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PubMed1991

Clover leaf inferior vena cava filter: in vitro evaluation of filter deployment and comparison of emboli-capturing ability.

A new self-centering stainless steel inferior vena cava filter (clover leaf filter) that can be delivered percutaneously through a 10-F catheter has been developed. The filter is loaded into an angiographic catheter as a set of straightened wires that are mechanically deformed into a predetermined clover leaf shape when the device is delivered. The filter can be deployed easily and effectively into the simulated vena cava. Emboli-capturing efficiency of the new filter was compared in vitro to that of the Greenfield and Bird's Nest inferior vena cava filters. All three filters captured all large 5 x 100-mm, potentially fatal emboli. The clover leaf filter captures more of the smaller (5 x 20-mm and 3 x 20-mm) emboli than the Greenfield filter and less than the Bird's Nest filter.

Embolism↗
PubMedDate not supplied

In vitro and in vivo experimental evaluation of a new vena caval filter.

PURPOSE: A new stainless steel (MP35N alloy) vena cava filter without a central stasis point was evaluated in vitro and in vivo. MATERIALS AND METHODS: The clot-trapping efficiency and hemodynamic flow pattern of the filter were assessed in a flow model and were compared with those of currently available commercial filters including the Vena Tech-LGM, Simon nitinol, Greenfield, and Bird's Nest filters. The new filter was placed in the inferior vena cava (IVC) of 31 dogs; 21 of the 31 dogs were followed up with cavography for up to 3 months. At the termination of the study, the filters and IVCs were examined grossly and histologically. An in vivo clot-trapping test was carried out in five dogs. RESULTS: The least turbulence was noted with the new filter and the titanium Greenfield filter. The stainless steel Greenfield and Simon nitinol filters caused major flow disturbances. Migration within 5 cm of initial placement occurred in two animals (9.5%). There were no IVC thromboses, perforations, or filter embolizations. An in vivo clot-trapping study showed an 80% efficiency for small thrombi (3 x 20 mm) and 100% efficiency for large thrombi (6 x 20 mm) with the new filter. The Simon and the new filter had the best clot-trapping capabilities. The Vena Tech-LGM and Bird's Nest filters were slightly inferior and the Greenfield filter demonstrated by far the lowest trapping capacity. CONCLUSION: The new vena cava filter is easily introduced percutaneously through a 12-F sheath and appears to be very promising due to its high filtering capability, low turbulence, nonmagnetic properties, good mechanical stability, and hypothrombogenicity. Clinical trials are warranted.

Alloys↗
PubMed1996

The hemodynamics of transjugular intrahepatic portosystemic shunts: investigations with Doppler sonography and development of an in vitro model.

RATIONALE AND OBJECTIVES: We evaluated Doppler sonography-based measurements of transjugular intrahepatic portosystemic shunt (TIPS) function and developed an in vitro model of normal TIPS hemodynamics. METHODS: We reviewed retrospectively the results of all trans-TIPS manometries (N = 116) performed during a 24-month period. Portosystemic pressure gradient was compared with peak stent velocity as measured by angle-corrected Doppler sonography. A flow phantom simulating TIPS was created using 8-, 10-, and 12-mm-diameter wire-mesh stents placed in cylindrical channels with lengths ranging from 3.4 to 6.0 cm. RESULTS: Among 50 trans-TIPS manometries with corresponding Doppler sonography performed on well-functioning shunts, measured portosystemic pressure gradient and peak velocity were not correlated (R2 = .014). On the basis of a regression of measurements in the flow phantom, pressure loss in a stented cylindrical channel was estimated as follows: delta p = rho.(0.145 -0.001.Rey + 0.816.L/D).(Vmean2/2), where rho is the fluid density, Rey is the Reynolds number, L is the channel length, D is the stent diameter, and Vmean is the time-averaged velocity within the stent. Predicted and measured pressure gradients were correlated (R2 = .91). CONCLUSION: Peak velocity in patients with a normally functioning TIPS does not predict the magnitude of the portosystemic pressure gradient.

Blood Flow Velocity↗
PubMed2003

Correlation of intimal hyperplasia development and shear stress distribution at the distal end-side-anastomosis, in vitro study using particle image velocimetry.

