Biomedical subjects

C D Bertram

Publications and source records attributed to C D Bertram.

At least 19 recordsLinked to original sources

PubMed2005

Measurement for implantable rotary blood pumps.

Rotary blood pumps offer a cost-effective way to assist the failing heart. Relative to their pulsatile cousins, they can consist of remarkably few moving parts, with attendant advantages in reliability. These advantages are realized in full only if the entire assist system is kept maximally simple. Control of the pump must therefore be based on a minimum number of measurement devices. This paper reviews the measurements that are made in the wide range of implantable rotary blood pump designs that are in development for ventricular assist. In a number of these, fluid-mechanical variables are estimated indirectly from measurements of motor speed and current or power. The introduction explains the goals of rotary blood pump control by comparison to the innate properties of the natural heart. Then motor and fluid-mechanical variables that may be transduced are discussed. Methods of indirect estimation of pressure drop and flow-rate are dealt with, followed by ways of detecting unusual states such as inflow obstruction. It is found that detection of these alone can be the basis of an adequate control strategy. Some groups have estimated variables pertaining to the heart that is being assisted, and there has also been work on monitoring the ongoing health of the assist system itself. The review concludes with a brief look at the wider measurement context for the intensive-care facility that proposes to use such devices to provide circulatory support.

Computer-Aided Design↗

PubMed2005

The flow field downstream of an oscillating collapsed tube.

A two-component laser Doppler anemometer was used to determine the velocity of aqueous flow in the region from 0.25 to 2.5 diameters downstream of a collapsible tube while the tube was executing vigorous repetitive flow-induced oscillations. The Reynolds number for the time-averaged flow was 10,750. A simultaneous measurement of the pressure at the downstream end of the tube was used to align all the results in time at sixty locations in each of the two principal planes defined by the axes of collapse of the flexible tube upstream. The raw data of seed-particle velocity were used to create a periodic waveform for each measured velocity component at each location by least-squares fitting of a Fourier series. The results are presented as both velocity vectors and interpolated contours, for each of ten salient instants during the cycle of oscillation. In the plane of the collapse major axis, the dominant feature is the jet which emerges from each of the two tube lobes when it collapses, but transient retrograde flow is observed on both the central and lateral edges of this jet. In the orthogonal, minor-axis plane, the dominant feature is the retrograde flow, which during part of the cycle extends over the whole plane. All these features are essentially confined to the first 1.5 diameters of the rigid pipe downstream of the flexible tube. These data map the temporal and spatial extent of the highly three-dimensional reversing flow just downstream of an oscillating collapsed tube.

Animals↗

PubMed2005

The origins of syringomyelia: numerical models of fluid/structure interactions in the spinal cord.

A two-dimensional axi-symmetric numerical model is constructed of the spinal cord, consisting of elastic cord tissue surrounded by aqueous cerebrospinal fluid, in turn surrounded by elastic dura. The geometric and elastic parameters are simplified but of realistic order, compared with existing measurements. A distal reflecting site models scar tissue formed by earlier trauma to the cord, which is commonly associated with syrinx formation. Transients equivalent to both arterial pulsation and percussive coughing are used to excite wave propagation. Propagation is investigated in this model and one with a central canal down the middle of the cord tissue, and in further idealized versions of it, including a model with no cord, one with a rigid cord, one with a rigid dura, and a double-length untapered variant of the rigid-dura model. Analytical predictions for axial and radial wave-speeds in these different situations are compared with, and used to explain, the numerical outcomes. We find that the anatomic circumstances of the spinal cerebrospinal fluid cavity probably do not allow for significant wave steepening phenomena. The results indicate that wave propagation in the real cord is set by the elastic properties of both the cord tissue and the confining dura mater, fat, and bone. The central canal does not influence the wave propagation significantly.

Cerebrospinal Fluid↗

PubMed2004

Aperiodic flow-induced oscillations of collapsible tubes: a critical reappraisal.

