Pressure-flow Relationship Research Articles

Arterial Properties and Ventricular-Arterial Interactions in Severe Aortic Stenosis: Impact on Prognosis.

Systemic arterial properties contribute to clinical heterogeneity and outcomes in degenerative calcific aortic stenosis (AS). Lumped parameters of afterload have previously been associated with adverse left ventricular remodeling, mortality, and poor exercise tolerance in this population, but most studies did not assess pulsatile aortic pressure-flow relations, the gold standard method for assessing arterial load. Moreover, arterial compliance is highly dependent on non-pulsatile (mean) arterial pressure, which influences prognosis in this population. We retrospectively studied 135 patients with severe AS with same-day catheterization and echocardiogram. Invasive aortic pressures and echocardiographic flow waveforms were used to assess pressure-flow and pressure-volume relations using Windkessel modeling and wave separation analyses. We used Cox regression to assess the relationship between pulsatile load and time to death and heart failure hospital admission (DHFA). Total arterial compliance accounting for pressure-dependence (PD-TAC) was independently predictive of all-cause mortality (HR=0.80, 95%CI=0.66-0.97; p=0.023) and DHFA (HR=0.70; 95%CI=0.50-0.97; p=0.031) even after adjustment for age, race, gender, BMI, and comorbidities, while other arterial parameters were not. In patients with severe AS, pressure-dependent arterial compliance predicts adverse outcomes, while traditional pulsatile arterial load measures do not. Our findings suggest that methods accounting for pressure load on the arterial wall are advantageous in this population in which lower mean pressure can result from severe stenosis and ventricular dysfunction.

Randomized Crossover Trial of 2-Week Ketone Ester Treatment in Patients With Type 2 Diabetes and Heart Failure With Preserved Ejection Fraction.

Heart failure with preserved ejection fraction (HFpEF) is a major cause of morbidity and mortality in patients with type 2 diabetes (T2D). Acute increases in circulating levels of ketone body 3-hydroxybutyrate have beneficial acute hemodynamic effects in patients without T2D with chronic heart failure with reduced ejection fraction. However, the cardiovascular effects of prolonged oral ketone ester (KE) treatment in patients with T2D and HFpEF remain unknown. A total of 24 patients with T2D and HFpEF completed a 6-week randomized, double-blind crossover study. All patients received 2 weeks of KE treatment (25 g D-ß-hydroxybutyrate-(R)-1,3-butanediol × 4 daily) and isocaloric and isovolumic placebo, separated by a 2-week washout period. At the end of each treatment period, patients underwent right heart catheterization, echocardiography, and blood samples at trough levels of intervention, and then during a 4-hour resting period after a single dose. A subsequent second dose was administered, followed by an exercise test. The primary end point was cardiac output during the 4-hour rest period. During the 4-hour resting period, circulating 3-hydroxybutyrate levels were 10-fold higher after KE treatment (1010±56 µmol/L; P<0.001) compared with placebo (91±55 µmol/L). Compared with placebo, KE treatment increased cardiac output by 0.2 L/min (95% CI, 0.1 to 0.3) during the 4-hour period and decreased pulmonary capillary wedge pressure at rest by 1 mm Hg (95% CI, -2 to 0) and at peak exercise by 5 mm Hg (95% CI, -9 to -1). KE treatment decreased the pressure-flow relationship (∆ pulmonary capillary wedge pressure/∆ cardiac output) significantly during exercise (P<0.001) and increased stroke volume by 10 mL (95% CI, 0 to 20) at peak exercise. KE right-shifted the left ventricular end-diastolic pressure-volume relationship, suggestive of reduced left ventricular stiffness and improved compliance. Favorable hemodynamic responses of KE treatment were also observed in patients treated with sodium-glucose transporter-2 inhibitors and glucagon-like peptide-1 analogs. In patients with T2D and HFpEF, a 2-week oral KE treatment increased cardiac output and reduced cardiac filling pressures and ventricular stiffness. At peak exercise, KE treatment markedly decreased pulmonary capillary wedge pressure and improved pressure-flow relationship. Modulation of circulating ketone levels is a potential new treatment modality for patients with T2D and HFpEF. URL: https://www.clinicaltrials.gov; Unique Identifier: NCT05236335.

