The cooling effect of blood flow during hyperthermia treatment.

The amount and duration of the applied heat in hyperthermia treatment are critical for cancer survivors. The challenge is to use a mechanism dealing with the tumor cells only while keeping healthy tissues unharmed. The aim of this paper is to predict the blood temperature distribution in main dimensions during hyperthermia process by deriving a new analytical solution of unsteady flow that adequately covers the cooling factor. We adopted a separation of variable method to solve the bio-heat transfer problem of unsteady blood flow. The solution is similar to Pennes' equation, except that it is for blood rather than tissue. We also performed computational simulations with varied flow conditions and thermal energy transports. The blood cooling effects were calculated with vessel's diameter, tumor's zone length, pulsating period and flow velocity. The cooling rate rises by around 133% if the tumor zone's length is extended four times the diameter of 0.5 (mm), but it is seemingly fixed with this distance if the diameter is equal or larger than 4 (mm). Likewise, the temporal variations of temperature disappear if the blood vessel has a diameter of 4 (mm) or more. Pre-heating or post-cooling techniques perform effectively given the theoretical solution; under particular conditions, the reduction percentages of the cooling effect are between 130% and 200%, respectively.

Comparative Analysis of Temperature Rise between Convective Heat Transfer Method and Computational Fluid Dynamics Method in an Anatomy-Based Left Atrium Model during Pulsed Field Ablation: A Computational Study.

The non-thermal effects are considered one of the prominent advantages of pulsed field ablation (PFA). However, at higher PFA doses, the temperature rise in the tissue during PFA may exceed the thermal damage threshold, at which time intracardiac pulsatile blood flow plays a crucial role in suppressing this temperature rise. This study aims to compare the effect of heat dissipation of the different methods in simulating the pulsatile blood flow during PFA. This study first constructed an anatomy-based left atrium (LA) model and then applied the convective heat transfer (CHT) method and the computational fluid dynamics (CFD) method to the model, respectively, and the thermal convective coefficients used in the CHT method are 984 (W/m2*K) (blood-myocardium interface) and 4372 (W/m2*K) (blood-catheter interface), respectively. Then, it compared the effect of the above two methods on the maximum temperature of myocardium and blood, as well as the myocardial ablation volumes caused by irreversible electroporation (IRE) and hyperthermia under different PFA parameters. Compared with the CFD method, the CHT method underestimates the maximum temperature of myocardium and blood; the differences in the maximum temperature of myocardium and blood between the two methods at the end of the last pulse are significant (>1 °C), and the differences in the maximum temperature of blood at the end of the last pulse interval are significant (>1 °C) only at a pulse amplitude greater than 1000 V or pulse number greater than 10. Under the same pulse amplitude and different heat dissipation methods, the IRE ablation volumes are the same. Compared with the CFD method, the CHT method underestimates the hyperthermia ablation volume; the differences in the hyperthermia ablation volume are significant (>1 mm3) only at a pulse amplitude greater than 1000 V, a pulse interval of 250 ms, or a pulse number greater than 10. Additionally, the hyperthermia ablation isosurfaces are completely wrapped by the IRE ablation isosurfaces in the myocardium. Thus, during PFA, compared with the CFD method, the CHT method cannot accurately simulate the maximum myocardial temperature; however, except at the above PFA parameters, the CHT method can accurately simulate the maximum blood temperature and the myocardial ablation volume caused by IRE and hyperthermia. Additionally, within the range of the PFA parameters used in this study, the temperature rise during PFA may not lead to the appearance of additional hyperthermia ablation areas beyond the IRE ablation area in the myocardium.

Study of Non-Newtonian blood flow - heat transfer characteristics in the human coronary system with an external magnetic field.

This paper proposes a novel mathematical model of non-Newtonian blood flow and heat transfer in the human coronary system with an external magnetic field. As the blood viscosity is assumed to depend not only on shear rate but also on temperature and magnet strength, the modified Carreau-Yasuda viscosity model is formulated. The computational domain includes the base of the aorta, the right coronary artery, and the left coronary artery, with the left circumflex and left anterior descending arteries. The element-based finite volume method is derived for the solution of the proposed model. Numerical simulations are carried out to investigate the magnetic field effect on the blood flow-heat transfer characteristic in the human coronary system. It is found that the magnetic field has a significant impact on fluid viscosity, leading to enhanced fluid velocity.

