Alteration in facial skin blood flow during acute exposure to -10 and -30° head-down tilt in young human volunteers

NEW FINDINGS

What is the central question of this study? Facial skin blood flow (SBF) might increase during head-down tilt (HDT). However, the effect of HDT on facial SBF remains controversial. In addition, the changes in facial SBF in the cheek (cheek SBF) during a steeper angle of HDT (>-12° HDT) have not been investigated. What is the main finding and its importance? This study showed that cheek SBF decreased during -30° HDT, alongside increased vascular resistance. Furthermore, vascular impedance was suggested to be elevated, accompanied by an increased hydrostatic pressure gradient caused by HDT. Constriction of the facial skin vascular bed and congestion of venous return owing to the steep angle of HDT can decrease facial SBF.

ABSTRACT

Head-down tilt (HDT) has been used to simulate microgravity in ground-based studies and clinical procedures including the Trendelenburg position or in certain surgical operations. Facial skin blood flow (SBF) might be altered by HDT, but the effect of a steeper angle of HDT (>-12° HDT) on facial SBF remains unclear. We examined alterations in facial SBF in the cheek (cheek SBF) using two different angles (-10 and -30°) of HDT and lying horizontal (0°) in a supine position for 10 min, to test the hypothesis that cheek SBF would increase with a steeper angle of HDT. Cheek SBF was measured continuously by laser Doppler flowmetry. Cheek skin vascular resistance and the pulsatility index of cheek SBF were calculated to assess the circulatory effects on the facial skin vascular bed in the cheek. Cheek SBF decreased significantly during -30° HDT. In addition, the resistance in cheek SBF increased significantly during -30° HDT. The pulsatility index of cheek SBF increased during both -10 and -30° HDT. Contrary to our hypothesis, cheek SBF decreased during -30° HDT along with increased skin vascular resistance. Vascular impedance, estimated by the pulsatility index in the cheek SBF, was elevated during both -10 and -30° HDT, and elevated vascular impedance would be related to increased hydrostatic pressure induced by HDT. Skin vascular constriction and venous return congestion would be induced by -30° HDT, leading to deceased cheek SBF. The present study suggested that facial SBF in the cheek decreased during acute exposure to a steep angle of HDT (∼-30° HDT).

Short-Term Volume Loading Effects on Estimated Intracranial Pressure in Human Volunteers

BACKGROUND: Short-term fluid loading is used as part of post-spaceflight medical procedures and clinical treatment in hospitals. Hypervolemia with hemodilution induced by rapid fluid infusion reportedly impaired dynamic cerebral autoregulation. However, the effects on intracranial pressure (ICP) remain unknown. Therefore, we estimated ICP noninvasively (nICP) to examine whether rapid fluid infusion would raise ICP.METHODS: Twelve healthy male volunteers underwent two discrete normal saline (NS) infusions (15 and 30 ml · kg^-1 stages, NS-15 and NS-30, respectively) at a rate of 100 ml · min^-1. The cerebral blood flow (CBF) velocity (CBFv) waveform from the middle cerebral artery obtained by transcranial Doppler ultrasonography was recorded, as was the arterial blood pressure (ABP) waveform at the radial artery obtained by tonometry. We then used these waveforms to calculate nICP, cerebral artery compliance, and the pulsatility index (PI) in an intracranial hydraulic model.RESULTS: nICP increased significantly in both infusion stages from preinfusion (preinfusion: 7.6 ± 3.4 mmHg; NS-15: 10.9 ± 3.3 mmHg; NS-30: 11.7 ± 4.2 mmHg). No significant changes were observed in cerebral artery compliance or PI. Although ABP did not change in any stage, CBFv increased significantly (preinfusion: 67 ± 10 cm · s^-1; NS-15: 72 ± 12 cm · s^-1; NS-30: 73 ± 12 cm · s^-1).DISCUSSION: Hypervolemia with hemodilution induced by rapid fluid infusion caused increases in nICP and CBFv. No changes were observed in cerebral artery compliance or PI related to cerebrovascular impedance. These findings suggest that rapid fluid infusion may raise ICP with increased CBF.Kurazumi T, Ogawa Y, Takko C, Kato T, Konishi T, Iwasaki K. Short-term volume loading effects on estimated intracranial pressure in human volunteers. Aerosp Med Hum Perform. 2022; 93(4):347-353.

Effects of -10° and -30° head-down tilt on cerebral blood velocity, dynamic cerebral autoregulation and non-invasively estimated intracranial pressure

Steady-state cerebral blood flow (CBF) and dynamic cerebral autoregulation are reportedly maintained during −10° head-down tilt (HDT) despite slight increases in intracranial pressure (ICP). However, the higher ICP during −30° HDT may alter steady-state CBF and dynamic cerebral autoregulation. The present study hypothesized that steady-state CBF and dynamic cerebral autoregulation would be altered by higher ICP during −30° HDT than during 0° and −10° HDT. Seventeen healthy participants were positioned horizontal (0°) and in −10° HDT and −30° HDT for 10 min in random order on separate days. The arterial blood pressure waveform was obtained using a finger blood pressure device and the cerebral blood velocity waveform in the middle cerebral artery was obtained using transcranial Doppler sonography (TCD) for the last 6 min in each position. ICP was estimated using noninvasive ICP (nICP) based on TCD. Dynamic cerebral autoregulation was evaluated by spectral and transfer function analysis. Although nICP was significantly higher during −30° HDT (12.4 mmHg) than during −10° HDT (8.9 mmHg), no significant differences in steady-state mean cerebral blood velocity or transfer function gain in any frequency ranges were seen among all angles of HDT. Counter to our hypothesis, the present results suggest that steady-state CBF and dynamic cerebral autoregulation may be preserved during short-term −30° HDT despite the higher ICP compared with that during −10° HDT.

