Doppler echocardiographic estimation of left ventricular end-diastolic pressure after MI in rats

The spectral Doppler mitral flow pattern, alone or combined with tissue Doppler mitral annulus velocity, can be used to predict left ventricular (LV) filling pressure in humans, whereas invasive hemodynamic measurements are still required in the rat. This study was undertaken to assess whether LV end-diastolic pressure (LVEDP) can be estimated using Doppler echocardiography in the rat after myocardial infarction (MI). Thirty-seven rats (23 rats with MI after left coronary artery ligation and 14 sham-operated rats) were evaluated 3 mo after surgery with echo-Doppler and invasive hemodynamic measurements. Pulse wave spectral Doppler at the mitral valve tip was used to measure the E wave, the E wave deceleration time (DT), and the A wave; spectral Doppler tissue imaging was used to measure the early diastolic lateral mitral annulus velocity (E(a)). We found weak correlations between LVEDP and the peak velocity of the early mitral inflow (E), E/peak velocity of the late mitral inflow, and DT, and strong correlations with E(a) and especially with E/E(a) [R(2) = 0.89, LVEDP (in mmHg) = 0.987E/E(a) - 4.229]. Longitudinal followup of a subgroup of rats with MI revealed a marked rise of E/E(a) between days 7 and 21 in rats with heart failure only. We conclude that Doppler echocardiography can be used for serial assessment of LV diastolic function in rats with MI.     

A theoretical model for the noninvasive assessment of the transmitral pressure-flow relation.

The purpose of this paper is to formulate from the equations of fluid mechanics an equation which describes the transmitral pressure-flow relationship. According to the linear momentum equation applied to the atrioventricular coupling, the left-atrium-left-ventricle pressure difference (Pa-Pv) can be written as Pa-P v = A delta v/delta t + B v 2 + C v, where v is the transmitral blood velocity and A, B, and C are variables related to the geometry of the atrium, ventricle and mitral orifice, respectively. Based on this theory, Pa-Pv is calculated noninvasively in a patient with a nonobstructive mitral valve. Mitral flow and cardiac dimensions recorded by Doppler echocardiography are digitized and analyzed. Calculation shows that Pa-Pv reaches its peak value at the time of flow peak acceleration and has already considerably decreased at the time of peak velocity. The time course of calculated Pa-Pv is in close agreement with the published experimental catherization data. Numerical computation of early diastolic left atrium and left ventricle pressure curves based on the experimental data of others for the time constant of left ventricular relaxation, left atrial and ventricular chambers stiffness constants, combined with sine-waveform-simulated mitral flow, verifies the time course and the magnitude of Pa-Pv as predicted from flow equations. This paper provides a theoretical method for the noninvasive assessment of the transmitral pressure-flow relationship using ultrasound technique and might help to achieve a better understanding of the diastolic function as assessed by Doppler echocardiography.     

Left ventricular diastolic filling and systolic function of young and older trained and untrained men.

Aging is associated with impaired early diastolic filling; however, the effect of endurance training on resting diastolic function in older subjects is unclear. Heart rate and ventricular loading conditions affect mitral inflow velocities measured by Doppler echocardiography; therefore, tissue Doppler imaging of mitral annular velocity, which is relatively preload independent, was combined with mitral inflow velocity and maximal oxygen consumption (V(o2 max)) in young (20-35 yr) and older (60-80 yr) trained and untrained men to determine whether endurance training is associated with an attenuation of age-associated changes in diastolic filling. As expected, V(o2 max) was higher in trained men (P < 0.01) and lower in older men (P < 0.01). Peak early mitral inflow velocity (E) and early-to-late mitral inflow velocity ratios were lower in older vs. young men (P < 0.01); however, there was no training effect (P > 0.05). Peak early mitral annular velocity (E') was higher and peak late mitral annular velocity (A') was lower in young vs. older men (P < 0.01). A significant interaction effect was found for A', E'/A', and peak systolic mitral annular velocity (S'). Training was associated with lower A' in young and higher A' in older men. S' was greater in trained vs. untrained older men (P < 0.05), but it was similar in trained and untrained young men. These findings suggest that early diastolic filling is not affected by training in older men, and the effect of training on A' and S' is different in young and older men.     

