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PubMedDate not supplied

Two-component laser Doppler anemometer for measurement of velocity and turbulent shear stress near prosthetic heart valves.

The velocity and turbulent shear stress measured in the immediate vicinity of prosthetic heart valves play a vital role in the design and evaluation of these devices. In the past hot wire/film and one-component laser Doppler anemometer (LDA) systems were used extensively to obtain these measurements. Hot wire/film anemometers, however, have some serious disadvantages, including the inability to measure the direction of the flow, the disturbance of the flow field caused by the probe, and the need for frequent calibration. One-component LDA systems do not have these problems, but they cannot measure turbulent shear stresses directly. Since these measurements are essential and are not available in the open literature, a two-component LDA system for measuring velocity and turbulent shear stress fields under pulsatile flow conditions was assembled under an FDA contract. The experimental methods used to create an in vitro data base of velocity and turbulent shear stress fields in the immediate vicinity of prosthetic heart valves of various designs in current clinical use are also discussed.

Blood Flow Velocity↗

PubMed2002

Implantation of the permanent Jarvik-2000 left ventricular assist device: a single-center experience.

OBJECTIVES: We sought to evaluate the surgical results and effects of continuous support with the permanent Jarvik-2000 left ventricular assist device (LVAD). We report the early outcomes. BACKGROUND: A shortage of transplant donors necessitates the testing of alternative treatments. The Jarvik-2000 is an axial flow pump with a percutaneous retro-auricular power connector, designed for permanent use. METHODS: Patients with severe heart failure (HF), unsuitable for heart transplantation or conventional LVAD support, were offered implantation. The surgical approach included a left lateral thoracotomy. The device was implanted into the left ventricular apex on femoro-femoral bypass. It is set to allow pulsatile flow with an aortic valve opening. Anticoagulation is adjusted the same as for patients with a heart valve. RESULTS: Between May 2001 and August 2001, we implanted the Jarvik-2000 in two patients with dilated cardiomyopathy and in one with cardiac amyloidosis, all with severe HF (cardiac index 1.8 +/- 0.3 l/m(2) per min). One patient required preoperative inotropic support. All patients did well, with no repeat operations or infections. Patients received 4.3 +/- 3.2 packed red blood cells and were intubated at 14 +/- 3 h, and the intensive care unit stay was 7.0 +/- 0.5 days. The cardiac index increased from 3.7 +/- 1.5 l/min per m(2) at 8,000 rpm to 5.9 +/- 2.9 l/min per m(2) at 12,000 rpm. All patients currently have mild hemolysis not requiring transfusion. The following postoperative events were recorded: a transient ischemic attack with complete recovery, a short re-intubation due to ventricular arrhythmia, loss of consciousness with a battery change while standing, knee-joint effusion after ergometry training, a minor wound problem and a short hospital re-admission due to dehydration. Patients were discharged home after 49 +/- 7 days; one has returned to work. All quality-of-life scores have improved. CONCLUSIONS: The permanent Jarvik-2000 appears safe. It can be used for dilative or restrictive disease. The Jarvik-2000 might prove a valid option for the long-term treatment of patients with severe HF.

Amyloidosis↗

PubMed2000

Estimation of aortic compliance using magnetic resonance pulse wave velocity measurement.

A method for compliance estimation employing magnetic resonance pulse wave velocity measurement is presented. Time-resolved flow waves are recorded at several positions along the vessel using a phase contrast sequence, and pulse wave velocity is calculated from the delay of the wave onsets. Using retrospective cardiac gating in combination with an optically decoupled electrocardiogram acquisition, a high temporal resolution of 3 ms can be achieved. A phantom set-up for the simulation of pulsatile flow in a compliant vessel is described. In the phantom, relative errors of pulse wave velocity estimation were found to be about 15%, whereas in a volunteer, larger errors were found that might be caused by vessel branches. Results of pulse wave velocity estimation agree with direct aortic distension measurements which rely on a peripheral estimate of aortic pressure and are therefore less accurate. Studies in 12 volunteers show values of pulse wave velocity consistent with the literature; in particular the well-known increase in pulse wave velocity with age was observed. Preliminary results show that the method can be applied to aortic aneurysms.