Stress distribution in the walls of major arteries: implications for atherogenesis.

BACKGROUND: There is a correlation between the sites of atheroma development and stress points in the arterial system. Generally, pulse pressure results in stresses acting on the vascular vessel, including longitudinal stress, radial or normal stress, tangential stress or hoop stress and shear stress. This paper explores the relationship between arterial wall shear stress and pulsatile blood pressure with the aim of furthering the understanding of atherogenesis and plaque progression. METHODS: We computed the magnitude of the shear stresses within the carotid bifurcation geometry of a patient and calculated the increase in shear stress levels that would occur when the blood pressure and pulse pressures rise during exertion. We also determined in which layer of the artery wall the maximum shear stress is located, and computed the shear stress at different levels within the media. We used the theory of laminate analysis, (Classical Laminate Plate Theory), to analyse the stress distribution on the carotid artery wall. Computational Fluid Dynamics (CFD) analysis was used on anatomy based on a CT angiogram of the carotid bifurcation of a patient with a 90% stenosis on the right side and 10% on the left. The pulsatile non-Newtonian blood flow with a resting blood pressure of 120/80 mmHg and an exertion pressure of 200/100 mmHg was simulated and the resultant forces were transferred to an ANSYS Composite PrepPost (ACP) model for wall shear stress analysis. A multilayer elastic, anisotropic, and inhomogeneous arterial wall (intima, internal elastic lamina, media, external elastic lamina, and adventitial layers) was modelled and the shear stress magnitudes and change over time between the layers was calculated. RESULTS: Shear stress in the individual composite layers is far greater than that acting on the endothelium (less than 5 Pa). At rest, the maximum variation of shear stress in the arterial wall occurs in the intima (138 Pa) and adventitia (135 Pa). The medial layer has the lowest variation of shear stress. Under severe exertion, the maximum shear stress magnitude in the intimal layer and the adjacent medial layer is near the ultimate stress level. The maximum/minimum shear stress ratios during the cardiac cycle vary most widely in the innermost part of the media, adjacent to the intima, with a four-fold ratio increase. This compares with a less than two-fold increase in all the other layers including the intima and adventitia, making the inner media the most vulnerable layer to mechanical injury. CONCLUSIONS: This study showed that the magnitude of exertion-induced shear stress approaches the ultimate stress limit in the intima and the immediate adjacent medial layer. The variation in stress is maximal in the inner layer of the media. These findings correlate the site of atheroma development with the most vulnerable site for injury in the media and emphasise the impact of pulse pressure. Further biological studies are required to ascertain whether this leads to injury that initiates atheroma that then precipitates an injury/healing cycle.

Superconducting nanowire single-photon sensing of cerebral blood flow.

Significance: The ability of diffuse correlation spectroscopy (DCS) to measure cerebral blood flow (CBF) in humans is hindered by the low signal-to-noise ratio (SNR) of the method. This limits the high acquisition rates needed to resolve dynamic flow changes and to optimally filter out large pulsatile oscillations and prevents the use of large source-detector separations ( ≥ 3 cm ), which are needed to achieve adequate brain sensitivity in most adult subjects. Aim: To substantially improve SNR, we have built a DCS device that operates at 1064 nm and uses superconducting nanowire single-photon detectors (SNSPD). Approach: We compared the performances of the SNSPD-DCS in humans with respect to a typical DCS system operating at 850 nm and using silicon single-photon avalanche diode detectors. Results: At a 25-mm separation, we detected 13 ± 6 times more photons and achieved an SNR gain of 16 ± 8 on the forehead of 11 subjects using the SNSPD-DCS as compared to typical DCS. At this separation, the SNSPD-DCS is able to detect a clean pulsatile flow signal at 20 Hz in all subjects. With the SNSPD-DCS, we also performed measurements at 35 mm, showing a lower scalp sensitivity of 31 ± 6 % with respect to the 48 ± 8 % scalp sensitivity at 25 mm for both the 850 and 1064 nm systems. Furthermore, we demonstrated blood flow responses to breath holding and hyperventilation tasks. Conclusions: While current commercial SNSPDs are expensive, bulky, and loud, they may allow for more robust measures of non-invasive cerebral perfusion in an intensive care setting.

