Blood Velocity Measurements Research Articles

Feasibility of conjunctival hemodynamic measurements in rabbits: reproducibility, validity, and response to acute hypotension.

To evaluate the feasibility of conjunctival hemodynamic measurements based on assessment of reproducibility, validity, and response to acute hypotension. Image sequences of the conjunctival microvasculature of rabbits were captured using a slit lamp biomicroscope under a steady-state condition, after topical administration of phenylephrine, and after intravenous administration of esmolol. Venous hemodynamic parameters (diameter, blood velocity, blood flow, and wall shear stress) were derived. Conjunctival venous diameters ranged from 9 to 34 μm and blood velocities ranged from 0.08 to 0.95 mm/s. Coefficients of variation of venous diameter and blood velocity measurements were, on average, 6% and 14%, respectively. Automated and manual measurements of venous diameter and velocity were highly correlated (R = 0.97; p < 0.001; n = 16). With phenylephrine administration, diameter and velocity were reduced by 21% and 69%, respectively. Following esmolol administration, blood pressure was reduced with a concomitant decrease in velocity, followed by recovery to baseline. Venous blood velocity, flow, and WSS were correlated with blood pressure (R ≥ 0.52; p ≤ 0.01). The feasibility of quantifying alterations in microvascular hemodynamics in the bulbar conjunctiva was established. The method is of potential value in evaluating microcirculatory hemodynamics related to cardiovascular function.

The optimal measure of microvascular function with velocity time integral for cardiovascular risk prediction

Recent evidence suggests that microvascular function may be important in cardiovascular risk prediction. One measure of microvascular function is hyperaemic velocity time integral (VTI). We assessed whether the VTI of more than one beat of reactive hyperaemia would provide a stronger correlate to traditional cardiovascular risk factors using a subset of subjects from the Firefighters and Their Endothelium (FATE) study. Vascular function was assessed by measurement of hyperaemic blood velocity with high-resolution ultrasound of the brachial artery. We evaluated three measures in the current analysis: the VTI of the first beat, average VTI of 10 beats, and maximum VTI of 10 beats post-cuff release. A total of 399 male subjects (45.5 ± 10 years) were included in this analysis. Univariate correlations between the three end points and cardiovascular risk factors were calculated, and multivariable regression models constructed. Intra-observer variability was approximately equal for all VTI end points (coefficient of variation: first = 1.6%, average = 1.4%, maximum = 1.4%). Univariate correlations between VTI and cardiovascular risk factors were similar across all three end points. In multivariable analyses, there were no differences in the relationships between cardiovascular risk factors and the various VTI end points (R(2) from 0.090 to 0.102). Age, systolic blood pressure, and BMI were predictors of the three VTI end points (p < 0.05). In conclusion, the first beat of reactive hyperaemia remains the suitable measure of microvascular function.

Beyond a one‐track mind: understanding blood flow to the brain in humans

Beyond a one‐track mind: understanding blood flow to the brain in humans

Development of a Fluorescent Molecular Rotor‐Based Approach for Concurrent Measurement of Blood Viscosity and Hemodynamics

Shear stress measurements are routinely calculated using local fluid velocity gradients and estimated static viscosity values. However, since blood viscosity is dependent on local hematocrit and is affected by non‐Newtonian shear‐thinning, a methodology allowing concurrent blood velocity and viscosity measurement should improve intracardiac shear mapping accuracy. Our objective was to develop such a methodology using viscosity‐sensitive fluorescent molecular rotors coupled to inert microspheres and validate the approach in a zebrafish model. Microinjection parameters have been optimized for effective delivery of rotors into the zebrafish vascular bed. Age‐dependent injection targets have been identified to facilitate reproducible bolus delivery. Finally, FCVJ reflect local hemodynamic profiles demonstrating a functionality consistent with their in vitro performance. Taken together, our data indicate that fluorescent molecular rotors provide the spatial and temporal resolution required for use in planar and volumetric digital defocusing particle image velocimetry (DDPIV) technology to facilitate real‐time 3‐dimensional in vivo shear mapping. (This work funded by AHA 10IRG4020025.)

