Biomedical subjects

P Verdonck

Publications and source records attributed to P Verdonck.

At least 19 recordsLinked to original sources

PubMed2002

Validation of the coupling of magnetic resonance imaging velocity measurements with computational fluid dynamics in a U bend.

Magnetic resonance imaging (MRI) can be used in vivo in combination with computational fluid dynamics (CFD) to derive velocity profiles in space and time and accordingly, pressure drop and wall shear stress distribution in natural or artificial vessel segments. These hemodynamic data are difficult or impossible to acquire directly in vivo. Therefore, research has been performed combining MRI and CFD for flow simulations in flow phantoms, such as bends or anastomoses, and even in human vessels such as the aorta, the carotid, and the abdominal bifurcation. There is, however, no unanimity concerning the use of MRI velocity measurements as input for the inflow boundary condition of a CFD simulation. In this study, different input possibilities for the inflow boundary conditions are compared. MRI measurements of steady and pulsatile flow were performed on a U bend phantom, representing the aorta geometry. PAMFLOW (ESI Software, Krimpen aan den Ussel, The Netherlands), an industrial CFD software package, was used to solve the Navier-Stokes equations for incompressible flow. Three main parameters were found to influence the choice of an inflow boundary condition type. First, the flow rate through a vessel should be exact, since it proves to be a determining factor for the accuracy of the velocity profile. The other decisive parameters are the physiology of the flow profile and the required computer processing unit time. Our comparative study indicates that the best way to handle an inflow boundary condition is to use the velocities measured by MRI at the inflow plane as being fixed velocities. However, before using these MRI velocity data, they first should be corrected for the partial volume effect by filtering and second scaled in order to obtain the correct flow rate. This implies that a reliable flow rate measurement absolutely is needed for CFD calculations based on MRI velocity measurements.

Aorta↗

PubMed2002

In vitro evaluation of the hydraulic permeability of polysulfone dialysers.

An in vitro set-up has been designed to study the hydraulic permeability of hollow fiber dialysers. Forward and reverse dialysate ultrafiltration were determined using both sterile dialysers and samples with a protein layer settled on the membrane (Fresenius F6, F8, F60 and F80). The ultrafiltration coefficient KUF (ml/h.mmHg) was calculated as the ratio of volumetrical flow (QUF) and transmembrane pressure (TMP) measurements. The protein layer on the membrane was induced either by recirculating human plasma through the dialysers (in vitro) or by a standard hemodialysis session (in vivo). KUF is largely independent of TMP up to 600mmHg (low flux) and 60mmHg (high flux) for forward and reverse flow In sterile dialysers, backfiltration yields a significantly different KUF except for the F80. An in vitro induced protein layer on the membrane decreases KUF15-30% (forward) and 4-12% (backward) in low flux and 45-70% (forward) and 65-73% (backward) in high flux dialysers.

Biocompatible Materials↗

PubMed2001

Limitations of Doppler echocardiography for the post-operative evaluation of aortic coarctation.

Doppler blood flow measurements and derived pressure differences, through the Bernoulli equation, are used in the diagnosis of aortic coarctation, a congenital stenosis distal to the left subclavian artery. Doppler velocities remain elevated at the coarctation site after successful repair of coarctation, leading to high Doppler derived pressure differences without significant arm-leg pressure differences. We studied this apparent contradiction of two diagnostic methods, in vivo using patient and control data, and in vitro using a hydraulic model. Clinical and echocardiographic data from 31 patients, aged 13.0 +/- 4.0, 10.5 +/- 4.7 yr after coarctectomy by end-to-end anastomosis, and 18 age-matched healthy subjects were reviewed. Doppler peak velocities at the aortic isthmus were elevated in patients (2.2 +/- 0.4 vs. 1.2 +/- 0.2m/s, P < 0.001), corresponding to significant Doppler differences (20 +/- 7 mmHg), however, without significant arm-leg pressure differences. In all patients, a mild anatomic stenosis could still be observed. Local stiffness was increased. The hypothesis that the less distensible surgical scar in post-coarctectomy patients leads to a significant dynamic obstruction in systole was validated in a latex model of the aorta. Rigid rings (0.5-1.5 cm), matching the unloaded aortic diameter, were mounted around the aorta. Under loading conditions, Doppler peak velocities increased by 40 +/-7%, yielding Doppler differences of 21 +/- 3 mmHg, without a significant pressure drop. An alternative expression to calculate pressure differences, using both velocity and geometric information, was validated in the model. In conclusion, post-operatively, Doppler velocities remain elevated due to a mild anatomical and significant dynamic narrowing, but the specific geometry, resembling a tubular hypoplasia rather than an abrupt stenosis, permits an almost complete pressure recovery explaining the occurrence of Doppler differences in disagreement with the negligible arm-leg pressure difference.