Investigation of the plaque morphology effect on changes of pulsatile blood flow in a stenosed curved artery induced by an external magnetic field.

In a new therapeutic technique, called magnetic drug targeting (MDT), magnetic particles carrying therapeutic agents are directed to the target tissue by applying an external magnetic field. Meanwhile, this magnetic field also affects the blood as a biomagnetic fluid. Therefore, it is necessary to select a magnetic field with an acceptable range of influence on the blood flow. This study investigates the effect of an external magnetic field on the pulsatile blood flow in a stenosed curved artery to identify a safe magnetic field. The effects of a number of parameters, including the magnetic susceptibility of blood in oxygenated and deoxygenated states and the magnetic field strength, were studied. Moreover, the effect of the plaque morphology, including the occlusion percentage and the chord length of the stenosis, on changes in blood flow induced by the magnetic field was investigated. The results show that applying a magnetic field increases the wall shear stress (WSS) and the pressure of the deoxygenated blood. Comparing the wall shear stresses of the deoxygenated and oxygenated blood shows that the effect of magnetic field on the deoxygenated blood is more significant than its effect on the oxygenated blood due to its higher magnetic susceptibility. The study of the stenosis geometry shows that the influence of magnetic field on the blood flow is increased by decreasing the occlusion percentage of the artery. Furthermore, among the evaluated lengths, the 50° chord length results in the highest variation under the influence of the magnetic field. Finally, the magnetic field of Mn = 2.5 can be utilized as a safe field for MDT purposes in such a stenosed curved artery.

Cerebrovascular Impedance During Hemodynamic Change in Rabbits: A Pilot Study.

INTRODUCTION: Cerebrovascular impedance describes the relationship between pulsatile changes in arterial blood pressure (ABP) and cerebral blood flow (CBF). It is commonly defined by modulus and phase shift derived from Fourier spectra of ABP and CBF velocity (CBFV) signals under mostly steady-state conditions. The aim of this work was to assess heartbeat-to-heartbeat cerebrovascular impedance at heart rate frequency during controlled changes in mean ABP and intracranial pressure (ICP). MATERIAL AND METHODS: Recordings of ABP in the femoral artery, transcranial Doppler CBFV in the basilar artery, and subarachnoid ICP were obtained from anesthetized rabbits with induced arterial hypotension (n = 8 rabbits), arterial hypertension (n = 5), or intracranial hypertension (n = 7). Modulus of cerebrovascular impedance (|Z|) was estimated from amplitudes of ABP and CBFV. Phase shift of cerebrovascular impedance (PS) was estimated from time-frequency (TF) representations of phase shift between ABP and CBFV overlaid with a time-variant mask based on the fundamental frequency of ABP. RESULTS: Both |Z| and PS increased with increasing mean ABP. |Z| decreased with increasing mean ICP, but no change was observed in PS. CONCLUSIONS: The combined beat-to-beat and TF approach allows for the estimation of cerebrovascular impedance during transient hemodynamic changes. |Z| and PS follow the pattern of changes in CPP.

Frequency domain analysis and clinical outcomes of pulsatile and non-pulsatile blood flow energy during cardiopulmonary bypass.

