Mean Flow Reversal Research Articles

Multi-center experience with intravascular lithotripsy for treatment of severe calcification during transcarotid artery revascularization for high-risk patients

ObjectiveTranscarotid artery revascularization (TCAR) offers a safe alternative to carotid endarterectomy (CEA), but severe calcification is currently considered a contraindication in carotid artery stenting. This study aims to describe the safety and effectiveness of TCAR with intravascular lithotripsy (IVL) in patients with traditionally prohibitive calcific disease. MethodsAll consecutive patients who underwent TCAR+IVL from 2018-2022 at nine institutions were identified. IVL was combined with pre-dilatation angioplasty to treat calcified vessels before stent deployment. The primary outcome was a new ipsilateral stroke within 30 days. Secondary outcomes included any new ipsilateral neurologic event (stroke/transient ischemic attack [TIA]) at 30 days, technical success, and <30% residual stenosis. ResultsFifty-eight patients (62% male; mean age, 78 ± 6.6 years) underwent TCAR+IVL, with 22 (38%) for symptomatic disease. Fifty-seven patients (98%) met high-risk anatomical or physiologic criteria for CEA. Forty-seven patients had severely calcific lesions. Fourteen patients (30%) had isolated eccentric plaque, 20 patients (43%) had isolated circumferential plaque, and 13 (27%) had eccentric and circumferential calcification. Mean procedure and flow reversal times were 87 ± 27 minutes and 25 ± 14 minutes. The median number of lithotripsy pulses per case was 90 (range, 30-330), and mean contrast usage was 29 mL. No patients had electroencephalogram changes or new deficits observed intraoperatively. Technical success was achieved in 100% of cases, with 98% having <30% residual stenosis on completion angiography. One patient had an in-hospital post-procedural stroke (1.72%). Four patients total had any new ipsilateral neurologic event (stroke/TIA) within 30 days for an overall rate of 6.8%. One TIA and one stroke occurred during the index hospitalization, and two TIAs occurred after discharge. Preoperative mean stenosis in patients with any postoperative neurologic event was 93% (vs 86% in non-stroke/TIA patients; P = .32), and chronic renal insufficiency was higher in patients who had a new neurologic event (75% vs 17%; P = .005). No differences were observed in calcium, procedural, or patient characteristics between the two groups. The mean follow-up was 132 days (range, 19-520 days). Three stents developed recurrent stenosis (5%) on follow-up duplex; the remainder were patent without issue. There were no reported interventions for recurrent stenosis during the study period. ConclusionsIVL sufficiently remodels calcified carotid arteries to facilitate TCAR effectively in patients with traditionally prohibitive calcific disease. One patient (1.7%) suffered a stroke within 30 days, although four patients (6.8%) sustained any new neurological event (stroke/TIA). These results raise concerns about the risks of TCAR+IVL and whether it is an appropriate strategy for patients who could potentially undergo CEA.

On the low-frequency flapping motion in flow separation

Transitional separating flow induced by a rectangular plate subjected to uniform incoming flow at Reynolds number (based on the incoming velocity and half plate height) 2000 is investigated using direct numerical simulation. The objective is to unveil the long-lasting mystery of low-frequency flapping motion (FM) in flow separation. At a fixed streamwise-vertical plane or from the perspective of previous experimental studies using pointwise or planar measurements, FM manifests as a low-frequency periodic switching between low and high velocities covering the entire separation bubble. The results indicate that in three-dimensional space, FM reflects an intricate evolution of streamwise elongated streaky structures under the influence of separated shear layer and mean flow reversal. The FM is an absolute instability, and is initiated through a lift-up mechanism boosted by mean flow deceleration near the crest of the separating streamline. At this particular location, the shear bends the vortex filament abruptly, so that one end is vertically struck into the first half of the separation bubble, whereas the other end is extended in the streamwise direction in the second half of the separation bubble. These two ends of vortex filament are mutually sustained and also stretched by the vertical acceleration and streamwise acceleration in the first and second halves of the separation bubble, respectively. This process periodically switches the low-velocity (or high-velocity) streaky structure to a high-velocity (or low-velocity) streaky structure encompassing the entire separation bubble, and thus flaps the separated shear layer up and down in the vertical direction. A ‘large vortex’ shedding manifests when the streaky structure switches signs. This large vortex is fundamentally different from the spanwise vortex shedding residing in the shear layer originated from the Kelvin–Helmholtz instability and successive vortex amalgamation. It is also believed that the three-dimensional evolution of streaky structures in the form of FM is applicable for both geometry- and pressure-induced separating flows.

Use of a prosthetic conduit for stent delivery in transcarotid artery revascularization for patients with unfavorable anatomy.

