Researched Apps Used in Dementia Care for People Living With Dementia and Their Informal Caregivers: Systematic Review on App Features, Security, and Usability

BACKGROUND

Studies have shown that mobile apps have the potential to serve as nonpharmacological interventions for dementia care, improving the quality of life of people living with dementia and their informal caregivers. However, little is known about the needs for and privacy aspects of these mobile apps in dementia care.

OBJECTIVE

This review seeks to understand the landscape of existing mobile apps in dementia care for people living with dementia and their caregivers with respect to app features, usability testing, privacy, and security.

METHODS

ACM Digital Library, Cochrane Central Register of Controlled Trials, Compendex, Embase, Inspec, Ovid MEDLINE, PsycINFO, and Scopus were searched. Studies were included if they included people with dementia living in the community, their informal caregivers, or both; focused on apps in dementia care using smartphones or tablet computers; and covered usability evaluation of the app. Records were independently screened, and 2 reviewers extracted the data. The Centre for Evidence-Based Medicine critical appraisal tool and Mixed Methods Appraisal Tool were used to assess the risk of bias in the included studies. Thematic synthesis was used, and the findings were summarized and tabulated based on each research aim.

RESULTS

Overall, 44 studies were included in this review, with 39 (89%) published after 2015. In total, 50 apps were included in the study, with more apps developed for people living with dementia as end users compared with caregivers. Most studies (27/44, 61%) used tablet computers. The most common app feature was cognitive stimulation. This review presented 9 app usability themes: user interface, physical considerations, screen size, interaction challenges, meeting user needs, lack of self-awareness of app needs, stigma, technological inexperience, and technical support. In total, 5 methods (questionnaires, interviews, observations, logging, and focus groups) were used to evaluate usability. There was little focus on the privacy and security aspects, including data transfer and protection, of mobile apps for people living with dementia.

CONCLUSIONS

The limitations of this review include 1 reviewer conducting the full-text screening, its restriction to studies published in English, and the exclusion of apps that lacked empirical usability testing. As a result, there may be an incomplete representation of the available apps in the field of dementia care. However, this review highlights significant concerns related to the usability, privacy, and security of existing mobile apps for people living with dementia and their caregivers. The findings of this review provide a valuable framework to guide app developers and researchers in the areas of privacy policy development, app development strategies, and the importance of conducting thorough usability testing for their apps. By considering these factors, future work in this field can be advanced to enhance the quality and effectiveness of dementia care apps.

TRIAL REGISTRATION

PROSPERO CRD42020216141; https://www.crd.york.ac.uk/prospero/display_record.php?RecordID=216141.

INTERNATIONAL REGISTERED REPORT IDENTIFIER (IRRID)

RR2-10.1159/000514838.

Towards PPG-based anger detection for emotion regulation

BACKGROUND

Anger dyscontrol is a common issue after traumatic brain injury (TBI). With the growth of wearable physiological sensors, there is new potential to facilitate the rehabilitation of such anger in the context of daily life. This potential, however, depends on how well physiological markers can distinguish changing emotional states and for such markers to generalize to real-world settings. Our study explores how wearable photoplethysmography (PPG), one of the most widely available physiological sensors, could be used detect anger within a heterogeneous population.

METHODS

This study collected the TRIEP (Toronto Rehabilitation Institute Emotion-Physiology) dataset, which comprised of 32 individuals (10 TBI), exposed to a variety of elicitation material (film, pictures, self-statements, personal recall), over two day sessions. This complex dataset allowed for exploration into how the emotion-PPG relationship varied over changes in individuals, endogenous/exogenous drivers of emotion, and day-to-day differences. A multi-stage analysis was conducted looking at: (1) times-series visual clustering, (2) discriminative time-interval features of anger, and (3) out-of-sample anger classification.

RESULTS

Characteristics of PPG are largely dominated by inter-subject (between individuals) differences first, then intra-subject (day-to-day) changes, before differentiation into emotion. Both TBI and non-TBI individuals showed evidence of linear separable features that could differentiate anger from non-anger classes within time-interval analysis. However, what is more challenging is that these separable features for anger have various degrees of stability across individuals and days.

CONCLUSION

This work highlights how there are contextual, non-stationary challenges to the emotion-physiology relationship that must be accounted for before emotion regulation technology can perform in real-world scenarios. It also affirms the need for a larger breadth of emotional sampling when building classification models.

Dementia Care Apps for People with Dementia and Informal Caregivers: A Systematic Review Protocol

Dementia drastically impacts the quality of life (QOL) of both people living with dementia (PLwD) and their family caregivers. As dementia progresses and care needs escalate, the likelihood of institutionalization is increased, which is counter to the wishes of the majority of older adults and their family members. Dementia care apps can provide critical support and have the potential to improve the QOL of both PLwD and their family caregivers and reduce perceived caregivers' burden. However, there is a lack of understanding of the needs of both PLwD and their family caregivers related to dementia care apps. There is also a gap in understanding the privacy concerns in relation to the apps among older adults with dementia and their caregivers. As such, the main aims of this systematic review are to understand the landscape of dementia mobile apps targeting PLwD and their caregivers with respect to the features of the apps, usability testing, and the privacy and security aspects of the app from the perspective of both app developers/researchers and the end users (PLwD and family caregivers who provide care of PLwD). Extensive databases, including ACM Digital Library, Cochrane Central Register of Controlled Trials, Compendex, Embase, Inspec, Ovid MEDLINE(R) Daily, Proquest Dissertations and These Global, PsycINFO, and Scopus, have been searched. All searches are from the inception of the databases. All peer-reviewed studies and articles written in the English language are included. Two reviewers will independently screen and select the studies with the involvement of a third reviewer for disagreements. Data will be abstracted using a custom data extraction form that is made based on the research questions. Critical Appraisal Skills Programme (CASP) checklists will be used to assess the study quality. As the first review of its kind, the findings from this review will provide valuable insights related to the needs of the dementia care apps for both PLwD and their family caregivers. The review will be relevant to health providers who are interested in using technologies to promote the independence of PLwD and reduce the stress experienced from caregivers of PLwD. The review will also serve as a guide to app developers and researchers to design usable and acceptable apps. In addition, the review will provide critical knowledge of the privacy and security features of the app to reveal the valid concerns from the end users and thus help with the uptake and adoption of the dementia care apps.

Evaluation of an intelligent wheelchair system for older adults with cognitive impairments

BACKGROUND

Older adults are the most prevalent wheelchair users in Canada. Yet, cognitive impairments may prevent an older adult from being allowed to use a powered wheelchair due to safety and usability concerns. To address this issue, an add-on Intelligent Wheelchair System (IWS) was developed to help older adults with cognitive impairments drive a powered wheelchair safely and effectively. When attached to a powered wheelchair, the IWS adds a vision-based anti-collision feature that prevents the wheelchair from hitting obstacles and a navigation assistance feature that plays audio prompts to help users manoeuvre around obstacles.

METHODS

A two stage evaluation was conducted to test the efficacy of the IWS. Stage One: Environment of Use - the IWS's anti-collision and navigation features were evaluated against objects found in a long-term care facility. Six different collision scenarios (wall, walker, cane, no object, moving and stationary person) and three different navigation scenarios (object on left, object on right, and no object) were performed. Signal detection theory was used to categorize the response of the system in each scenario. Stage Two: User Trials - single-subject research design was used to evaluate the impact of the IWS on older adults with cognitive impairment. Participants were asked to drive a powered wheelchair through a structured obstacle course in two phases: 1) with the IWS and 2) without the IWS. Measurements of safety and usability were taken and compared between the two phases. Visual analysis and phase averages were used to analyze the single-subject data.

