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PubMed1987

Flush mounted hot film anemometer measurement of wall shear stress distal to a tri-leaflet valve for Newtonian and non-Newtonian blood analog fluids.

Wall shear stress has been measured by flush-mounted hot film anemometry distal to an Ionescu-Shiley tri-leaflet valve under pulsatile flow conditions. Both Newtonian (aqueous glycerol) and non-Newtonian (aqueous polyacrylamide) blood analog fluids were investigated. Significant differences in the axial distribution of wall shear stress between the two fluids are apparent in flows having nearly identical Reynolds numbers. The Newtonian fluid exhibits a (peak) wall shear rate which is maximized near the valve seat (30 mm) and then decays to a fully developed flow value (by 106 mm). In contrast, the shear rate of the non-Newtonian fluid at 30 mm is less than half that of the Newtonian fluid and at 106 mm is more than twice that of the Newtonian fluid. It is suggested that non-Newtonian rheology influences valve flow patterns either through alterations in valve opening associated with low shear separation zones behind valve leaflets, or because of variations in the rate of jet spreading. More detailed studies are required to clarify the mechanisms. The Newtonian wall shear stresses for this valve are low. The highest value observed anywhere in the aortic chamber was 2.85 N/m2 at a peak Reynolds number of 3694.

Aortic Valve↗
PubMed1994

A clinical study on the effects of pulsatile cardiopulmonary bypass on the blood endotoxin levels.

Levels of endogenous endotoxins have been reported to increase after cardiopulmonary bypass. Endotoxin levels have also been implicated in multiple organ failure and may contribute to the immunocompromised state seen after bypass. We evaluated the effects of pulsatile cardiopulmonary bypass circulation on endogenous endotoxin levels. The study population consisted of 15 consecutive adult patients who underwent cardiac operations with cardiopulmonary bypass. Pulsatile flow was used during aortic crossclamping in eight patients (group I) and nonpulsatile flow was used in the remaining seven patients (group II). Changes in blood endotoxin levels were monitored during aortic crossclamping, after release of the clamp, and after weaning from bypass. The blood endotoxin level at each stage was expressed as a percentage of the level at the beginning of bypass. Group I patients a significantly lower blood endotoxin percentage than group II (from 20 to 120 minutes after the initiation of aortic crossclamping). In group I, the blood endotoxin percentage was nearly constant during aortic crossclamping. After release of aortic crossclamping, group I also had a lower blood endotoxin percentage than group II. Endogenous endotoxin levels appear to increase in the presence of intestinal congestion and ischemia. Improvement in intestinal circulation by pulsatile cardiopulmonary bypass may prevent increases in endogenous endotoxin levels by reducing these factors.

Adult↗
PubMed1985

Mechanical assistance for biventricular failure following coronary bypass operation and heart transplantation.

From September 1983 to March 1985, five patients who could not be weaned from extracorporeal circulation or who deteriorated in the recovery room have been treated with biventricular mechanical support using two vortex pumps, standard cannulas and tubing. One patient was supported for six hours following heart transplantation and acute graft failure until another donor heart could be found. Intraaortic balloon pumping was utilized in each patient to augment the circulation and produce pulsatile flow. Four of the five patients were weaned from the device. Only one patient is currently a long-term survivor. Although long-term survival has been low, the ability of the myocardium to recover was impressive and warrants the efforts at assisting the heart in biventricular failure.

Adult↗
PubMed1989

Fibronectin enhances early shear stress resistance of seeded adult human venous endothelial cells.

An in vitro parallel plate perfusion chamber was used to study the shear stress resistance of seeded adult human saphenous vein endothelial cells (AHSVECs) on glass surfaces coated with different substrates. Endothelial cells were seeded onto glass slides precoated with these substrates and then exposed to pulsatile flow with an average shear stress of 8 dyn/cm2 for 1 hr. After AHSVEC attachment periods of 15 min, 1 hr, and 2 hr, flow dislodged all but 1.4, 30.4, and 72.2%, respectively, of cells that had been seeded onto 1% gelatin. Control slides that were not exposed to flow retained 12.9% (P less than 0.03), 49.8% (NS), and 95.2% (NS) of seeded cells. Precoating the slides with 10 micrograms/ml fibronectin resulted in 69.4, 89.5, and 97.7% of cells remaining after flow, compared with 6.4% (P less than 0.03), 53.7% (NS), and 93.3% (NS), respectively, when using matched slides coated with 1% gelatin. Results with 20% fetal bovine serum as the substrate were not statistically different from those obtained with 1% gelatin. We conclude that fibronectin enhances the early attachment of AHSVECs to artificial surfaces and is, therefore, potentially useful for increasing attached cell yields on arterial prostheses prepared with immediate cell seeding techniques.

