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At least 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
NASA NTRS2010

Characterizing Methods of Measuring Flow-Mediated Dilation in the Brachial Artery

Regulation of vascular tone is one of the many important functions of the vascular endothelium. Endothelial dysfunction is a critical early event in the pathogenesis of atherosclerosis and occurs in the absence of angiographic disease. Flow-Mediated Dilation (FMD) is a noninvasive technique commonly used to evaluate endothelium-dependent vasodilation in humans and gauge the health of the cardiovascular system. Reductions in brachial artery FMD have been strongly correlated with disease progression and are predictive of future cardiac events. The flow stimulus for brachial artery FMD occurs as a result of the increased shear stress following deflation of an occlusion cuff around the upper arm. Using 2-dimensional ultrasound, changes in arterial diameter up to 5-minutes following cuff deflation are calculated from baseline image measurements. Along with pulsed Doppler measures of flow velocity through the artery, flow-mediated, endothelium-dependent vasodilation can be assessed. There is debate among investigators, however, about the proper positioning of the occlusion cuff during FMD testing. It is thought that placement of the cuff around the upper arm may not accurately reflect the impact of nitric oxide, a critically important molecule released as a result of the increased shear stress created by the FMD technique. Data suggest that the production of other endogenous metabolites may also contribute to FMD-related changes when positioning the cuff around the upper arm. To overcome the potential influence of such molecules, researchers now suggest that the occlusion cuff be placed below the elbow allowing a more precise estimate of nitric oxide mediated dilation. The purpose of this study is to compare the differences in FMD between the two methodologies of occlusion cuff placement. In addition, this study will determine the method that is easier for ultrasound technicians to perform and will produce a low coefficient of variance between technicians. Ultimately, the results of this study will help in tracking any adverse cardiovascular effects of spaceflight.

Callender, Ariane R.↗
NASA NTRS2014

Analysis of Arterial Mechanics During Head-down Tilt Bed Rest

Arterial health may be affected by microgravity or ground based analogs of spaceflight, as shown by an increase in thoracic aorta stiffness1. Head-down tilt bed rest (HDTBR) is often used as a ground-based simulation of spaceflight because it induces physiological changes similar to those that occur in space2, 3. This abstract details an analysis of arterial stiffness (a subclinical measure of atherosclerosis), the distensibility coefficient (DC), and the pressure-strain elastic modulus (PSE) of the arterial walls during HDTBR. This project may help determine how spaceflight differentially affects arterial function in the upper vs. lower body.

Elliot, Morgan↗
DOE OSTI2021

The effect of entrance flow development on vortex formation and wall shear stress in a curved artery model

In this work, we numerically investigate the effect of entrance condition on the spatial and temporal evolution of multiple three-dimensional vortex pairs and the wall shear stress distribution in a curved artery model. We perform this study using a Newtonian blood-analog fluid subjected to a pulsatile flow with two inflow conditions. The first flow condition is fully developed while the second condition is undeveloped (i.e., uniform). We discuss the connection along the axial direction between regions of organized vorticity observed at various cross sections of the model and compare results between the different entrance conditions. We model a human artery with a simple, rigid 180° curved pipe with a circular cross section and constant curvature, neglecting the effects of taper, torsion, and elasticity. Numerical results are computed from a discontinuous high-order spectral element flow solver. The flow rate used in this study is physiological. We observe differences in secondary flow patterns, especially during the deceleration phase of the physiological waveform where multiple vortical structures of both Dean-type and Lyne-type coexist. The results indicate that decreased axial velocities under an undeveloped condition produce smaller secondary flows that ultimately inhibit growth of any interior flow vortices. We highlight the effect of the entrance condition on the formation of these structures and subsequent appearance of abnormal inner wall shear stresses, which suggest there may be a lower prevalence of cardiovascular disease in curved arteries where the flow is rather undeveloped—a potentially physiologically significant result to help understand the influence of blood flow development on disease.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2024

In vitro evidence that the vasorelaxant effects of 2‐nitro‐1‐phenyl‐1‐propanol on rat coronary arteries involve cyclic nucleotide pathways

