Background: the transition to microgravity eliminates the hydrostatic gradients in the vascular system. The resulting fluid redistribution commonly manifests as facial edema, engorgement of the external neck veins, nasal congestion, and headache. This experiment examined the responses to modified Valsalva and Mueller maneuvers measured by cardiac and vascular ultrasound (ECHO) in a baseline steady state and under the influence of thigh occlusion cuffs available as a countermeasure device (Braslet cuffs). Methods: nine International Space Station crewmember subjects (expeditions 16–20) were examined in 15 experiment sessions 101 ± 46 days after launch (mean ± SD; 33–185). Twenty-seven cardiac and vascular parameters were obtained with/without respiratory maneuvers before and after tightening of the Braslet cuffs (162 parameter states/session). Quality of cardiac and vascular ultrasound examinations was assured through remote monitoring and guidance by investigators from the NASA Telescience Center in Houston, TX, and the Mission Control Center in Korolyov, Moscow region, Russia. Results: 14 of 81 conditions (27 parameters measured at baseline, Valsalva, and Mueller maneuver) were significantly different when the Braslet was applied. Seven of 27 parameters were found to respond differently to respiratory maneuvers depending on the presence or absence of thigh compression. Conclusions: acute application of Braslet occlusion cuffs causes lower extremity fluid sequestration and exerts commensurate measurable effects on cardiac performance in microgravity. Ultrasound techniques to measure the hemodynamic effects of thigh cuffs in combination with respiratory maneuvers may serve as an effective tool in determining the volume status of a cardiac or hemodynamically compromised patient at the “microgravity bedside.”
- occlusion cuffs
- cardiac function
an immediate effect of the transition to microgravity during early spaceflight is a loss of the hydrostatic gradient in the cardiovascular system, resulting in a cephalad venous and lymphatic fluid shift (11, 15). The loss of lower extremity volume is on the order of 1–2l (11, 51), i.e., larger than on transition to supine or even head-down-tilt (HDT) posture on Earth (4, 6, 8, 52). Notwithstanding the massive fluid shift, invasive central venous pressure (CVP) measurements in spaceflight have shown it to decrease to 2 mmHg with no clinically significant long-term changes in cardiac output (12, 13). Increased jugular venous distension in spaceflight is seen consistently and persists throughout the mission (30), but it is no longer a reliable index of right atrial pressure or right ventricular preload.
The effect of fluid redistribution in astronauts is commonly observed as visible facial edema (“puffiness”) and engorgement of the external neck veins, with a concomitant decrease in leg size. Subjectively, crewmembers report “fullness in the head,” nasal congestion, and diminished sense of taste and smell (42).
Occlusive inflatable cuffs and the Braslet have been investigated in an attempt to reverse the fluid shifts seen during HDT and microgravity (4, 5, 7). Investigations have documented the peripheral vascular effects; however, ultrasound technology at the time was not adequate to confirm the effect on cardiac preload (30). The International Space Station (ISS) ultrasound system operated with real-time remote guidance has heightened both the scope and the fidelity of imaging studies in space (16, 18, 19, 30, 47, 48); furthermore, the ISS crew represents an unprecedented cohort of long-duration microgravity test subjects for prospective investigations. In a recent ISS-based study, Hamilton et al. (30) presented a set of standardized methods and echocardiographic “normal” results in six subjects during long-duration spaceflight.
The Braslet-M thigh cuff countermeasure device is intended to ameliorate the symptoms associated with the microgravity-induced cephalad volume shifts in the early hours and days of microgravity exposure by impeding the venous outflow and creating commensurate fluid sequestration in the lower extremities. The cuffs are custom built for each crewmember before the mission and consist of segments of elastic and nonelastic materials to conform to the shape of the upper thigh (Fig. 1). Each device contains an adjustment belt that can be tightened to achieve the desired degree of compression selected in preflight calibration, which consists of a special negative 30° tilt-table procedure based on subject feedback and cranial impedance rheography to determine the appropriate compression of the extremity.
The purpose of this study is to determine if the Braslet consistently changes the responses of the cardiovascular system to the Valsalva and Mueller maneuvers and whether combinations of Braslet and maneuvers, compared with the Braslet alone, have utility for the assessment of the fluid status in microgravity.