RESULTS

Stage One: The IWS performed correctly for all environmental anti-collision and navigation scenarios. Stage Two: Two participants completed the trials. The IWS was able to limit the number of collisions that occurred with a powered wheelchair and lower the perceived workload for driving a powered wheelchair. However, the objective performance (time to complete course) of users navigating their environment did not improve with the IWS.

CONCLUSIONS

This study shows the efficacy of the IWS in performing with a potential environment of use, and benefiting members of its desired user population to increase safety and lower perceived demands of powered wheelchair driving.

Anchoring barbs and balloon expandable stents: what is the risk of perforation and failed stent deployment?

PURPOSE

Balloon expandable stents may on occasion be deployed in close proximity to the anchoring barbs of endovascular grafts. The aim of this study was to determine the risk and effect of balloon perforation by anchoring barbs and to assess whether these risks are different if the balloon is protected by a covered stent mounted upon it.

METHODS

A bench-top model was developed to mimic the penetration of anchoring barbs into the lumen of medium sized blood vessels. The model allowed variation of angle and depth of vessel penetration. Both bare balloons and those with covered stents mounted upon them were tested in the model to determine whether there was a risk of perforation and which factors increased or decreased this risk.

RESULTS

All combinations of barb angle and depth caused balloon perforation but this was most marked when the barb was placed perpendicular to the long axis of the balloon. When the deployment of covered stents was attempted balloon perforation occurred in some cases but full stent deployment was achieved in all cases where the perforation was in the portion of the balloon covered by the stent. The only situation in which stent deployment failed was where the barb was intentionally placed in the uncovered portion of the balloon. This resulted in only partial deployment of the stent.

CONCLUSIONS

Balloon rupture is a distinct possibility when deploying balloon-expandable stents in close proximity to anchoring barbs. Care should be taken in this circumstance to ensure that the barb is well away from the uncovered portion of the balloon.

How secure is the anastomosis between the proximal and distal body components of a fenestrated stent-graft?

OBJECTIVES

To examine the longitudinal migratory force required to cause disconnection of the bifurcated distal body component from the tubular proximal body of a fenestrated stent-graft.

METHODS

Using a previously reported mathematical model distal distraction forces were calculated prior to performing in vitro pullout testing. The top end of the proximal body and the iliac limbs of the distal body were attached to the grips of a tensile tester via plastic sealing plugs and pneumatic clamps. Channels within the plugs allowed pressurisation of the inside of the stent-graft. Pullout tests were conducted in the vertical plane. Force and displacement data were recorded and tests repeated 8 times at room temperature with the stent-grafts either dry or wet and unpressurized, at 100 mmHg or at 120 mmHg.

RESULTS

The median maximum pullout force was 2.9 N (2.6-4.1) when dry, 3.9 N (3.5-5.4) when wet and unpressurized, 6.3 N (4.8-8.3) when wet and pressurized at 100 mmHg and 6.5 N (4.8-7.2) when wet and pressurized at 120 mmHg. There was a significant difference between pressurized and unpressurized conditions (P < 0.01).

CONCLUSIONS

The force required to distract the distal bifurcated component of a fenestrated stent graft is much lower than the reported proximal fixation strength of both a standard and fenestrated Zenith stent graft. Although this helps protect the fenestrated proximal body from the effects of longitudinal migration forces in vivo the current strength of the body overlap zone may actually be unnecessarily weak and requires careful surveillance in follow up.

Reprinted article "Factors associated with early failure of arteriovenous fistulae for haemodialysis access"

The radiocephalic arteriovenous fistula remains the method of choice for haemodialysis access. In order to assess their suitability for fistula formation, the radial arteries and cephalic veins were examined preoperatively by ultrasound colour flow scanner in conjunction with a pulse-generated run-off system. Intraoperative blood flow was measured after construction of the fistulae. Post-operative follow-up was performed at various intervals to monitor the development of the fistulae. Radial artery and cephalic vein diameter less than 1.6 mm was associated with early fistula failure. The intraoperative fistula blood flow did not correlate with the outcome of the operation probably due to vessel spasm from manipulation. However, blood flow velocities measured non-invasively 1 day after the operation were significantly lower in fistulae that failed early compared with those that were adequate for haemodialysis. Most of the increase in fistula diameter and blood flow occur within the first 2 weeks of surgery.

Measurement of pulsatile haemodynamic forces in a model of a bifurcated stent graft for abdominal aortic aneurysm repair

The longitudinal haemodynamic force (LF) acting on a bifurcated stent graft for abdominal aortic aneurysm repair has been estimated previously using a simple one-dimensional analytical model based on the momentum equation which assumes steady flow of an inviscid fluid. Using an instrumented stent-graft model an experimental technique was developed to measure the LF under pulsatile flow conditions. The physical stent-graft model, with main trunk diameter of 30mm and limb diameters of 12 mm, was fabricated from aluminium. Strain gauges were bonded on to the main trunk to determine the longitudinal strain which is related to the LF. After calibration, the model was placed in a pulsatile flow system with 40 per cent aqueous glycerol solution as the circulating fluid. The LF was determined using a Wheatstone bridge signal-conditioning circuit. The signals were averaged over 590 cardiac cycles and saved to a personal computer for subsequent processing. The LF was strongly dependent on the pressure but less so on the flowrate. The measured forces were higher than those predicted by the simplified mathematical model by about 6-18 per cent during the cardiac cycle. The excess measured forces are due to the viscous drag and the effect of pulsatile flow. The peak measured LF in this model of 30 mm diameter may exceed the fixation force of some current clinical endovascular stent grafts.

Physics-based virtual environment for training core skills in vascular interventional radiological procedures

Recent years have seen a significant increase in the use of Interventional Radiology (IR) as an alternative to open surgery. A large number of IR procedures commences with needle puncture of a vessel to insert guidewires and catheters: these clinical skills are acquired by all radiologists during training on patients, associated with some discomfort and occasionally, complications. While some visual skills can be acquired using models such as the ones used in surgery, these have limitations for IR which relies heavily on a sense of touch. Both patients and trainees would benefit from a virtual environment (VE) conveying touch sensation to realistically mimic procedures. The authors are developing a high fidelity VE providing a validated alternative to the traditional apprenticeship model used for teaching the core skills. The current version of the CRaIVE simulator combines home made software, haptic devices and commercial equipments.

A new device for in vitro evaluation of thrombogenicity

A device to measure the time of coagulation of whole blood has been designed in order to facilitate testing of thrombogenicity of biomaterials. The principle of operation of the apparatus is to record the time taken for a sphere to fall through a sample of blood. The coagulation time is defined here as the time from collection of the blood sample to coagulation, coagulation having deemed to have occurred when the sphere is prevented from falling by the presence of the fibrin-cell network. The device was tested with homogeneous fluids of different viscosity, milk containing different amounts of rennin and calcium chloride and non anti-coagulated whole blood obtained from five volunteers. Repeat measurements made with each homogeneous fluid show that the variance in the transit time of the sphere is small and consistent with small dispersion. In contrast, the onset of coagulation in milk and blood samples was readily detected. The clotting of milk was highly dependant on the concentration of rennin and calcium chloride. The coagulation time of blood samples from five individuals ranged from 23 to 33 min but simultaneous measurements of samples from the same individual in two identical devices agreed to within +/-1 min in all cases. This device may be easily adapted for use in studies to determine the thromboresistance of biomaterials where the onset of coagulation of whole blood in contact with different materials may be readily compared.

