Compliant Tube Research Articles

Rapid aortic wave velocity measurement with MR imaging.

The utility of a one-dimensional magnetic resonance (MR) sequence to rapidly and accurately measure wave velocity in vivo was evaluated. Studies were conducted in the thoracic aortas of 20 healthy subjects of varying age, and the MR method was validated in a compliant tube model. Aortic wave velocity ranged from 3.8 to 9.7 m/sec and demonstrated a positive correlation with subjects' age. Peak blood velocity ranged from 47 to 125 cm/sec and exhibited a strong negative correlation with subjects' age.

EFFECT OF VESSEL COMPLIANCE ON THE IN-VITRO PERFORMANCE OF A PULSATING RESPIRATORY SUPPORT CATHETER

Intravena caval respiratory support (or membrane oxygenation) is a potential therapy for patients with acute respiratory insufficiency. A respiratory support catheter is being developed that consists of a bundle of hollow fiber membranes with a centrally positioned pulsating balloon to enhance gas exchange. This study examined the influence of vessel compliance on the gas exchange performance of the pulsating respirator, support catheter. Polyurethane elastic tubes were fabricated with compliance comparable to that measured in bovine vena cava specimens. The gas exchange performance of the respiratory catheter was studied in an in-vitro flow loop using either the model compliant tube or a rigid tube as a mock vena cava. Balloon pulsation enhanced gas exchange comparably in both rigid and model compliant vessels up to 120 bpm pulsation frequency. Above 120 bpm gas exchange increased with further pulsation in the rigid tube, but no additional increase in gas exchange was seen in the compliant tube. The differences above 120 bpm may reflect differences in the compliance of the elastic tube versus the natural vena cava.

In vitro model of intravenous fluid administration: analysis of vein resistance to rapid fluid delivery

Rapid fluid administration is the cornerstone of successful trauma resuscitation of patients in a state of shock. Intravenous (IV) fluid delivery is a physical intrusion into a vein which results in a complex interaction between the rigid catheter and the compliant vein. We present an experimental model of IV infusion into a vein-like compliant tube that (a) demonstrated the interdependence between fluid administration and blood flow in a compliant tube and (b) allowed investigation of the contribution of the central venous system (between the infusion site and the heart) to the total resistance to infusion flow rate. The results show that in cases with very high resistance in the central venous system a significant increase of infusion flow rate cannot be achieved just by increasing the infusion pressure. Similarly, in cases of small veins when only small catheters can be used, infusate flow rate may be increased only by using two independent infusion ports. In cases with increased tissue pressure due to edema, gravity-driven infusion may not produce sufficient perfusion of the vascular compartments. It was also shown that the vein valves do not always close, and that peripheral blood flow may continue together with the infusate fluid (e.g., when there is a small downstream resistance and infusion with a small catheter).

Interaction between Wall Shear Stress and Circumferential Strain Affects Endothelial Cell Biochemical Production

The wall shear stress (WSS) of flowing blood and the circumferential strain (CS) driven by the pressure pulse interact to impose a dynamic force pattern on endothelial cells (ECs) which can be characterized by the temporal phase angle between WSS and CS, a quantity which varies significantly throughout the circulation. To study the interaction of WSS and CS on endothelial production of vasodilators (prostacyclin and nitric oxide) and a vasoconstrictor (endothelin-1), bovine aortic ECs were cultured on the inner surface of compliant tubes and subjected to various flow conditions: steady shear (10 dyn/cm²), oscillatory shear (10 ± 10 dyn/cm², rigid tube), and oscillatory shear (10 ± 10 dyn/cm²) with CS (8%) either in or out of phase with shear. The 4-hour production rates of vasoactive agents show that steady shear stimulates the highest production of vasodilators whereas oscillatory shear stimulates the highest vasoconstrictor production. The addition of CS in concert with oscillatory shear enhances the production of vasodilators and inhibits the production of vasoconstrictors, and this effect is modulated by the phase angle between WSS and CS. These data suggest that the interactions of WSS and CS are important in vascular regulation and remodeling.