Cell Adhesion↗
PubMedDate not supplied

Three-dimensional time-of-flight MR angiography with a specialized gradient head coil.

A gradient head coil has been developed, incorporating two independent gradients within the conventional body coil of the magnetic resonance (MR) system, with reduced rise times (200 microseconds) and maximum amplitudes of 37 and 18 mT/m in the z and y directions, respectively. This gradient coil was systematically evaluated by testing two-dimensional (2D) and three-dimensional (3D) time-of-flight (TOF) MR angiography sequences applied to a pulsatile flow phantom simulating a carotid stenosis and the intracranial vasculature. When standard 2D and 3D TOF MR angiography techniques were used to image the carotid stenosis model, dramatic signal loss in the stenotic segment and a large flow void distal to the stenosis were seen. The shorter (3.8 msec) absolute echo times (TEs) achievable with the gradient coil in 3D sequences substantially reduced the phase dispersion and associated signal loss in the region of stenosis. Shorter TEs alone (3.2 msec) did not minimize signal loss, and first-order flow compensation in the read and section-select directions provided further improvements (despite slightly longer TEs). Reduction of TEs in 2D sequences yielded relatively poor results regardless of the refocusing scheme or TE. This study confirms the predicted benefits of a dedicated coil with improved gradient capabilities for 3D MR angiography. The study suggests the limitations of 2D TOF MR angiography in the evaluation of severe stenoses.

Carotid Arteries↗
PubMed1997

Quantitative acoustic characterization of a new surfactant-based ultrasound contrast agent.

The acoustic properties of a new ultrasound contrast agent, ST68, have been investigated. ST68 is a sonicated mixture of nonionic surfactants (Span-type and Tween-type) consisting of stabilized microbubbles with a mean diameter of 3.8 microns and a concentrations of 7.1 x 10(8) bubbles/mL. A pulsatile flow system was used to acquire data in vitro. The acoustic properties of ST68, as a function of time, frequency and dose, were calculated. Enhancement changed nonlinearly with contrast agent dose; maximum was 13.1 dB +/- 1.0 dB for a dose of 0.30 microL/mL of suspending medium. Attenuation reached approximately 11 dB/cm for dosages above 0.27 microL/mL and for frequencies between 2.5 and 6.0 MHz. In vivo, i.v. injections of ST68 were given to 4 rabbits (doses from 0.01 to 0.23 mL/kg). A clear increase in flow signal intensity was observed for 1 to 2 min. An in vivo dose-response curve was calculated from audio Doppler signals obtained with a 10-MHz cuff transducer placed around the distal aorta. Maximum enhancement was 18.3 dB +/- 3.13 dB for a 0.13 mL/kg dose. Moreover, ST68 appears to follow a simple relationship between in vivo enhancement and dose. In conclusion, ST68 is capable of producing marked vascular enhancement. Its acoustic properties have been characterized in vitro and in vivo.

Acoustics↗
PubMedDate not supplied

Hemodynamic comparisons of polyurethane trileaflet and bioprosthetic heart valves.

Hemodynamic comparison of two polyurethane prosthetic heart valves with a bioprosthetic valve is presented. The valves were incorporated in a pulse duplicator simulating physiologic pulsatile flow, and comparisons between the valves were made on the transvalvular pressure drop, percent regurgitation, valve orifice area, rate of opening and closing as well as the performance index. The results showed that the functional characteristics of the polyurethane valve compared favorably with that of the bioprosthetic valve. The polyurethane valve can be a viable and inexpensive alternative, especially for short-term use in a total artificial heart as a bridge to transplant.

Bioprosthesis↗
PubMedDate not supplied

Rotacor: a new rotary blood pump.

A new rotary blood pump was tested in calves for 6 hr. The pump consists of a rigid housing with a trochoidal internal surface, an inlet and outlet, and two lateral walls. A two-corner piston rotates on an eccentric shaft in a trochoidal path, thus creating a gap seal. The pump is driven by a water-cooled DC motor. For right ventricular assist, a cannula was inserted into the right ventricle through the right atrium, and into the left ventricle for left ventricular assist. From a total of 10 experiments, two left ventricular assists, two right ventricular assists, and three biventricular assists were evaluated. The pump produced a pulsatile flow of 3 L at 70 rpm. Energy requirements were 2.19 watts for left, 2.06 for right, and 7.26 for biventricular assists. Plasma hemoglobin remained as low as 10 mg/dl during monoventricular, and increased during biventricular assists to 20 mg/dl after 3 hr, when it started to chop again; after 6 hr it was 16 mg/dl. From these preliminary results it is concluded that this new type of blood pump may be suitable as a circulatory assist device.

