STEM Today

STEM TODAY March 2018, No. 30

STEM TODAY March 2018, No. 30

CONTENTS CV3: Is orthostatic intolerance a potential hazard?

Post­flight orthostatic intolerance, the inability to maintain blood pressure while in an upright position, is an established, spaceflight­related medical problem. Risk definition work was largely accomplished prior to establishment of HRP risks/gaps.

Current gaps are heavily focused on mitigation.

Editorial Editor: Mr. Abhishek Kumar Sinha Editor / Technical Advisor: Mr. Martin Cabaniss

STEM Today, March 2018, No.30

Disclaimer ( Non-Commercial Research and Educational Use ) STEM Today is dedicated to STEM Education and Human Spaceflight. This newsletter is designed for Teachers and Students with interests in Human Spaceflight and learning about NASA’s Human Research Roadmap. The opinion expressed in this newsletter is the opinion based on fact or knowledge gathered from various research articles. The results or information included in this newsletter are from various research articles and appropriate credits are added. The citation of articles is included in Reference Section. The newsletter is not sold for a profit or included in another media or publication that is sold for a profit.

Cover Page Beaming with the Light of Millions of Suns Researchers using Chandra data have identified a fourth ultraluminous X-ray source (ULX), dubbed ULX8, as being a neutron star. These results provide clues about how these objects can shine so brightly in X-rays. The newly characterized ULX is located in the Whirlpool galaxy, also known as M51. These images of the Whirlpool show X-rays from Chandra and optical data from the Hubble Space Telescope. The ULX is marked with a circle.

Image Credit: X-ray: NASA/CXC/Caltech/M. Brightman et al.; Optical: NASA/STScI

Back Cover Optical Images of M51 Researchers using Chandra data have identified a fourth ultraluminous X-ray source (ULX), dubbed ULX8, as being a neutron star. These results provide clues about how these objects can shine so brightly in X-rays. The newly characterized ULX is located in the Whirlpool galaxy, also known as M51. These images of the Whirlpool show X-rays from Chandra and optical data from the Hubble Space Telescope. The ULX is marked with a circle.

Image Credit: Optical: NASA/CXC/Caltech/M. Brightman et al.; Optical: NASA/STScI

STEM Today , March 2018

Editorial Dear Reader

STEM Today, March 2018, No.30

All young people should be prepared to think deeply and to think well so that they have the chance to become the innovators, educators, researchers, and leaders who can solve the most pressing challenges facing our world, both today and tomorrow. But, right now, not enough of our youth have access to quality STEM learning opportunities and too few students see these disciplines as springboards for their careers.

According to Marillyn Hewson, "Our children - the elementary, middle and high school students of today - make up a generation that will change our universe forever. This is the generation that will walk on Mars, explore deep space and unlock mysteries that we can’t yet imagine". "They won’t get there alone. It is our job to prepare, inspire and equip them to build the future."

STEM Today will inspire and educate people about Spaceflight and effects of Spaceflight on Astronauts.

Editor Mr. Abhishek Kumar Sinha

Editorial Dear Reader The Science, Technology, Engineering and Math (STEM) program is designed to inspire the next generation of innovators, explorers, inventors and pioneers to pursue STEM careers.

According to former President Barack Obama, " Science is more than a school subject, or the periodic table, or the properties of waves. It is an approach to the world, a critical way to understand and explore and engage with the world, and then have the capacity to change that world..."

STEM Today addresses the inadequate number of teachers skilled to educate in Human Spaceflight. It will prepare , inspire and educate teachers about Spaceflight. STEM Today will focus on NASA’S Human Research Roadmap. It will research on long duration spaceflight and put together latest research in Human Spaceflight in its monthly newsletter.

Editor / Technical Advisor Mr. Martin Cabaniss

STEM Today, March 2018, No.30

Human Health Countermeasures (HHC) CV3: Is orthostatic intolerance a potential hazard?

Post- ight orthostatic intolerance, the inability to maintain blood pressure while in an upright position, is an established, space ight-related medical problem. Current gaps are heavily focused on mitigation. Countermeasures have been identi ed and implemented with some success ( uid loading, compression garments). It is currently unknown if partial gravity (moon, Mars) will be protective against OI.

Midodrine as a Countermeasure against Postflight Orthostatic Hypotension

After space flight the capability to remain upright (standing) may be compromised by an inability to maintain adequate cerebral perfusion. This condition, termed orthostatic hypotension, may result in presyncope (lightheadedness) or syncope (loss of consciousness) during reentry or egress from the space vehicle and for several days after landing.

Approximately 20% of crewmembers on short-duration missions and 80% of those on long-duration missions experience presyncope during testing on landing day. To date, the potential countermeasures that have been tested (including lower-body negative pressure, fluid loading, Florinef, exercise) have not eliminated postflight orthostatic hypotension.

STEM Today, March 2018, No.30

Midodrine is a selective alpha-1 adrenergic agonist that is used clinically to treat orthostatic hypotension. It is almost completely absorbed after oral administration and is hydrolyzed enzymatically to its active metabolite, desglymidodrine, which has a bioavailability of 93%. Midodrine acts by increasing vaso- and venoconstriction, thereby decreasing peripheral venous capacity and blood pooling, but does not pass the blood-brain barrier and therefore has no central stimulant effects. The effect of midodrine as an alpha-adrenergic agonist may be particularly protective of orthostatic tolerance in astronauts who become presyncopal on landing day due to inadequate release of norepinephrine.

