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PubMedDate not supplied

Doppler color flow mapping of the proximal isovelocity surface area: a new method for measuring volume flow rate across a narrowed orifice.

This manuscript describes a new method, validated in in vitro models, for quantitating volume flow rate across an orifice with Doppler color flow mapping. Flow through a narrowed orifice is characterized by the convergence of radial streamlines proximal to the orifice. In this color Doppler method, one or more isovelocity surface areas (PISA), delineated by blue and red aliasing velocity interfaces, can be identified proximal to the narrowed orifice. Volume flow rate (in milliliters per second) can then be calculated as PISA (in square centimeters) multiplied by the isovelocity of the PISA (in centimeters per second). Doppler color flow mapping was performed in in vitro models of constant and pulsatile flow through an orifice in a wall. The first proximal isovelocity surface area, with an isovelocity corresponding to the aliasing velocity, that is, one half the Nyquist sampling limit, could be identified as a blue and red color interface proximal to the orifice. Over a range of circular orifice diameters from 3 mm to 16 mm and flow rates from 0.5 to 18.7 L/min, the proximal isovelocity surface area could be imaged in two planes. This PISA was best described by a hemielliptic mathematical model with two different radii measured from long-axis and short-axis views. In the constant flow model, volume flow rate calculated from the Doppler PISA correlated well with actual volume flow rate measured simultaneously with a cylinder and stopwatch (r = 0.98, p less than 0.001, standard error of the estimate [SEE] = 0.36 L/min). In the pulsatile flow model, with jet velocities ranging from 2.6 to 7.7 m/sec and flow volume ranging from 1.0 to 10.3 L/min, calculated volume flow rate also demonstrated an excellent correlation with actual volume flow rate (r = 0.99, p less than 0.001, SEE = 0.53 L/min). Findings from these in vitro models suggest that quantification of the proximal isovelocity surface area by Doppler color flow mapping appears to be a promising technique for estimating volume flow rate across a narrowed orifice. This new color Doppler flow method may have advantages over previous Doppler methods in estimating volume flow rate in various clinical situations, for example, valvular regurgitation and shunt lesions.

Blood Flow Velocity↗
PubMed1981

The PO2 in venous valve pockets: its possible bearing on thrombogenesis.

The PO2 in the lumen and valve pockets of veins in 2 patients and 8 dogs was measured during streamline blood flow and in conditions of intermittent pulsatile flow. The blood within the valve pockets became rapidly hypoxic when undisturbed during streamline flow (i.e. when "static"), but the PO2 in the pockets rose to that of the lumenal blood when the column of venous blood was made to pulsate and so empty the valve pockets at short intervals. The observations suggest that the endothelium covering the valve cusps is entirely dependent on pocket or lumenal blood for its oxygen supply. The endothelium facing the pocket can therefore become hypoxic during non-pulsatile blood flow when that facing the lumen is adequately to oxygenated. Early thrombus formation was seen to develop on a valve cusp after only 2 h non-pulsatile flow. The demonstration that localized hypoxaemia occurs readily and can produce endothelial damage, in circumstances and situations where thrombi are commonly found to originate, is additional circumstantial evidence that hypoxia may trigger thrombogenesis.

Adult↗
PubMed1990

Analysis of pulmonary arterial pressure profile after occlusion of pulsatile blood flow.

In isolated canine lung lobes perfused with a pulsatile pump, arterial occlusions were performed and the postocclusion arterial pressure profiles were analyzed to estimate the pulmonary capillary pressure. A solenoid valve interposed between the pump and the lobar artery was used to perform arterial occlusions at several instants equally distributed within a pressure cycle. Double occlusions were also accomplished by simultaneously activating the solenoid valve and clamping the venous outflow of the lung lobe. To analyze an arterial occlusion pressure profile, we computed the best monoexponential fit of the pressure decay over a short period of time after the occlusion maneuvers. Two estimates of the capillary pressure were derived from this analysis: 1) the extrapolation of the exponential fit to the instant of occlusion, and 2) the point at which the recorded pressure decay curve merges with the exponential fit. The pressures thus determined were compared with the double occlusion pressure that provided an independent estimate of the pulmonary capillary pressure. Our results show that, under a wide range of conditions, the estimates of the capillary pressure obtained from the arterial occlusion data are nearly equal to the double occlusion pressures. Additionally, we estimated the capillary pressure variations within a pressure cycle by examining the occlusion pressures sampled at different instants of the cycle. The pulsatility of the pulmonary microvascular pressure varied with the pump frequency as well as the state of arterial and venous vasoaction. These variations are consistent with the representation of the lung vasculature as a low-pass filter.