Phrenic Ampulla Emptying Dysfunction in Patients with Esophageal Symptoms.

Pharyngeal pump, esophageal peristalsis, and phrenic ampulla emptying play important roles in the propulsion of bolus from the mouth to the stomach. There is limited information available on the mechanism of normal and abnormal phrenic ampulla emptying. The goal of our study is to describe the relationship between bolus flow and esophageal pressure profiles during the phrenic ampulla emptying in normal subjects and patient with phrenic ampulla dysfunction. Pressure (using topography) and bolus flow (using changes in impedance) relationship through the esophagus and phrenic ampulla were determined in 15 normal subjects and 15 patients with retrograde escape of bolus from the phrenic ampulla into esophagus during primary peristalsis. During the phrenic ampulla phase, 2 high pressure peaks (proximal, related to lower esophageal sphincter and distal, related to crural diaphragm) were observed in normal subjects and patients during the phrenic ampulla emptying phase. The proximal was always higher than the distal one in normal subjects; in contrast, reverse was the case in patients with the retrograde escape of bolus from the phrenic ampulla into the esophagus. We propose that a strong after-contraction of the lower esophageal sphincter plays an important role in the normal phrenic ampullary emptying. A defective lower esophageal after-contraction, along with high crural diaphragm pressure are responsible for the phrenic ampulla emptying dysfunction.

Exploring the Noninvasive Evaluation of the Pulmonary Pressure-Flow Relationship During Exercise.

Exploring the Noninvasive Evaluation of the Pulmonary Pressure-Flow Relationship During Exercise.

Efficient uncertainty quantification in a spatially multiscale model of pulmonary arterial and venous hemodynamics

Pulmonary hypertension (PH) is a debilitating disease that alters the structure and function of both the proximal and distal pulmonary vasculature. This alters pressure-flow relationships in the pulmonary arterial and venous trees, though there is a critical knowledge gap in the relationships between proximal and distal hemodynamics in disease. Multiscale computational models enable simulations in both the proximal and distal vasculature. However, model inputs and measured data are inherently uncertain, requiring a full analysis of the sensitivity and uncertainty of the model. Thus, this study quantifies model sensitivity and output uncertainty in a spatially multiscale, pulse-wave propagation model of pulmonary hemodynamics. The model includes fifteen proximal arteries and twelve proximal veins, connected by a two-sided, structured tree model of the distal vasculature. We use polynomial chaos expansions to expedite sensitivity and uncertainty quantification analyses and provide results for both the proximal and distal vasculature. We quantify uncertainty in blood pressure, blood flow rate, wave intensity, wall shear stress, and cyclic stretch. The latter two are important stimuli for endothelial cell mechanotransduction. We conclude that, while nearly all the parameters in our system have some influence on model predictions, the parameters describing the density of the microvascular beds have the largest effects on all simulated quantities in both the proximal and distal arterial and venous circulations.

Exercise Hemodynamics and Mitochondrial Oxidative Capacity in Disease Stages of Wild-Type Transthyretin Amyloid Cardiomyopathy.

