Flow reversal during stroke thrombectomy

When performing mechanical thrombectomy for stroke patients, some physicians use balloon guide catheters (BGCs) in order to achieve flow reversal and thereby improve reperfusion quality. There is substantial evidence favoring the use of BGCs to improve reperfusion rates and clinical outcomes for thrombectomy patients; however, as we will outline in this review, there is also evidence that BGCs do not achieve reliable flow reversal in many circumstances. Therefore, if we are able to modify our techniques to improve the likelihood of flow reversal during thrombectomy maneuvers, we may be able to further improve reperfusion quality and clinical outcomes. This paper provides an overview of concepts on this topic and outlines some potential techniques to facilitate flow reversal more consistently, including a method to visually confirm it, with the aim of making iterative improvements towards optimal reperfusion for stroke patients.

Configuration-independent thermal invariants under flow reversal in thin vascular systems

Modulating temperature fields is indispensable for advancing modern technologies: space probes, electronic packing, and implantable medical devices, to name a few. Bio-inspired thermal regulation achieved via fluid flow within a network of embedded vesicles is notably desirable for slender synthetic material systems. This far-reaching study-availing theory, numerics, and experiments-reveals a counter-intuitive yet fundamental property of vascular-based fluid-flow-engendered thermal regulation. For such thin systems, the mean surface temperature and the outlet temperature-consequently, the heat extracted by the flowing fluid (coolant)-are invariant under flow reversal (i.e. swapping the inlet and outlet). Despite markedly different temperature fields under flow reversal, our newfound invariance-a discovery-holds for anisotropic thermal conductivity, any inlet and ambient temperatures, transient and steady-state responses, irregular domains, and arbitrary internal vascular topologies, including those with branching. The reported configuration-independent result benefits thermal regulation designers. For instance, the flexibility in the coolant's inlet location eases coordination challenges between electronics and various delivery systems in microfluidic devices without compromising performance (e.g. soft implantable coolers for pain management). Last but not least, the invariance offers an innovative way to verify computer codes, especially when analytical solutions are unavailable for intricate domain and vascular configurations.

Transfemoral Flow-Reversal for Carotid Artery Stenting with Balloon Guide Catheter: Proof of Concept with Robotic Transcranial Doppler

In this video, we demonstrate our technique for transfemoral carotid artery stenting (CAS) with flow-reversal through a Walrus balloon guide catheter (BGC) using robotic transcranial Doppler (rTCD) monitoring. Before crossing the plaque for distal filter placement and/or angioplasty, the BGC is inflated and the three-way stopcock opened, allowing back-bleeding. Immediately, the rTCD shows a change in blood-flow direction, indicating flow-reversal, which likely occurs due to a passive pressure gradient between the intracranial compartment and the atmospheric pressure. Then, the filter is placed with reduced risk of displacing plaque fragments, and angioplasty and stenting are performed with dual protection afforded by the BGC and filter. This technique may confer greater safety for CAS. Although in this case rTCD was used for proof of concept, it could also be used as a tool to monitor embolic load during CAS procedures.

Assessing the Suitability of the Carotid Bifurcation for Stenting: Anatomic and Morphologic Considerations

OBJECTIVE

Over the years where stents have been used to treat carotid lesions, a great deal has been learned about which anatomical characteristics lead to adverse outcomes. This review summarizes the anatomic and morphologic characteristics of the carotid vasculature that can help guide patient selection and clinical decision-making.

METHODS

Each of the carotid artery anatomy and lesion characteristics that are relevant to carotid stenting are described in detail. These are accompanied with evidence-based outcomes and results.

RESULTS

Data on the prevalence of carotid artery lesions that are unsuitable for stenting are summarized and the implications of these data for practice are discussed, especially as they pertain to transcarotid artery revascularization.

CONCLUSIONS

CAS can be viable option for carotid revascularization, but the lesion must be acceptable and safe for stent placement. There should be thorough assessment to rule out the presence of severe tortuosity, long-segment disease, severe calcification (circumferential or exophytic), mobile-plaque, swollen ICA sign, and carotid diameters outside the acceptable range. In carefully chosen lesions with the absence of the unfavorable characteristics described-TCAR may offer improved periprocedural success, and CAS may attain better long-term durability.