Adult↗

PubMed1994

Doppler in non-immune hydrops fetalis.

Fetal ultrasound studies were performed on 24 fetuses with non-immune hydrops to evaluate echocardiographic and cardiovascular Doppler parameters that may be useful in assessing hemodynamics and in predicting outcome. Of all cardiovascular parameters analyzed, only the presence of abnormal pulsations in the umbilical vein (p < 0.001) was found to be significantly different between the 11 survivors and 13 non-survivors. In a smaller subset of 12 fetuses, in whom inferior vena caval waveforms were recorded, survivors (n = 6) had a significantly lower percentage of retrograde flow in the inferior vena cava (p < 0.001) and higher inferior vena caval E/V velocity ratio (p < 0.001) than non-survivors (n = 6). Sixteen of the 24 cases examined had abnormal umbilical venous pulsations; 12 of the 16 (75%) died including all fetuses with hydrops due to twin-to-twin transfusion or congenital heart disease. When fetuses with pulsatile flow in the umbilical vein were compared with fetuses with normal umbilical venous flow, the following significant differences were found: lower right and left ventricular output velocities, larger inferior vena caval diameter, decreased shortening fractions of the right and left ventricles, and lower peak velocities at the aortic and pulmonary valves and in the ductus arteriosus.

Journal Article↗

PubMedDate not supplied

MR imaging of low signal intensity pulmonary lesions using flow-sensitive techniques.

Various spin echo techniques can be used to define the nature of low signal intensity lesions of the lung. A pulmonary arteriovenous malformation and a chronic pulmonary hematoma had a similar appearance on standard spin echo magnetic resonance (MR) images. Both lesions demonstrated central absence of signal surrounded by a thin rim of tissue, suggesting a vascular lesion. For further characterization of these lesions, a rotating gated MR technique was used in which images are obtained at different points in the cardiac cycle, such that pulsatile flow alters signal intensity with velocity changes during the cardiac cycle. Phase images also discriminated flowing blood in the arteriovenous malformation from stationary lung tissues, whereas the hematoma was not distinguished from lung parenchyma. Rotating gated sequences and reconstructed phase images may be useful in determining the etiology of low signal intensity pulmonary lesions.

Adult↗

PubMed1993

Focal nodular hyperplasia of the liver: value of color Doppler US in association with MR imaging.

Seventeen patients with 19 lesions of focal nodular hyperplasia (FNH) underwent prospective examination with color Doppler ultrasound (US) and magnetic resonance (MR) imaging. In 13 lesions, color Doppler US demonstrated central spots with an arterial Doppler spectrum. The frequency shifts ranged from 1.02 to 1.94 kHz (mean, 1.56 kHz). On flow-sensitive unenhanced gradient-echo images, hyperintense central dots were depicted in only five of these 13 lesions. All the MR features of FNH were present in 12 lesions. Among the seven lesions without all of the MR features, central color spots were present with an arterial Doppler spectrum in four lesions. In the remaining three lesions, color Doppler examination demonstrated either peritumoral pulsatile flow (two lesions) or no flow (one lesion). Color Doppler US adds information about the intratumoral flow characteristics of this vascular malformative lesion and appears to be superior to MR imaging in detection of the arterial intratumoral abnormalities.

Adult↗

PubMed2003

Rapid left-to-right shunt quantification in children by phase-contrast magnetic resonance imaging combined with sensitivity encoding (SENSE).

