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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 145 records · Page 8
All sourcesNASA NTRSDOE OSTI
NASA NTRS1996

Augmented baroreflex heart rate gain after moderate-intensity, dynamic exercise

The occurrence of a sustained vasodilation and hypotension after acute, dynamic exercise suggests that exercise may alter arterial baroreflex mechanisms. Therefore, we assessed systemic hemodynamics, baroreflex regulation of heart rate, and cardiac vagal tone after 60 min of cycling at 60% peak oxygen consumption in 12 healthy, untrained men and women (ages 21-28 yr). We derived sigmoidal carotid-cardiac baroreflex relations by measurement of R-R interval changes induced by ramped, stepwise, R-wave-triggered changes in external neck pressure from 40 to -65 mmHg. We estimated tonic cardiac vagal control with power spectral analysis of R-R interval variability in the respiratory frequency band (0.2-0.3 Hz) during frequency- and tidal volume-controlled breathing. Both mean arterial pressure and total peripheral resistance were reduced postexercise [pressure: from 86 +/- 2 (mean +/- SE) to 81 +/- 2 mmHg; resistance: from 23 +/- 2 to 16 +/- 1 units; both P < 0.05]. Cardiac output was increased postexercise (from 3.9 +/- 0.3 to 5.5 +/- 0.5 l/min, P < 0.05). Both slope and range of the carotid-cardiac baroreflex relation were increased postexercise (slope: from 4.7 +/- 0.7 to 6.1 +/- 0.9 ms/mmHg; range: from 186 +/- 23 to 238 +/- 30 ms, P < 0.05). Respiratory R-R interval variability (cardiac vagal tone) was not changed at any time after exercise, whereas heart rate and plasma norepinephrine levels were elevated. Thus moderate-intensity, dynamic exercise increases heart rate and cardiac output, reduces peripheral vascular resistance, and augments baroreflex responsiveness. Our data suggest that augmented baroreflex heart rate gain restrains rather than contributes to postexercise hypotension, which appears to be mediated predominately by vasodilation.

NASA Discipline Cardiopulmonary↗
NASA NTRS1995

Rheoencephalographic and electroencephalographic measures of cognitive workload: analytical procedures

This investigation demonstrates the feasibility of mental workload assessment by rheoencephalographic (REG) and multichannel electroencephalographic (EEG) monitoring. During the performance of this research, unique testing, analytical and display procedures were developed for REG and EEG monitoring that extend the current state of the art and provide valuable tools for the study of cerebral circulatory and neural activity during cognition. REG records are analyzed to provide indices of the right and left hemisphere hemodynamic changes that take place during each test sequence. The EEG data are modeled using regression techniques and mathematically transformed to provide energy-density distributions of the scalp electrostatic field. These procedures permit concurrent REG/EEG cognitive testing not possible with current techniques. The introduction of a system for recording and analysis of cognitive REG/EEG test sequences facilitates the study of learning and memory disorders, dementia and other encephalopathies.

Arousal/physiology↗
NASA NTRS1994

Cardiovascular responses to repetitive exposure to hyper- and hypogravity states produced by parabolic flight

Physiologic changes to repetitive hyper- and hypogravity stresses occurring during eight to ten parabolas on NASA's KC-135 aircraft were studied. Hemodynamic responses in 11 subjects in 4 different postures (supine, standing, sitting, and semisupine Space Shuttle launch position) were determined using noninvasive impedance cardiography. Five seconds of heart rate, cardiac index, thoracic fluid index, stroke index, ejection velocity index, and ventricular ejection time data were averaged during four different gravity (g) states: 1.3g (before parabola onset); 1.9g (parabola entry); 0g (parabola peak); and 1.7g (parabola exit) for each subject. The standing position was associated with the largest changes in the cardiovascular response to hypo- and hypergravity. The thoracic fluid index did not indicate a headward redistribution during transition from a simulated launch position to weightlessness. Analysis of the eight to ten parabolas revealed that, in general, values obtained at 1.8g differed from 1.6g, 0g differed from 1.6 and 1.3g, and 1.6g differed from 1.3g. The factors of gravity, thoracic fluid index, and cardiac index exhibited significant differences that were most likely to occur between parabola 1 versus parabolas 6, 7, and 8, and parabola 2 versus parabolas 4 through 8. Only the parameter of thoracic fluid index exhibited significance for parabolas 3 versus parabolas 6 and 7.