Never Change a Flowing System? The Effects of Retrograde Flow on Isolated Perfused Lungs and Vessels

Retrograde perfusion may occur during disease, surgery or extracorporeal circulation. While it is clear that endothelial cells sense and respond to changes in blood flow, the consequences of retrograde perfusion are only poorly defined. Similar to shear stress or disturbed flow, retrograde perfusion might result in vasomotor responses, edema formation or inflammation in and around vessels. In this study we investigated in rats the effects of retrograde perfusion in isolated systemic vessels (IPV) and in pulmonary vessels of isolated perfused lungs (IPL). Anterograde and retrograde perfusion was performed for 480 min in IPV and for 180 min in the IPL. Perfusion pressure, cytokine levels in perfusate and bronchoalveolar lavage fluid (BALF), edema formation and mRNA expression were studied. In IPV, an increased perfusion pressure and initially also increased cytokine levels were observed during retrograde perfusion. In the IPL, increased edema formation occurred, while cytokine levels were not increased, though dilution of cytokines in BALF due to pulmonary edema cannot be excluded. In conclusion, effects of flow reversal were visible immediately after initiation of retrograde perfusion. Pulmonary edema formation was the only effect of the 3 h retrograde perfusion. Therefore, further research should focus on identification of possible long-term complications of flow reversal.

Overscreening, Co-Ion-Dominated Electroosmosis, and Electric Field Strength Mediated Flow Reversal in Polyelectrolyte Brush Functionalized Nanochannels

Controlling the direction and strength of nanofluidic electrohydrodyanmic transport in the presence of an externally applied electric field is extremely important in a number of nanotechnological applications. Here, we employ all-atom molecular dynamics simulations to discover the possibility of changing the direction of electroosmotic (EOS) liquid flows by merely changing the electric field strength in a nanochannel functionalized with polyelectrolyte (PE) brushes. In exploring this, we have uncovered three facets of nanoconfined PE brush behavior and resulting EOS transport. First, we identify the onset of an overscreening effect: such overscreening refers to the presence of more counterions (Na⁺) within the brush layer than needed to neutralize the negative brush charges. Accordingly, as a consequence of the overscreening, in the bulk liquid outside the brush layer, there is a greater number of co-ions (Cl^-) than counterions in the presence of an added salt (NaCl). Second, this specific ion distribution ensures that the overall EOS flow is along the direction of motion of the co-ions. Such co-ion-dictated EOS transport directly contradicts the notion that EOS flow is always dictated by the motion of the counterions. Finally, for large-enough electric fields, the brush height reduces significantly, causing some of the excess overscreening-inducing counterions to squeeze out of the PE brush layer into the brush-free bulk. As a result, the overscreening effect disappears and the number of co-ions and counterions outside the PE brush layer become similar. Despite that there is an EOS transport, this EOS transport, unlike the standard EOS transport that occurs due to the imbalance of the co-ions and counterions, occurs since a larger residence time of the water molecules in the first solvation shell of the counterions (Na⁺) ensures a water transport in the direction of motion of the counterions. The net effect is the reversal of the direction of the EOS transport by merely changing the strength of the electric field.

Fully Developed Opposing Mixed Convection Flow in the Inclined Channel Filled with a Hybrid Nanofluid

This paper studies the convective heat transfer of a hybrid nanofluid in the inclined channel, whose walls are both heated by the uniform heat flux. The governing ordinary differential equations are made nondimensional and solved analytically, in which explicit distributions of velocity, temperature and pressure are obtained. The effects of flow reversal, wall skin friction and Nusselt number with the hybrid nanofluid depend on the nanoparticle volume fractions and pressure parameters. The obtained results indicate that the nanoparticle volume fractions play a key role in delaying the occurrence of the flow reversal. The hybrid nanofluids hold more delayed range than conventional nanofluids, which is about 2.5 times that of nanofluids. The calculations have been compared with the base fluid, nanofluid and two kinds of hybrid models (type II and type III). The hybrid model of type III is useful and simplified in that it omits the nonlinear terms due to the interaction of different nanoparticle volumetric fractions, with the relative error less than 3%. More results are discussed in the results section below.

