The effects of external compression on venous blood flow and tissue deformation in the lower leg

Spatio-temporal Relationship between Three-Dimensional Deformations of a Collapsible Tube and the Downstream Flowfield

The interactions between fluid flow and structural components of collapsible tubes are representative of those in several physiological systems. Although extensively studied, there exists a lack of characterization of the three-dimensionality in the structural deformations of the tube and its influence on the flow field. This experimental study investigates the spatio-temporal relationship between 3D tube geometry and the downstream flow field under conditions of fully open, closed, and slamming-type oscillating regimes. A methodology is implemented to simultaneously measure three-dimensional surface deformations in a collapsible tube and the corresponding downstream flow field. Stereophotogrammetry was used to measure tube deformations, and simultaneous flow field measurements included pressure and planar Particle Image Velocimetry (PIV) data downstream of the collapsible tube. The results indicate that the location of the largest collapse in the tube occurs close to the downstream end of the tube. In the oscillating regime, sections of the tube downstream of the largest mean collapse experience the largest oscillations in the entire tube that are completely coherent and in phase. At a certain streamwise distance upstream of the largest collapse, a switch in the direction of oscillations occurs with respect to those downstream. Physically, when the tube experiences constriction downstream of the location of the largest mean collapse, this causes the accumulation of fluid and build-up of pressure in the upstream regions and an expansion of these sections. Fluctuations in the downstream flow field are significantly influenced by tube fluctuations along the minor axes. The fluctuations in the downstream flowfield are influenced by the propagation of disturbances due to oscillations in tube geometry, through the advection of fluid through the tube. Further, the manifestation of the LU-type pressure fluctuations is found to be due to the variation in the propagation speed of the disturbances during the different stages within a period of oscillation of the tube.

Venous Thromboembolism: Review of Clinical Challenges, Biology, Assessment, Treatment, and Modeling

Venous thromboembolism (VTE) is a massive clinical challenge, annually affecting millions of patients globally. VTE is a particularly consequential pathology, as incidence is correlated with extremely common risk factors, and a large cohort of patients experience recurrent VTE after initial intervention. Altered hemodynamics, hypercoagulability, and damaged vascular tissue cause deep-vein thrombosis and pulmonary embolism, the two permutations of VTE. Venous valves have been identified as likely locations for initial blood clot formation, but the exact pathway by which thrombosis occurs in this environment is not entirely clear. Several risk factors are known to increase the likelihood of VTE, particularly those that increase inflammation and coagulability, increase venous resistance, and damage the endothelial lining. While these risk factors are useful as predictive tools, VTE diagnosis prior to presentation of outward symptoms is difficult, chiefly due to challenges in successfully imaging deep-vein thrombi. Clinically, VTE can be managed by anticoagulants or mechanical intervention. Recently, direct oral anticoagulants and catheter-directed thrombolysis have emerged as leading tools in resolution of venous thrombosis. While a satisfactory VTE model has yet to be developed, recent strides have been made in advancing in silico models of venous hemodynamics, hemorheology, fluid-structure interaction, and clot growth. These models are often guided by imaging-informed boundary conditions or inspired by benchtop animal models. These gaps in knowledge are critical targets to address necessary improvements in prediction and diagnosis, clinical management, and VTE experimental and computational models.

Multiple blood flow surges during intermittent pneumatic compression: The origins and their implications

Intermittent pneumatic compression (IPC) therapy has been used to enhance peripheral blood flow for prevention and rehabilitation of ischemic-related vascular diseases. A novel phenomenon has been reported that multiple blood flow surges appeared in the skin blood flow signal during each compression, but its mechanism has not been fully revealed. This study aimed to gain insights into the origins of these blood flow surges through experiment and biomechanical modeling methods. Foot skin blood flow (SBF) signals of 13 healthy adults (23.8 ± 0.5 yr old, 7 males) and air cuff pressure signals were recorded during IPC. Lumped parameter modeling and wavelet analysis were adopted to investigate the multiple blood flow surges (named as Peak₁, Peak₂ and Peak₃). The results of the simulated Peak₁ and Peak₂ were in good agreements with the experiment results, suggesting that IPC could enhance foot SBF not only by deflation, but also by inflation. Statistical analysis demonstrated that high frequency compression with more frequent occurrence of Peak₁ and Peak₂ lead to significantly higher (Friedman test, p < 0.001) time-averaged SBF enhancement than the traditional mode. In addition, wavelet analysis showed that the major frequency component of the Peak₃ (0.059 Hz) was within the range of the vascular myogenic activity, suggesting a vascular regulation process triggered by intravascular pressure changes. Our study provide new insights into the mechanism of how IPC enhance foot SBF.

The effects of gravity and compression on interstitial fluid transport in the lower limb

Edema in the limbs can arise from pathologies such as elevated capillary pressures due to failure of venous valves, elevated capillary permeability from local inflammation, and insufficient fluid clearance by the lymphatic system. The most common treatments include elevation of the limb, compression wraps and manual lymphatic drainage therapy. To better understand these clinical situations, we have developed a comprehensive model of the solid and fluid mechanics of a lower limb that includes the effects of gravity. The local fluid balance in the interstitial space includes a source from the capillaries, a sink due to lymphatic clearance, and movement through the interstitial space due to both gravity and gradients in interstitial fluid pressure (IFP). From dimensional analysis and numerical solutions of the governing equations we have identified several parameter groups that determine the essential length and time scales involved. We find that gravity can have dramatic effects on the fluid balance in the limb with the possibility that a positive feedback loop can develop that facilitates chronic edema. This process involves localized tissue swelling which increases the hydraulic conductivity, thus allowing the movement of interstitial fluid vertically throughout the limb due to gravity and causing further swelling. The presence of a compression wrap can interrupt this feedback loop. We find that only by modeling the complex interplay between the solid and fluid mechanics can we adequately investigate edema development and treatment in a gravity dependent limb.

