Design and evaluation of a novel subatmospheric pressure bioreactor for the preconditioning of tissue-engineered vascular constructs.

PURPOSE: The pre-conditioning of tissue-engineered vascular scaffolds with mechanical stimuli is being recognised as an essential step in producing a functional vascular construct. In this study we design and evaluate a novel bioreactor, which exerts a mechanical strain on developing vascular scaffolds via subatmospheric pressure. METHODS: We design and construct a bioreactor, which exerts subatmospheric pressure via a vacuum assisted closure unit. Vascular scaffolds seeded with human umbilical endothelial cells were evaluated for structural integrity, microbial contamination, cellular viability, von Willebrand factor (VWF) production, cell proliferation and morphology under a range of subatmospheric pressures (75-200mmHg). RESULTS: The bioreactor produced sustained subatmospheric pressures, which exerted a mechanical strain on the vascular scaffold. No microbial contamination was found during the study. The structural integrity of the vascular construct was maintained. There was no difference in cellular viability between control or subatmospheric pressure groups (p = 0.817). Cells continued to produce VWF under a range of subatmospheric pressures. Cells subjected to subatmospheric pressures of 125mmHg and 200mmHg exhibited higher levels of growth than cells in atmospheric pressure at 24 (p≤0.016) and 48 hour (p≤0.001). Negative pressure affected cellular morphology, which were more organised, elongated and expanded when exposed to subatmospheric pressure. CONCLUSIONS: We have constructed and validated a novel subatmospheric bioreactor. The bioreactor maintained a continuous subatmospheric pressure to the vascular scaffolds in a stable, sterile and constant environment. The bioreactor exerted a strain on the vascular sheets, which was shown to alter cellular morphology and enhance cellular proliferation.

Fine-wire electromyography response to neuromuscular electrical stimulation in the triceps surae.

Neuromuscular electrical stimulation (NMES) has previously been used to enhance venous return from the lower leg. By artificially activating lower leg muscles, venous blood may be effectively ejected from the muscle and adjacent veins. It could easily be assumed that combined NMES of the gastrocnemius and soleus would be the most effective single-channel application in this regard, as these muscles represent the largest muscular bulk in the lower leg. However, we have previously reported that soleus stimulation in isolation is substantially more effective. To understand why this is the case, we recorded fine-wire electromyography during NMES of the gastrocnemius and soleus muscles. We found that gastrocnemius and soleus stimulation are effective in eliciting selective stimulation of these muscles. However, combined stimulation of these muscles using a single set of electrodes was only capable in generating ∼ 50% of the response in each muscle, insufficient to generate their theoretical maximum venous return.

Remote ischaemic preconditioning as a method for perioperative cardioprotection: concepts, applications and future directions.

Remote ischaemic preconditioning (RIPC) is a phenomenon whereby brief episodes of non-lethal ischaemia in one organ or tissue can render a distant organ or tissue resistant to subsequent longer ischaemic insults. It represents an exciting perioperative risk reduction strategy as it allows cardioprotection (and organ protection in general) from injuries that are caused by multiple mechanisms. Several proof of concept studies show benefits in cardiovascular interventions and in a variety of other procedures. However convincing and consistent evidence of benefits in patient important outcomes is lacking but may emerge with the completion of large scale studies. This article aims to provide a concise review of the origins and concepts of RIPC. It will revisit the biological theories of RIPC and the clinical applications thus far. The article concludes by discussing the current status of multi-centre cardiovascular RIPC research and the future challenges that investigators must overcome.

The biaxial mechanical behaviour of abdominal aortic aneurysm intraluminal thrombus: classification of morphology and the determination of layer and region specific properties.