Activity pulses induce spontaneous flow reversals in viscoelastic environments

Complex interactions between cellular systems and their surrounding extracellular matrices are emerging as important mechanical regulators of cell functions, such as proliferation, motility and cell death, and such cellular systems are often characterized by pulsating actomyosin activities. Here, using an active gel model, we numerically explore spontaneous flow generation by activity pulses in the presence of a viscoelastic medium. The results show that cross-talk between the activity-induced deformations of the viscoelastic surroundings and the time-dependent response of the active medium to these deformations can lead to the reversal of spontaneously generated active flows. We explain the mechanism behind this phenomenon based on the interaction between the active flow and the viscoelastic medium. We show the importance of relaxation time scales of both the polymers and the active particles and provide a phase space over which such spontaneous flow reversals can be observed. Our results suggest new experiments investigating the role of controlled pulses of activity in living systems ensnared in complex mircoenvironments.

Achieving Independent Control over Surface and Bulk Fluid Flows in Microchambers

To fully realize the potential of microfluidic platforms as useful diagnostic tools, the devices must be sufficiently portable that they function at the point-of-care, as well as remote and resource-poor locations. Using both modeling and experiments, here we develop a standalone fluidic device that is driven by light and operates without the need for external electrical or mechanical pumps. The light initiates a photochemical reaction in the solution; the release of chemical energy from the reaction is transduced into the spontaneous motion of the surrounding fluid. The generated flow is driven by two simultaneously occurring mechanisms: solutal buoyancy that controls the motion of the bulk fluid and diffusioosmosis that regulates motion near the bottom of the chamber. Consequently, the bulk and surface fluid flows can be directed independently of one another. We demonstrate that this exceptional degree of spatiotemporal control provides a new method for autonomously transporting different-sized particles in opposite directions within the chamber. Thus, one device can be used to both separate the particles and drive them to different locations for further processing or analysis. This property is particularly useful for analyzing fluids that contain multiple contaminants or disease agents. Because this system relies on intrinsic hydrodynamic interactions initiated by a portable, small-scale source of light, the device provides the desired level of mobility vital for the next generation of functional fluidic platforms.

Modelling foam improved oil recovery: towards a formulation of pressure-driven growth with flow reversal

The pressure-driven growth model that describes the two-dimensional (2-D) propagation of a foam through an oil reservoir is considered as a model for surfactant-alternating-gas improved oil recovery. The model assumes a region of low mobility, finely textured foam at the foam front where injected gas meets liquid. The net pressure driving the foam is assumed to reduce suddenly at a specific time. Parts of the foam front, deep down near the bottom of the front, must then backtrack, reversing their flow direction. Equations for one-dimensional fractional flow, underlying 2-D pressure-driven growth, are solved via the method of characteristics. In a diagram of position versus time, the backtracking front has a complex double fan structure, with two distinct characteristic fans interacting. One of these characteristic fans is a reflection of a fan already present in forward flow mode. The second fan however only appears upon flow reversal. Both fans contribute to the flow's Darcy pressure drop, the balance of the pressure drop shifting over time from the first fan to the second. The implications for 2-D pressure-driven growth are that the foam front has even lower mobility in reverse flow mode than it had in the original forward flow case.

One-Way Valve Method for Achieving Effective Flow Reversal on Carotid Artery Stenting

Objective

We report a simplified method of manual transfusion with a one-way valve during carotid artery stenting (CAS) with flow reversal.

Case Presentation

A 77-year-old man with cervical internal carotid artery stenosis who developed vulnerable plaques was scheduled for CAS using flow reversal. Blood flow reversal was naturally caused by the arteriovenous pressure gradient, and a method with a one-way valve, which enables simplified manual transfusion using a syringe technique, was used for the patient. CAS was completed without distal embolization.

Conclusion

Manual transfusion can be simplified using a one-way valve in cases of flow reversal during CAS, which often require complicated procedures.