Animals↗
PubMed1998

Computed numerical analysis of the biomechanical effects on coronary atherogenesis using human hemodynamic and dimensional variables.

The objectives of this investigation were to evaluate biomechanical factors in the atherosclerotic process using human in vivo hemodynamic parameters and computed numerical simulation qualitatively and quantitatively. The three-dimensional spatial patterns of steady and pulsatile flows in the left coronary artery were simulated, using a finite volume method. Coronary angiogram and Doppler ultrasound measurement of the proximal left coronary flow velocity were performed in humans. Inlet wave velocity distribution obtained from in vivo data of the intravascular Doppler study allowed for input of in vitro numerical simulation. Hemodynamic variables, such as flow velocity, pressure and shear stress of the left anterior descending coronary bifurcation site were calculated. We found that there were spatial fluctuation of flow-velocity and recirculation areas at the curved outer wall of the left anterior descending coronary artery, which were due to the differences of flow-velocity and shear stress, especially during the declaration phase of pulsatile flow. This study suggests that rheologic properties may be a part of the atherogenic process in the coronary bifurcated and curved areas.

Biomechanical Phenomena↗
PubMed1992

Effect of oxygenator type and bypass flow pattern on the P(a-ET)CO2 gradient.

Changes in pre-bypass and post-bypass P(a-ET)CO2 gradients were evaluated regarding the type of bypass flow (pulsatile or nonpulsatile) and oxygenator (membrane or bubble). Duration of bypass and hemodynamic changes were analyzed also to determine their possible influence on PaCO2, PETCO2, and P(a-ET)CO2. A total of 36 adult patients undergoing cardiopulmonary bypass were anesthetized using a sufentanil-pancuronium-oxygen technique. Patients were divided into three groups based on the type of oxygenator and pump flow: group 1 (control group) consisted of a bubble oxygenator with nonpulsatile flow (BN), group 2 consisted of a bubble oxygenator with pulsatile flow (BP), and group 3 consisted of a membrane oxygenator with nonpulsatile flow (MN). Cardiac parameters (MAP, CI, SVR, and PVR) PaCO2, PETCO2, and P(a-ET)CO2 were determined pre-bypass and post-bypass following steady-state conditions. For the entire group there was a trend for the P(a-ET)CO2 gradient to increase in the post-bypass period (pre-bypass = 3.5 +/- 0.5 mm Hg, post-bypass = 4.3 +/- 0.5 mm Hg.). However, this increase was not statistically significant. Pulsatile flow (group 2) demonstrated a significant correlation with the change in P(a-ET)CO2 gradients from the pre-bypass to the post-bypass period (r = 0.85) when compared with the other two groups (group 1: r = -0.09 and group 3: r = 0.37). Thus, the P(a-ET)CO2 gradient tended to remain constant from the pre-bypass to the post-bypass period in group 2, whereas it increased in groups 1 and 3. Changes in MAP, CI, SVR, and PVR and the duration of CPB did not influence the P(a-ET)CO2 gradient.

Adult↗
PubMed1997

Regional blood flow during pulsatile cardiopulmonary bypass and after circulatory arrest in an infant model.

