Spontaneous arterial BP and cerebral blood flow velocity in both bilateral MCAs were recorded using a servo-controlled plethysmograph and TCD, respectively. Transfer function analysis was used to derive the autoregulatory parameters, including phase difference (PD), and gain.ResultsThe cerebral augmentation index in patients with stroke was significantly higher on both the ipsilateral and contralateral sides than that in controls, while the PD in patients with stroke was significantly lower on both sides than those in controls (all P < 0.05). The cerebral augmentation index did not correlate with PD and gain on either the ipsilateral or contralateral side of patients with stroke or in controls (all P > 0.05). The cerebral augmentation index of patients with stroke was significantly related to mean BP change on the ipsilateral side (R2 = 0.108, P = 0.038).ConclusionThe degree of ECP-induced cerebral augmentation effects as measured by the cerebral augmentation index did not correlate with the magnitude of impaired dynamic cerebral autoregulation.

Functional transcranial Doppler: Selection of methods for statistical analysis and representation of changes in flow velocity.

IntroductionTranscranial Doppler (TCD) is a method used to study cerebral hemodynamics. In the majority of TCD studies, regression analysis and analysis of variance are the most frequently applied statistical methods. However, due to the dynamic and interdependent nature of flow velocity, nonparametric tests may allow for better statistical analysis and representation of results.MethodThe sample comprised 30 healthy participants, aged 33.87 ± 7.48 years; with 33% (n = 10) females. During a visuo‐motor task, the mean flow velocity (MFV) in the middle cerebral artery (MCA) was measured using TCD. The MFV was converted to values relative to the resting state. The results obtained were analyzed using the general linear model (GLM) and the general additional model (GAM). The fit indices of both analysis methods were compared with each other.ResultsBoth MCAs showed a steady increase in MFV during the visuo‐motor task, smoothly returning to resting state values. During the first 20 seconds of the visuo‐motor task, the MFV increased by a factor of 1.06 ± 0.07 in the right‐MCA and by a factor of 1.08 ± 0.07 in the left‐MCA. GLM and GAM showed a statistically significant change in MFV (GLM:F(2, 3598) = 16.76, P < .001; GAM:F(2, 3598) = 21.63, P < .001); together with effects of hemispheric side and gender (GLM:F(4, 3596) = 7.83, P < .005; GAM:F(4, 3596) = 2.13, P = .001). Comparing the models using the χ2 test for goodness of fit yields a significant difference χ2 (9.9556) = 0.6836, P < .001.ConclusionsBoth the GLM and GAM yielded valid statistical models of MFV in the MCA in healthy subjects. However, the model using the GAM resulted in improved fit indices. The GAM's advantage becomes even clearer when the MFV curves are visualized; yielding a more realistic approach to brain hemodynamics, thus allowing for an improvement in the interpretation of the mathematical and statistical results. Our results demonstrate the utility of the GAM for the analysis and representation of hemodynamic parameters.

Use of Transcranial Doppler as a Biomarker of CAR T Cell-Related Neurotoxicity.

To examine the relationship between transcranial Doppler (TCD) mean flow velocity (MFV) and the severity and temporal onset of neurotoxicity after chimeric antigen receptor (CAR) T-cell therapy in patients with relapsed lymphoma. We identified a cohort of 165 patients with relapsed or refractory B-cell lymphoma who received CAR T-cell therapy. TCDs were performed at baseline, treatment day 5, and throughout hospitalization based on development of neurologic symptoms. We assessed the percent change in velocity from baseline in each of the 6 major supratentorial arteries and the relationship of these values to development and timing of neurotoxicity. Our cohort was 30% female with an average age of 60 years. Of patients with TCDs performed, 63% developed neurotoxicity, and 32% had severe neurotoxicity. The median time of neurotoxicity onset was day 7. Higher maximum percent change in MFV across all vessels was significantly associated with likelihood of developing neurotoxicity (p = 0.0002) and associated with severe neurotoxicity (p = 0.0421). We found that with increased percent change in MFV, the strength of correlation between day of TCD velocity change and day of neurotoxicity onset increased. There was no single vessel in which increase in MFV was associated with neurotoxicity. Our study demonstrates an association between increase in TCD MFV and the development of neurotoxicity, as well as timing of neurotoxicity onset. We believe that TCD ultrasound may be used as a bedside functional biomarker in CAR T-cell patients and may guide immunologic interventions to manage toxicity in this complex patient group.