Resection of Internal Carotid Artery Aneurysm Under Neuroprotection With Flow Reversal Technique

Extracranial internal carotid artery (ICA) aneurysms account for <1.0% all aneurysms and a rare indication for carotid intervention. Causes include atherosclerotic degeneration, trauma, dissection, previous carotid surgery, connective tissue disorders, and infection. Authors report a case of a middle-aged male found to have a large aneurysm of the left ICA who underwent repair by resection and reconstruction with end-to-end anastomosis under neuroprotection with flow reversal. Our discussion includes a recommendation for this particular surgical repair. The patient in this case report has granted the authors consent for review of records and subsequent publication submission.

Utility of Intravascular Ultrasound During Carotid Angioplasty and Stenting with Proximal Protection

Carotid artery stenting (CAS) is an established treatment for patients at high-risk for endarterectomy. Patients who undergo CAS have been shown to have periprocedural microembolic events on transcranial Doppler ultrasonography. Flow reversal is often applied in these situations to prevent distal emboli and concurrently allow blood to flush into the common carotid artery. Patients who demonstrate soft plaque morphology that may embolize distally during CAS benefit from flow reversal. Even so, the all-stroke risk in these patients is nearly 1.4%. High-risk patients typically have more difficult plaque morphology; flow reversal decreases the rate of distal emboli but does not offer the intraprocedural visualization seen with intravascular ultrasound (IVUS). In this paper, we illustrate potential periprocedural outcomes associated with stenting of the stenotic carotid bifurcation under flow reversal and how IVUS influenced endovascular management. Three high-risk patients who underwent CAS with direct common carotid artery cutdown approaches due to common carotid ostia disease with flow-reversal proximal embolic protection also had intraprocedural IVUS performed to evaluate plaque morphology and stability before the protection system was removed. Case 1 illustrates no intraluminal thrombus on IVUS, requiring no further intervention after stent placement. Case 2 demonstrates intraluminal thrombus on IVUS requiring a second stent to stabilize plaque. Case 3 shows the inadequate resolution of thrombus after a second stent, which was addressed with balloon angioplasty. In our experience, using IVUS as an adjunct to CAS under proximal embolic protection helped demonstrate plaque morphology and plaque fragmentation after stent placement. These cases illustrate the potential benefit of allowing stabilization of the plaque before flow reversal is stopped.

Voxel-by-voxel 4D flow MRI-based assessment of regional reverse flow in the aorta

BACKGROUND

Complex and reverse flow in the aorta has been implicated in aneurysm development and stroke via retrograde embolization.

PURPOSE

To evaluate global and regional differences between standard 2D plane-based and volumetric voxel-based quantification of regional forward/reverse flow, and reverse flow fraction (RFF) in the aorta.

STUDY TYPE

Retrospective.

SUBJECTS

In all, 35 subjects: 10 healthy controls (age: 57 ± 7 years, nine male), nine patients without aortic valve regurgitation (AR) (age: 63 ± 10 years, seven male), six patients with mild AR (age: 66 ± 6 years, five male), and 10 with moderate or severe AR (age: 60 ± 16 years, eight male).

FIELD STRENGTH/SEQUENCE

4D flow MRI (3T and 1.5T) was employed to acquire 3D blood flow velocities with entire thoracic aorta in all subjects.

ASSESSMENT

Data analysis included standard 2D plane-based quantification of forward/reverse flow, and RFF-plane. In addition, a new semiautomatic workflow based on 3D segmentation and extraction of an aorta centerline was developed for voxel-by-voxel visualization (forward/reverse flow and RFF-voxel maps) and quantification of regional voxel-by-voxel forward/reverse flow in the entire thoracic aorta.

STATISTICAL TESTS

Kruskal-Wallis tests were performed to test for differences between groups. A two-sample t-test or Wilcoxon rank sum test was used to compare voxel-based and plane-based results.

RESULTS

Semiautomatic plane-based analysis showed excellent agreement with standard manual plane-based analysis for net flow and RFF-plane (RFF-plane: y = 0.99x-0.0, net flow: y = 1.00x-0.21, R > 0.99, P < 0.0001). Voxel-by-voxel maps demonstrated marked regional flow reversal in the ascending aorta in all patients and RFF-voxel was significantly increased (P < 0.001) compared to RFF-plane for all four groups, with the most pronounced differences for mild AR (18.0 ± 15.2% vs. 4.7 ± 5.4%). Voxel-based flow and RFF-voxel along the aorta showed areas with marked regional flow reversal (eg, vortex flow) compared to plane-based analysis.