Adolescent↗

PubMed2001

Diastolic filling and pressure imaging: taking advantage of the information in a colour M-mode Doppler image.

The ability to derive non-invasively information on left ventricular diastolic function on one hand and pressure gradients on the other hand, makes Doppler ultrasound a very attractive tool in clinical practice. However, the limitations of the standard Doppler approaches in differentiating between normal and pseudonormal filling patterns, together with the limitations of the simplified Bernoulli equation for assessing pressure gradients, are well described. In this manuscript the role of colour M-mode Doppler echocardiography as a tool that can overcome these limitations is discussed. Relevant key concepts of the haemodynamics of left ventricular filling and its relationship with colour M-mode Doppler echocardiography are introduced.

Coronary Circulation↗

PubMed2001

Fixed region of nondistensibility after coarctation repair: in vitro validation of its influence on Doppler peak velocities.

After coarctectomy, local loss of distensibility is noted in addition to mild anatomic narrowing. We hypothesize that the increased Doppler peak velocities measured at the aortic isthmus in these patients partly reflect obstruction secondary to the stiff surgical scar. The hypothesis was studied in a pulsatile hydraulic model. Thirty-one patients (13.0 +/- 4.0 years of age), 10.5 +/- 4.7 years after coarctectomy by end-to-end anastomosis, were studied clinically and echocardiographically. Indexes of distensibility were calculated. The effect of isolated increased stiffness was studied in vitro with a stiff and a compliant 1:1 scale latex model of the aorta mounted in a pulsatile full-scale circulation loop. Local stiffening was obtained by a rigid ring mounted around the aorta, fitted to the dimension of the unloaded aorta. For different pressure and flow regimens, pressures and Doppler velocities were measured across the ring. Mean peak velocities at the surgical scar were 2.2 +/- 0.4 m/s. Mild anatomic stenosis was present. All distensibility indexes indicated locally increased stiffness (P <.001). In the stiff latex model, Doppler peak velocities increased from 1.89 +/- 0.04 m/s to 2.32 +/- 0.06 m/s (P <.03); in the compliant model, from 1.15 +/- 0.03 m/s to 1.79 +/- 0.05 m/s (P <.001). The increase of Doppler peak velocities depends on model compliance only and is independent of flow rate, length of the noncompliant segment, and viscosity of the perfusion fluid. Velocities do not change when semicircular stiffening is applied. We have demonstrated in vitro that isolated local nondistensibility leads to vessel narrowing during vascular distension. The relative contribution of local scar stiffness in the increase of Doppler peak velocities after coarctectomy was hereby assessed.

Adolescent↗

PubMed2001

Peripheral "oscillatory" compliance is associated with aortic augmentation index.

