Cerebral blood flow velocity increases with acute head-down tilt of humans
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NASA Center ARC↗
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Effect of 1% Inspired CO2 During Head-Down Tilt on Ocular Structures, Cerebral Blood Flow, and Visual Acuity in Healthy Human Subjects
The cephalad fluid shift induced by microgravity has been hypothesized to elevate intracranial pressure (ICP) and contribute to the development of the visual impairment/intracranial pressure (VIIP) syndrome experienced by many astronauts during and after long-duration space flight. In addition, elevated ambient partial pressure of carbon dioxide (PCO2) on the International Space Station (ISS) has also been hypothesized to contribute to the development of VIIP. We seek to determine if an acute, mild CO2 exposure, similar to that occurring on the ISS, combined with the cephalad fluid shift induced by head-down tilt will induce ophthalmic and ICP changes consistent with the VIIP syndrome.
Laurie, S. S.↗
Impaired Vestibular Function affects Orthostatic Cerebral Blood Flow Response
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Gosala Gopalakrishnan↗
The effect of resting blood flow occlusion on exercise tolerance and W′
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Ryan M. Broxterman↗
Medical support and technology for long-duration space missions
The current philosophy and development directions being taken towards realization of medical systems for use on board space stations are discussed. Data was gained on the performance of physical examinations, venipuncture and blood flow, blood smear and staining, white blood cell differential count, throat culture swab and colony count, and microscopy techniques during a 28-day period of the Skylab mission. It is expected that the advent of Shuttle flights will rapidly increase the number of persons in space, create a demand for in-space rather than on-earth medical procedures, and necessitate treatments for disorders without the provision for an early return to earth. Attention is being given to pressurized environment and extravehicular conditions of treatment, the possibilities of the use of the OTV for moving injured or ill crewmembers to other space stations, and to isolation of persons with communicable diseases from station crews.
Furukawa, S.↗
Improved Calibration Of Acoustic Plethysmographic Sensors
Improved method of calibration of acoustic plethysmographic sensors involves acoustic-impedance test conditions like those encountered in use. Clamped aluminum tube holds source of sound (hydrophone) inside balloon. Test and reference sensors attached to outside of balloon. Sensors used to measure blood flow, blood pressure, heart rate, breathing sounds, and other vital signs from surfaces of human bodies. Attached to torsos or limbs by straps or adhesives.
Zuckerwar, Allan J.↗
Cardiovascular responses of men and women to lower body negative pressure
Changes in blood flow and blood redistribution were measured by impedance plethysmography in the pelvic and leg regions of six male and four female subjects during three 5-min exposures to -20, -40, and -60 mm Hg lower body negative pressure (LBNP). Female subjects demonstrated significantly higher mean heart rate and lower leg blood flow indices than the male subjects during the recumbent control periods. Men had slightly higher mean resting systolic and diastolic blood pressures and higher mean control pelvic blood indices. Women demonstrated significantly less blood pooling in the legs and slightly less in the pelvic region than the men. All of the 18 tests with male subjects at -60 mm Hg were completed without initial signs of syncope, while only two of the tests with women were completed successfully without the subject exhibiting presyncopal conditions. Results indicate that impedance plethysmography can be used to measure segmental cardiovascular responses during LBNP and that females may be less tolerant to -60 mm Hg LBNP than males.
Montgomery, L. D.↗
Effect of radial acceleration on venous flow in the cerebral blood vessels of rabbits
Radial acceleration effect on venous blood flow in cerebral veins of rabbits
ANIMAL STUDY↗
Raised cerebrovascular resistance in idiopathic orthostatic intolerance: evidence for sympathetic vasoconstriction
Patients with idiopathic orthostatic intolerance (IOI) exhibit symptoms suggestive of cerebral hypoperfusion and an excessive decrease in cerebral blood flow associated with standing despite sustained systemic blood pressure. In 9 patients (8 women and 1 man aged 22 to 48 years) with IOI, we tested the hypothesis that volume loading (2000 cc normal saline) and alpha-adrenoreceptor agonism improve systemic hemodynamics and cerebral perfusion and that the decrease in cerebral blood flow with head-up tilt (HUT) could be attenuated by alpha-adrenoreceptor blockade with phentolamine. At 5 minutes of HUT, volume loading (-20+/-3.2 bpm) and phenylephrine (-18+/-3.4 bpm) significantly reduced upright heart rate compared with placebo; the effect was diminished at the end of HUT. Phentolamine substantially increased upright heart rate at 5 minutes (20+/-3.7 bpm) and at the end of HUT (14+/-5 bpm). With placebo, mean cerebral blood flow velocity decreased by 33+/-6% at the end of HUT. This decrease in cerebral blood flow with HUT was attenuated by all 3 interventions. We conclude that in patients with IOI, HUT causes a substantial decrease in cerebrovascular blood flow velocity. The decrease in blood flow velocity with HUT can be attenuated with interventions that improve systemic hemodynamics and therefore decrease reflex sympathetic activation. Moreover, alpha-adrenoreceptor blockade also blunts the decrease in cerebral blood flow with HUT but at the price of deteriorated systemic hemodynamics. These observations may suggest that in patients with IOI, excessive sympathetic activity contributes to the paradoxical decrease in cerebral blood flow with upright posture.