Low shear areas at the distal anastomosis of peripheral bypasses are thought to promote neointimal hyperplasia. In this study we evaluated the fluid dynamic environment at the distal anastomosis of peripheral bypasses by means of a new method for in vitro flow visualization and quantitative velocity field measurement. A silastic model of a distal end-side anastomosis was attached to a mock circulation loop driven by an artificial heart. High resolution velocity fields were measured by means of particle image velocimetry (PIV). The velocity vector data were used to calculate vorticity omega, strain rates ex, shear rates h and shear stresses tau. Two separations and a stagnation zone were identified by means of flow visualization. Measured velocities inside the three zones were significantly lower than in the high velocity mainstream. Calculated shear rates and shear stresses inside the zones were significantly lower than human wall shear rates. At the transition between the effective mainstream and the boundary layers high vorticity and compressive strain fields existed, indicating the presence of high shear forces. The locations of these areas corresponded to the well known zones of intimal hyperplasia. The high resolution shear stress analysis supports the low shear theory of intimal hyperplasia development. A wall diversion angle greater than 6 degrees leads to flow separation and presumed IH promotion until high shear transition areas are reached.

Anastomosis, Surgical↗
PubMed2003

Local haemodynamics and shear stress in cuffed and straight PTFE-venous anastomoses: an in-vitro comparison using particle image velocimetry.

OBJECTIVES: To use particle image velocimetry (PIV) to study the haemodynamics and shear stress associated with cuffed and straight PTFE-venous anastomoses. METHODS: Silastic models of a straight and cuffed (Venaflo) PTFE-venous anastomoses were attached to a pulsatile flow 'Berlin Heart' circuit filled with glycerine/water and hollow glass tracer spheres. Instantaneous velocity fields were obtained PIV and shear rates and patterns calculated from frame-by-frame analysis. RESULTS: A high velocity jet struck the anastomotic 'floor' and was deflected toward the venous outflow. Shear stresses near the floor were significantly higher, in the straight anastomosis. Sites of high shear stress correlated well with the known sites of intimal hyperplasia. CONCLUSIONS: A cuffed anastomosis type may be favourable in terms of local haemodynamics so enhancing the long-term patency of PTFE-venous grafts.

Anastomosis, Surgical↗
PubMed2003

The haemodynamics of multiple sequential stenoses and the criteria for a critical stenosis.

INTRODUCTION: Occlusive arterial disease is usually multi-focal but the cumulative functional effect of multiple arterial stenoses is not fully understood. We tested the hypothesis that a non-linear pressure/flow model (DeltaP=k1Q+k2Q2) that has been validated for single stenoses is also valid for multiple stenoses arranged in series. METHOD: The pressure/flow characteristics of three dissimilar modelled stenoses were measured individually and in different combinations using a hydraulic flow rig. RESULTS: The combined effect of multiple stenoses fitted the non-linear model accurately (R2=0.99) and approximated to the sum of the k1 and k2 parameters for each individual stenosis. For multiple stenoses the cumulative k1 was less than the predicted and the series order of the individual stenoses consistently altered the combined effect. CONCLUSIONS: A sequence of multiple stenoses is functionally equivalent to a single equivalent stenosis of greater functional severity and can be represented by a single viscous (k1) and inertial (k2) pressure loss coefficient. This finding allows the term 'critical stenosis' to be defined precisely in terms of the functional effect rather than the anatomical appearance, particularly where disease is multi-focal.

Arterial Occlusive Diseases↗
PubMed2003

Endotension as a result of pressure transmission through the graft following endovascular aneurysm repair--an in vitro study.

BACKGROUND: endovascular aneurysm repair (EVAR) significantly reduces, but does not abolish aneurysm sac pressure, possibly because of trans-fabric transmission. OBJECTIVE: to investigate how blood pressure is transmitted through different types of grafts into the aneurysm sac. DESIGN: experimental study, in vitro. METHODS: a latex aneurysm was inserted into an in vitro circulation model. The systemic mean pressure (SPmean) was varied from 50 to 120 mmHg. The grafts used for aneurysm exclusion were: thin wall polyethylene (PE), thick wall polyethylene (PE) and thin wall ePTFE. Mean aneurysm sac pressure (ASPmean) was measured, as was pulse pressure (ASPpulse). RESULTS: at an SPmean of 70 mmHg, the ASPmean was 34 +/- 0.8 mmHg (polyethylene knitted, thick wall), 30 +/- 1.0 mmHg (polyethylene woven, thin wall), and 17 +/- 0.6 mmHg (thin wall ePTFE). The ASPmean increased with SPmean, the relationship depending on the graft material. Stiffer grafts were associated with lower ASPmean and ASPpulse (p<0.001). CONCLUSIONS: the relationship between aneurysm sac mean pressure and systemic pressure (SP) depends on the graft material. These data highlights the need for further studies regarding endotension.