Evaluation of exercise capacity using wave intensity in chronic heart failure with normal ejection fraction

Impaired exercise capacity has been found in patients with diastolic dysfunction with preserved systolic function. Although conventional transthoracic echocardiography (TTE) provides useful clinical information about systolic and diastolic cardiac function, its capability to evaluate exercise capacity has been controversial. The inertia force of late systolic aortic flow is known to have a tight relationship with left ventricular (LV) performance during the period from near end-systole to isovolumic relaxation. The inertia force and the time constant of LV pressure decay during isovolumic relaxation can be estimated noninvasively using the second peak (W(2)) of wave intensity (WI), which is measured with an echo-Doppler system. We sought to determine whether W(2) is associated with exercise capacity in patients with chronic heart failure with normal ejection fraction (HFNEF) and to compare its ability to predict exercise capacity with parameters obtained by conventional TTE including tissue Doppler imaging. Sixteen consecutive patients with chronic HFNEF were enrolled in this study. Wave intensity was obtained with a color Doppler system for measurement of blood velocity combined with an echo-tracking system for detecting changes in vessel diameter. Concerning conventional TTE, we measured LV ejection fraction (EF), peak velocities of early (E) and late (A) mitral inflow using pulse-wave Doppler, and early (Ea) and late (Aa) diastolic velocities using tissue Doppler imaging. Left ventricular EF, E/A ratio, Ea, and E/Ea ratio did not correlate with exercise capacity, whereas W(2) significantly correlated with peak VO(2) (r = 0.54, p = 0.03), VE/VCO(2) slope (r = -0.53, p = 0.03), and ΔVO(2)/ΔWR (r = 0.56, p = 0.02). W(2) was associated with exercise capacity in patients with chronic HFNEF. In conclusion, W(2) is considered to be clinically more useful than conventional TTE indices for evaluating exercise capacity in patients with chronic HFNEF.

Chaotic frequency-modulating continuous wave for an ultrasonic doppler blood flow velocity measurement system

Although a pulsed-wave (PW) Doppler system can measure blood velocity with range information, measurable velocity is limited by frequency aliasing, and signal-noise-ratio (SNR) is poor. Continuous wave (CW) Doppler system offers a higher SNR in Doppler signal and no measurable velocity limitations but does not provide ranging information. Linear frequency-modulated continuous wave (LFMCW) and sine frequency-modulating continuous wave are able to provide high SNR Doppler signals with ranging information, and low clutter power, but with a certain range ambiguity due to signal periodicity of the Doppler signal spectrum. Various efforts have been made to improve the SNR, range resolution, and velocity limitations with FMCW Doppler system. An ultrasonic doppler blood flow velocity measurement system based on chaotic FMCW is proposed which has a higher SNR with no range ambiguity because of the aperiodicity of the Doppler signal spectrum. Principle analysis and experimental simulation show that the effectiveness of the new system is verified.

Development of Miniaturized Fiber-Optic Laser Doppler Velocimetry Sensor for Measuring Local Blood Velocity: Measurement of Whole Blood Velocity in Model Blood Vessel Using a Fiber-Optic Sensor with a Convex Lens-Like Tip

A miniaturized fiber-optic laser Doppler velocimetry sensor has been developed to measure the local blood velocity in vivo. The laser beam emitted from the sensor tip can be focused at any distance between 0.1 and 0.5 mm from the tip. Consequently, the sensor has a sufficiently high signal-to-noise ratio to measure the local velocity in almost any opaque fluid, including blood. The sensor head is inserted in an injection needle or a catheter tube. In the former case, it is inserted at an angle to the wall of a vessel and is scanned across the vessel to measure the velocity distribution. In the latter case, it is aligned parallel with the flow in a vessel. For all flows of whole human blood, whole caprine blood, and 69% hematocrit of bovine blood, the velocity distribution across the vessel could be measured very accurately. The insertion angle of the fiber into the flow significantly affects the measurement accuracy; an angle of about 50° is suitable when an injection needle is used. When a catheter is employed, an insertion direction opposite to the flow direction is better than parallel to the flow due to the generation of a wake behind the fiber.