BACKGROUND: Parallel imaging by sensitivity encoding (SENSE) may considerably reduce scan time in MRI. For rapid flow quantification in children with congenital heart disease, we evaluated phase-contrast MRI (PC-MRI) techniques combined with SENSE. METHODS AND RESULTS: In 22 pediatric patients (mean age, 7.2+/-6.2 years) with cardiac left-to-right shunt, blood flow rate in the pulmonary artery (Qp) and ascending aorta (Qs) and flow ratio Qp/Qs were determined by PC-MRI with SENSE reduction-factor 2 and 3 (SF-2 and SF-3). Additionally, we used PC-MRI with higher spatial in-plane resolution (1.6x2.1 versus 2.3x3.1 mm) with and without SF-3. Results were compared with a recently validated standard PC-MRI protocol and tested in vitro using a pulsatile flow phantom. Reduction of signal averages from 2 to 1 and application of SENSE accelerated flow measurements by a factor of 3.5 (5.2) using PC-MRI with SF-2 (SF-3) compared with standard PC-MRI. For blood flow rate through the pulmonary artery and aorta, as well as for the Qp/Qs ratio we found negligible differences of +/-3%, lower limits of agreement (mean+/-2 SD) of -7% to -18%, and upper limits of agreement (mean+/-2 SD) of +3 to +24%, demonstrating good agreement with standard PC-MRI. Mean Qp/Qs ratio by standard PC-MRI was 1.69+/-0.45 (range, 1.27 to 2.79). Interobserver variability was low, and high accuracy was confirmed in vitro for all protocols. CONCLUSIONS: PC-MRI for flow quantitation may be combined with SENSE to achieve a substantive reduction of scanning time. In children with left-to-right shunt, Qp/Qs quantification is possible by PC-MRI+SF-3 in <60 seconds. Use of higher in-plane resolution did not improve measurement results.

Adolescent↗

PubMed1996

Control of skeletal muscle blood flow during dynamic exercise: contribution of endothelium-derived nitric oxide.

Traditional explanations for the hyperaemia which accompanies exercise have invoked the 'metabolic theory' of vasodilation, whereby contractile activity in the active muscle gives rise to metabolic by-products which dilate vessels bathed in interstitial fluid. Whilst metabolites with vasodilator properties have been identified, this theory does not adequately explain the magnitude of hyperaemia observed in active skeletal muscle, principally because large increases in flow are dependent on dilation of 'feed' arteries which lie outside the tissue parenchyma and are not subjected to changes in the interstitial milieu. Coordinated resistance vessel dilation during exercise is therefore dependent on a signal which 'ascends' from the microvessels to the feed arteries located upstream. Recent studies of ascending vasodilation have concentrated on the possible contribution of the endothelium, a monolayer of flattened squamous cells which lie at the interface between the circulating blood and vascular wall. These cells are uniquely positioned to respond to changes in rheological and humoral conditions within the cardiovascular system, and to transduce these changes into vasoactive signals which regulate blood flow, vascular tone and arterial pressure. Endothelial cells produce nitric oxide (NO), a rapidly diffusing labile substance which relaxes adjacent vascular smooth muscle. NO is released basally and contributes to the regulation of vascular tone by acting as a functional antagonist to sympathetic neural constriction. In addition, NO is spontaneously released in response to deformation of the endothelial cell membrane, indicating that changes in pulsatile flow and wall shear stress are likely physiological stimuli. Since the dilation of microvessels in response to exercise increases blood flow through the upstream feed arteries, which subsequently dilate, one explanation for ascending vasodilation is that NO release is stimulated by flow-induced shear stress. Evidence that NO contributes to ascending vasodilation is reviewed, along with studies which indicate that NO mediates exercise hyperaemia, that physical conditioning upregulates NO production and that NO controls blood flow by modifying other physiological mechanisms.

Animals↗

PubMed2003

Myocardial reperfusion: its assessment and its relation to clinical outcomes.

It has become increasingly apparent that epicardial blood flow restoration is necessary, but not sufficient, to achieve optimal clinical outcomes in the setting of acute coronary syndromes. Indeed, clinical outcomes are strongly associated with myocardial perfusion. The diagnostic tools available to assess myocardial perfusion following acute myocardial infarction and their association with clinical outcomes are reviewed here. Many simple angiographic markers--the Thrombolysis in Myocardial Infarction (TIMI) count frame, TIMI myocardial perfusion grade, pulsatile flow--are all readily available measures and are strongly related to clinical outcomes. Other tools, while accurate, remain limited by their high cost, low availability, and complexity for routine use.

Angiography, Digital Subtraction↗

PubMed1997

Changes with age in the influence of endogenous nitric oxide on transport properties of the rabbit aortic wall near branches.