Aortic Aneurysm, Abdominal↗
PubMed1996

Detection of sub-critical arterial stenoses by hyperaemic Doppler.

OBJECTIVES: This study assessed the potential of hyperaemic Doppler to detect sub-critical stenoses using a flowrig model. METHODS: Pulsatile flow of a blood substitute was produced in a compliant circuit. A cadaver carotid artery, constricted by a silk suture produced a variable, focal stenosis. Forty-seven stenoses were created in five arteries. Pressure gradients and Doppler measurements were recorded simultaneously across each stenosis at low (200 ml/min) and high (400 ml/min) flow rates. The change in peak velocities between the arterial segment 2cm proximal to the stenosis (V1), and the stenotic jet (V2) were used to calculate three Doppler indices: (i) V2/V1 ratio, (ii) V2-V1 difference, (iii) a modified 'Bernoulli' value. A high flow pressure gradient of > or = 15% of the resting distal pressure (% delta P), represented a significant stenosis. RESULTS: There was improved correlation between Doppler indices and % delta P at high flow (r = 0.87 to 0.88) compared to low flow rates (r = 0.81 to 0.84). Optimum V2/V1 cut off values were determined by received operator characteristics (ROC) curve analysis. At low flow five sub-critical stenoses were not detected (sensitivity 82.8%) yet all but one of these lesions were identified at high flow (sensitivity of 96.6%). The V2-V1 and Bernoulli indices did not improve on the discriminant ability of the V2/V1 ratio. CONCLUSIONS: The V2/V1 ratio is sensitive to haemodynamic changes at enhanced flow rates across ideal arterial stenoses. The potential of hyperaemic Doppler to detect sub-critical lesions and so avoid intraarterial pressure measurements deserves further in vivo study.

Arterial Occlusive Diseases↗
PubMed2001

Platelet deposition in stagnation point flow: an analytical and computational simulation.

A mathematical and numerical model is developed for the adhesion of platelets in stagnation point flow. The model provides for a correct representation of the axi-symmetric flow and explicitly uses shear rate to characterise not only the convective transport but also the simple surface reaction mechanism used to model platelet adhesion at the wall surface. Excellent agreement exists between the analytical solution and that obtained by the numerical integration of the full Navier--Stokes equations and decoupled conservation of species equations. It has been shown that for a constant wall reaction rate modelling platelet adhesion the maximum platelet flux occurs at the stagnation point streamline. This is in direct contrast to that found in experiment where the maximum platelet deposition occurs at some distance downstream of the stagnation point. However, if the wall reaction rate is chosen to be dependent on the wall shear stress then the analysis shows that the maximum platelet flux occurs downstream of the stagnation point, providing a more realistic model of experimental evidence. The analytical formulation is applicable to a large number of two-dimensional and axi-symmetrical surface reaction flows where the wall shear stress is known a priori.

Arterial Occlusive Diseases↗
PubMed2002

Dynamic model of the role of platelets in the blood coagulation system.

In order to confirm which process is the most important in the blood coagulation cascade, a dynamic model of the function of platelets in blood coagulation is provided based on biochemical experiments. A series of conclusions based on qualitative analysis and mathematical simulation are drawn about the influence of the activation rate of factor VIII and factor IX on the generation of thrombin (IIa). It is evident that the pro-coagulation stimulus must exceed a threshold value to initiate the coagulation cascade. The value is related to the rate of platelet activation, the binding constant d2. The stability of the fixed value is also related to the pro-coagulation stimulus. This article also evaluates the influence of the stimulus strength and the activated rate parameter of platelets on thrombin. The proportion of platelets activated at any given time is designated c. To each c, we obtain a maximum concentration of thrombin. It is evident that when the level of factor IX is below 1% of normal levels, the rate of thrombin generation reduces dramatically resulting in severe bleeding tendency.

Blood Coagulation↗
PubMed2002

On-line novelty detection for artefact identification in automatic anaesthesia record keeping.

We report the design of a kernel-based on-line novelty detector (ADDaM - Automatic Dynamic Data Mapper) and its use in the detection of artefacts in an automatic anaesthesia record keeper (AARK).ADDaM produces a partitioned history of any ordered data stream and constructs a probability distribution function (PDF) from that history using Gaussian kernels. Two forms of PDF are possible: a static PDF where the prior probability of each kernel is determined by the number of observations it represents and a temporal PDF where more recent observations have a higher prior probability. Testing against the current PDF assesses the novelty of the next point entering the stream. The performance of this method for artefact detection in heart rate data was compared to Kalman, ARIMA and moving mean filters using receiver operator characteristic (ROC) curves. Performance was measured using the area under the curves (AUC), and the false positive rate (FPR) and positive predictive value (PPV) calculated at the optimal cost-point on the curves. The results obtained were: ADDaM (Static PDF) AUC 0.92, FPR 0.12, PPV 0.12 and ADDaM (Temporal PDF) AUC 0.97, FPR 0.12, PPV 0.15. Both ADDaM-based methods out performed all other on-line methods tested.