Wild-type transthyretin amyloid (ATTRwt) cardiomyopathy is increasingly recognized in the development of heart failure. The link between cardiac performance, hemodynamics, and mitochondrial function in disease stages of ATTRwt has not previously been studied but may provide new insights into the pathophysiology and clinical performance of the patients. The study investigated 47 patients diagnosed with ATTRwt at Aarhus University Hospital, Denmark. Patients were stratified according to the disease stages of the National Amyloidosis Centre (NAC) as NAC I with low levels of NT-proBNP (N-terminal pro-B-type natriuretic peptide) (NAC I-L, n=14), NAC I with high levels NT-proBNP (NAC I-H, n=20), and NAC II-III (n=13). Exercise testing with simultaneous right heart catheterization was performed in all patients. Endomyocardial biopsies were collected from the patients and the mitochondrial oxidative phosphorylation capacity was assessed. All NAC disease groups, even in the NAC I-L group, a significant abnormal increase in biventricular filling pressures were noted during exercise while the filling pressures was normal or near normal at rest. The inotropic response to exercise was reduced with diminished increase in cardiac output which was significantly more pronounced in the NAC I-H (Diff. -2.4, 95% CI (-4.2: -0.7), P=0.00) and the NAC II-III group (Diff: -3.1 L/min, 95% CI (-5.2: -1.1), P=0.00) compared with the NAC I-L group. The pulmonary artery wedge pressure to cardiac output ratio at peak exercise was significantly different between NAC I-L and NAC II-III (Diff: 1.6 mm Hg*min/L, 95% CI (0.01:3.3, P=0.04)). Patients with ATTRwt had a reduced oxidative phosphorylation capacity which correlated to left ventricular mass but not to cardiac output capacity. An abnormal restrictive left ventricle and right ventricle response to exercise was demonstrated, even present in patients with early-stage ATTRwt. In more advanced disease stages a progressive impairment of the pressure-flow relationship was noted. The myocyte energetics is deranged but not associated to the contractile reserve or restrictive filling characteristics in ATTRwt.

Rapid and Precise Computation of Fractional Flow Reserve from Routine Two-Dimensional Coronary Angiograms Based on Fluid Mechanics: The Pilot FFR2D Study.

Objective: To present a novel pipeline for rapid and precise computation of fractional flow reserve from an analysis of routine two-dimensional coronary angiograms based on fluid mechanics equations (FFR2D). Material and methods: This was a pilot analytical study that was designed to assess the diagnostic performance of FFR2D versus the gold standard of FFR (threshold ≤ 0.80) measured with a pressure wire for the physiological assessment of intermediate coronary artery stenoses. In a single academic center, consecutive patients referred for diagnostic coronary angiography and potential revascularization between 1 September 2020 and 1 September 2022 were screened for eligibility. Routine two-dimensional angiograms at optimal viewing angles with minimal overlap and/or foreshortening were segmented semi-automatically to derive the vascular geometry of intermediate coronary lesions, and nonlinear pressure-flow mathematical relationships were applied to compute FFR2D. Results: Some 88 consecutive patients with a single intermediate coronary artery lesion were analyzed (LAD n = 74, RCA n = 9 and LCX n = 5; percent diameter stenosis of 45.7 ± 11.0%). The computed FFR2D was on average 0.821 ± 0.048 and correlated well with invasive FFR (r = 0.68, p < 0.001). There was very good agreement between FFR2D and invasive-wire FFR with minimal measurement bias (mean difference: 0.000 ± 0.048). The overall accuracy of FFR2D for diagnosing a critical epicardial artery stenosis was 90.9% (80 cases classified correctly out of 88 in total). FFR2D identified 24 true positives, 56 true negatives, 4 false positives, and 4 false negatives and predicted FFR ≤ 0.80 with a sensitivity of 85.7%, specificity of 93.3%, positive likelihood ratio of 13.0, and negative likelihood ratio of 0.15. FFR2D had a significantly better discriminatory capacity (area under the ROC curve: 0.95 [95% CI: 0.91-0.99]) compared to 50%DS on 2D-QCA (area under the ROC curve: 0.70 [95% CI: 0.59-0.82]; p = 0.0001) in predicting wire FFR ≤ 0.80. The median time of image analysis was 2 min and the median time of computation of the FFR2D results was 0.1 s. Conclusion: FFR2D may rapidly derive a precise image-based metric of fractional flow reserve with high diagnostic accuracy based on a single two-dimensional coronary angiogram.