Space Flight↗
NASA NTRS1995

Simultaneous cerebrovascular and cardiovascular responses during presyncope

BACKGROUND AND PURPOSE: Presyncope, characterized by symptoms and signs indicative of imminent syncope, can be aborted in many situations before loss of consciousness occurs. The plasticity of cerebral autoregulation in healthy humans and its behavior during this syncopal prodrome are unclear, although systemic hemodynamic instability has been suggested as a key factor in the precipitation of syncope. Using lower body negative pressure (LBNP) to simulate central hypovolemia, we previously observed falling mean flow velocities (MFVs) with maintained mean arterial blood pressure (MABP). These findings, and recent reports suggesting increased vascular tone within the cerebral vasculature at presyncope, cannot be explained by the classic static cerebral autoregulation curve; neither can they be totally explained by a recent suggestion of a rightward shift in this curve. METHODS: Four male and five female healthy volunteers were exposed to presyncopal LBNP to evaluate their cerebrovascular and cardiovascular responses by use of continuous acquisition of MFV from the right middle cerebral artery with transcranial Doppler sonography, MABP (Finapres), and heart rate (ECG). RESULTS: At presyncope, MFV dropped on average by 27.3 +/- 14% of its baseline value (P < .05), while MABP remained at 2.0 +/- 27% above its baseline level. Estimated cerebrovascular resistance increased during LBNP. The percentage change from baseline to presyncope in MFV and MABP revealed consistent decreases in MFV before MABP. CONCLUSIONS: Increased estimated cerebrovascular resistance, falling MFV, and constant MABP are evidence of an increase in cerebral vascular tone with falling flow, suggesting a downward shift in the cerebral autoregulation curve. Cerebral vessels may have a differential sensitivity to sympathetic drive or more than one type of sympathetic innervation. Future work to induce dynamic changes in MABP during LBNP may help in assessing the plasticity of the cerebral autoregulation mechanism.

Non-NASA Center↗
NASA NTRS1993

Effect of upright tilt on ventricular/vascular coupling in chronically instrumented primates

Studies of the hydraulic loading conditions on the heart in humans, especially pulsatile load, have primarily been limited to the supine state. Therefore, we have chosen a nonhuman primate model, the baboon, to assess left ventricular/vascular coupling in both supine and upright positions. Primate subjects were studied by catheterization under sedation and then after surgical implantation of transducers. This allowed the evaluation of postural stress in the chronically instrumented conscious baboon and then after light dissociative doses of ketamine. Basic hemodynamic variables were evaluated for baboons in supine and upright positions. Fourier analysis was applied to aortic pressure and flow to obtain input and characteristic impedance and the ratio of pulsatile (Wp) to total (Wt) left ventricular power (Wp/Wt). The aortic reflected, or backward, pressure was also calculated. Peripheral resistance increased (P = 0.01) and reflected pressure decreased (17.74 +/- 1.50 vs. 15.21 +/- 2 mmHg; P < 0.01) in upright subjects. Characteristic impedance and Wp/Wt were unchanged. Postoperatively, peripheral resistance increased (2,651 +/- 311 vs. 3,667 +/- 276; P < 0.05) and mean power and Wt decreased (P < 0.01) without changes in reflected pressure. All variables were unchanged after light dissociative doses of ketamine. Thus there is no significant change in efficiency of left ventricular/vascular coupling formulated in terms of Wp/Wt or input impedance with postural stress.

NASA Program Space Physiology and Countermeasures↗
NASA NTRS1989

Beat to beat variability in cardiovascular variables: noise or music?

Cardiovascular variables such as heart rate, arterial blood pressure, stroke volume and the shape of electrocardiographic complexes all fluctuate on a beat to beat basis. These fluctuations have traditionally been ignored or, at best, treated as noise to be averaged out. The variability in cardiovascular signals reflects the homeodynamic interplay between perturbations to cardiovascular function and the dynamic response of the cardiovascular regulatory systems. Modern signal processing techniques provide a means of analyzing beat to beat fluctuations in cardiovascular signals, so as to permit a quantitative, noninvasive or minimally invasive method of assessing closed loop hemodynamic regulation and cardiac electrical stability. This method promises to provide a new approach to the clinical diagnosis and management of alterations in cardiovascular regulation and stability.