Cyclic variation of the Doppler power from porcine blood under pulsatile flow in tubes of varying compliance

The cyclic variation of the Doppler power backscattered from porcine whole blood and red blood cell suspensions was investigated under pulsatile flow in a mock flow loop of varying compliance. Experiments were performed using five different hematocrit values at four different stroke rates. A significant cyclic variation of Doppler power was measured in all experiments but the pattern of the cyclic variation was different. At high stroke rates (40 beats/min or more), the cyclic variation of the Doppler power showed a small peak during acceleration of flow and reached a minimum at peak systole and another peak was observed during deceleration. This minimum power phenomenon during peak systole became more prominent as the stroke rate was increased up to 60 beats/min. At a low stroke rate (20 beats/min), the Doppler power increased during early systole and remained the same until late diastole. The cyclic variation using whole porcine blood and red cell suspension showed the same pattern and suggested that aggregation of RBC did not affect the pattern of the cyclic variation. In addition to these general features, the differences for different experimental scenarios (whole blood versus red cell suspensions, and rigid tube versus compliant tube) are discussed.

Studies in Calf Venous Pump Function Utilizing a Two-valve Experimental Model

Objectives: to explore the hydrodynamic mechanisms involved in the regulation of ambulatory venous pressure. Design: an experimental model of calf venous pump was constructed with collapsible tubes and valves.Material: the model consisted of a conduit and a pump with an intervening competent valve. Another valve that could allow reflux into the pump was mounted above the pump. Methods: conduit pressure and recovery times were monitored under conditions of different degrees of ejection fraction and reflux into the pump. Model variables included using poorly compliant tubes for the pump, the conduit and for both the pump and conduit. Results: the latex tube exhibited a non-linear volume–pressure relationship and a bi-modal regimen of compliance. This bestowed pressure-buffering properties. Ambulatory venous hypertension resulted when reflux beyond buffering capacity occurred. Substituting less compliant PTFE for latex at the pump had a relatively minor effect on post-ejection pressure and recovery times. Using PTFE at the conduit had a profound but divergent effect on both of these parameters. Conduit capacitance reduction had a similar effect. Conclusion: conduit elastance plays a significant role in the regulation of ambulatory venous pressure in this experimental model. The hydrodynamic principles illustrated by the model may enhance our understanding of the human calf venous pump.

The effect of hemodynamics, vessel wall compliance and hematocrit on ultrasonic Doppler power: an in vitro study

Previous in vitro studies in rigid tubes under pulsatile flow conditions have reported a lack of a cyclic variation in blood echogenicity that contradicts in vivo results. To investigate whether or not these variations can be attributed to the compliance of the vessel wall, a series of in vitro experiments with compliant tubes, under pulsatile flow conditions, was performed. Two important factors that may affect the Doppler power were investigated: 1. the dependence on hematocrit and 2. the effect of the vessel wall elasticity. In the present study, it is shown that, at the low beat rates, the peak of the mean Doppler power within the flow cycle depends on the vessel wall compliance. When the vessel becomes more compliant, the peak is shifted from the early to the late systole. Additionally, there is a correlation between the power peak and hematocrit that is more evident in compliant vessels. At a higher pulsation rate of 37 beats/min, a different variation is observed. A drop in the power occurs near peak systole in compliant tube experiments and is more pronounced as the vessel becomes more constricted. The observed power drop agrees with previously reported in vivo results, but is not seen in rigid tube experiments. The results of this study suggest that proper interpretation of cyclic variations in Doppler power requires a knowledge of hemodynamic parameters, such as the modulus of elasticity of the vessel wall, propagation velocity or, possibly, the phase angle of input impedance.

Composite hydrophone array assembly and shading

A composite hydrophone array assembly is made from a compliant mandrel suchs a hollow tube and at least one wrap of piezoelectric film adhered to the compliant hollow tube at a plurality of locations thereon. Each location defines a composite hydrophone channel. The centers of adjacent hydrophone channels are separated along the length of the compliant hollow tube. To shade a hydrophone channel, the wall thickness of the hollow tube can be varied along its length, the circumferential area of coverage of the wraps of piezoelectric film can be varied along their length, the longitudinal lengths of the wraps of piezoelectric film are varied, or a combination of these approaches can be used. Unwanted noise is rejected by eliminating hydrophone channel grating lobes and overlapping portions of the wraps of piezoelectric film of the adjacent hydrophone channels to create a wavenumber response for the composite hydrophone array that significantly reduces array grating lobes.