Influence of erodible layer on granular column collapse using discrete element analysis

ABSTRACT Landslides, debris flows and avalanches are geophysical hazards, which require sophisticated multi-scale analysis to understand the nature of the flow behaviour. This article discusses numerical investigation of the behaviour of granular flows through granular column collapse model. The column is allowed to collapse on an erodible bed of granular particles to understand the mechanism of entrainment. Discrete Element Method (DEM) is used to model the column collapse to investigate the run-out length and velocity of flow. Simulations were performed at the particulate level in this method. Particle–particle interaction in DEM helps in better understanding of the entrainment phenomenon. Run-out lengths increase with the increase in the initial column aspect ratio of the granular column with rigid bed. In this study, substantial variations were observed in the final run-out lengths in the presence of erodible bed in the model. The force of impact induces frontal ploughing giving rise to the entrainment of particles into the flowing mass and increased mean flow velocity. Column collapse over erodible bed resulted in thicker final deposits, which is evident from the increased coordination number.

Effects of Mean Inflow Velocity and Droplet Diameter on the Propagation of Turbulent V-Shaped Flames in Droplet-Laden Mixtures

Three-dimensional carrier phase Direct Numerical Simulations of V-shaped n-heptane spray flames have been performed for different initially mono-sized droplet diameters to investigate the influence of mean flow velocity on the burning rate and flame structure at different axial locations from the flame holder. The fuel is supplied as liquid droplets through the inlet and an overall (i.e., liquid + gaseous) equivalence ratio of unity is retained in the unburned gas. Additionally, turbulent premixed stoichiometric V-shaped n-heptane flames under the same turbulent flow conditions have been simulated to distinguish the differences in combustion behaviour of the pure gaseous phase premixed combustion in comparison to the corresponding behaviour in the presence of liquid n-heptane droplets. It has been found that reacting gaseous mixture burns predominantly under fuel-lean mode and the availability of having fuel-lean mixture increases with increasing mean flow velocity. The extent of flame wrinkling for droplet cases has been found to be greater than the corresponding gaseous premixed flames due to flame-droplet-interaction, which is manifested by dimples on the flame surface, and this trend strengthens with increasing droplet diameter. As the residence time of the droplets within the flame decreases with increasing mean inflow velocity, the droplets can survive for larger axial distances before the completion of their evaporation for the cases with higher mean inflow velocity and this leads to greater extents of flame-droplet interaction and droplet-induced flame wrinkling. Mean inflow velocity, droplet diameter and the axial distance affect the flame brush thickness. The flame brush thickens with increasing droplet diameter for the cases with higher mean inflow velocity due to the predominance of fuel-lean gaseous mixture within the flame. However, an opposite behaviour has been observed for the cases with lower mean inflow velocity where the smaller extent of flame wrinkling due to smaller values of integral length scale to flame thickness ratio arising from higher likelihood of fuel-lean combustion for larger droplets dominates over the thickening of the flame front. It has been found that the major part of the heat release arises due to premixed mode of combustion for all cases but the contribution of non-premixed mode of combustion to the total heat release has been found to increase with increasing mean inflow velocity and droplet diameter. The increase in the mean inflow velocity yields an increase in the mean values of consumption and density-weighted displacement speed for the droplet cases but leads to a decrease in turbulent burning velocity. By contrast, an increase in droplet diameter gives rise to decreases in turbulent burning velocity, and the mean values of consumption and density-weighted displacement speeds. Detailed physical explanations have been provided to explain the observed mean inflow velocity and droplet diameter dependences of the flame propagation behaviour.