Prediction of applied pressure on model lower limb exerted by an air pneumatic device

Air pneumatic compression is a concept used for management of venous disease, including oedema. A typical air pneumatic compression device (PCD) consists of an inflatable sleeve composed of either single or multiple pressure chambers that encircle a limb. The aim of this research was to develop a mathematical model to predict the pressure applied by an air pneumatic device to an irregular cross-sectional lower limb manikin. The radius of curvature at any cross-section of the lower limb (i.e. calf (gastrocnemius), tibial crest (anterior edge of the tibia bone)) is irregular, and differs amongst individuals and populations. The effectiveness of air pneumatic devices is difficult to predict with these irregular cross sections. A theoretical model was developed to calculate pressure applied by compression sleeves on a lower leg manikin and results compared against experimental pressure exerted on the manikin by a silicone-based PCD. This prediction was made at each of three positions. The theoretical model developed based on elliptical shaped forms predicted the pressure more accurately for the ankle to above ankle position, whereas the model based on circular shaped forms predicted the pressure more accurately for below the calf to below the knee position. Refinements to the theoretical model to predict the pressure applied by PCD are recommended.

One-Step Formation of Protein-Based Tubular Structures for Functional Devices and Tissues

Tubular biological structures consisting of extracellular matrix (ECM) proteins and cells are basic functional units of all organs in animals and humans. ECM protein solutions at low concentrations (5-10 milligrams per milliliter) are abundantly used in 3D cell culture. However, their poor "printability" and minute-long gelation time have made the direct extrusion of tubular structures in bioprinting applications challenging. Here, this limitation is overcome and the continuous, template-free conversion of low-concentration collagen, elastin, and fibrinogen solutions into tubular structures of tailored size and radial, circumferential and axial organization is demonstrated. The approach is enabled by a microfabricated printhead for the consistent circumferential distribution of ECM protein solutions and lends itself to scalable manufacture. The attached confinement accommodates minute-long residence times for pH, temperature, light, ionic and enzymatic gelation. Chip hosted ECM tubular structures are amenable to perfusion with aqueous solutions and air, and cyclic stretching. Predictive collapse and reopening in a crossed-tube configuration promote all-ECM valves and pumps. Tissue level function is demonstrated by factors secreted from cells embedded within the tube wall, as well as endothelial or epithelial barriers lining the lumen. The described approaches are anticipated to find applications in ECM-based organ-on-chip and biohybrid structures, hydraulic actuators, and soft machines.

Mechanical compression augments venous flow equal to intermittent pneumatic compression

Intermittent pneumatic compression is part of the current standard of care model for preventing venous thromboembolic events (VTE) after total joint arthroplasty. Pneumatic motors limit the rate of inflation resulting in bulky devices with uncomfortable sleeves that inhibited patient compliance and mobility. Nonpneumatic mechanical devices are an alternative for providing mobile, graded, intermittent, sequential, rapid, and monitorable compression posthospitalization. Fifteen healthy volunteers underwent mechanical compression using the Cirvo (Radial Medical, Mountain View, CA) as well as pneumatic compression with four commercially available systems (VenaFlow Elite, Kendall SCD Compression System, ActiveCare DVT, Vasculaire Compression System) and manual calf compression. Peak flow velocity (PFV) was measured by ultrasound of the femoral vein during compression and at baseline. Mechanical compression for 1 second resulted in a significant increase in femoral venous PFV to 107.8 ± 38.2 cm/s from 17.1 ± 4.7 cm/s at baseline (P < .001). The change in femoral venous PFV with mechanical compression for 1 second (90.7 ± 34.9 cm/s) was not statistically different from pneumatic compression from VenaFlow system (106.0 ± 35.6 cm/s, P = .124) and statistically lower than manual calf compression (115.5 ± 26.8 cm/s, P = .015). Pneumatic compression from the VenaFlow system produced the largest change in femoral venous PFV of all commercial pneumatic systems tested. Mechanical compression replicates or exceeds femoral venous PFV available from currently available intermittent pneumatic compression.

Fluid-Structure Coupling Model and Experimental Validation of Interaction Between Pneumatic Soft Actuator and Lower Limb

Pneumatic soft actuators (PSAs) are components that produce predesigned motion or force in different end-use devices. PSAs are lightweight, flexible, and compatible in human-machine interaction. The use of PSAs in compression therapy has proven promising in proactive pressure delivery with a wide range of dosages for treatment of chronic venous insufficiency and lymphedema. However, effective design and control of PSAs for dynamic pressure delivery have not been fully elaborated. The purpose of this study is to explore interactive working mechanisms between a PSA and lower limbs through establishing fluid-structure coupling models, an intermittent pneumatic compression (IPC) testing system, and conducting experimental validation. The developed IPC testing system consisted of a PSA unit (multichambered bladders laminated with an external textile shell), a pneumatic controller, and various real-time pressure monitoring sensors and accessory elements. The established coupling model characterized the dynamic response process with varying design parameters of the PSA unit, and demonstrated that the design of initial thickness, stiffness, and air mass flow of the PSA, as well as stiffness of limb tissues of the users, influenced PSA-lower limb interactions and resultant pressure dosages. The simulated results presented a favorable agreement with the experimental data collected by the IPC testing system. This study enhanced understanding of PSA-lower limb interactive working mechanisms and provided an evidence-based technical guidance for functional design of PSA. These results contribute to improving the efficacy of dynamic compression therapy for promotion of venous hemodynamics and user compliance in practice.

Pulse flow of liquid in flexible tube

The simulation of liquid flow in significantly deformed elastic material is one of the more challenging tasks. Tube wall motion prediction implemented directly into CFD software can noticeably reduce the computational and time demands of such problems. The FSI simulation of a liquid-flowed flexible plastic tube was analyzed on the FEA and CFD solvers coupling basis. The flexible tube is the basic symmetric test body that could be appropriately tested on the experimental stand. A comparison of experimental data and FSI problem using commercial code and one-dimensional tube models was made by evaluating the tube wall deformation magnitudes at defined flow ratios. The type of tube material, which can be understood as a nonlinear from the stress and deformation point of view, was considered. The paper shows several possibilities of tube modeling using the main constitutive relations of linear and nonlinear mechanics. The hyperelastic material models such as neo-Hookean and Mooney-Rivlin were tested. The results represent differences in impacts on the tube liquid flow and differences in the magnitudes of the wall tube deformations. Based on these findings it should be possible to simulate the problems of liquid flow in more complicated shape flow zones, such as arteries affected by various defects, in our future research.