NEW & NOTEWORTHY This appears to be the first study to evaluate steady-state cerebral blood flow (CBF), dynamic cerebral autoregulation, and intracranial pressure (ICP) during −30° head-down tilt (HDT) compared with those during −10° HDT using noninvasive measurements. The results suggest that steady-state CBF and dynamic cerebral autoregulation are preserved despite the higher ICP during short-term −30° HDT compared with −10° HDT.

Long-duration spaceflight alters estimated intracranial pressure and cerebral blood velocity

Key points

During long‐duration spaceflights, some astronauts develop structural ocular changes including optic disc oedema that resemble signs of intracranial hypertension.

In the present study, intracranial pressure was estimated non‐invasively (nICP) using a model‐based analysis of cerebral blood velocity and arterial blood pressure waveforms in 11 astronauts before and after long‐duration spaceflights.

Our results show that group‐averaged estimates of nICP decreased significantly in nine astronauts without optic disc oedema, suggesting that the cephalad fluid shift during long‐duration spaceflight rarely increased postflight intracranial pressure.

The results of the two astronauts with optic disc oedema suggest that both increases and decreases in nICP are observed post‐flight in astronauts with ocular alterations, arguing against a primary causal relationship between elevated ICP and spaceflight associated optical changes.

Cerebral blood velocity increased independently of nICP and spaceflight‐associated ocular alterations. This increase may be caused by the reduced haemoglobin concentration after long‐duration spaceflight.

Abstract

Persistently elevated intracranial pressure (ICP) above upright values is a suspected cause of optic disc oedema in astronauts. However, no systematic studies have evaluated changes in ICP from preflight. Therefore, ICP was estimated non‐invasively before and after spaceflight to test whether ICP would increase after long‐duration spaceflight. Cerebral blood velocity in the middle cerebral artery (MCAv) was obtained by transcranial Doppler sonography and arterial pressure in the radial artery was obtained by tonometry, in the supine and sitting positions before and after 4−12 months of spaceflight in 11 astronauts (10 males and 1 female, 46 ± 7 years old at launch). Non‐invasive ICP (nICP) was computed using a validated model‐based estimation method. Mean MCAv increased significantly after spaceflight (ANOVA, P = 0.007). Haemoglobin decreased significantly after spaceflight (14.6 ± 0.8 to 13.3 ± 0.7 g/dL, P < 0.001). A repeated measures correlation analysis indicated a negative correlation between haemoglobin and mean MCAv (r = −0.589, regression coefficient = −4.68). The nICP did not change significantly after spaceflight in the 11 astronauts. However, nICP decreased significantly by 15% in nine astronauts without optic disc oedema (P < 0.005). Only one astronaut increased nICP to relatively high levels after spaceflight. Contrary to our hypothesis, nICP did not increase after long‐duration spaceflight in the vast majority (>90%) of astronauts, suggesting that the cephalad fluid shift during spaceflight does not systematically or consistently elevate postflight ICP in astronauts. Independently of nICP and ocular alterations, the present results of mean MCAv suggest that long‐duration spaceflight may increase cerebral blood flow, possibly due to reduced haemoglobin concentration.

During long‐duration spaceflights, some astronauts develop structural ocular changes including optic disc oedema that resemble signs of intracranial hypertension.

In the present study, intracranial pressure was estimated non‐invasively (nICP) using a model‐based analysis of cerebral blood velocity and arterial blood pressure waveforms in 11 astronauts before and after long‐duration spaceflights.

Our results show that group‐averaged estimates of nICP decreased significantly in nine astronauts without optic disc oedema, suggesting that the cephalad fluid shift during long‐duration spaceflight rarely increased postflight intracranial pressure.

The results of the two astronauts with optic disc oedema suggest that both increases and decreases in nICP are observed post‐flight in astronauts with ocular alterations, arguing against a primary causal relationship between elevated ICP and spaceflight associated optical changes.

Cerebral blood velocity increased independently of nICP and spaceflight‐associated ocular alterations. This increase may be caused by the reduced haemoglobin concentration after long‐duration spaceflight.

Changes in cerebral oxygen saturation and cerebral blood flow velocity under mild +Gz hypergravity

We previously reported that cerebral blood flow (CBF) was reduced by even mild +Gz hypergravity. Regional cerebral oxygen saturation as measured by near-infrared spectroscopy (C-rSO₂) has been widely used to detect cerebral ischemia in clinical practice. For example, decreases in C-rSO₂ reflect reduced CBF or arterial oxygen saturation. Thus it was hypothesized that C-rSO₂ would decrease in association with reduced CBF during mild hypergravity. To test this hypothesis, we measured CBF velocity by transcranial Doppler ultrasonography and C-rSO₂ during mild +Gz hypergravity while participants were in a sitting position. Among 17 male participants, 15 completed 21 min of exposure to +1.5 Gz generated by short-arm centrifuge. C-rSO₂ and mean CBF velocity in the middle cerebral artery (MCBFV_MCA) during centrifugation were averaged every 5 min and compared with pre-hypergravity (+1.0 Gz). C-rSO₂ did not change significantly throughout centrifugation, although MCBFV_MCA gradually decreased from the beginning (−1.2% at 0–5 min), and significantly decreased at 5–10 min (−4.8%), 10–15 min (−6.7%), and 15–20 min (−7.4%). Contrary to our hypothesis, decreases in C-rSO₂ were not detected, despite reductions in CBF velocity during hypergravity. Since some assumptions, such as unaltered arteriovenous volume ratio, hemoglobin concentration, extracranial blood flow, and brain activity, need to be satisfied to monitor cerebral ischemia by C-rSO₂, the present results suggest that these necessary assumptions for near-infrared spectroscopy are not always applicable, and that cerebral oxygenation may not precisely reflect decreases in CBF under mild +Gz hypergravity.

NEW & NOTEWORTHY To our knowledge, this is the first study to evaluate simultaneously cerebral oxygenation monitored by near-infrared spectroscopy and cerebral blood flow (CBF) monitored by transcranial Doppler under +1.5 Gz hypergravity. Contrary to our hypothesis, there was no significant correlation between CBF velocity and regional cerebral oxygen saturation (C-rSO₂). However, an incomplete case nearly involving syncope suggests the possibility that C-rSO₂ can detect a remarkable decrease in CBF with development of presyncope during +Gz hypergravity.