Intraluminal thrombus (ILT) is present in 75% of clinically-relevant abdominal aortic aneurysms (AAAs) yet, despite much research effort, its role in AAA biomechanics remains unclear. The aim of this work is to further evaluate the biomechanics of ILT and determine if different ILT morphologies have varying mechanical properties. Biaxial mechanical tests were performed on ILT samples harvested from 19 patients undergoing open surgical repair. ILT were separated into luminal, medial and medial/abluminal layers. A total of 356 tests were performed and the Cauchy stress (σ) and tangential modulus (TM) at a stretch ratio (λ) of 1.14 were recorded for each test in both the circumferential (θ) and longitudinal (L) directions. Our data revealed three distinct types of ILT morphologies, each with a unique set of mechanical properties. All ILT layers were found to be isotropic and inhomogeneous. Type 1 (n=10) was a multi-layered ILT (thick medial/abluminal layer) whose strength and stiffness decreased gradually from the luminal to the medial/abluminal layer. Type 2 (n=6) was a multi-layered ILT (thin/highly degraded medial/abluminal layer) whose strength and stiffness decreased abruptly between the luminal and medial/abluminal layer and Type 3 (n=3) is a single layered ILT with a lower strength and stiffness than Types 1 and 2. In a sub-study, we found the luminal layer to be stronger and stiffer in the posterior than the anterior region. This work provides further insights to the biomechanical behaviour of ILT and the use of our ILT classification may be useful in future studies.

Systematic review and meta-analysis of routine total body CT compared with selective CT in trauma patients.

BACKGROUND: Full-body CT scanning is increasingly being used in the initial evaluation of severely injured patients. We sought to analyse the literature to determine the benefits of full-body scanning in terms of mortality and length of time spent in the emergency department (ED). METHODS: A systematic search of the Pubmed and Cochrane Library databases was performed. Eligible studies compared trauma patients managed with selective CT scanning with patients who underwent immediate full-body scanning. Using random effects modelling, the pooled OR was used to calculate the effect of routine full-body CT on mortality while the pooled weighted mean difference was used to analyse the difference in ED time. RESULTS: Five studies (8180 patients) provided mortality data while four studies (6073 patients) provided data on ED time. All were non-randomised cohort studies and were prone to several sources of bias. There was no mortality difference between groups (pooled OR=0.68; 95% CI 0.43 to 1.09, p=0.11). There was a significant reduction in the time spent in the ED when patients underwent full-body CT (pooled effect size of weighted mean difference=-32.39 min; 95% CI -51.78 to -13.00; p=0.001). CONCLUSIONS: We eagerly await the results of randomised controlled trials. Firm clinical outcome data are expected to emerge in the near future, though data on cost and radiation exposure will be needed before definitive conclusions can be made.

Randomised controlled trial comparing European standard class 1 to class 2 compression stockings for ulcer recurrence and patient compliance.

The aim of this study was to determine the rate of venous ulcer recurrence and the level of compliance in patients wearing European class 1 or class 2 compression stockings. A total of 100 patients with healed venous leg ulcers were recruited, and were randomised to either class 1 (n = 50) or class 2 (n = 50) compression stockings. Follow-up was at 1 week, 3, 6, 9 and 12 months to monitor ulcer recurrence and compliance. Patients had a duplex scan to identify the source of venous incompetence. The rate of ulcer recurrence after 12 months was 16·1%, and the difference in recurrence rate between classes was not statistically significant (P = 0·287) although greater numbers in class 1 developed a recurrence. Participants (88·9%) were compliant; non-compliant patients were at a significantly greater risk of recurrence (P≤ 0·0001). Thirteen patients had both superficial and deep incompetence; those randomised to class 1 stockings (n = 4) developed ulcer recurrence. Patients with a history of multiple episodes of ulceration were more likely to develop a recurrence (P = 0·001). The lowest venous ulcer recurrence rates were seen in patients who were compliant with hosiery regardless of the compression level. Patients with both superficial and deep incompetence had a lower rate of recurrence with class 2 compression.

A systematic review of preoperative duplex ultrasonography and arteriovenous fistula formation.

BACKGROUND: Arteriovenous fistula (AVF) formation for dialysis access is a common procedure. Fistula maturation is unpredictable. Preoperative duplex mapping may increase procedural success. We undertook a systematic review to assess the effect of preoperative duplex mapping on subsequent AVF patency. METHODS: The published literature was searched on PubMed and the Cochrane Library using the following keywords: 'arteriovenous fistula,' 'venous mapping,' 'ultrasound,' 'hemodialysis,' 'vascular access,' and 'perioperative vessel mapping.' Conference proceedings were hand searched for otherwise unpublished trials. Only randomized controlled trials in which preoperative duplex mapping was compared with clinical evaluation were eligible. RESULTS: Three trials (402 patients) were identified. More patients who underwent ultrasound successfully started using their fistula for dialysis access, although the difference did not reach statistical significance (174/214 vs 130/188; pooled odds ratio, 1.96; P = .11). CONCLUSIONS: Preoperative duplex mapping may improve fistula maturation rates. However, the results do not reach statistical significance and there are no cost-effectiveness data. Further work is required.