Non-NASA Center↗
Potential Mechanism Leading to Impaired Thermoregulation Following Microgravity Exposure
Prolonged exposure to microgravity or its analogues impairs thermoregulation in humans evidenced by higher internal temperatures following the exposure during a thermal challenge. Although the mechanism leading to this response has not been clearly delineated, we identified that prolonged head-down tilt (HDT) markedly impairs thermoregulatory reflex control of skin blood flow, as demonstrated by an increased internal temperature threshold for cutaneous vasodilation, and by a reduced slope of the relationship between the elevation in skin blood flow relative to the elevation in internal temperature. Recently, Fortney et al. identified similar responses in two individuals following 115 days of microgravity exposure. One possible mechanism leading to altered cutaneous vasodilation during a thermal challenge following actual or simulated microgravity exposure may be associated with baroreflex-mediated attenuation in the elevation of skin blood flow. During a heat stress the elevation in skin blood flow is accomplished through a combination of increased cutaneous vascular conductance and cardiac output, both of which result in central venous pressure (CVP) decreasing 2-6 mmHg. Reductions in CVP of this magnitude in normothermia decrease muscle blood flow and skin blood flow presumably through unloading the cardiopulmonary baroreceptors. It is unclear whether the reduction in CVP, and accompanying cardiopulmonary baroreceptor unloading, during passive heating buffers the elevation in skin blood flow. That is, would the elevation in skin blood flow be greater if CVP did not decrease, or decreased to a lesser extent during the heat stress? Conversely, if CVP decreased to a greater extend during a thermal challenge following a perturbation such as prolonged HDT, would the elevation in skin blood flow be attenuated during that thermal challenge? Given that prolonged HDT decreases plasma volume and central venous pressure, such a finding would provide a plausible hypothesis to explain why skin blood flow does not increase to the same extent during a heat stress following simulated or actual microgravity exposure. Thus, the purpose of this project was to identify whether cardiopulmonary baroreceptor unloading coincident with heat stress buffers the elevation in skin blood flow.
Crandall, C. G.↗
Proposed law of nature linking impacts, plume volcanism, and Milankovitch cycles to terrestrial vertebrate mass extinctions via greenhouse-embryo death coupling
A greenhouse-physiological coupling killing mechanism active among mammals, birds, and reptiles has been identified. Operating via environmental thermal effects upon the maternal core-skin blood flow critical to the survival and development of embryos, it reduces the flow of blood to the uterine tract. Today, during hot summers, this phenomena kills embryos on a vast, global scale. Because of sensitivity of many mammals to modern heat, a major modern greenhouse could reduce population numbers on a global scale, and potentially trigger population collapses in the more vulnerable parts of the world. In the geological past, the killing mechanism has likely been triggered into action by greenhouse warming via impact events, plume volcanism, and Earth orbital variations (Milankovitch cycles). Earth's biosphere is maintained and molded by the flow of energy from the solar energy source to Earth and on to the space energy sink (SES). This SES energy flow maintains Earth's biosphere and its living components, as open, intermediate, dissipative, nonequilibrium systems whose states are dependent upon the rate of energy flowing through them. Greenhouse gases such as CO2 in the atmosphere influence the SES energy flow rate. Steady-state flow is necessary for global ecological stability (autopoiesis). Natural fluctuations of the C cycle such as rapid releases of CO2 from the mantle, or oceans, disrupt steady-state SES flow. These fluctuations constantly challenge the biosphere; slowdown of SES energy flow drives it toward thermodynamical equilibrium and stagnation. Fluctuations induced by impact event, mantle plume volcanism, and Milankovitch cycles can grow into structure-breaking waves triggering major perturbations of Earth's C cycle and mass extinctions. A major C cycle perturbation involves readjustment of the outer physiochemical spheres of the Earth: the atmosphere, hydrosphere, and lithosphere; and by necessity, the biosphere. A greenhouse, one manifestation of a major C cycle perturbation, is the most dangerous natural phenomenon that life on Earth can experience. Greenhouse conditions existed during the KT mass extinctions of 65 m.y. ago, and the Pleistocene-Holocene (P-H) mammalian extinctions of 10,000-12,000 yr ago. Coupling climatology to reproductive physiology via effects of ambient air temperature upon uterine blood flow to developing embryos accounts for the extinctions via established physiological principles.