Transcarotid artery revascularization (TCAR) provides a safe alternative to carotid endarterectomy. The anatomic requirements include a 5-cmminimum clavicle to carotid bifurcation distance for sheath access proximal to the lesion. In the present report, we describe our experience with conduit use for patients not meeting that requirement. Patients undergoing elective TCAR with a conduit from 2021 to 2022 were retrospectively identified. After carotid artery exposure, a 6-mm prosthetic graft was anastomosed to the common carotid artery in an end-to-side fashion. After stent delivery, the conduit was ligated and oversewn. The patient demographics, procedural details, and outcomes were recorded and compared with our nonconduit TCAR experience. A total of 11 patients (64% male; age, 75± 5years) underwent TCAR with a conduit, 5 (46%) for symptomatic disease, and 77 patients underwent TCAR with no conduit, 52 (60%) with symptomatic disease (P= .50). Other than a higher rate of prior coronary interventions in the conduit group (55% vs 47%; P= .007), no significant differences were found in age, gender, race, comorbidities, or high risk for carotid endarterectomy criteria. In the conduit group, the average skin to carotid artery depth was 4.2cm (range, 1.9-6.1cm). The average clavicle to bifurcation distance was 4.4cm (range, 3.3-4.9cm) vs 6.5cm (range, 3.3-9.7cm; P= .002) in the nonconduit group. Dacron was the most common conduit material used (73%). No differences were found in the mean procedure times (121± 32 vs 129± 53minutes; P= .785) or flow reversal times (14± 5 vs 19± 13minutes; P=.989) for the conduit and nonconduit cohorts, respectively. Technical success was achieved in 100% of the conduit and nonconduit cases. Excluding one outlier of a prolonged stay (7days) for management of unrelated medical issues (gastrostomy tube placement for chronic dysphagia after mass resection and neck radiation), the mean hospital stay was 2days (1.2± 0.4 intensive care unit days) compared with 3.8± 5.7days for our nonconduit cohort (P= .2). Hypotension was the most common reason for delayed discharge for the conduit group (n=3; 27%). The average follow-up was 2.7months (range, 1-10months). For all 11 conduit patients, the stent remained patent without stenosis, thrombus, or pseudoaneurysm at the conduit stump site on surveillance duplex ultrasound. No strokes or complications had occurred at 30days in the conduit group compared with four strokes or transient ischemic attacks (P= .469) and 18 minor complications in the nonconduit group (P= .091). For patients lacking a sufficient distance between the clavicle and carotid artery bifurcation, a prosthetic conduit facilitates safe use of flow reversal for stent delivery and can be ligated at procedural completion without consequences.

Impact of Flow Reversal Duration on Neurological Outcomes of Transcarotid Artery Revascularization (TCAR)

Flow reversal is a key component of transcarotid artery revascularization (TCAR). However, the impact of flow reversal duration on neurological outcomes and the duration of flow reversal which optimizes TCAR's outcomes is not known. We evaluated the association of flow reversal time with the intraoperative and postoperative neurological outcomes of TCAR. We studied all patients undergoing TCAR from September 2016 to October 2021. The exposure of interest was the duration of flow reversal. Multivariable logistic and fractional polynomial models were used to study the impact of flow reversal duration on in-hospital stroke, intraoperative neurological change/intolerance and stroke/death following TCAR and to identify the flow reversal time above which significant perioperative neurological events occur. The study included 19,462 patients with mean age of 73.4years who were mostly Caucasian (91%) and male (63%). The mean flow reversal time was 10.7minutes, and the overall stroke rate was 1.4%. The odds of intraoperative neurological change increased by 3.6% per minute increase in flow reversal time (odds ratio (OR), 1.04; 95%, 1.01-1.06; P<0.002). Flow reversal duration >10minutes was associated with 78% increased odds of neurological changes compared to flow reversal duration <10minutes. There was no significant association between flow reversal duration and stroke, and stroke/death in the first 5minutes after initiation of flow reversal. The odds of stroke increased by 2.7% per minute increase in flow reversal time >5minutes (OR, 1.03; 95%, 1.01-1.04; P<0.001), with flow reversal duration >10minutes associated with 38% increased odds of stroke compared to flow reversal duration ≤10minutes (OR, 1.37, 95% confidence interval (CI), 1.09-1.73, P=0.006). The odds of stroke/death increased by 2.5% per minute increase in flow reversal time >5minutes (OR, 1.03; 95%, 1.01-1.04; P<0.001). Flow reversal duration >10minutes was associated with 25% increased odds of stroke/death compared to flow reversal duration <10minutes (OR, 1.25, 95% CI, 1.01-1.53, P=0.038). Symptomatic status did not modify outcomes. Our findings suggest that outcomes following TCAR are optimal if the duration of flow reversal is minimized. A clinical cutoff time of 10minutes is suggested by this study and recommended as a guide. Further studies targeted at the flow reversal component of TCAR are needed to solidify the evidence regarding the clinical effects of temporarily induced retrograde cerebral blood flow during TCAR.

