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PubMed2000

Improved phase-contrast flow quantification by three-dimensional vessel localization.

In this paper, a method of three-dimensional (3D) vessel localization is presented to allow the identification of a vessel of interest, the selection of a vessel segment, and the determination of a slice orientation to improve the accuracy of phase-contrast magnetic resonance (PCMR) angiography. A marching-cube surface-rendering algorithm was used to reconstruct the 3D vasculature. Surface-rendering was obtained using an iso-surface value determined from a maximum intensity projection (MIP) image. This 3D vasculature was used to find a vessel of interest, select a vessel segment, and to determine the slice orientation perpendicular to the vessel axis. Volumetric flow rate (VFR) was obtained in a phantom model and in vivo using 3D localization with double oblique cine PCMR scanning. PCMR flow measurements in the phantom showed 5. 2% maximum error and a standard deviation of 9 mL/min during steady flow, 7.9% maximum error and a standard deviation of 13 mL/min during pulsatile flow compared with measurements using an ultrasonic transit-time flowmeter. PCMR VFR measurement error increased with misalignment at 10, 20, and 30 degrees oblique to the perpendicular slice in vitro and in vivo. The 3D localization technique allowed precise localization of the vessel of interest and optimal placement of the slice orientation for minimum error in flow measurements.

Blood Flow Velocity↗
PubMed1999

Doppler waveform analysis of the intertwin blood flow in acardiac pregnancy: implications for pathogenesis.

OBJECTIVE: To investigate interfetal hemodynamics in acardiac twins and the implications for pathogenesis. DESIGN: A retrospective study. SUBJECTS: All acardiac twins involved in this study were identified by B-mode ultrasound at a teaching hospital in Taiwan. METHODS: Color Doppler imaging and, in one case, color power angiography were used to assess umbilical blood flow between acardiac twins and their normal co-twins. Placental anastomoses were examined by pathologists after delivery. RESULTS: In total five sets of acardiac twins and their normal co-twins were enrolled. Autonomous cardiac activity in the form of a contractile pocket was detected in two cases. Analysis of the pattern of the Doppler waveforms in each case enabled us to classify the type of hemodynamics into one of three categories: 'collision-summation', typified by a pattern of cyclic alternations of bidirectional flow; 'twin-pulse', which described the simultaneous recordings of two opposite constant flows with different pulsating rates; and the 'pump in' pattern, which indicated pulsatile flow in the reversed direction towards the acardiac mass. In two cases we detected, we believe for the first time, an artery-to-vein placental anastomosis between the acardiac twin and its co-twin. The nature of the vascular connections were confirmed on pathological follow-up. CONCLUSIONS: Our observations suggest that acardiac twins may be not only the result of but also the cause of placental vascular anastomoses, which may involve either artery-to-artery or artery-to-vein anastomoses.

Angiography↗
PubMed2001

Development of the MEDOS/HIA DeltaStream extracorporeal rotary blood pump.

The DeltaStream blood pump has been developed for extracorporeal circulation with one focus on potential integration into simplified bypass systems (SBS). Its small size and an embedded electric motor are the basic pump properties. A variation of the impeller design has been performed to optimize hydraulic and hematologic characteristics. A simple impeller design was developed which allows flow and pressure generation for cardiopulmonary bypass applications. The option of a pulsatile flow mode for ventricular assist device applications also was demonstrated in vitro. Impeller washout holes were implemented to improve nonthrombogenicity. The pump was investigated for potential thermal hazards for blood caused by the integrated electric motor. It could be demonstrated that there is no thermal risk associated with this design. Durability tests were performed to assess the lifetime of the pump especially with regard to the incorporated polymeric seal. Seal lifetimes of up to 28 days were achieved using different blood substitutes. In animal tests using either the pump as a single device or in an SBS setup, biocompatibility, low hemolysis, and nonthrombogenicity were demonstrated. In summary, the DeltaStream pump shows great potential for different extracorporeal perfusion applications. Besides heart-lung machine and SBS applications, ventricular assist and extracorporeal membrane oxygenation up to several days also appear promising as potential applications.