Asymmetric temporal variation of oscillating AC electroosmosis with a steady pressure-driven flow

In most electrokinetic flows in micro-/nanofluidics, electroosmotic flow (EOF) is always a fundamental phenomenon. When an external AC electric field is applied, there will be a transient EOF velocity, which is crucial for understanding the mechanism of the unsteady AC EOF. However, the transient flow velocity cannot be experimentally measured with sufficiently and simultaneously temporal and spatial resolution. Here we report that under a strong AC electric field, an asymmetric temporal variation of oscillating EOF (OEOF) can be generated near the electric double layer (EDL) on the bottom wall of a microchannel. The flow is measured by laser-induced fluorescence photobleaching anemometer (LIFPA). Both the flow velocity of the OEOF and the linearly related internal electric field in the microchannel are theoretically governed by an inviscid Burgers’ equations with an external forcing term and controlled by two coefficients which represent the magnitudes of linear ( $${Z}_{l}$$ ) and nonlinear ( $${Z}_{nl}$$ ) terms. These two coefficients are separately determined by five dimensionless parameters, including Stokes number ( $${St}$$ ), aspect ratio ( $$\gamma$$ ), Schmidt number ( $${Sc}$$ ), Peclet number of mean flow velocity ( $${P}_{e, U}$$ ) and Peclet number of electrophoresis ( $${P}_{e, {EP}}$$ ). In this work, the influence of $${P}_{e, U}$$ and $${P}_{e, {EP}}$$ which are tightly related to the local mean flow velocity and the amplitude of AC electric field is investigated. When the electric field is low, $${P}_{e, U}\gg {P}_{e,{EP}}$$ and accordingly $${Z}_{l}\gg {Z}_{nl}$$ , a temporal symmetry of the time series of velocity is observed. However, when the electric field is high, $${P}_{e, U}\ll {P}_{e, {EP}}$$ and $${Z}_{l}\ll {Z}_{nl}$$ , then the periodicity and temporal symmetry of OEOF can be significantly broken and a temporal asymmetry of the time series of velocity is resulted in, especially under the square wave external electric field. We also find increased mean flow velocity and saturation of velocity fluctuations with the increasing electric field. This work provides a new methodology to study unsteady electrokinetic flows.

Powered Air Purifying Respirator (PAPR) restores the N95 face mask induced cerebral hemodynamic alterations among Healthcare Workers during COVID-19 Outbreak

Background and aimCOVID-19 pandemic has resulted in an unprecedented increased usage of Personal protective equipment (PPE) by healthcare-workers. PPE usage causes headache in majority of users. We evaluated changes in cerebral hemodynamics among healthcare-workers using PPE. MethodsFrontline healthcare-workers donning PPE at our tertiary center were included. Demographics, co-morbidities and blood-pressure were recorded. Transcranial Doppler (TCD) monitoring of middle cerebral artery was performed with 2-MHz probe. Mean flow velocity (MFV) and pulsatility index (PI) were recorded at baseline, after donning N95 respirator-mask, and after donning powered air-purifying respirator (PAPR), when indicated. End-tidal carbon-dioxide (ET-CO2) pressure was recorded for participants donning PAPR in addition to the N95 respirator-mask. ResultsA total of 154 healthcare-workers (mean age 29 ± 12 years, 67% women) were included. Migraine was the commonest co-morbidity in 38 (25%) individuals while 123 (80%) developed de-novo headache due to N95 mask. Donning of N95 respirator-mask resulted in significant increase in MFV (4.4 ± 10.4 cm/s, p < 0.001) and decrease in PI (0.13 ± 0.12; p < 0.001) while ET-CO2 increased by 3.1 ± 1.2 mmHg (p < 0.001). TCD monitoring in 24 (16%) participants donning PAPR and N95 respirator mask together showed normalization of PI, accompanied by normalization of ET-CO2 values within 5-min. Combined use of N95 respirator-mask and PAPR was more comfortable as compared to N95 respirator-mask alone. ConclusionUse of N95 respirator-mask results in significant alterations in cerebral hemodynamics. However, these effects are mitigated by the use of additional PAPR. We recommend the use of PAPR together with the N95 mask for healthcare-workers doing longer duties in the hospital wards.