Animals↗
PubMed1998

Vasculature in hepatocellular carcinoma after transcatheter arterial chemoembolization: comparison of power and color Doppler sonography.

The purpose of this study was to compare power Doppler sonography with conventional color Doppler sonography for the detection of the vascularity of hepatocellular carcinomas after transcatheter arterial chemoembolization. Of the 93 embolized hepatocellular carcinomas, hypervascularity was demonstrated in 36 on angiography; power Doppler sonography correctly identified pulsatile flow signals in 33 (92%) of these 36, whereas color Doppler sonography identified flow signals in 24 (67%). A statistically significant difference was noted when the sizes of the nodules were 30 mm or less. Despite technical difficulties, such as flash artifact, power Doppler sonography is superior to color Doppler sonography for detection of hypervascularity, especially in small embolized nodules of hepatocellular carcinoma (30 mm or less in diameter).

Adult↗
PubMedDate not supplied

Pulsation artifact in short TR MR imaging and angiography: exacerbation with signal averaging.

Averaging the signals from more than one excitation per phase-encoding view increases the signal-to-noise ratio and, in conventional spin-echo magnetic resonance imaging, reduces most motion artifacts. To determine the effects of signal averaging on two-dimensional gradient-echo images, acquisitions with different TRs and with no averaging versus multiple-signal averaging were compared in a pulsatile flow phantom and the human abdominal aorta. Intraview (each view repeated before changing the phase-encoding value) and interview (obtaining all views sequentially and then repeating the entire set) averaging methods were used. Pulsation artifacts were present on all images of the flow phantom and the aorta. Intraview signal averaging, the method most commonly used, exacerbated rather than ameliorated pulsation artifacts with short TR sequences. Pulsation artifacts on two-dimensional images obtained with a short TR can be minimized by completing the acquisition as rapidly as possible, avoiding signal averaging. If signal averaging is used for short TR images, it should be interview averaging.

Algorithms↗
PubMed1994

Influence of angle on wall shear stress distribution for an end-to-side anastomosis.

PURPOSE: The purpose of this article was to study the effects of anastomotic angle on the wall shear stress distribution for end-to-side anastomosis models under pulsatile flow conditions. METHOD: The photochromic tracer technique was used to visualize the flow field and to determine the instantaneous wall shear stress at multiple locations simultaneously. Models with angles of 20, 30, 45, and 60 degrees were examined. RESULTS: For all angles, low shear stress was present at the heel and on the bed opposite the heel of the anastomosis apparently as a result of the complete occlusion of the proximal end of the host vessel. Near the toe, increased flow separation occurred with increasing angle. On the bed across from the toe, increasing the angle led to increased shear stress. In addition, in this region the anastomotic angle significantly altered other properties of the shear stress field such as the mean and peak-to-peak magnitudes and cycle-to-cycle fluctuations. CONCLUSIONS: This study provides quantitative data on the wall shear stress distribution within an end-to-side anastomosis and its relation to the anastomotic angle. The results are discussed in terms of possible roles of shear-induced intimal hyperplasia.

Anastomosis, Surgical↗
PubMed2001

Tissue engineering of small caliber vascular grafts.

OBJECTIVE: Previous tissue engineering approaches to create small caliber vascular grafts have been limited by the structural and mechanical immaturity of the constructs. This study uses a novel in vitro pulse duplicator system providing a 'biomimetic' environment during tissue formation to yield more mature, implantable vascular grafts. METHODS: Vascular grafts (I.D. 0.5 cm) were fabricated from novel bioabsorbable polymers (polyglycolic-acid/poly-4-hydroxybutyrate) and sequentially seeded with ovine vascular myofibroblasts and endothelial cells. After 4 days static culture, the grafts (n=24) were grown in vitro in a pulse duplicator system (bioreactor) for 4, 7, 14, 21, and 28 days. Controls (n=24) were grown in static culture conditions. Analysis of the neo-tissue included histology, scanning electron microscopy (SEM), and biochemical assays (DNA for cell content, 5-hydroxyproline for collagen). Mechanical testing was performed measuring the burst pressure and the suture retention strength. RESULTS: Histology showed viable, dense tissue in all samples. SEM demonstrated confluent smooth inner surfaces of the grafts exposed to pulsatile flow after 14 days. Biochemical analysis revealed a continuous increase of cell mass and collagen to 21 days compared to significantly lower values in the static controls. The mechanical properties of the pulsed vascular grafts comprised supra-physiological burst strength and suture retention strength appropriate for surgical implantation. CONCLUSIONS: This study demonstrates the feasibility of tissue engineering of viable, surgically implantable small caliber vascular grafts and the important effect of a 'biomimetic' in vitro environment on tissue maturation and extracellular matrix formation.