Animals↗
PubMed1994

In vitro pressure drop comparison between two mechanical valve prostheses.

An hemodynamic evaluation of two mechanical heart valves is presented. A tilting disc valve and a bileaflet valve were incorporated in a mock circulatory system which consists of a closed flow loop with a pneumatically driven flexible diaphragm to simulate the physiologic pulsatile flow. Comparisons between the valves were made on the aortic pressure, ventricular pressure, as well as mean pressure gradient at a systolic duration of 45% and a heart rate of 90 beats per minute. The results showed that the tilting disc valve has higher ventricular pressure and mean pressure gradient than that of the bileaflet valve. This indicates that the tilting disc valve has higher transvalvular flow resistance and energy loss than that of the bileaflet valve. From this study it is demonstrated that the mock circulatory system can be a very useful device to evaluate the prosthetic heart valves in vitro.

Heart Valve Prosthesis↗
PubMed2002

A 3D unsteady flow analysis in a doubly constricted arterial vessel.

The aim of this study is to examine the interaction between two mild atherosclerotic proliferations spaced apart by a distance S by analyzing their influence on flow structure, pressure drop and stress field in an arterial vessel under pulsatile flow conditions. This has been achieved numerically by employing a time accurate, cell centered finite volume method in solving the Navier-Stokes equations governing the 3D unsteady flow dynamics in a conceptual model of an multiply constricted arterial vessel. In comparison to the pressure drop across a single stenosis, nearly a 50% increase in the late systolic and early diastolic pressure drops has been observed across the two mild constrictions when they are spaced within a distance of S<or=4. When S<or=4, more than a 25% reduction in the peak systolic wall shear stress (WSS) on the downstream constriction is noted.

Arteries↗
PubMed1982

Open versus closed position fixation of bioprosthesis. Comparative in vitro studies from the viewpoint of durability.

In an effort to normalize the dynamic leaflet behavior of a porcine bioprosthesis, an experimental attempt was made to treat a tissue heart valve with 0.625% glutaraldehyde in the open position ("open-type valve"). Eighteen open-type valves, having fixed openings of various sizes, were examined with our pulse simulators. Eight closed-type valves, which were treated in the closed position, were studied for comparison. The results showed that closed-type valve tended to offer an immobile cusp at a low flow state; at a pump pressure of 90 mm Hg in a pulsatile flow or under steady flows of no more than 10 L/min. By contrast, the open-type valve showed the symmetric cusps opening with circular orifices at any given flow rates. Analysis of the pressure loss coefficient revealed that the leaflets prepared in the open position produced only a minimal impediment to the forward flow. One half of the anatomic orifice of a valve was considered to be the optimal size for the fixed opening. It is concluded that open position fixation is expected to provide the bioprosthesis with improved fatigue durability in patients with low cardiac output syndrome and in small children.

Aortic Valve↗
PubMed2006

Effects of annular size, transmitral pressure, and mitral flow rate on the edge-to-edge repair: an in vitro study.

