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PubMed2004

Intra-aneurysmal flow with helix and mesh stent placement across side-wall aneurysm pore of a straight parent vessel.

Pulsatile flow fields in a cerebrovascular side-wall aneurysm model with a wide ostium after stenting are presented in terms of particle tracking velocimetry measurements and flow visualization. Among the stent parameters the shape, helix versus mesh, was selected to study its effect on the changes of intraaneurysmal hemodynamics for the reference of minimally invasive endovascular aneurysm treatment. The blocking ratio of the stents was fixed at 30%. The Womersley number was 3.9 and the mean, peak, and minimal Reynolds numbers based on the bulk average velocity and diameter of the parent vessel were 600, 850, and 300, respectively. Four consecutive flow-rate phases were selected to characterize the intra-aneurysmal flow. The results are characterized in terms of velocity vector field, regional average velocity, and intra-aneurysmal vorticity/circulation/wall shear stress. It is found that the hemodynamic features inside the aneurysm alter markedly with the shape of the stent and the size of the orifice. Both stents investigated induce favorable changes in the intra-aneurysmal flow stasis as well as direction and undulation of wall shear stresses. A comparison of the results of the helix to mesh stent shows that the former is more favorable for endovascular treatment.

Animals↗
PubMed2003

Transfer function analysis of ultrasonic time-intensity measurements.

Time-intensity measurements of ultrasonic-contrast microbubbles based on the dilution theory have been used to assist blood flow estimation. The compartment model has been employed to describe the dilution process. Under the linear and time-invariant assumption, the time-intensity curve measured at the output of a compartment (i.e., blood mixing chamber) is the convolution of the input time-intensity curve with the compartment's transfer function. Thus, transfer function analysis is possible using deconvolution when the temporal variations in both the input and the output intensities are available. Note that the linear and time-invariant assumption requires a constant flow rate because, with flow pulsation, the flow rate changes with time and the mixing process becomes time varying. Thus, the purpose of this paper was to study the effects of flow pulsation on time-intensity measurements. In addition, a deconvolution technique based on a recursive least squares approach is used for transfer function analysis. Both simulations and experiments were performed; the results from which indicate that the pulsation generally does not affect the validity of time-intensity-based flow estimation. The proposed deconvolution technique is also effective for both constant and pulsatile flows; thus, permitting transfer function analysis in various flow conditions. One potential application of this transfer function analysis is to remove the effects of a noninstantaneous input function. The results from this paper lead to future work in brain-perfusion estimation based on extracranial time-intensity measurements.

Blood Flow Velocity↗
PubMedDate not supplied

Physiology of nonpulsatile circulation: acute versus chronic support.

Mammals seem to be able to adapt to nonpulsatile circulation. For chronic support, pressure and flow, not the presence of a pulse, are the major requirements to sustain normal organ function. Pulsatile flow, however, seems to offer advantages over nonpulsatile flow for acute support in maintaining lymphatic flow, decreasing systemic vascular resistance, improving peripheral and pulmonary capillary perfusion, and reversing shock. These advantages may not be seen with chronic support with adaptation to nonpulsatile flow.

Animals↗
PubMed1975

Xylose absorption test before and after surgical repair of coarctation of the aorta.

The aim of the study was to ascertain whether the acceleration of physical development in children after operation for coarctation of the aorta is due to the improved intestinal absorption. Lowered values of the oral xylose absorption test were found in all 9 patients before surgery. In all the patients who had been operated on, sphigmo-oscillometry revealed an increased pulsatile character of the blood flow, pulsatile volume in the lower extremities, and a rise of mean arterial blood pressure in this area. On the other hand, surgical repair had no influence on the renal blood flow as evidenced by PAH clearance and isotope renography. There was no correlation of diuresis and the xylose excretion test. Increased values of the xylose test after operation for coarctation of the aorta are due to the increased intestinal absorption rather than to improved renal function. Improved intestinal absorption may be a factor responsible for the accelerated physical development of children operated on for coarctation of the aorta.

Adolescent↗
PubMed1963

Partial carotid occlusion as treatment for intracranial internal carotid aneurysms.