INTRODUCTION: The superiority of pulsatile perfusion during cardiopulmonary bypass remains controversial. We analyzed the frequency-domain characteristics and organ protection of pulsatile and nonpulsatile flow in adult patients with valvular disease. METHODS: EEP and SHE were used to calculate blood flow energy in 60 patients. The Fast Fourier Transform was employed to analyze the power spectral density and power density ratio (Rvpd) of flow energy. Changes in endothelin-1, nitric oxide, interleukin-6,10, tumor necrotic factor, S100ß, NSE, blood and urinary ß2-microglobulin levels were investigated to assess the endothelial function, inflammatory reaction, kidney and brain injury during CPB. RESULTS: EEP and SHE in PP group at each time point were 1.52-1.62 times and 2.03-2.22 times higher respectively compared with NP group. Power spectral density analysis demonstrated that the blood flow energy frequencies in each group were all within 40 Hz and the low frequency energy (0-5 Hz) was dominant in physiological perfusion (>90%). The energy ratio of 0-5 Hz at radial artery was significantly decreased compared with that of post arterial filter in PP (81% vs 64%) and NP (63% vs 37%) group. The power density ratio (Rvpd) was higher than that of NP in all frequency ranges at the radial artery (9.51 vs 4.68 vs 3.59) and arterial filter (3.87 vs 2.69 vs 2.38). The S100ß, NSE Urinary and plasma ß2-microglobulin level were significantly increased at 6 and 24 hours after surgery in two group, and significantly higher in group NP. CONCLUSION: PP provided more energy than NP. The proportion of low frequency energy in the pulsatile or nonpulsatile flow is significantly reduced. The low-frequency energy is significantly attenuated during conduction to peripheral tissues in nonpulsatile flow. The surplus pulsatile energy influences the secretion of endothelial and inflammatory factors, and demonstrate better cerebral and kidney protective effect at the biological marker level.

Particle swarm optimizer for arterial blood flow models.

BACKGROUND AND OBJECTIVE: Mathematical modeling and computational simulations of arterial blood flow network can offer an insilico platform for both diagnostics and therapeutic phases of patients that suffer from cardiac diseases. These models are normally complex and involve many unknown parameters. For physiological relevance, these parameters should be optimized using in-vivo human/animal data sets. The main goal of this work is to develop an efficient, yet an accurate optimization algorithm to compute parameters in the arterial blood flow models. METHODS: The particle swarm optimization (PSO) method is proposed herein for the first time, as an accurate algorithm that applies to computing parameters in the Windkessel type model of blood flow in the arterial system. We begin by defining a 6-element Windkessel (WK6) arterial flow model, which is then implemented and validated using multiple flow rate and aortic pressure measurements obtained from different subjects including dogs, pigs and humans. The parameters in the model are obtained using the PSO technique which minimizes the pressure root mean square (P-RMS) error between the computed and the measured aortic pressure waveform. RESULTS: Model parameters obtained using the proposed PSO method were able to recover the pressure waveform in the aorta during the cardiac cycle for both healthy and diseased species (animals/humans). The PSO method provides an accurate approach to solve this challenging multi-dimensional parameter identification problem. The results obtained by PSO algorithm was compared with the classical gradient-based, namely the non-linear square fit (NLSF) algorithm. CONCLUSIONS: The results indicate that the PSO method offers alternative and accurate method to find optimal physiological parameters involved in the Windkessel model for the study of arterial blood flow network. The PSO method has performed better than the NLSF approach as depicted from the P-RMS calculations. Finally, we believe that the PSO method offers a great potential and could be used for many other biomedicine optimization problems.

Detection of localized pulsatile motion in cutaneous microcirculation by speckle decorrelation optical coherence tomography angiography.

SIGNIFICANCE: Pulsatility is a vital characteristic of the cardiovascular system. Characterization of the pulsatility pattern locally in the peripheral microvasculature is currently not readily available and would provide an additional source of information, which may prove important in understanding the pathophysiology of arterial stiffening, vascular ageing, and their linkage with cardiovascular disease development. AIM: We aim to confirm the suitability of speckle decorrelation optical coherence tomography angiography (OCTA) under various noncontact/contact scanning protocols for the visualization of pulsatility patterns in vessel-free tissue and in the microvasculature of peripheral human skin. RESULTS: Results from five healthy subjects show distinct pulsatile patterns both in vessel-free tissue with either noncontact or contact imaging and in individual microvessels with contact imaging. Respectively, these patterns are likely caused by the pulsatile pressure and pulsatile blood flow. The pulse rates show good agreement with those from pulse oximetry, confirming that the pulsatile signatures reflect pulsatile hemodynamics. CONCLUSIONS: This study demonstrates the potential of speckle decorrelation OCTA for measuring localized peripheral cutaneous pulsatility and defines scanning protocols necessary to undertake such measurements. Noncontact imaging should be used for the study of pulsatility in vessel-free tissue and contact imaging with strong mechanical coupling in individual microvessels. Further studies of microcirculation based upon this method and protocols are warranted.