Lower Limb Deep Vein Diameters Beneath Medical Compression Stockings in the Standing Position

OBJECTIVES

The mechanism by which compression therapy works is still discussed, especially at calf level. Whether lower limb deep vein diameters change under compression stockings is a matter of debate: no change versus great change. New study material helps to address this question.

METHODS

This was an experimental single centre controlled study on nine selected patients with mild to moderate superficial venous disease. A total of 34 deep vein segments were examined. A new hybrid (elastic + non-elastic materials) cuff pressure device enabled the deep vein diameter changes from baseline to occlusion similar to that which could be observed under stockings. The deep vein diameters were measured through the device with the patients in a standing position and their body weight distributed equally on both legs. This was compared to a 20-35 mm Hg medical compression stocking. The diameter change when patients put their whole body weight on the tested leg was also measured.

RESULTS

A pressure of 25.3 ± 6.4 mm Hg (mean, SD) was required to ovalise lower leg deep veins and a pressure of 43.1 ± 16.2 mm Hg (mean, SD) to occlude them. Both pressures were significantly different from baseline: p = .003 and p < .0001, respectively. No diameter reduction was achieved when the stockings were worn, and occlusion of deep veins occurred when the patients transferred their body weight onto the examined leg.

CONCLUSION

In the standing position, deep vein diameter reduction is not caused by compression stockings but may be due to the isometric muscle contractions required to support the patient's body weight.

A reduced-order model-based study on the effect of intermittent pneumatic compression of limbs on the cardiovascular system

This work investigates the effect that the application of intermittent pneumatic compression to lower limbs has on the cardiovascular system. Intermittent pneumatic compression can be applied to subjects with reduced or null mobility and can be useful for therapeutic purposes in sports recovery, deep vein thrombosis prevention and lymphedema drainage. However, intermittent pneumatic compression performance and the effectiveness are often difficult to predict. This study presents a reduced-order numerical model of the interaction between the cardiovascular system and the intermittent pneumatic compression device. The effect that different intermittent pneumatic compression operating conditions have on the overall circulation is investigated. Our findings confirm (1) that an overall positive effect on hemodynamics can be obtained by properly applying the intermittent pneumatic compression device and (2) that using intermittent pneumatic compression for cardiocirculatory recovery is feasible in subjects affected by lower limb disease.

A Current Review of Mechanical Compression and Its Role in Venous Thromboembolic Prophylaxis in Total Knee and Total Hip Arthroplasty

Interest in mechanical compression for venous thromboembolic disease prophylaxis has increased over the last several years because of concerns related to bleeding complications associated with chemoprophylaxis. However, the research evaluating compression is clearly not definitive. Therefore, this review aims to: (1) summarize methods of compression; (2) compare AAOS, ACCP, and SCIP guidelines; and (3) make recommendations regarding usage. Below-the-knee devices have demonstrated the most efficacy with multiple guidelines recommending usage. Efficacy and compliance may be improved with the use of mobile devices.

Engineering cancer microenvironments for in vitro 3-D tumor models

The natural microenvironment of tumors is composed of extracellular matrix (ECM), blood vasculature, and supporting stromal cells. The physical characteristics of ECM as well as the cellular components play a vital role in controlling cancer cell proliferation, apoptosis, metabolism, and differentiation. To mimic the tumor microenvironment outside the human body for drug testing, two-dimensional (2-D) and murine tumor models are routinely used. Although these conventional approaches are employed in preclinical studies, they still present challenges. For example, murine tumor models are expensive and difficult to adopt for routine drug screening. On the other hand, 2-D in vitro models are simple to perform, but they do not recapitulate natural tumor microenvironment, because they do not capture important three-dimensional (3-D) cell-cell, cell-matrix signaling pathways, and multi-cellular heterogeneous components of the tumor microenvironment such as stromal and immune cells. The three-dimensional (3-D) in vitro tumor models aim to closely mimic cancer microenvironments and have emerged as an alternative to routinely used methods for drug screening. Herein, we review recent advances in 3-D tumor model generation and highlight directions for future applications in drug testing.

Viscous flow past a collapsible channel as a model for self-excited oscillation of blood vessels

Motivated by collapse of blood vessels for both healthy and diseased situations under various circumstances in human body, we have performed computational studies on an incompressible viscous fluid past a rigid channel with part of its upper wall being replaced by a deformable beam. The Navier-Stokes equations governing the fluid flow are solved by a multi-block lattice Boltzmann method and the structural equation governing the elastic beam motion by a finite difference method. The mutual coupling of the fluid and solid is realized by the momentum exchange scheme. The present study focuses on the influences of the dimensionless parameters controlling the fluid-structure system on the collapse and self-excited oscillation of the beam and fluid dynamics downstream. The major conclusions obtained in this study are described as follows. The self-excited oscillation can be intrigued by application of an external pressure on the elastic portion of the channel and the part of the beam having the largest deformation tends to occur always towards the end portion of the deformable wall. The blood pressure and wall shear stress undergo significant variations near the portion of the greatest oscillation. The stretching motion has the most contribution to the total potential elastic energy of the oscillating beam.

The geko™ electro-stimulation device for venous thromboembolism prophylaxis: a NICE medical technology guidance

The geko™ device is a single-use, battery-powered, neuromuscular electrostimulation device that aims to reduce the risk of venous thromboembolism (VTE). The National Institute for Health and Care Excellence (NICE) selected the geko™ device for evaluation, and invited the manufacturer, Firstkind Ltd, to submit clinical and economic evidence. King's Technology Evaluation Centre, an External Assessment Centre (EAC) commissioned by the NICE, independently assessed the evidence submitted. The sponsor submitted evidence related to the geko™ device and, in addition, included studies of other related devices as further clinical evidence to support a link between increased blood flow and VTE prophylaxis. The EAC assessed this evidence, conducted its own systematic review and concluded that there is currently limited direct evidence that geko™ prevents VTE. The sponsor's cost model is based on the assumption that patients with an underlying VTE risk and subsequently treated with geko™ will experience a reduction in their baseline risk. The EAC assessed this cost model but questioned the validity of some model assumptions. Using the EACs revised cost model, the cost savings for geko™ prophylaxis against a 'no prophylaxis' strategy were estimated as £197 per patient. Following a second public consultation, taking into account a change in the original draft recommendations, the NICE medical technologies guidance MTG19 was issued in June 2014. This recommended the adoption of the geko™ for use in people with a high risk of VTE and when other mechanical/pharmacological methods of prophylaxis are impractical or contraindicated in selected patients within the National Health Service in England.