A study was undertaken to investigate the possibility of using partial carotid occlusion instead of complete carotid ligation for the treatment of intracranial internal carotid aneurysms with a view to avoiding such complications of the latter procedure as neurological deficit resulting from cerebral ischemia, and ascending thrombus formation. The beneficial effect of carotid ligation has been explained by the interruption of pulsatile flow which can cause rupture of an aneurysm by resonance phenomena. Studies on blood flow in the aorta in dogs, as well as in a human carotid artery in vivo and in vitro, showed that the same object could be achieved by the use of constriction by a Poppen clamp. This changed the pulsatile blood flow to a relatively non-pulsatile state with slight diminution in mean flow. Partial occlusion of the common carotid artery is recommended for those cases of intracranial aneurysm in which complete carotid occlusion would not likely be tolerated.

Aneurysm↗
PubMed2002

Numerical analysis of flow through a severely stenotic carotid artery bifurcation.

The results of computational simulations may supplement MR and other in vivo diagnostic techniques to provide an accurate picture of the hemodynamics in particular vessels, which may help demonstrate the risks of embolism or plaque rupture posed by particular plaque deposits. In this study, a model based on an endarterectomy specimen of the plaque in a carotid bifurcation was examined. The flow conditions include steady flow at Reynolds numbers of 300, 600, and 900 as well as unsteady pulsatile flow. Both dynamic pressure and wall shear stress are very high, with shear values up to 70 N/m2, proximal to the stenosis throat in the internal carotid artery, and both vary significantly through the flow cycle. The wall shear stress gradient is also strong along the throat. Vortex shedding is observed downstream of the most severe occlusion. Two turbulence models, the Chien and Goldberg varieties of k-epsilon, are tested and evaluated for their relevance in this geometry. The Chien model better captures phenomena such as vortex shedding. The flow distal to stenosis is likely transitional, so a model that captures both laminar and turbulent behavior is needed.

Arteriosclerosis↗
PubMed1996

Pulsatile venous Doppler flow in lower limbs: highly indicative of elevated right atrium pressure.

OBJECTIVE: The purpose of this study was to determine if pulsatile flow in lower limbs as seen on venous Doppler waveforms correlates with increased right atrium pressure. MATERIALS AND METHODS: Of 429 patients who, over a 14-month period, underwent venous Doppler imaging of the lower limb to exclude deep venous thrombosis, 343 records were available for review at the time of the study. Of these, 74 had right atrium pressure measurements available for correlation. Seventeen patients were excluded because of thrombosis in the common femoral vein. Six other patients were also excluded because the time difference between the Doppler and the correlative studies was more than 4 weeks. The remaining 51 patients constituted the study subjects. In 18 of these, the right atrium pressure was measured within 1 week, 31 within 2 weeks, 42 within 3 weeks, and 51 within 4 weeks. In the study, we evaluated the three major veins of the lower limb (the common femoral, superficial femoral, and popliteal) by venous Doppler imaging. Data from only the common femoral vein were included in our analysis because this vessel was the least involved with thrombosis. The findings were correlated with the presence or absence of right-sided heart failure as determined by right atrium pressure measurement. A Doppler waveform was considered pulsatile when it had a cyclic retrograde component. A right atrium pressure of more than 8 mm Hg was considered elevated. RESULTS: Of 51 patients, 17 (33%) had pulsatile lower limb venous Doppler flow waveforms and 33 (65%) had elevated right atrium pressure. We found a statistically significant correlation between the presence of these abnormal waveforms and elevated right atrium pressure. The sensitivity of lower limb venous Doppler imaging for detecting right-sided heart failure as determined by right atrium pressure measurement was 46%, specificity was 94%, positive predictive value was 94%, negative predictive value was 50%, and accuracy was 65%. CONCLUSION: Pulsatile lower limb venous Doppler waveform correlates well with right-sided heart failure, as indicated by a right atrium pressure measurement of more than 8 mm Hg. However, because of its low sensitivity, lower limb venous Doppler imaging cannot be used to screen for right-sided heart failure.

Adult↗
PubMed1989

Estimation of pulsatile ocular blood flow from intraocular pressure.

A relationship has been derived between intraocular pressure and pulsatile blood flow in the eye. Measurements of intraocular pressure show a time variation that is associated with the pulsatile component of arterial pressure. Experimental results provide a means of transforming intraocular pressure changes into ocular volume changes. The eye is represented by a chamber with elastic walls, a pulsatile incoming flow of incompressible fluid (blood), and a steady outgoing flow of blood. Under these conditions, the rate of pulsatile blood flow through the eye can be approximated from the instantaneous intraocular pressure measurements. Data from a healthy human eye are used to illustrate the analysis.