Swirling flow pattern in a non-planar model of an interposition vein cuff anastomosis

One of the main causes of long-term failure of ePTFE grafts is the development of anastomotic intimal hyperplasia which leads to graft thrombosis. Experimental studies with bypass grafts have shown an inverse relationship between mean wall shear stress and intimal hyperplasia. The geometry of the anastomosis has a strong influence on the flow pattern and wall shear stress distribution. The aim of this in vitro study was to investigate the influence of non-planarity in a model of a distal anastomosis with interposition vein cuff, an anastomosis configuration that is increasingly being used because of improved clinical results. Laser Doppler anemometer measurements were carried out in silicone rubber models of interposition vein cuff anastomoses with planar and non-planar inflow. The pulsatile flow waveforms were typical of those found in femoro-infrapopliteal bypass. Axial and radial velocities were measured in the proximal and distal outflow segments. As expected a symmetrical helical flow pattern (Dean flow) was evident in the planar model. The model with non-planar inflow, however, gave rise to swirling flow in both the distal and proximal artery outflow segments for during the systolic phase. In patients, the anastomosis is usually non-planar. Since the configuration depends in part upon the tunnelling of the graft, this may be altered to some extent. Non-planar anastomotic configurations induce a swirling flow pattern, which may normalise wall shear stress, thereby potentially reducing intimal hyperplasia.

Attachment, morphology and adherence of human endothelial cells to vascular prosthesis materials under the action of shear stress

In an effort to improve the long-term patency of vascular prostheses several groups now advocate seeding autologous endothelial cells (ECs) onto the lumen of the vessel prior to implantation, a procedure that involves pre-treating the prosthesis material with fibrin, collagen and/or other matrix molecules to promote cell attachment and retention. In this study, we examined the degree to which human umbilical venous endothelial cells (HUVECs) adhered to three materials commonly used polymeric vascular prosthesis that had been coated with the same commercial extra cellular matrix proteins, and after exposure to fluid shear stresses representative of femoro-distal bypass in a cone-and-plate shearing device. We quantified cell number, area of coverage and degree of cell spreading using image analysis techniques. The response of cells that adhered to the surface of each material, and following exposure to fluid shear stress, depended on surface treatment, topology and cell type. Whereas collagen coating improved primary cellular adhesion and coverage significantly, the degree of spreading depended on the underlying surface structure and on the application of the shear stress. In some cases, fewer than 30% of cells remained on the surface after only 1-h exposure to physiological levels of shear stress. The proportion of the surface that was covered by cells also decreased, despite an increase in the degree to which individual cells spread on exposure to shear stress. Moreover, the behaviour of HUVECs was distinct from that of fibroblasts, in that the human ECs were able to adapt to their environment by spreading to a much greater extent in response to shear. The quality of HUVEC attachment, as measured by extent of cell coverage and resistance to fluid shear stress, was greatest on expanded polytetrafluoroethylene samples that had been impregnated with Type I/III collagen.

In vitro assessment of a continuous cardiac output catheter system

Continuous measurement of cardiac output (CCO) is useful in assessing the cardiovascular status of patients during cardiac surgery and in intensive care. Recently, a CCO system (truCCOMS, Aortech, UK), capable of detecting rapid changes in cardiac output (CO) was introduced. The method is based on the energy required to maintain an integral heat-transfer device at constant temperature above the ambient value. The aim of this study was to assess the performance of this CCO system in vitro under in steady as well as pulsatile flow conditions representative of those in the pulmonary artery. In order to determine the sensitivity of the system to changes in vessel cross-sectional area and therefore local flow velocity, the catheter was deployed in a linear-tapered tube. Steady and pulsatile flows were generated, and the electrical power at various locations along the tapered tube was recorded. The results show significant differences in the performance under the two different flow conditions. In steady flow, the CO was highly dependent on the local velocity whereas in pulsatile flow, CO varied much less with local velocity. The sensitivity expressed as a percentage increase in CO per 100% increase in velocity at a CO of 5 l min(-1) was 87% in steady flow and 24% in pulsatile flow. Experiments carried out with three fluids with different viscosity show that the errors in determining CO in the tapered tube were also dependent on the Reynolds number and flow regime. The mean errors ranged from about 50% at 2 l min(-1) to less than 10% at 8 l min(-1). The correlation between the predicted and actual CO was generally good. In conclusion, the pulmonary artery catheter is not recommended in situations where blood flow is expected to be steady or of low pulsatility. It may, however, be suitable under normal pulsatile flow conditions in the pulmonary artery.

Outflow distribution at the distal anastomosis of infrainguinal bypass grafts

Outflow distribution at the distal anastomosis of infrainguinal bypass grafts remains unquantified in vivo, but is likely to influence flow patterns and haemodynamics, thereby impacting upon graft patency. This study measured the ratio of distal to proximal outflow in 30 patients undergoing infrainguinal bypass for lower limb ischaemia, using a flow probe and a transit-time ultrasonic flow meter. The mean outflow distribution was approximately 75% distal to 25% proximal, with above knee anastomoses having a greater proportion of distal flow (84%) compared to below knee grafts (73%). These in vivo flow characteristics differ significantly from those used in theoretical models studying flow phenomena (50:50 and/or 100:0), and should be incorporated into future research.

Flow patterns in an endovascular stent-graft for abdominal aortic aneurysm repair

Endovascular exclusion of the abdominal aortic aneurysm (AAA) has been carried out in selected patients during the past decade. The deployment of a complex multicomponent endovascular device in an aneurysmal aorta may alter the local haemodynamics and lead to thrombosis and intimal hyperplasia development. The aim of this in vitro study was to investigate the flow patterns using flow visualisation and laser Doppler anemometry in a commercial bifurcated stent-graft. Two configurations of the stent-graft, endo-stent and exo-stent, were investigated in an idealised planar AAA model. The flow structures in the main trunk in both configurations of the stent-graft are three-dimensional with complex secondary structures. However, these flow structures were not entirely caused by the stent-graft. The stent struts in the endo-stent configuration cause localised alteration in the flow pattern but the overall flow structures were not significantly affected. Low velocity regions in the main trunk and flow separation in the stump region and the curved segment of the iliac limbs were observed. These areas are associated with thrombosis in the clinical situation. Improvements in the design of endovascular devices may remove these areas of unfavourable flow patterns and lead to better clinical performance.

Aortic side branch perfusion alone does not account for high intra-sac pressure after endovascular repair (EVAR) in the absence of graft-related endoleak

OBJECTIVE

to examine the effect of stent-graft deployment on pressure within an aneurysm sac and to investigate the potential sources of intra-sac pressure.

MATERIAL AND METHODS

intra-sac pressure was monitored during and immediately after endovascular repair via an indwelling catheter. Intra-sac pressure was also monitored during conventional open repair and was compared with the pressure measured within patent lumbar and inferior mesenteric side-branches, both before and after restoration of iliac arterial blood flow. Intra-sac and side-branch pressures were recorded and expressed as ratios of simultaneously measured radial artery pressure.

RESULTS

in the absence of a graft-related endoleak (23/25 patients), endovascular repair resulted in a significant reduction in intra-sac pulse pressure (median ratio 0.31 IQR 0.10-0.46). There was no corresponding reduction in mean intra-sac pressure (median ratio 0.91; IQR 0.83-1.00). Application of clamps at conventional open repair resulted in a fall in both intra-sac pressure (median ratio 0.39, IQR 0.32-0.64) and pressure within side-branches (median ratio 0.45, IQR 0.33-0.64). Restoration of iliac blood flow resulted in a modest recovery of the side-branch pressure (median ratio 0.63, IQR 0.57-0.81), which nonetheless remained significantly less than the intra-sac pressure recorded after EVAR (p=0.01).

CONCLUSION

reperfusion of the aneurysm sac through patent side-branches seems insufficient to account for persistent pressurisation of the aneurysm after endovascular repair. This finding supports the hypothesis that pressure may be transmitted directly through stent-graft fabric.

The distaflo graft: a valid alternative to interposition vein?

INTRODUCTION

the rationale behind the Distaflo graft is inhibition of myointimal hyperplasia through optimisation of haemodynamic forces at the distal anastomosis. This prospective study reports our early clinical results.