The augmentation index (AIx) and "oscillatory" compliance (C(2)) are wave contour analysis parameters for the central aorta (P(ao)) and radial artery pressure wave (P(rad)), respectively. Both are sensitive to cardiovascular risk factors such as aging, hypertension, and diabetes and have been proposed as prognostic markers for cardiovascular disease. In this work, we studied the relation between both. We first calculated P(rad) corresponding to a typical aortic A-type (AIx >0.15) and C-type wave (AIx <0), taken from the literature, by using a generalized aorta-radial pressure transfer function. P(rad) corresponding to C-type waves yielded the highest C(2) value. We further used simultaneously measured aortic and radial artery pressure in 45 human subjects age 34 to 84 years (63+/-12 [SD]) at baseline and after administration of nitroglycerin to calculate AIx(meas) and C(2), respectively. Transfer function was used to calculate reconstructed aortic pressure and AIx(rec). AIx(rec) underestimates AIx(meas) by 0.03+/-0.16, but both values correlate well (r=0.64; P<0.001). C(2) and AIx were inversely correlated (r=-0.36; P<0.001 for AIx(meas); r=-0.30; P<0.01 for AIx(rec)). Both AIx(meas) (0.06+/-0.17 versus 0.20+/-0.21; P<0.01) and AIx(rec) (0.04+/-0.12 versus 0.16+/-0.16; P<0.001) were lower after nitroglycerin, whereas C(2) increased only nonsignificantly (0.080+/-0.036 versus 0.071+/-0.042). C(2) is related to AIx and reflects, at least in part, hemodynamic changes affecting central aortic pressure. Nevertheless, given the model assumptions and computational steps associated with calculating C(2), AIx could be a more appropriate parameter to use in the clinical setting because it is determined directly from the pressure wave contour.

Adult↗

PubMed2001

Hydrodynamic characterisation of ventricular assist devices.

A new mock circulatory system (MCS) was designed to evaluate and characterise the hydraulic performance of ventricular assist devices (VADs). The MCS consists of a preload section and a multipurpose afterload section, with an adjustable compliance chamber (C) and peripheral resistor (Rp) as principal components. The MCS was connected to a pulse duplicator system for validation, simulating a wide range of afterload conditions. Both pressure and flow were measured, and the values of the different components calculated. The data perfectly fits a 4-element electrical analogon (EA). The MCS was further used to assess the hydrodynamic characteristics of the Medos VAD as an example of a displacement pump. Data was measured for various MCS settings and at different pump rates, yielding device specific pump function graphs for water and pig blood. Our data demonstrate (i) flow sensitivity to preload and afterload and (ii) the effect of test fluid on hemodynamic performance.

Equipment Design↗

PubMed2000

Role of tapering in aortic wave reflection: hydraulic and mathematical model study.

Pressure and flow have been measured simultaneously at six locations along the aorta of an anatomically correct 1:1 scale hydraulic elastic tube model of the arterial tree. Our results suggest a discrete reflection point at the level of the renal arteries based on (i) the quarter-wavelength formula and (ii) the comparison of foot-to-foot (c(ff)) and apparent phase velocity (c(app)). However, separation of the pressure wave into an incident and reflected wave at all six locations indicates continuous reflection: a reflected wave is generated at each location as the forward wave passes by. We did a further analysis using a mathematical transmission line model with a simple tapering geometry (length 50 cm, 31 and 11 mm proximal and distal diameter, respectively) for a low (0.32 ml/mmHg), normal (1.6 ml mmHg) and high (8 ml/mmHg) value of total arterial compliance. Using the quarter-wavelength formula, a discrete reflection point is found at x = 33 cm, the level of the renal arteries, independent of the value of total compliance. However, local analysis comparing c(ff) and c(app) does not reveal a marked reflection site, and the analysis of incident and reflected waves merely suggests a continuous reflection. We therefore conclude that the measured in vivo aortic wave reflection indices are the result of at least two interacting phenomena: a continuous wave reflection due to tapering, and local reflections arising from branches at the level of the diaphragm. The continuous reflection is hidden in the input impedance pattern. Using the quarter-wavelength formula or the classical wave separation theory, it appears as a reflection coming from a single discrete site, confusingly also located at the level of the diaphragm. Therefore, the quarter-wavelength formula and the linear wave separation theory should be used with caution to identify wave reflection zones in the presence of tapering, i.e., in most mammalian arteries.

Animals↗

PubMed2000

Individualizing the aorto-radial pressure transfer function: feasibility of a model-based approach.