Orthostatic Hypotension after Long-Duration Space Flight: NASA’s Experiences from the International Space Station The incidence of orthostatic hypotension (OH) was greater after long- than short-duration spaceflight in astronauts who participated in Mir Space Station and Space Shuttle missions. Twenty ISS and 66 Shuttle astronauts participated in 10-min 80◦ head-up tilt tests 10 d before launch (L-10), on landing day (R+0) or 1 d after landing (R+1). Data from 5 ISS astronauts tested on R+0 or R+1 who used non-standard countermeasures were excluded. Many astronauts repeated the test 3 d (R+3) after landing. Fisher’s Exact Test was used to compare the ability of ISS and Shuttle astronauts to complete the tilt test on R+0. Cox regression was used to identify cardiovascular parameters that were associated with test completion across all tests, and mixed model analysis was used to compare the change and recovery rates between ISS and Shuttle astronauts. In these analyses, ISS data from R+0 and R+1 were pooled to provide sufficient statistical power.

The proportion of astronauts who completed the tilt test on R+0 without OH was less in ISS than in Shuttle astronauts (p=0.03). On R+0, only 2 of 6 ISS astronauts completed the test compared to 53 of 66 (80%) Shuttle astronauts. However, 8 of 9 ISS astronauts completed the test on R+1. On R+3, 13 of 15 (87%) of the ISS and 19 of 19 (100%) of the Shuttle astronauts completed the 10-min test. An index comprised of stroke volume and diastolic blood pressure provided a very good prediction of overall tilt survival. This index was altered by spaceflight similarly for both groups soon after landing (pooled R+0 and R+ 1), but ISS astronauts did not recover at the same rate as Shuttle astronauts (p = 0.007).

The proportion of ISS astronauts who could not complete the tilt test on R+0 due to OH (4 of 6) is similar to that reported in astronauts who flew on Mir (5 of 6). Further, cardiovascular parameters most closely associated with OH recover more slowly after long- compared to short-duration spaceflight.

Hemodynamic effects of midodrine after space flight in astronauts without orthostatic hypotension This study included both retrospective and prospective aspects. Retrospectively, authors used data from tilt or stand tests that had been performed on astronauts as a part of an earlier research protocol or as part of a test performed for medical operations. From those results, astronauts were identified who had completed landing day tilt tests without symptoms of hypotension or presyncope. Then, as those crew members were assigned to upcoming flights, authors asked them to take midodrine on landing day as the prospective part of the study.

Thus tilt/stand test data from the prior flights (control flight) were compared with tilt test data from the new flights (midodrine flight).

Authors identified astronauts who had previously flown in space and for whom they had tilt test data, and thus knew if they were susceptible to postflight orthostatic hypotension and presyncope. From this group, five veteran, male astronauts volunteered to participate in this study. Subjects returned to earth in the seated position following short duration space flights averaging 11.86 ± 0.81 days. Upon arrival, astronauts were assisted off the orbiter and onto the crew transport vehicle where they were de-suited. Crew members were then trans-

Page 4

ported back to the data collection facility. During transport crew members were ambulatory and were allowed to consume water ad libitum. Midodrine was given as soon as the crew members arrived at the data collection facility (about 2 hours after landing). Post-flight activities were similar for all flights.

There were no changes in either arterial pressure or heart rate during the midodrine tolerance tests. The time between the 1st flight (control flight) and the 2nd flight (midodrine flight) was 53.8 ± 10.7 months. Median flight duration for the midodrine flights was 10.3 days while the median for the control flights was 12.8 days.

While height and weight did not vary between flights, BMI’s were significantly higher following the midodrine flight (24.22 ± 1.52 vs 26.42 ± 1.7, p ≤ 0.05) as was age (45.62 ± 1.3 vs 50.2 ± 1.42 years p ≤ 0.05) . Other than mild piloerection, no side effects associated with midodrine were observed in the course of this study.

STEM Today, March 2018, No.30

Tilt/stand tests were performed ten days before flight and about 3 hours after landing. During a 5 minute supine rest period, ECG, finger blood pressure (beat-to-beat), brachial artery blood pressure (every minute) and stroke volume (Doppler ultrasound of ascending aorta) were measured. Then the subjects were moved to the upright posture by one of two methods: following the control flight, three subjects had their shoulders lifted off the bed, while their feet were swept off the bed by investigators (stand test), The remaining two subjects had tilt tests done after the control flight where an automatic tilt table was used to tilt the subjects passively to 80◦ for 10 minutes, during which all measurements continued. All subjects underwent tilt testing before and after the midodrine flight. Previous published work from this laboratory has shown that the stand tests and tilt tests elicit orthostatic hypotension and presyncope at statistically indistinguishable rates For all flights, the astronauts performed the standard oral fluid load (equivalent to isotonic saline at 15 ml/kg within 2 hours) prior to landing, and had their anti-gravity suits fully inflated (1.5 psi). The subjects abstained from caffeine, alcohol, and medications for 12 hours before the test session; were at least 2 hours postprandial; and had not exercised maximally 24 hours before testing.

The main differences in protocols between the control and midodrine flights were as follows. After the control flights, no pharmacological countermeasures were used; after the midodrine flights a single 10 mg dose of midodrine was given following landing, and ∼1 hour before the tilt test was performed. Because there is some evidence that space flight can effect Q-Tc interval, a conservative approach was taken and after the midodrine flights, an additional 12-lead ECG was performed before midodrine administration to verify that the Q-Tc interval was < 0.46 seconds. A catheter was inserted into an antecubital vein for blood collection. Blood was collected for a baseline measure and then every 15 minutes to measure the concentration of midodrine and de-glymidodrine, the active metabolite. The subjects were placed supine on the table and the tilt/stand test was performed so that they were tilted as close to 1 hour after midodrine ingestion as possible .