Influence of Hemodialysis Catheter Insertion on Hemodynamics in the Central Veins.

Central venous catheter (CVC) related thrombosis is a major cause of CVC dysfunction in patients under hemodialysis. The aim of our study was to investigate the impact of CVC insertion on hemodynamics in the central veins and to examine the changes in hemodynamic environments that may be related to thrombus formation due to the implantation of CVC. Patient-specific models of the central veins with and without CVC were reconstructed based on computed tomography images. Flow patterns in the veins were numerically simulated to obtain hemodynamic parameters such as time-averaged wall shear stress (TAWSS), oscillating shear index (OSI), relative residence time (RRT), and normalized transverse wall shear stress (transWSS) under pulsatile flow. The non-Newtonian effects of blood flow were also analyzed using the Casson model. The insertion of CVC caused significant changes in the hemodynamic environment in the central veins. A greater disturbance and increase of velocity were observed in the central veins after the insertion of CVC. As a result, TAWSS and transWSS were markedly increased, but most parts of OSI and RRT decreased. Newtonian assumption of blood flow would overestimate the increase in TAWSS after CVC insertion. High wall shear stress (WSS) and flow disturbance, especially the multidirectionality of the flow, induced by the CVC may be a key factor in initiating thrombosis after CVC insertion. Accordingly, approaches to decrease the flow disturbance during CVC insertion may help restrain the occurrence of thrombosis. More case studies with pre-operative and postoperative modeling and clinical follow-up need to be performed to verify these findings. Non-Newtonian blood flow assumption is recommended in computational fluid dynamics (CFD) simulations of veins with CVCs.

Numerical investigation of unsteady pulsatile Newtonian/non-Newtonian blood flow through curved stenosed arteries.

A numerical investigation of Newtonian/non-Newtonian unsteady pulsatile entry blood flow inside a 3D curved stenosed artery is presented. For considering the non-Newtonian effect (shear thinning or shear thickening behavior), the blood viscosity is characterized by the power-law model (Ostwald de Waele Equation). At the inlet of the artery, a realistic pulsatile waveform is utilized according to the experimental data reported by other researchers. This study belongs to the analysis of the curvature ratios, percentage and length ratio of stenosis, and blood thickening on hemodynamic characteristics of the flow. The results emphasize that the maximum wall shear stress happens near the stenosis neck and as expected, by decreasing the stenosis length, the maximum value of wall shear stress increases. In addition, the results indicate that the shear thickening fluid shows a more stable velocity profile rather than the shear thinning fluid flow.

Characterization of blood flow patterns and endothelial shear stress during flow-mediated dilation.

INTRODUCTION: Endothelial dysfunction is considered the first step in the development of atherosclerosis. Flow-mediated dilation (FMD) has been the most common assessment of endothelial function in research but it has failed in obtaining a widespread use in clinical settings due to a lack of standardization and a large inter-subject variability. Normalization of FMD to endothelial shear stress (ESS) has been proposed to solve its technical limitations. However, studies have not considered the characteristic of the blood flow during FMD under pulsatile conditions in their ESS estimations. METHODS: A total of 26 young healthy subjects (15 females and 11 males) underwent FMD testing. Microhematocrit measurement was used to determine blood viscosity (µ). ESS was calculated by Womersley's approximation, ESS = µ*2K*Velocity/Diameter, where K is a function of Womersley's parameter (α). Blood flow patterns were determined by critical Reynolds number. Statistical analysis included repeated measures ANOVA to detect ESS differences during FMD until peak dilation. Significance was established at P≤0.05. RESULTS: The mean (SD) FMD% and time to peak dilation were 7·4 (3·1) % and 35 (9·3) seconds, respectively. ESS was significantly reduced during FMD until peak dilation (P<0·001). Turbulent blood flow was the only pattern observed until peak dilation in 96·15% of the sample. CONCLUSION: Peak FMD dilation in a young healthy population is triggered mostly by high-ESS under turbulent flow conditions. Due to the pulsatile nature of blood flow and the appearance of a turbulent pattern during FMD, ESS should be estimated by Womersley's approximation rather than Poiseuille's law.