Turbulent separations beneath semi-submerged bluff bodies with smooth and rough undersurfaces

The spatio-temporal characteristics of turbulent separations beneath semi-submerged bluff bodies with different undersurface roughness conditions are studied using a time-resolved particle image velocimetry. The Reynolds number based on the free-stream velocity and submergence depth was fixed to 14 400. Three different undersurface conditions – smooth, sandpaper roughness and cube roughness – were examined. The results showed that wall roughness reduces the mean reattachment length, and suppresses the Reynolds stresses in the second half of the mean separation bubble. The Kelvin–Helmholtz instability is observed at the leading edge of the smooth bluff body, but is bypassed in the rough cases. In the first half of the mean separation bubble, the frequencies in the separated shear layer migrate to lower values in a discrete manner through the vortex pairing mechanism. Consequently, multiple vortex shedding motions at different frequencies are nested in the separated shear layer, and the cores of shed vortices are aligned near the isopleth of free-stream velocity. The shed vortex is accompanied with multiple vortices along the edge of mean flow reversal in the upstream locations. These vortices are influenced significantly by wall roughness. A low-frequency flapping motion manifests as enlargement/shrinkage of reverse flow areas in the first half of the mean separation bubble. The frequencies of flapping motion in the smooth and sandpaper cases are similar, but are relatively lower than that in the cube roughness case. This flapping motion is associated with an extremely large vortex shed from the mean reattachment point to the free-stream region.

A single-center experience of 30-day perioperative and one year clinical outcomes of transcarotid artery revascularization in 100 consecutive patients.

Transcarotid Artery Revascularization (TCAR) using the ENROUTE system (Silk Road) has been proposed as a safe and effective alternative to both carotid endarterectomy (CEA) and transfemoral carotid artery stenting (TF-CAS). Two large registries (ROADSTER 1 and ROADSTER 2) have shown that TCAR has acceptable/low rates of perioperative stroke/death. This study will analyze the 30-day perioperative and 1-year clinical outcomes from a single-center. This is a retrospective analysis of prospectively collected data from SVS/VQI TCAR surveillance project (TSP) of 100 consecutive patients (102 TCAR procedures) done in our institution. These procedures were done for high-risk patients for CEA, which included anatomical (previous CEA, high cervical lesion, neck radiation, stoma, arch type, etc.), physiological (CHF, severe coronary artery disease, COPD on O2 therapy, etc.) and combined anatomical/physiological reasons. These procedures were done by vascular surgeons after receiving the appropriate training. The perioperative stroke, death, and MI rates were analyzed. Kaplan Meyer analysis was used to estimate rate of freedom from stroke/death and the incidence of ≥50% and ≥80% in-stent restenosis at 1year. 100 consecutive high-risk patients for CEA included: 38% anatomical, 44% physiological, and 18% combined anatomical and physiological reasons. The mean age was 72.5years (range 52-90years). Indications for TCAR were 34% for symptomatic lesions (TIA/stroke) and 66% for asymptomatic lesions. Mean ipsilateral treated stenosis was 80.4%. Contralateral ≥50% stenosis/occlusion was present in 31% of patients. Technical success rate was 100%. 92% had pre-stenting PTA and 26% had post-stenting PTA. The mean flow reversal time was 8.5min (range 3-26min). The 30-day perioperative stroke rate was 2.9% (1/67, 1.5% for asymptomatic patients), the stroke/death rate was 2.9%, and stroke/death and MI rate was 3.9% (4/102). Other perioperative complications included cranial nerve injury 3/102 (2.9%), carotid artery dissection (2%), and major hematoma (necessitated operation evacuation) (5.9%). Freedom from stroke rates and stroke/death rates at 1year were: 90% and 89%. Freedom from ≥50% and ≥80% in-stent restenosis rates at 1year were 82% and 90%, respectively. None of these restenosis were symptomatic except two (2/13). Freedom from reintervention rate at 1 year was 98%. Although the perioperative events were somewhat higher than what has been reported in previous registries, TCAR for patients who are high-risk for CEA has a low perioperative stroke and stroke/death rates with satisfactory outcome at 1year. Further long-term data is probably needed to verify long-term outcome.

Transcarotid Arterial Revascularization Adoption Should not Be Hindered by a Concern for a Long Learning Curve