DATA CONCLUSION

Voxel-based analysis demonstrated regional flow reversal that was not detected by plane-based analysis.

LEVEL OF EVIDENCE

3 Technical Efficacy: Stage 1 J. Magn. Reson. Imaging 2018;47:1276-1286.

Convective flow reversal in self-powered enzyme micropumps

Surface-bound enzymes can act as pumps that drive large-scale fluid flows in the presence of their substrates or promoters. Thus, enzymatic catalysis can be harnessed for “on demand” pumping in nano- and microfluidic devices powered by an intrinsic energy source. The mechanisms controlling the pumping have not, however, been completely elucidated. Herein, we combine theory and experiments to demonstrate a previously unreported spatiotemporal variation in pumping behavior in urease-based pumps and uncover the mechanisms behind these dynamics. We developed a theoretical model for the transduction of chemical energy into mechanical fluid flow in these systems, capturing buoyancy effects due to the solution containing nonuniform concentrations of substrate and product. We find that the qualitative features of the flow depend on the ratios of diffusivities δ=D(P)/D(S) and expansion coefficients β=β(P)/β(S) of the reaction substrate (S) and product (P). If δ>1 and δ>β (or if δ<1 and δ<β ), an unexpected phenomenon arises: the flow direction reverses with time and distance from the pump. Our experimental results are in qualitative agreement with the model and show that both the speed and direction of fluid pumping (i) depend on the enzyme activity and coverage, (ii) vary with the distance from the pump, and (iii) evolve with time. These findings permit the rational design of enzymatic pumps that accurately control the direction and speed of fluid flow without external power sources, enabling effective, self-powered fluidic devices.

Transcervical Carotid Stenting With Dynamic Flow Reversal Demonstrates Embolization Rates Comparable to Carotid Endarterectomy

PURPOSE

To evaluate a series of patients treated electively with carotid endarterectomy (CEA), transfemoral carotid artery stenting with distal filter protection (CASdp), and transcervical carotid stenting with dynamic flow reversal (CASfr) monitored continuously with transcranial Doppler (TCD) during the procedure to detect intraoperative embolization rates.

METHODS

Thirty-four patients (mean age 67.6 years; 24 men) with significant carotid stenosis underwent successful TCD monitoring during the revascularization procedure (10 CEA, 8 CASdp, and 16 CASfr). Ipsilateral microembolic signals were segregated into 3 phases: preprotection (until internal carotid artery cross-shunted or clamped if no shunt was used, filter deployed, or flow reversal established), protection (until clamp/shunt was removed, filter retrieved, or antegrade flow reestablished), and postprotection (after clamp/shunt or filter removal or restoration of normal flow).

RESULTS

CASdp showed higher embolization rates than CEA or CASfr in the preprotection phase (p<0.001). In the protection phase, CASdp was again associated with more embolization compared with CEA and CASfr (p<0.001). In the postprotection phase, no differences between the revascularization therapies were observed. CASfr and CEA did not show significant differences in intraoperative embolization during any of the phases.

CONCLUSION

TCD recordings demonstrated a significant reduction in embolization to the brain during transcervical carotid artery stent placement with the use of dynamic flow reversal compared to transfemoral CAS using distal filters. No significant differences in microembolization could be detected between CEA and CASfr. The observed lower embolization rates and lack of adverse events suggest that transcervical CAS with dynamic flow reversal is a promising technique and may be the preferred method when performing CAS.

End-diastolic flow reversal limits the efficacy of pediatric intra-aortic balloon pump counterpulsation

BACKGROUND

Counterpulsation with an intra-aortic balloon pump (IABP) has not achieved the same success or clinical use in pediatric patients as in adults. In a pediatric animal model, IABP efficacy was investigated to determine whether IABP timing with a high-fidelity blood pressure signal may improve counterpulsation therapy versus a low-fidelity signal.