BACKGROUND: Pulsatile perfusion systems have been proposed as a means of improving end-organ perfusion during and after cardiopulmonary bypass. Few attempts have been made to study this issue in an infant model. METHODS: Neonatal piglets were subjected to nonpulsatile (n = 6) or pulsatile (n = 7) cardiopulmonary bypass and 60 minutes of circulatory arrest. Cerebral, renal, and myocardial blood flow measurements were obtained at baseline, on bypass before and after circulatory arrest, and after bypass. RESULTS: Cerebral blood flow did not differ between groups at any time and was diminished equally in both groups after circulatory arrest. Renal blood flow was diminished in both groups during bypass but was significantly better in the pulsatile group than in the nonpulsatile group prior to, but not after, circulatory arrest. Myocardial blood flow was maintained at or above baseline in the pulsatile group throughout the study, but in the nonpulsatile group, it was significantly lower than baseline during CPB prior to circulatory arrest and lower compared with baseline and with the pulsatile group 60 minutes after CPB. CONCLUSIONS: Pulsatile bypass does not improve recovery of cerebral blood flow after circulatory arrest, may improve renal perfusion during bypass but does not improve its recovery after ischemia, and may have beneficial effects on myocardial blood flow during bypass and after ischemia compared with nonpulsatile bypass in this infant model.

Animals↗
PubMed1998

Evaluation of a novel hirudin-coated polyester graft to physiologic flow conditions: hirudin bioavailability and thrombin uptake.

PURPOSE: Our laboratory has developed methods required to covalently bind recombinant hirudin (rHir) to the surface of polyester vascular grafts. Using alkaline hydrolysis of the polyester surface, carboxyl-binding sites are created on the outer periphery of each fiber. A series of static, in vitro experiments have demonstrated that surface-bound rHir rapidly removes and inhibits activated human alpha-thrombin from the reaction system; however, the performance of this modified graft material under physiologic flow conditions was undefined. METHODS: An in vitro flow loop was used to evaluate structural stability of the 125I-rHir and 131I-albumin covalently bound to the surface of 6 mm interior diameter crimped polyester grafts exposed to either constant flow (n = 4; shear rate, 300 sec(-1)) or pulsatile flow (n = 4; maximum shear rate, 780 sec(-1)) conditions for a 7-day period. In a separate series of experiments, the kinetics of thrombin-rHir interaction were evaluated through perfusion of 125I-rHir-coated grafts (n = 6) with 131I-thrombin for a 27-hour period under constant flow conditions. Identically prepared 125I-albumin-coated grafts (n = 3) were used as controls. RESULTS: Results of the stability experiments were independent of flow conditions, demonstrating moderate loss of both proteins, with rHir and albumin losses of 52.1% and 19.9% under constant flow and 49.1% and 21.6% under pulsatile flow, respectively. With results comparable with those of previous static experiments, rHir-coated grafts were significantly more effective at removing thrombin from the perfusion stream with 131I-thrombin binding densities of 3.08 +/- 0.61 and 0.64 +/- 0.04 NIHU/cm2 (p < 0.01) for rHir-coated and albumin-coated grafts, respectively. Estimates of the total amount of thrombin inactivated during the perfusion period similarly demonstrated a marked difference between the rHir-coated and control graft segments (125 +/- 8 vs. 3 +/- 14 NIHU; p < 0.005). CONCLUSIONS: These in vitro flow results illustrate that polyester grafts with covalently bound rHir can provide significant reductions in local thrombin concentration under physiologic flow conditions, and can serve as a foundation with which to understand the performance of these grafts when implanted in vivo under physiologic flow and shear rates.

Biocompatible Materials↗
PubMedDate not supplied

Characteristics of pulsatile blood flow through the curved bileaflet mechanical heart valve installed in two different types of blood vessels: velocity and pressure of blood flow.

The aim of this study was to investigate the flow fields of blood flowing through the curved bileaflet mechanical heart valve. A numerical analysis was carried out with the fluid-structure interaction between the blood flow and the motion of leaflets in two different types of blood vessels (type A, with sinus blood vessel, and type B, without sinus blood vessel). When the leaflet was fully opened, a fluttering phenomenon was detected in association with the blood flow, and recirculation flows were observed in the sinus region of the blood vessel for type A. During the closing phase, regurgitation was formed between the ring and the edge of the each leaflet for both types. When the leaflet came into contact with the valve ring at the end of the closing phase, rebound of the leaflet occurred. In consideration of the entire domain, the pressure drop occurs mainly in the valve region. The present results showed tendencies similar to those obtained by previous experiments for blood flow and contribute to the development of the curved bileaflet mechanical heart valve prostheses.

Bioprosthesis↗
PubMed1997

Umbilical venous pulsation associated with hypercoiled cord in growth-retarded fetuses.