A new theoretical model for noninvasive quantification of mitral regurgitation

The most common objective assessments of mitral regurgitation are limited by their invasive or semiquantitative nature. Recent attempts at correlation with jet size from Doppler flow maps have failed to produce a direct measure of regurgitant volume and are fundamentally limited by the dependence of jet dimensions on factors other than flow volume. The purpose of this paper was to develop an equation, based on the physics of turbulent regurgitant jets, for calculating regurgitant volume from quantities that can be measured by Doppler ultrasound. The result is an equation forw flow rate Q as a function of orifice velocity Uo, a downstream centerline velocity Um and the intervening distance chi: Q = pi U2m chi 2/160Uo. This equation can also be modified to obtain total regurgitant volume in clinical pulsatile flow. The assumptions made demand a free turbulent jet for which momentum is conserved, but should otherwise be physiologically applicable. The advantage of this technique compared to correlations with jet size are its theoretical justification and ability to quantify regurgitant volume directly.     

The relationship among central obesity, systemic inflammation, and left ventricular diastolic dysfunction as determined by structural equation modeling

The purpose of this study was to investigate the associations among central obesity, inflammation, and left ventricular (LV) diastolic dysfunction by structural equation modeling. Echocardiographic parameters were assessed in 102 otherwise-healthy adults over age 30. The participants were classified as having LV diastolic dysfunction by echocardiographic findings including mitral inflow E/A ratio <1, deceleration time >220 cm/s, or decreased peak annular early diastolic velocity in tissue Doppler imaging or otherwise the control group. Serum C-reactive protein (CRP) and lipid profile were also measured. The homeostasis model of insulin resistance (HOMA) was calculated. Central obesity was assessed by computerized tomography (CT) at the L4 level. In a multivariate regression analysis, the relationship between visceral adipose tissue (VAT) and LV diastolic dysfunction became insignificant when CRP was introduced into the model, although CRP itself was significantly associated with LV diastolic dysfunction (odds ratio (OR): 1.32, 95% confidence interval (CI): 1.01-1.72, P = 0.04). A significant correlation was also found between VAT and CRP (r = 0.70; P < 0.001). We then performed path analysis as illustrated by the structural equation model. This proved our hypotheses that VAT might affect LV diastolic dysfunction through the effect of CRP (total fat load with inflammation (B = 1.133, P < 0.001) and that inflammation might affect LV diastolic dysfunction (B = 0.373. P < 0.001)). Using structural equation modeling, we concluded that higher amounts of VAT were associated with low-grade inflammation and this may lead to subclinical LV diastolic dysfunction in otherwise-healthy subjects.     

Fluid dynamics of the left ventricular filling in dilated cardiomyopathy

Modifications in diastolic function occur in a broad range of cardiovascular diseases and there is an increasing evidence that abnormalities in left ventricular function may contribute significantly to the symptomatology. The flow inside the left ventricle during the diastole is here investigated by numerical solution of the Navier-Stokes equations under the axisymmetric assumption. The equation are written in a body-fitted, moving prolate spheroid, system of coordinates and solved using a fractional step method. The system is forced by a given volume time-law derived from clinical data, and varying the two-degrees-of-freedom ventricle geometry on the basis of a simple model. The solution under healthy conditions is analysed in terms of vorticity dynamics, showing that the flow field is characterised by the presence of a vortex wake; it is attached to the mitral valve during the accelerating phase of the E-wave, and it detaches and translate towards the ventricle apex afterwards. The flow evolution is discussed, results are also reported as an M-mode representation of colour-coded Doppler velocity maps. In the presence of ventricle dilatation the mitral jet extends farther inside the ventricle, propagation velocity decreases, and the fluid stagnates longer at the apex.     