Near-Infrared Diffuse Correlation Spectroscopy (DCS) for Assessing Deep Tissue Blood Flow

Many diseases are associated with abnormal blood flow such as stroke, peripheral arterial disease (PAD), and cancer. An occlusive stroke/PAD is an interruption of the blood flow to the brain/skeletal muscle, which may impair or even destroy the cerebral/muscular function. The abnormal vasculature and/or high metabolic demand in tumors may generate abnormal blood flow compared to the surrounding normal tissues. The various treatments for these diseases share the common goal of modifying blood flow in some way. These treatments include improving blood flow to the ischemic tissues or shutting down the blood flow to cancer cells. Therefore, measurement of tissue blood flow may provide information for diagnosis of various tissue diseases, and for monitoring of therapeutic effects. A variety of methods are currently used for the measurement of blood flow and velocity [1]. In the clinic, however, blood flow measurements are required to be noninvasive, continuous, and fast while still providing accurate and useful quantitative flow information in deep tissues. Duplex ultrasonography can noninvasively image large and deep vessels in three dimensions with relatively high spatial (~mm) and temporal (~ms) resolution. Imaging modalities for evaluation of deep tissue blood flow at the level of microvasculature include positron emission tomography (PET), single photon emission computed tomography (SPECT), Xenon-enhanced computed tomography (Xenon-CT), dynamic perfusion computed tomography (PCT), dynamic susceptibility contrast magnetic resonance imaging (DSC-MRI), and arterial spin labeling MRI (ASL-MRI). These techniques use endogenous (ASL-MRI) or exogenous tracers (PET, SPECT, XeCT, PCT, DSC-MRI), and their temporal (seconds to minutes) and spatial (several millimeters) resolutions vary. However, these imaging modalities have several limitations that preclude their routine use in clinics. PET, SPECT, and Xenon-CT require exposure to ionizing radiation, and MRI methods cannot be used in patients with pacemakers, metal implants, or claustrophobia. Furthermore, most of these methods require large and costly instrumentation, and are largely incompatible with serial/continuous measurements. Other surfacesensitive imaging techniques for measurement of microvascular flow include scanning laser Doppler, laser speckle imaging and Doppler optical coherence tomography (DOCT). These methods are used primarily for noninvasive monitoring of blood flow in tissues located at a few hundred microns below the tissue surface. Apparently, there is a critical need to develop bedside techniques that can monitor microvascular blood flow in deep tissues noninvasively, frequently and inexpensively. Recently, an emerging near-infrared (NIR) diffuse correlation spectroscopy (DCS) was developed which can directly measure the motions of moving scatterers (primarily red blood cells) in biological tissues [2,3]. DCS uses a coherent NIR light source to penetrate deep tissue, and monitors temporal light intensity fluctuations caused by moving red blood cells in tissue microvasculature to calculate flow index. DCS offers several attractive new features for blood flow measurement such as noninvasiveness, high temporal resolution (up to 100 Hz) [4], portability [5], and relatively inexpensive and large penetration depth (up to several centimeters) [3]. Furthermore, DCS

Middle cerebral artery resistivity and pulsatility indices in systemic lupus erythematosus: evidence for hyperperfusion

Systemic lupus erythematosus (SLE) is associated with significant cerebrovascular and neuropsychiatric disease for which multiple pathogeneses have been proposed. Although global cerebral hypoperfusion has been proposed, there are limited data about intracerebral arterial hemodynamics. Transcranial Doppler (TCD) allows portable, high temporal and spatial resolution, noninvasive blood velocity measurements in the middle cerebral arteries, and calculations of standard resistivity (RI) and pulsatility (PI) indices. RI and PI correlate with cerebral hemispheric arteriolar tone, blood flow resistances, and impedances. Accordingly, we hypothesized that there would be significant differences (p < 0.05) in RI and PI between SLE patients and healthy, age and gender matched controls. TCD was used to measure RI and PI bilaterally on 34 stable SLE patients (35 ± 11 years) and 15 control subjects (34 ± 10 years). Patients and controls had similar, normal blood pressures and were examined in the supine position during normal, resting respiration. RI and PI were determined by a blinded, experienced observer. There were no significant differences in RI and PI bilaterally within each cohort. However, SLE patients had significantly lower average RI and PI values compared with controls: 0.45 ± 0.10 versus 0.52 ± 0.05 (p < 0.05); and 0.65 ± 0.19 versus 0.77 ± 0.12, (p < 0.05); respectively. These preliminary data suggest that RI and PI values in the human middle cerebral artery are significantly lower in SLE compared with controls. These indices indicate that middle cerebral arterial resistances and impedances are decreased in SLE. Under normotensive conditions, the results are consistent with hyperperfusion in SLE with increased arteriolar dilation and increased cerebral blood flow.