From Experiments to Simulation: Shear-Induced Responses of Red Blood Cells to Different Oxygen Saturation Levels.

Red blood cells (RBC) carry and deliver oxygen (O2) to peripheral tissues through different microcirculatory regions where they are exposed to various levels of shear stress (SS). O2 affinity of hemoglobin (Hb) decreases as the blood enters the microcirculation. This phenomenon determines Hb interactions with RBC membrane proteins that can further regulate the structure of cytoskeleton and affect the mechanical properties of cells. The goal of this study is to evaluate shear-induced RBC deformability and simulate RBC dynamics in blood flow under oxygenated and deoxygenated conditions. Venous blood samples from healthy donors were oxygenated with ambient air or deoxygenated with 100% nitrogen gas for 10 min and immediately applied into an ektacytometer (LORRCA). RBC deformability was measured before and after the application of continuous 5 Pa SS for 300 s by LORRCA and recorded as elongation index (EI) values. A computational model was generated for the simulation of blood flow in a real carotid artery section. EI distribution throughout the artery and its relationships with velocity, pressure, wall SS and viscosity were determined by computational tools. RBC deformability significantly increased in deoxygenation compared to oxygenated state both before and after 5 Pa SS implementation (p < 0.0001). However, EI values after continuous SS were not significant at higher SS levels (>5.15 Pa) in deoxygenated condition. Simulation results revealed that the velocity gradient dominates the generation of SS and the shear thinning effect of blood has a minor effect on it. Distribution of EI was calculated during oxygenation/deoxygenation which is 5-10 times higher around the vessel wall compared to the center of the lumen for sections of the pulsatile flow profile. The extent of RBC deformability increases as RBCs approach to the vessel wall in a real 3D artery model and this increment is higher for deoxygenated condition compared to the oxygenated state. Hypoxia significantly increases shear-induced RBC deformability. RBCs could regulate their own mechanical properties in blood flow by increasing their deformability in hypoxic conditions. Computational tools can be applied for defining hypoxia-mediated RBC deformability changes to monitor blood flow in hypoxic tissues.

Non-pulsatile blood flow is associated with enhanced cerebrovascular carbon dioxide reactivity and an attenuated relationship between cerebral blood flow and regional brain oxygenation.

BACKGROUND: Systemic blood flow in patients on extracorporeal assist devices is frequently not or only minimally pulsatile. Loss of pulsatile brain perfusion, however, has been implicated in neurological complications. Furthermore, the adverse effects of absent pulsatility on the cerebral microcirculation are modulated similarly as CO2 vasoreactivity in resistance vessels. During support with an extracorporeal assist device swings in arterial carbon dioxide partial pressures (PaCO2) that determine cerebral oxygen delivery are not uncommon-especially when CO2 is eliminated by the respirator as well as via the gas exchanger of an extracorporeal membrane oxygenation machine. We, therefore, investigated whether non-pulsatile flow affects cerebrovascular CO2 reactivity (CVR) and regional brain oxygenation (rSO2). METHODS: In this prospective, single-centre case-control trial, we studied 32 patients undergoing elective cardiac surgery. Blood flow velocity in the middle cerebral artery (MCAv) as well as rSO2 was determined during step changes of PaCO2 between 30, 40, and 50 mmHg. Measurements were conducted on cardiopulmonary bypass during non-pulsatile and postoperatively under pulsatile blood flow at comparable test conditions. Corresponding changes of CVR and concomitant rSO2 alterations were determined for each flow mode. Each patient served as her own control. RESULTS: MCAv was generally lower during hypocapnia than during normocapnia and hypercapnia (p < 0.0001). However, the MCAv/PaCO2 slope during non-pulsatile flow was 14.4 cm/s/mmHg [CI 11.8-16.9] and 10.4 cm/s/mmHg [CI 7.9-13.0] after return of pulsatility (p = 0.03). During hypocapnia, non-pulsatile CVR (4.3 ± 1.7%/mmHg) was higher than pulsatile CVR (3.1 ± 1.3%/mmHg, p = 0.01). Independent of the flow mode, we observed a decline in rSO2 during hypocapnia and a corresponding rise during hypercapnia (p < 0.0001). However, the relationship between ΔrSO2 and ΔMCAv was less pronounced during non-pulsatile flow. CONCLUSIONS: Non-pulsatile perfusion is associated with enhanced cerebrovascular CVR resulting in greater relative decreases of cerebral blood flow during hypocapnia. Heterogenic microvascular perfusion may account for the attenuated ΔrSO2/ΔMCAv slope. Potential hazards related to this altered regulation of cerebral perfusion still need to be assessed. TRIAL REGISTRATION: The study was retrospectively registered on October 30, 2018, with Clinical Trial.gov (NCT03732651).