REVERSAL OF THE BERNOULLI EFFECT AND CHANNEL FLUTTER

This paper is concerned with the collapse and subsequent self-excited oscillation of compliant tubes conveying fluids. Our model considers a two-dimensional, inviscid, shear flow in a flexible channel of infinite length subject to linear, travelling varicose waves. Analysis of the boundary-value problem leads to two findings which do not seem to have been noticed before, despite the close attention this kind of fluid–structure interaction has attracted on account of its medical significance. The pressure perturbation on the wall has two components, the first is in-phase with the wall displacement and the second with the velocity of the wall motion. For potential flow, the first component is the only one tending to destabilize and is known as the Bernoulli effect. For shear flow, however, the sign of the pressure is reversed as the Bernoulli effect is overcome by the perturbations of the vorticity field. Streamline patterns show that Kelvin's “cats' eyes” are sheltered in the wider channel sections, rendering the effective flow passage smaller where the physical width is larger. The second component produces a wave drag, hence irreversible transfer of energy from the flow to waves. We argue that this is a possible mechanism for the self-excited oscillation observed in experiments. This mechanism is similar to Miles's (1957) mechanism of water wave generation by wind, which is a class B instability according to the Benjamin–Landahl categorization, but the accompanying reversal of the Bernoulli effect is different and depends essentially on the presence of a second boundary. The eigenvalue problem is also considered and it is shown that dynamic instability of long but finite wavelength could be experienced by compliant channels with thick walls, a typical application being the respiratory flow in the upper airways. The critical flow speed is given in terms of the channel properties.

Comments on “Flexural wave reduction using a compliant tube baffle” [J. Acoust. Soc. Am. 99(2), 691–699 (1996)] and “Signal pressure received by a hydrophone placed on a plate backed by a compliant baffle” [J. Acoust. Soc. Am. 89(2), 559–564 (1991)

The cited formulations for studying the baffling of flexural waves by a compliant tube grating [S. H. Ko, J. Acoust. Soc. Am. 99, 691–699 (1996)] and for determining the acoustic receive sensitivity of a hydrophone placed near a plate backed by a compliant tube grating [S. H. Ko and H. H. Schloemer, J. Acoust. Soc. Am. 89, 559–564 (1991)] are dependent on the validity of the lumped-fluid parameter approximations developed by Junger [M. C. Junger, J. Acoust. Soc. Am. 78, 1010–1012 (1985)] for a grating of closely spaced compliant tubes between two semi-infinite fluid half-spaces. If the fluid between the plate and the grating is great enough to minimize their interaction, the incorporation of the effective layer parameters developed by Junger should be reasonably valid. Unfortunately, the sample calculations in Ko (Ref. 4) and Ko and Schloemer (Ref. 5) have been made for the limiting case of the compliant tube grating in direct contact with the plates where the use of Junger’s approximations is inappropriate.

Input impedance and reflection coefficient in fractal-like models of asymmetrically branching compliant tubes.

A mathematical model is described, based on linear transmission line theory, for the computation of hydraulic input impedance spectra in complex, dichotomously branching networks similar to mammalian arterial systems. Conceptually, the networks are constructed from a discretized set of self-similar compliant tubes whose dimensions are described by an integer power law. The model allows specification of the branching geometry, i.e., the daughter-parent branch area ratio and the daughter-daughter area asymmetry ratio, as functions of vessel size. Characteristic impedances of individual vessels are described by linear theory for a fully constrained thick-walled elastic tube. Besides termination impedances and fluid density and viscosity, other model parameters included relative vessel length and phase velocity, each as a function of vessel size (elastic nonuniformity). The primary goal of the study was to examine systematically the effect of fractal branching asymmetry, both degree and location within the network, on the complex input impedance spectrum and reflection coefficient. With progressive branching asymmetry, fractal model spectra exhibit some of the features inherent in natural arterial systems such as the loss of prominent, regularly-occurring maxima and minima; the effect is most apparent at higher frequencies. Marked reduction of the reflection coefficient occurs, due to disparities in wave path length, when branching is asymmetric. Because of path length differences, branching asymmetry near the system input has a far greater effect on minimizing spectrum oscillations and reflections than downstream asymmetry. Fractal-like constructs suggest a means by which arterial trees of realistic complexity might be described, both structurally and functionally.

Flexural wave reduction using a compliant tube baffle

A theoretical model was developed to evaluate the reduction of flexural wave that is generated by a line force applied on an infinite plate using a compliant baffle. The compliant baffle layer placed between the plate excited by a line force and a signal conditioning plate is designed for reducing pressure fluctuations from the vibrating plate. The baffle layer considered here is the compliant‐tube array, modeled by Junger [J. Acoust. Soc. Am. 78, 1010 (1985)] to represent a homogeneous (dispersive) fluid layer. Effects of various parameters such as the aspect ratio of the compliant tube, the distance between tubes, and the damping of the tube material on the flexural wave reduction are presented. Calculations of noise reductions were made for both the dispersive and nondispersive fluid baffles.