Robust method for 3D arterial spin labeling in mice

Arterial spin labeling is a versatile perfusion quantification methodology, which has the potential to provide accurate characterization of cerebral blood flow (CBF) in mouse models. However, a paucity of physiological data needed for accurate modeling, more stringent requirements for gradient performance, and strong artifacts introduced by magnetization transfer present special challenges for accurate CBF mapping in the mouse. This article describes robust mapping of CBF over three-dimensional brain regions using amplitude-modulated continuous arterial spin labeling. To provide physiological data for CBF modeling, the carotid artery blood velocity distribution was characterized using pulsed-wave Doppler ultrasound. These blood velocity measurements were used in simulations that optimize inversion efficiency for parameters meeting MRI gradient duty cycle constraints. A rapid slice positioning algorithm was developed and evaluated to provide accurate positioning of the labeling plane. To account for enhancement of T(1) due to magnetization transfer, a binary spin bath model of magnetization transfer was used to provide a more accurate estimate of CBF. Finally, a study of CBF was conducted on 10 mice with findings of highly reproducible inversion efficiency (mean ± standard-error-of-the-mean, 0.67 ± 0.03), statistically significant variation in CBF over 12 brain regions (P < 0.0001) and a mean ± standard-error-of-the-mean whole brain CBF of 219 ± 6 mL/100 g/min.

The impact of cataract on the quantitative, non-invasive assessment of retinal blood Flow

The aim of the study was to determine the impact of cataract on the quantitative, non-invasive assessment of retinal blood flow assessed by bidirectional laser Doppler flowmetry and simultaneous vessel densitometry. Ten patients scheduled for extracapsular cataract extraction using phacoemulsification and intraocular lens implantation between the ages of 61 and 84 (mean age 73 years, SD ± 8) were prospectively recruited. Two visits were required to complete the study; one visit prior to extracapsular cataract extraction and one at least 6 weeks after the surgery to allow for sufficient postoperative recovery. The severity of cataract was documented using the Lens Opacity Classification System (LOCS, III) at the first visit. Retinal arteriolar hemodynamics were measured at both visits using the high-intensity setting of the Canon Laser Blood Flowmeter. All eyes showed no clinical signs of postoperative intraocular inflammation. The quantitative assessment of retinal arteriolar diameter and blood flow were reduced following extracapsular cataract extraction (Wilcoxon signed-rank test, p = 0.022 and p=0.028, respectively); however, centreline blood velocity was not significantly changed (Wilcoxon signed-rank test, p=0.074). Intraocular pressure was unchanged pre- and postcataract extraction. Retinal vessel densitometry assessment in the presence of cataract results in the erroneous elevation of the diameter measurement and thereby the calculation of blood flow. The bidirectional Doppler assessment of blood velocity appears to be more robust to light scatter induced by cataract. Care needs to be exercised in the interpretation of studies of retinal vessel diameter or blood flow that utilize similar densitometry techniques.