A study of protoplasmic streaming inPhysarum by laser Doppler spectroscopy

Summary Protoplasmic streaming in the slime moldPhysarum polycephalum has been characterized using laser Doppler spectroscopy. Measurement of the spectrum of scattered laser light permits simultaneous determination of the velocities of all particles in the laser beam, with the relative intensity from each particle proportional to its light scattering cross-section. Simple experimental modifications allow the tracking of the oscillations of the streaming velocities. Rhythmic wall contractions can be monitored simultaneously with the flow velocities. Interpretation of the Doppler spectra shows that a small fraction of the particles in the flowing protoplasm are moving with velocities two to four times greater than the characteristic velocities reported by optical microscopy. Transverse velocities in the tubes are nearly as great as the longitudinal velocities. The shape of the Doppler spectrum at the maximum of the oscillation cycle is consistent with a spatial velocity profile which is sharper than parabolic, presumably because of a viscosity gradient from the center to the walls of the plasmodial tubes. The shape of the Doppler spectrum of depolarized scattered light is of approximately the same form. The response of the plasmodium to increased temperature is an increase in the frequency of the velocity oscillations with little change in the magnitude of the velocities. The response of the plasmodium to very high intensities of laser light is to gel at the point of incidence.     

Mitral and tricuspid annular velocities determined by Doppler tissue imaging in hypopituitary, growth hormone-deficient patients

BACKGROUND: The aim of this study is to determine systolic and diastolic velocity profiles of the left and right ventricles by tissue Doppler imaging (TDI) and to reveal the associations between TDI parameters and early atherosclerotic changes in adult hypopituitary patients with GH deficiency. PATIENTS AND METHODS: The study group is composed of 16 hypopituitary, GH-deficient patients and 13 healthy controls. All patients had been receiving adequate substitution therapy other than GH at stable doses for at least 6 months. Conventional Doppler echocardiography and TDI of the mitral and tricuspid annulus were performed. Intima-media thickness (IMT) of the common carotid artery was calculated. RESULTS: IMT was significantly higher in the hypopituitary group compared with controls (0.83 +/- 0.25 vs. 0.51 +/- 0.14 mm, p < 0.001). Hypopituitary patients had significantly lower peak early diastolic (Em) mitral annular velocity (11.2 +/- 3.0 vs. 13.9 +/- 2.8 cm/s, p < 0.05). Multiple regression analysis revealed that age was the only independent variable significantly associated with Em and IMT in the patients. CONCLUSION: Diastolic abnormalities on TDI of the mitral annulus and early atherosclerotic changes occur concurrently in asymptomatic hypopituitary patients with GH deficiency. Aging may have a more deleterious effect on ventricular function and atherogenesis in this group of patients.     

Three-Directional Evaluation of Mitral Flow in the Rat Heart by Phase-Contrast Cardiovascular Magnetic Resonance

Purpose

Determination of mitral flow is an important aspect in assessment of cardiac function. Traditionally, mitral flow is measured by Doppler echocardiography which suffers from several challenges, particularly related to the direction and the spatial inhomogeneity of flow. These challenges are especially prominent in rodents. The purpose of this study was to establish a cardiovascular magnetic resonance (CMR) protocol for evaluation of three-directional mitral flow in a rodent model of cardiac disease.

Materials and Methods

Three-directional mitral flow were evaluated by phase contrast CMR (PC-CMR) in rats with aortic banding (AB) (N = 7) and sham-operated controls (N = 7). Peak mitral flow and deceleration rate from PC-CMR was compared to conventional Doppler echocardiography. The accuracy of PC-CMR was investigated by comparison of spatiotemporally integrated mitral flow with left ventricular stroke volume assessed by cine CMR.

Results

PC-CMR portrayed the spatial distribution of mitral flow and flow direction in the atrioventricular plane throughout diastole. Both PC-CMR and echocardiography demonstrated increased peak mitral flow velocity and higher deceleration rate in AB compared to sham. Comparison with cine CMR revealed that PC-CMR measured mitral flow with excellent accuracy. Echocardiography presented significantly lower values of flow compared to PC-CMR.

Conclusions

For the first time, we show that PC-CMR offers accurate evaluation of three-directional mitral blood flow in rodents. The method successfully detects alterations in the mitral flow pattern in response to cardiac disease and provides novel insight into the characteristics of mitral flow.     