Dynamic cerebral autoregulation after confinement in an isolated environment for 14 days

BACKGROUND

To develop human space exploration, it is necessary to study the effects of an isolated and confined environment, as well as a microgravity environment, on cerebral circulation. However, no studies on cerebral circulation in an isolated and confined environment have been reported. Therefore, we investigated the effects of a 14-day period of confinement in an isolated environment on dynamic cerebral autoregulation.

METHODS

We participated in an isolation and confinement experiment conducted by the Japan Aerospace Exploration Agency in 2016. Eight healthy males were isolated and confined in a facility for 14 days. Data were collected on the days immediately before and after confinement. Arterial blood pressure waveforms were obtained using a finger blood pressure monitor, and cerebral blood flow velocity waveforms in the middle cerebral artery were obtained using transcranial Doppler ultrasonography for 6 min during quiet rest in a supine position. Dynamic cerebral autoregulation was evaluated by transfer function analysis between spontaneous variability of beat-to-beat mean arterial blood pressure and mean cerebral blood flow velocity.

RESULTS

Transfer function gain in the low- and high-frequency ranges increased significantly (0.54 ± 0.07 to 0.69 ± 0.09 cm/s/mmHg and 0.80 ± 0.05 to 0.92 ± 0.09 cm/s/mmHg, respectively) after the confinement.

CONCLUSION

The increases observed in transfer function gain may be interpreted as indicating less suppressive capability against transmission from arterial blood pressure oscillation to cerebral blood flow velocity fluctuation. These results suggest that confinement in an isolated environment for 14 days may impair dynamic cerebral autoregulation.

TRIAL REGISTRATION

UMIN000020703 , Registered 2016/01/22.

Time-Dependent Changes in Cerebral Blood Flow and Arterial Pressure During Mild +Gz Hypergravity

BACKGROUND

Artificial hypergravity has been proposed to prevent or treat various forms of physiological deconditioning experienced during spaceflight. We have previously reported that cerebral blood flow decreased at 15-21 min of +1.5-Gz centrifugation without decreases in arterial pressure at heart level. We reanalyzed our previous data to clarify time-dependent changes in cerebral blood flow and arterial pressure during mild +Gz hypergravity.

METHOD

We reanalyzed data for 0-20 min during +1.5-Gz centrifugation on 13 male subjects for whom physiological data were steadily recorded. Mean cerebral blood flow velocity in the middle cerebral artery (MCBFVMCA), mean arterial pressure at heart level (MAPheart), and middle cerebral artery level (MAPMCA) during centrifugation were averaged every 5 min and compared with prehypergravity data (+1.0 Gz, 5 min).

RESULTS

MAPheart did not change significantly, but MAPMCA decreased significantly throughout centrifugation compared to prehypergravity data (-16.7% to -24.7%). MCBFVMCA tended to be decreased at 0-5 min of +1.5-Gz centrifugation (-3.3%), but this was not statistically significant. MCBFVMCA was significantly decreased at 5-10 min (-5.5%). MCBFVMCA at 10-15 min and 15-20 min were also significantly decreased to almost the same level (-6.9% and -6.8%, respectively).

DISCUSSION

No significant change in MAPheart was detected, whereas MAPMCA decreased significantly from the beginning of +1.5-Gz centrifugation. On the other hand, MCBFVMCA gradually decreased and became roughly flat in the latter half of 20-min centrifugation. Understanding the different time-dependent changes in cerebral blood flow and arterial pressure under mild +Gz hypergravity might be important for implementation of centrifuging as a countermeasure for spaceflight-induced deconditioning.Konishi T, Kurazumi T, Kato T, Takko C, Ogawa Y, Iwasaki K. Time-dependent changes in cerebral blood flow and arterial pressure during mild +Gz hypergravity. Aerosp Med Hum Perform. 2018; 89(9):787-791.

Non-Invasive Intracranial Pressure Estimation During Combined Exposure to CO₂ and Head-Down Tilt

BACKGROUND

Exposure to carbon dioxide (CO2) and cephalad fluid shift are considered factors that affect intracranial pressure (ICP) during spaceflight. Increases in ICP were reported during cephalad fluid shift induced by head-down tilt (HDT), while little is known regarding the effect of additional CO2 during HDT on ICP. Therefore, we tested the hypothesis that this combination increases ICP more than HDT alone.

METHODS

There were 15 healthy male volunteers who underwent 4 types of 10-min interventions consisting of Placebo/Supine (air and supine), CO2/Supine (3% CO2 and supine, CO2 alone), Placebo/HDT (air and -10° HDT, HDT alone), and CO2/HDT (3% CO2 and -10° HDT, combination). Using arterial blood pressure (ABP) and cerebral blood flow velocity waveforms, ICP was estimated noninvasively before and during the four interventions. Two calculation methods were employed. One is based on the signal transformation from ABP to ICP with the intracranial component as a "black box" system (nICP_BB), and the other is based on the equation ICP = ABP - cerebral perfusion pressure, reflecting critical closing pressure (nICP_CrCP).

RESULTS

Both nICP_BB and nICP_CrCP significantly increased during Placebo/HDT and CO2/HDT, although there was no statistically significant difference between the nICP indexes of these two interventions.

DISCUSSION

Increases in ICP were observed during both Placebo/HDT and CO2/HDT. Contrary to our hypothesis, the combination of 3% CO2 and -10° HDT did not increase ICP remarkably compared to -10° HDT alone. Therefore, the addition of 3% CO2 is considered to have little effect on increasing ICP during cephalad fluid shift.Kurazumi T, Ogawa Y, Yanagida R, Morisaki H, Iwasaki K. Non-invasive intracranial pressure estimation during combined exposure to CO2 and head-down tilt. Aerosp Med Hum Perform. 2018; 89(4):365-370.