Remembering one of the great pioneers

Sadly, Professor John MacVicar, the pioneering obstetrician in ultrasound died in May this year. His contributions to obstetrics and gynaecology, and to ultrasound were immeasurable. John was one of the co-authors, along with Donald and Brown, of the seminal paper ‘Investigation of abdominal masses using pulsed ultrasound’ published in The Lancet in 1958, which helped to secure ultrasound’s future as a serious imaging modality – not that the sketchy line ‘images’ obtained back then would have meant much to any of us practising today. In view of John’s achievements and of the subsequent technological developments in ultrasound in the last 50 years, the title of his MD thesis in 1959 could be considered one of the great understatements of the 20th century; ‘Ultrasound as a diagnostic aid in obstetrics and gynaecology’. I am grateful to one of John MacVicar’s former colleagues, John Fleming, for writing a fine tribute to this great man (p 118). These are exciting times to be involved in ultrasound education. It is undergoing many changes as higher education institutions, professional organizations and private companies attempt to meet the ultrasound training needs of growing numbers of individuals working in and outside the National Health Service. In the UK various guidelines, standards and recommendations have been set. However, as more focused applications of ultrasound emerge it may be that these standards are not appropriate for all users. It may be timely to consider revisions or the development of new guidance while ensuring, of course, that clinical competence is still achieved and measured. An illustration of this idea is included in this issue of Ultrasound; an article by Bidwai and Casserley discusses the need for focused ultrasound training for orthopaedic surgeons. Arguably their proposal could be echoed by a number of practitioners from other specialties. I anticipate their paper will generate debate and I look forward to hearing your views on this important subject. Still on the theme of focused education, we also include the next two articles in the Point of Care Ultrasound (PoCUS) series aimed specifically at emergency physicians. There is a special feature by Pat Ward, Director of the Fetal Anomaly Screening Programme, who reviews the current situation regarding combined screening for Down’s syndrome. Staying with first trimester ultrasound imaging, Chudleigh et al. offer guidance on how to date pregnancies for combined screening where head circumference and crown rump length measurements do not correlate. They also provide advice on how to date a pregnancy when parents have declined combined screening. Practitioners who have previously been unsure as to whether to employ head circumference or crown rump length measurements towards the end of the first trimester may well find this information useful. A team from South Africa report on the value of abdominal ultrasound in children with tuberculosis. Hoskins reviews errors in blood velocity and flow measurements, and suggests that they may be minimized by the development of standard protocols. A collaboration from India presents a technical paper on digital scan converters and signal processing in relation to B-mode imaging. As well as the usual book reviews and Echoes, this edition also contains a thought-provoking letter by Tom Brown, the retired engineer and former colleague of John MacVicar, who suggests a method for reducing work-related musculoskeletal disorders in sonographers. Again, I welcome your response.

Doppler velocity measurements from large and small arteries of mice

With the growth of genetic engineering, mice have become increasingly common as models of human diseases, and this has stimulated the development of techniques to assess the murine cardiovascular system. Our group has developed nonimaging and dedicated Doppler techniques for measuring blood velocity in the large and small peripheral arteries of anesthetized mice. We translated technology originally designed for human vessels for use in smaller mouse vessels at higher heart rates by using higher ultrasonic frequencies, smaller transducers, and higher-speed signal processing. With these methods one can measure cardiac filling and ejection velocities, velocity pulse arrival times for determining pulse wave velocity, peripheral blood velocity and vessel wall motion waveforms, jet velocities for the calculation of the pressure drop across stenoses, and left main coronary velocity for the estimation of coronary flow reserve. These noninvasive methods are convenient and easy to apply, but care must be taken in interpreting measurements due to Doppler sample volume size and angle of incidence. Doppler methods have been used to characterize and evaluate numerous cardiovascular phenotypes in mice and have been particularly useful in evaluating the cardiac and vascular remodeling that occur following transverse aortic constriction. Although duplex ultrasonic echo-Doppler instruments are being applied to mice, dedicated Doppler systems are more suitable for some applications. The magnitudes and waveforms of blood velocities from both cardiac and peripheral sites are similar in mice and humans, such that much of what is learned using Doppler technology in mice may be translated back to humans.

Doppler velocity detection limitations in spectrometer-based versus swept-source optical coherence tomography

Recent advances in Doppler techniques have enabled high sensitivity imaging of biological flow to measure blood velocities and vascular perfusion. Here we compare spectrometer-based and wavelength-swept Doppler OCT implementations theoretically and experimentally, characterizing the lower and upper observable velocity limits in each configuration. We specifically characterize the washout limit for Doppler OCT, the velocity at which signal degradation results in loss of flow information, which is valid for both quantitative and qualitative flow imaging techniques. We also clearly differentiate the washout effect from the separate phenomenon of phase wrapping. We demonstrate that the maximum detectable Doppler velocity is determined by the fringe washout limit and not phase wrapping. Both theory and experimental results from phantom flow data and retinal blood flow data demonstrate the superiority of the swept-source technique for imaging vessels with high flow rates.