Relationship between medical compression and intramuscular pressure as an explanation of a compression paradox

Background

Using standing magnetic resonance imaging (MRI), we recently showed that medical compression, providing an interface pressure (IP) of 22 mmHg, significantly compressed the deep veins of the leg but not, paradoxically, superficial varicose veins.

Objective

To provide an explanation for this compression paradox by studying the correlation between the IP exerted by medical compression and intramuscular pressure (IMP).

Material and methods

In 10 legs of five healthy subjects, we studied the effects of different IPs on the IMP of the medial gastrocnemius muscle. The IP produced by a cuff manometer was verified by a Picopress® device. The IMP was measured with a 21G needle connected to a manometer. Pressure data were recorded in the prone and standing positions with cuff manometer pressures from 0 to 50 mmHg.

Results

In the prone position, an IP of less than 20 did not significantly change the IMP. On the contrary, a perfect linear correlation with the IMP ( r = 0.99) was observed with an IP from 20 to 50 mmHg. We found the same correlation in the standing position.

Conclusion

We found that an IP of 22 mmHg produced a significant IMP increase from 32 to 54 mmHg, in the standing position. At the same time, the subcutaneous pressure is only provided by the compression device, on healthy subjects. In other words, the subcutaneous pressure plus the IP is only a little higher than 22 mmHg—a pressure which is too low to reduce the caliber of the superficial veins. This is in accordance with our standing MRI 3D anatomical study which showed that, paradoxically, when applying low pressures (IP), the deep veins are compressed while the superficial veins are not.

A model-based method for the design of intermittent pneumatic compression systems acting on humans

Intermittent pneumatic compression is a well-known technique, which can be used for several therapeutic treatments like sports recovery, lymphoedema drainage, deep vein thrombosis prevention or others, which may require very different operating characteristics as regards the desired pressure values and the operating velocity. The performance and the effectiveness of the device are often difficult to predict and must be usually optimized through empirical adjustments. This article presents a general method based on the mathematical modelling of a generic IPC system, aimed at studying and developing such a device with physical and dynamical characteristics suitable for the intended application.

Simultaneous measurement of deep tissue blood flow and oxygenation using noncontact diffuse correlation spectroscopy flow-oximeter

We report a novel noncontact diffuse correlation spectroscopy flow-oximeter for simultaneous quantification of relative changes in tissue blood flow (rBF) and oxygenation (Δ[oxygenation]). The noncontact probe was compared against a contact probe in tissue-like phantoms and forearm muscles (n = 10), and the dynamic trends in both rBF and Δ[oxygenation] were found to be highly correlated. However, the magnitudes of Δ[oxygenation] measured by the two probes were significantly different. Monte Carlo simulations and phantom experiments revealed that the arm curvature resulted in a significant underestimation (~−20%) for the noncontact measurements in Δ[oxygenation], but not in rBF. Other factors that may cause the residual discrepancies between the contact and noncontact measurements were discussed, and further comparisons with other established technologies are needed to identify/quantify these factors. Our research paves the way for noncontact and simultaneous monitoring of blood flow and oxygenation in soft and vulnerable tissues without distorting tissue hemodynamics.

Finite element analysis of the deformation of deep veins in the lower limb under external compression

Compression devices applied to the lower limb can help improve venous flow by deforming the muscles and veins. In this study, a two-dimensional finite element model using displacement boundary conditions was employed to simulate the deformations of deep veins in the calf under external compression. Magnetic resonance (MR) scans were performed on four healthy volunteers with and without compression stockings. The displacement boundary condition was extracted from the corresponding deformed (with stocking) and undeformed (without stocking) MR images by using a non-rigid image registration procedure. In the finite element model, soft tissues of the calf were simplified as a homogenous material with hyperelastic properties. The effective bulk modulus of the material was evaluated at a chosen transverse section for each subject, which was also applied to three other locations at 2mm, 6mm and 10mm below the original section. Comparison between the simulation and measurement showed good agreement in area reduction of deep veins (discrepancy=8.7±6.4%), especially for sections close to the chosen location where the effective bulk modulus was evaluated. Beyond a certain distance, the discrepancies increased and became quite variable. The reason for this is that the architecture and stiffness of the calf tissues vary along the leg. The results also showed considerable variation in effective bulk modulus among the four subjects examined.

A novel method for computerized measurement of episcleral venous pressure in humans

Episcleral venous pressure (EVP) is an important determinant of intraocular pressure (IOP) and can be estimated by the pressure required to compress an episcleral vein. However, the lack of objective measurement endpoints makes EVP measurements in humans uncertain. To address this issue, we developed a new method to measure EVP objectively and reproducibly, and demonstrated its utility on a group of normal subjects. Our system for pressure chamber based venomanometry included a computer-controlled motor drive to increase pressure automatically, a transducer to record pressure, and a high-definition video camera to record vein collapse. Pressure measurements were synchronized with the video stream to determine the pressure required to collapse the vein to a specific pre-determined degree. This system was used to measure EVP in 10 eyes from 5 young healthy volunteers. Episcleral veins were selected in each of 4 quadrants. EVP was calculated to be the pressure in the chamber that compressed the vein by 0% (by back-projection), 10% or 50% as determined by using image analysis of the video stream. For this group of subjects, mean EVP was 6.3 ± 2.8 mmHg (mean ± SD, n = 40 measurements), 7.0 ± 2.6 mmHg, and 9.6 ± 2.6 mmHg using the 0%, 10% and 50% reduction endpoints, respectively. Pressures and standard deviations determined from these endpoints were significantly different from each other (p < 0.001). Coefficients of variation between right and left eyes were 12.7%, 10.2%, and 6.8% using the 0%, 10% and 50% endpoints, respectively. Based on previous research and theoretical considerations, the 0% endpoint is assumed to provide the most accurate estimate of baseline EVP, and can only be estimated by analyzing the brightness profiles of the vessels in the video stream. Objective measurement of EVP is important for understanding normal aqueous humor dynamics and its changes in disease states and with therapies. EVP has typically been assumed to be constant because of the lack of a convenient means of its measurement. This new method provides a precise means to assess EVP based on specific endpoints of vessel collapse, and enables, for the first time, objective and non-invasive measurements of EVP changes.