METHOD

patients with critical limb ischaemia, but no autologous vein, underwent infrainguinal bypass using Distaflo. Clinical and Duplex assessment provided prospective data from which one year cumulative patency, limb salvage and survival rates were calculated using Kaplan-Meier analysis. Log rank test enabled comparison with an historical control group of Miller cuff grafts.

RESULTS

fifty Distaflo were inserted over 29 months into 46 patients, median age 68.5 years, 27 male (59%), of which 27 (54%) were re-do procedures. Proximal anastomoses were to common femoral arteries in 40 cases (80%); distal anastomoses were to popliteal vessels in 20 (40%), and tibial vessels in 30 (60%). The Distaflo graft had patency, limb salvage and survival rates of 39, 50 and 82% respectively compared to 49, 56 and 85% respectively in the control group, with no statistical difference (p = 0.39; 0.65; 0.67 respectively; log rank).

CONCLUSION

in this non-randomised study, the Distaflo has similar one year patency, limb salvage and survival rates to the Miller cuff, potentially justifying its use an alternative in distal prosthetic arterial reconstruction for critical limb ischaemia.

Steal phenomenon in radiocephalic arteriovenous fistula. In vitro haemodynamic and electrical resistance simulation studies

OBJECTIVE

steal phenomenon following an arteriovenous fistula (AVF) creation is characterised by retrograde flow in the artery segment distal to the anastomosis and occurs in the majority of patients with radiocephalic AVF although this rarely leads to distal ischaemia. To investigate the local haemodynamics after the creation of an AVF, a simple electrical resistance model which assumes time-independent flow was used. The applicability of this model to pulsatile flow conditions was verified using an in vitro flow circuit. The effects of stenoses in various artery segments were also investigated.

DESIGN OF THE STUDY

the electrical analogue model consists of a pressure source, constant resistances that represent the resistance to flow of various arterial segments and the fistula. The stenosis was modelled by a resistor and a non-linear term is simulated by a current-controlled voltage source. In vitro experiments were performed in pulsatile and steady flow and the results were compared with electrical simulations. The effects of fistula flow and the presence and severity of a stenosis on flow distribution, particularly the direction of flow in the distal radial artery and flow into the hand were assessed.

RESULTS

steady and pulsatile time-averaged flows measured in vitro compared well with the results of electrical circuit simulations for cases without a stenosis. When a stenosis was present comparisons were made only in steady flow and these show good agreement for stenoses of 75% area reduction. The direction of flow in the distal radial artery was antegrade (towards the hand) at low fistula flow and became retrograde as fistula flow increased. The presence of a severe stenosis in the brachial artery was found to have the strongest influence on flow to the hand.

CONCLUSIONS

an electrical resistance model of a radiocephalic AVF has been validated with an in vitro pulsatile flow circuit. One of the benefits of this model is that it can be easily analysed using standard circuit simulation software. The model also provide insights into the possible haemodynamics consequences of creating an AVF with or without the presence of a stenosis in the arterial segments.

Distal anastomotic environment: the influence of intrinsic and extrinsic factors on flow structure

Background

Improved patency of the Miller cuff may be attributable to the modification of haemodynamics at the anastomosis inhibiting the accretion of myointimal hyperplasia. The influence of the distal anastomotic environment on flow patterns may therefore have important clinical significance.

Methods

Anastomotic models of Miller cuff, end-to-side (ETS) and precuffed grafts (Distaflo™, Impra, Tempe, AZ, USA) were perfused under physiological conditions. Visualization studies and Doppler colour flow mapping (P700, Philips Medical Systems, Reigate, UK) enabled qualitative flow analysis of the influence of anastomotic geometry, configuration and outflow distribution. Intraoperative flow distribution was measured with a Doppler transit time probe. Flow within in vivo precuffed grafts was analysed by means of colour flow mapping and velocity profiles.

Results

ETS anastomosis demonstrated a large area of flow separation at the graft toe, except when distal outflow approached 100 per cent, when laminar flow prevailed. The Miller cuff created a cohesive vortex within the proximal anastomosis, with reduced areas of flow separation. Outflow distribution had little effect upon the integrity of this vortex, although helical flow occurred in the distal vessel when proximal run-off was occluded. Cuff geometry, expressed as aspect ratio (cuff length: height) was important, with long and low cuffs demonstrating detrimental flow structures, including complex short-lived vortices and large areas of separation. The precuffed model produced a robust vortex, resistant to flow distribution. In vitro Doppler images of the vortex were reproduced accurately in 77 per cent of patients with precuffed grafts in situ (n = 32). Mean intraoperative flow distributions of 76 per cent distal: 24 per cent proximal in infragenicular grafts were significantly different from anticipated ratios of 50: 50 and 100: 0 (P &lt; 0·001, single-sample t test). In above-knee grafts, however, distribution approached purely distal flow (mean 85: 15 versus 100: 0; P = 0·046).

Conclusion

Anastomotic flow structures are determined by the geometry of the anastomosis and the relative flow distribution proximally and distally in the recipient artery. A cuffed configuration of anastomosis is likely to be of most benefit when there is substantial flow proximally as well as distally in the run-off vessel, a situation that applies most often in infragenicular reconstruction. In addition, variable cuff geometry may detract from the full potential of the Miller cuff. A precuffed graft of optimum geometry may negate this concern, with the majority of patients demonstrating accurate replication of potentially beneficial flow patterns.

Experimental evaluation of flow patterns in interposition vein cuff anastomosis. The influence of non-planarity of the inflow

Clinical evidence suggests that the development of myointimal hyperplasia in prosthetic femorodistal bypass grafts may be reduced by the interposition of a cuff of autologous vein between the graft and the recipient artery. Previous experimental work has shown that some of the benefits may be attributed to the geometry of the cuffed anastomosis. Since the distal anastomosis in vivo is often non-planar we have carried out a preliminary study in a model where the graft is at an angle of 45 degrees to the anterior-posterior plane of the anastomosis. This out-of-plane angulation produces highly asymmetric flow patterns in the anastomosis with significant flow separation on the ipsilateral side of the cuff. In the proximal and distal outflow, however, the velocity vectors show significant helical motion with temporal instability in the distal outflow.

Harnessing haemodynamic forces for the suppression of anastomotic intimal hyperplasia: the rationale for precuffed grafts

OBJECTIVES

Standardisation of cuff geometry by manufacturing prosthetic precuffed grafts (PCG) theoretically optimises haemodynamic forces. This study was designed in order to determine whether these beneficial flow patterns are replicated in vivo in PCG.

PATIENTS AND METHODS

Flow visualisation and Doppler studies performed on anatomically accurate PCG models characterised in vitro anastomotic flow patterns. Thirty-two patients (median age 68 years) in whom autologous vein was unavailable, underwent bypass using PCG. Post-operative analysis included qualitative assessment of flow within the distal anastomosis using Doppler colour flow mapping. Cardiac gating techniques and assessment of velocity distribution were performed to gain additional information. These in vivo results were validated against the bench studies.

RESULTS

A cohesive vortex was identified within the distal anastomosis of in vitro models and had an integral relationship with the cardiac cycle. This flow structure was also characterised using Doppler colour flow mapping in both longitudinal and transverse planes, confirming the location of the vortex within the body and proximal part of the anastomosis. Twenty-two patients (69%) undergoing bypass with a PCG underwent successful Doppler assessment one week post-operatively, of whom 17 (77%) had a vortical flow structure identified at the distal anastomosis, similar to that characterised in vitro. Cardiac gating verified the same integral relationship of the vortex with the cardiac cycle as that described in vitro.

CONCLUSION

The geometric configuration of precuffed grafts induced vortices within the distal anastomoses in 17 out of 22 patients undergoing arterial reconstruction, thereby harnessing the haemodynamic forces that may suppress anastomotic hyperplasia and improve patency rates.