Blood Flow Velocity↗
PubMed1992

The design of pulse sequences employing spatial presaturation for the suppression of flow artifacts.

The use of spatial presaturation to suppress the signal, and therefore also the artifacts, from flowing blood has become an important tool in the arsenal of techniques to suppress pulsatile flow artifacts in magnetic resonance images. However, a detailed theoretical analysis of the behavior of these flow artifact suppression pulses and of the important aspects of implementing suppression pulses in combination with particular imaging sequences has yet to be presented. In this paper we present a general theoretical framework to describe the flow artifact suppression technique. This analysis addresses the following four major issues: (1) the spin washout characteristics of the imaging sequence, (2) the interference between the flow signal suppression pulses and the imaging sequence, (3) the flow velocity range for a single application of the suppression pulse, and (4) the total flow velocity range for a suppression pulse repeated with a constant time interval between applications of the pulse. The predictions of our theoretical model are confirmed by experimental measurements made with stationary and flow phantoms. The results of this investigation provide guidelines for the design of flow artifact suppression pulse sequences and, in addition, should aid in the future development and refinement of the spatial presaturation technique as applied to flow signal suppression.

Artifacts↗
PubMed1998

In vitro analysis of regurgitant fraction using Doppler power-weighted sum of velocities.

The power-weighted sum of velocities (PWS) is the sum of each velocity component of the Doppler signal multiplied by its power. The purpose of this study was to determine (1) whether PWS is linearly related to volume flow and (2) whether PWS can predict the regurgitant fraction in an in vitro pulsatile flow system simulating aortic regurgitation. Doppler analysis of aortic flow was performed with an intact valve and two regurgitant valves. For each valve a linear relation between the forward flow PWS and forward flow volume was demonstrated, with excellent correlation (r = 0.99). For the valves with regurgitant orifices, the values for the PWS-derived regurgitant fraction were compared with measured regurgitant fraction. A fair correlation was demonstrated (r = 0.59), with low accuracy in prediction (error 44% +/- 24%). The PWS was inaccurate in predicting flow ratios in our in vitro system despite the strong relation with forward flow volume. The error incurred may be due to effects of filters that remove low velocity and low amplitude information.

Aortic Valve↗
PubMed1999

Single-loop coil concepts for intravascular magnetic resonance imaging.

Compared with other coil designs that have been investigated for intravascular use, the single-loop coil can be designed with a very small diameter for insertion into small vessels and with a longitudinal extent over several centimeters for multislice imaging. If it designed to be expandable inside the target vessel, then it combines these features with increased signal-to-noise ratio (SNR) and penetration depth. Expandable single-loop coils that are capable of meeting these requirements were developed and integrated into two different commercial catheter-based delivery systems: a self-expandable, single-loop made from NiTinol and a single-loop coil mounted on an inflatable balloon. The influence of a small-diameter coaxial cable for remote tuning and matching on the coil performance was investigated. Calculations showed the dependence of the signal on the separation between the conductors. The comparison of both catheter approaches in in vitro flow experiments and in an in vivo pig experiment revealed the influence of pulsatile flow on image quality during intravascular imaging with these designs.

Animals↗
PubMed1999

Pressure and flow fields in the hinge region of bileaflet mechanical heart valves.