Dynamic Cerebral Autoregulation During the Combination of Mild Hypercapnia and Cephalad Fluid Shift

BACKGROUND

Mild hypercapnia combined with a cephalad fluid shift [e.g., that occurring during spaceflight or laparoscopic surgery with head-down tilt (HDT)] might affect cerebral autoregulation. However, no reports have described the effects of the combination on dynamic cerebral autoregulation. Therefore, we tested the hypothesis that the combination of mild hypercapnia and a cephalad fluid shift would attenuate dynamic cerebral autoregulation.

METHODS

There were 15 healthy male volunteers who were exposed to 4 10-min protocols in which they received air in the supine position (Placebo/Supine), 3% carbon dioxide (CO2) in the supine position (CO2/Supine), air with -10° HDT (Placebo/HDT) and 3% CO2 with -10° HDT (CO2/HDT). Dynamic cerebral autoregulation was evaluated using a transfer function analysis of the beat-to-beat variability in mean arterial blood pressure (ABP) and mean cerebral blood flow (CBF) velocity.

RESULTS

The phase in the low-frequency range was significantly lower during CO2/HDT than all other protocols, where CO2/HDT was -25% lower than Placebo/Supine (CO2/HDT, 0.49 ± 0.21; Placebo/Supine, 0.65 ± 0.16 radians). The transfer function gain in the low-frequency range was significantly higher during CO2/HDT than all other protocols, where CO2/HDT was 26% higher than Placebo/Supine (CO2/HDT, 1.08 ± 0.34; Placebo/Supine, 0.86 ± 0.28 cm · s-1 · mmHg-1). However, neither the CO2/Supine nor Placebo/HDT showed significant differences compared with the Placebo/Supine.

DISCUSSION

Even short-term exposure to 3% CO2 plus HDT increased synchrony and the magnitude of transmission between ABP and CBF in the low-frequency range. Thus, the combination of mild hypercapnia and a cephalad fluid shift attenuated dynamic cerebral autoregulation.Kurazumi T, Ogawa Y, Yanagida R, Morisaki H, Iwasaki K. Dynamic cerebral autoregulation during the combination of mild hypercapnia and cephalad fluid shift. Aerosp Med Hum Perform. 2017; 88(9):819-826.

Speed ratio but cabin temperature positively correlated with increased heart rates among professional drivers during car races

OBJECTIVES

The present study measures heart rate (HR) on a number of professional race-car drivers during actual car races through annual seasons to test hypotheses that faster relative speed and higher cabin temperature would induce higher HR.

METHODS

Heart rates in fifteen male drivers (31.2 ± 5.5 years old) were obtained by chest-strap sensors during official-professional 13 races. Average HR was calculated while the driver was racing from the start to the end of each race.

RESULTS

The average HR during races was 164.5 ± 15.1 beats min-1 and the average amount of time each driver spent driving per race was 54.2 ± 13.7 min. Average HR significantly and positively correlated with mean speed ratio (P < 0.001), but not with the average cabin temperatures (P = 0.533, range 25.6-41.8 °C) by the multiple linear regression analysis. Both average HR and mean speed ratio were significantly lower under wet, than dry conditions (151.9 ± 16.5 vs. 168.3 ± 12.5 beats min-1, 86.9 ± 4.4 vs. 93.4 ± 1.5 %).

CONCLUSIONS

The cardiovascular system of drivers is considerably stressed at extremely high HR. This high average HR positively correlated with mean speed ratio, suggesting that faster driving speed would induce greater cardiovascular stress to professional drivers during actual races. However, contrary to our hypothesis, cabin temperature was not significantly correlated with average HR. It is speculated that direct body cooling systems used in this professional race category work well against increases in HR by thermal stress under the temperature range found herein.

The relationship between widespread changes in gravity and cerebral blood flow

OBJECTIVES

We investigated the dose-effect relationship between wide changes in gravity from 0 to 2.0 Gz (Δ0.5 Gz) and cerebral blood flow (CBF), to test our hypothesis that CBF has a linear relationship with levels of gravity.

SUBJECTS AND METHODS

Ten healthy seated men were exposed to 0, 0.5, 1.0, 1.5, and 2.0 Gz for 21 min, by using a tilt chair and a short-arm human centrifuge. Steady-state CBF velocity (CBFV) in the middle cerebral artery by transcranial Doppler ultrasonography, mean arterial pressure (MAP) at the heart level (MAPHeart), heart rate, stroke volume, cardiac output and respiratory conditions were obtained for the last 6 min at each gravity level. Then, MAP in the middle cerebral artery (MAPMCA), reflecting cerebral perfusion pressure, was estimated.

RESULTS

Steady-state CBFV decreased stepwise from 0.5 to 2.0 Gz. Steady-state heart rate, stroke volume, estimated MAPMCA and end-tidal carbon dioxide pressure (ETCO2) also changed stepwise from hypogravity to hypergravity. On the other hand, steady-state MAPHeart and cardiac output did not change significantly. Steady-state CBFV positively and linearly correlated with estimated MAPMCA and ETCO2 in most subjects.

CONCLUSION

The present study demonstrated stepwise gravity-induced changes in steady-state CBFV from 0.5 to 2.0 Gz despite unchanged steady-state MAPHeart. The combined effects of reduced MAPMCA and ETCO2 likely led to stepwise decreases in CBFV. We caution that a mild increase in gravity from 0 to 2.0 Gz reduces CBF, even if arterial blood pressure at the heart level is maintained.

