Impairment of Cerebral Blood Flow Regulation in Astronauts With Orthostatic Intolerance After Flight
Background and Purpose—We investigated cerebral blood flow regulation in astronauts before and after flights. We hypothesized that autoregulation would be different before flight and after flight between nonfinishers and the finishers of a stand test.
Methods—Twenty-seven astronauts from shuttle missions lasting 8 to 16 days underwent a 10-minute stand test: 10 days before flight, 1 to 2 hours and 3 days after landing. Mean blood flow velocity of the middle cerebral artery (MCA) was measured using transcranial Doppler; Mean arterial pressure was measured using a Finapres (Ohmeda, Englewood, CO) and was adjusted to the level of the MCA (BPMCA). Cross-spectral power, gain, phase, and coherence were determined for the relation between BPMCA and the cerebrovascular resistance index mean blood flow velocity/BPMCA.
Results—BPMCA was reduced with stand (P<0.001). Differences between finishers and nonfinishers (P=0.011) and over test days (P=0.004) were observed. Cerebrovascular conductance was affected by stand (P<0.001), by group (P<0.001) with a group by stand, and test day interaction (P<0.01). Preflight data suggest that the nonfinishers were operating at a higher cerebral vasodilation than finishers for a given BPMCA, and on landing day the nonfinishers had a greater decrease in mean blood flow velocity as a function of BPMCA with standing compared to finishers and preflight. There was a significant interaction effect of gender over the test days and from supine to stand (P=0.035).
Conclusions—Our results indicate that the cause of presyncope in astronauts may be related to a mismatch of cerebral blood flow with blood pressure. Astronaut gender may also play a role in susceptibility to orthostatic intolerance after flight.
After space flight, there is a wide range of susceptibility to orthostatic intolerance. It is possible that cerebral autoregulation, which ensures that cerebrovascular conductance is changed to maintain a relatively constant level of perfusion in case of fluctuations in blood pressure,1 could be compromised by the effects of weightlessness.
Investigation of cerebral autoregulation after head-down bed rest (HDBR) studies has revealed conflicting results. After 4 days of HDBR, Arbeille et al2 found no change in dynamic cerebral regulation during head-up tilt. Conversely, Zhang et al3 found an earlier and greater decline in cerebral blood flow during lower body negative pressure after HDBR and speculated that impairment of cerebral autoregulation may contribute to orthostatic intolerance after bed rest. Pavy Le-Trao4 showed no major impairment of cerebral autoregulation with HDBR; however, there was a slower response in vasodilation to a sudden decline in blood pressure in participants who had orthostatic intolerance. In the only reported study of astronauts, Iwasaki et al5 reported improved cerebral autoregulation on landing day; however, all 6 astronauts who participated in the study did not have presycopal symptoms after their 16-day space flight.
In this study we investigated for the first time to our knowledge the possible importance of cerebral autoregulation on orthostatic intolerance after space flight in a sample that contained presyncopal astronauts on landing day. We hypothesized that autoregulation would be negatively affected by space flight in those astronauts who did not complete a 10-minute stand test (nonfinishers) when compared to those did (finishers). Although some studies have examined cerebral autoregulation after space flight, we believe that this is the first time that it has been evaluated in astronauts who were, or were not, able to complete a stand test after space flight.
Materials and Methods
This protocol was approved by the Johnson Space Center Human Research Policy and Procedures Committee. Data before and after flights were recorded from astronauts who took part in shuttle missions lasting 8 to 16 days. Data were collected 10 days before launch (baseline, before flight), on landing day (1 to 2 hours after landing), and 3 days after landing (after flight).6
Cerebral blood flow in the M1 segment of the middle cerebral artery (MCA) was measured using transcranial Doppler ultrasound and blood pressure via noninvasive finger photoplethysmography. The dynamic autoregulatory gain was determined using the noninvasive transfer function method.7 Astronauts were classified as finishers (completed the 10-minute stand test) or as nonfinishers (presyncopal during the stand test). Detailed data collection and analysis procedures can be found in the Supplement (http://stroke.ahajournals.org).
