Nick Cheshire

Fluid structure interaction of patient specific abdominal aortic aneurysms: a comparison with solid stress models

BackgroundAbdominal aortic aneurysm (AAA) is a dilatation of the aortic wall, which can rupture, if left untreated. Previous work has shown that, maximum diameter is not a reliable determinant of AAA rupture. However, it is currently the most widely accepted indicator. Wall stress may be a better indicator and promising patient specific results from structural models using static pressure, have been published. Since flow and pressure inside AAA are non-uniform, the dynamic interaction between the pulsatile flow and wall may influence the predicted wall stress. The purpose of the present study was to compare static and dynamic wall stress analysis of patient specific AAAs.MethodPatient-specific AAA models were created from CT scans of three patients. Two simulations were performed on each lumen model, fluid structure interaction (FSI) model and static structural (SS) model. The AAA wall was created by dilating the lumen with a uniform 1.5 mm thickness, and was modeled as a non-linear hyperelastic material. Commercial finite element code Adina 8.2 was used for all simulations. The results were compared between the FSI and SS simulations.ResultsResults are presented for the wall stress patterns, wall shear stress patterns, pressure, and velocity fields within the lumen. It is demonstrated that including fluid flow can change local wall stresses slightly. However, as far as the peak wall stress is concerned, this effect is negligible as the difference between SS and FSI models is less than 1%.ConclusionThe results suggest that fully coupled FSI simulation, which requires considerable computational power to run, adds little to rupture risk prediction. This justifies the use of SS models in previous studies.

Preliminary comparative study of small amplitude helical and conventional ePTFE arteriovenous shunts in pigs

Intimal hyperplasia (IH), which causes occlusion of arterial bypass grafts and arteriovenous (A-V) shunts, develops preferentially in low wall shear, or stagnation, regions. Arterial geometry is commonly three-dimensional, generating swirling flows, the characteristics of which include in-plane mixing and inhibition of stagnation. Clinical arterial bypass grafts are commonly two-dimensional, favouring extremes of wall shear. We have developed small amplitude helical technology (SwirlGraft) devices and shown them to generate physiological-type swirling flows. Expanded polytetrafluorethylene (ePTFE) grafts, although widely used as A-V shunts for renal dialysis access, are prone to thrombosis and IH. In a small preliminary study in pigs, we have implanted SwirlGraft ePTFE carotid artery-to-jugular vein shunts on one side and conventional ePTFE carotid artery-to-jugular vein shunts contralaterally. There was consistently less thrombosis and IH in the SwirlGraft than conventional shunts. At eight weeks (two animals), the differences were marked, with virtually no disease in the SwirlGraft devices and occlusion of the conventional grafts by thrombosis and IH. The study had limitations, but the lesser pathology in the SwirlGraft devices is likely to have resulted from their geometry and the associated swirling flow. The results could have implications for vascular biology and prolongation of the patency of arterial bypass grafts and A-V shunts.

Smoking and plasma fibrinogen, lipoprotein (a) and serotinin are markers for postoperative infrainguinal graft stenosis

OBJECTIVESnA number of systemic variables are associated with infrainguinal graft failure and also with experimental smooth muscle hyperplasia. Stenosis is the most common cause of infrainguinal graft thrombosis but it is not known if systemic variables are associated with stenosis.nnnDESIGN, MATERIALS, AND METHODSnIn this study, clinical and serological factors were measured and correlated with stenosis development in 81 infrainguinal bypass grafts (52 vein, 29 PTFE; 28 with stenosis) in prospective (n=46) and retrospective (n=35) groups. Pre-existing stenosis was excluded by perioperative graft assessment.nnnRESULTSnThere was a significantly greater proportion of smokers in the patients who developed stenosis (11/18; 61%) compared with those who did not (6/28; 21%, p=0.006; chi 2). Patients who developed stenosis also had significantly (Mann Whitney U-tests), higher circulating levels of [median (interquartile range)] fibrinogen (412.5 (356-484.5) vs. 339 (300-397.7) mg/100ml, p=0.003), Lipoprotein (a) (0.20 (0.05-0.45) vs. 0.085 (0.05-0.23) g/l, p=0.03) and 5-hydroxytryptamine (14.1 (6.6-45) vs. 4.41 (3-8.39) nmol/l, p=0.005), than those without stenosis. By logistic regression, these associations were independent of graft material and whether grafts were studied prospectively or retrospectively.nnnCONCLUSIONSnSmoking and plasma fibrinogen, Lp(a) and 5-hydroxytryptamine are markers for postoperative infrainguinal graft stenosis.