Results Tilt test responses Systolic pressure (108 ± 2.87 vs 119 ± 3.99 mmHg) , diastolic pressure (74.8 ± 4.55 vs 80.4 ± 1.63 mmHg) and cardiac output (2.82 ± 0.40 vs 2.45 ± 0.26 l/min) during tilt/stand tests were similar between control and midodrine flights respectively (10 minute post flight data shown). There were no statistically significant differences between blood pressure or cardiac output. No subject experienced hypotension or presyncope during any test. Hemodynamic responses were very similar between flights. Of note, however, is that mean postflight upright heart rate was significantly higher than the preflight baseline for the control flight (p = 0.001), but was not after the midodrine flight (p = 0.185), compared to their respective preflight upright heart rates.

Importantly, midodrine did not result in significantly increased supine or upright blood pressure. Additionally, supine stroke volume (P = 0.056) tended to be higher after midodrine, compared to preflight.

Drug clearance Plasma concentrations of midodrine and the active metabolite de-glymidodrine both peaked at times that were reported previously. Midodrine peaked at 30 minutes and de-glymidodrine peaked at 90 minutes.

There were no striking hemodynamic differences between the control and midodrine tilt responses, but the data suggest that midodrine had a modest beneficial effect. Although no astronauts were hypotensive following their control flights, increases in heart rates during tilt on landing day indicate that they were less tolerant of the procedure than before flight. They had a less pronounced increase in heart rate following midodrine and standing systolic pressure tended to be higher after midodrine.

Effects of Promethazine and Midodrine on Orthostatic Tolerance This study evaluated the interactive effects of midodrine and promethazine on hemodynamic responses to

Page 5

STEM Today, March 2018, No.30

upright tilt. This study was approved by the National Aeronautics and Space Administration (NASA) Johnson Space Center Committee for the Protection of Human Subjects and all subjects gave written, informed consent.

A total of eight volunteers (5 men and 3 women) who passed a modified Air Force Class III physical examination were studied. The average (Mean ± SEM) age was 42 ± 2 yr (range: 35-48), weight was 73.1 ± 5.4 kg (range: 47-91), and height was 175.6 ± 4.0 cm (range: 155-188). Subjects abstained from caffeine and alcohol for 12 h, all medications for 24 h, and had eaten a snack of complex carbohydrates 2 h prior to each test session.

For each test session, the subject was placed on a tilt table in a quiet, temperature controlled room (23-25◦ C).

Intravenous catheters were inserted into antecubital veins in each arm: one for drug administration, the other for blood collection. The subject was instrumented for automatic brachial arterial pressure and pulse, electrocardiogram, and beat-to-beat finger arterial pressure. Doppler ultrasound of the ascending aorta was used to determine stroke volume.

The subject was transferred to a comfortable reclining chair and given either a midodrine or placebo capsule. After 40 min the subject was placed on a tilt table and remained supine for another 20 min (for a total of 60 min after midodrine administration). Then the intravenous promethazine or saline alone was infused.

Heart rate and arterial pressure were measured every minute during the infusion, and a blood draw was taken for later analyses of norepinephrine, epinephrine, plasma renin activity, and aldosterone. Supine hemodynamic data were collected for 6 min and the table was then tilted to 80◦ head-up. Upright tilt was maintained for 20 min unless presyncopal symptoms caused early termination. Hemodynamic measurements were taken at 1-min intervals. A final blood sample was drawn at the end of the tilt period.

Tilt test outcomes are presented in Table I . All subjects completed the tilt test during midodrine alone, but no subject completed the tilt test during promethazine alone. Standing minus supine (delta) systolic and diastolic pressures, heart rate, stroke volume, cardiac output, and total peripheral resistance for all four treatments are shown in Fig.1. During upright tilts, systolic pressure fell more with promethazine alone than during any other treatment (Fig.1A), and diastolic pressure fell more with promethazine alone than with midodrine alone or with no drug (Fig.1B). Cardiac output fell more with promethazine alone than with no drug or with midodrine plus promethazine (Fig.1E). No other differences were found although total peripheral vascular resistance tended to be lower with promethazine.

Interestingly, heart rate responses with promethazine were not greater even given the huge falls in arterial pressure, indicating a loss of baroreflex mediated increases in cardiac sympathetic activity. Plasma catecholamines, plasma renin activity, and aldosterone are shown in Fig.2. Norepinephrine responses to tilt with promethazine alone were signifi cantly lower than with midodrine alone or with no drug (Fig.2A). No differences were seen in epinephrine responses (Fig.2B). Tilt induced increases in plasma renin activity (Fig.2C) and aldosterone (Fig.2D) were virtually blocked by promethazine. They were both significantly reduced with promethazine alone and with midodrine plus promethazine.

Midodrine can be 100% effective in supporting arterial pressure during a 20 min tilt test. Promethazine caused 100% failure during a 20 min tilt test and even overrode the protective effect of midodrine in over half of the subjects. Promethazine is the most common drug prescribed to treat space motion sickness. Perhaps flumazenil, which has minimal side effects, might be given in combination to block some of the sedative effects of promethazine. Even though flumazenil is only approved for intravenous administration, this should not be a limitation for postflight use, as flight surgeons routinely rehydrate crewmembers intravenously. In addition to relevance to post-spaceflight orthostatic intolerance, this finding is also related to Earth-based medical practice.

Page 6

Page 7

STEM Today, March 2018, No.30

STEM Today, March 2018, No.30

Promethazine significantly increased the incidence of orthostatic hypotension in subjects, even when combined with midodrine. Inhibition of sympathetic responses, likely via enhancement of the inhibitive effects of GABA, by promethazine may underlie the increased orthostatic hypotension. Promethazine also appears to inhibit responses of the renin angiotensisn system during orthostatic challenge.