Characterization of vortices using pulsed-wave Doppler ultrasound

The detection and characterization of vortices from a Kaman vortex generator by means of a 20 MHz pulsed-wave Doppler ultrasound system were assessed. Measurements were made at different steady flowrates in a 10 mm internal diameter polyurethane tube, 14 mm distal to a circular cylinder of diameter 2 mm, placed across the tube inlet. The results were compared with those obtained with a two-component laser Doppler anemometer system. There was generally good agreement between the two techniques in the measurement of convective flow velocity, frequency of vortex shedding and the circulation velocity of the vortices. It is concluded that pulsed-wave Doppler ultrasound is a suitable technique for investigating vortical flow structures.

Optimising Miller Cuff Dimensions. The Influence of Geometry on Anastomotic Flow Patterns

OBJECTIVES

since cuff dimensions are variable, we studied the influence of cuff geometry on flow mechanics, in an attempt to identify the optimum configuration.

MATERIALS AND METHODS

bench studies involved the manufacture of anatomically accurate models of varying cuff dimensions, perfused in a specifically designed flow rig, simulating physiological conditions. Flow visualisation studies incorporating laser illumination of tracer particles enabled accurate analysis of flow patterns.

RESULTS

the vortex created within the proximal cuff of each model during the deceleration phase of the cardiac cycle was strongly influenced by the aspect ratio (AR=cuff height:length). The standard and high cuffs (AR=1.63 and 1.18, respectively) demonstrated cohesive vortices and stable flow patterns. Low and long cuffs (AR=2.6 and 2.25, respectively) created more complex vortices with large areas of flow separation and low velocities.

CONCLUSIONS

aspect ratio has an important influence on flow within the distal anastomosis, with cuff dimensions of 13 mm long and 8-11 mm high (standard and high cuffs) creating beneficial flow patterns anticipated to optimise wall shear stress and inhibit myointimal hyperplasia.

Interposition vein cuff anastomosis alters wall shear stress distribution in the recipient artery

OBJECTIVE

Interposition of a vein cuff between a prosthetic infrainguinal bypass graft and a recipient infrageniculate artery can improve graft patency. There is evidence that the improved performance may be explained by a redistribution of myointimal hyperplasia (MIH) away from the critical areas at the heel and toe of the cuff-artery anastomosis. It is widely accepted that there is an association between hemodynamic forces, more specifically, low wall shear stress (WSS), and the development of MIH. The aim of this study was to determine whether the reported redistribution of MIH in the interposition vein cuff (IVC) may be explained by differences in magnitude and distribution of WSS. Design of Study and Method: Detailed flow velocity measurements were made in life-size models of conventional end-to-side (ETS) and IVC anastomoses using a two-component laser Doppler anemometer under pulsatile flow conditions. Velocity vectors were determined in the plane of symmetry of the anastomosis, and the variation of WSS was estimated from near-wall velocity measurements on the floor and upper wall of the artery.

RESULTS

The main flow features in the ETS anastomosis were flow separation at the graft hood, strong radial velocity at the heel, and a stagnation point on the floor of the artery that moved slightly during the flow cycle. In the IVC anastomosis, a coherent vortex that occupied most of the cuff volume was present from the systolic deceleration phase to end diastole. A stagnation point on the anastomosis floor was found to oscillate by about 4 mm. Critical regions of low mean WSS (ie, below 0.5 N/m(2)) were identified. In the ETS anastomosis, they were found at the heel and along the floor. In the IVC anastomosis, low mean WSS was found only on the floor, and it was generally less extensive than in the ETS anastomosis.

CONCLUSION

The vein cuff anastomosis alters the mean WSS distribution within the recipient artery and removes the area of low WSS at the heel. This may explain the redistribution of MIH away from important sites in the recipient artery.

Local haemodynamics of arterial bypass graft anastomoses

One of the main causes of failure of expanded polytetrafluoroethylene (PTFE) bypass grafts used in the lower limbs is the development of myointimal hyperplasia (MIH). Clinical studies show that higher patency rates can be obtained with the use of an autologous vein cuff (the Miller cuff) interposed between the graft and artery. The reasons for the improved performance are still unclear, but preliminary studies suggest that the change in local haemodynamics due to the cuff geometry may be the significant factor rather than the presence of autologous material. If this is the case, then PTFE grafts can be produced with an integral cuff, i.e. a precuffed graft, with similar haemodynamic patterns to that of the Miller cuff. In this paper, two different types of precuffed graft are presented and their flow patterns are compared with those recorded in the Miller cuff and the conventional end-to-side anastomosis. The haemodynamic studies were carried out using optically clear silicone rubber models under simulated in vivo pulsatile flow conditions. Flow structures similar to those observed in the Miller cuff were seen in the precuffed grafts.

Flow patterns in the radiocephalic arteriovenous fistula: an in vitro study

A significant number of late failures of arteriovenous fistulae for haemodialysis access are related to the progression of intimal hyperplasia. Although the aetiology of this process is still unknown, the geometry of the fistula and the local haemodynamics are thought to be contributory factors. An in-vitro study was carried out to investigate the local haemodynamics in a model of a Cimino-Brescia arteriovenous (AV) fistula with a 30 degrees anastomotic angle and vein-to-artery diameter ratio of 1.6. Flow patterns were obtained by planar illumination of micro-particles suspended in the fluid. Steady and pulsatile flow studies were performed over a range of flow conditions corresponding to those recorded in patients. Quantitative measurements of wall shear stress and turbulence were made using laser Doppler anemometry. The flow structures in pulsatile flow were similar to those seen in steady flow with no significant qualitative changes over the cardiac cycle. This was probably the result of the low pulsatility index of the flow waveform in AV fistulae. Turbulence was the dominant feature in the vein, with relative turbulence intensity > 0.5 within 10 mm of the suture line decreasing to a relatively constant value of about 0.10-0.15 between 40 and 70 mm from the suture line. Peak and mean Reynolds shear stress of 15 and 20 N/m2, respectively, were recorded at the suture line. On the floor of the artery, peak values of temporal mean and oscillating wall shear stress of 9.22 and 29.8 N/m2, respectively. In the vein, both mean and oscillating wall shear stress decreased with distance from the anastomosis.

Characterisation of vortex shedding in vascular anastomosis models using pulsed Doppler ultrasound

Vortex shedding at vascular anastomoses were investigated in vitro using a 20 MHz pulsed-wave Doppler velocimeter. Centreline velocity measurements were made at various axial distances in simplified polyurethane models of proximal and distal end-to-side anastomoses of angles 15, 30, 45, 60 and 80 degrees using pulsatile flow waveforms similar to those in femoropopliteal bypass grafts. The in-phase and quadrature Doppler signals were recorded and the maximum frequency waveform, averaged over 64 cycles, was obtained using short-time Fourier transform. A fourth-order Butterworth low-pass filter was employed to separate the vortex velocity signal from the convective velocity. The vortex signal envelope was calculated using a Hilbert transform method and the vortex amplitude was taken as the maximum of this envelope. The results show that higher vortex amplitude were found in the proximal anastomoses and under resting flow conditions. Although the vortex amplitudes generally increased with angles of anastomosis, they were found to be higher in the 60 degrees than in the 80 degrees proximal anastomosis. The vortex structures were investigated using spectrograms and these show prominent features at 40-50 Hz indicative of the short-duration oscillatory signals during the decelerative phase of systole expected from the passage of vortices. The study indicates that flow disturbances due to vortex shedding may be a common feature in femoropopliteal bypass grafts.

Computer aided design and fabrication of models for in vitro studies of vascular fluid dynamics

An integrated computer aided design/computer aided manufacture system has been used to model the complex geometry of blood vessel anastomoses. Computer models are first constructed with key dimensions derived from radiological images of bypass grafts, and from casts of actual blood vessel anastomoses. Physical models are then fabricated in one of two ways: the surface geometry data can be used to control the movement of a three-axis milling machine; alternatively, the same data can be exported in a form that can be interpreted by a stereolithography apparatus. Both methods produce geometrically defined solid investments that can be used in a multistep casting process that yields high-quality physical models for vascular fluid dynamic studies. This technique is useful for parametric studies.