We fitted a three-segment transmission line model for the radial-carotid/aorta pressure transfer function (TFF) in 31 controls and 30 patients with coronary artery disease using noninvasively measured (tonometry) radial and carotid artery pressures (P(car)). Except for the distal reflection coefficient (0.85 +/- 0.21 in patients vs. 0.71 +/- 0.25 in controls; P < 0.05), model parameters were not different between patients or controls. Parameters were not related to blood pressure, age, or heart rate. We further assessed a point-to-point averaged TFF (TFF(avg)) as well as upper (TFF(max)) and lower (TFF(min)) enveloping TFF. Pulse pressure (PP) and augmentation index (AIx) were derived on original and reconstructed P(car) (P(car,r)). TFF(avg) yielded closest morphological agreement between P(car) and P(car,r) (root mean square = 4.3 +/- 2.3 mmHg), and TTF(avg) best predicted PP (41.5 +/- 11.8 vs. 41.1 +/- 10.0 mmHg measured) and AIx (-0.02 +/- 0.19 vs. 0.01 +/- 0.19). PP and AIx, calculated from P(car) or P(car,r), were higher in patients than in controls, irrespectively of the TFF used. We conclude that 1) averaged TFF yield significant discrepancies between reconstructed and measured pressure waveforms and subsequent derived AIx; and 2) different TFFs seem to preserve the information in the pressure wave that discriminates between controls and patients.

Adult↗

PubMed1999

Assessing coronary artery stenosis severity: in vitro validation of the concept of fractional flow reserve.

Fractional flow reserve (FFR) is an index to assess the functional obstruction of an isolated coronary artery stenosis. It can be measured using only pressure measurements proximal (Pa) and distal (Pd) to the stenosis: FFR = P(d)/P(a). We studied the relation of pressure and flow-derived measurements of FFR in a pulsatile, hydraulic model of a coronary artery under physiological aorta pressure (80-110 mm Hg) and coronary flow (140-260 ml/min) conditions. Measurements were done at baseline and for several stenosis levels obtained with an external occluder. We found good correlations (r2 > 0.95) between pressure and flow-derived FFR, irrespective of isolated changes in myocardial resistance or aortic perfusion pressure. The basic assumption, i.e., that myocardial resistance is constant, was identified as most crucial in the validity of the concept of pressure-derived FFR. The agreement between our data and published animal and human studies indicates that this is most probably the case in hyperaemic conditions.

Coronary Disease↗

PubMed1999

Mechanism of pulmonary venous pressure and flow waves.

The pulmonary venous systolic flow wave has been attributed both to left heart phenomena, such as left atrial relaxation and descent of the mitral annulus, and to propagation of the pulmonary artery pressure pulse through the pulmonary bed from the right ventricle. In this study we hypothesized that all waves in the pulmonary veins originate in the left heart, and that the gross wave features observed in measurements can be explained simply by wave propagation and reflection. A mathematical model of the pulmonary vein was developed; the pulmonary vein was modeled as a lossless transmission line and the pulmonary bed by a three-element lumped parameter model accounting for viscous losses, compliance, and inertia. We assumed that all pulsations originate in the left atrium (LA), the pressure in the pulmonary bed being constant. The model was validated using pulmonary vein pressure and flow recorded 1 cm proximal to the junction of the vein with the left atrium during aortocoronary bypass surgery. For a pressure drop of 6 mmHg across the pulmonary bed, we found a transit time from the left atrium to the pulmonary bed of tau approximately 150ms, a compliance of the pulmonary bed of C approximately 0.4 ml/mmHg, and an inertance of the pulmonary bed of 1.1 mmHgs2/ml. The pulse wave velocity of the pulmonary vein was estimated to be c approximately 1m/s. Waves, however, travel both towards the left atrium and towards the pulmonary bed. Waves traveling towards the left atrium are attributed to the reflections caused by the mismatch of impedance of line (pulmonary vein) and load (pulmonary bed). Wave intensity analysis was used to identify a period in systole of net wave propagation towards the left atrium for both measurements and model. The linear separation technique was used to split the pressure into one component traveling from the left atrium to the pulmonary bed and a reflected component propagating from the pulmonary bed to the left atrium. The peak of the reflected pressure wave corresponded well with the positive peak in wave intensity in systole. We conclude that the gross features of the pressure and flow waves in the pulmonary vein can be explained in the following manner: the waves originate in the LA and travel towards the pulmonary bed, where reflections give rise to waves traveling back to the LA. Although the gross features of the measured pressure were captured well by the model predicted pressure, there was still some discrepancy between the two. Thus, other factors initiating or influencing waves traveling towards the LA cannot be excluded.