Mclean, D. M.↗
Theoretical considerations on the flow of blood through a capillary
Theoretical considerations on flow of blood through capillary
RHEOLOGY↗
Age alters the cardiovascular response to direct passive heating
During direct passive heating in young men, a dramatic increase in skin blood flow is achieved by a rise in cardiac output (Qc) and redistribution of flow from the splanchnic and renal vascular beds. To examine the effect of age on these responses, seven young (Y; 23 +/- 1 yr) and seven older (O; 70 +/- 3 yr) men were passively heated with water-perfused suits to their individual limit of thermal tolerance. Measurements included heart rate (HR), Qc (by acetylene rebreathing), central venous pressure (via peripherally inserted central catheter), blood pressures (by brachial auscultation), skin blood flow (from increases in forearm blood flow by venous occlusion plethysmography), splanchnic blood flow (by indocyanine green clearance), renal blood flow (by p-aminohippurate clearance), and esophageal and mean skin temperatures. Qc was significantly lower in the older than in the young men (11.1 +/- 0.7 and 7.4 +/- 0.2 l/min in Y and O, respectively, at the limit of thermal tolerance; P < 0. 05), despite similar increases in esophageal and mean skin temperatures and time to reach the limit of thermal tolerance. A lower stroke volume (99 +/- 7 and 68 +/- 4 ml/beat in Y and O, respectively, P < 0.05), most likely due to an attenuated increase in inotropic function during heating, was the primary factor for the lower Qc observed in the older men. Increases in HR were similar in the young and older men; however, when expressed as a percentage of maximal HR, the older men relied on a greater proportion of their chronotropic reserve to obtain the same HR response (62 +/- 3 and 75 +/- 4% maximal HR in Y and O, respectively, P < 0.05). Furthermore, the older men redistributed less blood flow from the combined splanchnic and renal circulations at the limit of thermal tolerance (960 +/- 80 and 720 +/- 100 ml/min in Y and O, respectively, P < 0. 05). As a result of these combined attenuated responses, the older men had a significantly lower increase in total blood flow directed to the skin.
Clinical Trial↗
Cerebral blood velocity and other cardiovascular responses to 2 days of head-down tilt
Spaceflight induces a cephalad redistribution of fluid volume and blood flow within the human body, and space motion sickness, which is a problem during the first few days of space flight, could be related to these changes in fluid status and in blood flow of the cerebrum and vestibular system. To evaluate possible changes in cerebral blood flow during simulated weightlessness, we measured blood velocity in the middle cerebral artery (MCA) along with retinal vascular diameters, intraocular pressure, impedance cardiography, and sphygmomanometry on nine men (26.2 +/- 6.6 yr) morning and evening for 2 days during continuous 10 deg head-down tilt (HDT). When subjects went from seated to head-down bed rest, their heart rate and retinal diameters decreased, and intraocular pressures increased. After 48 h of HDT, blood flow velocity in the MCA was decreased and thoracic impedance was increased, indicating less fluid in the thorax. Percent changes in blood flow velocities in the MCA after 48 h of HDT were inversely correlated with percent changes in retinal vascular diameters. Blood flow velocities in the MCA were inversely correlated (intersubject) with arterial pressures and retinal vascular diameters. Heart rate, stroke volume, cardiac output, systolic arterial pressure, and at times pulse pressure and blood flow velocities in the MCA were greater in the evening. Total peripheral resistance was higher in the morning. Although cerebral blood velocity is reduced after subjects are head down for 2 days, the inverse relationship with retinal vessel diameters, which have control analogous to that of cerebral vessels, indicates cerebral blood flow is not reduced.
Frey, Mary A. B.↗
Rheoencephalographic (REG) Assessment of Head and Neck Cooling for use with Multiple Sclerosis Patients
We have prepared a computer program (RHEOSYS:RHEOencephalographic impedance trace scanning SyStem) that can be used to automate the analysis of segmental impedance blood flow waveforms. This program was developed to assist in the post test analysis of recorded impedance traces from multiple segments of the body. It incorporates many of the blood flow, segmental volume, and vascular state indices reported in the world literature. As it is currently programmed, seven points are selected from each blood flow pulse and associated ECG waveforrn: 1. peak of the first ECG QRS complex, 2. start of systolic slope on the blood flow trace, 3. maximum amplitude of the impedance pulse, 4. position of the dicrotic notch, 5. maximum amplitude of the postdicrotic segment, 6. peak of the second ECG QRS complex, and 7. start of the next blood flow pulse. These points we used to calculate various geometric, area, and time-related values associated with the impedance pulse morphology. RHEOSYS then calculates a series of 34 impedance and cardiac cycle parameters which include pulse amplitudes; areas; pulse propagation times; cardiac cycle times; and various measures of arterial and various tone, contractility, and pulse volume. We used this program to calculate the scalp and intracranial blood flow responses to head and neck cooling as it may be applied to lower the body temperatures of multiple sclerosis patients. Twelve women and twelve men were tested using a commercially available head and neck cooling system operated at its maximum cooling capacity for a period of 30 minutes. Head and neck cooling produced a transient change in scalp blood flow and a significant, (P<0.05) decrease of approx. 30% in intracranial blood flow. Results of this experiment will illustrate how REG and RHEOSYS can be used in biomedical applications.