Highly Viscous Pipe Flow with Least Entropy Generation under Magnetic Field Effect in Low Stokes Number

This study aims to express an optimize temperature profile with the most heat transfer and least work loss by minimizing the total entropy generation of a pipeline under external magnetic field. This study has investigated in low Stokes number and low Reynolds number conditions. It shows that, the fluid’s shear stress increases around the wall (r* > 0.8), due to presence of magnetic field. As a consequence, the magnetic field boosts the dissipation term. However, the velocity gradient over a wide range of radius 0 < r∗ < 0.8 decreases and hence the mean flow velocity increases. Our goal is to use this analysis to manage our magnetic energy better. The entropy generation for fluid flow is minimized in the circular tube with a fix wall temperature under constant magnetic field effect and considering heat dissipation, by applying the calculus of variations. By solving the normalized energy and momentum equations with added source terms due to magnetic field, the real temperature distribution is obtained. Taking the variational of the entropy generation function in the integral form, a nonlinear differential equation with respect to temperature is obtained, which is difficult to be solved analytically. However, series method used to obtain approximate optimize temperature profile.

Increased Blood Flow Velocity in Middle Cerebral Artery and Headache Upon Ingestion of Ice Water.

Introduction: “Headache attributed to ingestion or inhalation of a cold stimulus” (HICS) is one of the most common primary headache disorders. Little is known about the pathophysiology of HICS and other headache disorders. The aim of this study was to analyze mean flow velocity (MFV) and cerebrovascular resistance (RI) in both middle cerebral arteries (MCA) upon ingestion of ice water.Methods: The MFV and RI in both MCAs was continuously measured by transcranial sonography. HICS was induced by drinking 200 ml of ice water.Results: In all volunteers, the ingestion of ice water led to a decrease in RI, which was accompanied by an increase in MFV. In volunteers with induced HICS, MFV were significantly higher compared to volunteers that did not experience HICS. In volunteers with HICS, MFV increased even more significantly when lacrimation occurred compared to volunteers in which it did not. In volunteers without induced HICS, MFV was higher in those volunteers with a positive history of HICS than in those with a negative HICS history.Conclusion: This study revealed a raised MFV upon ingestion of ice water. Volunteers with a provoked case of HICS had a higher MFV than volunteers without HICS. The increase in MFV was even higher when the headache was accompanied by lacrimation. This may indicate an involvement of the trigeminal-parasympathetic vasodilator reflex.

Cerebrovascular reactivity recovery following carotid angioplasty and stenting

BackgroundInternal carotid artery high-grade stenosis is a major cause of stroke. Carotid angioplasty and stenting (CAS) is a reasonable approach for treatment of certain patients. Cerebrovascular reactivity (CVR) evaluated through breath holding index test can assess the outcome following CAS.AimEvaluate changes in CVR before and after CAS.Subjects and methodThirty-two patients with 35 symptomatic internal carotid artery high-grade stenosis (> 70%) were enrolled by the mean of ultrasound screening. Breath holding index test was done using transcranial Doppler ultrasound before and after CAS.ResultsThe study includes 32 patients (22 males and 10 females). The mean age was 61.91 ± 6.59 years. The main risk factor was hypertension (75.3%). All patients showed an increase in mean flow velocity and BHI following CAS (P < 0.001) on both ipsilateral and contralateral side. Both groups (70–90% and > 90% stenosis) showed similar improvement of CVR.ConclusionCAS improves CVR in all high grades of internal carotid artery stenosis.