Recent advancements in medical simulation: patient-specific virtual reality simulation

BackgroundPatient-specific virtual reality simulation (PSVR) is a new technological advancement that allows practice of upcoming real operations and complements the established role of VR simulation as a generic training tool. This review describes current developments in PSVR and draws parallels with other high-stake industries, such as aviation, military, and sports.MethodsA review of the literature was performed using PubMed and Internet search engines to retrieve data relevant to PSVR in medicine. All reports pertaining to PSVR were included. Reports on simulators that did not incorporate a haptic interface device were excluded from the review.ResultsFifteen reports described 12 simulators that enabled PSVR. Medical procedures in the field of laparoscopy, vascular surgery, orthopedics, neurosurgery, and plastic surgery were included. In all cases, source data was two-dimensional CT or MRI data. Face validity was most commonly reported. Only one (vascular) simulator had undergone face, content, and construct validity. Of the 12 simulators, 1 is commercialized and 11 are prototypes. Five simulators have been used in conjunction with real patient procedures.ConclusionsPSVR is a promising technological advance within medicine. The majority of simulators are still in the prototype phase. As further developments unfold, the validity of PSVR will have to be examined much like generic VR simulation for training purposes. Nonetheless, similar to the aviation, military, and sport industries, operative performance and patient safety may be enhanced by the application of this novel technology.

Role of Virtual Reality Simulation in Teaching and Assessing Technical Skills in Endovascular Intervention

Training in endovascular intervention ultimately aims to produce interventionalists who demonstrate competence in technical skills. Herein, the authors investigate the rationale for simulation-based training by providing an overview of the psychological theories underpinning acquisition of technical skills, training and assessment history, recent advances in simulation technology, and a critical appraisal of their role in training and assessment in endovascular intervention. Simulators have potential for training and assessment and promise solution to many shortcomings of traditional apprenticeship training models. Before inclusion into the curriculum, further work is needed regarding fidelity, validity, reliability, and design of simulators to ensure accurate transfer of acquired endovascular skills from simulator to patient.

Renal dysfunction and prolonged visceral ischaemia increase mortality rate after suprarenal aneurysm repair

Elective juxtarenal abdominal aneurysm repair has a significantly lower mortality rate than suprarenal repair. Identification of factors affecting outcome may lead to a reduction in mortality rate for suprarenal repair.

Contemporary management of splanchnic and renal artery aneurysms: results of endovascular compared with open surgery from two European vascular centers.

INTRODUCTIONnSplanchnic and renal artery aneurysms (SRAAs) are uncommon but potentially life-threatening in case of rupture. Whether these aneurysms are best treated by open repair or endovascular intervention is unknown. The aim of this retrospective study is to report the results of open and endovascular repairs in two European institutions over a fifteen-year period. We have reviewed the available literature published over the 10 last years.nnnMETHODSnAll patients with SRAAs diagnosed from 1995 to 2010 in St Marys Hospital (London, UK) and Henri Mondor Hospital (Créteil, France) were reviewed. Preoperative clinical and anatomical data, operative management and outcomes were recorded from the charts and analyzed.nnnRESULTSn40 patients with 51 SRAAs were identified. There were 21 males and 19 females with a mean age of 57 ± 14.9 years. The aneurysms locations were: 14 (27%) renal, 11 (22%) splenic, 7 (14%) celiac trunk, 7 (14%) superior mesenteric artery, 4 (8%) hepatic, 4 (8%) pancreaticoduodenal arcades, 3 (6%) left gastric and 1 (2%) gastroduodenal. 4 patients presented with a ruptured SRAA. 17 SRAAs in 16 patients were treated by open repair, 15 in 15 patients were treated endoluminally and 17 (mean diameter: 18 mm, range: 8-75 mm) were managed conservatively. One patient with metastatic pulmonary cancer with two mycotic aneurysms of the superior mesenteric artery (75 mm) and celiac trunk (15 mm) was palliated. After endovascular treatment, the immediate technical success rate was 100%. There was no significant difference between open repair and endovascular patients in terms of 30-day post-operative mortality rate and peri-operative complications. No in-hospital death occurred in patients treated electively. Postoperatively, four patients (1 ruptured and 3 elective) suffered non-lethal mild to severe complication in the open repair group, as compared with one in the endovascular group (p = .34). The mean length of stay was significantly higher after open repair as compared with endovascular repair (17 days, range: 8-56 days vs. 4 days, range: 2-6; p < .001). The mean follow-up time was 17.8 months (range: 0-143 months) after open repair, 15.8 months (range: 0-121 months) after endovascular treatment, and 24.8 (range: 3-64 months) for patient being managed conservatively. No late death related to the VAA occurred. In each group, 2 successful reoperations were deemed necessary. In the endovascular group, two patients presented a reperfusion of the aneurysmal sac at 6 and 24 months respectively.nnnCONCLUSIONnNo significant difference in term of 30-day mortality and post-operative complication rates could be identified between open repair and endovascular treatment in the present series. Endovascular treatment is a safe alternative to open repair but patients are exposed to the risk of aneurysmal reperfusion. This mandates careful long-term imaging follow up in patients treated endoluminally.