On the influence of patient-specific material properties in computational simulations: a case study of a large ruptured abdominal aortic aneurysm.

Patient-specific modelling of abdominal aortic aneurysm has been shown to have clinical potential. This paper examines a large ruptured abdominal aortic aneurysm where the tissue from the diseased wall and the intraluminal thrombus was excised during open surgical repair and experimentally characterised. The mechanical data were used to develop material parameters that were incorporated into finite element models with measured nonuniform wall thickness. Implementation of the material data into the numerical model increased peak wall stress by 67%, wall strain by 320% and displacement by 177%, when compared with simulations based on material properties available in the literature. Distributions of numerical results were similar for both material data. Magnitudes of numerical results can differ significantly when using patient-specific material properties and therefore, care should be taken when interpreting numerical results derived from population-based data.

Ultrasound guidance for difficult peripheral venous access: systematic review and meta-analysis.

BACKGROUND: Establishing intravenous access is often vital in an acute hospital setting but can be difficult. Ultrasound-guided cannulation increases success rates in prospective studies. However, these studies have often lacked a comparative group. This systematic review and meta-analysis aimed to determine the clinical effectiveness of Ultrasound-guided peripheral intravenous cannulation compared with the standard technique in patients known to have difficult access. METHODS: Electronic abstract databases, trial registries, article reference lists and internet repositories were searched using the following search terms: 'peripheral venous cannulation', 'peripheral venous access'. Studies meeting the following criteria were included: randomised controlled trial patients of all ages who required peripheral intravenous access; interventions were Ultrasound-guided versus standard cannulation technique; patients were identified as having difficult venous access; inclusion of at least one defined outcome (procedural success time to cannula placement; number of attempts). RESULTS: 7 trials were identified (289 participants). Ultrasound guidance increases the likelihood of successful cannulation (pooled OR 2.42; 95% CI 1.26 to 4.68; p=0.008). There were no differences in time to successful cannulation, or number of percutaneous skin punctures. CONCLUSION: Ultrasound guidance increases the likelihood of successful peripheral cannulation in difficult access patients. We recommend its use in patients who have difficult venous access, and have failed venous cannulation by standard methods. Further randomised controlled trials (RCTs) with larger sample sizes would be of benefit to investigate if Ultrasound has any additional advantages in terms of reducing the procedure time and the number of skin punctures required for successful venous cannulation.

Comparison of single- and two-channel neuromuscular electrical stimulation sites for enhancing venous return.

Neuromuscular electrical stimulation (NMES) has previously been used to activate the musculature of the lower leg and increase venous return to the heart. However, there is little evidence to suggest the superiority of one particular stimulation site over another. In this paper, we aim to reveal the optimal stimulation site on the lower leg in a group of healthy adults. Doppler ultrasound measurements of venous blood volume expelled and peak venous velocity in response to various single and two-channel applications of NMES were taken for each subject. We found that soleus NMES is the most effective single-channel stimulation method, capable of expelling 58.3% of the blood achieved during a voluntary contraction, alternatively soleus plus tibialis posterior stimulation is capable of expelling 76.5%. Based on these and other factors we suggest that the soleus and soleus plus tibialis posterior are the most effective NMES sites for improving venous return.

Hemodynamic effects of habituation to a week-long program of neuromuscular electrical stimulation.

OBJECTIVES: Neuromuscular electrical stimulation (NMES) of the calf muscles has been shown to cause instantaneous increases in venous outflow from the lower leg and could be used as an adjunct to current gold-standard compression therapies for the prevention of venous stasis and its related pathologies. However, little is known about the effects of NMES in combination with compression therapies on subject comfort, compliance and popliteal venous blood flow over the course of a week-long NMES protocol. This study aimed to assess the effects of a NMES and compression protocol for the prevention of venous stasis on the compliance, comfort and venous blood flow of healthy volunteers over the course of seven days. DESIGN: Twenty-four healthy subjects were assigned to either a stimulation or control group. The stimulation group received 1.5 h of NMES daily while the control group received none. Daily measures of popliteal venous blood flow, subject compliance and comfort were recorded over 7 days. RESULTS: Ejected blood flow volumes and peak velocities in the popliteal vein during NMES were sustained over a 30-min stimulation session and increased by approximately 100% over the course of seven days. Mean stimulation intensities increased progressively throughout the week, while perceived pain during NMES decreased significantly. Mean compliance to the 7-day protocol was 100%. CONCLUSION: User habituation to a combined NMES and compression protocol resulted in significant increases in ejected venous volume and peak velocity over the course of 7 days. This resulted in the highest ejected venous volume reported from a single NMES induced contraction of the calf muscles to date which was twice the magnitude of values previously reported in the literature. These findings suggest that NMES based protocols applied over an extended period of days, weeks or months may provide greater hemodynamic effect for the prevention of venous stasis than previously observed during NMES sessions lasting less than a few hours.