BACKGROUND: Although edge-to-edge repair is an established adjunctive procedure, there is still debate on its long-term durability and efficacy. METHODS: Fifteen porcine mitral valves were studied in a physiologic left heart simulator with a variable size annulus (dilated = 8.22 cm2, normal = 6.86 cm2, contracted = 5.5 cm2). Mitral valves were tested under steady and physiologic pulsatile flow conditions (cardiac outputs: 4 to 6 L/min), at peak transmitral pressures between 100 mm Hg and 140 mm Hg. A miniature force transducer was used to measure the Alfieri stitch force (F(A)). Mitral flow rate (MFR), transmitral pressure, effective orifice area, mitral regurgitation, and F(A) were monitored. RESULTS: The edge-to-edge repair led to a decrease in effective orifice area of 16.55% +/- 8.22%; further reduction in effective orifice area was attained with annular contraction. Mitral regurgitation after the edge-to-edge repair was significantly higher (p <0.05) with annular dilation. In the pulsatile experiments, two peaks in F(A) were observed: one during systole (F(A) = 0.059 +/- 0.024 N) and a second during diastole (F(A) = 0.072 +/- 0.021 N). Multivariate analysis of variance analysis showed that during systole, transmitral pressure and mitral annular area (MAA) had significant effects on F(A) [F(A) = (4.40 x 10(-4)) transmitral pressure (mm Hg) + (5.0 x 10(-3)) MAA (cm2) - 0.05 (R2 = 0.80)], whereas during diastole MFR and MAA had significant effects on F(A) [F(A) = (1.03 x 10(-4)) MFR2 (L/min) - (1.60 x 10(-3)) MAA (cm2) + 0.02 (R2 = 0.90)]. CONCLUSIONS: With annular dilation, mitral regurgitation persisted even after the edge-to-edge repair. The edge-to-edge repair does not cause clinically relevant mitral valve stenosis in a normal size mitral valve. Mitral flow rate and transmitral pressure are the main determinants of F(A) during the cardiac cycle. Increasing annular area increases F(A) during systole but decreases F(A) during diastole. Systolic F(A) may become dominant with increases in MAA or peak transmitral pressure, or both.

Animals↗
PubMed2004

Consequences of an increase in the ocular perfusion pressure on the pulsatile ocular blood flow.

PURPOSE: In previous studies we have demonstrated that physical exertion constricted the major retinal arteries, increased the redness of the optic nerve head, and attenuated scotopic white flash oscillatory potentials. These anatomic and functional changes in the human retina suggested that blood flow to the innermost retinal layers was modified in some way to cause these changes. The objective of this study was to determine whether physical exertion also affected blood flow in the choroid, the exclusive source of blood to the outermost layers of the retina. METHODS: Eighteen healthy adults volunteered for this study. An OBF Tonograph system (OBF Laboratories UK Ltd., Malmesbury, Wiltshire, U.K.) was used to derive the pulsatile ocular blood flow, which reflects the pulsatile component of blood flow in the choroid, at rest and 20 minutes after biking at a heart rate of 140 beats/min. RESULTS: At the systemic level, biking increased the blood pressure and the heart rate. At the ocular level, the duration of the systolic and diastolic phases of the intraocular pulse was shortened, and the pulse amplitude and volume were reduced. Despite the attenuation of the intraocular pulse parameters, the pulsatile ocular blood flow increased by some 18% after exercise largely because of the much larger increase in heart rate. The ocular perfusion pressure increased, whereas the intraocular pressure decreased. CONCLUSIONS: Physical exertion in the form of aerobic exercise increased the pulsatile component of blood flow in the choroid. Because the choroid is the sole blood supply to the outer retina, it was concluded that the degree of perfusion of the photoreceptors necessary for vision is increased by physical exertion. This increased choroidal blood flow is presumably to sustain vision as blood is directed to the large muscle groups involved in the physical activity.

Adult↗
PubMed2001

The acute effect of pilocarpine on pulsatile ocular blood flow in ocular hypertension.