Intermittent pneumatic leg compressions acutely upregulate VEGF and MCP-1 expression in skeletal muscle

Application of intermittent pneumatic compressions (IPC) is an extensively used therapeutic strategy in vascular medicine, but the mechanisms by which this method works are unclear. We tested the hypothesis that acute application (150 min) of cyclic leg compressions in a rat model signals upregulation of angiogenic factors in skeletal muscle. To explore the impact of different pressures and frequency of compressions, we divided rats into four groups as follows: 120 mmHg (2 s inflation/2 s deflation), 200 mmHg (2 s/2 s), 120 mmHg (4 s/16 s), and control (no intervention). Blood flow and leg oxygenation (study 1) and the mRNA expression of angiogenic mediators in the rat tibialis anterior muscle (study 2) were assessed after a single session of IPC. In all three groups exposed to the intervention, a modest hyperemia (approximately 37% above baseline) between compressions and a slight, nonsignificant increase in leg oxygen consumption (approximately 30%) were observed during IPC. Compared with values in the control group, vascular endothelial growth factor (VEGF) and monocyte chemotactic protein-1 (MCP-1) mRNA increased significantly (P < 0.05) only in rats exposed to the higher frequency of compressions (2 s on/2 s off). Endothelial nitric oxide synthase, matrix metalloproteinase-2, and hypoxia-inducible factor-1alpha mRNA did not change significantly following the intervention. These findings show that IPC application augments the mRNA content of key angiogenic factors in skeletal muscle. Importantly, the magnitude of changes in mRNA expression appeared to be modulated by the frequency of compressions such that a higher frequency (15 cycles/min) evoked more robust changes in VEGF and MCP-1 compared with a lower frequency (3 cycles/min).

A miniature and mobile intermittent pneumatic compression device for the prevention of deep-vein thrombosis after joint replacement

The WizAir-DVT is a miniature, lightweight (690 g), battery-operated and mobile intermittent pneumatic compression device (ICD), which enables continuous intraoperative use and immediate patient mobilization postoperatively. We compared its efficacy with a commonly used ICD, the Kendall SCD. Peak femoral vein flow velocity was measured in 20 apparently healthy volunteers at rest and with each device: we found no significant differences between them. A second prospective, randomized, clinical trial was used to compare the efficiency of the device in preventing deep venous thrombosis (DVT) after joint replacement in 50 patients (n=25/group). None developed DVT. Doppler ultrasonography revealed no significant differences. The WizAir-DVT antithrombotic compression device is as safe and effective as the Kendall SCD.

Effects of elastic compression stockings on wall shear stress in deep and superficial veins of the calf

The purpose of this study was to estimate wall shear stress (WSS) in individual vessels of the venous circulation of the calf and quantify the effects of elastic compression based on change of vessel geometry and velocity waveform. The great saphenous vein and either a peroneal or posterior tibial vein have been imaged in four healthy subjects using magnetic resonance imaging, with and without the presence of a grade 1 medical stocking. Flow through image-based reconstructed geometries was numerically simulated for both a range of steady flow rates and ultrasound-derived transient velocity waveforms, scaled to give a standardized time averaged flow rate. For steady flow, the stocking produced an average percentage increase in mean WSS of approximately 100% in the great saphenous vein across a range of 0.125-1.25 ml/s. The percentage increase in the peroneal/posterior tibial veins varied from 490 to 650% across a range of 0.5-5 ml/s. In addition, application of the stocking eliminated periods of very low or zero flow from the transient waveforms. The average minimum value of WSS in all vessels without the stocking was <0.1 Pa. With the stocking, this was increased to 0.7 Pa in the great saphenous and 0.9 Pa in the peroneal/posterior tibial veins. The pathophysiological effects of these changes are discussed. In conclusion, the flight stocking was effective in raising venous WSS levels in prone subjects, and this effect was much more pronounced in the deep vessels. The stocking also tended to prevent cessation of flow during periods of increased downstream pressure produced by respiration.

The role of venous valves in pressure shielding

Background

It is widely accepted that venous valves play an important role in reducing the pressure applied to the veins under dynamic load conditions, such as the act of standing up. This understanding is, however, qualitative and not quantitative. The purpose of this paper is to quantify the pressure shielding effect and its variation with a number of system parameters.

Methods

A one-dimensional mathematical model of a collapsible tube, with the facility to introduce valves at any position, was used. The model has been exercised to compute transient pressure and flow distributions along the vein under the action of an imposed gravity field (standing up).

Results

A quantitative evaluation of the effect of a valve, or valves, on the shielding of the vein from peak transient pressure effects was undertaken. The model used reported that a valve decreased the dynamic pressures applied to a vein when gravity is applied by a considerable amount.

Conclusion

The model has the potential to increase understanding of dynamic physical effects in venous physiology, and ultimately might be used as part of an interventional planning tool.

Intermittent pneumatic compression in fracture and soft-tissue injuries healing

INTRODUCTION

Current methods of fracture care use various adjuncts to try and decrease time to fracture union, improve fracture union rates and enhance functional recovery. Intermittent pneumatic compression (IPC), one such modality, is used in the management of both fractures and soft-tissue injuries.

METHODS AND RESULTS

A search of PubMed, Medline, CINAHL, DH data and Embase databases was performed using the following keywords 'intermittent pneumatic compression', 'fracture healing' and 'soft tissue healing'. Sixteen studies on the use of IPC in fracture and soft-tissue healing were identified. These studies demonstrated that IPC facilitates both fracture and soft-tissue healing with rapid functional recovery.

CONCLUSIONS

IPC appears to be an effective modality to enhance fracture and soft-tissue healing. However, the number of subjects in human studies is small, and adequately powered randomized controlled trials in humans are required to produce stronger clinically relevant evidence.

Role of MRI in investigating the effects of elastic compression stockings on the deformation of the superficial and deep veins in the lower leg

PURPOSE

To evaluate the potential of MRI to investigate the mechanical effects of compression stockings on the veins of the lower limb.