The physiological basis of renal nuclear medicine.

Renal physiology underpins renal nuclear medicine, both academic and clinical. Clearance, an important concept in renal physiology, comprises tissue uptake rate of tracer (tissue clearance), disappearance rate from plasma (plasma clearance), appearance rate in urine (urinary clearance) and disappearance rate from tissue. In clinical research, steady-state plasma clearances of para-amino-hippurate and inulin have been widely used to measure renal blood flow (RBF) and glomerular filtration rate (GFR), respectively. Routinely, GFR is measured at non-steady state as plasma clearance of a filtration agent, such as technetium-99m diethylenetriaminepentaacetic acid. Scaled to three-dimensional whole body metrics rather than body surface area, GFR in women is higher than in men but declines faster with age. Age-related decline is predominantly from nephron loss. Tubular function determines parenchymal transit time, which is important in renography, and the route of uptake of technetium-99m dimercaptosuccinic acid, which is via filtration. Resistance to flow is defined according to the pressure-flow relationship but in renography, only transit time can be measured, which, being equal to urine flow divided by collecting system volume, introduces further uncertainty because the volume is also unmeasurable. Tubuloglomerular feedback governs RBF and GFR, is regulated by the macula densa, mediated by adenosine and renin, and can be manipulated with proximal tubular sodium-glucose cotransporter-2 inhibitors. Other determinants of renal haemodynamics include prostaglandins, nitric oxide and dopamine, while protein meal and amino acid infusion are used to measure renal functional reserve. In conclusion, for measuring renal responses to exogenous agents, steady-state para-amino-hippurate and inulin clearances should be replaced with rubidium-82 and gallium-68 EDTA for measuring RBF and GFR.

Constrictive versus compressive bladder outflow obstruction in men: Does it matter?

Bladder outflow obstruction (BOO) is a urethral resistance (UR) at a level above a clinically relevant threshold. UR is currently graded in terms of the existence and severity of the BOO based on maximum flowrate and associated detrusor pressure only. However, the pressure-flow relation throughout the course of voiding includes additional information that may be relevant to identify the type of BOO. This study introduces a new method for the distinction between the provisionally called compressive and constrictive types of BOO and relates this classification to underlying patient and urodynamic differences between those BOO types. In total, 593 high-quality urodynamic pressure-flow studies in men were included in this study. Constrictive BOO was identified if the difference Δp between the actual minimal urethral opening pressure (pmuo) and the expected pmuo according to the linearized passive urethral resistance relation (linPURR) nomogram was >25 cmH2O. Compressive BOO is identified in the complementary case where the pressure difference Δp ≤ 25 cmH2O. Differences in urodynamic parameters, patient age, and prostate size were explored. In 81 (13.7%) of the cases, constrictive BOO was found. In these patients, the prostate size was significantly smaller when compared to patients diagnosed with compressive BOO, while displaying a significantly lower maximum flowrate, higher detrusor pressure at maximal flowrate and more postvoid residual (PVR). This study is an initial step in the validation of additional subtyping of BOO. We found significant differences in prostate size, severity of BOO, and PVR, between patients with compressive and constrictive BOO. Subtyping of voiding-outflow dynamics may lead to more individualized management in patients with BOO.

In vivo fluid dynamics of the Ventura interatrial shunt device in patients with heart failure.