Differential effects of mild central hypovolemia with furosemide administration vs. lower body suction on dynamic cerebral autoregulation

Diuretic-induced mild hypovolemia with hemoconcentration reportedly improves dynamic cerebral autoregulation, whereas central hypovolemia without hemoconcentration induced by lower body negative pressure (LBNP) has no effect or impairs dynamic cerebral autoregulation. This discrepancy may be explained by different blood properties, by degrees of central hypovolemia, or both. We investigated the effects of equivalent central hypovolemia induced by furosemide administration or LBNP application on dynamic cerebral autoregulation to test our hypothesis that mild central hypovolemia due to furosemide administration enhances dynamic cerebral autoregulation in contrast to LBNP. Seven healthy male subjects received 0.4 mg/kg furosemide and LBNP, with equivalent decreases in central venous pressure (CVP). Dynamic cerebral autoregulation was assessed by spectral and transfer function analysis between beat-to-beat mean arterial blood pressure (MAP) and mean cerebral blood flow velocity (MCBFV). CVP decreased by ∼3-4 mmHg with both furosemide administration (∼26 mg) and LBNP (approximately -20 mmHg). Steady state MCBFV remained unchanged with both techniques, whereas MAP increased significantly with furosemide administration. Coherence and transfer function gain in the low and high frequency ranges with hypovolemia due to furosemide administration were significantly lower than those due to LBNP (ANOVA interaction effects, P < 0.05), although transfer function gain in the very low frequency range did not change. Our results suggest that although the decreases in CVP were equivalent between furosemide administration and LBNP, the resultant central hypovolemia differentially affected dynamic cerebral autoregulation. Mild central hypovolemia with hemoconcentration resulting from furosemide administration may enhance dynamic cerebral autoregulation compared with LBNP.

Subfoveal choroidal thickness and foveal retinal thickness during head-down tilt

INTRODUCTION

To reveal subtle morphological changes in the eye during simulated microgravity for spaceflights, we measured subfoveal choroidal thickness and foveal retinal thickness during 10 degrees head-down tilt (HDT). We hypothesized that elevated ophthalmic vein pressure during simulated microgravity increases subfoveal choroidal thickness via enlargement of the choroidal vasculature and greater choroidal blood volume.

METHODS

The right eyes of nine healthy subjects (seven men, two women) were examined. Subfoveal choroidal thickness and foveal retinal thickness were measured using spectral domain-optical coherence tomography in the sitting position, and after 15 and 30 min of 10 degrees HDT. Intraocular pressure was also measured.

RESULTS

Mean subfoveal choroidal thickness (+/- SEM) increased from 300 +/- 31 microm in the sitting position to 315 +/- 31 microm with 15-min HDT, and 333 +/- 31 microm with 30-min HDT. However, no change in foveal retinal thickness was observed (228 +/- 9 microm in the sitting position, 228 +/- 10 microm with 15-min HDT and 228 +/- 9 microm with 30-min HDT). Intraocular pressure increased from 14 +/- 1 mmHg in the sitting position to 21 +/- 2 mmHg with 30-min HDT (54 +/- 6%, N = 5).

DISCUSSION

Subfoveal choroidal thickness and intraocular pressure were increased by HDT during simulated microgravity, although no change in foveal retinal thickness was observed.

Cerebral circulation during mild +Gz hypergravity by short-arm human centrifuge

We examined changes in cerebral circulation in 15 healthy men during exposure to mild +Gz hypergravity (1.5 Gz, head-to-foot) using a short-arm centrifuge. Continuous arterial pressure waveform (tonometry), cerebral blood flow (CBF) velocity in the middle cerebral artery (transcranial Doppler ultrasonography), and partial pressure of end-tidal carbon dioxide (ETco(2)) were measured in the sitting position (1 Gz) and during 21 min of exposure to mild hypergravity (1.5 Gz). Dynamic cerebral autoregulation was assessed by spectral and transfer function analysis between beat-to-beat mean arterial pressure (MAP) and mean CBF velocity (MCBFV). Steady-state MAP did not change, but MCBFV was significantly reduced with 1.5 Gz (-7%). ETco(2) was also reduced (-12%). Variability of MAP increased significantly with 1.5 Gz in low (53%)- and high-frequency ranges (88%), but variability of MCBFV did not change in these frequency ranges, resulting in significant decreases in transfer function gain between MAP and MCBFV (gain in low-frequency range, -17%; gain in high-frequency range, -13%). In contrast, all of these indexes in the very low-frequency range were unchanged. Transfer from arterial pressure oscillations to CBF fluctuations was thus suppressed in low- and high-frequency ranges. These results suggest that steady-state global CBF was reduced, but dynamic cerebral autoregulation in low- and high-frequency ranges was improved with stabilization of CBF fluctuations despite increases in arterial pressure oscillations during mild +Gz hypergravity. We speculate that this improvement in dynamic cerebral autoregulation within these frequency ranges may have been due to compensatory effects against the reduction in steady-state global CBF.

[Approach to elucidating the influences and factors affecting circulation system in humans in space environment]

Many physiological changes associated with spaceflight, including decreases in orthostatic tolerance, exercise capacity, and blood volume have been reported. Orthostatic intolerance is a problem affecting many astronauts immediately postspaceflight. In particular, the relationship between orthostatic intolerance and cerebral autoregulation has been the focus of study in our research group. Although impairment of cerebral autoregulation was speculated to be one of the factors resulting in reduced post flight orthostatic tolerance, a 2-wk spaceflight study revealed that human cerebral autoregulation is preserved or even improved during and immediately after spaceflight in nonsymptomatic astronauts. To investigate the influences of the different kinds of reduction in central blood volume, we performed two ground-based studies. It is suggested that the mild intravascular dehydration partly explains the improved dynamic cerebral autoregulation observed during and immediately after a short-term spaceflight. Moreover, we also studied the relationship between orthostatic intolerance and cerebral autoregulation under hyperthermic conditions, because hyperthermia leads to orthostatic intolerance. Furthermore, we planned to conduct a study at the International Space Station (ISS) and ground-based studies to elucidate the influences and factors affecting the circulation system in humans in a space environment.