PURPOSE: To determine the acute effects of application of 2% pilocarpine on pulsatile ocular blood flow. METHODS: In a randomised prospective controlled study of an exploratory nature, 18 subjects with ocular hypertension had pilocarpine 2% eye drops instilled into a randomly chosen eye three times at 10 min intervals. Physiological saline was instilled into the contralateral control eye. Intraocular pressure (IOP) and pulsatile ocular blood flow (POBF) measurements were taken before the first application and 90 min after the last application using the OBF tonometer (OBF Laboratory, Wilts, UK). Statistical analysis was performed using the Wilcoxon signed rank test. RESULTS: Of the 18 patients who entered the trial, 2 were suggested by the OBF system software as having 'poorly reliable' data. The analysis was made on the remaining 16. There was a significant reduction in IOP at 90 min for the treated eye in comparison with the contralateral control eye (p = 0.001; median difference -4.25 mmHg; 95% confidence interval, -5.85 to -2.40). There was a significant increase in POBF at 90 min in the treated eye in comparison with the contralateral control eye (p < 0.001; median difference 4.60 microl/s; 95% confidence interval, 2.35 to 6.75). CONCLUSION: Acute application of pilocarpine 2% drops increased POBF to a significant extent in untreated ocular hypertension.

Aged↗
PubMed2004

Intrasac pressure waveforms after endovascular aneurysm repair (EVAR) are a reliable marker of type I endoleaks, but not type II or combined types: an experimental study.

PURPOSE: To ascertain the nature of the pressure wave transmitted through aneurysm thrombus and the changes produced after endovascular repair and the development of type I and II endoleaks. METHODS: A 25 mm Talent endovascular graft was deployed in a latex model of an abdominal aortic aneurysm, which was incorporated in a pulsatile flow unit. The graft was surrounded by thrombus analogue to simulate conditions in vivo. Pressure waveforms in the sac were captured over 5s at 1000 Hz in these settings: (i) no endoleaks (baseline), after introduction of (ii) type I (iii) type II and (iv) combined type I and II endoleaks. The arterial blood pressure settings used were 140/100 and 130/90 mmHg, denoted the high and low settings, respectively. ANOVA in Minitab 13 was applied for statistical analysis. RESULTS: Pulsatile waveforms were transmitted through the thrombus. Intrasac pressure after stent-grafting reduced to 110/107, 99/96 mmHg (p<0.001) (high, low settings, respectively). Introduction of a type I endoleak caused this to rise to 120/112, 115/107 mmHg (p<0.001, vs. baseline); after producing a type II endoleak these were 101/98, 91/88 mmHg (p<0.001, vs. baseline). A combined type I and II endoleak produced intrasac pressures identical to that of a type I endoleak. CONCLUSIONS: Intrasac pressure waveforms following EVAR are easily defined following a type I endoleak. Waveforms obtained following type II endoleak simulation resemble the baseline waveform in an attenuated form. Intrasac pressures are, therefore, a reliable marker for type I, but not a type II endoleak. In the case of a combined endoleak, the type I endoleak waveform effectively masks that of the type II. Intrasac thrombus faithfully transmits intrasac pressures.

Aortic Aneurysm, Abdominal↗
PubMed1996

[Flow contrast behavior in fast MR angiography with an ECG-triggered, slice-selective inversion recovery turbo-FLASH technique].

PURPOSE: To optimise and evaluate fast "inflow"-based contrast-prepared gradient-echo magnetic resonance angiography (MRA) of blood vessels with pulsatile flow. MATERIAL AND METHOD: We used a standard 2-D Turbo-FLASH sequence with an acquisition time of 0.9 s and a slice-selective hyperbolic secant inversion (SSIR) pulse for contrast preparation. Trigger delay and inversion time were systematically varied. RESULTS: Abdominal blood vessels of volunteers showed high signal intensity compared to stationary tissue without pulsatile artifacts. However, depending on cardiac cycle and inversion time, venous vessels in particular exhibited a periodic signal drop by maximum 70% of the peak value. Using optimised parameters SSIR turbo-FLASH images of patients could be obtained with high flow contrast between normal and pathological structures. CONCLUSION: With ECG triggering SSIR Turbo-FLASH MRA enables fast imaging of pulsatile vessels with optimised flow contrast.

Abdomen↗
PubMed1998

Indices for decreased cerebral blood flow control--a modelling study.