BACKGROUND AND AIM OF THE STUDY: Recent clinical thrombotic experiences with the Medtronic Parallel (MP) bileaflet heart valve have highlighted the need for new methods to assess preclinical valve hinge flow. The aim of the current study was to investigate hinge pivot flow fields in bileaflet mechanical heart valves using flow visualization in scaled x5 magnification transparent polymer models and computational fluid dynamic (CFD) analysis using CFD 2000 STORM code. METHODS: Polymeric x5 flow models of the On-X, St. Jude Medical (SJM) and MP bileaflet heart valves were constructed using laser stereolithography to replicate the interior geometry while maintaining realistic manufacturing tolerances. Each hinge flow experiment was carried out by installing the transparent x5 model in a pulsatile flow loop, which was designed according to Womersley number similitude requirements. Motions of suspended microparticles in the valve hinge area, recorded by laser imaging techniques, were used to visualize hinge flow. Experimentally measured parameters were used as input for CFD analysis. CFD simulations were made by solving the Navier-Stokes equation using a finite volume method with the pressure-based algorithm for continuity, and a pressure-implicit with splitting of operators (PISO) algorithm for pressure-velocity coupling. Moving grid methodology was employed to simulate periodic motion of the valve leaflets. CFD hinge flow results were visualized on four parallel planes at different depths in the hinge socket. The hinge flow patterns of the three types of bileaflet heart valve design are discussed. RESULTS: Prominent vortex formation and stagnant flow areas were noticed in the pivot region of the MP valve. Vortices persisted throughout both the forward- and reverse-flow phases. These flow structures were not observed in the hinge areas of the SJM and On-X valves. CONCLUSIONS: Vortex formation observed in the MP valve may contribute to the high thrombogenic potential of this valve. The absence of such vortices and areas of stagnant flow in the On-X and SJM valves indicate that hinge flow conditions in these valves do not favor mechanically induced thrombogenesis or thromboembolic events.

Biocompatible Materials↗
PubMed1996

Pulmonary blood supply in bidirectional cavopulmonary anastomosis with pulsatile pulmonary blood flow: quantitative analysis using radionuclide angiocardiography.

OBJECTIVE: To establish a non-invasive method for quantitative analysis of pulmonary perfusion in patients with bidirectional cavopulmonary anastomosis (BCPA) and sources of pulsatile blood flow. The method should quantify left to right lung flow ratio and relative contribution of BCPA and sources of pulsatile blood flow to perfusion of each lung. DESIGN: A pilot study using radionuclide angiocardiography for quantitative analysis and for visualisation of cavo-caval collaterals. No criterion standard is available. SETTING: Tertiary care centre, ambulatory and hospital inpatient care. PATIENTS: Consecutive sample of 18 patients with BCPA and sources of pulsatile blood flow. RESULTS: In eight patients (44%) cavocaval collaterals prevented quantitative analysis. In 10 patients without cavo-caval collaterals, BCPA provided 42.3 (SEM 3.4)% of total pulmonary blood flow. From the total BCPA flow, 67.2 (4.3)% was directed to the ipsilateral lung. This lung received only 16.5 (3.3)% of all the blood from sources of pulsatile blood flow. The blood flow to the lung at the side of BCPA accounted for 35.3 (1.7)% of the total pulmonary blood flow. CONCLUSIONS: Radionuclide angiocardiography allows the quantitative analysis of pulmonary blood supply in BCPA with sources of pulsatile blood flow except in patients with cavo-caval collaterals or bilateral BCPA. Non-pulsatile flow from BCPA is mainly directed to the ipsilateral lung, whereas pulsatile flow to the contralateral lung. Total perfusion of the ipsilateral lung is less than the perfusion of the contralateral lung.

Adult↗
PubMed2005

A thin film nitinol heart valve.

In order to create a less thrombogenic heart valve with improved longevity, a prosthetic heart valve was developed using thin film nitinol (NiTi). A "butterfly" valve was constructed using a single, elliptical piece of thin film NiTi and a scaffold made from Teflon tubing and NiTi wire. Flow tests and pressure readings across the valve were performed in vitro in a pulsatile flow loop. Bio-corrosion experiments were conducted on untreated and passivated thin film nitinol. To determine the material's in vivo biocompatibility, thin film nitinol was implanted in pigs using stents covered with thin film NiTi. Flow rates and pressure tracings across the valve were comparable to those through a commercially available 19 mm Perimount Edwards tissue valve. No signs of corrosion were present on thin film nitinol samples after immersion in Hank's solution for one month. Finally, organ and tissue samples explanted from four pigs at 2, 3, 4, and 6 weeks after thin film NiTi implantation appeared without disease, and the thin film nitinol itself was without thrombus formation. Although long term testing is still necessary, thin film NiTi may be very well suited for use in artificial heart valves.

Alloys↗
PubMed1992

Pulmonary arterial impedance after single lung transplantation.