Absorbable Implants↗
PubMed1989

Duplex Doppler ultrasound in determination of renal artery stenosis.

Twenty-six cases of renal artery stenosis were evaluated with Doppler ultrasound using the pulsatile flow index (PFI). To establish normal values, the PFI in 60 renal arteries in 30 healthy volunteers was obtained. Normal values by PFI ranged between 0.48 and 0.71 (mean +/- SD: 0.6 +/- 0.06). In renal artery stenosis the PFI range was 0.72-0.79. The normal upper limit was 0.71. The PFI failed in three patients; however, an ultrasound examination showed secondary renal disease (two patients with stenosis on both sides with a shrunken kidney on one side, and one patient with hypernephroma on the opposite side). The PFI was normal (14 of 14 patients) in patients without angiographic evidence of stenosis or after successful dilatation.

Humans↗
PubMedDate not supplied

MR angiography with a cardiac-phase--specific acquisition window.

A method for cardiac-phase-specific magnetic resonance (MR) angiography is presented. An electronics module permits incrementing of phase-encoding gradients and storage of incoming data only during a chosen portion of the cardiac cycle. Suppression of stationary material is maintained by delivering radio-frequency pulses at constant TR throughout the cycle. Imaging of a pulsatile flow phantom demonstrates that acquiring data only during systole substantially increases the signal intensity of flowing material. In addition, phase-encoding ghost artifacts are eliminated from the neighborhood of the vessel. Image acquisition time is minimized by acquiring only the low-frequency phase-encoding lines in the cardiac-phase-specific mode. In healthy volunteers, greatly improved MR angiograms of the lower extremities are obtained. Fat saturation and magnetization transfer further enhance vessel/background contrast. Acquiring data only during systole ensures rapid inflow for all phase-encoding lines, permitting a near-longitudinal section orientation without in-plane saturation. This substantially reduces total acquisition time relative to axial acquisition.

Blood Vessels↗
PubMed1997

The influence of free hand suturing technique and zero pressure fixation on the hydrodynamic function of aortic root and aortic valve leaflets.

OBJECTIVE: There is increasing interest in the use of stentless porcine bioprostheses implanted by the free hand suturing technique. However, preservation of the normal valve geometry and function is a matter of concern. This study has been conducted to investigate the influence of free hand insertion on the hydrodynamic function of the aortic valve. In particular, free sewn valves have been compared to whole root replacements for both fresh valves and zero pressure fixed bioprostheses. METHODS: Four sets of porcine aortic roots with six aortic roots in each set consisting of (A) fresh aortic roots, (B) fresh aortic valve inserted by free hand suturing technique in fresh aortic valve fixed in glutaraldehyde under zero pressure, inserted by free hand suturing technique in fresh aortic roots, and (D) aortic roots fixed in glutaraldehyde under zero pressure, were studied. Hydrodynamic function and leaflet bending deformations were analysed in a modified pulsatile flow simulator which incorporates the elastic aortic roots. RESULTS: The glutaraldehyde-treated roots were found to be less distensible than the fresh aortic roots, with a mean dilatation of only 8% compared with 43% for the fresh roots at 120 mmHg pressure. The other two groups of free sewn valves showed reasonably good dilatation of 38%. There was no significant difference between the two sets. The hydraulic effective orifice area (EOA) of the valve, which was derived from the flow and pressure measurements, was divided by the actual orifice area for the annulus to arrive at the performance index. The performance index for glutaraldehyde-fixed aortic root was the lowest, with an index of 0.47 compared with 0.91 for that of fresh roots, 0.61 for the fresh valve in fresh root and 0.50 for the glutaraldehyde-treated valve in fresh root. The mean pressure difference during the peak flow also associated the same order with 12 mmHg for the glutaraldehyde-fixed roots, 1.8 mmHg for the fresh roots, 5.3 mmHg for the fresh valve in fresh root and 7.7 mmHg for the glutaraldehyde-treated valve in fresh root. The free hand suturing technique altered the leaflet geometry and caused increased open leaflet bending deformations compared to the fresh root. Both groups of free sewn valves behaved better than the zero pressure fixed root. CONCLUSIONS: Alteration in leaflet biomechanics is likely to influence long-term valve durability.