A repeated-measures analysis of variance with factors of test day (before flight, landing day, 3 days after flight), stand test (supine, stand), and group (finishers/nonfinishers or male/female) was used with group nested within participants. The JMP-IN statistical package (SAS Institute) was used.
Of the 27 astronauts (20 male, 7 female) who were analyzed in this study, 19 (17 male) were classified as finishers and 8 (5 female) were classified as nonfinishers. Mean arterial pressure at the level of the MCA (BPMCA) was significantly reduced by the stand test (P<0.001). This was combined with differences between finishers and nonfinishers (P=0.011) and over test days (P=0.004). On landing day, finishers had a higher BPMCA than those before flight and the nonfinishers (Figure 1). Three days after landing, the finishers had returned to preflight values whereas the nonfinishers had elevated supine BPMCA.
Mean cerebral blood flow velocity (MFV) was affected by test day (P<0.001) and by the stand test (P<0.01). Preflight nonfinishers had higher MFV than finishers. Compared to the preflight group, the supine MFV of the finisher group was elevated on landing day, whereas in the nonfinisher group MFV was unchanged. Unlike preflight MFV, on landing day both finishers and nonfinishers had a reduction in MFV with standing. Only the nonfinishers had MFV reduced lower than preflight values. Three days after landing, both supine and stand MFV were not different from preflight values for the finishers but were significantly elevated in the nonfinishers.
Cerebrovascular conductance was affected by stand (P<0.001) and by group (P<0.001), and it was different between finishers and nonfinishers during stand and over test days (P<0.01). Before flight, conductance increased with the stand test; however, nonfinishers had higher conductance than finishers (Figure 1, bottom). On landing day, supine conductance was unchanged from preflight values, but the response to standing was different. On standing, conductance did not change in the finishers and decreased in the nonfinishers to a value lower than that before flight. Three days after flight, supine conductance was again higher in the nonfinishers compared to the finishers, but it did not increase on standing.
There were significant gender effects with BPMCA (P=0.035), MFV (P=0.044), and conductance (P=0.041), which are represented inFigure 1. Female astronauts had an increase in supine BPMCA 3 days after flight and in overall MFV 3 days after flight compared to preflight values. In general, the largest change in conductance occurred in females 3 days after flight, when cerebral conductance was greater than preflight.
The effects of space flight on blood pressure at head level and cerebral blood flow velocity can be seen readily in the beat-by-beat tracing of a nonfinisher astronaut (Figure 2). Although the blood pressures in the supine and early standing conditions were similar, MFV was drastically lower. In the expanded view of presyncope, beat-by-beat differences can be seen in signals from preflight to 2 hours after landing.
To further explore impairment of autoregulation on landing day, we plotted the average MFV against average BPMCA (Figure 3) in each of the groups. The large decline of MFV on landing day in the nonfinishers is visible, and this decrease occurred within the same blood pressure range seen before flight in both finishers and nonfinishers. A standardized autoregulation curve1 with arbitrary upper and lower limits was superimposed over the preflight data with the supine and standing data on the plateau of the curve (Figure 3, dashed lines). The curve was adjusted to the landing day results through a simple increase in the slope (slope, curved arrow,Figure 3) of the plateau region. The data from 3 days after flight also could be represented by a simple upward shift (plateau, vertical arrow,Figure 3) of the preflight autoregulation curve. The same pattern of increased slope of the plateau on landing day and upward shift of the curve 3 days after flight also could be observed in the group of nonfinishers but with larger changes in slope on landing day and upward shift (plateau) 3 days after flight.
Before flight, the change in MFV as a function of BPMCA (slope of solid lines,Figure 4) was not different between males (0.16±0.22 cmṡ−1ṁm Hg−) and females (0.01±0.51 cmṡ−1ṁm Hg−1; P=0.76); however, it was greater in females on landing day (1.10±0.35 cmṡ−1ṁm Hg−1) when compared to preflight values (P=0.048) and compared to males on landing day (0.19±0.21 cmṡ−1ṁm Hg−1; P=0.06).