Hypergravity modulates vitamin D receptor target gene mRNA expression in mice

The possibility of pathological calcium metabolism is a critical health concern introduced by long-term space travel. Because vitamin D plays an important role in calcium homeostasis, we evaluated the effects of hypergravity on the expression of genes involved in vitamin D and calcium metabolism in ICR mice. When exposed to 2G hypergravity for 2 days, the mRNA expression of renal 25-hydroxyvitamin D 24-hydroxylase (Cyp24a1) was increased and that of 25-hydroxyvitamin D 1alpha-hydroxylase (Cyp27b1) was decreased. Although hypergravity decreased food intake and increased the expression of starvation-induced genes, the changes in Cyp24a1 and Cyp27b1 expression were not due to starvation, suggesting that hypergravity affects these genes directly. Hypergravity decreased plasma 1alpha,25-dihydroxyvitamin D(3) levels in ICR mice, suggesting a consequence of decreased Cyp27b1 and increased Cyp24a1 expression. Although 1alpha-hydroxyvitamin D(3) [1alpha(OH)D(3)] treatment induced the expression of vitamin D receptor (VDR) target genes in the kidney of 2G-exposed ICR mice to similar levels as controls, 1alpha(OH)D(3) increased the intestinal expression of Cyp24a1 in ICR mice. Hypergravity-dependent changes of Cyp24a1 and Cyp27b1 expression were diminished in mice exposed to hypergravity for 14 days, which may represent an adaptation to hypergravity stress. Hypergravity exposure also increased Cyp24a1 expression in the kidney of C57BL/6J mice. We examined the effects of hypergravity on VDR-null mice and found that renal Cyp27b1 expression in VDR-null mice was decreased by hypergravity while renal Cyp24a1 expression was not detected in VDR-null mice. Thus hypergravity modifies the expression of genes involved in vitamin D metabolism.

Cascade model of ventricular-arterial coupling and arterial-cardiac baroreflex function for cardiovascular variability in humans

Cardiovascular variability reflects autonomic regulation of blood pressure (BP) and heart rate (HR). However, systolic BP (SBP) variability also may be induced by fluctuations in stroke volume through left ventricular end-diastolic pressure (LVEDP) variability via dynamic ventricular-arterial coupling during respiration. We hypothesized that dynamic ventricular-arterial coupling is modulated by changes in left ventricular compliance associated with altered preload and that a cascade control mechanism of ventricular-arterial coupling with arterial-cardiac baroreflex function contributes to the genesis of cardiovascular variability at the respiratory frequency. Seven healthy young subjects underwent 6-min recordings of beat-by-beat LVEDP, SBP, and HR in the supine position with controlled respiration at 0.2 Hz during hyper- and hypovolemia. Spectral and transfer function analysis of these variables was conducted between 0.18 and 0.22 Hz. Dynamic ventricular-arterial coupling gain (Gain LVEDP-SBP) was smaller by 25% (P = 0.009) during hypervolemia than during hypovolemia, whereas arterial-cardiac baroreflex function gain (Gain SBP-HR) was similar. As predicted from a cascade model, a linear relationship between Gain LVEDP-HR and LVEDP-SBP times Gain SBP-HR was identified (R(2) = 0.93, P < 0.001). Gain LVEDP-HR was smaller by 40% (P = 0.04) during hypervolemia than during hypovolemia, leading to a reduction in spectral power of HR variability by 45% (P = 0.08). We conclude that dynamic ventricular-arterial coupling gain is reduced during hypervolemia because of a decrease in left ventricular compliance. A cascade model of ventricular-arterial coupling with the arterial-cardiac baroreflex contributes to the genesis of cardiovascular variability at the respiratory frequency.

Cardiovascular neuroregulation during acute exposure to 40, 70, and 100% oxygen at sea level

BACKGROUND

Humans encounter increased partial pressures of inspired oxygen in some kinds of diving as well as during use of hyperoxic mixtures to shorten decompression times and hyperbaric oxygen therapy for decompression sickness or other clinical conditions. Although it is known that hyperoxia may affect cardiovascular regulation, such effects are generally obscured by stress and the diving reflex. In this study, we evaluated cardiovascular neuroregulation for various levels of hyperoxia in a laboratory setting.

METHODS

There were 10 healthy adults who were exposed to 21, 40, 70, and 100% oxygen administered via mask as a series of stepwise increases. Subjects breathed at a fixed respiratory rate of 15 breaths x min(-1) while their heart rate (HR) and blood pressure (BP) were measured continuously over 5-min intervals.

RESULTS

HR decreased with increasing fraction of inspired oxygen (FIO2) (21%: 65 +/- 9, 40%: 63 +/- 9, 70%: 61 +/- 8, 100%: 60 +/- 8 bpm) and the high-frequency power of HR variability (an index of cardiac parasympathetic activity) increased as FIO2 rose (21%: 773 +/- 565, 40%: 880 +/- 590, 70%: 966 +/- 681, 100%: 1114 +/- 715 ms2); both changes were significant at the 70% and 100% oxygen levels. The low-frequency power of systolic BP variability (an index of vasomotor sympathetic activity) did not change. Low- and high-frequency transfer function gains (indices of arterial-cardiac baroreflex function) increased with FIO2.

CONCLUSION

Parasympathetic activity and arterial-cardiac baroreflex function increased with hyperoxia in a dose-dependent manner. This increase may help reduce the likelihood of arrhythmias during diving.

Effect of a repeated series of intermittent hypoxic exposures on ventilatory response in humans

The purpose of this study was to elucidate the magnitude and the time course of ventilatory changes resulting from a repeated series of hypoxic exposures. Eight healthy males participated in the present study. The subjects spent 1 h/day in normobaric hypoxia (12% inspired oxygen). Inspired minute ventilation (V(I)), end-tidal partial pressure of carbon dioxide (P(ET(CO2))), and arterial oxygen saturation (SaO2) were measured in a hypoxic tent. These measurements were taken for 10 consecutive days (series 1), and were taken again after the subjects had been away from hypoxic exposure for 1 month (series 2). P(ET(CO2)) decreased and SaO2 increased progressively in the hypoxic tent during the 10 days of intermittent hypoxia in series 1. At the onset of series 2 (days 1 to 3), P(ET(CO2)) was significantly lower and SaO2 was significantly higher than those on day 1 during series 1. These results suggest that humans who have had previous hypoxic exposure adapt sooner to hypoxic condition due to an increase in the magnitude of hyperventilation in the first few days of a series of reexposures to hypoxia.