Technique for the computation of lower leg muscle bulk from magnetic resonance images.

An unsupervised technique to estimate the relative size of a patient's lower leg musculature in vivo using magnetic resonance imaging (MRI) in the context of venous insufficiency is presented. This post-acquisition technique was designed to segment calf muscle bulk, which could be used to make inter- or intra-patient comparisons of calf muscle size in the context of unilateral leg ulcers and venous return. Pre-processing stages included partial volume reduction, intensity inhomogeneity correction and contrast equalization. The algorithm created a binary mask of voxels that fell within a computed threshold designated as representing muscle based on a 3-class fuzzy clustering approach. The segmentation was improved using a set of morphological operations to remove adipose tissue, spongy bone and cortical bone. The technique was evaluated for accuracy against a manual segmented ground truth. Results showed that the automatic technique performed sufficiently well in terms of accuracy and efficacy. The automatic method did not suffer from intra-observer variability.

The anatomy and physiology of the venous foot pump.

The presence of a venous pumping mechanism in the foot may be significant for venous return in the lower extremities. However, there has been a lack of conclusive research in the area to date and controversy still exists over the detailed anatomy and physiologic mechanism of the venous foot pump. A full understanding of the anatomy and physiology of the venous foot pump is essential for designing effective interventions for the prevention, treatment, and management of venous disease in the lower limbs. This article highlights and discusses the relevant literature relating to the anatomy and physiology of the venous foot pump. In addition, the plantar aspects of 10 cadaveric feet were dissected. These dissections revealed the presence of a previously unreported secondary deep plantar arch and/or deep system of venous connections in the foot and facilitated a more detailed description of the patterns of doubling and branching of the primary veins of the foot. The results of these dissections are discussed within the context of previous work in the field with the aid of detailed diagrams of the dissected feet and may provide a backdrop for the physiology of the venous foot pump and its potential role in lower limb circulation. This is discussed in the last section of the article, which also highlights existing controversy regarding the role of weight bearing and muscular contraction as the dominant mechanisms for venous pumping in the foot.

New pulsatile hydrostatic pressure bioreactor for vascular tissue-engineered constructs.

Mechanical conditioning represents a potential means to enhance the biochemical and biomechanical properties of tissue-engineered cell constructs. Bioreactors that can simulate physiologic conditions can play an important role in the preparation of tissue-engineered constructs. Although various forms of bioreactor systems are currently available, these have certain limitations, particularly when these are used for the creation of vascular constructs. The aim of the present report is to describe and validate a novel pressure bioreactor system for the creation of vascular tissue. Here, we present and discuss the design concepts, criteria, as well as the development of a novel pressure bioreactor. The system is compact and easily housed in an incubator to maintain sterility of the construct. Moreover, the proposed bioreactor, in addition to mimicking in vivo pressure conditions, is flexible, allowing different types of constructs to be exposed to various physiologic pressure conditions. The core bioreactor elements can be easily sterilized and have good ergonomic assembly characteristics. This system is a fundamental tool, which may enable us to make further advances in bioreactor technology and tissue engineering. The novel system allows for the application of pressure that may facilitate the growth and development of constructs needed to produce a tissue-engineered vascular graft.

The influence of a eutectic mixture of lidocaine and prilocaine on minor surgical procedures: a randomized controlled double-blind trial.