Shear-layer detection in poststenotic flow by spectrum analysis of Doppler signals

Spectrum analysis of the Doppler signals was performed 0.5 tube diameters downstream from an axisymmetric constriction with an area reduction of 80 percent in steady flow at a jet Reynolds number of 2840. Both pulsed and continuous wave (CW) Doppler spectra showed significant reverse flow components in the separated flow. The pulsed Doppler spectra exhibited sudden changes when the sample volume crossed the shear layer between the center jet and the separated flow. A power spectrum equation was theoretically derived from continuity of flow to define the Doppler shift frequency for the shear layer velocity. The CW Doppler spectrum showed a minimum spectrum density at a frequency which equalled the shear layer Doppler shift frequency derived from the equation. The pulsed spectra exhibited the sudden changes at the same frequency as well.     

Effects of modulation of left ventricular contractile state and loading conditions on tissue Doppler myocardial performance index

The Tei index is clinically useful to quantify left ventricular (LV) function, but it requires sequential Doppler recordings from two different views. A related myocardial performance index (MPI) using tissue Doppler (TD) can be rapidly calculated from a single beat; however, its ability to quantify contractility and the effects of acute changes in loading have not been determined. Our aim was to test the hypothesis that TD MPI can quantify contractile state but is affected by acute alterations in loading, using LV pressure-volume relations in an animal model. Eight dogs were studied by using mitral annular TD, high-fidelity pressure, and conductance catheters. TD MPI was calculated as (a' - b')/b', where a' was the duration of mitral annular velocity during diastole and b' was the duration of the systolic wave. End-systolic elastance (Ees), the time constant of isovolumic relaxation (tau), and peak positive and negative first derivative of pressure (dP/dtmax and dP/dtmin, respectively) were used as measures of LV function. Data were obtained at baseline, at dobutamine and esmolol infusion to alter contractile state, and at inferior vena cava and aortic occlusion to alter preload and afterload. TD MPI decreased from 0.83 (SD 0.19) to 0.62 (SD 0.20) with dobutamine and increased to 1.19 (SD 0.26) with esmolol. TD MPI significantly correlated with dP/dtmax (r = -0.76), Ees (r = -0.68), dP/dtmin (r = 0.82), and tau (r = 0.78); however, it was affected by acute decreases in preload [from 0.83 (SD 0.19) to 1.09 (SD 0.36)] and acute increases in afterload [to 1.23 (SD 0.17)]. All the above increases and decreases and r values were significant (P < 0.05 vs. baseline). In conclusion, TD MPI can rapidly quantify alterations in LV contractile state but is affected by acute alterations in preload and afterload.     

Radial and longitudinal compartmental analysis of gas transport during high-frequency ventilation

A model of gas exchange by low-tidal-volume (VT), high-frequency ventilation (HFV) is presented, based on the physical principles of dispersion. These are the nonuniformity of the velocity profile and the nonreversible mixing of fluid components in a diffusive manner. A numerical method was used to incorporate these principles into a quantitative model. The airways of a symmetrically bifurcating bronchial-tree model were partitioned in the radial direction into two concentric layers representing the kinematic dispersion by nonuniformity of the velocity profile. Mixing between the layers was invoked in proportion to the diffusivity and local dimensions. The effects of frequency (f), VT, shape of the velocity profile, and bronchial-model configuration were tested in the model, with favorable comparison to available experimental data. The model predicts that for a frequency-dependent velocity profile, the rate of tracer exchange is proportional to the square root of f and to the square of VT-V0, where V0 is a constant small volume under which gas exchange was nil. Intracycle asymmetric mixing is predicted to have a stronger effect on gas exchange than asymmetric velocity profile. Gas exchange when turbulent-flow regime is assumed is predicted to be less for the higher VT values than with laminar flow and with mixing by molecular diffusivity. This model was found to be didactic, flexible, and capable of modeling combinations of factors affecting either one of the two fundamental processes of dispersion.     