Hypergravity exercise against bed rest induced changes in cardiac autonomic control

An intermittent exposure to artificial hypergravity with physical exercise by a human centrifuge may provide a countermeasure against various physiological problems after space flight. To test the effects of hypergravity with ergometric exercise on dynamic regulation of heart rate during weightlessness, we quantified autonomic cardiovascular control before and after head-down-tilt bed rest (HDBR) with and without the countermeasure. Twelve male subjects underwent a 14-day period of HDBR. Six of them were exposed to a hypergravity (+1.2 Gz acceleration at heart level) for 30 min with ergometric exercise (60 W, n=4; 40 W, n=2) as a countermeasure on day 1, 2, 3, 5, 7, 9, 11, 12, 13 and 14, during HDBR (CM group). The remaining six were not exposed to a hypergravity exercise during HDBR (control group). Blood pressure and ECG were recorded at a supine position before and after HDBR. The high frequency power of R-R interval (HFRR; 1,008+/-238 to 353+/-56 ms(2) P<0.05) as an index of cardiac parasympathetic activity, and transfer function gain between BP and R-R interval in the high frequency range (GainHF; 21.9+/-5.4 to 14.5+/-4.2 ms/mmHg, P<0.01) as an index of vagally mediated arterial-cardiac baroreflex, decreased significantly after HDBR in the control group. However, these changes were not statistically significant in the CM group (HFRR, 1,150+/-344 to 768+/-385 ms(2); GainHF, 21.5+/-3.3 to 18.6+/-3.4 ms/mmHg). Moreover, baroreflex gain by sequence analysis showed similar results. This observation suggests that the intermittent exposure to hypergravity with ergometric exercise may attenuate the decreases in the parasympathetic activity and the spontaneous arterial-cardiac baroreflex function after weightlessness.

Reduced baroreflex control of heart period after bed rest is normalized by acute plasma volume restoration

Adaptation to spaceflight or head-down-tilt bed rest leads to hypovolemia and an apparent abnormality of baroreflex regulation of cardiac period. In a previous study, we demonstrated that both chronic (2 wk) head-down-tilt bed rest and acute induced hypovolemia led to similar impairments in spontaneous baroreflex control of cardiac period, suggesting that a reduction in plasma volume may be responsible for this abnormality after bed rest. Therefore we hypothesized that this reduced "baroreflex function" could be restored by intravenous volume infusion equivalent to the reduction in plasma volume after bed rest. Six healthy subjects underwent 2 wk of -6 degrees head-down bed rest. Beat-by-beat arterial blood pressure and ECG were recorded during 6 min of spontaneous respiration and fixed-rate breathing (0.2 Hz), and transfer function analysis between systolic blood pressure and R-R interval was performed. Plasma volume was measured with Evans blue dye, and cardiac filling pressures were directly measured (Swan-Ganz catheter). After bed rest, studies were repeated before and after plasma volume restoration, with which both plasma volume and left ventricular end-diastolic pressure were restored to pre-bed rest levels by intravenous dextran40 infusion (288 +/- 31 ml). Transfer function gain in the high-frequency range, used as an index of vagally mediated arterial-cardiac baroreflex function, decreased significantly (13.4 +/- 3.1 to 8.1 +/- 2.9 ms/mmHg, P < 0.05) after bed rest. However, reduced transfer function gain was normalized to the pre-bed rest level (12.2 +/- 3.6 ms/mmHg) after precise plasma volume restoration. This result confirms that reductions in plasma volume, rather than a unique autonomic nervous system adaptation to bed rest, are largely responsible for the observed changes in spontaneous arterial-cardiac baroreflex function after bed rest.

Dose-response relationship of the cardiovascular adaptation to endurance training in healthy adults: how much training for what benefit?

Occupational or recreational exercise reduces mortality from cardiovascular disease. The potential mechanisms for this reduction may include changes in blood pressure (BP) and autonomic control of the circulation. Therefore, we conducted the present long-term longitudinal study to quantify the dose-response relationship between the volume and intensity of exercise training, and regulation of heart rate (HR) and BP. We measured steady-state hemodynamics and analyzed dynamic cardiovascular regulation by spectral and transfer function analysis of cardiovascular variability in 11 initially sedentary subjects during 1 yr of progressive endurance training sufficient to allow them to complete a marathon. From this, we found that 1) moderate exercise training for 3 mo decreased BP, HR, and total peripheral resistance, and increased cardiovascular variability and arterial baroreflex sensitivity; 2) more prolonged and intense training did not augment these changes further; and 3) most of these changes returned to control values at 12 mo despite markedly increased training duration and intensity equivalent to that routinely observed in competitive athletes. In conclusion, increases in R-wave-R-wave interval and cardiovascular variability indexes are consistent with an augmentation of vagal modulation of HR after exercise training. It appears that moderate doses of training for 3 mo are sufficient to achieve this response as well as a modest hypotensive effect from decreasing vascular resistance. However, more prolonged and intense training does not necessarily lead to greater enhancement of circulatory control and, therefore, may not provide an added protective benefit via autonomic mechanisms against death by cardiovascular disease.

Usefulness of daily +2Gz load as a countermeasure against physiological problems during weightlessness

Adaptation to head-down-tilt bed rest as a simulated microgravity leads to an abnormality of reflex control of circulation, hypovolemia and reduction of exercise capacity. We hypothesized that this cardiovascular deconditioning and reduction of exercise capacity could be prevented by a daily 1 hr centrifugation at +2Gz. To test this hypothesis, twenty healthy male subjects underwent 4 day of 6 degrees head-down-tilt bed rest. Ten of them were exposed to a +2Gz load for up to 30 min twice per day (the Gz group). The remaining 10 were not exposed to a Gz load (the no-Gz group). We estimated autonomic cardiovascular control by power spectral analysis of blood pressure and R-R interval variability, and baroreflex regulation by the transfer function analysis and the sequence method, before and after bed rest. Further, we measured hematocrit as an index of changes in plasma volume and maximal oxygen consumption as an index of exercise capacity, before and after bed rest.

