Key Pointsr Astronauts have recently been discovered to have impaired vision, with a presentation that resembles syndromes of elevated intracranial pressure on Earth.r Gravity has a profound effect on fluid distribution and pressure within the human circulation.In contrast to prevailing theory, we observed that microgravity reduces central venous and intracranial pressure.r This being said, intracranial pressure is not reduced to the levels observed in the 90 deg seated upright posture on Earth. Thus, over 24 h in zero gravity, pressure in the brain is slightly above that observed on Earth, which may explain remodelling of the eye in astronauts.Abstract Astronauts have recently been discovered to have impaired vision, with a presentation that resembles syndromes of elevated intracranial pressure (ICP). This syndrome is considered the most mission-critical medical problem identified in the past decade of manned spaceflight. We recruited five men and three women who had an Ommaya reservoir inserted for the delivery of prophylactic CNS chemotherapy, but were free of their malignant disease for at least 1 year. ICP was assessed by placing a fluid-filled 25 gauge butterfly needle into the Ommaya reservoir. Subjects were studied in the upright and supine position, during acute zero gravity (parabolic flight) and prolonged simulated microgravity (6 deg head-down tilt bedrest). ICP was lower when seated in the 90 deg upright posture compared to lying supine (seated, 4 ± 1 vs. supine, 15 ± 2 mmHg). Whilst lying in the supine posture, central venous pressure (supine, 7 ± 3 vs. microgravity, 4 ± 2 mmHg) and ICP (supine, 17 ± 2 vs. microgravity, 13 ± 2 mmHg) were reduced in acute zero gravity, although not to the levels observed in the 90 deg seated upright posture on Earth. Prolonged periods of simulated microgravity did not cause progressive elevations in ICP (supine, 15 ± 2 vs. 24 h head-down tilt, 15 ± 4 mmHg). Complete removal of gravity does not pathologically elevate ICP but does prevent the normal lowering of ICP when upright. These findings suggest the human brain is protected by the daily circadian cycles in regional ICPs, without which pathology may occur.

High-intensity interval exercise (HIIE) improves cerebral executive function (EF), but the improvement in EF is attenuated after repeated HIIE, perhaps because of lower lactate availability for the brain. This investigation examined whether improved EF after exercise relates to brain lactate uptake. Fourteen healthy, male subjects performed 2 HIIE protocols separated by 60 min of rest. Blood samples were obtained from the right internal jugular venous bulb and from the brachial artery to determine arterial-venous differences across the brain for lactate (a-v diff), glucose (a-v diff), oxygen (a-v diff), and brain-derived neurotrophic factor (BDNF; a-v diff). EF was evaluated by the color-word Stroop task. The first HIIE improved EF for 40 min, whereas the second HIIE improved EF only immediately after exercise. The a-v diff was unchanged, whereas the a-v diff increased similarly after both HIIEs, and the a-v diff increased, but the increase was attenuated after the second HIIE, compared with the first HIIE ( P < 0.05). The EF after HIIE correlated with the a-v diff ( r = 0.62; P < 0.01). We propose that attenuated improvement in EF after repeated HIIE relates to reduced cerebral lactate metabolism and is, thereby, linked to systemic metabolism as an example of the lactate shuttle mechanism.-Hashimoto, T., Tsukamoto, H., Takenaka, S., Olesen, N. D., Petersen, L. G., Sørensen, H., Nielsen, H. B., Secher, N. H., Ogoh, S. Maintained exercise-enhanced brain executive function related to cerebral lactate metabolism in men.