Non-NASA Center↗
NASA NTRS2005

Consequences of cardiovascular adaptation to spaceflight: implications for the use of pharmacological countermeasures

There is little evidence obtained from space flight to support the notion that occurrence of cardiac dysrhythmias, impaired cardiac and vascular function, and manifestation of asymptomatic cardiovascular disease represent serious risks during space flight. Therefore, the development of orthostatic hypotension and instability immediately after return from spaceflight probably reflect the most significant operational risks associated with the cardiovascular system of astronauts. Significant reductions in stroke volume and lower reserve for increasing peripheral vascular resistance contribute to ineffective maintenance of systemic arterial blood pressure during standing after spaceflight despite compensatory elevations in heart rate. The primary mechanism underlying reduced stroke volume appears to be a reduction in preload associated with less circulating blood volume while inadequate peripheral vasoconstriction may be caused partly by hyporeactivity of receptors that control arterial smooth muscle function. A focus for development of future countermeasures for hemodynamic responses to central hypovolemia includes the potential application of pharmacological agents that specifically target and restore blood volume (e.g., fludrocortisone, electrolyte-containing beverages) and reserve for vasoconstriction (e.g., midodrine, vasopressin). Based on systematic evaluations, acute physical exercise designed to elicit maximal effort or inspiratory resistance have shown promise as successful countermeasures that provide protection against development of orthostatic hypotension and intolerance without potential risks and side effects associated with specific pharmacological interventions.

Non-NASA Center↗
NASA NTRS1972

Action of acetylstrophanthidin on experimental myocardial infarction.

An experimental animal model with acute myocardial infarction of a size insufficient to produce profound heart failure or shock was used to study the effects of acute infarction on digitalis tolerance and the hemodynamic changes produced by moderate and large doses of acetylstrophanthidin. With acute myocardial infarction, digitalis toxic arrhythmias could be precipitated with significantly lower doses of digitalis than in animals without myocardial infarction. There was no precise correlation between the size of infarction and the toxic dose of glycoside. Coronary artery ligation produced a stable but relatively depressed circulatory state, as evidenced by lowered cardiac output and stroke volume and elevated systemic vascular resistance and left atrial mean pressure. When digitalis was infused, the following significant changes were observed at nontoxic doses: (1) elevation of aortic and left ventricular pressures; (2) further decline in cardiac output; and (3) decreased left atrial mean pressure.

Nola, G. T.↗
NASA NTRS1972

Cardiovascular responses to glucagon - Physiologic measurement by external recordings.

Assessment by noninvasive polygraphic techniques of the cardiovascular responses of normal subjects to intravenous injections of glucagon and glucagon diluent. A blinding procedure which eliminated observer bias was used during the reading of tracings. Analysis of group results showed that glucagon provoked uniformly significant changes, including increase in heart rate, blood pressure, pressure-rate product, and ejection time index, and decrease in prejection period, mechanical and electromechanical systole, left ventricular ejection time, and the ratio PEP/LVET. The principal results correlated well with those of previous studies of the hemodynamic effects of glucagon.

Byrne, M. J.↗
NASA NTRS1972

Physiological responses to environmental factors related to space flight

The research is reported for establishing physiological base line data, and for developing procedures and instrumentation necessary for the automatic measurement of hemodynamic and metabolic parameters. The work in the following areas is discussed: biochemistry, bioinstrumentation, nutrition, physiology, experimental surgery, and animal colony.

Pace, N.↗
NASA NTRS1973

Functional changes in systemic and regional (intracranial) circulation accompanying low accelerations

Functional changes in systemic and cerebral hemodynamics were studied with respect to vestibular stresses. The main types of responses, differing qualitatively with respect to the tolerance of test subjects to low accelerations (particularly to Coriolis accelerations), were established. This is of practical importance in the selection of aircraft and space pilots. The data presented sheds light on the physiological mechanisms of adaptation and disturbed compensation during vestibular stimulation. Further studies in this important field of aerospace medicine are outlined.

Usachev, V. V.↗