Transcarotid arterial revascularization (TCAR) offers a novel technique for carotid artery stenting (CAS) that provides flow reversal in the carotid artery and avoids aortic arch manipulation, thus, potentially lowering ipsilateral and contralateral periprocedural stroke rates. As a new technology, adoption may be limited by concern for learning a new technique. This study seeks to examine the number of cases needed for a surgeon to reach technical proficiency. Retrospective analysis was performed using a prospectively collected database of all TCAR procedures performed in a tertiary health care system between 2016 and 2020. Patient demographics and anatomic characteristics were collected. Intraoperative variables and perioperative outcomes were examined. These variables were collated into groups for the first 4 procedures, procedures 5-8, and after 8. Independent Samples t test, 1-way ANOVA, and logarithmic regression were used to statistically analyze the data. One-hundred and eighty-seven TCARs were performed by 14 surgeons. One hundred and twenty-two (65%) were male, 59 (32%) were older than 75 years, and 83 (44%) were symptomatic. The most common indications were high-lesions in 87 patients (47%) and recurrent stenosis after CEA in 37 patients (20%). Significant differences were found between the first and second groups of 4 cases when comparing mean operative time (71 vs. 58 min; P=0.001) and flow reversal time (10.8 vs. 7.9 min; P= 0.004). similar significant differences were found between the first and third groups of 4 cases but not between the second and third groups. There was a reduction in contrast usage and fluoroscopy time after the first 4 cases, however, this did not reach statistical significance. There was no ipsilateral perioperative strokes. One patient had a contralateral stroke on postoperative day 2 due to intracranial atherosclerosis, and there was one perioperative mortality that occurred on postoperative day 3 after discharge. Procedural and flow reversal times significantly shorten after 4 TCAR procedures are performed. Other metrics, such as fluoroscopy time and contrast usage, are also decreased. Complications, in general, are minimal. Proficiency in TCAR, as measured by these metrics, is met after performing only 4 procedures.

Explicit Algebraic Reynolds-Stress Modeling of Pressure-Induced Separating Flows in the Presence of Sidewalls

AbstractReynolds-averaged Navier–Stokes (RANS) approach is used to simulate the steady-state of a family of pressure-induced turbulent separation bubbles in the presence of sidewalls. Different turbulence models are employed with a specific emphasis on the baseline explicit algebraic Reynolds stress model (BSL-EARSM) and the simulations are compared with experimental data. The separation and reattachment of a flat-plate turbulent boundary layer are generated through a combination of adverse and favorable pressure gradients (APG-FPG) by numerically reproducing the geometry of the wind-tunnel test section used for the experiments. Three cases are considered a large (LB) and a medium (MB) bubble presenting mean backflow, and a small bubble (SB) without mean-flow reversal. This is achieved by varying the streamwise position of the APG/FPG transition. Good agreement between the BSL-EARSM-computed solutions and the experimental results are obtained for wall-pressure and skin-friction distributions on the centerline plane of the test section as well as for the overall three-dimensional flow topology. However, both detachment and reattachment are predicted too early and the bubble length is slightly overestimated for cases LB and MB. For case LB, the streamwise Reynolds stress is estimated fairly well but its production is somewhat delayed. Normal and shear stresses are in good agreement with the experiments in the upstream part of the bubble but are significantly over-estimated in the reattachment region. The k−ω shear-stress transport (SST) model with the so-called reattachment modification performs better than the other tested linear-eddy-viscosity models but it is still unable to reproduce accurately the three-dimensional flow topology even for the “simplest” case SB. Overall, the results suggest that BSL-EARSM is the most suitable turbulence model for this flow configuration.

On the Holton–Lindzen–Plumb model for mean flow reversals in stratified fluids

AbstractThe Holton–Lindzen–Plumb (HLP) model describes the spontaneous emergence of mean flow reversals in stratified fluids. It has played a central role in understanding the Quasi‐Biennial Oscillation of equatorial winds in Earth's stratosphere and has arguably become a linchpin of wave–mean flow interaction theory in geophysical and astrophysical fluid dynamics. The derivation of the model's equation from primary equations follows from several assumptions, including quasi‐linear approximations, WKB expansion of the wavefield, simplifications of boundary‐layer terms, among others. Starting from the two‐dimensional, non‐rotating, Boussinesq equations, we present in this paper a self‐consistent derivation of the HLP model and show the existence of a distinguished limit for which all approximations remains valid. We furthermore discuss the important role of boundary conditions, and the relevance of this model to describe secondary bifurcations associated with a quasi‐periodic route to chaos.

Outcomes of transcarotid artery revascularization with dynamic flow reversal in patients with contralateral carotid artery occlusion