Hypovolemia As a Model of Space Flight: Cardiovascular Effects and Countermeasures

Circulating blood volume is reduced during spaceflight, leaving astronauts hemodynamically compromised after

Page 8

landing. Because of this hypovolemia, crew members are able to withstand a postflight 10 minute upright tilt test only if they are able to mount a hyper-sympathetic response. Previous work from this laboratory has shown that about 30% of astronauts, primarily female, have postflight sympathetic responses to tilt that are equal to or less than their preflight responses and thus, they become presyncopal. Part of the mission of the cardiovascular lab at the Johnson Space Center is to identify susceptible crewmembers before flight so that individualized countermeasures can be prescribed.

STEM Today, March 2018, No.30

The goal of this study was to develop a ground based model of hypovolemia that could be used for this purpose.

A major physiological adaptation to spaceflight is a reduction in circulating blood volume, which has a significant effect on orthostatic tolerance. Other systems must compensate for the volume loss if orthostatic tolerance is to be maintained after landing. During postflight testing of orthostatic tolerance, only those who can mount an extra-ordinary sympathetic response are able to maintain upright blood pressures during stand or tilt tests. A substantial percentage of astronauts cannot mount such a response and experience severe orthostatic hypotension and presyncope. The ability to predict, before flight, those astronauts that will have orthostatic hypotension on landing day would be a major breakthrough in the effort to develop appropriate countermeasures. To date, the only predictor of postflight orthostatic hypotension and presyncope in an individual astronaut has been the tilt test outcome from a prior flight.

Preflight identification of susceptibility is difficult because, prior to flight, large sympathetic responses are not needed to maintain upright posture. Thus incidents of presyncope preflight are very rare in these normal healthy astronauts. Authors theorized that, since loss of blood volume is a large driver of postflight orthostatic hypotension, it may be possible to reproduce the landing day presyncope by reproducing the landing day hypovolemia.

In a ground-based study, authors used a regimen of an intravenous dose of furosemide plus a low salt diet to induce hypovolemia. They tested the hypothesis that the incidence of presyncope during upright tilt testing would be the same during hypovolemia as it is on landing day, and that the underlying cause of presyncope would be inadequate compensatory sympathetic responses. If true, we would be able to predict before flight which individuals would become presyncopal after spaceflight, so that countermeasures could be prescribed prospectively for them.

Subjects were 12 men and 5 women, aged 40.9 ± 2 years. Six of these were astronauts (five men and one woman), for whom authors were able to compare their previous preflight and postflight tilt test outcomes with their current normovolemia and hypovolemia tilt test outcomes.

Results Plasma Volumes Plasma volume index was significantly decreased on the hypovolemia day (P < 0.0001). The loss ranged from -0.02 to -0.59 L/m2 . There were no differences in arterial pressure between the normovolemia and hypovolemia days, but heart rate was higher during hypovolemia. Baseline systolic and diastolic pressures were 114 ± 3 mmHg and 66 ± 2 mmHg during normovolemia and 114 ± 3 mmHg and 69 ± 2 mmHg during hypovolemia (p = NS). Baseline heart rate was 56 ± 2 bpm during normovolemia and 63 ± 3 bpm during hypovolemia (P < 0.0005).

Orthostatic Tolerance Of the six astronaut subjects who participated in this study, the four who became presyncopal during tilt tests on landing day also became presyncopal during tilt tests on the hypovolemia day. The two astronauts who did not become presyncopal on landing day did not become presyncopal during tests with hypovolemia. Unfortunately, no preflight or postflight plasma volume or norepinephrine data were available so comparisons were not possible.

Norepinephrine Responses During normovolemia the two subjects had similar increases in norepinephrine levels during upright tilt. During hypovolemia, they had similar supine plasma norepinephrine levels, but the presyncopal subject had no additional increase in response to tilt, while the non-presyncopal subject had a greater than two-fold increase.

This same trend holds true for the entire group of subjects.

Combined Responses The survival curves in Figure 5 show that the probability of surviving a 15 minute tilt test decreases with increasing plasma volume loss and increases with the increasing norepinephrine release. Plasma volume losses of 0.2, 0.4 and 0.6 L/m2 , and plasma NE increase of 0 (panel A), 250 (panel B), 500 (panel C) and 750 pg/ml (panel D) were chosen for illustration. The summary of Figure 5 is that adequate sympathetic compensation can protect tilt survival even in the face of profound hypovolemia.

Page 9

This study describes a very important finding; that if astronauts and normal subjects are subjected to experimentallyinduced hypovolemia that reproduces spaceflight-induced hypovolemia, they will self-separate into two groups, those who do and those who do not become presyncopal during an upright tilt test. In the astronauts whom authors tested under both conditions, the hypovolemia reproduced the occurrence of post-spaceflight presyncope with 100% fidelity. Moreover, the mechanism of presyncope in both conditions appears to be the same; the failure to mount the hyper-sympathetic response needed to overcome the hemodynamic compromise caused by hypovolemia. Using this model, authors explained that even with a plasma volume loss of 30%, subjects who can release enough norepinephrine still have an 80% probability of completing 15 minutes of upright tilt.

This model can provide new opportunities for the study of mechanisms of blood pressure control. By driving subjects into a compromised hemodynamic state, we can measure the limits of sympathetic responsiveness. The model offers the means to determine which astronauts may be susceptible to postflight orthostatic hypotension prior to their first flight, so that countermeasure treatments can be individualized and prescribed prospectively.