Validation of echocardiographic and Doppler indexes of left ventricular relaxation in adult hypertensive and normotensive rats

This study was performed to validate echocardiographic and Doppler techniques for the assessment of left ventricular (LV) diastolic function in spontaneously hypertensive rats (SHR) and normotensive Wistar rats. In 11 Wistar rats and 20 SHR, we compared 51 sets of invasive and Doppler LV diastolic indexes. Noninvasive indexes of LV relaxation were related to the minimal rate of pressure decline (-dP/dt(min)), particularly isovolumic relaxation time (IVRT), the Tei index, the early velocity of the mitral annulus (E(m)) using Doppler tissue imaging, and early mitral flow propagation velocity using M-mode color (r = 0.28-0.56 and P < 0.05-0.0001). When the role of systolic load was considered, the correlation between Doppler indexes of LV diastolic function and relaxation rate [(-dP/dt(min))/LV systolic pressure] improved (r = 0.48-0.86 and P = 0.004-0.0001, respectively). Similarly, Doppler indexes of LV diastolic function and the time constant of isovolumic LV relaxation (tau) correlated well (r = 0.50-0.84 and P = 0.0002-0.0001, respectively). In addition, eight SHR and eight Wistar rats were compared; their LV end-diastolic diameters were similar, whereas the SHR LV mass was greater. Furthermore, IVRT and Tei index were significantly higher and E(m) was lower in SHR. Moreover, tau was higher in SHR, demonstrating impaired LV relaxation. In conclusion, LV relaxation can be assessed reliably using echocardiographic and Doppler techniques, and, using these indexes, impaired relaxation was demonstrated in SHR.     

Hemodynamic changes in apolipoprotein E-knockout mice

Apolipoprotein E-knockout (ApoE-KO) mice develop advanced atherosclerotic lesions by 1 yr of age and have been well characterized pathologically and morphologically, but little is known regarding their cardiovascular physiology and hemodynamics. We used noninvasive Doppler ultrasound to measure aortic and mitral blood velocity and aortic pulse-wave velocity in 13-mo-old ApoE-KO and wild-type (WT) mice anesthetized with isoflurane. In other mice from the same colony, we measured systolic blood pressure, body weight, heart weight, cholesterol, and hematocrit. Heart rate and blood pressure were comparable (P = not significant) between ApoE-KO and WT mice, but significant decreases (P < 0.001) were found in body weight (-22%) and hematocrit (-11%), and significant increases were found in heart weight (+23%), aortic velocity (+60%), mitral velocity (+81%) (all P < 0.001), and pulse-wave velocity (+13%, P < 0.05). We also found inflections in the aortic arch velocity signal consistent with enhanced peripheral wave reflection. Thus ApoE-KO mice have phenotypic alterations in indexes of peripheral vascular resistance and compliance and significantly elevated cardiac outflow velocities and heart weight-to-body weight ratios.     

Mitral inertance in humans: critical factor in Doppler estimation of transvalvular pressure gradients

The pressure-velocity relationship across the normal mitral valve is approximated by the Bernoulli equation DeltaP = 1/2 rhoDeltav(2) + M. dv/dt, where DeltaP is the atrioventricular pressure difference, rho is blood density, v is transmitral flow velocity, and M is mitral inertance. Although M is indispensable in assessing transvalvular pressure differences from transmitral flow, this term is poorly understood. We measured intraoperative high-fidelity left atrial and ventricular pressures and simultaneous transmitral flow velocities by using transesophageal echocardiography in 100 beats (8 patients). We computed mean mitral inertance (M) by M = integral((DeltaP)-(1/2 x rho v(2))dt/integral(dv/dt)dt and we assessed the effect of the inertial term on the transmitral pressure-flow relation. ranged from 1.03 to 5.96 g/cm(2) (mean = 3.82 +/- 1.22 g/cm(2)). DeltaP calculated from the simplified Bernoulli equation (DeltaP = 1/2. rhov(2)) lagged behind (44 +/- 11 ms) and underestimated the actual peak pressures (2.3 +/- 1.1 mmHg). correlated with left ventricular systolic pressure (r = -0.68, P < 0.0001) and transmitral pressure gradients (r = 0.65, P < 0.0001). Because mitral inertance causes the velocity to lag significantly behind the actual pressure gradient, it needs to be considered when assessing diastolic filling and the pressure difference across normal mitral valves.     