Interatrial shunts are under evaluation as a treatment for heart failure (HF); however, their in vivo flow performance has not been quantitatively studied. We aimed to investigate the fluid dynamics properties of the 0.51cm orifice diameter Ventura shunt and assess its lumen integrity with serial transesophageal echocardiography (TEE). Computational fluid dynamics (CFD) and bench flow tests were used to establish the flow-pressure relationship of the shunt. Open-label patients from the RELIEVE-HF trial underwent TEE at shunt implant and at 6 and 12month follow-up. Shunt effective diameter (Deff) was derived from the vena contracta, and flow was determined by the continuity equation. CFD and bench studies independently validated that the shunt's discharge coefficient was 0.88 to 0.89. The device was successfully implanted in all 97 enrolled patients; mean age was 70±11years, 97% were NYHA class III, and 51% had LVEF ≤40%. Patency was confirmed in all instances, except for one stenotic shunt at 6months. Deff remained unchanged from baseline at 12months (0.47±0.01cm, P=0.376), as did the trans-shunt mean pressure gradient (5.1±3.9mmHg, P=0.316) and flow (1137±463mL/min, P=0.384). TEE measured flow versus pressure closely correlated (R2≥0.98) with a fluid dynamics model. At 12months, the pulmonary/systemic flow Qp/Qs ratio was 1.22±0.12. When implanted in patients with advanced HF, this small interatrial shunt demonstrated predictable and durable patency and performance.

Effects of Coronary Stenosis on Coronary Vascular Impedance

Introduction: The impedance analysis for circulation system is a powerful tool to comprehensively measure the afterload of the entire circulation system. The spectrum of vascular mechanical parameters derived from the impedance analysis can reveal the compliance and resistance changes in proximal and distal vessels, as well as the wave reflections. However, the similar investigation in the opposition of the coronary circulation is limited, and it is unknown how coronary stenosis affects the vascular impedance of the coronary circulation. In this study, we aimed to quantify and compare the changes in the vascular impedance of coronary circulation from the dynamic pressure and flow relations in healthy and patients with stenosis. Method: The pressure and flow rate waveforms from the main coronary artery were obtained from a lumped-parameter computational model that coupled the systemic circulation with the coronary perfusion within a closed system. Both healthy and diseased (coronary stenosis) pressure-flow relations were examined. A Fourier transform was performed to analyze the pressure and flow data, and the resistance (Z0), characteristic impedance (ZC), wave reflections (RW), total arterial compliance (C), and other hemodynamic parameters were derived. Results: Compared to the healthy subject, the diseased subject presented an increased resistance (Z0 was elevated from 0.44 mmHg ▪ min/ml to 0.59 mmHg ▪ min/ml), increased characteristic impedance (ZC was elevated from 0.07 mmHg ▪ min/ml to 0.14 mmHg ▪ min/ml), and reduced wave reflection (RW was decreased from 0.72 to 0.6). These changes indicate elevated distal arteries and arterioles narrowing and proximal artery stiffening in the entire coronary vasculature. Moreover, the coronary flow and the total arterial compliance (stroke volume divided by pulse pressure) were reduced with the disease progression. These data suggest a reduced coronary perfusion and stiffening of the artery in the coronary circulation. Discussion: The impedance analysis has been widely used for systemic and pulmonary circulations previously to quantify the total afterload of the left and right ventricles, respectively. The distribution of the impedance in different frequencies reflects the anatomical and mechanical properties of the vasculature in the circulation system. This study originally found that the coronary artery impedance is altered by coronary stenosis. The key characteristics include the narrowing of distal coronary arteries and stiffening of proximal coronary arteries. However, since both proximal and distal arteries are altered, the wave reflection was reduced, and this may explain a reduced pulse pressure observed in the diseased subject. The reduction in overall coronary perfusion indicates an impaired blood supply to the ventricle and may be responsible for the dysfunction of the heart. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Increased left atrial pressure distorts pulmonary arterial impedance estimates in the low-range of frequency in rats