Time-dependent interactions between pressure, flow and volume of cerebral blood and cerebrospinal fluid were mathematically modelled. The model was designed to simulate blood inflow and storage, arteriolar and capillary blood circulation controlled by cerebral autoregulation, venous blood outflow and storage modulated by intracranial pressure, and cerebrospinal fluid production, storage and reabsorption. The software implementation of the model was used to calculate the response to a gradual decrease in cerebral perfusion pressure corresponding to either systemic hypotension or intracranial hypertension. We computed flow pulsatility index (PI) and short range correlation coefficients between systolic, diastolic and mean flow velocity (FVs,d,m) and mean cerebral perfusion pressure (CPP). In simulation, the changes in cerebral flow produced by intracranial hypertension and systemic hypotension were practically indistinguishable. The relationship between PI and CPP was reciprocal, independent of the state of autoregulation. The short range running correlation coefficients between FVs, FVm and CPP indicated both combined safe CPP range and preserved autoregulation, promising a clear clinical detection of "non-worsening" blood supply conditions. A similar procedure was applied to selected clinical data to illustrate the theoretical considerations.

Adolescent↗
PubMed1988

Laminar and turbulent flow in the mammalian aorta: Reynolds number.

Laminar to turbulent flow transition in the mammalian aorta is generally characterized by Reynolds number. When dimensional analysis is applied to obtain the Reynolds number in allometric form, it is found that this number is not invariant of body weight but is approximately proportional to body length dimensions. This implies that flow in the aorta of large mammals is turbulent and laminar in smaller mammals during most of ventricular ejection. Since Reynolds number is defined for steady flow through rigid tubes, it may not reflect the actual fluid behavior of pulsatile flow in compliant vessels such as the aorta. In addition, turbulence is frequency dependent. The larger compliance of the aorta and the slower heart rate in larger mammals and the shorter entrance length and higher heart rate in smaller mammals lead to equal prevalence of turbulence. The consequence is that the aortic flow waveforms remain similar in all mammals.

Anatomy, Comparative↗
PubMed1986

Visualization of moving fluid: quantitative analysis of blood flow velocity using MR imaging.

A new method for the measurement of blood flow using magnetic resonance imaging has been developed. The flow velocities are calculated from the distances that the fluid has moved. The distances are directly visualized by a new pulse sequence. In a phantom study, the measured flow rates showed very good correlation with actual flow rates of up to 20 l/min (3 m/sec). In a volunteer study, pulsatile flow velocities of a large artery were measured with electrocardiographic gating. The flow pattern of a cardiac cycle at the abdominal aorta is similar to that revealed by other methods of measurement, such as Doppler ultrasound. This method allows reasonably accurate quantitative analysis of blood flow in the large arteries.

Blood Flow Velocity↗
PubMed1976

Pulmonary vein flow pattern in man during thoracotomy.

Pulmonary vein flow (PVF) pattern was traced with an electromagnetic flowmeter in 10 patients with normal hearts undergoing operations for pulmonary diseases. Instantaneous pulmonary artery flow, ECG, pulmonary artery pressure and left atrial pressure were recorded simultaneously. The pulsatile flow patterns obtained ocrresponded well to those previously found in dogs, and were remarkably similar to caval flow pattern. A constant and inverse relationship between the contour of the right atrial pressure andthe PVF pattern was found. In 3 patients, PVF was reversed by atrial systole; otherwise the flow was forward throughout the cardiac cycle. The influence of positive pressure ventilation was small, consisting of a slight initial inspiratory increase followed by a decrease during the latter half of inspiration.

Adult↗
PubMed1992

Angiographic methods for the study of fluid mechanical factors in atherogenesis.