Effects of the arterial radius and the center-line velocity on the conductivity and electrical impedance of pulsatile flow in the human common carotid artery.

In order to investigate the contribution of arterial radius and center-line velocity to the blood conductivity and electrical impedance of pulsatile flow in the human common carotid artery, we proposed three simplified mathematical models to describe the relationship between the center-line velocity, the arterial radius, and the blood conductivity. By comparing the fitting results with those obtained from our previously proposed elastic-tube hemodynamic model, we found that the change in center-line velocity had more notable effect on the blood conductivity than the change in arterial radius. Moreover, the change in arterial radius contributed much more to the electrical impedance than the change in blood conductivity induced by the center-line velocity. Graphical abstract Graphical abstract contains poor-quality text inside the artwork. Please do not re-use the file that we have rejected or attempt to increase its resolution and re-save. It is originally poor; therefore, increasing the resolution will not solve the quality problem. We suggest that you provide us the original format. We prefer replacement figures containing vector/editable objects rather than embedded images. Preferred file formats are eps, ai, tiff and pdf.The new graphical abstract have been attached. ᅟ.

A hybrid Windkessel-Womersley model for blood flow in arteries.

A hybrid Windkessel-Womersley (WK-W) coupled mathematical model for the study of pulsatile blood flow in the arterial system is proposed in this article. The model consists of the Windkessel-type proximal and distal compartments connected by a tube to represent the aorta. The blood flow in the aorta is described by the Womersley solution of the simplified Navier-Stokes equations. In addition, we defined a 6-elements Windkessel model (WK6) in which the blood flow in the connecting tube is modeled by the one-dimensional unsteady Bernoulli equation. Both models have been applied and validated using several aortic pressure and flow rate data acquired from different species such as, humans, dogs and pigs. The results have shown that, both models were able to accurately reconstruct arterial input impedance, however, only the WK-W model was able to calculate the radial distribution of the axial velocity in the aorta and consequently the model predicts the time-varying wall shear stress, and frictional pressure drop during the cardiac cycle more accurately. Additionally, the hybrid WK-W model has the capability to predict the pulsed wave velocity, which is also not possible to obtain when using the classical Windkessel models. Moreover, the values of WK-W model parameters have found to fall in the physiologically realistic range of values, therefore it seems that this hybrid model shows a great potential to be used in clinical practice, as well as in the basic cardiovascular mechanics research.

Cerebrovascular Compliance Within the Rigid Confines of the Skull.

Pulsatile blood flow is generally mediated by the compliance of blood vessels whereby they distend locally and momentarily to accommodate the passage of the pressure wave. This freedom of the blood vessels to exercise their compliance may be suppressed within the confines of the rigid skull. The effect of this on the mechanics of pulsatile blood flow within the cerebral circulation is not known, and the situation is compounded by experimental access difficulties. We present an approach which we have developed to overcome these difficulties in a study of the mechanics of pulsatile cerebral blood flow. The main finding is that while the innate compliance of cerebral vessels is indeed suppressed within the confines of the skull, this is compensated somewhat by compliance provided by other "extravascular" elements within the skull. The net result is what we have termed "intracranial compliance," which we argue is more pertinent to the mechanics of pulsatile cerebral blood flow than is intracranial pressure.