Biomechanical Phenomena↗

PubMed1999

Left-ventricular pressure gradients: a computer-model simulation.

Both invasive left-ventricular pressure measurements and non-invasive colour M-mode echographic measurements have shown the existence of intraventricular pressure gradients (IVPGs) during early filling. The mechanisms responsible for these IVPG cannot be completely explained by the experiments. Therefore a one-dimensional numerical model is developed and validated. The model describes filling (both velocities and pressures) along a left ventricular (LV) base-apex axis. Blood-wall interaction in the left ventricle with moving boundaries is taken into account. The computational results for a canine heart indicate that the observed IVPGs during filling are the consequence of a complex interaction between, on the one hand, pressure waves travelling in the LV and, on the other hand, LV geometry, relaxation and compliance. The computational results indicate the pressure dependency of wavespeed (0.77-1.90 m-1 s) for different mean intraventricular pressures (0.88-5.00 mmHg) and IVPGs up to 2 mmHg, independent of the ratio of end systolic volume and equilibrium volume. Increasing relaxation rate not only decreases minimum basal pressure (2.8 instead of 3.6 mmHg) but also has a strong influence on the time delay between the minimum basal and apical pressures (14 ms instead of 49 ms). The results sustain the hypothesis that pressure-wave propagation determines IVPGs and that IVPGs are no proof of elastic recoil.

Blood Pressure↗

PubMed1999

Pulmonary arterial compliance in dogs and pigs: the three-element windkessel model revisited.

In six dogs and six weight-matched miniature pigs at baseline and after pulmonary embolization, pulmonary arterial compliance was determined using the pulse pressure method (C(PPM)), the three-element windkessel model (C(WK-3)), and the ratio of stroke volume to pulse pressure (SV/PP). C(PPM) was lower in pigs than in dogs at baseline (0.72 +/- 0.23 vs. 1.14 +/- 0.29 ml/mmHg, P < 0.05) and after embolism (0.37 +/- 0.14 vs. 0.54 +/- 0.16 ml/mmHg, P = 0. 07) at matched flow, but not at matched flow and pressure. C(PPM) showed the expected inverse relation with pressure and a direct relation with flow. C(WK-3) was closely correlated with C(PPM), except for all dogs at baseline where C(WK-3) was up to 100% higher than C(PPM). Excluding these data, regression analysis yielded C(WK-3) = -0.01 + 1.30. C(PPM) (r(2) = 0.97). C(WK-3) was found to be unreliable when input impedance first harmonic modulus was close to characteristic impedance, i.e., when reflections were small. SV/PP correlated well with C(PPM) (SV/PP = -0.10 + 1.76. C(PPM), r(2) = 0.89). We conclude that 1) C(PPM) is a consistent estimate of pulmonary arterial compliance in pigs and dogs, 2) C(WK-3) and SV/PP overestimate compliance, and 3) C(WK-3) is unreliable when wave reflections are small.

Animals↗

PubMed1999

Distortion of the stentless porcine valve induces accelerated leaflet fibrosis and calcification in juvenile sheep.