Noninvasive Measurements and Analysis of Blood Velocity Profiles in Human Retinal Vessels

To quantitatively model the changes in blood velocity profiles for different cardiac phases in human retinal vessels. An adaptive optics scanning laser ophthalmoscope (AOSLO) was used to measure blood velocity profiles in three healthy subjects. Blood velocity was measured by tracking erythrocytes moving across a scanning line. From the radial position of the cells within the lumen, the blood velocity profile was computed. The cardiac pulsatility was recorded with a cardiac signal monitor. The shape of the blood velocity profile in retinal arteries changed systematically during the cardiac cycle, with the flattest profile occurring during the diastolic phase. The measured blood velocity profiles were typically flatter than the commonly assumed parabolic shape. The flatness increased with decreasing vessel size. For the large veins (>80 μm), the ratio of the centerline velocity to the cross-sectional average velocity was between 1.50 and 1.65. This ratio decreased to 1.36 in the smallest vein studied (32 μm). Velocity profiles downstream from a venous confluence showed two peaks at 120 μm from the confluence, but a single velocity peak 500 μm downstream from the confluence. The cardiac cycle influences the blood flow velocity profiles systematically in retinal arteries but not in veins. Parabolic flow was not found in even the largest vessels studied, and deviations from parabolic flow increased in smaller vessels. The measurements are sensitive enough to measure the dual-humped blood velocity profile at a vein confluence.

Biofluid mechanics and the circulatory system

A fluid is a medium which deforms, or undergoes motion, continuously under the action of a shearing stress and includes liquids and gases. Applying biofluid mechanics to the cardiovascular system requires knowledge of anatomy and geometry, pressure data and blood flow, volume and velocity measurements. A good example is the assessment of the haemodynamics of biological and mechanical heart valves.

Automatic tracking and measurement of the motion of blood cells in microvessels based on analysis of multiple spatiotemporal images

Automatic blood cell tracking and velocity measurement in microvessels is a crucial task in biomedical and physiological research. For the analysis of the motion of blood cells in microvessels, a commonly used method for blood cell tracking and velocity estimation is spatiotemporal image-based analysis. However, in the process of the spatiotemporal image generation, a single spatial path is used, i.e. the centreline, which is not suitable for many situations in which cells do not move strictly along the central axis of the microvessel. In this paper, we propose a new method for automatic tracking and measurement of the motion of blood cells in a microvessel based on multiple spatiotemporal images analysis. First, the proposed method adopts three spatial paths (the centreline, inner and outer contour of the microvessel) to generate three spatiotemporal images; then, the traces of blood cells in the spatiotemporal images are extracted and subsequently trace grouping and fusion processes are developed for tracking cell trajectories. For extracting traces in spatiotemporal images, a steerable filter is employed to enhance the traces in raw spatiotemporal images, and then the noise suppression function and orientation-filtering function are designed to extract trace candidates. In the subsequent grouping and fusion process, trace candidates are grouped by the proposed trace grouping rule, and then the trajectories are calculated by the proposed trace fusion approach. The results validate the proposed method for blood cell tracking and the accuracy for blood cell velocity measurement. Moreover, for the larger microvessels, we discuss the criterion of the number selection of the optimal spatial path by using both simulated and real experiments, and it can be used as the criterion for blood cell tracking in microvessels.

Comparison of blood velocity measurements between ultrasound Doppler and accelerated phase-contrast MR angiography in small arteries with disturbed flow