We evaluated postural effects on intracranial pressure (ICP) and cerebral perfusion pressure [CPP: mean arterial pressure (MAP) - ICP] in neurosurgical patients undergoing 24-h ICP monitoring as part of their diagnostic workup. We identified nine patients (5 women, age 44 ± 20 yr; means ± SD), who were "as normal as possible," i.e., without indication for neurosurgical intervention (e.g., focal lesions, global edema, abnormalities in ICP-profile, or cerebrospinal fluid dynamics). ICP (tip-transducer probe; Raumedic) in the brain parenchyma (n = 7) or in the lateral ventricles (n = 2) and cardiovascular variables (Nexfin) were determined from 20° head-down tilt to standing up. Compared with the supine position, ICP increased during 10° and 20° of head-down tilt (from 9.4 ± 3.8 to 14.3 ± 4.7 and 19 ± 4.7 mmHg; P < 0.001). Conversely, 10° and 20° head-up tilt reduced ICP to 4.8 ± 3.6 and 1.3 ± 3.6 mmHg and ICP reached -2.4 ± 4.2 mmHg in the standing position (P < 0.05). Concordant changes in MAP maintained CPP at 77 ± 7 mmHg regardless of body position (P = 0.95). During head-down tilt, the increase in ICP corresponded to a hydrostatic pressure gradient with reference just below the heart, likely reflecting the venous hydrostatic indifference point. When upright, the decrease in ICP was attenuated, corresponding to formation of a separate hydrostatic gradient with reference to the base of the skull, likely reflecting the site of venous collapse. ICP therefore seems to be governed by pressure in the draining veins and collapse of neck veins may protect the brain from being exposed to a large negative pressure when upright. Despite positional changes in ICP, MAP keeps CPP tightly regulated.

Abstract-During everyday life, gravity constantly stresses the cardiovascular system in upright humans by diminishing venous return. This decreases cardiac output and induces systemic vasoconstriction to prevent blood pressure from falling. We therefore tested the hypothesis that entering weightlessness leads to a prompt increase in cardiac output and to systemic vasodilatation and that these effects persist for at least a week of weightlessness in space. Cardiac output and mean arterial pressure were measured in 8 healthy humans during acute 20-s periods of weightlessness in parabolic airplane flights and on the seventh and eighth day of weightlessness in 4 astronauts in space. The seated 1-G position acted as reference. Entering weightlessness promptly increased cardiac output by 29Ϯ7%, from 6.6Ϯ0.7 to 8.4Ϯ0.9 L min Ϫ1 (meanϮSEM; Pϭ0.003), whereas mean arterial pressure and heart rate were unaffected. Thus, systemic vascular resistance decreased by 24Ϯ4% (Pϭ0.017). After a week of weightlessness in space, cardiac output was increased by 22Ϯ8% from 5.1Ϯ0.3 to 6.1Ϯ0.1 L min Ϫ1 (Pϭ0.021), with mean arterial pressure and heart rate being unchanged so that systemic vascular resistance was decreased by 14Ϯ9% (Pϭ0.047). In conclusion, entering weightlessness promptly increases cardiac output and dilates the systemic circulation. This vasorelaxation persists for at least a week into spaceflight. Thus, it is probably healthy for the human cardiovascular system to fly in space. Key Words: blood pressure Ⅲ cardiac output Ⅲ cardiovascular diseases Ⅲ heart rate Ⅲ vascular resistance T hroughout evolution of mankind, gravity has constantly stressed the cardiovascular system by diminishing venous return of blood to the heart. 1 This gravity-induced decrease in venous return, and, thus in cardiac output, is detected by cardiopulmonary and arterial baroreflexes, which initiate constriction of the vasculature to prevent blood pressure from falling. Therefore, gravity is a chronic systemic vasoconstrictor in upright humans during normal everyday life.Previous cardiovascular measurements in astronauts in space indicate that cardiac output is increased by some 18% by weightlessness compared with upright standing or sitting on the ground and more so during the initial days of flight than at the end. 2,3 In these studies, blood pressure was not measured. In another study, Fritsch-Yelle et al 4 observed in 12 astronauts over several flights that the mean 24-hour diastolic arterial pressure, but not systolic pressure, was significantly decreased in space by some 5 mm Hg. This decrease was evident at the beginning and at the end of flight. However, cardiac output was not measured. Therefore, it is not known whether the unchanged systolic and decreased diastolic pressure in space is accounted for by systemic vasodilatation and whether systemic vasodilatation is evident from the very beginning and to the end of flight.We therefore tested the hypothesis that the weightlessnessinduced increase in cardiac output leads to systemic vasodilata...