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).

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.

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.

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.

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.

Blunted cutaneous vasoconstriction and increased frequency of presyncope during an orthostatic challenge under moderate heat stress in the morning.

PURPOSE: In normothermia, the tolerance time to presyncope during an orthostatic challenge is shortened during the early morning. Heat stress reduces tolerance to presyncope and the degree of cutaneous vasoconstriction prior to presyncope. However, whether these changes show diurnal variations remains unknown. Therefore, we examined diurnal changes in orthostatic tolerance and cutaneous vascular conductance (CVC) during an orthostatic challenge under moderate heat stress. METHODS: Each lower body negative pressure (LBNP) under normothermia and whole body heat stress was applied for 7 min or until the appearance of presyncopal symptoms in 16 males at both 08:00 (a.m.) and 17:00 hours (p.m.). Measurements included internal and skin temperatures, forearm skin blood flow, arterial pressure, and heart rate. CVC was calculated as skin blood flow/mean arterial pressure, normalized to CVC prior to LBNP and expressed as %CVC. RESULTS: The average tolerance time in eight subjects exhibiting presyncopal symptoms due to LBNP and moderate heat stress was significantly shorter in the a.m. than in the p.m. (3.7 ± 0.8 versus 6.7 ± 0.3 min, respectively; P = 0.005). Neither %CVC during LBNP in these subjects under moderate heat stress nor normothermia were significantly decreased in the a.m. (P > 0.05, respectively). CONCLUSIONS: These findings indicate an orthostatic challenge even during moderate heat stress that led to an increase in the frequency of presyncope, especially in the morning. The reduction in tolerance was accompanied by blunted cutaneous vasoconstriction prior to presyncope. Hence, diurnal changes in cutaneous vascular responses during combined orthostatic and heat stresses should contribute, at least partly, to heat-induced orthostatic intolerance in the morning.

Steady-state cerebral blood flow and dynamic cerebral autoregulation during neck flexion and extension in seated healthy young adults.

Neck flexion and extension show differences in various physiological factors, such as sympathetic nerve activity and intracranial pressure (ICP). We hypothesized that differences would exist in steady-state cerebral blood flow and dynamic cerebral autoregulation between neck flexion and extension in seated, healthy young adults. Fifteen healthy adults were studied in the sitting position. Data were collected during neck flexion and extension in random order for 6 min each on the same day. Arterial pressure at the heart level was measured using a cuff sphygmomanometer. Mean arterial pressure at the middle cerebral artery (MCA) level (MAPMCA ) was calculated by subtracting the hydrostatic pressure difference between heart and MCA levels from mean arterial pressure at the heart level. Non-invasive cerebral perfusion pressure (nCPP) was estimated as the MAPMCA minus the non-invasive ICP as determined from transcranial Doppler ultrasonography. Waveforms of arterial pressure in the finger and blood velocity in the MCA (MCAv) were obtained. Dynamic cerebral autoregulation was evaluated by transfer function analysis between these waveforms. The results showed that nCPP was significantly higher during neck flexion than during neck extension (p = 0.004). However, no significant differences were observed in mean MCAv (p = 0.752). Likewise, no significant differences were observed in any of the three indices of dynamic cerebral autoregulation in any frequency range. Although non-invasively estimated cerebral perfusion pressure was significantly higher during neck flexion than during neck extension, no differences in steady-state cerebral blood flow or dynamic cerebral autoregulation were evident between neck flexion and extension in seated healthy adults.

Effect of subthalamic nucleus deep brain stimulation on the autonomic nervous system in Parkinson's disease patients assessed by spectral analyses of R-R interval variability and blood pressure variability.

OBJECTIVE: Autonomic nervous system impairment is an untoward symptom that is typically observed in advanced Parkinson's disease (PD) patients. However, details of the effects of subthalamic nucleus deep brain stimulation (STN-DBS) on the autonomic nervous system remain unclear. METHODS: Twenty-eight patients with advanced PD (12 males and 16 females) who underwent bilateral STN-DBS and 13 age-matched healthy controls were included in this study. We analyzed the dynamic cardiovascular autonomic function regulating the R-R interval and blood pressure by spectral and transfer function analyses of cardiovascular variability before and after STN-DBS. RESULTS: Vagally mediated arterial-cardiac baroreflex function improved after STN-DBS compared to that before STN-DBS (p < 0.05). However, there were no statistically significant differences in the results of the comparison of vagally mediated arterial-cardiac baroreflex function between on-stimulation and off-stimulation. CONCLUSIONS: The vagal component in cardiac autonomic dysfunction associated with PD is expected to improve after STN-DBS. We considered that the patients improved their lifestyle; in particular, increasing the amount of exercise by STN-DBS and the best pharmachological treatment may have positive effects on parasympathetic activities.