Montogomery, Leslie D.↗
Skeletal muscle fiber type composition and performance during repeated bouts of maximal, concentric contractions
Force output and fatigue and recovery patterns were studied during intermittent short-term exercise. 27 men performed three bouts of 30 maximal unilateral knee extensions on 2 different occasions. Blood flow was maintained or occluded during recovery periods (60 s). Blood flow was restricted by inflating a pneumatic cuff placed around the proximal thigh. Muscle biopsies from vastus lateralis were analyzed for identification of fast twitch (FT) and slow twitch (ST) fibers and relative FT area. Peak torque decreased during each bout of exercise and more when blood flow was restricted during recovery. Initial peak torque (IPT) and average peak torque (APT) decreased over the three exercise bouts. This response was 3 fold greater without than with blood flow during recovery. IPT and APT decreased more in individuals with mainly FT fibers than in those with mainly ST fibers. It is suggested that performance during repeated bouts of maximal concentric contractions differs between individuals with different fiber type composition. Specifically, in high intensity, intermittent exercise with emphasis on anaerobic energy release a high FT composition may not necessarily be advantageous for performance.
Muscle Contraction↗
Influence of Exercise Modality on Cerebral-Ocular Hemodynamics and Pressures
Background: Moderate and high intensity aerobic or resistance exercise has clearly identified benefits for cardiac, muscle, and bone health. However, the impact of such exercise - either as a mitigating or an exacerbating factor - on the development of the visual impairment and intracranial pressure syndrome (VIIP) is unknown. Accordingly, our aim was to characterize the effect of an acute bout of resistance (RE), moderate-intensity continuous (CE), and high-intensity interval exercise (IE) during a cephalad fluid shift on cerebral-ocular hemodynamics and pressures. Methods: 10 male subjects (36 plus or minus 9 years) completed 4 testing days in a 15 degree head-down tilt (HDT): (1) assessment of maximum volume of O (sub 2), (2) RE session (4 sets of 12 repetition maximum leg press exercise), (3) CE session (30 minutes of cycling at 60 percent maximum volume of O (sub 2)), and (4) IE session (4 by 4-minute intervals of exercise at 85 percent maximum volume of O (sub 2) with 3-minute active rest periods). During each session, blood flow (Vivid-e, GE Healthcare) in extracranial arteries (common carotid artery, CCA; internal carotid artery, ICA; external carotid artery, ECA and vertebral artery, VA), and mean blood flow velocity in middle cerebral artery (MCA), internal jugular pressure (IJP; VeinPress), and intraocular pressure (IOP; Icare PRO) were measured at rest, at the end of each resistance or interval set, and every 5 minutes during continuous exercise. Translaminar pressure gradient (TLPG) was estimated by subtracting IJP from IOP. Results: There were no differences across days in pre-exercise resting blood flows or pressures. IOP decreased slightly from HDT rest (20.2 plus or minus 2.3 millimeters of mercury) to exercise (RE: 19.2 plus or minus 2.8 millimeters of mercury; CE: 18.9 plus or minus 3.2 millimeters of mercury; IE: 20.1 plus or minus 2.8 millimeters of mercury), while IJP decreased during CE (31.6 plus or minus 9.5 millimeters of mercury) and RE (32.0 plus or minus 8.1 millimeters of mercury), and increased during IE (35.1 plus or minus 9.5 millimeters of mercury) from HDT rest (33.3 plus or minus 6.5 millimeters of mercury). Estimated TLPG was increased during IE only. Compared to RE and CE, IE resulted in the greatest increase in MCA blood flow velocity and extracranial artery blood flow. Conclusions: These preliminary results suggest that high-intensity IE acutely increases cerebral blood flow, IJP, and TLPG. Alterations in TLPG is one mechanism that may contribute to optic nerve sheath edema in astronauts. Accordingly, acutely raising IOP and/or orbital pressure during exercise could optimize cerebral-ocular pressures during spaceflight.
Scott, J.↗