BackgroundThe outcomes of carotid revascularization in patients with contralateral carotid artery occlusion (CCO) are controversial. CCO has been defined by the Centers for Medicare and Medicaid Services as a high-risk criterion and is used as an indication for transfemoral carotid artery stenting. With the promising outcomes associated with transcarotid artery revascularization (TCAR), we aimed to study the perioperative outcomes of TCAR in patients with CCO and to assess the feasibility of TCAR in these high-risk patients. MethodsAll patients in the Vascular Quality Initiative database who underwent TCAR with flow reversal between September 2016 and May 2019 were included. Patients with trauma, dissection, or more than two treated lesions were excluded. Univariable and multivariable logistic analyses were used to compare the primary outcome of in-hospital stroke or death after TCAR in patients with CCO and those without CCO (patent and <99% stenosis). Secondary outcomes included intraoperative neurologic changes and the individual outcomes of in-hospital stroke, death, and myocardial infarction as well as 30-day mortality. ResultsA total of 5485 TCAR cases were included, of which 593 (10.8%) had CCO. In patients with CCO, mean flow reversal time was shorter (10.1 ± 6.7 minutes vs 11.1 ± 7.8 minutes; P < .01); intraoperative neurologic changes occurred in 1% of these patients compared with 0.7% of those with patent contralateral carotid arteries (P = .43). On univariable analysis, no significant difference in in-hospital stroke or death was shown between patients with and patients without CCO (1.7% vs 1.5%; P = .65). Similarly, no significant differences were noted between the groups in terms of in-hospital death (0.7% vs 0.4%; P = .27), stroke (1.7% vs 1.2%; P = .32), and stroke/death/myocardial infarction (2.2% vs 1.8%; P = .53) as well as 30-day mortality (0.8% vs 0.6%; P = .55). The results remained statistically nonsignificant after adjustment for baseline differences between the groups; the adjusted odds ratio (OR) of in-hospital stroke/death in patients with CCO compared with those with patent contralateral carotid arteries was not significant (OR, 1.39; 95% confidence interval, 0.65-3.0; P = .40). In symptomatic patients presenting with prior stroke, CCO was associated with significantly higher odds of stroke or death (OR, 4.63; 95% confidence interval, 1.39-15.4; P = .01) compared with no CCO. On the other hand, in asymptomatic patients, no significant difference in outcomes was observed between the groups. ConclusionsIn this analysis, TCAR seems to be safe in patients with CCO. Caution should be taken in symptomatic patients with CCO and a history of prior stroke as they might have worse outcomes compared with patients with patent contralateral carotid arteries. Studies with larger sample size and longer follow-up are needed to assess the perioperative and long-term outcomes of TCAR in patients with CCO in comparison to other procedures.

General Anesthesia for Transcarotid Artery Revascularization Offers Favorable Outcomes in a Symptomatic and High-Risk Population of Patients

Transcarotid artery revascularization (TCAR) was developed as a treatment option for patients deemed to be at high risk for traditional surgical revascularization. Previous reports have suggested superior outcomes with avoidance of general anesthesia, although outcomes beyond the perioperative period have not been published. We have adopted TCAR using general anesthesia for all patients, which include those meeting high-risk physiologic and anatomic criteria for TCAR. The purpose of this study was to evaluate outcomes of this strategy with short-term follow-up. This is a retrospective, observational cohort study of all patients undergoing TCAR at a single tertiary care institution from July 2018 to July 2019. Patients undergoing concurrent operations were excluded. All TCAR patients met high-risk profile criteria as required for reimbursement. Data were obtained from detailed chart review. Descriptive statistics were used to assess primary end points of operative and flow reversal time and morbidity, including death, stroke, myocardial infarction, and cranial nerve injury, as well as length of stay and readmissions for comparison to historical controls. Ninety-one patients underwent TCAR, all under general anesthesia. Mean age was 73 ± 9 years, with 39% of patients older than 75 years. There were 66 patients (72.5%) who met high-risk criteria on the basis of their physiologic condition and 37 (40.7%) by anatomic considerations only. Stenosis of 80% to 99% was observed in 71 (78.0%) patients; and overall, 39 (43%) patients were symptomatic. Mean operative and flow reversal times were 55.0 ± 14.3 minutes and 8.8 ± 3.8 minutes, respectively. There were two postoperative strokes (2.2%), both within 30 days. At a mean follow-up of 119 ± 111 days, there were no deaths, myocardial infarction, or cranial nerve injuries. Three patients (3.3%) required short-term vasopressor support and three (3.3%) required prolonged intravenous antihypertensive management. Mean length of stay was 1.5 ± 1.3 days, and unplanned readmission rate was 1.1%. Exclusive use of general anesthesia for TCAR provides expeditious carotid revascularization and minimal short-term morbidity and length of stay in this high-risk population. On short-term follow-up, we observed excellent outcomes including no additional cardiopulmonary morbidity and only one readmission. Additional study in larger, controlled populations is needed to define TCAR patients who may benefit from an anesthetic strategy other than general anesthesia.

Periodicity Disruption of a Model Quasibiennial Oscillation of Equatorial Winds.

The quasibiennial oscillation (QBO) of equatorial winds on Earth is the clearest example of the spontaneous emergence of a periodic phenomenon in geophysical fluids. In recent years, observations have revealed intriguing disruptions of this regular behavior, and different QBO-like regimes have been reported in a variety of systems. Here, we show that part of the variability in mean-flow reversals can be attributed to the intrinsic dynamics of wave-mean-flow interactions in stratified fluids. Using a constant-in-time monochromatic wave forcing, bifurcation diagrams are mapped for a hierarchy of simplified models of the QBO, ranging from a quasilinear model to fully nonlinear simulations. The existence of new bifurcations associated with faster and shallower flow reversals, as well as a quasiperiodic route to chaos are reported in these models. The possibility for periodicity disruptions is investigated by probing the resilience of regular wind reversals to external perturbations.