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.

The effect of dexmedetomidine on arterial-cardiac baroreflex function assessed by spectral and transfer function analysis.

PURPOSE: The α(2)-adrenergic receptor agonist dexmedetomidine reportedly weakens heart rate (HR) responses to 'rapid' (during a few seconds) reduction in arterial pressure, but does not affect HR responses to 'gradual' (during 60 s) reduction in arterial pressure. As the speed of neurotransmission along the parasympathetic nerve is relatively rapid, alteration of parasympathetic-mediated arterial-cardiac baroreflex function plays a more important role in HR responses to 'rapid' changes in arterial pressure. We therefore hypothesized that dexmedetomidine attenuates parasympathetic-mediated arterial-cardiac baroreflex function. METHODS: Twelve healthy men received placebo, low-dose (loading, 3 µg/kg/h for 10 min; maintenance, 0.2 µg/kg/h for 60 min) (low-DEX), or moderate-dose (loading, 6 µg/kg/h for 10 min; maintenance, 0.4 µg/kg/h for 60 min) (moderate-DEX) dexmedetomidine infusions in a randomized, double-blind, crossover study. Before and after 70 min of infusion, arterial-cardiac baroreflex function was assessed by spectral and transfer function analysis between arterial pressure variability and HR variability. RESULTS: The high-frequency power of systolic arterial pressure (SAP) variability increased significantly with low-DEX and moderate-DEX infusions (significant interaction effects, P = 0.005), whereas the high-frequency power of R-wave-R-wave interval (RRI) variability (as an index of cardiac parasympathetic activity) did not change significantly at any dose infusions. Then, transfer function gain in the high-frequency range (as an index of parasympathetic arterial-cardiac baroreflex) decreased significantly with low-DEX and moderate-DEX infusions (significant interaction effects, P = 0.007). CONCLUSIONS: The present results suggest that dexmedetomidine attenuates parasympathetic-mediated arterial-cardiac baroreflex function, implying weakened HR response to 'rapid' reduction in arterial pressure.

[Relationship between baroreflex function and training effects on altitude training].

OBJECTIVE: Altitude training is frequently used for athletes requiring competitive endurance in an attempt to improve their sea-level performance. However, there has been no study in which the mechanisms by which spontaneous arterial-cardiac baroreflex function changes was examined in responders or nonresponders of altitude training. The purpose of this study was to clarify the different effects of altitude training on baroreflex function between responders and nonresponders. METHODS: Twelve university student cross-country skiers (6 men, 6 women; age, 19±1 years) participated in the altitude training in a camp for 3 weeks, which was carried out in accordance with the method of Living High-Training Low. Baroreflex function was estimated by transfer function analysis before and after the training. RESULTS: The responders of the training were 3 men and 2 women, and the nonresponders were 3 men and 4 women. In the responders, the transfer function gain in the high-frequency range significantly increased after the training (28.9→46.5 ms/mmHg p=0.021). On the other hand, no significant change in this index was observed in the nonresponders (25.9→21.2 ms/mmHg p=0.405). CONCLUSION: As indicated by the results of transfer function gain in the high-frequency range, the baroreflex function in the responders increased significantly after the altitude training, whereas no significant change was observed in the nonresponders.

Effects of -10° and -30° head-down tilt on cerebral blood velocity, dynamic cerebral autoregulation, and noninvasively 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.

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.

[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.

[The advantage and harmful effects of nitrous oxide in dental management].

Frequency of using nitrous oxide (N2O) in anesthetic field decreased recently by the influence of the environmental problems and the widespread use of intravenous agents. In dental treatment, however, inhalation sedation using low concentration of N2O has been effectively used to relax dental patients. Nitrous oxide inhalation sedation for dental treatment generally involves the use of a combination of low-dose N2O and high-dose oxygen. Low-dose N2O induces the relaxation of psychological tension in dental patients who have "dental phobia". Also, high-dose oxygen would contribute to manage "(pre) syncope" due to pain stimuli. Thus, N2O inhalation sedation is effective for the dental treatment, although the leak of N2O affects the health of dental staffs. The present review described the advantage and harmful effects of N2O in dental management.

[Effects of the induction of anesthesia with propofol on hemodynamics in patients with Parkinson's disease].