METHODS

In Yorkshire piglets (n = 19; weight, 13.0 ± 0.5 kg) with coronary ligation-induced acute ischemic left ventricular failure, pediatric IABPs (5 or 7 mL) were placed in the descending thoracic aorta. Inflation and deflation were timed with traditional criteria from low-fidelity (fluid-filled) and high-fidelity (micromanometer) blood pressure signals during 1:1 support. Aortic, carotid, and coronary hemodynamics were measured with pressure and flow transducers. Myocardial oxygen consumption was calculated from coronary sinus and arterial blood samples. Left ventricular myocardial blood flow and end-organ blood flow were measured with microspheres.

RESULTS

Despite significant suprasystolic diastolic augmentation and afterload reduction at heart rates of 105 ± 3 beats per minute, left ventricular myocardial blood flow, myocardial oxygen consumption, the myocardial oxygen supply/demand relationship, cardiac output, and end-organ blood flow did not change. Statistically significant end-diastolic coronary, carotid, and aortic flow reversal occurred with IABP deflation. Inflation and deflation timed with a high-fidelity versus low-fidelity signal did not attenuate systemic flow reversal or improve the myocardial oxygen supply/demand relationship.

CONCLUSIONS

Systemic end-diastolic flow reversal limited counterpulsation efficacy in a pediatric model of acute left ventricular failure. Adjustment of IABP inflation and deflation timing with traditional criteria and a high-fidelity blood pressure waveform did not improve IABP efficacy or attenuate flow reversal. End-diastolic flow reversal may limit the efficacy of IABP counterpulsation therapy in pediatric patients with traditional timing criteria. Investigation of alternative deflation timing strategies is warranted.

Flow reversal versus filter protection: a pilot carotid artery stenting randomized trial

BACKGROUND

Carotid artery stenting (CAS) has become an alternative treatment for patients presenting symptomatic carotid artery stenosis. The improvement in clinical outcomes with CAS has been associated with the development of embolic protection devices. The trial aim is to compare flow reversal versus filter protection during CAS through femoral access.

METHODS AND RESULTS

Patients were randomly enrolled in CAS using flow reversal or filter protection. The primary end points were the incidence, number, and size of new ischemic brain lesions after CAS. The secondary end points included major adverse cardiac and cerebrovascular events, transient ischemic attack, and definitive ischemic brain lesions on fluid-attenuated inversion recovery magnetic resonance image at a 3-month follow-up. Ischemic brain lesions were assessed by a 3T magnetic resonance image. Neurological outcomes were evaluated by means of the National Institutes of Health Stroke Scale and the modified Rankin Scale (mRS). Forty consecutive patients were randomly assigned. Compared with flow reversal (n=21), filter protection (n=19) resulted in a significant reduction in the incidence (15.8% versus 47.6%, P=0.03), number (0.73 versus 2.6, P=0.05), and size (0.81 versus 2.23 mm, P=0.05) of new ischemic brain lesions. Two patients, 1 from each group, presented transient ischemic attack at 3-month follow-up. There were no major adverse cardiac and cerebrovascular events in the hospital or at 3-month follow-up.

CONCLUSIONS

In this small sample size trial, filter protection was more effective than flow reversal in reducing ischemic brain lesions during CAS through femoral approach.

CLINICAL TRIAL REGISTRATION URL

http://portal2.saude.gov.br/sisnep/. Unique identifier: 0538.0.004.000-10.

Carotid intervention 3: the evidence for cerebral protection

Carotid stenting is a safe alternative option to conventional carotid endarterectomy in the treatment of carotid artery stenosis in patients considered poor candidates for surgery or who choose not to have open surgery. During the stenting procedure, however, distal embolization may occur with neurological sequelae. To reduce the incidence of this, several cerebral-protection devices (CPDs) have been developed. Different types of CPDs are now commercially available: distal occlusion balloons, distal filters, and proximal protection devices with or without reversal of flow. But complications can occur with their use and are usually associated with an inability to cross the lesion, failure to capture the emboli, vasospasm, and vessel wall injury. Because protection devices are currently the focus of interest by manufacturers and physicians, several trials are going on worldwide to analyze the characteristics of each of them and to evaluate their efficacy in reducing the rate of distal embolization.