Sites of stenosis in AV fistulae for haemodialysis access

BACKGROUND

A large proportion of late failures of radiocephalic arteriovenous fistulae are related to the progression of intimal hyperplasia. The aetiology of this process is still unknown but the fistula configuration and resultant haemodynamics have been implicated. This clinical study was devised to identify sites of stenosis in patients with fistulae and relate the findings to various clinical and geometrical parameters.

METHOD

Measurement of anastomotic length and angle was made intraoperatively in 25 consecutive fistulae. Post-operative assessment was carried out at regular intervals using duplex and colour-flow ultrasonography.

RESULTS

Stenoses were present in all 25 of the fistulae studied at 3 months. The stenoses could be classified to three specific sites: at the anastomosis (Type 1), on the inner wall of the curved region of the cephalic vein (Type 2) and just proximal to this curved segment where the vein straightens out (Type 3). Most of Type 1 and Type 2 stenoses were not progressive while Type 3 stenoses were generally progressive.

CONCLUSION

These findings emphasize the need for an effective surveillance programme of AV fistulae.

Characterizing flow distributions in AV fistulae for haemodialysis access

BACKGROUND

The arteriovenous fistula is the most widely used means of enabling vascular access for chronic haemodialysis. When interest in vascular access was at its height, in the 1970s, a substantial amount of work was carried out on the physiology of arteriovenous fistulae and on the relationship between fistula flow and patency. One omission in the catalogue of studies was research into the distribution of flow in the fistula and the effects on surgical outcome.

METHOD

To address this issue, 30 patients with end-to-side radiocephalic fistulae were studied. Flow measurements were made intraoperatively, and at various intervals postoperatively using colour-flow ultrasonography.

RESULTS

In 22 cases, the development of the fistula was monitored at regular intervals for periods of up to 27 months. In seven of the 30 fistulae, flow in the distal artery was antegrade, while in the remaining 23, distal artery flow was retrograde. The proportion of fistula flow supplied by the proximal and distal arteries varied considerably.

CONCLUSION

Despite the wide range of flow distributions in the fistulae studied, flow distribution did not appear to affect fistula maturation or long-term function.

Development of a simulator for endovascular repair of abdominal aortic aneurysms

The design and development of a simulator for endovascular repair of abdominal aortic aneurysm (AAA) is described. The simulator consists of an interchangeable model of a human AAA based on computed tomography data and is produced by means of computer-aided design and manufacture (CAD/CAM) techniques. The model has renal, iliac, and femoral arteries, and is perfused with a temperature controlled blood-analog fluid under simulated physiological flow conditions. "Fluoroscopic imaging" is simulated by a computerized imaging system that uses visible light. A movable video camera relays images in the antero-posterior and lateral planes of the AAA to a monitor. The imaging system allows "arteriography" and "road-mapping" to be performed so as to facilitate accurate deployment of endovascular stent-grafts. The system has been used for teaching and demonstrating endovascular techniques to clinicians, as well as the evaluation of new stent-graft devices. Its successful incorporation into endovascular workshops has demonstrated its role in the training of clinicians in endovascular repair of AAA.

Stable vortices within vein cuffs inhibit anastomotic myointimal hyperplasia?

OBJECTIVES

Interposition vein cuffs improve the patency of below-knee ePTFE arterial grafts, and there is evidence that they do so, at least in part, by modifying the distribution of myointimal hyperplasia (MIH) at the distal anastomosis. Alteration of local haemodynamics is one of the mechanisms which might be involved. The purpose of this study was to characterise the local haemodynamics within an interposition vein cuff.

MATERIAL AND METHODS

Flow patterns have been analysed in a laboratory model of cuffed anastomosis and compared with observations made in patients by cine intra-arterial digital subtraction angiography (IA DSA) and dynamic colour duplex scanning.

RESULTS

In contrast to non-cuffed anastomoses in which the flow is predominantly laminar, cuffed anastomoses are associated with the formation of a coherent vortex.

CONCLUSION

High frictional forces or shear stress exerted upon the arterial wall by the vortex could explain the beneficial effect of a cuff upon anastomotic MIH, in which case the optimal configuration of small vessel anastomoses would be that which most effectively promotes the formation of this type of vortex.

A prototype simulator for endovascular repair of abdominal aortic aneurysms

A prototype simulator for training in endovascular repair of abdominal aortic aneurysms (AAA) has been developed. Employing transparent models of human AAA complete with renal, iliac and femoral arteries, this system allows accurate simulation of aortography, road-mapping, catheter guidewire manipulation and stent-graft deployment while obviating the need for ionising radiation.

Chemical and morphological analysis of explanted polyurethane vascular prostheses: the challenge of removing fixed adhering tissue

During in vivo experiments to evaluate the biocompatibility and biostability of alternative biomaterials, the ideal protocol for the handling and preservation of the explanted material is often compromised in order to meet the needs of both the pathologist and the materials scientist. Explants surrounded by tissue are often fixed in formalin or glutaraldehyde to facilitate later pathological and histological analysis, but the subsequent removal of such fixed tissue from thermally sensitive and less chemically stable polymers, such as polyurethanes, poses major problems for the materials scientist, who does not wish to modify the chemical, physical or morphological characteristics of the underlying biomaterial. The present study has attempted to find a solution to this problem by exposing virgin specimens of the microporous polyurethane Vascugraft vascular prosthesis to six different cleaning conditions, all known to be effective in removing fixed tissue. These conditions included the use of 20% aqueous potassium hydroxide solution for 48 h at room temperature, 5% sodium bicarbonate solution for 5 min at the boil, and 9, 10, 11 and 12N hydrochloric acid for 48 h at room temperature. The appearance and chemical properties of the virgin and treated specimens were compared using electron spectroscopy for chemical analysis, Fourier transform infrared spectroscopy, gel permeation chromatography for molecular weight and differential scanning calorimetry techniques. The use of temperatures close to the boil resulted in the formation of a translucent, rubbery material with gross changes in the microporous and microfibrous structure. The strongly acidic and alkaline conditions caused a loss in the surface carbonate group content. In addition, 12N hydrochloric acid reduced the molecular weight and urethane content. Consequently, 9N hydrochloric acid is recommended as the cleaning agent of choice for removing fixed tissue from this type of microporous polyurethane. Control experiments on virgin material should also be included in any cleaning protocol.

Factors associated with early failure of arteriovenous fistulae for haemodialysis access

The radiocephalic arteriovenous fistula remains the method of choice for haemodialysis access. In order to assess their suitability for fistula formation, the radial arteries and cephalic veins were examined preoperatively by ultrasound colour flow scanner in conjunction with a pulse-generated run-off system. Intraoperative blood flow was measured after construction of the fistulae. Post-operative follow-up was performed at various intervals to monitor the development of the fistulae. Radial artery and cephalic vein diameter less than 1.6 mm was associated with early fistula failure. The intraoperative fistula blood flow did not correlate with the outcome of the operation probably due to vessel spasm from manipulation. However, blood flow velocities measured non-invasively 1 day after the operation were significantly lower in fistulae that failed early compared with those that were adequate for haemodialysis. Most of the increase in fistula diameter and blood flow occur within the first 2 weeks of surgery.