Why is flow-mediated dilation dependent on arterial size? Assessment of the shear stimulus using phase-contrast magnetic resonance imaging

Flow-mediated dilation (FMD) is strongly dependent on arterial size, but the reasons for this phenomenon are poorly understood. We have previously shown that FMD is greater in small brachial arteries because the shear stress stimulus is greater in small brachial arteries. However, it is unclear why the shear stimulus is greater in small arteries. Furthermore, this relationship has not been investigated in other, differently sized arterial beds. Postischemic systolic shear stress and resulting FMD were evaluated in the brachial and femoral arteries of 24 young, healthy adults using phase-contrast magnetic resonance imaging. Arterial shear and radius were calculated from the velocity profile via a best-fit parabola before and after occlusion. Summing the velocity pixels provided hyperemic systolic flow. FMD was proportional to hyperemic shear in the brachial and femoral arteries (P < 0.0001, r = 0.60). Hyperemic systolic flow was proportional to radius2 (P < 0.0001, r = 0.93). Applying this relationship to the Poiseuille equation (shear is proportional to flow/radius3) shows that hyperemic shear is proportional to radius2/radius3 and, therefore, explains why hyperemic shear is proportional to 1/radius. We conclude that FMD is proportional to hyperemic systolic shear stress in both the brachial and the femoral arteries. The hyperemic shear stimulus for FMD is greater in small arteries due to the dependence of postischemic systolic flow on radius squared. Therefore, greater FMD in small arteries does not necessarily reflect better conduit artery endothelial function. Evaluating the shear stimulus using phase-contrast magnetic resonance imaging enhances the understanding of mechanisms underlying FMD.     

The effect of body weight on the degree of blood velocity profile skewness in the aortic annulus in domestic pigs

We investigated the blood velocity profile in the aortic annulus (AA) in two groups of domestic pigs using epicardial Doppler echocardiography. The velocity profile skewness in terms of max/mean TVI (the ratio of maximal to cross-sectional mean time-velocity integral along the diameter) was 1.107 +/- 0.01 in the small pigs (n = 10; body weight: 24.6 +/- 0.8 kg) and 1.216 +/- 0.026 in the large pigs (n = 8; body weight: 50.6 +/- 2.5 kg) (P = 0.002). The velocity profile in the AA is more skewed in large animals than in small animals and the skewness in the larger animals is similar to that in normal adult humans. This study shows the importance of choosing animals of sufficient size if flow method investigations are to be performed. This is particularly important for ultrasound Doppler investigations based on a limited sample of velocities across the flow channel.     

What measurements are necessary for a comprehensive evaluation of the peripheral arterial circulation?

Several methods are available to detect atherosclerotic lesions with a severe degree of stenosis (>70%), but the diagnosis of atherosclerotic lesions with no stenosis or with a minor degree of stenosis (<20%), is problematic. Hemodynamics associated with stenotic lesions are well described by the relationship of blood pressure and blood flow velocity, both as a function of time and localization (along the length and cross-section of the vessel). The use of this relationship in the clinic is difficult because no precise information is available about the geometry and branching of arteries, blood viscosity, and the velocity distribution over the cross-sectional area of the blood vessel. Besides, the invasiveness of the technique to measure arterial pressure as a function of time and localization does not allow routine application in patients. Because of these limitations, alternative methods have been developed. The degree and extensiveness of atherosclerotic disease can, for instance, be estimated from the changes in maximum blood flow velocity and in velocity profile, i.e., velocity distribution along the cross-section of the vessel. Moreover, the delay between simultaneously recorded arterial blood flow velocity tracings (pulse-wave velocity determination) is used to assess the elastic properties of the vessel. Changes in velocity profile occur at relatively slight degrees of arterial stenosis (around 20%), so that determination of these profiles along diseased arteries may contribute to the early diagnosis of atherosclerotic lesions. In man, transcutaneous information about the maximum and mean blood flow velocities over the cross-sectional area of the artery as an instantaneous function of time as well as the flow pattern can be obtained online with continuous wave Doppler flowmeters, at least when audio spectrum analysis is used as a processing technique. Velocity profiles can be determined with multichannel pulsed Doppler systems if the resolution of the system is adequate and a sufficient number of sample volumes can be obtained, limiting the interpolation between these samples. The on-line recording of velocity profiles can be facilitated by combining the pulsed Doppler device with either a velocity imaging system or a B-mode scan. In systems with a high resolution (sample distance 0.5 mm), one should be able to detect local disturbances in the velocity profile at the site of the lesion (due to local increases in shear stress) and proximal to the lesion (due to reflections), so that lesions with a minor degree of stenosis can be detected. In resistive systems (e.g., internal carotid arteries) in which the relationship between pressure and velocity changes during the cardiac cycle is relatively simple, the elasticity of the arterial wall can be determined by relating the relative diameter changes of the vessel, determined on-line with multichannel pulsed Doppler systems, to the instantaneous velocity pulse. Although the detection of atherosclerotic lesions at an early stage of the disease with sophisticated Doppler devices looks promising, further clinical evaluation is required.     