Within patient populations the model could be useful in the study of the interaction of volemic state and the sympathetic nervous system in both hypotension and hypertension.

STEM Today, March 2018, No.30

Effects of Hypovolemia on Cerebral Blood Velocity and Autoregulation During Upright Tilt: Implications for Post-Spaceflight Orthostasis Orthostatic stability depends on maintenance of adequate cerebral blood flow. Orthostatic instability experienced by returning astronauts is associated with microgravity-induced hypovolemia, suggesting that hypovolemia may disrupt the ability of the cerebral vasculature to regulate blood flow.

In this study, authors test the hypothesis that hypovolemia reduces cerebral blood velocity and impairs cerebral autoregulation (CA) during upright tilt.

Nine males (age 23 ± .5 yrs; height 172 ± 2 cm; weight 87 ± 3 kg; mean ± SE) were tilted head-up to 70◦ on two occasions separated by at least 5 days under euhydration (EUH) and dehydration (DEH) conditions.

Dehydration was induced with 40 mg Furosemide and 8 h water restriction. Plasma volumes (PV) and blood volumes (BV) were estimated from venous hemoglobin and hematocrit. ECG, beat-by-beat finger arterial pressures, and cerebral blood velocity (CBV) were measured during a five min supine baseline, and during the first (T1) and last (T2) five min of upright tilt. Dynamic CA was assessed in the frequency domain with cross-spectral analysis of mean arterial pressure (MAP) and mean CBV within the frequency range of 0.07-0.2 Hz Furosemide reduced PV by 10 ± 2 % and BV by 6 ± 2 % (P = .005 and P = .07). MAP decreased during tilt (P < .007), but the reduction was similar between hydration conditions. CBV during DEH was lower during the entire 10-min tilt by about 7 cm/s (P < .004) compared with EUH. Low frequency coherence was higher during DEH T1 compared with EUH T1 (.67 ± .04 vs .51 ± .04; P =.02), but coherence decreased as tilt continued, and was similar to EUH during T2 (P =0.7).

Increased coherence during the first 5 min of tilt suggests that reductions of CBV with hypovolemia might be explained by a reduced autoregulatory capacity. However, maintenance of lower CBV despite reduced coherence during the second 5 min of tilt suggests that disruptions of autoregulatory capacity with hypovolemia are transient. Our results provide evidence that hypovolemic astronauts may be at greatest risk for orthostatic intolerance immediately upon assumption of upright posture.

In-flight Use of Florinef to Improve Orthostatic Intolerance Postflight Hypovolemia may be one cause of the orthostatic intolerance that is common after space flight. Space flight is known to cause a shift in fluid away from the lower body and a decrease in body fluid. These effects are also produced by head-down tilt bed rest. Fludrocortisone (Florinef) is a drug that increases plasma volume by causing the kidneys to retain sodium. It has shown some ability to restore plasma volume and orthostatic tolerance after bed rest. However, the absorption of a drug may be altered in microgravity, and a dosage regimen that is effective on the ground may not be effective in space. The objective of this study was to test the use of fludrocortisone during space flight to expand plasma volume and improve postflight orthostatic tolerance, comparing three dosage regimens.

There were 25 male astronauts who were randomized into 2 groups: placebo (n = 18) and fludrocortisone (n = 7), and participated in stand tests 10 days before launch and 2-4 hours after landing. Three regimens of fludrocortisone were used: (1) 0.2 mg twice daily (B.I.D.) for the last 5 days of flight,(2) 0.1 mg twice a day for the last 5 days of flight, or (3) a single dose of 0.3 mg taken 7 hours before landing. Supine blood volume, supine and standing heart rate and arterial pressure, as well as plasma catecholamines were measured before

Page 10

and after flight.

Only limited conclusions could be made because participation in the protocols was incomplete, and were also limited by subjective evaluation of the medication. The first two protocols (five-day regimens) were not welltolerated by the crew, being found to cause unacceptable side effects such as congestion, head fullness, and headache. The third regimen (single-dose) did not restore plasma volume and did not seem to improve orthostatic tolerance on landing day.

None of the protocols restored blood volume. The percent change in plasma and red blood cell volumes from preflight to postflight was not significantly different in the fludrocortisone vs. non-fludrocortisone group. Fludrocortisone subjects did not have greater orthostatic tolerance than control subjects on landing day.

On landing day, 2 of 18 in the placebo group and 1 of 7 in the fludrocortisone group became presyncopal (chi2 = 0.015, p = 0.90). Plasma volumes were significantly decreased after flight in the placebo group, but not in the fludrocortisone group. During postflight stand tests, standing plasma norepinephrine was significantly less in the fludrocortisone group compared with the placebo group.

STEM Today, March 2018, No.30

The conclusion therefore, was that treatment with a single dose of fludrocortisone results in protection of plasma volume but no protection of orthostatic tolerance. Fludrocortisone was, therefore, not recommended as a countermeasure for spaceflight-induced orthostatic intolerance.

Use of Fludrocortisone or Salt Load to Expand Plasma Volume in Ambulatory Subjects It is known that a fall in blood pressure occurs after space flight on return to Earthâ&#x20AC;&#x2122;s gravity. This is associated with a reduced volume of plasma, an adaptation made for the weightless environment, and currently restored by the ingestion of large volumes of water and salt tablets on landing. Earlier work revealed that the synthetic corticosteroid, flurocortisone, clinically useful in patients with low blood pressure, could expand plasma volume in subjects exposed to the bed rest model for space flight. The purpose of this study was to compare the effectiveness of salt ingestion with the administration of a synthetic corticosteroid to prevent orthostatic hypotension.