Time and Frequency Characteristics of Cavitation Activity Enhanced by Flowing Phase-Shift Nanodroplets and Lipid-Shelled Microbubbles During Focused Ultrasound Exposures

This study investigated and compared the time and frequency characteristics of cavitation activity between phase-shift nanodroplets (NDs) and lipid-shelled microbubbles (MBs) exposed to focused ultrasound (FUS) under physiologically relevant flow conditions. Root-mean-square (RMS) of broadband noise, spectrograms of the passive cavitation detection signals and inertial cavitation doses (ICDs) were calculated during FUS at varying mean flow velocities and two different peak-rarefactional pressures. At a lower pressure of 0.94 MPa, the mean values of the RMS amplitudes versus time for the NDs showed an upward trend but slowed down as the mean flow velocity increased. For flowing NDs, the rate of growth in RMS amplitudes within 2–5 MHz decreased more obviously than those within 5–8 MHz. At a higher pressure of 1.07 MPa, the increase in RMS amplitudes was accelerated as the mean flow velocity increased from 0 to 10 cm/s and slowed down as the mean flow velocity reached 15 cm/s. The general downward trends of RMS amplitudes for the MBs were retarded as the mean flow velocity increased at both acoustic pressures of 0.94 MPa and 1.07 MPa. At 0.94 MPa, the mean ICD value for the NDs decreased from 57 to 36 as the mean flow velocity increased from 0 to 20 cm/s. At 1.07 MPa, the mean ICD value initially increased from 45 to 57 as the mean flow velocity increased from 0 to 10 cm/s and subsequently decreased to 43 as the mean flow velocity reached 20 cm/s. For the MBs, the mean ICD value increased with increasing mean flow velocity at both acoustic pressures. These results could aid in future investigations of cavitation-enhanced FUS with the flowing phase-shift NDs and encapsulated, gas-filled MBs for various applications.

Gel-on-a-chip: continuous, velocity-dependent DNA separation using nanoscale lateral displacement.

We studied the trajectories of polymers being advected while diffusing in a pressure driven flow along a periodic pillar nanostructure known as nanoscale deterministic lateral displacement (nanoDLD) array. We found that polymers follow different trajectories depending on their length, flow velocity and pillar array geometry, demonstrating that nanoDLD devices can be used as a continuous polymer fractionation tool. As a model system, we used double-stranded DNA (dsDNA) with various contour lengths and demonstrated that dsDNA in the range of 100-10 000 base pairs (bp) can be separated with a size-selective resolution of 200 bp. In contrast to spherical colloids, a polymer elongates by shear flow and the angle of polymer trajectories with respect to the mean flow direction decreases as the mean flow velocity increases. We developed a phenomenological model that explains the qualitative dependence of the polymer trajectories on the gap size and on the flow velocity. Using this model, we found the optimal separation conditions for dsDNA of different sizes and demonstrated the separation and extraction of dsDNA fragments with over 75% recovery and 3-fold concentration. Importantly, this velocity dependence provides a means of fine-tuning the separation efficiency and resolution, independent of the nanoDLD pillar geometry.

Cerebral Vasoreactivity in Children with Sickle Cell Disease: A Transcranial Doppler Study

BackgroundImpairment of vasodilatory capacity reflecting reduced cerebrovascular reserve was previously shown in adults with sickle cell disease (SCD) and might play a role in the pathophysiology of stroke in such patients. We examined the hypothesis that children with SCD would also have a higher frequency of impaired cerebral vasoreactivity when compared with healthy age- and gender-matched controls. MethodsPatients were recruited from our hematology outpatient clinic. All SCD patients aged 10-18 years without a history of symptomatic stroke as well as age- and gender-matched healthy children were evaluated with transcranial Doppler (TCD) ultrasonography, with breath-holding maneuver. Breath-holding index (BHI) was calculated by dividing the percentage increase in mean flow velocity occurring during breath holding by the length of time subjects hold their breath after a normal inspiration. BHI was considered abnormal if less than .69. ResultsTCD was performed in 42 patients (mean age 12.7 ± 2.2 years) and 20 controls (mean age 13.90 ± 3.04 years). Blood flow velocities were higher in patients with SCD than in controls in all arteries evaluated (P < .001). BHI values in patients with SCD were significantly lower than in control subjects (1.27 ± .65 versus 1.74 ± .15, P = .013 on the left and 1.16 ± .45 versus 1.61 ± .11, P = .002 on the right). BHI was abnormal in 19% of the patients and in none of the controls, P = .036. ConclusionsChildren with SCD may have impaired cerebral vasoreactivity, with low BHI values suggesting a reduced autoregulation capacity.