Uptake of circulating albumin by the aortic wall is greater downstream than upstream of branches in immature rabbits, but the opposite pattern occurs in mature animals. We investigated the role of NO in determining these variations. Descending thoracic aortas of rabbits were cannulated using techniques that avoid depressurization, overstretching, and excessive fluid dynamic stresses at the endothelial surface. They were perfused in situ at a constant pressure and flow rate with oxygenated, protein-containing physiological buffer, with or without N omega-monomethyl-L-arginine, an inhibitor of NO synthesis. Aortas were fixed 7 to 8 minutes after the addition of rhodamine-labeled albumin to this perfusate, and uptake of the tracer near intercostal ostia was measured by digital imaging fluorescence microscopy of sections through the wall. Despite the absence of pulsatile flow, blood cells, and many plasma components, patterns of transport in control experiments were the same as those occurring in vivo; uptake was greatest downstream of ostia in immature vessels and upstream in mature ones, although mean uptake was higher than previously reported. In the presence of the inhibitor, mean uptake in immature arteries was elevated threefold and the maximum tracer concentration occurred deeper in the wall, but there was no change in the fractional difference between regions. Conversely, the reverse of the control pattern of transport was observed in mature arteries exposed to the inhibitor, but there was no change in mean uptake. The reversal was almost entirely prevented by adding excess L-arginine to the perfusate and was largely stereospecific. Endogenous NO thus appears to determine the mature pattern of transport near branches and helps to maintain the barrier function of the immature wall.

Age Factors↗

PubMed1999

Effect of aortic compliance on Doppler diastolic flow pattern in coarctation of the aorta.

The spectral Doppler pattern at the site of an aortic coarctation (CoA) generally displays increased maximal velocity (Vmax) during systole with a slow velocity decay, resulting in the characteristic "sawtooth" pattern. If there is rapid velocity decay, the obstruction is often judged to be mild. The purpose of this study was to investigate if velocity decay is affected by proximal aortic compliance (C(p)). The relation between the velocity decay measured from the Doppler pattern and C(p) was studied with the use of an in vitro pulsatile flow model. The time (tau) between Vmax and 33% Vmax was the measure of velocity decay. The C(p) was varied from 0.7 to 2.6 mL/mm Hg for each of 4 levels of CoA severity. The various obstructions produced a Vmax range of 2.7 to 5.5 m/s. There was a positive linear relation between tau and C(p) (r(2) = 0.76). For a low C(p) (compliance = 0.7 mL/mm Hg), velocity decay was rapid (tau = 0.2 to 0.3 seconds) with no diastolic gradient. For equivalent obstructions, a high C(p) (2.6 mL/mm Hg) produced a persistent diastolic gradient and slow velocity decay (tau = 0.5 to 0.6 seconds). The Doppler pattern across a CoA is affected by C(p). Therefore, the absence of a sawtooth pattern should not exclude the diagnosis of significant CoA obstruction.

Aorta↗

PubMed1987

The use of laser Doppler flowmetry to evaluate anterior cruciate blood flow.

The purpose of this study was to quantitate the blood flow of the anterior cruciate ligament in vivo. Functional flow was evaluated using laser Doppler flowmetry (LDF), for which the output signal, blood cell flux (BCF), is expressed in terms of volts. Ten patients undergoing routine arthroscopic surgery with clinically intact anterior cruciate ligaments were selected at random for participation in the study. Under arthroscopic visualization, a 2.2-mm probe was placed through a trocar sleeve into the anterior cruciate ligament after the arthroscopic procedure. Pulsatile flow within the ligament was observed in all patients. The mean maximum BCF value ranged from 101 to 274 mV; SD range was +/- 3-9 mV. The mean minimum BCF ranged from 75 to 197 mV; SD range was +/- 0 to 9 mV. Laser Doppler flowmetry offers significant promise as a method for measurement of in vivo anterior cruciate and cruciate substitution blood flow.

Arthroscopy↗

PubMed1977

The management of stenotic and obstructive lesions of the aortic arch branches.