A previous study (HR77) had tested 4 subjects with fludrocortisone (Florinef) and 4 subjects with a saline load after 7 days of - 6o bed rest. Florineff restored plasma volume to pre-bedrest levels, but a saline load did not. The response in ambulatory subjects was opposite to this post bed rest response. This study plans to repeat the earlier study in the same subjects, reversing the protocol (giving Florinef to subjects who were treated with a saline load before and giving a saline load to subjects treated with Florinef earlier). A second group of tests was done at night to understand if the lack of effect of a salt load in some astronauts was related to a difference in the regulation of salt excretion during the evening. The saline load was 8 g NaCl, (1 g/tablet) given with 960 ml water and the dose of Florinef was 0.2 mg every 8 hrs for 3 doses.

Four of 5 subjects in the saline group experienced syncope and could not complete 15 minutes of standing after bed rest, while 5 of the 6 subjects given Florinef showed no syncope. After 7 days of bed rest, the increase in heart rate was similar in Florinef and saline treated subjects (to 24 bpm) and greater than the heart rate response before bed rest (to 13 bpm, p<0.05). However the saline load failed to maintain mean arterial pressure during standing post-bedrest, while Florinef treatment did maintain blood pressure. Plasma volume decreased approximately 12% after 7 days bed rest. Following treatments, Florinef returned plasma volume to within 4% of baseline whereas saline failed to restore plasma volume. Urine volume increased during 2 hours of standing after the saline load and decreased after Florinef compared with the pre-bed rest response. Urine Na and the Na/K ratio was similar before and after bed rest following the salt load, but urine Na was decreased and the Na/K ratio reduced after bed rest following Florinef.

Acute treatment with Florinef appears to have distinct advantages over the administration of a saline load as a countermeasure for post-bed rest orthostatic intolerance. Florinef restored plasma volume, maintained mean arterial pressure, and reduced the renal excretion of Na and the Na/K ratio whereas a saline load did not.

Vestibular-Cerebrovascular Interactions and Their Contribution to Post-Spaceflight Orthostatic Intolerance

Page 11

Post-spaceflight orthostatic intolerance, a principal NASA safety concern, is a complex multi-factorial problem that continues to be poorly understood. Recent evidence clearly suggests that the vestibular otolith system, which is directly affected by spaceflight, assists in both autonomic and blood pressure regulation during orthostatic stress. Vestibular activation has also has direct effects on cerebral blood flow suggesting that vestibular inputs also affect the cerebrovascular response to orthostasis.

STEM Today, March 2018, No.30

Vestibular effects on cerebral blood flow To maintain arterial pressure when upright, humans must respond to a translocation of blood from the upper into the lower body. Responses to this translocation include baroreflex-mediated increases in heart rate and peripheral vasoconstriction, thus compensating for reduced venous return and minimizing pooling of blood in the lower body.

Anatomical evidence in animals demonstrates that neural connections are present between the vestibular nuclei and cerebral vessels through two possible pathways (Figure 1). Connections have been found between the Vestibular Nuclei and the Fastigial Nucleus, then to the Rostral Ventrolateral Medulla, followed by vasodilatory connections to the cerebral vessels. Similarly, neurons travel from the Vestibular Nuclei to the Nucleus Tractus Solitarius and then to the Pterygopalatine Ganglion, resulting in cerebral vasodilation. However, the role these connections play in human postural adjustments remains to be determined.

Caloric vestibular stimulation in humans that activates the semicircular canals, involved with detection of movement (i.e. angular acceleration), has been found to increase blood flow in the basilar and middle cerebral arteries as well as the parietal lobe while decreasing flow in the posterior cerebral artery. However, it remains unclear whether these changes were due to functional activation of vestibular and other centers, rather than general vascular changes.

Authors previously found that subjects exposed to 30 minutes of hypergravity demonstrate impaired cerebral blood flow regulation that returned to normal upon assumption of the upright posture . Furthermore, this impairment was found to correlate with non-invasive measures of otolith sensitivity, providing indirect evidence of

Page 12

a role for otolith activation. Similarly, using head down neck flexion, they found a modulation of cerebrovascular resistance (CVR) that may have been due to otolith activation. Finally, in subjects that developed nausea during centrifugation, cerebral blood flow was reduced almost two minutes prior to actual nausea. Since centrifugation was performed in the dark with no visual cues, these data suggest a role for vestibular inputs in affecting the cerebrovasculature.

In this study, authors examined the affects of vestibular stimuli on cerebral blood flow by utilizing: 1) prolonged variable-radius centrifugation (to elicit otolith activation without stimulating the semicircular canals); and 2) a pitch tilt stimulus about an Earth-horizontal axis (to elicit cues from both the otolith organs and the semicircular canals).

Twenty four healthy, non-smoking subjects (29 ± 9 years, 72 ± 12 kg, 173 ± 9 cm, 11 females) were recruited.

STEM Today, March 2018, No.30

Results Table 1 demonstrates that the baseline values for each dependent measure were not significantly different between the tilt and centrifuge test sessions. Both centrifugation and tilt resulted in a modulation of CFV as measured by transcranial Doppler that were linked to stimulation frequency (Figure 3). CFV responses were dependent on frequency of stimulation (P < 0.01) and demonstrated significant changes within each frequency cycle (P < 0.01) that differed by frequency (Frequency X Cycle interaction, P < 0.01). While CFV during centrifugation at 0.5 Hz tended to be lower than during tilt (P =0.07), the change within the cycle was very similar.