The evidence for the aperiodic self-excited oscillations of flow-conveying collapsible tubes being mathematically chaotic is re-examined. Many cases which powerfully suggest nonlinear deterministic behaviour have not been recorded over time-spans which allow their exhaustive examination. The present investigation centred on a previously recorded robust and generic oscillation, but more recent and more discerning tests were applied. Despite hints that a low embedding dimension might suffice, the data appeared on most indices high-dimensional. A U-shaped return map was found and modelled using both radial basis functions and polynomials, but lack of detailed structure in the map prevented effective parameter estimation. On the basis of power-law rather than exponential divergence of nearby trajectories, and of inability to discriminate against behaviour which would also be manifested by a surrogate consisting of a noise-perturbed nonlinear periodic oscillator, it is concluded that the data do not support the idea that the aperiodicity in the particular oscillation examined is caused by deterministic chaos. There was evidence that the distributed nature of the physical system might underlie aspects of the high dimensionality. We advocate equally searching testing of any future candidate chaotic oscillations in the investigation of collapsed-tube flows.

Biomedical Engineering↗

PubMed2001

Measurements of steady turbulent flow through a rigid simulated collapsed tube.

Axial and transverse components of liquid velocity are measured by laser Doppler anemometer in a perspex tube that has been deformed at one point to resemble the shape of the throat of a partially collapsed flexible tube, conveying fluid while being compressed externally. The Reynolds number is 5900. The flow down-stream of the throat consists of two side-jets with reverse flow extending all across the cross-section between them. The jets spread out around the central retrograde-flow zone, initially forming crescents of high-speed forward flow and then, at three diameters downstream, an almost complete annulus of forward flow around a central zone of lower-speed but now forward flow. Comparison is made between the features of this turbulent flow and those of a previously investigated laminar flow through the same geometry. In both, retrograde flow ceases between two and three diameters downstream of the centre of the throat. However, the laminar flow is annular at three diameters downstream, whereas here the jets remain influential at that station. The maximum normalised turbulence intensity exceeds 1.35.

Humans↗

PubMed2001

Computational fluid dynamics performance prediction for the hydrodynamic bearings of the ventrassist rotary blood pump.

Finite-volume computations are described for laminar flow in the hydrodynamic bearings supporting 2 different versions of the impeller of the VentrAssist rotary pump. Pressure boundary conditions are taken from prior computations of turbulent flow in the whole pump with frictionless sliding of the impeller on the inside of the pump body. By investigating various impeller positions, the true ride height is determined. Net lift and combined drag from all 8 bearings of the 4-bladed impeller are compared with predictions based on 2-D theory. The computations also reveal the extent of net force and moment acting to move the impeller away from its nominal axis of rotation.

Computational Biology↗

PubMed2001

Fluid dynamics of a textured blood-contacting surface.

This study examined the fluid dynamics of a textured blood-contacting surface using a computational fluid-dynamic modeling technique. The texture consisted of a regular array of microfibers of length 50 or 100 microm, spaced 100 microm apart, projecting perpendicularly to the surface. The results showed that the surface texture served as a flow-retarding solid boundary for a laminar viscous flow, resulting in a lowered wall shear stress on the hase-plane surface. However, the maximum wall shear stress on the fibers was much higher than the shear stress on the nontextured phase plane. At all fractions of fiber height down past 10 microm, the permeability of the textured region greatly exceeded the analytically predictable permeability of an equivalent array of infinite-height fihers. The lowered suiface shear stress appears to explain in part the enhanced deposition of formed blood elements on the textured surface.

Biocompatible Materials↗

PubMed2001

Laser-Doppler measurements of velocities just downstream of a collapsible tube during flow-induced oscillations.

The flow field less than one diameter downstream of the end of a collapsible tube executing self excited oscillations was examined using a two-component fiber-optic laser-Doppler anemometer. The time-averaged Reynolds number of the flow was 11,000. With the tube oscillating periodically, results obtained during many cycles of oscillation were combined to yield surface plots of the axial component over the cross section at 16 phases of the cycle. By combining measurements obtained with the laser probe in two different orientations, secondary flow vectors over the cross section were likewise constructed for 16 phases. The measurements showed strongly phasic turbulence intensity, with the phase of high intensity coinciding with the time of maximal tube collapse. Reverse flow occurred during much of the cycle, at places in the cross section that agree with our previous observations of laminar and turbulent steady flow through a rigid simulated collapsed tube.