Abstract The synthetic nitro‐alcohol 2‐nitro‐1‐phenyl‐1‐propanol (NPP) has endothelium‐independent relaxing properties in isolated preparations of rat aorta and mesenteric artery. In this study, we investigated whether the vasodilator effects occur in coronary vessels and explored whether hyperpolarization is involved in the underlying mechanism of NPP‐induced smooth muscle relaxation. The relaxing responses were studied in isolated preparations of the left anterior descending coronary (ADC) and the septal coronary (SC) arteries, which had been previously maintained under sustained contraction induced by the thromboxane A 2 analogue U‐46619. Administered cumulatively, NPP elicited concentration‐dependent vasorelaxation with similar potency in both vessels. The relaxant effect remained unaffected by the nitric oxide synthase inhibitor L‐NAME, the protein kinase C inhibitor bisindolylmaleimide IV and the Rho‐associated protein kinase inhibitor Y‐27632. However, it was significantly diminished by the adenylyl cyclase inhibitor MDL‐12,330A, the guanylyl cyclase inhibitor ODQ, as well as the K + channel inhibitors tetraethylammonium and CsCl. In ADC preparations impaled with intracellular micropipettes, NPP hyperpolarized the vascular preparation. When the isolated preparation was precontracted by 5‐hydroxytryptamine or 80 mM KCl, NPP‐induced relaxation with lower pharmacological potency compared to the vessels contracted by U‐46619. In conclusion, NPP exhibits vasorelaxant effects on rat coronary arteries, likely involving pathways that include cyclic nucleotide production and membrane hyperpolarization.

Vasconcelos‐Silva, Alfredo Augusto↗
NASA NTRS1971

Arterial and grooved cryogenic heat pipes.

Tests were conducted on two cryogenic heat pipes using nitrogen as the working fluid. Both pipes are 1.27 cm in diameter. The first pipe is 141 cm long and uses longitudinal grooves in the aluminum wall as the wick structure. The second pipe is 91 cm long and has an arterial wick adjacent to the wall. In addition, circumferential screw threads are machined along the entire length of the inside pipe wall. Both heat pipes primed with no difficulty in the horizontal position. Start-up after burn-out was obtained repeatedly. However, more testing is required to determine whether the artery pipe is as reliable as the grooved pipe with respect to priming. Once primed, the artery heat pipe is much less sensitive to elevation than the grooved design and, consequently, offers a significant advantage with respect to ground testing. Heat-transfer characteristics of the two pipes were comparable, with both pipes achieving the initial goal of 1000 watt-cm of heat-transport capability.

Brennan, P.↗
NASA NTRS1972

A nonlinear analysis of pulsatile flow in arteries.

An approximate numerical method for calculating flow profiles in arteries is developed. The theory takes into account the nonlinear terms of the Navier-Stokes equations as well as the nonlinear behaviour and large deformations of the arterial wall. Through the locally measured values of the pressure, pressure gradient, and pressure-radius function, the velocity distribution and wall shear at a given location along the artery can be determined. The computed results agree well with the corresponding experimental data.

Ling, S. C.↗
NASA NTRS1974

Investigation of arterial gas occlusions

The effect of noncondensable gases on high-performance arterial heat pipes was investigated both analytically and experimentally. Models have been generated which characterize the dissolution of gases in condensate, and the diffusional loss of dissolved gases from condensate in arterial flow. These processes, and others, were used to postulate stability criteria for arterial heat pipes under isothermal and non-isothermal condensate flow conditions. A rigorous second-order gas-loaded heat pipe model, incorporating axial conduction and one-dimensional vapor transport, was produced and used for thermal and gas studies. A Freon-22 (CHCIF2) heat pipe was used with helium and xenon to validate modeling. With helium, experimental data compared well with theory. Unusual gas-control effects with xenon were attributed to high solubility.

Saaski, E. W.↗
NASA NTRS1975

Arterial gas occlusions in operating heat pipes

The effect of noncondensable gases on high performance arterial heat pipes has been investigated both analytically and experimentally. Models have been generated which characterize the dissolution of gases in condensate and the diffusional loss of dissolved gases from condensate in arterial flow. These processes, and others, have been used to postulate stability criteria for arterial heat pipes. Experimental observations of gas occlusions were made using a stainless steel heat pipe equipped with viewing ports, and the working fluids methanol and ammonia with the gas additives helium, argon, and xenon. Observations were related to gas transport models.

Saaski, E. W.↗
NASA NTRS1976

Role of arterial baroreceptors in mediating cardiovascular response to exercise

Experiments were conducted to define the role of the major arterial baroreceptors during moderately severe exercise by comparing the responses of untethered conscious dogs instrumented for the measurement of aortic pressure and cardiac output with those of dogs with total arterial baroreceptor denervation. The reflex heart rate responses to intravenous bolus doses of methoxamine were also examined in intact animals, both at rest and during exercise. Methoxamine is found to cause striking bradycardia at rest, but little bradycardia during exercise. Experimental findings suggest that the arterial baroreceptor reflex is normally inhibited during severe exercise and therefore plays little role in modulating the cardiovascular response to exercise.