BACKGROUND AND AIM OF THE STUDY: Stent mounting of a porcine aortic valve induces loss of mobility and reduces the effective orifice area. Any stentless design conserves the flexibility of the valvular apparatus, but unfortunately, these valves require a more elaborate implantation technique which is considered a major drawback by many surgeons. In an attempt to see if the insertion of the Toronto SPV stentless valve could be made easier, we altered the configuration by lowering the profile at the depth of both coronary sinuses. Theoretically, this could enable insertion of the valve by a single suture layer under the two coronary ostia, in addition to some fixation points at the commissures. METHODS: Two modified 20 mm Toronto SPV valves were tested in vitro for hydrodynamics in a computer-controlled pulse duplicator system and compared with two standard 20 mm valves. Animal implant studies using three standard versus three modified valves (21, 23 and 25 mm) were conducted in juvenile sheep for durability and biocompatibility over a period of three months. RESULTS: The standard Toronto SPV valve provided excellent hemodynamics. The altered configuration performed less optimally during hydrodynamic testing with increased transvalvular gradients. In animal implant studies, cusps adjacent to all reduced-height sinuses showed markedly accelerated fibrosis and substantial calcification, in contrast to only mild fibrosis on the inflow aspects of the standard leaflets. CONCLUSIONS: The more pronounced the deformation of the stentless valve, the faster the calcification of the leaflets adjacent to the distorted sinuses. As both valves types were prepared using an identical preservation technique, the role of loss of mobility in leading to early failure is clearly demonstrated. These findings also underline the extreme importance of correct implantation technique for all stentless valves in order to prevent degeneration.

Animals↗

PubMed1998

Quantification of mitral regurgitation by the automated cardiac output method: an in vitro and in vivo study.

BACKGROUND: Recently, the automated cardiac output method (ACM) was introduced for the calculation of blood flow at the left ventricular outflow tract (LVOT). This study was performed to examine the possibility of using ACM for flow calculation at the level of the mitral valve and for the quantification of mitral regurgitation (MR) in vitro and in vivo. METHODS AND RESULTS: In a computer-controlled in vitro model of the human heart, aortic and mitral normal bioprosthetic valves were inserted. ACM and electromagnetic probe flow measurements correlated well at the LVOT and at the mitral level (r2 = 0.79 and 0.77, respectively). For stroke volumes ranging from 30 to 100 ml/beat, there was no statistically significant bias between ACM and electromagnetic flow probe (-1.5 and 1.3 ml for LVOT and mitral level, respectively). Limits of agreement were [-14; +11] ml and [-18; +16] ml, respectively. We evaluated 68 patients in our in vivo study. They were divided into three groups according to the results of "standard" echocardiographic Doppler methods for the semiquantification of MR: echocardiographic color Doppler cartography, intensity of the continuous wave Doppler spectra, and in some patients, pulmonary venous flow, conventional Doppler, and proximal isovelocity surface area quantitative data. Group 1 consisted of 35 patients without MR or a physiologic one; the 17 patients in group 2 had a mild MR (1-2/4) and in group 3, 16 patients with MR 3-4/4 were included. Regurgitant volume (RV) was calculated as the difference between ACM mitral flow and ACM aortic flow, and regurgitant fraction (RF) was defined as the ratio between RV and ACM mitral flow. When mitral flow was measured only from the four-chamber view, we found in group 1, RV = -0.57 (0.67) L/min and RF = -16% (19%); in group 2, RV = -0.31 (1.06) L/min and RF = -8% (19%); and in group 3, RV = 1.53 (0.94) L/min and RF = 23% (13%). RV and RF were statistically higher in group 3 compared with group 2 or group 1 (p < 0.0005), but no significant difference was found between groups 1 and 2. When mitral flow was measured by the mean value of ACM four-chamber and two-chamber views, this resulted in group 1, RV = -0.26 (0.63) L/min and RF = -8% (15%); in group 2, RV = 0.01 (1.04) L/min and RF = -2% (18%); and in group 3, RV = 2.07 (1.21) L/min and RF = 34% (19%). RV and RF were again significantly higher in group 3 (p < 0.0001). There was no significant difference between group 1 and group 2, but in group 1 RF was no longer statistically different from 0%. CONCLUSIONS: (1) In our in vitro setting, ACM is reliable both at the LVOT and at the mitral valve. (2) In the in vivo situation, some overlapping does exist between the three groups of MR. However, ACM is a very easy, rapid, and objective method to differentiate hemodynamic nonsignificant (<3/4) from significant (> or =3/4) MR. Together with other well-known methods for the quantification of MR, it should facilitate the gradation of MR in the clinical setting. The absence of significant differences between group 1 and group 2 proves that the accuracy of ACM measurements at the mitral valve needs to be ameliorated before ACM can be used as a gold standard for the noninvasive measurement of RV and RF.