Biomechanical Phenomena↗

PubMed2000

Acute cellular interaction with textured surfaces in blood contact.

Textured blood-contacting surfaces can promote the formation of a blood-compatible pseudo-neointima. We hypothesized that by controlling the surface texturing, the pseudo-neointima thickness could be controlled. The hypothesis was tested experimentally by fabricating the polyurethane textured surfaces with three different fiber lengths, and exposing them simultaneously to the flowing blood in an ovine ex vivo carotid-jugular series shunt for periods up to 4 h. The textured surface consisted of regularly spaced tapered micro-fibers of defined length on a smooth base-plane surface. Because of the simple surface topography, detailed computational fluid-dynamic modeling of the surface could be obtained as a parallel study. Experimental results showed that white cell was the predominant cell type deposited on the textured surfaces, whereas macroscopic thrombus formation occurred only in one of nine blood-contacting experiments. White cell density on the textured base-plane surface was subsequently quantified by image-analyzing the electron micrographs of blood-contacted textured surfaces. The statistical analysis of cell densities on individual textured surfaces showed effects of wall shear stress on the textured base plane (which was obtained from the fluid-dynamic modeling), the longitudinal position of the test section in the series shunt, and blood-contact time.

Animals↗

PubMed2000

Interactions of pulsatile upstream forcing with flow-induced oscillations of a collapsed tube: mode-locking.

Interest in flow through collapsible tubes derives from their prevalence in the human body. In the circulation at least, the driving pressure for flow is pulsatile, and it is accordingly appropriate to investigate the response of a collapsible tube to a pulsatile head. A servo-controlled hydraulically powered piston pump was used to add pulsation to the head propelling aqueous flow through a silicone rubber tube. The pump was commanded to execute a sinusoidal variation of piston position at various frequencies on either side of the natural frequency of an operating point where slow (3.75 Hz) repetitive self-excited oscillations in the absence of such forcing could be guaranteed. The resulting time series were characterised by the ratio of the number of collapse cycles to the number of forcing cycles in one overall interaction waveform period. Nonlinear interaction rather than linear superposition was dominant. Many different mode-locked interactions were recorded, and are here systematised in Farey series and plots which show the extent of entrainment. A few of the time series were aperiodic over the time scales investigated, suggesting possible chaos.

Biomechanical Phenomena↗

PubMed2000

Computational fluid dynamics analysis of hydrodynamic bearings of the VentrAssist rotary blood pump.

The computational fluid dynamics (CFD) package CFX-TASCflow was applied to simulate the flows through the blood pump hydrodynamic bearings. The three-dimensional flow patterns through the bearings were predicted and the hydraulic performance analyzed. The computations were carried out at 3 axial positions of the pump impeller. Net lift force away from the nearer part of the housing increased when the impeller moved closer to this part. Radial force and drag force were also found. Separated flows were observed at the leading and trailing edge of the bearing gap. To test the CFD package, a series of two-dimensional computations were also carried out for various bearing geometries. The results were compared with published experimental data.

Equipment Design↗

PubMed1999

A novel textured surface for blood-contact.

Blood-contacting surface modifications aimed at reduction of thromboembolic complications have included the texturing of surfaces so as to promote the formation of a stable pseudo-neointima. A technique has been developed whereby a textured surface consisting of regularly spaced micro-fibres was produced on a smooth base plane. Polyurethane vascular patches with and without the textured luminal surface were fabricated and implanted bilaterally in ovine carotid arteries for 1- and 3-week implantation periods (n = 6 per period). One of 6 arteries with textured patches in the 1-week group was occluded. All other arteries were patent. At 1 week, all patent textured patches had adherent thrombus covering the entire patch surface. By 3 weeks, the thrombus had organised to form a stable pseudo-neointima. Non-textured patches at 1 week had only partial surface coverage of thrombus. At 3 weeks, 4 of 6 non-textured patches had significant red thrombus in the lumen. At 3 weeks, there was also evidence of cellular migration from artery onto both textured and non-textured patches. These findings suggest that the major role of the textured surface was as a promoter of a stabilised thrombus base onto which subsequent cellular migration and tissue healing occurred more rapidly than onto a smooth polyurethane surface.