Mcritchie, R. J.↗
NASA NTRS1977

Development of a jet pump-assisted arterial heat pipe

The development of a jet pump assisted arterial heat pipe is described. The concept utilizes a built-in capillary driven jet pump to remove vapor and gas from the artery and to prime it. The continuous pumping action also prevents depriming during operation of the heat pipe. The concept is applicable to fixed conductance and gas loaded variable conductance heat pipes. A theoretical model for the jet pump assisted arterial heat pipe is presented. The model was used to design a prototype for laboratory demonstration. The 1.2 m long heat pipe was designed to transport 500 watts and to prime at an adverse elevation of up to 1.3 cm. The test results were in good agreement with the theoretical predictions. The heat pipe carried as much as 540 watts and was able to prime up to 1.9 cm. Introduction of a considerable amount of noncondensible gas had no adverse effect on the priming capability.

Bienert, W. B.↗
NASA NTRS1978

Influence of central venous pressure upon sinus node responses to arterial baroreflex stimulation in man

Measurements were made of sinus node responses to arterial baroreceptor stimulation with phenylephrine injection or neck suction, before and during changes of central venous pressure provoked by lower body negative pressure or leg and lower truck elevation. Variations of central venous pressure between 1.1 and 9.0 mm Hg did not influence arterial baroreflex mediated bradycardia. Baroreflex sinus node responses were augmented by intravenous propranolol, but the level of responses after propranolol was comparable during the control state, lower body negative pressure, and leg and trunk elevation. Sinus node responses to very brief baroreceptor stimuli applied during the transitions of central venous pressure also were comparable in the three states. The authors conclude that physiological variations of central venous pressure do not influence sinus node responses to arterial baroreceptor stimulation in man.

Mark, A. L.↗
NASA NTRS1980

Effects of hypokinesia and hypodynamia on the intraorganic arteries of the heart

The experiments were performed on rabbits kept in small cages for 2-12 weeks. Their motor activity was greatly restricted. The arteries of the heart were injected with an India ink-gelatine mass; injection was directly into the coronary arteries. After fixation in 10% formaline solution 120 micron sections were cut. These were cleared by the Malygin method and examined with a light microscope. After exposure to hypokinetic conditions the picture of the cardiac arterial bed changed. During the first weeks the capillaries of the myocardium were significantly constricted, their course becoming sinuous. With prolonged exposure to hypokinetic conditions the vessels were found to dilate gradually and myocardial architectonics changed.

Aleksina, L. A.↗
NASA NTRS1980

Depriming of arterial heat pipes: An investigation of CTS thermal excursions

Four thermal excursions of the Transmitter Experiment Package (TEP) were the result of the depriming of the arteries in all three heat pipes in the Variable Conductance Heat Pipe System which cooled the TEP. The determined cause of the depriming of the heat pipes was the formation of bubbles of the nitrogen/helium control gas mixture in the arteries during the thaw portion of a freeze/thaw cycle of the inactive region of the condenser section of the heat pipe. Conditions such as suction freezeout or heat pipe turn-on, which moved these bubbles into the active region of the heat pipe, contributed to the depriming mechanism. Methods for precluding, or reducing the probability of, this type of failure mechanism in future applications of arterial heat pipes are included.

Antoniuk, D.↗
NASA NTRS1993

Acceleration induced depriming and capillary rewetting of external artery heat pipes - Comparison with SHARE-II flight experiment

A combined analytical and numerical model for the analysis of the deprime and reprime/rewetting characteristics of two high-capacity external artery heat pipe designs undergoing externally induced accelerations was developed using several previously derived analytical expressions. Three distinct phases of the deprime and reprime/rewetting process were analyzed: (1) the effect of longitudinal accelerations on the depriming, (2) the time required for repriming of the liquid artery once the longitudinal acceleration has been terminated, and (3) the rewetting characteristics of the circumferential wall grooves. Combining the three processes, a technique was developed allowing the prediction of the effect of external acceleration on the characteristics of the external artery heat pipes. The predictions made with this technique agreed well with the microgravity flight results.

Ochterbeck, J. M.↗
NASA NTRS2003

Summary of Research Adaptions of Visceral and Cerebral Resistance Arteries to Simulated Microgravity

The proposed studies were designed address the effects of simulated microgravity on vascular smooth muscle and endothelial cell function in resistance arteries isolated from visceral tissues (spleen, mesentery and kidneys) and cerebrum. Alterations in vascular function induced by microgravity are particularly relevant to the problems of orthostatic intolerance and reduced exercise capacity experienced by astronauts upon re-entry into the earth's gravitational field. Decrements in contractile function or enhanced vasodilatory responsiveness of peripheral resistance arteries could lead to decreased peripheral resistance and orthostatic hypotension. Alternatively, augmentation of contractile function in cerebral resistance arteries could lead to increased cerebral vascular resistance and diminished perfusion of the brain. The Specific Aims and hypotheses were proposed in this grant. Following each of the Specific Aims, progress toward addressing that specific aim is presented. With the exception of Specific Aim VI (see aim for details), all aims have been experimentally addressed as proposed. The final six months of the granting period will be used for manuscript preparation; manuscripts in preparation will contain results from Specific Aims I-IV. Results from Specific Aims V and VI have been published.