A new approach for the evaluation of the severity of coarctation of the aorta using Doppler velocity index and effective orifice area: in vitro validation and clinical implications

Early detection and accurate estimation of COA severity are the most important predictors of successful long-term outcome. However, current clinical parameters used for the evaluation of the severity of COA have several limitations and are flow dependent. The objectives of this study are to evaluate the limitations of current existing parameters for the evaluation of the severity of coarctation of the aorta (COA) and suggest two new parameters: COA Doppler velocity index and COA effective orifice area. Three different severities of COAs were tested in a mock flow circulation model under various flow conditions and in the presence of normal and stenotic aortic valves. Catheter trans-COA pressure gradients and Doppler echocardiographic trans-COA pressure gradients were evaluated. COA Doppler velocity index was defined as the ratio of pre-COA to post-COA peak velocities measured by Doppler echocardiography. COA Doppler effective orifice area was determined using continuity equation. The results show that peak-to-peak trans-COA pressure gradient significantly increased with flow rate (from 83% to 85%). Peak Doppler pressure gradient also significantly increased with flow rate (80-85%). A stenotic or bicuspid aortic valve increased peak Doppler pressure gradient by 20-50% for a COA severity of 75%. Both COA Doppler velocity index and COA effective orifice area did not demonstrate significant flow dependence or dependence upon aortic valve condition. As a conclusion, COA Doppler velocity index and COA effective orifice area are flow independent and do not depend on aortic valve conditions. They can, then, more accurately predict the severity of COA.     

Spatial velocity profile in mouse embryonic aorta and Doppler-derived volumetric flow: a preliminary model

Characterizing embryonic circulatory physiology requires accurate cardiac output and flow data. Despite recent applications of high-frequency ultrasound Doppler to the study of embryonic circulation, current Doppler analysis of volumetric flow is relatively crude. To improve Doppler derivation of volumetric flow, we sought a preliminary model of the spatial velocity profile in the mouse embryonic dorsal aorta using ultrasound biomicroscopy (UBM)-Doppler data. Embryonic hematocrit is 0.05-0.10 so rheologic properties must be insignificant. Low Reynolds numbers (<500) and Womersley parameters (<0.76) suggest laminar flow. UBM demonstrated a circular dorsal aortic cross section with no significant tapering. Low Dean numbers (<100) suggest the presence of minimal skewing of the spatial velocity profile. The inlet length allows for fully developed flow. There is no apparent aortic wall pulsatility. Extrapolation of prior studies to these vessel diameters (300-350 microm) and flow velocities (~50-200 mm/s) suggests parabolic spatial velocity profiles. Therefore, mouse embryonic dorsal aortic blood flow may correspond to Poiseuille flow in a straight rigid tube with parabolic spatial velocity profiles. As a first approximation, these results are an important step toward precise in utero ultrasound characterization of blood flow within the developing mammalian circulation.