Initial Clinical Application of a Robotically Steerable Catheter System in Endovascular Aneurysm Repair

Purpose: To report the initial clinical use of a robotically steerable catheter during endovascular aneurysm repair (EVAR) in order to assess this novel and innovative approach in a clinical setting. Technique: Following a series of in-vitro studies and procedure rehearsals using a pulsatile silicon aneurysm model, a 78-year-old man underwent robot-assisted EVAR of a 5.9-cm infrarenal abdominal aortic aneurysm. During the standard procedure, a 14-F remotely steerable robotic catheter was used to successfully navigate through the aneurysm sac, cannulate the contralateral limb of a bifurcated stent-graft under fluoroscopic guidance, and place stiff wires using fine and controlled movements. The procedure was completed successfully. There were no postoperative complications, and computed tomographic angiography prior to discharge and at 3 months confirmed that the stent-graft remained in good position, with no evidence of an endoleak. Conclusion: EVAR using robotically-steerable catheters is feasible. This technology may simplify more complex procedures by increasing the accuracy of vessel cannulation and perhaps reduce procedure times and radiation exposure to the patient and operator.

Simulated procedure rehearsal is more effective than a preoperative generic warm-up for endovascular procedures.

Introduction:Patient-specific simulated rehearsal (PsR) of a carotid artery stenting procedure (CAS) enables the interventionalist to rehearse the case before performing the procedure on the actual patient by incorporating patient-specific computed tomographic data into the simulation software. This study aimed to evaluate whether PsR of a CAS procedure can enhance the operative performance versus a virtual reality (VR) generic CAS warm-up procedure or no preparation at all. Methods:During a 10-session cognitive/technical VR course, medical residents were trained in CAS. Thereafter, in a randomized crossover study, each participant performed a patient-specific CAS case 3 times on the simulator, preceded by 3 different tasks: a PsR, a generic case, or no preparation. Technical performances were assessed using simulator-based metrics and expert-based ratings. Results:Twenty medical residents (surgery, cardiology, radiology) were recruited. Training plateaus were observed after 10 sessions for all participants. Performances were significantly better after PsR than after a generic warm-up or no warm-up for total procedure time (16.3 ± 0.6 vs 19.7 ± 1.0 vs 20.9 ± 1.1 minutes, P = 0.001) and fluoroscopy time (9.3 ± 0.1 vs 11.2 ± 0.6 vs 11.2 ± 0.5 minutes, P = 0.022) but did not influence contrast volume or number of roadmaps used during the “real” case. PsR significantly improved the quality of performance as measured by the expert-based ratings (scores 28 vs 25 vs 25, P = 0.020). Conclusions:Patient-specific simulated rehearsal of a CAS procedure significantly improves operative performance, compared to a generic VR warm-up or no warm-up. This technology requires further investigation with respect to improved outcomes on patients in the clinical setting.

Efficient Implementation of Patient-specific Simulated Rehearsal for the Carotid Artery Stenting Procedure: Part-task Rehearsal

OBJECTIVE(S)nPatient-specific simulated rehearsal (PsR) is a technological advance within the domain of endovascular virtual reality (VR) simulation. It allows incorporation of patient-specific computed tomography Digital Imaging and Communications in Medicine (CT DICOM) data into the simulation and subsequent rehearsal of real patient cases. This study aimed to evaluate whether a part-task rehearsal (PTr) of a carotid artery stenting procedure (CAS) on a VR simulator is as effective as a full-task (FTr) preoperative run through.nnnMETHODSnMedical trainees were trained in the CAS procedure and randomised to a PTr or FTr of a challenging CAS case (Type-II arch). PTr consisted of 30 min of repeated catheterisations of the common carotid artery (CCA). Thereafter, both groups performed the CAS procedure in a fully functional simulated operating suite (SOS) with an interventional team. Technical performances were assessed using simulator-based metrics and expert ratings. Other aspects of performance were assessed using the Non-Technical Skills for Surgeons (NOTSS) scoring.nnnRESULTSnTwenty trainees were evenly randomised to either PTr or FTr. No differences in performance were seen except for the total time the embolic protection device (EPD) was deployed (9.4 min for the PT vs. 8.1 min for the FT, p = 0.02). Total time (26.3 vs. 25.5 min, p = 0.94), fluoroscopy time (15.8 vs. 14.4 min, p = 0.68), number of roadmaps (10.5 vs. 11.0, p = 0.54), amount of contrast (53.5 vs. 58.0 ml, p = 0.33), time to deploy the EPD (0.9 vs. 0.8 min, p = 0.31) and time to catheterise the CCA (9.2 vs. 8.9 min, p = 0.94) were similar. Qualitative performances as measured by expert ratings (score 24 vs. 24, p = 0.49) and NOTSS (p > 0.05 for all categories) were also comparable.nnnCONCLUSIONSnPart- and full-task rehearsals are equally effective with respect to the operative performance of a simulated CAS intervention. This finding makes a patient-specific rehearsal more efficient and may increase the feasibility of implementation of this technology into medical practice.