Relationship between Aortic Compliance and Impact of Cerebral Blood Flow Fluctuation to Dynamic Orthostatic Challenge in Endurance Athletes.

Aorta effectively buffers cardiac pulsatile fluctuation generated from the left ventricular (LV) which could be a mechanical force to high blood flow and low-resistance end-organs such as the brain. A dynamic orthostatic challenge may evoke substantial cardiac pulsatile fluctuation via the transient increases in venous return and stroke volume (SV). Particularly, this response may be greater in endurance-trained athletes (ET) who exhibit LV eccentric remodeling. The aim of this study was to determine the contribution of aortic compliance to the response of cerebral blood flow fluctuation to dynamic orthostatic challenge in ET and age-matched sedentary (SED) young healthy men. ET (n = 10) and SED (n = 10) underwent lower body negative pressure (LBNP) (-30 mmHg for 4 min) stimulation and release the pressure that initiates a rapid regain of limited venous return and consequent increase in SV. The recovery responses of central and middle cerebral arterial (MCA) hemodynamics from the release of LBNP (~15 s) were evaluated. SV (via Modeflow method) and pulsatile and systolic MCA (via transcranial Doppler) normalized by mean MCA velocity (MCAv) significantly increased after the cessation of LBNP in both groups. ET exhibited the higher ratio of SV to aortic pulse pressure (SV/AoPP), an index of aortic compliance, at the baseline compared with SED (P < 0.01). Following the LBNP release, SV was significantly increased in SED by 14 ± 7% (mean ± SD) and more in ET by 30 ± 15%; nevertheless, normalized pulsatile, systolic, and diastolic MCAv remained constant in both groups. These results might be attributed to the concomitant with the increase in aortic compliance assessed by SV/AoPP. Importantly, the increase in SV/AoPP following the LBNP release was greater in ET than in SED (P < 0.01), and significantly correlated with the baseline SV/AoPP (r = 0.636, P < 0.01). These results suggest that the aortic compliance in the endurance athletes is able to accommodate the additional SV and buffer the potential increase in pulsatility at end-organs such as the brain.

Effects of Pulsatile Blood Flow on Oxygenator Performance.

Extracorporeal membrane oxygenation (ECMO) is mainly used for the therapy of acute respiratory distress syndrome and chronic obstructive lung disease. In the last years, the development of these systems underwent huge steps in optimization, but there are still problems with thrombus formation, clogging, and thus insufficient gas exchange. One idea of ECMO optimization is a pulsatile blood flow through the oxygenator, but this is still a controversy discussion. Analyzing available publications, it was not possible to identify a general statement about the effect of pulsatile blood flow on the gas exchange performance. The variety of parameters and circuit components have such a high influence on the outcome that a direct comparison of the studies is difficult. For this reason, we performed a structured study to evaluate the effects of pulsatile blood flow on the gas exchange performance of oxygenator. In in vitro tests according to DIN EN ISO 7199, we tested a small oxygenator (0.25 m2 exchange surface, polymethylpentene fibers, 33 mL priming volume) with constant and pulsatile blood flow in comparison. Therefore, we varied the mean blood flow from 250 to 1200 mL/min, the amplitude of 0, 20, and 50%, and the frequency of 30, 60, and 90 bpm. The results demonstrate that the gas transfer for pulsatile and constant blood flow was similar (oxygen: 36-64 mLO2 /LBlood ; carbon dioxide 35-80 mLCO2 /LBlood ) for the same mean blood flow ranges. Over all, the results and analyses showed a statistically nonsignificant difference between pulsatile and nonpulsatile flow. Consequently, we conclude that the implementation of pulsatile blood flow has only a small to no effect on the gas exchange performance in an oxygenator. As the results were obtained using an oxygenator with a coiled fiber bundle, the test must be verified for a stacked fiber oxygenator.