Algorithms↗
PubMed2003

Quantitative assessment of cerebral autoregulation from transcranial Doppler pulsatility: a computer simulation study.

Transcranial Doppler (TCD) ultrasonography is largely used today to achieve non-invasive assessment of cerebral autoregulation and cerebrovascular reactivity in neurosurgical patients. Recent experimental and clinical studies suggest that not only the pattern of mean velocity, but also velocity pulse amplitude alterations during changes in cerebral perfusion pressure (CPP) contain information on autoregulation status. The aim of this work is to investigate the relationship between cerebral autoregulation and TCD pulsatility by means of a comprehensive mathematical model of intracranial dynamics and cerebrovascular regulation. Simulation results, performed using different values of the most important clinical parameters of the model (autoregulation strength, cerebrospinal fluid (CSF) outflow resistance and intracranial elastance coefficient) show that velocity pulse amplitude increases with a reduction in CPP in patients with intact autoregulation, whereas changes in velocity pulsatility are modest in patients with weak autoregulation. Finally, velocity pulse amplitude decreases during a CPP reduction in patients with impaired autoregulation. Moreover, the relationship between the velocity pulse amplitude changes and autoregulation strength is almost linear in a wide range of CPP values, and is scarcely affected by changes in CSF circulation and intracranial elasticity. Starting from these results, we suggest a new quantitative index to assess autoregulation strength, i.e. G(aut)% = (s-b)/a, where G(aut)% is autoregulation strength (100% means intact autoregulation, 0% means impaired autoregulation), a approximately -0.03; b approximately 1.5 and s is the slope of the relationship ' percentage changes of velocity pulse amplitude to arterial pressure pulse amplitude vs. CPP changes'.

Adaptation, Physiological↗
PubMed2003

Dynamic cerebral autoregulation assessment using an ARX model: comparative study using step response and phase shift analysis.

Middle cerebral arterial blood velocity (MCAv) response to spontaneous and manipulated changes of arterial blood pressure (ABP) was studied in eight subjects using a linear autoregressive with exogenous input (ARX) model. ABP and MCAv were measured non-invasively by photoplethysmograph and transcranial Doppler ultrasound, respectively. Data were recorded at rest (spontaneous changes in ABP) and during thigh cuff (step-wise changes) and lower body negative pressure (sinusoidal changes of 1/12 Hz) tests in both normocapnia and hypercapnia (5% CO2). Since autoregulation is modulated by CO2, respiratory CO2 was simultaneously monitored to allow comparison of cerebral autoregulation status with different CO2 levels. ABP and MCAv were fitted by ARX models and dynamic cerebral autoregulation was estimated by analysing both the step responses and phase shift at the 1/12 Hz of the corresponding ARX models. The ARX model consistently modelled the phase lead of MCAv to ABP and it showed that the phase shift at 1/12 Hz of ARX model is consistent with the real phase shift of the data (p=0.59). Strong linear relationships between pCO2 and gradient of the step response (r=-0.58, p<0.0001) and between pCO2 and phase shift (r=-0.76, p<0.0001) were observed, which suggests that cerebral autoregulation can be assessed by step response or phase shift analysis of the ARX model fitted to ABP and MCAv data with spontaneous changes.

Adaptation, Physiological↗
PubMed2003

The influence of mean heart rate on measures of heart rate variability as markers of autonomic function: a model study.

Some studies have demonstrated that the assessments of autonomic activities from the alterations of heart rate variations (HRVs) after autonomic blockade and during exercise of high intensity by the spectral analysis of HRV seemed inconsistent with actual situation. The inconsistency is probably caused by the contributions of fluctuating magnitudes and mean levels of autonomic activities on HRV having not been clarified. The alterations of HRV after autonomic blockade and during exercise of high intensity using a mathematical model were simulated. The autonomic activity in normal condition was assumed first according to some experimental evidence. Then autonomic activities after sympathetic blockade, vagal blockade and during exercise of high intensity were appropriately adjusted accordingly. The HRVs in response to these given autonomic activities were simulated. We found that the effect on HRV influenced by the mean level of autonomic activity is helpful to explain alterations of HRV in these conditions. After vagal blockade, a largely reduced low frequency (LF) power could be caused by the reduced mean heartbeat interval induced by a decreased mean level of vagal activity. Increased low and high frequency powers after sympathetic blockade could be caused by the increased mean heartbeat interval induced by a decreased mean level of sympathetic activity. A decreased LF power during exercise of high intensity, in addition to the withdrawal of vagal activity, could also be caused by the decreased mean heartbeat interval induced by an increased mean level of sympathetic activity.