Numerical investigation of steady flow in proximal and distal end-to-side anastomoses

Steady flow in model proximal and distal end-to-end bypass anastomoses were simulated numerically. The predictions were compared to whole field measurements of the flow in in vitro models, and were shown to match well the general features of the measured flows. The predictions confirmed that the flows in end-to-side anastomoses are complex and three dimensional, and contain areas that could allow long residence times. Careful examination of the predictions revealed certain features of the flows not seen easily in the experiments. Shear stress and pressure on the vessel walls were predicted, and areas known to be prone to intimal hyperplasia were shown to correspond to areas of high spatial gradient of shear stress. Two anastomosis angeles, 30 and 45 deg, were considered, and it was shown that the more acute angle may have some benefit in terms of the levels of shear gradients and the power required to drive the flow through the anastomosis.

Effects of geometry and flow division on flow structures in models of the distal end-to-side anastomosis

Flow structures in models of the distal end-to-side anastomosis were visualised under steady and pulsatile flow conditions using planar illumination of suspended tracer particles. The effects of anastomosis geometry and flow in the proximal artery were investigated in models with anastomosis angles of 15, 30 or 45 degrees. The flow patterns in steady flow were highly three-dimensional and comprised two helical vortices in the distal artery, a recirculation vortex in the occluded proximal arterial segment and a stagnation point on the floor of the artery. Flow separation was observed at the toe of the anastomosis in the 30 and 45 degree models only. A second separation point was also found on the near wall of the 30 degree models at higher flow rates. Downstream flow in the proximal artery reduced and even eliminated the flow recirculation at the heel of the anastomosis, while upstream flow resulted in a captive vortex at the heel and flow reversal at the toe. In pulsatile flow, the secondary flow components in the distal artery became more pronounced during flow deceleration, particularly at higher Reynolds numbers. Significant flow reversal was observed at the toe of the anastomosis and this extended several vessel diameters along the near wall of the artery and upstream into the hood of the graft. The floor of the artery was subjected to a continually varying shear rate caused by the movement of the stagnation point during the pulsatile cycle. The results are in agreement with the observation that intimal hyperplasia occurs in regions of flow separation at the toe and the heel, and flow stagnation on the floor of the anastomosis.

Visualization of vortex shedding at the proximal side-to-end artery-graft anastomosis

The motion of guanine particles was recorded by video to visualize transitional flow phenomena in models of a proximal side-to-end anastomosis. Close examination of successive video fields revealed that above a critical Reynolds number, particles were periodically shed into the graft from a vortex situated near the anastomosis heel, and this disturbed the flow patterns in the graft causing vortex shedding to occur near to the toe of the anastomosis. The images clearly demonstrated that periodic flow structures propagated distally along the graft for over 15 tube diameters from the proximal anastomosis. The frequency of the vortex shedding was found to increase with Reynolds number. Under pulsatile conditions, the primary vortex at the heel of the anastomosis became unstable during the deceleration phase of the flow cycle and particles were shed downstream into the graft. Although it was possible briefly to observe the characteristic banded structure in the bypass graft, the flow patterns were highly three-dimensional and were quickly broken up by the accelerating flow. Dynamic flow visualization using guanine particles was found to be a complementary technique to particle tracer flow visualization and was highly effective in identifying transitional flow phenomena and the mass transport mechanisms associated with them.

In vivo analysis and three-dimensional visualisation of blood flow patterns at vascular end-to-side anastomoses

OBJECTIVES

The aim of this study was to describe the velocity fields at distal vascular end-to-side anastomoses with different anastomosis angles in vivo.

MATERIALS AND METHODS

The abdominal aorta of ten 90 kg pigs was exposed from the superior mesenteric artery to the trifurcation. A segment of the aorta was bypassed using a polyurethane graft. Three anastomosis angles: 90 degrees (n = 3), 45 degrees (n = 3) and 15 degrees (n = 4) were studied. The bypass length, the anatomical position and the geometry of the anastomoses were standardised. During measurements, the proximal outflow segment was occluded and the flow rate was controlled by reversible iliac artery cross-clamping. Using a colour Doppler system the velocity fields were measured at various positions in the anastomosis. The colour Doppler velocity data were transferred to a computer for dynamic three-dimensional visualisation of the velocity profiles.

RESULTS

The angulation was reproduced within 10%. During the experiment, the flow rate was kept constant with Reynold's numbers typical for peripheral arteries. In the 90 degrees anastomoses very disturbed flow fields were seen. The 45 degrees anastomoses were characterised by: (1) low antegrade and retrograde velocities at the heel and (2) a zone of reverse and oscillating velocities at the toe and at one diameter downstream of the toe (1DDD) during deceleration. In the 15 degrees anastomoses no flow disturbances were seen either at the toe or at 1DDD. The velocity profiles were close to parabolic at peak flow at both positions.

CONCLUSIONS

It is concluded that the 15 degrees anastomosis is preferable from a haemodynamic point of view.

Flow structures at the proximal side-to-end anastomosis. Influence of geometry and flow division

Flow structures were visualized in transparent polyurethane models of proximal side-to-end vascular anastomoses, using planar illumination of suspended tracer particles. Both the effects of geometry and flow division were determined under steady and pulsatile flow conditions, for anastomosis angles of 15, 30, and 45 degrees. The flow patterns were highly three-dimensional and were characterized by a series of vortices in the fully occluded distal artery and two helical vortices aligned with the axis of the graft. In steady flow, above a critical Reynolds number, the flow changed from a laminar regime to one displaying time-dependent behavior. In particular, significant fluctuating velocity components were observed in the distal artery and particles were shed periodically from the occluded artery into the graft. Pairs of asymmetric flow patterns were also observed in the graft, before the onset of the time-dependent flow regime. The critical Reynolds number ranged from 427 to 473 and appeared to be independent of anastomosis angle. The presence of a patent distal artery had a significant effect on the overall flow pattern and led to the formation of a large recirculation region at the toe of the anastomosis. The main structures observed in steady flow, such as vortices in the distal artery and helical flow in the graft, were also seen during the pulsatile cycle. However, the secondary flow components in the graft were more pronounced in pulsatile flow particularly during deceleration of the flow waveform. At higher mean Reynolds numbers, there was also a greater mixing between fluid in the occluded arterial section and that in the graft.

Removing fresh tissue from explanted polyurethane prostheses: which approach facilitates physico-chemical analysis?

Chemical, physical and structural analyses of polymers from explanted vascular prostheses are frequently jeopardized because of incomplete removal of the encroaching host tissue. In this study, microporous polyurethane arterial prostheses implanted as a canine thoraco-abdominal bypass were explanted after 1 and 12 months and were cleaned without fixation using four different digesting enzyme treatments, including collagenase, pancreatin and trypsin alone and collagenase and pancreatin in series, followed by washing in a solution of Triton X-100 detergent. By following this approach all the fresh tissue attached to the external and internal walls of the prostheses was removed with minimal damage to the underlying synthetic polymer. The morphology of the explanted and cleaned polyurethane prostheses could be obtained readily by light and scanning electron microscopy. Surface microporous features and the presence of polyurethane microfibres that had experienced in vivo biodegradation could therefore be identified easily. The surface and bulk physico-chemical properties of the polyurethane polymer were determined by electron spectroscopy for chemical analysis, attenuated total reflectance-Fourier transform infrared spectroscopy and differential scanning calorimetry. It was found that the most successful approach for removing fresh tissue and exposing a clean and uncontaminated polyurethane surface was to incubate the explanted samples first in collagenase followed by digestion in pancreatin. This particular cleaning technique has proved valuable in enabling us to monitor small in vivo changes in the surface chemistry and in the bulk microphase segmented structure of polyurethane biomaterials.

An in vivo model for studying the local haemodynamics of end-to-side anastomoses

OBJECTIVES

To develop an in-vivo model to study the anastomotic flow patterns.

DESIGN

Prospective, open, animal study.

METHODS

Polyurethane grafts with an internal diameter equal to the abdominal aorta (8 mm) of 90 kg pigs were implanted as bypass grafts from the supra-renal to the infra-renal level. A novel technique for constructing anastomoses with different anastomosis angles and only slight dilatation was used. The proximal outflow segment was occluded and the flow rate through the graft controlled by clamping the iliac arteries. Visualisation of the flow-fields at the distal end-to-side anastomosis was achieved by a comprehensive colour Doppler mapping protocol.