Ultrasound Doppler (UD) velocity measurements are commonly used to quantify blood flow velocities in vivo. The aim of our work was to investigate the accuracy of in vivo spectral Doppler measurements of velocity waveforms. Waveforms were derived from spectral Doppler signals and corrected for intrinsic spectral broadening errors by applying a previously published algorithm. The method was tested in a canine aneurysm model by determining velocities in small arteries (3–4 mm diameter) near the aneurysm where there was moderately disturbed flow. Doppler results were compared to velocity measurements in the same arteries acquired with a rapid volumetric phase contrast MR angiography technique named phase contrast vastly undersampled isotropic projection reconstruction magnetic resonance angiography (PC-VIPR MRA). After correcting for intrinsic spectral broadening, there was a high degree of correlation between velocities obtained by the real-time UD and the accelerated PC-MRA technique. The peak systolic velocity yielded a linear correlation coefficient of r = 0.83, end diastolic velocity resulted in r = 0.81, and temporally averaged mean velocity resulted in r = 0.76. The overall velocity waveforms obtained by the two techniques were also highly correlated (r = 0.89 ± 0.06). There were, however, only weak correlations for the pulsatility index (PI: 0.25) and resistive index (RI: 0.14) derived from the two techniques. Results demonstrate that to avoid overestimations of peak systolic velocities, the results for UD must be carefully corrected to compensate for errors caused by intrinsic spectral broadening.

Numerical assessment of time-domain methods for the estimation of local arterial pulse wave speed

A local estimation of pulse wave speed c, an important predictor of cardiovascular events, can be obtained at arterial locations where simultaneous measurements of blood pressure (P) and velocity (U), arterial diameter (D) and U, flow rate (Q) and cross-sectional area (A), or P and D are available, using the PU-loop, sum-of-squares (∑2), lnDU-loop, QA-loop or new D2P-loop methods. Here, these methods were applied to estimate c from numerically generated P, U, D, Q and A waveforms using a visco-elastic one-dimensional model of the 55 larger human systemic arteries in normal conditions. Theoretical c were calculated from the parameters of the model. Estimates of c given by the loop methods were closer to theoretical values and more uniform within each arterial segment than those obtained using the ∑2. The smaller differences between estimates and theoretical values were obtained using the D2P-loop method, with root-mean-square errors (RMSE) smaller than 0.18ms−1, followed by averaging the two c given by the PU- and lnDU-loops (RMSE <2.99ms−1). In general, the errors of the PU-, lnDU- and QA-loops decreased at locations where visco-elastic effects were small and nearby junctions were well-matched for forward-travelling waves. The ∑2 performed better at proximal locations.

In Vivo Measurement of Blood Velocity in Human Major Retinal Vessels Using the Laser Speckle Method

To develop a simple, noninvasive method of determining blood velocity and flow through human retinal vessels (RVs), by using the laser speckle method and validating the results by bidirectional laser Doppler velocimetry (LDV). The square blur rate (SBR), a quantitative blurring index of the laser speckle pattern that parallels the velocity of moving substances, obtained from blood flowing through glass capillary tubes (RV analogues), correlated with tube diameter, background reflectance and absorption, flow velocity, and the SBR obtained from blood flowing through underlying glass capillary tubes (choroidal vessel analogues). A nomogram was constructed to calculate the blood velocity in human RVs from the SBR values obtained in vivo. Blood velocities in RVs were determined in 12 normal eyes by using the laser speckle method and bidirectional LDV. Measurements were performed twice at the same site at 1-hour intervals. Measurements from a temporal superior artery (n = 12; mean ± SD) were blood velocity (V(mean)), 41.7 ± 4.2 mm/s; flow, 13.0 ± 3.2 μL/min; and diameter, 119.5 ±15.7 μm and time to complete one measurement, 65 ± 18 seconds, with the laser speckle apparatus; and V(mean), 37.7 ± 6.7 mm/s; flow, 11.7 ±3.0 μL/min; diameter, 111.1 ±16.6 μm; and measurement time, 112 ± 25 seconds, with the bidirectional LDV apparatus. The results obtained by the two methods correlated with each other (V(mean), r = 0.59, P = 0.023; flow, r = 0.83, P = 0.005; and diameter, r = 0.56, P = 0.032). The coefficients of reproducibility for V(mean), blood flow, and diameter measurement were 9.5% ± 2.5%, 10.5% ± 3.2%, and 5.3% ± 2.7% for the former and 15.3% ± 4.2%, 18.5% ± 4.1%, and 6.2% ± 2.2% for the latter, respectively. The laser speckle method accurately and reproducibly determines blood velocity in human RVs in less time than the LDV apparatus requires.