BACKGROUND: Patients with Parkinson's disease frequently suffer from impaired autonomic nervous function. To elucidate the effects of the induction of anesthesia with propofol on cardiovascular hemodynamics has become important, since the number of patients with Parkinson's disease undergoing deep brain stimulation under general anesthesia has increased recently. METHODS: Effects of induction with propofol in patients with Parkinson's disease on cardiovascular hemodynamics and autonomic nervous activity were compared with those of the control patients. Moreover, possible different effect on hemodynamics between the propofol alone and the combination of propofol and fentanyl for the induction were examined in patients with Parkinson's disease. RESULTS: Although heart rate or blood pressure was not different between patients with Parkinson's disease and the control patients before the induction, sympathetic vasomotor activity was lower in patients with Parkinson's disease than the control patients. The induction of anesthesia significantly decreased blood pressure in patients with Parkinson's disease. However the decreasing systolic blood pressure after the induction of anesthesia was more marked in patients with Parkinson's disease than the control patients. We did not find differences in the changes of blood pressure between the propofol alone and the combination of propofol and fentanyl in patients with Parkinson's disease. CONCLUSIONS: No abnormal responses to the induction of anesthesia with propofol were found in the patients with Parkinson's disease.

The different effects of midazolam and propofol sedation on dynamic cerebral autoregulation.

BACKGROUND: Although midazolam and propofol reduce cerebral blood flow (CBF) similarly, they generate different effects on the autonomic nervous system and endothelium-induced relaxation. Midazolam induces sympathetic dominance, whereas propofol induces parasympathetic dominance. Midazolam has no effect on endothelium-dependent relaxation, whereas propofol suppresses endothelium-dependent relaxation. Moreover, midazolam apparently constricts cerebral arterioles. We therefore hypothesized that midazolam and propofol have different effects on dynamic cerebral autoregulation. METHODS: Ten healthy male subjects received midazolam, propofol, and placebo administrations in a randomized, single-blind, crossover study. The modified Observer's Assessment of Alertness/Sedation scale was used to assess sedation depth. After reaching a target depth of sedation (Observer's Assessment of Alertness/Sedation scale score 3, responds only after name is called loudly and/or repeatedly) or after 15 minutes of normal saline administration as placebo, dynamic cerebral autoregulation was evaluated by spectral and transfer function analyses between mean arterial blood pressure variability in the radial artery measured by tonometry, and CBF velocity variability in the middle cerebral artery measured by transcranial Doppler ultrasonography. RESULTS: Steady-state CBF velocity decreased significantly with midazolam and propofol administration (significant interaction effects, P = 0.024). However, transfer function gain in the low-frequency range decreased significantly only with midazolam administration (significant interaction effects, P = 0.015), suggesting a reduced magnitude of transfer from mean arterial blood pressure oscillations to CBF fluctuations during midazolam sedation. CONCLUSION: Our results suggest that midazolam and propofol sedation have different effects on dynamic cerebral autoregulation despite causing equivalent decreases in steady-state CBF velocity. Only midazolam sedation is likely to improve dynamic cerebral autoregulation.

Dynamic cerebral autoregulation after mild dehydration to simulate microgravity effects.

INTRODUCTION: During and immediately following a 2-wk spaceflight, dynamic cerebral autoregulation is improved, leading to stable cerebral blood flow velocity (CBFV). However, the factors inducing this improved cerebral autoregulation remain unclear. Although mild reduction of plasma volume (one of the key adaptations to spaceflight) may lead to improved cerebral autoregulation, there have been no reports estimating the alteration of dynamic cerebral autoregulation during mild dehydration. We, therefore, investigated the effect of mild intravascular dehydration on dynamic cerebral autoregulation. METHODS: Furosemide, 0.2 mg x kg(-1), was administered to 14 men. Dynamic cerebral autoregulation was estimated before and approximately 45 min after furosemide administration using spectral and transfer function analyses between mean blood pressure (MBP) variability and mean CBFV variability. We then assessed the degree of dehydration as plasma volume changes, and/or central venous pressure (CVP). RESULTS: Furosemide administration caused an approximate 10% reduction in plasma volume, with CVP decreasing by 1.2 mmHg. Steady state MBP and mean CBFV did not change. However, transfer function gain in the low-frequency range significantly decreased, implying a decrease in the magnitude of transfer from MBP oscillations to mean CBFV fluctuations. DISCUSSION: The present degree of dehydration was similar to the alteration observed after short-term spaceflight, and enhanced dynamic cerebral autoregulation in the low-frequency range with unchanged steady state mean CBFV. Consequently, these results on cerebral circulation were consistent with those of a previous spaceflight study. Thus, it is suggested that mild intravascular dehydration partly explains the improved dynamic cerebral autoregulation observed during and immediately after 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 (MCBFVMCA) during centrifugation were averaged every 5 min and compared with pre-hypergravity (+1.0 Gz). C-rSO2 did not change significantly throughout centrifugation, although MCBFVMCA 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.