The purpose of this thesis is to investigate the feasibility of using angiographic methods to study, in vivo, fluid mechanical phenomena believed to influence the development and localization of atherosclerotic lesions, in particular, separated flow. This involved developing a method to recognize separated flow by digital analysis of cineangiography films, testing this method both with model measurements and in a clinical material, and a detailed analysis of certain methodological problems. In addition, methods have been developed to measure the tortuosity of arteries, a phenomenon which may, according to fluid mechanical theory, promote the occurrence of separated flow. In a glass model of an arterial bifurcation, a pump generated a pulsatile flow resembling that in large arteries. Using LDV (laser Doppler velocimetry) as a reference method, three velocity profiles in the symmetry plane of the model were recorded. During diastole, extensive separated flow was demonstrated in the larger branch of the bifurcation. The flow was then cineradiographed during injection of contrast medium, and the image sequence was transferred to an image analysis workstation. Treating the image sequence as a set of time-intensity curves, time parameters representing the arrival time of the contrast agent were computed. In the resulting parametric images, zones of delayed filling were identified and found to correspond to the separated flow. Viscosity was measured for seven radiographic contrast media and, as expected, the highest values were found for the largest molecules. For iohexol and ioxaglate, which were studied in detail, a linear relation to temperature and a quadratic relation to concentration were found. Whole-blood viscosity was measured for 5 healthy volunteers at high and low shear rates, before and after mixing with contrast agents in varying proportions. At low shear, viscosity decreased, while at high shear, it increased with increasing contrast concentration. The conclusion was that modern contrast media, despite their higher viscosity, seem to affect blood rheology less than older agents. In a study of the imaging characteristics of the digitization equipment for cinefilms, the resolution proved comparable to that of an entirely digital system, while the noise level was higher. Algorithms for the correction of variations in exposure and geometric distortion are also presented. The method for analysis of cinefilms, tried in the model study, was applied, with slight modifications, to a material of 26 patients with hyperlipidemia and slight or moderate atherosclerosis.(ABSTRACT TRUNCATED AT 400 WORDS)

Adult↗
PubMedDate not supplied

Hydrogen bubble visualization of the flow past aortic prosthetic valves.

The hydrogen bubble technique was used to visualize flow past two different aortic valve prostheses. The results reported in this paper clearly show the efficacy of the method and its main advantage, namely that tracers can be produced at any position and at a completely controlled rate. This fact allows the study of either selected moments or regions of the flow field, thus enhancing contrast and therefore increasing the signal-to-noise ratio of the picture. The possible perturbations introduced by the cathode may be decreased to a minimum by using small-diameter wires or wall flush-mounted thin plates. On the other hand, the buoyant velocity of the bubbles may be only a few percent of the fluid velocity and therefore not substantially affect the results. Within the limited scope of the present investigation, the xenographic valve seems to disturb the flow significantly less than the Björk-Shiley prosthesis, both for steady and pulsatile flow, as shown by the traverse velocity profiles or through turbulent stress mapping.

Aortic Valve↗
PubMed2000

Influence of biopump with and without intraaortic balloon on the coronary and carotid flow.

BACKGROUND: The objective of this study was to evaluate the influence of biopump used for left ventricular assistance on the coronary and carotid flows in dogs with normal heart. The efficacy of the simultaneous use of an intraaortic balloon pump to compensate the possible deleterious effects of the circulatory assistance with continuous flow was also analyzed. METHODS: Fifteen dogs were studied. The hemodynamic evaluation included serial measurements of the classic parameters. Carotid and coronary blood flows were obtained by electromagnetic transducers. RESULTS: The hemodynamic evaluation did not show significant statistical changes. The use of circulatory-isolated assistance with biopump shows reduction (24.6% +/- 6.1%) in coronary flow, in relation to the control situation and the concomitant use of biopump and intraaortic balloon pump showed similar coronary flow. Regarding carotid flow, a similar trend was observed in relation to the positive influence of the pulsatile flow with an intraaortic balloon pump without statistical significance (p = 0.0582). CONCLUSIONS: The biopump reduces the coronary flow in dogs. The use of intraaortic balloon pump with the biopump increases the coronary flow significantly, reaching similar values to those observed without the circulatory assistance.

Animals↗