Animals↗

PubMed1999

Pulse wave attenuation measurement by linear and nonlinear methods in nonlinearly elastic tubes.

Reasons for the continuing difficulty in making definitive measurements of pulse wave attenuation in elastic tubes and arteries in the presence of reflections are sought. The measurement techniques available were re-examined in elastic tubes mimicking the arterial compliance nonlinearity, under conditions of strong reflection. The pulse was of physiological shape, and two different pulse amplitudes in the physiological range were used. Measurements of pressure, flow-rate and diameter pulsation allowed the deployment of four of the classical linear methods of analysis. In addition, a method of separating the forward- and backward-travelling waves that does not require linearising assumptions was used, and the attenuation in the forward and reverse directions was calculated from the resulting waveforms. Overall, the results obtained here suggest that a fully satisfactory way of measuring arterial attenuation has yet to be devised. The classical linear methods all provided comparable attenuation estimates in terms of average value and degree of scatter across frequency. Increased scatter was generally found at the higher pulse amplitude. When the forward waveforms from the separation were similarly compared in terms of frequency components, the average value at energetic harmonics was similar to both the value indicated by the linear methods and the values predicted from linear theory on the basis of estimated viscous and viscoelastic parameter data. The backward waveforms indicated a physically unreasonable result, attributed as the expression for this technique of the same difficulties that normally manifest in scatter. Data in the literature suggesting that one of the classical methods, the three-point, systematically over-estimates attenuation were not supported, but it was confirmed that this method becomes prone to negative attenuation estimates at low harmonics as pulse amplitude increases. Although the goal of definitive attenuation measurement remains elusive, the task provides a sensitive tool for the examination of the effect of nonlinearities in the arterial system.

Arteries↗

PubMed1997

Comparison of different methods for the determination of the true wave propagation coefficient, in rubber tubes and the canine thoracic aorta.

The results from studies of wave propagation in large arteries carried out over the last 25 years have shown that there is a good agreement among values of the imaginary part of the complex propagation coefficient, as expressed by pressure or flow-rate wave propagation velocity. However, there is considerable disparity among estimations of the degree of wave attenuation, the real part of the propagation coefficient. In order to determine whether this disparity is due to differences inherent in the various methods used to measure true wave propagation coefficients or whether it is caused by differences in experimental conditions, we have compared three techniques for determining true pulse wave propagation coefficients the three-point method, the occlusion method and a recently described iterative procedure. In addition, the results were compared to apparent propagation coefficients calculated without accounting for reflections. Measurements were carried out using each method in turn on a rubber tube of known transmission characteristics in which the magnitude of reflections was small. The iterative procedure and the three-point method were also compared under conditions of strong reflection. In the tube, the values of propagation velocity and attenuation coefficient determined by each method were similar. Although some discrepancies were noted, they did not amount to a systematic trend. The iterative procedure and the occlusion method were also used to analyse measurements on the thoracic aorta of three anaesthetized greyhounds. In the animal experiments, in spite of increased scatter, partly due to the variation between dogs, the two methods for determining true pulse-wave propagation yielded similar results. Since the differences between our estimates of propagation coefficients obtained by the methods tested are small with respect to those found when comparing the results from several reports in the literature, we conclude that any discrepancies between studies cannot be due to problems associated with the methods themselves but must have been caused by variations in experimental conditions or by other unknown artefacts.

Animals↗

PubMed1997

LDA measurements of velocities in a simulated collapsed tube.