Aortic Valve↗

PubMed1998

Can an oxygenator design potentially contribute to air embolism in cardiopulmonary bypass? A novel method for the determination of the air removal capabilities of neonatal membrane oxygenators.

At present, air handling of a membrane oxygenator is generally studied by using an ultrasonic sound bubble counter. However, this is not a quantitative method and it does not give any information on where air was entrapped in the oxygenator and if it eventually was removed through the membrane for gas exchange. The study presented here gives a novel technique for the determination of the air-handling characteristics of a membrane oxygenator. The study aimed at defining not only the amount of air released by the oxygenator, but also the amount of air trapped within the oxygenator and/or removed through the gas exchange membrane. Two neonatal membrane oxygenators without the use of an arterial filter were investigated: the Polystan Microsafe and the Dideco Lilliput. Although the air trap function of both oxygenators when challenged with a bolus of air was similar, the Microsafe obtained this effect mainly by capturing the air in the heat exchanger compartment while the Lilliput did remove a large amount of air through the membrane. In conclusion, the difference in trap function was most striking during continuous infusion of air. Immediate contact with a microporous membrane, avoidance of high velocities within the oxygenator, pressure drop, transit time and construction of the fibre mat all contribute to the air-handling characteristics of a membrane oxygenator.

Air↗

PubMed1998

Applications of computer modelling for the design of orthopaedic, dental and cardiovascular biomaterials.

Biomaterials do not escape from the general trend present in all contemporary science and technology towards increasing use of computers and information technology. In this paper the use of computer modelling for the design of biomaterials is discussed. The word 'biomaterials' is interpreted in its broadest sense, i.e. referring to any foreign object brought into the body for temporary or permanent use. Computer modelling will first be discussed as a tool to model biological structures (bones, arteries) or to investigate and simulate biological interactions at implant-host interfaces. It will then be illustrated how computer modelling, using insights gained from the modelling of the biological structures themselves, is used in the design process of dental, orthopaedic and cardiovascular prostheses. The area of computer modelling for biomaterials applications has become so vast that an exhaustive overview is impossible in the framework of one paper. Rather, some illustrative case studies will be discussed which are, in the opinion of the authors, representative of general trends in this challenging domain of science on the boundary between engineering and medicine.

Cardiovascular Diseases↗

PubMed1997

Assessment of distributed arterial network models.

The aim of this study is to evaluate the relative importance of elastic non-linearities, viscoelasticity and resistance vessel modelling on arterial pressure and flow wave contours computed with distributed arterial network models. The computational results of a non-linear (time-domain) and a linear (frequency-domain) mode were compared using the same geometrical configuration and identical upstream and downstream boundary conditions and mechanical properties. pressures were computed at the ascending aorta, brachial and femoral artery. In spite of the identical problem definition, computational differences were found in input impedance modulus (max. 15-20%), systolic pressure (max. 5%) and pulse pressure (max. 10%). For the brachial artery, the ratio of pulse pressure to aortic pulse pressure was practically identical for both models (3%), whereas for the femoral artery higher values are found for the linear model (+10%). The aortic/brachial pressure transfer function indicates that pressure harmonic amplification is somewhat higher in the linear model for frequencies lower than 6 Hz while the opposite is true for higher frequencies. These computational disparities were attributed to conceptual model differences, such as the treatment of geometric tapering, rather than to elastic or convective non-linearities. Compared to the effect of viscoelasticity, the discrepancy between the linear and non-linear model is of the same importance. At peripheral locations, the correct representation of terminal impedance outweight the computational differences between the linear and non-linear models.

Aorta↗