BACKGROUND: A eutectic mixture of lidocaine and prilocaine (EMLA) has been shown to be effective in reducing pain from needle sticks, including those associated with blood sampling and intravenous insertion. OBJECTIVE: To evaluate the effectiveness of EMLA cream applied before needle puncture for local anesthetic administration before minor surgical procedures in this double-blind, randomized, controlled, parallel-group study. MATERIALS AND METHODS: Patients were randomly assigned to receive EMLA or placebo cream (Aqueous) applied under an occlusive dressing. After the procedure, patients were asked to rate the needle prick and procedure pain on a visual analog scale (0=no pain; 10=maximum pain). RESULTS: A total of 94 minor surgical procedures (49 in EMLA and 45 in control) were performed. The mean needle-stick pain score in the EMLA group was significantly lower than in the control group (2.7 vs. 5.7, p<.001, Mann-Whitney U-test). There was also significantly lower procedure pain in the EMLA group than in the control group (0.83 vs. 1.86, p=.009). There were no complications associated with the use of EMLA. CONCLUSION: EMLA effectively reduces the preprocedural needle-stick pain and procedural pain associated with minor surgical procedures.

Vessel asymmetry as an additional diagnostic tool in the assessment of abdominal aortic aneurysms.

OBJECTIVE: Abdominal aortic aneurysm (AAA) rupture is believed to occur when the local mechanical stress exceeds the local mechanical strength of the wall tissue. On the basis of this hypothesis, the knowledge of the stress acting on the wall of an unruptured aneurysm could be useful in determining the risk of rupture. The role of asymmetry has previously been identified in idealized AAA models and is now studied using realistic AAAs in the current work. METHODS: Fifteen patient-specific AAAs were studied to estimate the relationship between wall stress and geometrical parameters. Three-dimensional AAA models were reconstructed from computed tomography scan data. The stress distribution on the AAA wall was evaluated by the finite element method, and peak wall stress was compared with both diameter and centerline asymmetry. A simple method of determining asymmetry was adapted and developed. Statistical analyses were performed to determine potential significance of results. RESULTS: Mean von Mises peak wall stress +/- standard deviation was 0.4505 +/- 0.14 MPa (range, 0.3157-0.9048 MPa). Posterior wall stress increases with anterior centerline asymmetry. Peak stress increased by 48% and posterior wall stress by 38% when asymmetry was introduced into a realistic AAA model. CONCLUSION: The relationship between posterior wall stress and AAA asymmetry showed that excessive bulging of one surface results in elevated wall stress on the opposite surface. Assessing the degree of bulging and asymmetry that is experienced in an individual AAA may be of benefit to surgeons in the decision-making process and may provide a useful adjunct to diameter as a surgical intervention guide.

A review of the in vivo and in vitro biomechanical behavior and performance of postoperative abdominal aortic aneurysms and implanted stent-grafts.

Endovascular repair of abdominal aortic aneurysms has generated widespread interest since the procedure was first introduced two decades ago. It is frequently performed in patients who suffer from substantial comorbidities that may render them unsuitable for traditional open surgical repair. Although this minimally invasive technique substantially reduces operative risk, recovery time, and anesthesia usage in these patients, the endovascular method has been prone to a number of failure mechanisms not encountered with the open surgical method. Based on long-term results of second- and third-generation devices that are currently becoming available, this study sought to identify the most serious failure mechanisms, which may have a starting point in the morphological changes in the aneurysm and stent-graft. To investigate the "behavior" of the aneurysm after stent-graft repair, i.e., how its length, angulation, and diameter change, we utilized state-of-the-art ex vivo methods, which researchers worldwide are now using to recreate these failure modes.

Fibrin: a natural biodegradable scaffold in vascular tissue engineering.

Arterial occlusive disease remains a major health issue in the developed world and a rapidly growing problem in the developing world. Although a growing number of patients are now being effectively treated with minimally invasive techniques, there remains a tremendous pressure on the vascular community to develop a synthetic small-diameter vascular graft with improved long-term patency rates. The field of tissue engineering offers an exciting alternative in the search for living organ replacement structures. Several methodologies have emerged for constructing blood vessel replacements with biological functionality. Common strategies include cell-seeded biodegradable synthetic scaffolds, cell self-assembly, cell-seeded gels and xenogeneic acellular materials. A wide range of materials are being investigated as potential scaffolds for vascular tissue engineering applications. Some are commercialised and others are still in development. Recently, researchers have studied the role of fibrin gel as a three-dimensional scaffold in vascular tissue engineering. This overview describes the properties of fibrin gel in vascular tissue engineering and highlights some recent progress and difficulties encountered in the development of cell fibrin scaffold technology.