A perspex (plexiglas) tube was locally deformed into an almost bi-lobar interior cross section, representative of the localized throat at the downstream end of a collapsed tube conveying a flow. The axial and transverse (parallel to the long axis of the deformed cross section) components of fluid velocity were measured in a dense rectangular grid of points covering the whole cross section, at 15 axial sites between one diameter upstream of and three diameters downstream of the center of the constriction. The Reynolds number based on undeformed tube diameter and mean velocity was 705. Results are presented both as surfaces showing the variation of each component over the cross section and as velocity vector profiles. The overall changes in velocity in the streamwise direction are presented in terms of the variation of the maximum and minimum of each component with axial position. Flow downstream of the throat consisted of two parallel side-jets with a broad region of reverse flow in between. This pattern persisted until beyond 2.5 diameters downstream, by which point transverse inflow at the top and bottom of the cross section had converted the side jets into a complete annulus of axial velocity surrounding a central deficit. Jet velocities and reverse flow disappeared relatively abruptly before three diameters downstream.

Laser-Doppler Flowmetry↗

PubMed1996

Numerical simulation of collapsible-tube flows with sinusoidal forced oscillations.

Collapsible-tube flow with self-excited oscillations has been extensively investigated. Though physiologically relevant, forced oscillation coupled with self-excited oscillation has received little attention in this context. Based on an ODE model of collapsible-tube flow, the present study applies modern dynamics methods to investigate numerically the responses of forced oscillation to a limit-cycle oscillation which has topological characteristics discovered in previous unforced experiments. A devil's staircase and period-doubling cascades are presented with forcing frequency and amplitude as control parameters. In both cases, details are provided in a bifurcation diagram. Poincaré sections, a frequency spectrum and the largest Lyapunov exponents verify the existence of chaos in some circumstances. The thin fractal structure found in the strange attractors is believed to be a result of high damping and low stiffness in such systems.

Animals↗

PubMed1996

A study of the bifurcation behaviour of a model of flow through a collapsible tube.

Most of the elastic tubes found in the mammalian body will collapse from a distended circular cross section and when collapsed may undergo flow-induced oscillations. A mathematical model describing fluid flow in a collapsible tube is analysed using the software package AUTO-86. AUTO-86 is used for continuation and bifurcation problems in systems of non-linear ordinary differential equations. The model is third-order lumped-parameter type and is based on the classical "Starling resistor"; it describes the unsteady flow behaviour and, in particular, the experimentally observed self-excited oscillations, in a way which is simple enough to give physical understanding, yet still firmly based on fluid mechanical principles. Some of the bifurcation types found in this model bear close resemblance to the types suggested by experimental observations of self-excited oscillations in collapsible tubes; they thus shed some light on the various topological changes which occur in practice, particularly in view of the fact that some of the points found numerically are difficult to achieve experimentally, while the existence of others can only be inferred indirectly and uncertainly from experiment.

Animals↗

PubMed1996

Effects of friction and nonlinearities on the separation of arterial waves into their forward and backward components.

In this paper we examine the importance of fluid friction and nonlinearities due to the area-pressure relationship and to the convective acceleration on the separation of arterial pressure and flow waves into their forward and backward components. Experiments were run in straight uniform nonlinearly elastic tubes. Different degrees of fluid friction and nonlinearities, covering the physiological range, have been tested. We predicted the forward and backward running pressure components using two wave separation methods: the classical linear method (Westerhof et al., Cardiovasc. Res, 6,648-656, 1972) and the first order correction (FOC) method (Pythoud et al., Trans ASME J. Biomech. Engng, in press) which takes nonlinearities and fluid friction into account. We found that the two methods yield somewhat different predictions. The differences tend to increase with the degree of fluid friction and nonlinearities and are typically of the order of 4-8%. We further compared the transmission ratio of forward and backward waves predicted by both methods. The transmission ratio was found to be overestimated by 10% by the classical linear method. The nonlinear method gave more accurate estimates, consistent with theory. We conclude that, for in vivo applications, the classical linear method should be the method of choice because it is simpler to use and the erros involved (4-8%) are comparable to measurement erros.

Arteries↗