BACKGROUD: Post-capillary pulmonary hypertension (PH), characterized as increased downstream pressure such as left atrial pressure (LAP), is the most prevalent type of PH and a significant contributor to right heart failure. Accurate estimation of pulmonary arterial characteristics, serving as right ventricular afterload, is crucial for management of PH. These characteristics can be captured comprehensively through pulmonary arterial impedance (PAZ), which elucidates pulmonary arterial pressure-flow relationship within the frequency domain. Although LAP waveforms are potential to affect the contours of pulmonary arterial pressure (PAP) and flow (PAF) waveforms, it remains unknown whether increased LAP affects the accuracy of PAZ measurement. This study aimed to investigate the impact of LAP on the accuracy of PAZ using an acute rodent model. METHODS: We measured pulmonary arterial blood flow (PAF), PAP, and LAP simultaneously in eight male Sprague-Dawley rats. PAZ was calculated as a reciprocal of admittance. Admittance was calculated in the following two ways: 1) one-input (PAP) and one-output (PAF) analysis (I1O1 analysis): the ratio of PAP(f) to PAF(f) at each frequency, where PAP(f) and PAF(f) are frequency spectra of PAP and PAF, respectively, and 2) two-input (PAP and LAP) and one-output (PAF) analysis (I2O1 analysis): excluding the potential effect of LAP from PAP and PAF. We measured PAZ under both normal physiological condition and post-capillary PH condition. To induce acute post-capillary PH by raising LAP &gt;15 mmHg, we ligated aortic arch at the site between brachiocephalic artery and left common carotid artery and performed blood transfusion from a donor rat (TAL/BT). The accuracy of PAZ was examined as the coherence function representing the linearity between the input and output signals. Results: TAL/BT significantly increased LAP from 3 ± 1mmHg to 21 ± 4 mmHg and PAP from 17 ± 3 mmHg to 34 ± 3 mmHg (p&lt;0.01 for both). The I2O1 analysis provided a consistently accurate estimation of PAZ under both normal and elevated LAP conditions with high coherence function (&gt;0.9) in the wide range of frequency between 0.12 and 10.7 Hz. The I1O1 analysis also estimated the PAZ accurately in normal condition with high coherence function (&gt;0.9) in the wide range of frequency, however, the I1O1 analysis overestimated PAZ in the low-range frequency between 0.12 and 1.0 Hz compared with the I2O1 analysis (I1O1: 0.32±0.10 vs. I2O1 0.23±0.06 mmHg/ml/min, p&lt;0.01), with significantly lower coherence function (I1O1: 0.77±0.05 vs. I2O1: 0.96±0.01, p&lt;0.01). CONCLUSION: Increased LAP may have the impact on the accuracy of PAZ estimation in the low-range frequency. The I2O1 analysis can lead to more accurate assessment of right ventricular afterload in post-capillary PH, compared with the I1O1 analysis. Grant-in-Aid for Young Scientists of the Japanese Society of Cardiovascular Anesthesiologists Project Number 2271000048. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

A Condition Monitoring Method of Hydraulic Gear Pumps Based on Multilevel Mechanism-Data Fusion

Pumps are important components in aviation fuel hydraulic systems, and thanks to the development of sensor technology and industrial intelligence technology, it is possible to achieve efficient state monitoring of pumps. However, when data quality is poor or the amount of data is small, a single data-driven model may not be able to meet diagnostic accuracy. A condition monitoring method for hydraulic gear pumps based on mechanism-data fusion is proposed. The method combines a mechanism model based on the volumetric efficiency formula with a data-driven model based on vibration signals. First, the parameters of volumetric efficiency are solved by fitting the pressure–flow relationship. Subsequently, a multichannel fusion and multikernel function-weighted ensemble support vector classification (MCMK-SVC) is developed, to establish a data-driven model. Finally, through data-level fusion, feature-level fusion, and decision-level fusion, a condition monitoring model based on mechanism-data fusion is built. Experimental verification shows that the accuracy of the three levels of fusion models exceeds 96.9%. Compared to the single data-driven model or other traditional data-driven models, the accuracy of the proposed method has improved by 3% to 33%, demonstrating the effectiveness of the mechanism-data fusion model.