The CFV responses shown in Figure 3 may have been due to factors unrelated to the otoliths, such as driving pressure or changes in arterial CO2 levels. For example, there was a significant effect on blood pressure at brain level of both frequency of stimulation (P < 0.01) and position within cycle (P < 0.01) that differed by frequency (Frequency X Cycle interaction, P < 0.01). In addition, blood pressure was significantly higher during centrifugation than tilt at all frequencies (P < 0.01).

However, the patterns of the blood pressure and CFV changes were different. For example, as shown in the left panel (0.03125 Hz) of Figure 3, CFV increased during both the +25◦ and -25◦ tilt positions whereas blood pressure increased only during the +25◦ tilt position. Similarly at 0.25 and 0.5 Hz, blood pressure increased during the first half of the cycle (i.e. moving from upright to pitch forward and back to upright) at the same time CFV decreased. Thus CFV was decreasing even though driving pressure was increasing.

Thus, while blood pressure consistently increased somewhere between a quarter (slowest frequency) and half way into the cycle (fastest frequency), CFV demonstrated bimodal peaks at the slowest frequency and decreases at the fastest frequency (Figure 3). This would again suggest a disparity in the response of the two variables.

These data therefore suggest that otolith activation at various frequencies likely directly affects cerebral blood flow.

Changes in end tidal CO2 , an indicator of arterial CO2 , were similarly disparate from those in CFV. First, end tidal CO2 was also affected by both frequency of stimulation (P < 0.01) and position within cycle (P < 0.01) that differed by frequency (Frequency X Cycle interaction, P < 0.01). In addition end tidal CO2 was significantly lower during centrifugation with translation at 0.5 Hz. However, examination of individual frequencies showed differing patterns. For example, at 0.03125 Hz, end tidal CO2 increased at +25◦ and decreased at -25◦ , while CFV increased at both +25◦ and -25◦ . Similarly at 0.25 and 0.5 Hz, end tidal CO2 did not change within the cycle, while CFV decreased significantly. These data demonstrate that changes in end tidal CO2 cannot completely explain position-related changes in CFV.

Page 13

STEM Today, March 2018, No.30

While there was no significant difference in responses to either sinusoidal tilt or translation during centrifugation at the four slowest frequencies, CFV was 6.8 ± 0.3% lower during centrifugation. Since end tidal CO2 was also 1.6 ± 0.3 mmHg lower, based on the cerebrovascular reactivity of 2.6%/mmHg, approximately two thirds of the 6.8% decrease could be explained by the centrifugation-related hypocapnia.

Changes in CFV are likely mediated through changes in CVR. Since CFV is normally regulated to maintain flow relatively constant in the face of changing perfusion pressure –a phenomenon known as cerebral autoregulation – changes in CVR could result from changes in blood pressure (autoregulation) or changes in otolith afferent activity (vestibular).

Page 14

STEM Today, March 2018, No.30

Figure 3 demonstrates that changes in CVR within the motion cycle were similar to the changes in blood pressure, suggesting an autoregulatory response. However, if CVR changes were solely autoregulatory in nature, CFV would have remained constant throughout motion. The fact that CFV was changing throughout the cycles indicates CVR changes were not sufficient to maintain flow indicating that a non-autoregulatory component was influencing CVR and causing changes in flow.

To further explore the role of frequency in the response of CFV to vestibular activation, authors examined the correlation between CFV and either chair position or velocity of motion throughout the cycle (Figure 4).

As can be seen in Figure 4, during tilt, changes in CFV in the low frequency range (0.0625-0.125 Hz) were especially correlated to position (left panel), whereas those in the high frequency range (0.25-0.5 Hz) were especially correlated to the velocity of motion (right panel).

Correlations between CFV and the velocity of motion were significantly lower in the 0.03125-0.125 Hz ranges compared to the 0.25-0.5 Hz ranges for both tilt and centrifugation. A generally opposite pattern was demonstrated for position (left panel), where correlations between CFV and position were â&#x2C6;ź 0.6 for tilt at 0.0625 & 0.125 Hz, decreasing to 0.4 at the higher frequencies. Interestingly, at the lowest frequency for position, the correlation was only 0.2.

The overall results are consistent with vestibular activation at different frequencies of motion causing changes in cerebral blood flow that are in part independent of changes in both mean arterial pressure and end tidal CO2 . Interestingly, sinusoidal pitch tilt and centrifuge stimuli had two main effects.

First, changes in CFV in the low frequency range were generally significantly correlated to position. Regular otolith afferents across this frequency range are known to have constant sensitivity and small phase errors. This would suggest that otolith inputs of tilt angle were the predominant underlying factor in CFV changes at these frequencies.

Page 15

However, it remains unclear why the slowest frequency of motion during tilt (0.03125 Hz) produced the lowest correlations between position and CFV. One possible explanation is that at the lowest frequency the response depended on absolute tilt angle (i.e. irrespective of direction) rather than relative angle (i.e. forward tilt vs. backward tilt).

However, correlating CFV to absolute tilt at 0.03125 Hz only increased the R2 to 0.25 Âą 0.5. The changing correlations during tilt may reflect the influence of vertical canals indirectly through canal-otolith integration at the site of the vestibular nuclei.

Another explanation may be that during tilt at very low frequencies (i.e. 0.03125 Hz), other somatosensory cues also influence the CFV response, thus masking vestibular effects. These non-vestibular gravitational cues may have low-pass filtered dynamics that may differ between tilt and centrifugation paradigms.

STEM Today, March 2018, No.30

In a similar fashion, cardiovascular mechanisms, such as the baroreflex, may also be dominant at lower stimulus frequencies and sufficient to compensate for orthostatic challenges without feed-forward input from the otoliths. In contrast, during centrifugation the correlations with position did not change significantly across frequencies.