Venous pulsatile flows were detected by pulsed Doppler flow velocimetry throughout the entire umbilical cord in 2 cases of intrauterine growth retardation. Although the hearts of these fetuses had no anomalies and their inferior vena cava flows had normal flow velocity patterns, their cords were severely coiled. In these cases, the pulsatile flow in the umbilical cord vein was caused not by an increase in the preload of the fetal heart, but only by hypercoiling of the umbilical cord. This phenomenon also suggests that the coiled umbilical cord exerts a pump-like effect known as a 'pulsometer'.

Adult↗
PubMedDate not supplied

Errors in the estimation of arterial wall shear rates that result from curve fitting of velocity profiles.

An analysis was performed to determine the error that results from the estimation of the wall shear rates based on linear and quadratic curve-fittings of the measured velocity profiles. For steady, fully developed flow in a straight vessel, the error for the linear method is linearly related to the distance between the probe and the wall, dr1, and the error for the quadratic method is zero. With pulsatile flow, especially a physiological pulsatile flow in a large artery, the thickness of the velocity boundary layer, delta is small, and the error in the estimation of wall shear based on curve fitting is much higher than that with steady flow. In addition, there is a phase lag between the actual shear rate and the measured one. In oscillatory flow, the error increases with the distance ratio dr1/delta and, for a quadratic method, also with the distance ratio dr2/dr1, where dr2 is the distance of the second probe from the wall. The quadratic method has a distinct advantage in accuracy over the linear method when dr1/delta << 1, i.e. when the first velocity point is well within the boundary layer. The use of this analysis in arterial flow involves many simplifications, including Newtonian fluid, rigid walls, and the linear summation of the harmonic components, and can provide more qualitative than quantitative guidance.

Arteries↗
PubMed2003

Numerical simulation of carotid hemodynamics in patients with rotary blood pump cardiac assist.

In recipients of rotary blood pumps for cardiac assist, the pulsatility of arterial flow is considerably diminished. This influences the shear stress patterns and streamlines in the arterial bed, with potential influence on washout and subsequent plaque growth. To study these effects, a three-dimensional computer simulation of the carotid bifurcation at various levels of flow pulsatility was performed. The results showed that as expected pulsatile shear stress varied considerably, whereas local mean shear stress levels were nearly identical for all degrees of pulsatility. Particle residence time in the carotid bulb did only increase for less than 15%, with secondary washout patterns contributing to good washout also in nonpulsatile conditions. It is concluded that also under continuous pump support the local flow patterns in the carotids provide sufficient washout and fluid exchange to prevent excessive plaque growth.

Carotid Arteries↗
PubMedDate not supplied

Comparative in-vitro fluid dynamics characterization of heart valve bioprostheses under accelerated fatigue conditions and under physiologic conditions.

An accelerated fatigue testing system and a pulse duplicator (heart simulator) were used to evaluate the fluid dynamics characteristics of 12 cardiac bioprostheses. Pressure differences and pulsatile flow rates across the valves, as well as machine rates, were measured using a real-time on-line data-acquisition system. All other valve hydrodynamic parameters were internally calculated. For the same pressure difference, the pulsatile flow rate was higher at higher pulse rates. Regurgitation fraction values were higher for valves tested at higher speeds. Closing volumes, however, remained fairly constant. Mean transvalvular pressure difference (delta p) and root mean square pulsatile flow rate (QRMS) under accelerated testing conditions were related according to the particular case of a parabolic regression through the origin of the form (type 1), delta p = Co Q2RMS. Prosthetic valves tested in the fatigue tester presented in general a more effective area for flow than did valves tested in the pulse duplicator. Calculated discharge coefficients and performance indexes were accordingly higher. In both testing devices pericardial valves had higher effective orifice areas, discharge coefficients, and performance indexes than did porcine xenografts, and large valves performed more efficiently than small valves. Differences among regression coefficients for the same valves tested in both machines appeared to be significant in 100% of the cases. There were differences in the degrees of stenosis among valves tested in the pulse duplicator and in the fatigue tester. Understanding of fluid dynamics data obtained for undamaged valves at accelerated speeds and their relation to data obtained at physiologic speeds permitted the detection and quantification of rupture and malfunctioning of these valves without removing them from the fatigue tester.