The increased intrathoracic pressure during Valsalva maneuver (Valsalva) causes significant changes in the cardiovascular and cerebrovascular circulation (40). The “classic” Valsalva has four distinct phases: onset of strain, continued strain, release, and recovery (21). In microgravity and during HDT the internal jugular vein (IJV) is distended constantly (30, 49), allowing for only small, if any, added expansion upon Valsalva (30). The Braslet occlusion thigh cuff is expected to sequester fluid in the lower extremities and potentially reduce cardiac preload; this may result in a change to the effect of the Valsalva on both cardiac preload and IJV distention. The Mueller maneuver (Mueller) consists of inspiration with closed mouth and nose (or closed glottis) performed after a forced expiration; it causes reduction of intrathoracic pressure, which is the opposite of the Valsalva. In microgravity the IJV showed a trend to diminish in size when Mueller was performed (30). The expected effect of Braslet in reducing preload may desaturate the IJV distention, enhancing the effect of the Mueller on the IJV and making Mueller with optional Braslet application a clinically relevant tool for fluid status assessment in microgravity.
The current investigation is a collaborative effort between the NASA and the Federal Space Agency of Russia/Roscosmos investigators to observe the hemodynamic effects of the Braslet-M device in nominal steady-state condition, as well as during Mueller and Valsalva respiratory maneuvers performed before and during its application.
The Braslet-M device, when used per the current calibration and application procedures, will 1) consistently cause fluid sequestration and reduce flow velocities in the venous vascular bed of lower extremities in microgravity, acutely reducing the circulating volume; 2) reduce cardiac preload in microgravity for the duration of application; 3) facilitate the effects of the Valsalva and Mueller maneuvers on the filling (distention) and behavior of the IJV observed by real-time sonography.
All crewmembers were medically certified for spaceflight by the ISS Multilateral Space Medicine Board using a physical examination and screening that includes a comprehensive cardiovascular assessment with ECG, exercise stress test, transthoracic echocardiography, Holter monitoring, electron beam CT for coronary calcium scoring, a large assortment of laboratory tests, and a clinical cardiovascular risk assessment. Subjects and operators for this study were enlisted on a voluntary basis following written informed consent. The study was approved by the human research boards of each subject's agency and then by the ISS Human Research Multilateral Review Board.
Braslet-M sets were custom made by the manufacturer (Kentavr-Nauka, Moscow, Russia) for each subject and then individually calibrated as a nominal operational countermeasure in a standardized preflight tilt-table procedure. Cranial rheographic data were acquired in supine and −30° HDT. The Braslet was gradually tightened in a stepwise fashion while trained experts recorded subjective feedback, rheographic tracing, and the appearance of the subject. The device setting was determined to assure subjective improvement (reduction of fullness of the head and nasal congestion and often a perception of being untilted), as well as return of rheographic tracing to the pretilt pattern (Fig. 1). This setting is considered to provide appropriate compression of the extremity for countermeasure purposes; the same setting was used in this study.
Approximately 4 mo before launch, each ultrasound operator received 1 h of familiarization, including hands-on instruction by an expert sonographer using a simulated flight control environment. All training was performed in the US Destiny module Human Research Facility simulator (Johnson Space Center-Houston) containing a flight-modified ATL HDI-5000 (ATL/Philips, Seattle, WA) ultrasound system. The P4–2 phased array cardiac ultrasound probe (1–4 MHz) was used for cardiac imaging, and an L12–5 linear array probe was used to demonstrate the vascular imaging procedures. This configuration was identical to the one in the Destiny or Columbus modules of the ISS.
On-Orbit Data Acquisition
The study spanned three ISS expeditions with five and four sessions, respectively, involving nine crewmembers from 33 to 185 days after launch (101 ± 46, mean ± SD). No ground data were collected before or after the mission, as each crewmember served as their own control.
During each on-orbit session, real-time video from the ISS ultrasound system was transmitted to Mission Control Center-Houston (MCC-H), and two-way private audio channel with the crew was available for verbal discourse. The crewmembers were guided through each step of the study protocol, which typically took 60 min to perform (Fig. 2).
The astronauts were allowed to refine specific restraining techniques for both the subject and the operator to allow unrestricted use of the keyboard, access to the thoracic region and both thigh areas of the subject, and assure mutual stability for the examination in the microgravity environment. Attention was paid to the prevention of hand fatigue of the operator. In some cases, the subject assisted with ultrasound keyboard manipulations while simultaneously being scanned. The subject exerted very little effort to perform this task and therefore this activity would have had a negligible effect on cardiac performance.
The cardiac ultrasound examination was conducted using the four-chamber apical view primarily, with subcostal four-chamber view as the alternative, and involved B, M, pulsed-wave Doppler, and tissue Doppler (TM) modes. Probe repositioning and change-outs were minimized to enhance image acquisition and reduce examination duration (Fig. 3). The probe position was optimized per the remote sonographer's voice commands to achieve the desired images. The remote guidance team made real-time decisions on the quality of the apical view and the need for an alternative approach. All remote guiders were registered diagnostic cardiac sonographers and registered vascular technologists by the American Registry for Diagnostic Medical Sonography (2).