The learning curve of transcarotid artery revascularization

BackgroundIn the pivotal U.S. Food and Drug Administration approval trial, ROADSTER, transcarotid artery revascularization (TCAR) using the ENROUTE Transcarotid Neuroprotection System (Silk Road Medical Inc, Sunnyvale, Calif) was shown to have one of the lowest reported complication rates, not only for carotid artery stent placement, but also for any carotid intervention, including endarterectomy. The number of cases required for a surgeon to gain the proficiency to realize these outcomes has not been studied. Our objective was to determine the learning curve for vascular surgeons performing the TCAR procedure. We examined the effect of surgeon procedural experience on intraoperative data and postoperative outcome. MethodsThis retrospective review analyzed data from a prospectively maintained database of 188 consecutive patients from three large academic centers who underwent TCAR. Procedures were ordered chronologically for each surgeon and grouped into bins of five. Operative times and flow reversal times were analyzed by analysis of variance. Results led to comparison of surgeons' early experience (cases 1-15) with their later experience (cases 16-50). The primary outcome was postoperative stroke and death. ResultsThe mean procedural time for all cases was 75.0 ± 23.8 minutes. When the procedural time of cases 1 through 15 (mean, 79.0 ± 28.3 minutes) were compared with cases 16 through 50 (mean, 71.8 ± 19.0 minutes), a significant difference was noted (P = .02). The mean flow reversal time was 10.6 ± 6.8 minutes. When flow reversal times were compared using analysis of variance testing, cases 1 through 15 were statistically similar and cases 11 through 50 were also similar, but the two groups differed from each other (P < .001). For flow reversal, cases 1 through 15 had mean reversal times of 13.3 ± 8.8 minutes compared with 8.5 ± 3.5 minutes for cases 16 through 50 (P < .001). Postoperative stroke rates were similar in the 1 through 15 and 16 through 50 case groups (2.4% vs 1.0%; P = .59), as were death rates (0.0% vs 1.0%; P > .99). The combined stroke/death rates were also comparable at 2.4% in the early cohort vs 1.9% in the late cohort (P > .99). ConclusionsThere seems to be a relatively short learning curve for the TCAR procedure. After 15 cases, surgeons are able to reduce procedural decrease by 10% (from 79.0 to 71.8 minutes), and flow reversal times by an average of 40% (from 13.3 to 8.5 minutes). More important, the rates of stroke and death do not differ between early and late experience with TCAR. The TCAR procedure may be quickly and safely adopted by vascular surgeons for carotid intervention.

Temporary Reversal of Blood Flow During Transcarotid Artery Revascularization Does Not Change Brain Electrical Activity in Lead-In Cases of the ROADSTER 1 Multicenter Trial

Purpose: To evaluate any intraoperative electroencephalographic (EEG) changes accompanying reversed flow with the ENROUTE Transcarotid Neuroprotection System during transcarotid artery revascularization (TCAR). Methods: A post hoc analysis was performed of the first 81 consecutive lead-in patients (mean age 72.8±8 years; 61 men) enrolled in the ROADSTER 1 trial at 5 participating institutions. All patients had high-grade carotid artery stenosis [53 (59.3%) left sided; 12 (14.8%) contralateral occlusions] and high-risk criteria for carotid endarterectomy. A third had symptoms of either stroke (13, 16.0%) or transient ischemic attack (14, 17.3%). This subset of early patients underwent EEG monitoring to detect any cerebral changes during reversed flow as an added safety measure mandated by the ROADSTER 1 trial protocol. Results: Mean flow reversal time was 12.9±8.2 minutes. The goal mean arterial pressure during reversed flow was 100 mm Hg, but 7 (8.6%) patients suffered hypotension. One (1.2%) patient had slight EEG changes secondary to blood pressure fluctuation; these resolved with blood pressure elevation. No other EEG changes were noted. One (1.2%) patient had a postoperative stroke and another (1.2%) had postoperative myocardial infarction (MI), leading to 2.5% 30-day stroke/death/MI rate. Conclusion: Temporary reversal of blood flow during TCAR is a safe maneuver and does not cause cerebral ischemia in the vast majority of patients, including those with contralateral carotid occlusion. Carotid stenting performed with reversed blood flow mitigates cerebral embolization and periprocedural stroke without concern for brain ischemia.