The stenotic internal carotid can be managed in a variety of ways and number of tests can be utilized for assessing the collateral blood flow. Except in unusual situations, carotid thromboendarterectomy with or without a patch graft is generally employed. Although some surgeons use no protective shunt at all, or only upon specific indications, intraluminal shunting is utilized extensively. Our preference is to employ the customary Javid shunt technic except in unusual circumstances that suggest that added safety may be assured by shortening to a matter of seconds the period of interruption of carotid flow. In such cases, we believe the temporary axillary-internal carotid intraluminal shunt is of considerable value. Although mediastinal and thoracic procedures and bypass grafts delivering blood from the ascending aorta are not needed nearly as often as they were formerly, they are essential in certain cases. They yield excellent results and carry small risk. Carotid-subclavian grafts have proved quite valuable in restoring pulsatile flow to the subclavian and carotid systems. Our preference, however, because of technical simplicity, is the carotid-axillary bypass procedure. Subclavian-subclavian and axillary-axillary grafts have been employed successfully. When a carotid-axillary bypass is feasible, we would choose this method instead and reserve the others for unusual anatomic-pathologic situations.

Adult↗

PubMed1998

Verifying complete obliteration of carotid artery-cavernous sinus fistula: role of color Doppler ultrasonography.

The purpose of this study was to evaluate the role of color Doppler ultrasonography in verifying obliteration of carotid artery-cavernous sinus fistula before and after therapeutic embolization or gamma knife radiosurgery. Before treatment, carotid artery-cavernous sinus fistula showed the following data on color Doppler ultrasonography: (1) increased blood flow in the common carotid artery (220 to 1264 ml/min with mean+/-SD of 728+/-378 ml/min); internal carotid artery (435 to 1097 ml/min with mean+/-SD of 834+/-216 ml/min) in fistulas of the direct type; and external carotid artery (85 to 257 ml/min with mean+/-SD of 170+/-75 ml/min) in fistulas of the indirect type in comparison to the contralateral side; (2) reverse pulsatile flow or spiculated wave form with turbulent flow in the engorged superior ophthalmic vein on the lesion side in all patients. All of the above abnormal hemodynamic changes became normal in six patients immediately after first embolization, in two patients with balloon embolization combined with subsequent direct embolization by direct puncture through the superior orbital fissure or internal carotid artery embolization, and in five patients after gamma knife radiosurgery at 4, 4, 8, 9, and 9 months, respectively. Color Doppler ultrasonography might be a good modality in long-term follow-up of carotid artery-cavernous sinus fistula after gamma knife radiosurgery and embolization.

Adolescent↗

PubMed2003

Endoluminal aortic shunting for distal perfusion during thoracic aortal cross-clamping in a pig model.

PURPOSE OF THE STUDY: To investigate the haemodynamic properties of a direct endovascular aortic shunt to maintain distal aortic perfusion as an alternative of a distal shunt (left-left-, Gott shunt) in thoracic aortic aneurysm repair. METHODS: A shunt was developed and tested in an in vitro model which should be capable of transporting a flow of 3-4 L/min with a decrease in blood pressure < 20 mmHg. Thereupon the shunt was tested in an in vivo experiment in six pigs to assess the possibility of its use with normal distal blood pressure. The shunt was inserted in the thoracic aorta and stayed in place for 1.5 h. Parameters were measured at six time intervals to assess organ perfusion, -function, cardiac output, proximal- and distal blood pressure and aortic- and shunt flow. PRINCIPLE FINDINGS: The mean blood flow through the shunt was 2.5 L/min. The difference of the mean blood pressure over the shunt was on average 14.20 mmHg. Parameters for coagulation disturbance and organ ischaemia were tested. The decrease in mean thrombocyte count was 299-158 (p<0.02). The venous lactate and the venous mesenteric lactate as parameters for intestinal ischemia did not increase significantly. No significant changes occurred in angiotensin II levels. Pulsatile flow was maintained but significantly suppressed (60%) distal from the shunt. The clamp time needed to insert the shunt and the venous mesenteric lactate, as well as the venous lactate, showed high correlation, r(s) = 0.9 (p<0.05) and r(s) = 0.94 (p<0.01). This also accounted for the 2nd clamp time, both r(s) = 0.95 (p<0.05). CONCLUSION: The shunt is capable of transporting a blood flow of 2-4 L/min with an acceptable decrease in distal blood pressure. However, the time, needed to insert the shunt, was significantly associated with parameters of organ ischaemia.