BPMCA and Cerebrovascular Resistance Index Autospectral Data
Differences in BPMCA low-frequency (LF) (0.07–0.20 Hz) power were observed between finishers and nonfinishers during the stand tests (Figure 5). On average, nonfinishers had elevated (P<0.001) LF power (3.07±0.31 mm Hg2Hz−1) compared to finishers (1.49±0.18 mm Hg2Hz−1). There was also a significant overall difference between the finishers and nonfinishers with respect to the stand test (interaction, P<0.001).
There was a significant effect of test day on the cerebrovascular resistance index (CVRi) very-low-frequency (VLF) power between the finishers and nonfinishers (P=0.04); the nonfinishers had significantly higher CVRi VLF power 3 days after flight (Figure 5).
BPMCA and CVRi Cross-Spectral Data Gain and Phase
In the VLF region, there was a significant 3-way interaction (day, stand, presyncope; P=0.0037). The finisher group was characterized by constant autoregulation gain in the VLF region over all test conditions (Figure 6). Preflight, the nonfinisher group had a greater supine VLF gain than did the finisher group, which decreased on standing. On landing day, the supine VLF gain of the nonfinisher group was not different from that of the finishers; however, during the stand portion the gain was greater than that of the finishers.
Significant main effects were observed for the gain in the LF region of cerebral autoregulation. Overall, supine gain was higher than stand gain (0.043±0.002 CVRu mm Hg−1, 0.032±0.002 CVRu mm Hg−1, respectively; P=0.0004). An interaction was found between test day and astronaut presyncope (P=0.005). Astronauts who could finish the 10-minute stand test had a significant reduction in their LF gain from preflight to landing and 3 days after landing; however, the nonfinishers had a significant increase in LF gain on landing day that was greater than the finisher group.
In the VLF range, females had an overall larger gain (0.039±0.004 CVRu ṁm Hg−1) than males (0.022±0.002 CVRu ṁm Hg−1; P=0.03). Preflight, females had a larger supine VLF gain than males; on landing day, females had a higher VLF gain while standing compared to males (Figure 6). Males had a reduction in LF gain from preflight to landing and 3 days after landing. Males had lower LF gain than female astronauts on landing day and 3 days after landing (Figure 6). Both male and female astronauts had a significant main effect of a decrease in LF gain from supine to stand (P=0.036); however, visual inspection of the data (Figure 6) would suggest that this was not the case on landing day.
For the first time to our knowledge, we present cerebral autoregulation analyses for astronauts with and without orthostatic intolerance after flight. Our results confirm previous space flight data by Iwasaki et al,5 who showed that astronauts who do not exhibit orthostatic intolerance after flight have normal cerebral autoregulation on landing day; however, there was a severe impairment of cerebral blood flow velocity regulation in astronauts with orthostatic intolerance after flight.
Preflight, nonfinishers were operating at a higher resting cerebral blood flow velocity and similar mean supine blood pressure compared to finishers (Figure 1). On landing day, nonfinishers exhibited a large decrease in MFV associated with the same decline in BPMCA with standing as exhibited preflight. These results suggest that the cause of presyncope in astronauts may be linked with the loss of cardiovascular control of blood pressure and also may be related to a change in ability to autoregulate cerebral blood flow within the preflight blood pressure range.
Our analyses provide new information on cerebrovascular control during orthostatic hypotension after space flight. Preflight data (Figure 1) suggest that autoregulation characteristics of nonfinishers and finishers were different. MFV and conductance were much higher in the nonfinisher group in both the supine and standing positions. This could indicate that the nonfinishers were operating at a higher cerebral vasodilation for a given blood pressure. Although preflight BPMCA decreased from supine to stand, both finishers and nonfinishers had an increase in conductance, an appropriate response of a functional cerebral autoregulation system,8,9 and did not become presyncopal.