Bioprosthesis↗
PubMed1989

A new method for noninvasive quantification of valvular regurgitation based on conservation of momentum. In vitro validation.

The noninvasive Doppler assessment of regurgitant volume from jet size is limited by the fundamental inequality of jet volume and regurgitant volume and by the dependence of jet dimensions on driving pressure and instrument settings for a given flow volume. Therefore, this study addresses the hypothesis that an equation could be derived from basic physical principles to quantify regurgitant volume with velocities that can be directly measured by Doppler echocardiography. The principle of conservation of momentum for free turbulent jets resembling many cardiac lesions yields an equation for regurgitant volume as a function of maximum jet velocity, a distal centerline velocity, and the intervening distance. This theory was tested throughout a range of physiologic flow rates and pressures (orifice velocities) in steady flow for 0.08-0.40 cm2 circular orifices and a noncircular orifice and in physiologic pulsatile flow for 0.08 and 0.20 cm2 circular orifices. Plots of centerline velocities versus axial distance coincided with those expected for such jets. Calculated and actual volumetric flows agreed well by linear regression in the turbulent jet: for steady flow rates, y = 0.98x + 0.09 (r = 0.99, SEE = 0.14 l/min), with similar correlations for circular and noncircular orifices; for pulsatile flow, y = 1.02x + 0.03 for peak flow rate (r = 0.98, SEE = 0.18 l/min) and y = 1.02x + 0.58 for total regurgitant volume (r = 0.95, SEE = 0.81 ml). There was no significant effect of orifice size or location of velocity measurement within the turbulent jet. Therefore, for free jets resembling many clinical lesions, regurgitant flow rate and volume can be calculated noninvasively from Doppler velocities without planimetry of jet area. Because the required information is intrinsic to the jet, this method should apply regardless of associated valvular lesions. It should also apply to orifices of variable shape because turbulent eddies obliterate the details of flow at the orifice. The special case of jets impinging on walls must be considered separately for both this technique and flow mapping.

Blood Flow Velocity↗
PubMed1989

Flow visualization studies in a mold of the normal human aorta and renal arteries.

To study the flow behavior in regions where hemodynamic effects have been suggested to participate in atherogenesis, we evaluated flow in a mold of the aorta and renal arteries of a previously healthy 27-year-old woman who died of trauma. A birefringent solution (vanadium-pentoxide) was used. When diluted, this material behaves like a Newtonian fluid. This method gives a complete picture of the entire flow field. Zones of flow separation and disturbed flow can be seen and the location and size of disturbed areas observed. Unseparated flow regions downstream from disturbed zones can be properly visualized and the method can be used for pulsatile flow as well as steady flow. During steady flow (only at branch to-trunk flow ratios greater than 0.20), zones of flow separation were observed in the aorta distal to the renal arteries. During pulsatile flow, disturbances were found at nearly all branch-to-trunk flow ratios.

Aorta↗
PubMed1988

[Comparison of the clinical value of 2 frequently used indices of fetal blood flow analysis (pulsatility index PI and resistance index RI)].

249 blood-flow measurements were carried out with pulsed Doppler-ultrasound in the umbilical arteries and the descending aorta of 188 fetuses. The PI and the RI of the recorded flowpatterns were calculated. In a retrospective study we then calculated the validity of the prediction of a pathological fetal outcome, i.e. a SGA-fetus and/or perinatal morbidity, for both indices. The sensitivity of the PI is 63%, the specificity was 98%, the sensitivity of the RI was 40%, and the specificity 98%. Therefore the sensitivity of the PI is significantly higher than that of the RI (p less than 0.05). In the second, prospective part of the study these results could be verified by means of an equivalent number of measurements to the first group (n = 249) with 178 fetuses. All the jeopardized fetuses with IUGR and perinatal morbidity (n = 13) were detected by an increased PI, but even in this group the sensitivity of the RI was only 54%. The possible reasons for these results are discussed, taking into account the different characteristics of the two indices and the influence of the heart rate on both of them.

Asphyxia Neonatorum↗