Anastomosis, Surgical↗

PubMed1998

Branchial gas transfer models.

Gas transfer in fish gills is simulated by a simple counter-current model, with ventilation, water-blood transfer and blood flow characterized by conductances. The ventilation and perfusion conductances are products of flow rate and effective solubility. The diffusion conductance of water-blood transfer (diffusing capacity) is considered to depend on diffusion properties of both the water-blood tissue barrier and of interlamellar water. In the gills of the dogfish Scyliorhinus stellaris, more than half of the total resistance to O2 diffusion was located into interlamellar water. Complicating factors like water shunt, blood shunt, ventilation-perfusion maldistribution, pulsatile flow, diffusion in blood and reaction of O2 with hemoglobin may reduce the O2 transfer efficacy predicted by the simple model. In S. stellaris, the effect of such complicating factors appeared to be minor in most conditions, but in other species and/or conditions, more complex models might be required.

Animals↗

PubMed2001

A new design for polyurethane heart valves.

BACKGROUND AND AIM OF THE STUDY: The synthetic flexible tri-leaflet heart valve offers considerable potential for improvement in both hydrodynamic and biomechanical performance of replacement heart valves. To date, success with the synthetic leaflet heart valve has been limited, partly due to limitations in the biostability of the polyurethanes used. With the synthesis of new biostable polyurethanes, the integration of advancing technology, and better knowledge of the functional and biomechanical design requirements necessary to increase the long-term durability of the polyurethane heart valve, novel clinical solutions are now in sight. METHODS: This study describes the design characteristics, hydrodynamic and biomechanical performance of a new design of polyurethane heart valve. The function and durability characteristics of this novel design of heart valve, manufactured using a proven durable non-biostable polyurethane, was compared with that of a single AorTech porcine bioprosthetic heart valve and a single tilting disc mechanical heart valve, the Björk-Shiley Monostrut valve (BSM), of similar size. RESULTS: For equivalent sizes of valve, the new polyurethane heart valve design had significantly lower pressure gradients compared with the porcine valve at all flow rates and to the BSM valve at the higher flow rates. The effective orifice area of the polyurethane valve was greater than the other two valves studied; regurgitation and total energy loss were less. The new polyurethane valve design reached over 360 million cycles in an accelerated durability tester, without failure. CONCLUSION: This new design of polyurethane heart valve showed improved hydrodynamic function in comparison with either the porcine bioprosthetic or the BSM mechanical heart valve. The pulsatile flow results showed a lower total energy loss associated with this valve, indicating improved potential patient benefit. The durability of this new design of polyurethane heart valve was demonstrated when manufactured using a medical-grade polyurethane.

Biomechanical Phenomena↗

PubMed2002

Flow-induced wall shear stress in abdominal aortic aneurysms: Part I--steady flow hemodynamics.

Numerical predictions of blood flow patterns and hemodynamic stresses in Abdominal Aortic Aneurysms (AAAs) are performed in a two-aneurysm, axisymmetric, rigid wall model using the spectral element method. Homogeneous, Newtonian blood flow is simulated under steady conditions for the range of Reynolds numbers 10 < or =Re < or =2265. Flow hemodynamics are quantified by calculating the distributions of wall pressure (p(w)), wall shear stress (tau(w)), Wall Shear Stress Gradient (WSSG). A correlation between maximum values of hemodynamic stresses and Reynolds number is established, and the spatial distribution of WSSG is considered as a hemodynamic force that may cause damage to the arterial wall at an intermediate stage of AAA growth. The temporal distribution of hemodynamic stresses in pulsatile flow and their physical implications in AAA rupture are discussed in Part II of this paper.

Aorta, Abdominal↗