MATERIALS AND METHODS

The right calves of eight healthy volunteers were imaged in the prone position, with and without the presence of a compression stocking. Cross-sectional areas of all peroneal and posterior tibial veins, both saphenous veins, and any sufficiently large superficial veins were segmented in all subjects at mid-calf level in both cases. Variation in cross-sectional area along the axis of the great saphenous vein and a peroneal vein was also examined in three subjects.

RESULTS

The mean cross-sectional area reduction was found to be greater in the deep veins (64%) than in the superficial veins (39%). Deep-vein cross-sections were generally elliptical, while superficial veins were approximately circular. Significant axial fluctuations were found in the cross-sectional areas.

CONCLUSION

MRI offers a precise source of data on the mechanical effects of lower-limb compression. Ultrasound (US) may be more cost-effective, but the data acquired are less comprehensive. Future biomechanical studies of lower-limb compression should make use of MRI.

Rapid-inflation intermittent pneumatic compression for prevention of deep venous thrombosis

BACKGROUND

Current treatment regimens that are designed to prevent deep venous thrombosis in patients undergoing orthopaedic procedures rely predominantly on drug prophylaxis alone. The purpose of this randomized clinical study was to evaluate the effectiveness of a mechanical adjunct to chemoprophylaxis that involves intermittent compression of the legs.

METHODS

During a twenty-two month period, 1803 patients undergoing a variety of orthopaedic procedures were prospectively randomized to receive either chemoprophylaxis alone or a combination of chemoprophylaxis and mechanical prophylaxis. Nine hundred and two patients were managed with low-molecular-weight heparin alone, and 901 were managed with low-molecular-weight heparin and intermittent pneumatic compression of the calves for varying time periods. Twenty-four percent of the patients underwent total hip or knee joint replacement. Screening for deep venous thrombosis was performed on the day of discharge with duplex-color-coded ultrasound.

RESULTS

In the chemoprophylaxis-only group, fifteen patients (1.7%) were diagnosed with a deep venous thrombosis; three thromboses were symptomatic. In the chemoprophylaxis plus intermittent pneumatic compression group, four patients (0.4%) were diagnosed with deep venous thrombosis; one thrombosis was symptomatic. The difference between the groups with regard to the prevalence of deep venous thrombosis was significant (p = 0.007). In the chemoprophylaxis plus intermittent pneumatic compression group, no deep venous thromboses were found in patients who received more than six hours of intermittent pneumatic compression daily.

CONCLUSIONS

Venous thrombosis prophylaxis with low-molecular-weight heparin augmented with a device that delivers rapid-inflation intermittent pneumatic compression to the calves was found to be significantly more effective for preventing deep venous thrombosis when compared with a treatment regimen that involved low-molecular-weight heparin alone.

Mathematical model for the hemodynamic response to venous occlusion measured with near-infrared spectroscopy in the human forearm

We propose a mathematical model to describe the hemodynamic changes induced by a venous occlusion in a human limb. These hemodynamic changes, which include an increase in blood volume, a reduction in blood flow, and modifications to the oxygen saturation of hemoglobin, can all be measured noninvasively with near-infrared spectroscopy (NIRS). To test the model, we have performed NIRS measurements on the human forearm, specifically on the brachioradialis muscle, during venous occlusion induced by a pneumatic cuff inflated around the upper arm to pressures within the range 10-60 mmHg. We have found a good agreement between parameters measured by NIRS (total hemoglobin concentration and hemoglobin saturation) and the corresponding model parameters (capacitor voltage and arterial/capillary branch current). In particular, model and experiment indicate that the time constant for blood accumulation during venous occlusion (approximately 73-79 s) is much slower than the time constant for blood drainage following cuff release (approximately 5 s). These results indicate that this mathematical model can be a valuable analytical tool to characterize, optimize, and further develop diagnostic measurement schemes that use venous occlusion approaches.

Influence of intermittent compression cuff design on interface pressure and calf deformation: experimental results

Intermittent pneumatic compression (IPC) is widely used for deep vein thrombosis (DVT) prophylaxis. The technique involves periodic inflation of a compression cuff around a limb, which acts to simulate the muscle pump mechanism, encouraging venous blood flow. However, there is uncertainty regarding the relationship between compression, vascular effects and clinical outcomes. This study investigates calf compression provided by four IPC cuffs with different air bladder configurations. Interface pressure between the cuff and the skin surface is measured and magnetic resonance (MR) images are obtained showing the calf cross section before and during compression. The data will be used to inform numerical simulations of IPC, leading to increased understanding of the implications of cuff design in relation to IPC and DVT prophylaxis.

Multimodal prophylaxis for THA with mechanical compression

We used mechanical thromboembolism prophylaxis using intraoperative thigh-calf pneumatic compression and other measures in 1032 consecutive primary and revision total hip arthroplasties. No chemical prophylactic measures were used until after duplex ultrasonography was performed by experienced technologists before discharge. Asymptomatic proximal thrombi were treated with low molecular weight heparin and warfarin, whereas those patients with a negative scan or distal thrombi only were advised to take aspirin 325 mg twice a day for 6 weeks. Regional anesthesia was used in 95% of the arthroplasties. Using this protocol, the 30-day mortality was 0.3%. There was one autopsy-proven fatal pulmonary embolism (0.09%). One other patient died suddenly with cardiac arrest after abdominal pain and vomiting, but no autopsy was performed. Symptomatic pulmonary embolism occurred in seven patients (0.7%), four occurring early and three late. Only one of these seven patients had a positive duplex scan. Deep vein thrombosis occurred in 41 patients (3.9%) and 35 remained asymptomatic. We observed no association between type of surgery (primary or revision), age, gender or preoperative diagnosis and pulmonary embolism or deep vein thrombosis. The data confirm the efficacy of a multimodal protocol with thigh-calf mechanical prophylaxis for almost all patients undergoing primary or revision total hip arthroplasty.