Delp, Michael↗
NASA NTRS2001

Sequential modulation of cardiac autonomic control induced by cardiopulmonary and arterial baroreflex mechanisms

BACKGROUND: Nonhypotensive lower body negative pressure (LBNP) induces a reflex increase in forearm vascular resistance and muscle sympathetic neural discharge without affecting mean heart rate. We tested the hypothesis that a reflex change of the autonomic modulation of heartbeat might arise during low intensity LBNP without changes of mean heart rate. METHODS AND RESULTS: Ten healthy volunteers underwent plasma catecholamine evaluation and a continuous recording of ECG, finger blood pressure, respiratory activity, and central venous pressure (CVP) during increasing levels of LBNP up to -40 mm Hg. Spectrum and cross-spectrum analyses assessed the changes in the spontaneous variability of R-R interval, respiration, systolic arterial pressure (SAP), and CVP and in the gain (alpha(LF)) of arterial baroreflex control of heart rate. Baroreceptor sensitivity was also evaluated by the SAP/R-R spontaneous sequences technique. LBNP began decreasing significantly: CVP at -10, R-R interval at -20, SAP at -40, and the indexes alpha(LF) and baroreceptor sensitivity at -30 and -20 mm Hg, compared with baseline conditions. Plasma norepinephrine increased significantly at -20 mm Hg. The normalized low-frequency component of R-R variability (LF(R-R)) progressively increased and was significantly higher than in the control condition at -15 mm Hg. CONCLUSIONS: Nonhypotensive LBNP elicits a reflex increase of cardiac sympathetic modulation, as evaluated by LF(R-R), which precedes the changes in the hemodynamics and in the indexes of arterial baroreflex control.

Non-NASA Center↗
NASA NTRS2001

System identification of dynamic closed-loop control of total peripheral resistance by arterial and cardiopulmonary baroreceptors

Prolonged exposure to microgravity in space flight missions (days) impairs the mechanisms responsible for defense of arterial blood pressure (ABP) and cardiac output (CO) against orthostatic stress in the post-flight period. The mechanisms responsible for the observed orthostatic intolerance are not yet completely understood. Additionally, effective counter measures to attenuate this pathophysiological response are not available. The aim of this study was to investigate the ability of our proposed system identification method to predict closed-loop dynamic changes in TPR induced by changes in mean arterial pressure (MAP) and right atrial pressure (RAP). For this purpose we designed and employed a novel experimental animal model for the examination of arterial and cardiopulmonary baroreceptors in the dynamic closed-loop control of total peripheral resistance (TPR), and applied system identification to the analysis of beat-to-beat fluctuations in the measured signals. Grant numbers: NAG5-4989. c 2001. Elsevier Science Ltd. All rights reserved.

Non-NASA Center↗
NASA NTRS2001

Increases in intramuscular pressure raise arterial blood pressure during dynamic exercise

This investigation was designed to determine the role of intramuscular pressure-sensitive mechanoreceptors and chemically sensitive metaboreceptors in affecting the blood pressure response to dynamic exercise in humans. Sixteen subjects performed incremental (20 W/min) cycle exercise to fatigue under four conditions: control, exercise with thigh cuff occlusion of 90 Torr (Cuff occlusion), exercise with lower body positive pressure (LBPP) of 45 Torr, and a combination of thigh cuff occlusion and LBPP (combination). Indexes of central command (heart rate, oxygen uptake, ratings of perceived exertion, and electromyographic activity), cardiac output, stroke volume, and total peripheral resistance were not significantly different between the four conditions. Mechanical stimulation during LBPP and combination conditions resulted in significant elevations in intramuscular pressure and mean arterial pressure from control at rest and throughout the incremental exercise protocol (P < 0.05). Conversely, there existed no significant changes in mean arterial pressure when the metaboreflex was stimulated by cuff occlusion. These findings suggest that under normal conditions the mechanoreflex is tonically active and is the primary mediator of exercise pressor reflex-induced alterations in arterial blood pressure during submaximal dynamic exercise in humans.

Non-NASA Center↗