IP109. The Learning Curve of Transcarotid Artery Revascularization

Transcarotid artery revascularization (TCAR) is a relatively new procedure that involves access to the proximal cervical common carotid artery (CCA), cerebral blood flow reversal as neuroprotection, and stent delivery. The purpose of this study was to describe the learning curve of TCAR. A retrospective review of all patients sequentially undergoing TCAR at three institutions was ordered chronologically. Procedural time was defined as the time of skin incision to time of the final bandage placed on the neck wound. Flow reversal time was defined as the time when reverse flow was initiated (CCA clamped) until the reverse flow was terminated (CCA unclamped). Operative times were then grouped into bins of five and analyzed using one-way analysis of variance (ANOVA) with P < .05 considered statistically significant. Results from ANOVA led to comparison of cases 1 to 10 vs 11 to 50, accomplished using Wilcoxon signed rank test. Overall, 190 procedures were performed by 7 vascular surgeons at 3 institutions between 2013 and 2017. The mean procedural time for all cases was 68.9 ± 9.5 minutes (Fig). When the procedural time of cases 1 to 10 (mean, 74.8 ± 9.6 minutes) was compared with that of cases 11 to 50 (mean, 67.5 ± 9.0 minutes), a significant difference was noted (P = .014). ANOVA results between all bins of procedural times showed that the first 5 cases were statistically longer than cases 31 to 50. Mean flow reversal time was 9.7 ± 3.5 minutes. When flow reversal times were compared using ANOVA testing, cases 1 to 10 were statistically similar and all cases within 11 to 50 were also similar, but the two groups differed from each other (P < .001). For flow reversal time, the first 10 cases had mean reversal times of 14.5 ± 3.7 minutes compared with 8.6 ± 2.3 minutes for cases 11 to 50 (P < .001). Patients of cases 1 to 10 experienced postoperative stroke (2/68 [2.94%]) not significantly different from patients of later cases (1/122 [0.82%]; P = .292; Table). Similarly, postoperative mortality rates were similar in the <10 and >10 case groups (1.47% vs 1.64%; P = 1.00). All three strokes and two of the three deaths (both in cases 11-50) occurred in symptomatic patients, which represented 42% of the overall cohort. A learning curve does exist for the TCAR procedure, but it is relatively steep. After 10 cases, surgeons are able to reduce procedural times by 10% (74.8 to 67.5 minutes) and, more important, flow reversal times by an average of 40% (14.5 to 8.6 minutes). The critical steps in the TCAR procedure may be easily adopted by a broad group of vascular surgeons learning this technique.TablePostoperative outcomes for cases 0 to 10 vs cases >10Stroke, n/N (%)Death, n/N (%)Stroke and death combined, n/N (%)Cases 0-102/68 (2.94)1/68 (1.47)3/68 (4.41)Cases >101/122 (0.82)2/122 (1.64)3/122 (2.46)P = .2915P = 1.000P = .6684 Open table in a new tab

Electro-Magnetohydrodynamic Oscillatory Flow of a Dielectric Fluid Through a Porous Medium with Heat Transfer: Brinkman Model

The objective of the present study is to investigate the effect of electro-magnetic field and heat transfer on the oscillatory flow of a dielectric fluid through a Darcy’s Brinkman model in a symmetric flexible sinusoidal wavy channel. The equations which govern the Electro-Magneto hydro dynamic of oscillatory flow for a dielectric fluid are made non-dimensional and coordinate transformation is employed to convert the irregular boundary to a regular boundary. The obtained system of equations is solved analytically by using the regular perturbation method with a small amplitude ratio. Approximate solution for the mean axial velocity, the mean electric potential, the mean temperature, and the mean pressure gradient is obtained. Further, the effect of pertinent parameters is demonstrated and discussed. The phenomena of reflux (the mean flow reversal) are discussed. It is found that the critical reflux pressure is greater for a fluid without an electric field. Also, the increase of magnetic field decreases the flow rate which is helpful to control the blood flow during the surgeries.

Three-dimensional flow field around and downstream of a subscale model rotating vertical axis wind turbine

Three-dimensional, three-component mean velocity fields have been measured around and downstream of a scale model vertical axis wind turbine (VAWT) operated at tip speed ratios (TSRs) of 1.25 and 2.5, in addition to a non-rotating case. The five-bladed turbine model has an aspect ratio (height/diameter) of 1 and is operated in a water tunnel at a Reynolds number based on turbine diameter of 11,600. Velocity fields are acquired using magnetic resonance velocimetry (MRV) at an isotropic resolution of 1/50 of the turbine diameter. Mean flow reversal is observed immediately behind the turbine for cases with rotation. The turbine wake is highly three-dimensional and asymmetric throughout the investigated region, which extends up to 7 diameters downstream. A vortex pair, generated at the upwind-turning side of the turbine, plays a dominant role in wake dynamics by entraining faster fluid from the freestream and aiding in wake recovery. The higher TSR case shows a larger region of reverse flow and greater asymmetry in the near wake of the turbine, but faster wake recovery due to the increase in vortex pair strength with increasing TSR. The present measurement technique also provides detailed information about flow in the vicinity of the turbine blades and within the turbine rotor. The details of the flow field around VAWTs and in their wakes can inform the design of high-density VAWT wind farms, where wake interaction between turbines is a principal consideration.