Intermittent pneumatic compression regulates expression of nitric oxide synthases in skeletal muscles

This study investigated the effects of intermittent pneumatic compression (IPC) on expression of nitric oxide synthase (NOS) isoforms in compressed (anterior tibialis, AT) and uncompressed (cremaster muscles, CM) skeletal muscles. Following IPC application of 0.5, 1, and 5h on both legs of rats, the endothelial NOS (eNOS) mRNA expression was significantly up-regulated to 1.2-, 1.8, and 2.7-fold from normal, respectively, in both AT and CM, and protein expression increased more than 1.5-fold of normal at each time point. Similarly, neuronal NOS expression was up-regulated, but to a lesser degree. In contrast, inducible NOS expression was significantly and time-dependently down-regulated in both muscles. After IPC cessation, eNOS levels returned to normal in both AT and CM. The results confirm our hypothesis that IPC-induced vasodilation is mediated by regulating expression of NOS isoforms, in particular eNOS, in both compressed and uncompressed skeletal muscles. The results also suggest the importance of precisely characterizing expression of each NOS isoform in tissue pathophysiology.

Finite element modelling and simulations in cardiovascular mechanics and cardiology: A bibliography 1993–2004

The paper gives a bibliographical review of the finite element modelling and simulations in cardiovascular mechanics and cardiology from the theoretical as well as practical points of views. The bibliography lists references to papers, conference proceedings and theses/dissertations that were published between 1993 and 2004. At the end of this paper, more than 890 references are given dealing with subjects as: Cardiovascular soft tissue modelling; material properties; mechanisms of cardiovascular components; blood flow; artificial components; cardiac diseases examination; surgery; and other topics.

Simulation study of breast tissue hemodynamics during pressure perturbation

We simulated the effects of compression of the breast on blood volume and tissue oxygenation. We sought to answer the question: how does the compression during breast examination impact on the circulatory systems of the breast tissue, namely blood flow, blood pooling, and oxygen concentration? We assumed that the blood was distributed in two compartments, arterial and venous. All the parameters were expressed with oxy- and deoxyhemoglobin quantities and were measured with a non-invasive method, Near Infrared Spectroscopy (NIRS). The simulated data showed that the blood volume pool in the breast decreased due to lower arterial flow and higher venous outflow, as the breast was squeezed under 100 cm H2O with a 10 cm diameter probe (or 78 cm2). The blood volume was reversed when the pressure was released. The breast venous oxygen saturation dropped, but overall tissue saturation (presenting NIRS signal, volume weighted average saturation) was increased. The results showed that simulation can be used to obtain venous and average oxygen saturation as well as blood flow in compressed breast tissues.

Finite element analysis of active Eustachian tube function

The inability to open the collapsible Eustachian tube (ET) has been related to the development of chronic otitis media. Although ET dysfunction may be due to anatomic and/or mechanical abnormalities, the precise mechanisms by which these structural properties alter ET opening phenomena have not been investigated. Previous investigations could only speculate on how these structural properties influence the tissue deformation processes responsible for ET opening. We have, therefore, developed a computational technique that can quantify these structure-function relationships. Cross-sectional histological images were obtained from eight normal adult human subjects, who had no history of middle ear disease. A midcartilaginous image from each subject was used to create two-dimensional finite element models of the soft tissue structures of the ET. ET opening phenomena were simulated by applying muscle forces on soft tissue surfaces in the appropriate direction and were quantified by calculating the resistance to flow (Rv) in the opened lumen. A sensitivity analysis was conducted to determine the relative importance of muscle forces and soft-tissue elastic properties. Muscle contraction resulted in a medial-superior rotation of the medial lamina, stretching deformation in the Ostmann's fatty tissue, and lumen dilation. Variability in baseline Rv values correlated with tissue size, whereas the functional relationship between Rv and a given mechanical parameter was consistent in all subjects. ET opening was found to be highly sensitive to the applied muscle forces and relatively insensitive to cartilage elastic properties. These computational models have, therefore, identified how different tissue elements alter ET opening phenomena, which elements should be targeted for treatment, and the optimal mechanical properties of these tissue constructs.

Evidence-based compression: prevention of stasis and deep vein thrombosis

OBJECTIVE

To summarize the currently published scientific evidence for the venous flow effects of mechanical devices, particularly intermittent pneumatic compression, and the relation to prevention of deep vein thrombosis (DVT).

SUMMARY BACKGROUND DATA

While intermittent pneumatic compression is an established method of DVT prophylaxis, the variety of systems that are available can use very different compression techniques and sequences. In order for appropriate choices to be made to provide the optimum protection for patients, the general performance of systems, and physiological effects of particular properties, must be analyzed objectively.

METHODS

Medline was searched from 1970 to 2002, and all relevant papers were searched for further appropriate references. Papers were selected for inclusion when they addressed specifically the questions posed in this review.

RESULTS

All the major types of intermittent compression systems are successful in emptying deep veins of the lower limb and preventing stasis in a variety of subject groups. Compression stockings appear to function more by preventing distension of veins. Rapid inflation, high pressures, and graded sequential intermittent compression systems will have particular augmentation profiles, but there is no evidence that such features improve the prophylactic ability of the system.

CONCLUSIONS

The most important factors in selecting a mechanical prophylactic system, particularly during and after surgery, are patient compliance and the appropriateness of the site of compression. There is no evidence that the peak venous velocity produced by a system is a valid measure of medical performance.

Endothelial nitric oxide production during in vitro simulation of external limb compression

External pneumatic compression (EPC) is effective in preventing deep vein thrombosis (DVT) and is thought to alter endothelial thromboresistant properties. We investigated the effect of EPC on changes in nitric oxide (NO), a critical mediator in the regulation of vasomotor and platelet function. An in vitro cell culture system was developed to simulate flow and vessel collapse conditions under EPC. Human umbilical vein endothelial cells were cultured and subjected to tube compression (C), pulsatile flow (F), or a combination of the two (FC). NO production and endothelial nitric oxide synthase (eNOS) mRNA expression were measured. The data demonstrate that in the F and FC groups, there is a rapid release of NO followed by a sustained increase. NO production levels in the F and FC groups were almost identical, whereas the C group produced the same low amount of NO as the control group. Conditions F and FC also upregulate eNOS mRNA expression by a factor of 2.08 +/- 0.25 and 2.11 +/- 0.21, respectively, at 6 h. Experiments with different modes of EPC show that NO production and eNOS mRNA expression respond to different time cycles of compression. These results implicate enhanced NO release as a potentially important factor in the prevention of DVT.