RESULT

In the no-Gz group, heart rate increased after bed rest. The high frequency power of R-R interval variability as an index of cardiac parasympathetic nervous activity, baroreflex gains estimated by transfer function analysis and the sequence method as index of the integrated arterial-cardiac baroreflex function decreased significantly. Associated with these changes, the ratio of low to high frequency power of R-R as an indicator of cardiac sympathovagal balance tended to increase after bed rest in the no-Gz group. However, those showed no significant changes after bed rest in the Gz group. Hematocrit increased after bed rest in the no-Gz group. It also tended to increase in the Gz group, however it did not achieve statistical significance. Maximal oxygen consumption decreased significantly to similar extent in both the groups.

CONCLUSION

This result suggested that 1) a daily 1hr +2Gz load produced by a centrifuge might eliminate the changes in autonomic cardiovascular control during simulated weightlessness; 2) furthermore, it might partly reverse hypovolemia induced by bed rest; 3) however, it could not prevent the decreases in exercise capacity.

Effect of head-down-tilt bed rest and hypovolemia on dynamic regulation of heart rate and blood pressure

Adaptation to head-down-tilt bed rest leads to an apparent abnormality of baroreflex regulation of cardiac period. We hypothesized that this "deconditioning response" could primarily be a result of hypovolemia, rather than a unique adaptation of the autonomic nervous system to bed rest. To test this hypothesis, nine healthy subjects underwent 2 wk of -6 degrees head-down bed rest. One year later, five of these same subjects underwent acute hypovolemia with furosemide to produce the same reductions in plasma volume observed after bed rest. We took advantage of power spectral and transfer function analysis to examine the dynamic relationship between blood pressure (BP) and R-R interval. We found that 1) there were no significant differences between these two interventions with respect to changes in numerous cardiovascular indices, including cardiac filling pressures, arterial pressure, cardiac output, or stroke volume; 2) normalized high-frequency (0.15-0.25 Hz) power of R-R interval variability decreased significantly after both conditions, consistent with similar degrees of vagal withdrawal; 3) transfer function gain (BP to R-R interval), used as an index of arterial-cardiac baroreflex sensitivity, decreased significantly to a similar extent after both conditions in the high-frequency range; the gain also decreased similarly when expressed as BP to heart rate x stroke volume, which provides an index of the ability of the baroreflex to alter BP by modifying systemic flow; and 4) however, the low-frequency (0.05-0.15 Hz) power of systolic BP variability decreased after bed rest (-22%) compared with an increase (+155%) after acute hypovolemia, suggesting a differential response for the regulation of vascular resistance (interaction, P < 0.05). The similarity of changes in the reflex control of the circulation under both conditions is consistent with the hypothesis that reductions in plasma volume may be largely responsible for the observed changes in cardiac baroreflex control after bed rest. However, changes in vasomotor function associated with these two conditions may be different and may suggest a cardiovascular remodeling after bed rest.

Purification of pectate oligosaccharides showing root-growth-promoting activity in lettuce using ultrafiltration and nanofiltration membranes

Pectate oligosaccharides were separated from enzymatically hydrolyzed pectate by using ultrafiltration (UF) and nanofiltration (NF) membranes. The UF treatment was performed at a transmembrane pressure of 0.15 MPa and flow velocity of 0.6 m.s(-1), and nonhydrolyzed pectate was removed almost completely. The NF treatment was carried out at a transmembrane pressure of 0.5 MPa and flow velocity of 0.6 m.s(-1), and large amounts of monogalacturonic acid and sucrose, the contaminants included in the UF permeate were separated. Pectate oligosaccharides obtained by the diafiltration treatment of the NF concentrate were mainly composed of di- to pentasaccharides and exhibited root-growth-promoting activity in lettuce (approximately 1.8-fold) compared with the control. In particular, penta-, tetra-, and disaccharides were found to have strong activity.

Effects of daily 2-Gz load on human cardiovascular function during weightlessness simulation using 4-day head-down bed rest

An onboard short arm human centrifuge has been proposed as a countermeasure against physiological problems during long exposure to weightlessness in space and during extra planetary exploration. However, there are few studies on the effects of intermittent application of a Gz load via centrifuge during weightlessness. The present study evaluated the effects of a daily 2-Gz load on cardiovascular function during simulated weightlessness using a 4-day head-down bed rest (HDBR) period. Twelve young male subjects were exposed a HDBR period. Eight of them were exposed to a Gz load for up to 30 min twice per day (the Gz group). The remaining 4 were not exposed to a Gz load; they served as controls (the no-Gz group). Compared with the pre-HDBR period, the no-Gz group showed percent changes in the RR interval, the standard deviation (SD) of the RR interval, parasympathetic nervous activity, and baroreflex sensitivity (BRS) that progressively decreased and reached significance at the end of the HDBR period (-5.96 +/- 2.60%, -33.82 +/- 9.60%, -46.3 +/- 12.7% and -30.9 +/- 7.2%, respectively). In the Gz group, however, the percent changes in the RR interval, the SD of the RR interval, parasympathetic nervous activity, and BRS showed no significant changes throughout the HDBR period. At the end of the HDBR period, these indexes were 2.22 +/- 2.21%, -2.31 +/- 12.28%, 5.08 +/- 14.82% and 10.6 +/- 12.5%, respectively, and significantly greater than those of the no-Gz group. Sympathovagal balance indicators showed no significant change in the Gz and no-Gz groups (5.17 +/- 12.85% and 18.5 +/- 10.7%, respectively). These results indicate that a daily load of 2-Gz eliminates reduction of the RR interval, the SD of the RR interval, parasympathetic nervous activity, and BRS, and that it can maintain autonomic cardiovascular function in short-term weightlessness.