Animals↗
PubMed2001

Reversed portal vein pulsatility on Doppler ultrasound secondary to an iatrogenic mediastinal haematoma.

The Doppler ultrasound pattern of reversed pulsatile flow (RPF) of the portal vein (PV) is strongly associated with high atrial pressure. Tricuspid regurgitation is considered to be the main cause of RPF in patients with chronic heart disease, but the precise pathomechanism of this PV flow pattern has not yet been resolved. We describe for the first time a RPF of the PV in a young patient with a mediastinal haematoma after inadvertent puncture of the subclavian artery. In this patient, transcutaneous echocardiography demonstrated normal valves without any tricuspid regurgitation as well as normal diameters of the cardiac cavities. The RPF of the PV in this patient resolved spontaneously within 7 days. An increased hepatic outflow resistance with transmission of hepatic artery pulsations across arterioportal communications seems the most likely pathomechanism to explain our finding.

Adult↗
PubMed2004

Spatial comparison between wall shear stress measures and porcine arterial endothelial permeability.

A better understanding of how hemodynamic factors affect the integrity and function of the vascular endothelium is necessary to appreciate more fully how atherosclerosis is initiated and promoted. A novel technique is presented to assess the relation between fluid dynamic variables and the permeability of the endothelium to macromolecules. Fully anesthetized, domestic swine were intravenously injected with the albumin marker Evans blue dye, which was allowed to circulate for 90 min. After the animals were euthanized, silicone casts were made of the abdominal aorta and its iliac branches. Pulsatile flow calculations were subsequently made in computational regions derived from the casts. The distribution of the calculated time-dependent wall shear stress in the external iliac branches was directly compared on a point-by-point basis with the spatially varying in vivo uptake of Evans blue dye in the same arteries. The results indicate that in vivo endothelial permeability to albumin decreases with increasing time-average shear stress over the normal range. Additionally, endothelial permeability increases slightly with oscillatory shear index.

Albumins↗
PubMedDate not supplied

Evaluation of the iliac arteries: comparison of two-dimensional time of flight magnetic resonance angiography with cardiac compensated fast gradient recalled echo and contrast-enhanced three-dimensional time of flight magnetic resonance angiography.

We compared dynamic contrast-enhanced three-dimensional time of flight (3DTOF) magnetic resonance angiography (MRA) with two-dimensional time of flight (2DTOF) MRA with cardiac compensated fast gradient recalled echo (C-MON) and conventional angiography (CA) when it was available. C-MON re-orders the normal data acquisition to minimize ghosting artifacts generated by pulsatile flow. The initial phase of the study involved optimization of parameters and comparison C-MON with no C = MON in eight patients and volunteers. The final phase of the study involved 53 patients who were imaged with contrast-enhanced 3DTOF MRA and 2DTOF MRA with C-MON. Thirty of these patients also had CA. In the initial phase, 2DTOF MRA with C-MON was found to be equal (n = 3) or superior (n = 5) to 2DTOF without C-MON. In the final phase, the agreement among all imaging modalities varied from substantial to almost perfect (Cohen's kappa = .6-.83). The lowest agreement was using 2DTOF to evaluate the external iliac segments. The among suggested treatments varied from substantial to almost perfect for all imaging modalities (Cohen's kappa = .73-93). The diagnostic efficacies of 2DTOF with C-MON and contrast-enhanced 3DTOF were high overall, with the lowest value being a specificity of 63% for one reader in the evaluation of an external iliac segment using 2DTOF. In summary, 2DTOF with C-MON helped to eliminate artifacts due to pulsatility in the iliac arterial segments. In our experience, both dynamic contrast-enhanced 3DTOF MRA and 2DTOF MRA with C-MON performed well in the evaluation of the iliac arteries. Both studies have high interobeserver agreement and high diagnostic efficacy. Contrast-enhanced 3DTOF MRA should be reserved for situations in which the iliac vessels are extremely tortuous or occluded or the external iliac segments are poorly seen.

Adult↗