The effects of intermittent pneumatic compression on the arterial and venous system of the lower limb: a review

A better understanding of lower limb haemodynamics and the effects of intermittent pneumatic compression on the lower limb has led to an increasing awareness of the potential value of intermittent pneumatic compression in both venous and arterial disease. Intermittent pneumatic compression can be used in both the primary and secondary care settings, with its advantages being further enhanced by excellent patient compliance and very low rate of complications. Intermittent pneumatic compression has a proven role in the prophylaxis of deep vein thrombosis and there is some evidence that it is a useful adjunct in the management of venous ulcer disease. With laboratory and more recent clinical studies demonstrating augmentation of arterial inflow with intermittent pneumatic compression, its use in arterial diseases is being increasingly explored. Further studies are needed to define the precise role of intermittent pneumatic compression in arterial disease but any treatment regimen which reduces the need for referral or intervention in the claudicant or critically ischaemic limb will be a valuable addition to those managing arterial disease.

Role of nitric oxide in vasodilation in upstream muscle during intermittent pneumatic compression

This study investigated the dosage effects of nitric oxide synthase (NOS) inhibitor N(G)-monomethyl-L-arginine (L-NMMA) on intermittent pneumatic compression (IPC)-induced vasodilation in uncompressed upstream muscle and the effects of IPC on endothelial NOS (eNOS) expression in upstream muscle. After L-NMMA infusion, mean arterial pressure increased by 5% from baseline (99.5 +/- 18.7 mmHg; P < 0.05). Heart rate and respiratory rate were not significantly affected. One-hour IPC application on legs induced a 10% dilation from baseline in 10- to 20-microm arterioles and a 10-20% dilation in 21- to 40 microm arterioles and 41- to 70-microm arteries in uncompressed cremaster muscle. IPC-induced vasodilation was dose dependently reduced, abolished, or even reversed by concurrently infused L-NMMA. Moreover, expression of eNOS mRNA in uncompressed cremaster muscle was upregulated to 2 and 2.5 times normal at the end of 1- and 5-h IPC on legs, respectively, and the expression of eNOS protein was upregulated to 1.8 times normal. These increases returned to baseline level after cessation of IPC. The results suggest that eNOS plays an important role in regulating the microcirculation in upstream muscle during IPC.

Nonpharmacologic thromboembolic prophylaxis in total knee arthroplasty

Deep venous thrombosis is the most common complication in patients having elective total knee replacement. Pneumatic compression devices play an important role in the prophylaxis of deep venous thrombosis and effectively decrease the risk of distal deep venous thrombosis. The combination therapy with pharmacologic agents has the benefit of decreasing the rate of proximal deep venous thrombosis and therefore is recommended. In the absence of clinical data, recent in vivo flow studies suggest that calf or combined foot and calf compression are superior to foot compression alone. Epidural anesthesia in comparison with general anesthesia decreases the incidence of thromboembolic disease after total knee arthroplasty. Although hypotensive anesthesia and intraoperative heparin have been proven to substantially lower the incidence of deep venous thrombosis after total hip arthroplasty, the current literature does not support its application during the implantation of a total knee replacement. Pneumatic compression devices are an important part of deep venous thrombosis prophylaxis especially in the early postoperative period considering that pharmacologic anticoagulation is contraindicated in the first 12 hours after spinal anesthesia and in the presence of an epidural line.

Intermittent Pneumatic Compression Devices – Physiological Mechanisms of Action

There are many reports of how IPC is used effectively in the clinical setting; including the prevention of deep venous thrombosis, improvement of circulation in patients with lower extremity arterial diseases, reduction of lymphoedema, and the healing of venous ulcers. However, despite the widely accepted use of IPC, it is still unclear how IPC actually exerts its beneficial effects. The exact physiological mechanisms of action are unknown. The clinical utility of IPC and the putative mechanisms by which IPC could exert its therapeutic effect will be reviewed. The paper will examine the mechanical effects of IPC exerted on the lower extremity, and the subsequent biochemical changes in the circulation. In vitro studies of the effects of mechanical stress such as compressive strain and shear on cultured endothelial cells, and their clinical relevance to IPC will also be reviewed.

An in vitro cell culture system to study the influence of external pneumatic compression on endothelial function

PURPOSE

External pneumatic compression (EPC) is an effective means of prophylaxis against deep venous thrombosis. However, its mechanism remains poorly understood. Understanding of the biological consequences of EPC is an important goal for optimizing performance of the EPC-generating device and providing guidance for clinical use. We present a new in vitro cell culture system (Venous Flow Simulator) that simulates blood flow and vessel collapse conditions during EPC, and we examine the influence of these factors on endothelial cell (EC) fibrinolytic activity and vasomotor function.

METHODS

An in vitro cell culture system was designed to replicate the hemodynamic shear stress and vessel wall strain associated with induced blood flow during different modes of EPC. Human umbilical vein endothelial cells were cultured in the system and subjected to intermittent flow, vessel collapse, or a combination of the two. The biologic response was assessed through changes in EC morphology and the expression of fibrinolytic factors tissue plasminogen activator, plasminogen activator inhibitor type 1, profibrinolytic receptor (annexin II), and vasomotor factors endothelial nitric oxide synthase and endothelin-1.

RESULTS

The cells remained attached and viable after being subjected to intermittent pulsatile flow (F) and tube compression (C). In F and F + C, cells aligned in the direction of flow after 6 hours. Northern blot analysis of messenger RNA shows that there is an upregulation of tissue plasminogen activator expression (1.95 +/- 0.19 in F and 2.45 +/- 0.46 in FC) and endothelial nitric oxide synthase expression (2.08 +/- 0.25 in F and 2.11 +/- 0.21 in FC). Plasminogen activator inhibitor type 1, annexin II, and endothelin 1 show no significant change under any experimental conditions. The results also show that pulsatile flow, more than vessel compression, influences EC morphology and function.

CONCLUSION

Effects on ECs of intermittent flow and vessel collapse, either individually or simultaneously, were simulated with an in vitro system of new design. Initial results show that intermittent flow associated with EPC upregulates EC fibrinolytic potential and influences factors altering vasomotor tone. The system will facilitate future studies of EC function during EPC.