Animals↗
PubMed2003

The critical closing pressure of the cerebral circulation.

The critical closing pressure (CrCP) of the cerebral circulation indicates the value of arterial blood pressure (ABP) at which cerebral blood flow (CBF) approaches zero. Measurements in animals and in humans, have shown that the CrCP is significantly greater than zero. A simple mathematical model, incorporating the effects of arterial elasticity and active wall tension, shows that CrCP can be influenced by several structural and physiological parameters, notably intracranial pressure (ICP) and active wall tension. Due to the non-linear shape of the complete ABP-CBF curve, most methods proposed for estimation of CrCP can only represent the linear range of the pressure-flow (or velocity) relationship. As a consequence, only estimates of apparent CrCP can be obtained, and these tend to be significantly higher than the true CrCP. Estimates of apparent CrCP have been shown to be influenced by arterial PCO2, ICP, cerebral autoregulation, intra-thoracic pressure, and mean ABP. There is a lack of investigation, under well-controlled conditions, to assess whether CrCP is altered in disease states. Studies of the cerebral circulation need to take CrCP into account, to obtain more accurate estimates of cerebrovascular resistance changes, and to reflect the correct dynamic relationship between instantaneous ABP and CBF.

Animals↗
PubMed2003

Linearity and non-linearity in cerebral hemodynamics.

BACKGROUND: Transcranial Doppler ultrasound has been extensively used to study cerebral hemodynamics, and yet the basic characteristics of the input/output system of blood pressure/velocity are little known. We examine whether this system can best be considered linear or non-linear. METHODS: We assessed the adequacy of linear modeling in four ways: (1) Known properties of cerebral blood flow were reviewed and analyzed from a systems standpoint; (2) 1100 ARX & OE model types were tested with data from 29 normal subjects, with and without lowpass filtering; (3) time-frequency analysis was used to identify nonstationary behavior and markers of non-linearity (such as bifurcations, chirps, and intermittent autoregulatory impairment) in the same data sets; (4) simple computer models of autoregulation incorporating time delays and non-linear elements were tested for production of spontaneous oscillations. RESULTS: (1) Several aspects of cerebral hemodynamics are poorly described by linear models, (2) the ARX & OE models performed poorly, (3) time-frequency analysis showed non-linear and nonstationary behavior, (4) the computer models produced spontaneous oscillations similar to those observed in humans. CONCLUSIONS: There is strong evidence that the blood pressure/velocity system is non-linear.

Blood Flow Velocity↗
PubMed2003

Intracranial pressure dynamics: changes of bandwidth as an indicator of cerebrovascular tension.

The transmission bandwidth (BW) of arterial blood pressure (ABP) to intracranial pressure (ICP) was examined as a means of bedside monitoring of the state of cerebrovascular tension. Changes of BW of a black box identification model, relative arteriolar resistance and intracranial compliance were obtained from a piglet model equipped with a cranial window during induction of asphyxia, hypercapnia, and hypoxia. Changes of black box BW values and simulated changes of BW produced by a physiologically based lump parameter model of ICP dynamics are used to evaluate the hypothesis that during active cerebrovascular tension, changes of BW are inversely related to cerebral perfusion pressure (CPP), and during passive cerebrovascular tension, changes of BW are not inversely related to changes of CPP. Induction of asphyxia (n = 3) produced BW changes of the black box model that were simulated as an active cerebrovascular tension phase during decreasing CPP followed by a passive tension phase. Reventilation after prolonged asphyxia produced significant increases of BW that were simulated by a passive tension. Hypercapnic (n = 6) and hypoxic (n = 6) challenges produced: (1) significant changes of BW that were matched with simulations of the lumped parameter model for active tension; and (2) relationships between values of BW and relative average cerebral arteriolar resistance and intracranial compliance were inverse and correlated to a regression function of approximately x(-1). Changes of BW of the black box model and the simulations of the lumped parameter model support the feasibility of the stated hypothesis. As such, the evaluation of changes of BW of the black box model with respect to changes of CPP may be a useful method for monitoring the state of cerebrovascular tension.

Animals↗