Apparatus for transmitting two frequency signals with an acoustic projector

An acoustic projector wherein a unitary acoustic resonator produces active sonar signals in more than one frequency band. The double-slotted resonator is coupled to a transducer which is capable of exciting the resonator in two, distinct, volumetric modes of vibration using two asymmetric, generally arcuate vibrating members. Transduction techniques can include variable reluctance or piezoceramic transduction. Internal cavity pressure release may be achieved by bladders, compliant tubes, and the like.

Lateral movement induced in compliant tubes conveying pulsatile flow

Lateral movement induced in compliant tubes conveying pulsatile flow

Sonar baffles

Acoustic decoupling baffles are often used to minimize noise contamination at hydrophone and transducer arrays. To maintain sensitivity near the nominally pressure release surface of the baffle, hydrophones can be placed at an odd multiple of a quarter wavelength from the baffle or near a heavy signal conditioning plate inserted between the hydrophones and the baffle. In either case, coherent interference between the incident wave and the wave reflected from the baffle limits the bandwidth of high sensitivity. Wayne Reader reasoned that by inserting a broadband absorber between the hydrophones and the decoupling baffle, a smoother hydrophone response could be attained of potentially less weight with relatively small loss in sensitivity. The talk describes the use of gradual transition absorbers developed by Wayne for sonar applications and his interactions with the author to combine these materials with broadband decouplers such as arrays of compliant tubes. His careful and thoughtful approach to the theoretical and experimental aspects of acoustic and material research was inspirational and we miss his insight, advice, and encouragement.

F156. Effects of wall motion on pulsatile flow through compliant tubes

F156. Effects of wall motion on pulsatile flow through compliant tubes

Flexural wave reduction using a compliant baffle

A theoretical model was developed to evaluate the reduction of flexural waves which is generated by a line force applied on an infinite plate using a compliant baffle. The compliant baffle layer placed between the plate excited by a line force and a signal conditioning plate is designed for reducing pressure fluctuations from the vibrating plate. The baffle layer considered here is the compliant-tube array, modeled by Junger [J. Acoust. Soc. Am. 78, 1010 (1985)] to represent a homogeneous (dispersive) fluid layer. Effects of various parameters such as the aspect ratio of the compliant tube, the distance between tubes, and the damping of the tube material on the flexural wave reduction are presented. Calculations made for the nondispersive fluid layer are compared with those made for the dispersive fluid layer.

Complaint tube baffle

First Page

Finite element modeling of single periodic curved gratings

The analysis of the scattering of a plane acoustic wave by an in-plane periodic, active, or passive structure has been previously performed with the help of the finite element method [A. C. Hladky-Hennion etal., J. Acoust. Soc. Am. 94, 621–635 (1993)]. In that case, the compliant tubes grating is a single periodic structure that is periodic in one direction and infinite in the other one. The method previously described has been extended to take into account the effect of the curvature of the grating, periodic along the symmetry axis. Within this approach, only a bidimensional mesh is needed because the structure is axisymmetrical. Moreover, only one unit cell of the periodic structure, including a small part of the surrounding fluid domain, has to be meshed, due to the use of a classical Block-type relation between the displacement components of points that are separated by the grating spacing. Finally, the effect of the external fluid domain is accounted for by matching the pressure field in the finite element domain to plane waves expressed in terms of a series of Bessel or Hankel functions. This paper describes the general mathematical model and then provides the results obtained for the scattering of a plane wave form simple axisymmetrical structures, this validating the method. Next, the method is applied to the case of curved compliant tubes gratings, thus showing the influence of the curvature.

A Review of Pulse Propagation in Viscoelastic Arteries with Distributed Flow Leakage

Current mathematical models for describing pulse propagation in arteries are summarized. These models include the effects of viscoelasticity, frequency dependent friction, and other factors. One factor that has not been previously considered is the effect of distributed flow leakage. This leakage is known to have a significant effect on pulse propagation in rigid tubes and it has been assumed to have a similar effect in compliant tubes. These effects are described here, along with the results of a recent study by these authors on the effects of the distributed flow leakage in compliant tubes.