Finishers had neither an increase nor a decrease in conductance with stand on landing day. The finishers also had an increase in supine MFV compared to preflight values but without an increase in conductance, which is a strong indication that this was driven by the increase in BPMCA (Figure 1) after flight and that autoregulation was operating at a new set point. The lack of increase in conductance with the decline in BPMCA with standing also suggests some impairment in autoregulatory function before flight, although this was not serious enough to precipitate symptoms of presyncope.
In the nonfinishers, the decrease of landing day MFV with standing (Figure 1) was much greater than that of the finishers and those preflight. These data indicate that the nonfinishers were more severely challenged in terms of cerebral blood flow during stand than the finishers. This would indicate a severe impairment of cerebral autoregulation and may have precipitated presyncope in these astronauts. This can be seen in the sample landing day data from a nonfinisher (Figure 2); prolonged cerebral hypoperfusion attributable to loss of vasodilatory response (reduced blood flow) may be an important factor in the development of presyncope. A decrease in MCA conductance could be related to an increase in vascular stiffness, as seen in cerebral vessels after hind limb suspension in rats,10 or it could be that the cerebral vessels were close to maximal dilation and the decrease in MFV was directly attributable to the decline in blood pressure during standing.
Although it has been suggested that vasoconstriction, rather than vasodilation, could occur during presyncope because of increased sympathetic cerebral vasoconstriction,11,12 this mechanism may not play a role in the presyncopal astronauts because this group has been shown to have a less effective sympathetic response to standing.6,13 An increase of cerebral blood volume, caused by vasodilatation, could result in intracranial pressure elevation, which is positively correlated with cerebral blood flow velocity latency.14 Using our data, however, we are unable to speculate on this. The current data on cardiac output and blood pressure in these astronauts indicate hypovolemia and, therefore, a decrease in cerebral volume during standing. However, with respect to space flight, it has been speculated that the head ward fluid shift causes vasodilation in the cerebral vessels, which may cause similar velocity latency14 on landing day.
When plotted as a function of BPMCA ,the large decline of MFV on landing day in the nonfinishers is visible, as is the fact that this decrease occurred within the same blood pressure range seen before flight in both finishers and nonfinishers (Figure 3). This representation of data suggests a possible explanation of the change in autoregulation with space flight that is common between both finishers and nonfinishers, only with greater effect in the nonfinisher group; however, the steepness of the plateau region on landing day suggests a severe impairment or loss of autoregulation in this group.
Further evidence in the difference in the 2 groups is provided by the analysis of dynamic autoregulation. Overall, there was less variation in vascular resistance (lower CVRi autospectral power) in the nonfinisher group despite a greater variation in blood pressure (higher BPMCA autospectral power;Figure 4). This may indicate an overall reduced autoregulation capacity in the nonfinisher group.
Preflight, only the nonfinisher group had a decrease in dynamic autoregulation gain when going from supine to standing. In general, one can interpret a higher BPMCA→CVRi gain as an indication of a more effective filtering of blood pressure oscillations.8 When going rapidly from supine to stand, there is a sudden decrease in blood pressure at the level of vessels involved with the cerebral autoregulation and an effective autoregulation system should not be affected or at least should not have a decrease in gain.
In the finisher group, supine dynamic autoregulation gain was reduced on landing day and 3 days later (Figure 4). On landing day, supine blood pressure was higher than preflight values (Figure 1), yet MFV remained unchanged. These data would indicate that with the increased blood pressure, the system was within the limits of autoregulation. On landing day, supine blood pressure was higher in the finisher group compared to preflight values. It is possible that this may produce less defined, or passive, resting dynamic autoregulation with a wider variation of participant autoregulation curves.