Slip Effects on Peristaltic Transport of a Particle-Fluid Suspension in a Planar Channel.

Peristaltic pumping induced by a sinusoidal traveling wave in the walls of a two-dimensional channel filled with a viscous incompressible fluid mixed with rigid spherical particles is investigated theoretically taking the slip effect on the wall into account. A perturbation solution is obtained which satisfies the momentum equations for the case in which amplitude ratio (wave amplitude/channel half width) is small. The analysis has been carried out by duly accounting for the nonlinear convective acceleration terms and the slip condition for the fluid part on the wavy wall. The governing equations are developed up to the second order of the amplitude ratio. The zeroth-order terms yield the Poiseuille flow and the first-order terms give the Orr-Sommerfeld equation. The results show that the slip conditions have significant effect within certain range of concentration. The phenomenon of reflux (the mean flow reversal) is discussed under slip conditions. It is found that the critical reflux pressure is lower for the particle-fluid suspension than for the particle-free fluid and is affected by slip condition. A motivation of the present analysis has been the hope that such theory of two-phase flow process under slip condition is very useful in understanding the role of peristaltic muscular contraction in transporting biofluid behaving like a particle-fluid mixture. Also the theory is important to the engineering applications of pumping solid-fluid mixture by peristalsis.

Shock boundary layer interactions in a low aspect ratio duct

Experimental data acquired using high resolution two-component particle image velocimetry (PIV) are presented for shock boundary layer interactions (SBLIs) generated by a compression–expansion ramp geometry. The incident oblique shock wave is generated by a sub-boundary layer height ramp inclined 20° to the M∞=2.05 inflow. Results are presented for two different ramp sizes (hramp/δ0=0.56 and hramp/δ0=0.93), and compared to a previously documented hramp/δ0=0.20 case. For each case, mean velocitiy and turbulent statistics for both the SBLI at the foot of the compression ramp and the incident/reflected SBLI on the opposite wall are analyzed. Data are acquired in several streamwise-vertical planes across the span of the low aspect ratio duct in order to document spanwise non-uniformities and confinement effects, with the specific goal of producing a high quality experimental database for CFD validation. The angles of the incident and reflected shock waves become steeper as the side wall is approached, due to the lower velocities within the side wall boundary layer. Mean flow reversal is observed near the spanwise centerline of the duct, but only instantaneous flow reversal is seen closer to the side walls. These spanwise non-uniformities are more prominent for the stronger interactions caused by the larger ramp geometry. For this case there is no nominally two-dimensional region of the flow, and a Mach stem occurs in the core of the flow, causing a significant subsonic wake. The shock excursion length scale relative to the incoming boundary layer thickness, Lex/δ0, is measured for all of the shock features and found to be significantly lower than values reported in the literature for similar flows. Furthermore, Lex/δ0 for the reflected shock does not depend on the strength of the incident shock or the size of the separated zone.

Confinement effects in shock wave/turbulent boundary layer interactions through wall-modelled large-eddy simulations

AbstractWe present wall-modelled large-eddy simulations (WLES) of oblique shock waves interacting with the turbulent boundary layers (TBLs) (nominal$\def \xmlpi #1{}\def \mathsfbi #1{\boldsymbol {\mathsf {#1}}}\let \le =\leqslant \let \leq =\leqslant \let \ge =\geqslant \let \geq =\geqslant \def \Pr {\mathit {Pr}}\def \Fr {\mathit {Fr}}\def \Rey {\mathit {Re}}\delta _{99}=5.4\ \mathrm{mm}$and${\mathit{Re}}_{\theta }\approx 1.4\times 10^4$) developed inside a duct with an almost-square cross-section ($45\ \mathrm{mm}\times 47.5\ \mathrm{mm}$) to investigate three-dimensional effects imposed by the lateral confinement of the flow. Three increasing strengths of the incident shock are considered, for a constant Mach number of the incoming air stream$M\approx 2$, by varying the height (1.1, 3 and 5 mm) of a compression wedge located at a constant streamwise location that spans the top wall of the duct at a 20° angle. Simulation results are first validated with particle image velocimetry (PIV) experimental data obtained at several vertical planes (one near the centre of the duct and three near one of the sidewalls) for the 1.1 and 3 mm-high wedge cases. The instantaneous and time-averaged structure of the flow for the stronger-interaction case (5 mm-high wedge), which shows mean flow reversal, is then investigated. Additional spanwise-periodic simulations are performed to elucidate the influence of the sidewalls, and it is found that the structure and location of the shock system, as well as the size of the separation bubble, are significantly modified by the lateral confinement. A Mach stem at the first reflected interaction is present in the simulation with sidewalls, whereas a regular shock intersection results for the spanwise-periodic case. Low-frequency unsteadiness is observed in all interactions, being stronger for the secondary shock reflections of the shock train developed inside the duct. The downstream evolution of secondary turbulent flows developed near the corners of the duct as they traverse the shock system is also studied.