Cerebral border zone infarction: an etiological study

BackgroundBorder zone infarcts (BZI) are ischemic lesions at the junction between two main arterial territories which may be either cortical or internal BZI.MethodsThis study was conducted on 76 cerebral BZI patients and 20 healthy control subjects. Patients were divided to group I included 26 internal BZI, group II included 19 cortical BZI and group III included 21 mixed internal/cortical BZI patients. Included subjects were submitted to neurological examination, laboratory investigations, ECG, echocardiogram, brain CT and/or MRI and extra and intracranial blood vessels imaging by duplex and CT angiography.ResultsHypertension was significantly higher among groups I and III compared to group II while atrial fibrillation (AF) was significantly higher in groups II and III than group I (p < 0.05). Sonographic duplex assessment of extra and intracranial blood vessels revealed significant increase in mean flow velocities of CCA, ICC and MCA on both side in groups I and III compared to group II (p < 0.05). CT angiography revealed non-significant differences between BZI patients and control as well as in between the three BZI patient’s groups regarding the existence of vertebral artery hypoplasia and/or circle of Willis anomalies.ConclusionsVascular stenosis is the main etiological factor in internal BZI while AF is the predominant etiological factor of cortical BZI. Congenital vascular anomalies play roles in the localization of BZI but cannot predispose to it except when comorbid with hemodynamic disturbances.

Abstract TMP87: Non-Invasive Respiratory Impedance Enhances Cerebral Perfusion in Acute Stroke Patients

Introduction: Optimization of cerebral blood flow is a cornerstone of clinical management in a number of neurologic diseases, most notably acute ischemic stroke. Intra-thoracic pressure influences cardiac output and has the potential to impact cerebral blood flow (CBF). We aimed to quantify cerebral hemodynamic changes in acute ischemic stroke patients during increased respiratory impedance using a non-invasive oral device. Methods: Cerebral perfusion was measured during low (6cm H 2 0) and high (12cm H 2 0) levels of respiratory impedance in 20 patients within 72 hours of acute ischemic stroke. Microvascular CBF was measured by optical diffuse correlation spectroscopy, and middle cerebral artery mean flow velocity (MFV) was assessed by transcranial Doppler ultrasound. Results: High level respiratory impedance resulted in a 10% increase in MFV. Low level respiratory impedance resulted in a smaller (4%) non-significant change in MFV. Changes in cortical CBF were non-significant (figure). MFV varied across all studied levels (baseline vs low vs high, p=0.0017) with a significant test of trend (p=0.001), but this was not seen with microvascular CBF (p=0.33). Heart rate, cardiac output, and end tidal CO 2 remained stable during all levels of respiratory impedance. There was a small increase in mean arterial blood pressure at the low and high level of respiratory impedance, 4% (p=0.013) and 6% (p=0.017) respectively. All patients completed the protocol in its entirety, and the respiratory impedance exercise was well tolerated. Conclusions: Manipulating intrathoracic pressure via non-invasive respiratory impedance was safe and produced a small but measurable increase in cerebral blood flow in acute stroke patients. Future studies are warranted to assess whether respiratory impedance is feasible during hyperacute stroke management, and to determine the effect of volume status on the hemodynamic response to respiratory impedance.