This may have been due to the lack of canal inputs and the fact that the novel experience of centrifugation eliminated normal somatosensory cues which may have masked vestibular inputs. The second major effect of vestibular stimulation was the direction-dependent effect at frequencies greater than 0.03125 Hz. Movement from pitch forward (i.e. semiprone) to pitch backward (semi-supine) resulted in increases in CVR and decreases in CFV (Figure 3).

It remains unclear why there was a direction-dependent effect of pitch. Greater postural hypotension during roll vs. pitch tilts, suggesting that cardiovascular responses to tilt may be direction dependent. Changes in CFV during head position manipulation differed in the prone vs. supine position.

Since the plane of the otolith maculae is tilted back about 20â&#x2014;Ś relative to our upright position (Reidâ&#x20AC;&#x2122;s baseline parallel to Earth-horizontal plane), the pitch forward tilts would bring the otoliths into a position of increased sensitivity whereas the pitch backward would bring them into a position of reduced sensitivity. Thus, the differences in response may be due to differing otolith sensitivity in the two positions.

It is also interesting to note that irregular otolith afferents, which detect change in linear acceleration which is more associated with motion, become increasingly sensitive at higher frequencies. This response is manifested in the oculo-motor correlate that at higher frequencies, linear acceleration produces horizontal eye movements that compensates for retinal slip velocity, whereas at lower frequencies authors see ocular counter-roll, associated with tilt response.

Another possible explanation for the direction- dependent effect at higher frequencies is that by maintaining the equivalent tilt stimuli constant across frequencies, the velocity amplitude increased as a function of stimulus frequency.

Regardless of the exact etiology of these responses, the data are consistent with a vestibular mediated frequency dependent effect on the cerebral blood flow response to postural position changes relative to gravitoinertial forces.

These findings suggest a role for vestibular inputs in cerebral blood flow regulation.

Page 16

References Midodrine as a Countermeasure Against Postflight Orthostatic Hypotension (DSO 503S), LSDA, NASA.

Platts SH, Ziegler MG, Waters WW, Meck JV. Hemodynamic Effects of Midodrine After Spaceflight in Astronauts Without Orthostatic Hypotension. Aviation, Space, and Environmental Medicine. 2006 April; 77(4): 429-433. PMID: 16676655.

Platts SH, Ziegler MG, Waters WW, Meck JV. Midodrine Prescribed to Improve Recurrent Post-Spaceflight Orthostatic Hypotension. Aviation, Space, and Environmental Medicine. 2004 June; 75(6): 554-556. PMID: 15198285.

Waters WW, Ziegler MG, Meck JV. Postspaceflight orthostatic hypotension occurs mostly in women and is predicted by low vascular resistance. Journal of Applied Physiology. 2002 February; 92(2): 586-594. DOI: 10.1152/japplphysiol.00544.2001. PMID: 11796668.

STEM Today, March 2018, No.30

Shi S-J, Platts SH, Ziegler MG, Meck JV. Effects of promethazine and midodrine on orthostatic tolerance.

Aviat Space Environ Med 2011; 82:9-12.

Hypovolemia Induced Orthostatic Hypotension in Presyncopal Astronauts and Normal Subjects Relates to Hypo-Sympathetic Responsiveness, NTRS, NASA.

Romero, Steven A.; Sanborn, Eren R.; Moralez, Gilbert; and Cooke, William H. (2009) "Effects of Hypovolemia on Cerebral Blood Velocity and Autoregulation During Upright Tilt: Implications for Post-Spaceflight Orthostasis," International Journal of Exercise Science: Conference Proceedings: Vol. 2 : Iss. 1 , Article 8.

Garrett, Zachary K., James Pearson, and Andrew W. Subudhi. "Postural Effects on Cerebral Blood Flow and Autoregulation." Physiological Reports 5.4 (2017): e13150. PMC. Web. 26 Mar. 2018.

Romero SA, Moralez G, Rickards CA, Ryan KL, Convertino VA, Fogt DL, Cooke WH.Control of cerebral blood velocity with furosemide-induced hypovolemia and upright tilt. J Appl Physiol (1985). 2011 Feb;110(2):4928. doi:10.1152/japplphysiol.01060.2010. Epub 2010 Nov 25. PubMed PMID: 21109596.

Shi SJ, South DA, Meck JV. Fludrocortisone does not prevent orthostatic hypotension in astronauts after spaceflight. Aviat Space Environ Med. 2004 Mar;75(3):235-9.

Use of Fludrocortisone or Salt Load to Expand Plasma Volume in Ambulatory Subjects (ARCbedrest112),LSDA, NASA.

Noskov, V.B. Orthostatic tolerance after space flight and model experiments: new approaches to evaluation and prevention. Hum Physiol (2014) 40: 704. https://doi.org/10.1134/S0362119714070160 .

Simanonok KE, Fortney SM, Ford SR, Charles JB, Ward DF. The effect of dDAVP with saline loading on fluid balance during LBNP and standing after 24-hr head-down bedrest. J Gravit Physiol. 1994 May;1(1):P104-5.

PubMed PMID: 11538731.

Srinivasan RS, Simanonok KE, Charles JB. Computer simulation of the effect of dDAVP with saline loading on fluid balance after 24-hour head-down tilt. J Gravit Physiol. 1994 May;1(1):P106-7. PubMed PMID: 11538732.

Serrador, Jorge M et al. "Vestibular Effects on Cerebral Blood Flow." BMC Neuroscience 10 (2009): 119. PMC. Web. 26 Mar. 2018.