Directional sensitivity of the cerebral pressure-flow relationship during forced oscillations induced by oscillatory lower body negative pressure

A directional sensitivity of the cerebral pressure-flow relationship has been described using repeated squat-stands. Oscillatory lower body negative pressure (OLBNP) is a reproducible method to characterize dynamic cerebral autoregulation (dCA). It could represent a safer method to examine the directional sensitivity of the cerebral pressure-flow relationship within clinical populations and/or during pharmaceutical administration. Therefore, examining the cerebral pressure-flow directional sensitivity during an OLBNP-induced cyclic physiological stress is crucial. We calculated changes in middle cerebral artery mean blood velocity (MCAv) per alterations to mean arterial pressure (MAP) to compute ratios adjusted for time intervals (ΔMCAvT/ΔMAPT) with respect to the minimum-to-maximum MCAv and MAP, for each OLBNP transition (0 to −90 Torr), during 0.05 Hz and 0.10 Hz OLBNP. We then compared averaged ΔMCAvT/ΔMAPT during OLBNP-induced MAP increases (INC) (ΔMCAvT/ Δ MAP T INC ) and decreases (DEC) (ΔMCAvT/ Δ MAP T DEC ). Nineteen healthy participants [9 females; 30 ± 6 years] were included. There were no differences in ΔMCAvT/ΔMAPT between INC and DEC at 0.05 Hz. ΔMCAvT/ Δ MAP T INC (1.06 ± 0.35 vs. 1.33 ± 0.60 cm⋅s−1/mmHg; p = 0.0076) was lower than ΔMCAvT/ Δ MAP T DEC at 0.10 Hz. These results support OLBNP as a model to evaluate the directional sensitivity of the cerebral pressure-flow relationship.

The fluidic memristor as a collective phenomenon in elastohydrodynamic networks

Fluid flow networks are ubiquitous and can be found in a broad range of contexts, from human-made systems such as water supply networks to living systems like animal and plant vasculature. In many cases, the elements forming these networks exhibit a highly non-linear pressure-flow relationship. Although we understand how these elements work individually, their collective behavior remains poorly understood. In this work, we combine experiments, theory, and numerical simulations to understand the main mechanisms underlying the collective behavior of soft flow networks with elements that exhibit negative differential resistance. Strikingly, our theoretical analysis and experiments reveal that a minimal network of nonlinear resistors, which we have termed a ‘fluidic memristor’, displays history-dependent resistance. This new class of element can be understood as a collection of hysteresis loops that allows this fluidic system to store information, and it can be directly used as a tunable resistor in fluidic setups. Our results provide insights that can inform other applications of fluid flow networks in soft materials science, biomedical settings, and soft robotics, and may also motivate new understanding of the flow networks involved in animal and plant physiology.

Elastohydrodynamic interactions in soft hydraulic knots

Soft intertwined channel systems are frequently found in fluid flow networks in nature. The passage geometry of these systems can deform due to fluid flow, which can cause the relationship between flow rate and pressure drop to deviate from the Hagen–Poiseuille linear law. Although fluid–structure interactions in single deformable channels have been extensively studied, such as in Starling's resistor and its variations, the flow transport capacity of an intertwined channel with multiple self-intersections (a ‘hydraulic knot’), is still an open question. We present experiments and theory on soft hydraulic knots formed by interlinked microfluidic devices comprising two intersecting channels separated by a thin elastomeric membrane. Our experiments show flow–pressure relationships similar to flow limitation, where the limiting flow rate depends on the knot configuration. To explain our observations, we develop a mathematical model based on lubrication theory coupled with tension-dominated membrane deflections that compares favourably with our experimental data. Finally, we present two potential hydraulic knot applications for microfluidic flow rectification and attenuation.

Letter to the editor: Deriving transfer function analysis metrics from driven methods

Driven and spontaneous methods have been used to quantify the cerebral pressure-flow relationship via transfer function analysis (TFA). Commonly, TFA derived estimates are assessed using band averages within the very-low (0.02–0.07 Hz) and low (0.07–0.20 Hz) frequency during spontaneous oscillations but are quantified at frequencies of interest where blood pressure oscillations are driven (e.g., 0.05 and/or 0.10 Hz). Driven estimates more closely resemble the autoregulatory challenges individuals experience on a daily basis, while also eliciting higher levels of reliability. While driven estimates with point-estimates are not feasible for all clinical populations, these approaches increase the ability to understand pathophysiological changes.