Conversely, the nonfinisher group had an overall increase in dynamic autoregulatory gain on landing day (Figure 6), with no change in supine or standing blood pressure compared to preflight values. Although the increase in gain suggests improved autoregulation, greater decrease in MFV and a large decrease in cerebral conductance with standing compared to preflight (Figure 1) values suggest that on landing day these astronauts were unable to autoregulate cerebral blood flow with a normal decrease in BPMCA. This could indicate that the lower limit of autoregulation had shifted to the right (Figure 3, right), or that the changes in cerebrovascular resistance were ineffectual in maintaining normal cerebral perfusion. The incidence of presyncope was eliminated by day 3, most likely through changes in blood pressure regulation. The increase in blood pressure may have augmented blood flow to a region of effective autoregulation.
Our results indicate that female astronauts have different autoregulation characteristics, and on landing day females had reduced autoregulation compared to males. Astronaut gender may also play a role in susceptibility to orthostatic intolerance after flight. In this data set, 5 of 8 nonfinishers were female, whereas 17 of 19 finishers were male. A compilation of statistics (25 female, 140 male) on incidence of presyncope after short-duration space flight (5–16 days) showed a similar effect, with presyncope occurring in 28% of the females and in 7% of the males.3
The majority of cerebral autoregulation characteristics are similar between males and females; however, there are a few characteristics that are different and that may predispose females to orthostatic intolerance. These include differences in cerebral blood flow velocity and cerebrovascular reactivity.15 In our study, we did observe higher cerebral blood flow velocity in females 3 days after flight, but not during preflight or on landing day (Figure 1). However, preflight VLF gain was double that of males (Figure 6) and reduced on landing day. This may indicate a reduction in autoregulation on landing day compared to that of males. Pavy Le-Trao4 showed no major impairment of cerebral autoregulation with HDBR in females; however, there was a slower response in vasodilation to a sudden decline in blood pressure in participants who were orthostatic-intolerant.
Changes in velocity can only be equated to changes in flow if changes in the diameter of the MCA are minimal. Measurements of MCA caliber were not performed in the present study; however, previous invasive studies have shown minimal change in the M1 segment MCA diameter under a variety of conditions known to affect cerebral perfusion.16–19
Changes in blood CO2 are known to influence the set point for autoregulation level.20 Unfortunately, CO2 was not measured in the astronauts during the stand test, so its effect on the outcome of the present study is not known. Breathing rate was controlled in all tests at 15 breaths per minute; however, ventilatory effects on CO2 cannot be ruled out because no attempt was made to monitor tidal volume. There may be other factors besides CO2 that may contribute to the shift in the cerebral autoregulatory set point. This possibility cannot be ruled out in the present study either, because spaceflight may influence ≥1 of the factors that determine the level at which autoregulation tries to maintain cerebral blood flow and, hence, an individual's susceptibility to orthostatic intolerance after flight.
Cerebrovascular regulation was found to be different between astronauts who were able to remain standing (finishers) or not (nonfinishers) without assistance for 10 minutes on landing day and across gender. On landing day, nonfinishers had a decrease in conductance with standing, whereas the finishers did not. This was accompanied by an increase in VLF gain with stand and overall LF gain on landing day, which suggest that the autoregulation system was under greater stress in the nonfinishers and was unable to increase conductance in the face of declining blood pressure. Because the decrease in blood pressure during the time of these measurements was within the range of preflight and finisher values, these data indicate that a mismatch of cerebral blood flow and blood pressure control may be related to presyncope after flight. Astronaut gender may also play a role in susceptibility to orthostatic intolerance after flight.
Sources of Funding
National Aeronautics and Space Administration Contract NAS9-18942 and Natural Sciences and Engineering Research Council, Medical Research Council, Canadian Space Agency Industry University Partnership grant 669-008/93.
The authors thank the astronauts and the personnel in the Cardiovascular and Biochemistry Laboratories (Johnson Space Center) and the Microgravity Laboratory (Ryerson University, Toronto, Canada) for their excellent technical assistance.
The online-only Data Supplement is available at http://stroke.ahajournals.org/cgi/content/full/STROKEAHA.110.610576/DC1.
- Received December 8, 2010.
- Accepted February 17, 2011.
- © 2011 American Heart Association, Inc.
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