Single lung transplantation (SLT) is emerging as definitive therapy for end-stage pulmonary disease of varying etiology, yet a complete description of the hemodynamic properties of the transplanted lung has not been reported. In this study, Fourier analysis was used to calculate the pulmonary arterial (PA) impedance spectrum before and immediately after SLT to define precisely the pulmonary pressure-flow relationship. Median sternotomies were performed in 18 dogs (donors): an ultrasonic flow probe was placed around the PA and micromanometers were placed in the PA and left atrium (LA). Control PA pressure and flow (PAQ) and LA pressure were measured during transient occlusion of the right PA. The lungs were harvested using cold modified Euro-Collins solution for preservation. After thoracotomy and pneumonectomy, left SLT was performed in 18 recipient dogs with a mean ischemic time of 179 +/- 6 min. After reperfusion for 1 hr, PA pressure and flow data were again collected. Characteristic impedance (Z0), a measure of resistance to pulsatile flow, was compared to input resistance (Rin), a measure of resistance to mean flow, and pulmonary vascular resistance (PVR), the conventional index. Rin is defined as the zeroth harmonic of the impedance spectrum and Z0 as the mean of impedance moduli from 2-12 Hz. All recipients survived transplantation. Both PVR and Rin increased significantly after transplantation (11 +/- 1 vs 19 +/- 3 Wood U, P less than 0.05, and 1352 +/- 121 vs 1964 +/- 244 dyne.sec.cm-5, P less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1994

Experimental study of the effects of "fractional" gating on flow measurements.

Velocity encoded phase imaging is subject to errors from phase and amplitude variations of the k-space data caused by beat-to-beat variations of the flow. Fractional cardiac gating is defined as asynchronous gating with each phase encode step occupying a fixed fraction of the RR interval. The gating fraction is the inverse of the number of phase encode steps taken per RR interval. Studies in normal subjects show that deviations and standard errors of ascending and descending aorta flow measurements are significantly greater with decreased gating fraction. Significant errors occur when gating does not separate systolic and diastolic data. The studies establish a graded trade-off between flow measurement accuracy and precision with imaging time, and show that standard nongated phase contrast measurements of strongly pulsatile flow are unreliable.

Angiography↗
PubMed1989

In vitro comparison of velocity profiles and turbulent shear distal to polyurethane trileaflet and pericardial prosthetic valves.

A comparative study of flow dynamics past biomer trileaflet valves and a pericardial bioprosthetic valve under steady and physiological pulsatile flow conditions in vitro is reported in this paper. The velocity profiles and the turbulent shear stresses distal to the valves were measured using laser Doppler anemometry. The authors' results showed that the velocity profiles distal to the trileaflet valves were similar to that measured distal to the pericardial valve. Higher magnitudes of absolute turbulent shear stresses were measured distal to the synthetic valves in comparison to the pericardial valves. However, when the stresses were nondimensionalized with respect to the orifice diameter at the inlet aspect, the stresses were comparable for all of the three valves. With design modifications to increase the orifice diameter at the inlet aspect of the polyurethane valves, the turbulent stresses distal to the valves can be minimized. Such in vitro studies on the flow dynamics past the polyurethane valves can provide information towards design changes to improve the performance characteristics of these valves. Polyurethane valves with flow characteristics comparable to the pericardial valves can be manufactured relatively inexpensively compared to mechanical or tissue valve prosthesis. Hence, the synthetic valves may be a viable alternative for short-term use in total artificial heart devices as a bridge to transplant.

Bioprosthesis↗
PubMed1994

Carotid arterial impedance during oscillated blood flow.

Blood perfusion of systemic circulation is influenced by the physical characteristics of artery. Several investigations proposed that the physical parameters of vessels are determined by neurological factors, hormonal factors, and physical properties of arteries among others. In this study, arterial impedance was estimated because it may express the condition of vessels. To examine the change of arterial impedance according to the blood flow pattern, the typical sine wave blood flow (oscillated flow) was used during total cardiopulmonary bypass (CPB) because it is considered the most simple pulsatile flow. CPB using oscillated blood flow was performed in acute experiments on adult goats. Total systemic flow was controlled to remain at approximately 80 ml/min. Carotid arterial flow, aortic pressure, central venous pressure, and systemic flow were measured. Total peripheral resistance, carotid arterial resistance, systemic impedance, and carotid arterial impedance were calculated to evaluate blood flow distribution during CPB. This study suggested that the parameters of carotid arterial blood flow changed according to the change of flow frequency during oscillated blood flow. This change may occur because arterial impedance was influenced by flow frequency; therefore, the blood flow of the carotid artery was significantly changed according to the change of the frequency component of blood flow.

Animals↗