RESULTS

The angulation of the anastomoses was controllable and reproducible. Gross haemodynamic parameters were stable within physiological ranges and were typical for peripheral bypass grafts. The flow fields at the distal end-to-side anastomosis were visualised and found to be in accordance with those reported by in vitro studies. Using different angles of Doppler insonation the same flow field characteristics were found.

CONCLUSIONS

The model is an appropriate tool for studies of the effects of anastomotic geometry on local flow fields in vivo.

Effect of geometric taper on thrombosis in vascular prostheses: an ex vivo study

A common method of assessing the thrombogenicity of prosthetic arterial grafts is to determine the amount of platelets and fibrin that accumulates on their surfaces. This study was designed to compare thrombus formation in cylindrical and tapered vascular grafts (angle of taper 0.5 deg) made from the same biomaterial. Turbulence was generated upstream of the grafts by means of a 75% axi-symmetric stenosis. In general, turbulence is dissipated by the effect of blood viscosity, whereas in a tapered graft, there is an additional reduction in the intensity of turbulence owing to the effect of converging flow through the taper. The aim of this study was to assess the effect of turbulence on thrombus formation in cylindrical and tapered grafts. After blood had circulated for 2 and 4 h through the cylindrical and tapered grafts, which formed part of two parallel ex vivo circuits, platelet deposition and fibrinogen uptake were measured by means of a radioisotope technique. Although the flow disturbances were less in the tapered grafts, no significant difference in platelet and fibrin deposition in the two types of graft was observed at 2 h. For the longer perfusion time, however, increased platelet and fibrin activity was recorded in the annular vortex region in the tapered grafts. In general, neither platelets nor fibrin accumulated in areas of high turbulence intensity.

Input impedance of radiocephalic arteriovenous fistulae for haemodialysis access: its value in predicting early failure

This study is concerned with an assessment of the quality of the blood vessels used in the construction of radiocephalic arteriovenous fistulae for haemodialysis vascular access in 20 patients. Following non-invasive preoperative assessment of the cephalic vein by means of a colour Doppler scanner, input impedance was determined intraoperatively from blood flow and blood pressure measured about 2 cm downstream of the anastomosis. The patients were re-assessed 1 day and 2, 4, 6 and 12 weeks after surgery, and the diameter and the flow through the fistulae, the location of major branches, and the presence and severity of stenoses in the cephalic veins were determined non-invasively. Five fistulae failed within the 12 weeks period following surgery, mainly due to thrombosis. In a further 3 patients, the fistulae were patent but did not achieve an adequately high blood flow and dilate sufficiently for haemodialysis and were therefore considered to be essentially failed. The impedance moduli from 0 to 10 Hz as well as the average impedance modulus of successful fistulae were significantly lower than those of fistulae that subsequently failed. An attempt was made to locate the presence of reflection sites (i.e. stenoses and branches) from the minima in the impedance modulus spectra. Their locations were compared with those of the reflection sites detected by ultrasound 2 weeks after surgery. However, not all reflection sites detected 2 weeks after surgery were located by the impedance method. It is recommended that both preoperative examination and intraoperative haemodynamic measurements be made to improve the accuracy of the assessment.

Pulsatile velocity distribution and wall shear rate measurement using pulsed Doppler ultrasound

Instantaneous pulsatile velocity profiles were measured in a cylindrical tube using a 20 MHz pulsed Doppler ultrasound system. The spatial resolution of the device was improved with deconvolution using a procedure which employed frequency-domain windowing to suppress high-frequency noise. After deconvolution both the instantaneous velocity profiles and flow waveforms were compared with established theoretical predictions and good agreement was obtained. The average error in a velocity measurement was 7.9 +/- 0.9%, and the average error in the flow rate was 4.7 +/- 1.7%. The wall shear rate was determined from the gradient of the velocity profile both before and after deconvolution. The results were compared to the theoretical value of wall shear rate to quantify the accuracy of the technique under pulsatile flow conditions. Although the velocity profiles agreed well with theory, the wall shear rate was obtained less accurately. The average error in the mean wall shear rate and peak-to-peak wall shear rate was 28 +/- 7% and 11 +/- 3%, respectively. However, some of this discrepancy, was attributed to the method of evaluation. In all cases deconvolution significantly improved the accuracy of the wall shear rate determination.

Quantitative measurement of wall shear rate by pulsed Doppler ultrasound

A 20 MHz pulsed Doppler ultrasound system was used to obtain steady flow velocity profiles in cylindrical tubes. A deconvolution technique was employed to improve the spatial resolution and the corrected results were compared with Poiseuille's theory. Frequency domain windowing was used to suppress noise in the deconvolution procedure, using a method which reduced distortions at the window boundaries. Wall shear rate was calculated from the measured velocity profiles both before and after deconvolution, and compared with theory to quantify the accuracy of the technique. The results illustrate the significant improvement provided by deconvolution, with the average error in measured wall shear rate reducing from 46 to 12% after applying the technique. Some of the discrepancy is in part attributed to the method used to determine wall shear rate.

Engineering design of vascular prostheses

The replacement and bypass of arteries of diameter greater than 6 mm with textile vascular prostheses has proved very successful since they were first introduced forty years ago. Although manufacturers continue to improve their products and make them of consistent quality for increased safety and performance and to facilitate their use by surgeons, most of the research work in this area is concerned with the development of small-diameter prostheses. Current expanded PTFE and textile prostheses do not perform satisfactorily when their diameters are reduced to less than 6 mm. For the small-diameter prostheses it will be necessary to develop less thrombogenic materials and to design the structure of the prostheses more closely to match the mechanical properties of the natural arteries. The purpose of this paper is to discuss the design requirements and to review the development of large- and small-diameter vascular prostheses.

Exaggerated sensitivity to NE-induced vasoconstriction in IDDM patients with microalbuminuria. Possible etiology and diagnostic implications

Increased urinary albumin excretion rate (AER) in the microalbuminuric phase of diabetic nephropathy has been attributed to intraglomerular hypertension. This could be caused by constriction of efferent glomerular arterioles, which carry alpha-adrenoceptors. We tested the hypothesis that insulin-dependent diabetes mellitus (IDDM) patients with microalbuminuria are hypersensitive to vasoconstriction induced by norepinephrine (NE). We studied 15 IDDM patients with microalbuminuria (AER 32-295 mg/24 h), 13 IDDM patients with normal AER (5-24 mg/24 h), and 9 nondiabetic subjects (AER 8-22 mg/24 h). All were normotensive. NE-induced vasoconstriction was measured in dorsal hand veins, which carry alpha-receptors similar to those of glomerular efferent arterioles. Vein diameter was measured with a linear displacement probe during a stepped NE infusion (1-32 ng/min) into the vein, and venoconstriction was expressed as a percentage of the maximum passively distended venous diameter. Microalbuminuric IDDM patients exhibited significantly greater vasoconstriction (P less than 0.005) at all NE infusion rates than both other groups. The NE infusion rate producing 50% of maximal venoconstriction (ED50) in the microalbuminuric IDDM group (median 1.1 ng/min, range 0.2-25.2 ng/min) was significantly less than in both the normoalbuminuric IDDM group (median 12.5 ng/min, range 4.9-40.5 ng/min, P = 0.00007) and the nondiabetic group (median 17.7 ng/min, range 5.9-42.2 ng/min, P = 0.0003). Dose-response curves and ED50 did not differ significantly between normalbuminuric IDDM and nondiabetic groups. IDDM patients with microalbuminuria are hypersensitive to NE-induced vasoconstriction.(ABSTRACT TRUNCATED AT 250 WORDS)