Fully coupled simulation of dynamic characteristics of a backward bent duct buoy oscillating water column wave energy converter

In this paper, a fully coupled model for a backward bent duct buoy oscillating water column (BBDB-OWC) wave energy converter (WEC) is established, with the consideration of strong transient nonlinear interactions among ocean waves, BBDB hull motion, mooring system, internal water column oscillation, aerodynamics, turbine rotation and the damping effect of the generator. The model is validated by comparison with experimental results, and then used to investigate the transient characteristics and periodic average performance of the WEC under regular wave conditions. The differences in transient hydrodynamic characteristics and air pressure-flow rate relationship between the fully coupled model and the orifice model are compared. The transient characteristics of an impulse turbine under reciprocating and transient airflows are studied. And the effect of wave period, wave height and load torque coefficient on the average performance of the WEC is investigated.

ИССЛЕДОВАНИЕ КАВИТАЦИОННОГО ПРОЦЕССА В УЛЬТРАЗВУКОВОМ СМЕСИТЕЛЕ

An innovative way to solve pressing problems of energy saving and reducing environmental pollution during the operation of automotive equipment can be the use of a specially designed ultrasonic motor fuel mixer. The creation of cavitation areas in an ultrasonic mixer promotes the fragmentation of long hydrocarbon chains and thereby improves the quality of fuel (diesel and mixed diesel fuel), reduces fuel consumption, and prolongs the operation of the diesel engine fuel system. Possible modes of ultrasonic treatment were investigated in order to create an optimal cavitation region in the mixer. The establishment of treatment modes was carried out by changing the pressure-flow relationship in a laboratory installation, and the determination of cavitation areas based on the technique of numerical modeling of fuel flows in the structure under study. Numerical modeling made it possible to establish cavitation regions with different acoustic powers. The experimental studies of the hydrodynamic parameters of a real mixer indicate the adequacy of the developed model. 8 modes of ultrasonic exposure are defined depending on the pressure drop; they, in turn, are divided into pre-cavitation and cavitation. The operating pressure range was from 0 to 4 atm with a difference of 0.5 atm for each mode. The maximum reduction in kinematic viscosity (on average 23%) for all types of fuel is observed when the pressure drop reaches 2.5 atm. A further change in the cavitation regime towards an increase in the pressure drop does not lead to a significant change in the kinematic viscosity of motor fuels (about 2%).

Precision flow rate control during micro-scale material extrusion by iterative learning of pressure-flow rate relationships

Despite its low-cost and broad material capability, material extrusion-based additive manufacturing suffers from low process resolution and dimensional accuracy, due to challenges with precisely controlling the ink flow rate during the printing process, particularly while using small nozzles and complex inks. In this paper, we present a novel approach to control the ink flow rate by utilizing (1) a printhead capable of concurrently monitoring and controlling the printing pressure (P) and steady-state flow rate (Q) and (2) an automatic iterative learning approach to elucidate the flow rate-pressure relationships for the system in the form of a Q-P relator model. Our studies showed that flow rate accuracy within 10% can be achieved with nozzles as small as 100 microns in diameter through implementation of the trained Q-P relator with both pressure and piston velocity-controlled printing. Particularly, Q-P relator enables proper selection of printing pressure to achieve desired flow rate during pressure-controlled printing, while providing a means of smart-priming of the system prior to velocity controlled printing to overcome the system transients. We also characterized the performance of the iterative learning process, focusing on time to convergence, effect of steady-state detection, and accuracy variation across various time scales. Presented results indicate that the proposed method will increase the microscale material extrusion process resolution and accuracy, and minimize material waste and reduce time to production.