All sessions were conducted in a consistent manner with cardiac and peripheral vascular ultrasound being acquired first without Braslet during three test conditions (baseline, a modified Valsalva maneuver, and a modified Mueller maneuver) and then under the same conditions after the Braslet had been applied on both thighs for 10–30 min. The modified respiratory maneuvers were performed by exhaling or inhaling through a plastic tube of standardized length and diameter to limit airway pressures to ±40 cmH2O (D. Hamilton, unpublished results). The subjects were trained to spread the respiratory effort over 8–10 s starting at baseline, which was demonstrated in ground experiments to maintain a constant airway pressure within the above limits (Fig. 2). At the end of the “Braslet-On” session, a cardiac four chamber or right ventricular (RV) TD was obtained during the rapid release of the Braslet to observe the effect of the fluid sequestered in the lower extremities returning to the effective circulating volume.
All the ultrasound images were analyzed by registered sonographers and also reviewed by a cardiologist certified by the National Board of Echocardiography (41a) external to the investigator group to verify the diagnostic quality of the data. All parameters listed in Table 1 were measured in three different frames or cardiac cycles. In some subjects, data points from a particular step of the protocol were rejected due to poor quality; however, data of acceptable fidelity were obtained from all subjects.
Data analysis consisted of two phases. Phase I was an exploratory analysis designed to identify outcome parameters (Table 1) that showed evidence of being affected by the Braslet for any or all of the test conditions. For this phase, we made no distributional assumptions and used Somers D, a nonparametric measure of association to describe consistency of change with regard to Braslet status (on or off) taking into account differences between subjects and possibly flight days within a mission for the same subject. In Table 1, combinations of parameters and test conditions were flagged (★ in Table 1) as showing statistically significant change if P values for the test H0: D = 0 were below the critical threshold of 0.019, as determined by controlling the false discovery rate to 10% using the method of Benjamini and Yekutieli (10) with a nominal test level of α = 0.05. The false discovery rate is the expected proportion of rejected hypotheses that in fact were null and should not have been rejected.
In Phase II, we used a parametric mixed model to describe each parameter's response, with and without the Braslet, and under each of the three test conditions (baseline, Valsalva, Mueller). For some outcome parameters, log transformations were made before analysis. The model included random effects for subjects as well as sessions (i.e., flight days) for a given subject. From each fitted model, we made multiple comparisons between the interaction of Braslet (on, off) and test condition (baseline, Valsalva, Mueller) for each outcome parameter. As was the case in Phase I, correction for multiple inferences on three comparisons for each of 27 parameters had to be made. To do this, we again used the method of Benjamini and Yekutieli (10), but this time with the more conservative false discovery rate control of 5%. In this case the critical P value threshold was 0.0075, again with a nominal level of α = 0.05. Estimated values of Braslet effects (%change in mean ± SE) are shown in Table 1 for each parameter and test condition. Significant effects (P < 0.0075) of test condition as identified by the Benjamini and Yekutieli multiple-comparison procedure are shown on the right.
Table 1 summarizes results from 15 experiment sessions conducted on nine subjects. Out of the 17 sessions scheduled, 15 successfully produced a useful set of data. Some anomalies included the following.
One session not performed due to ultrasound hardware failure.
One session in which cardiac image data were not captured properly due to a procedural error.
One session in which no ECG tracing was obtained with vascular images.
One session in which partial data were lost due to hardware failure.
One session not performed due to scheduling constraints.
One session in which respiratory maneuvers were not properly synchronized, causing difficulties with data analysis.
Phase 1 analysis: effect of Braslet on cardiovascular parameters.
After correcting for multiple testing (see Statistical Methods), the Braslet cuff was identified as producing a statistically significant effect on the mean response during at least one type of maneuver (including baseline) in 10 of the 27 parameters measured. In particular, significant decreases with application of the cuff were observed in cardiac output, LV stroke volume, left lateral E′, mitral A and E wave velocity, and right isovolumic relaxation time (IVRT) during baseline; left lateral A′ and E′ during the modified Valsalva maneuver; and left lateral E′, right IVRT, and right Tei index (49a) during the modified Mueller maneuver. Significant increases were observed in mitral deceleration time (baseline) and in the femoral vein area (baseline) and Valsalva (★ in Table 1). Echocardiographic normal parameters were studied on the ISS by Hamilton et al. (30) and, using their data, the magnitude and direction of the changes reported in Table 1 can be approximated for these subjects.
Phase 2 analysis: comparison of Valsalva, Mueller, and baseline + maneuvers with and without Braslet.
Again after correction for multiple testing, we observed statistical evidence that the effect of the Braslet cuff (on vs. off) was significantly different in seven parameters between at least two of the maneuvers (including baseline; last column of Table 1). More specifically, parameters showing evidence of this differential change were cardiac output, heart rate, IJV area, LV diastolic volume, left lateral S′, mitral deceleration time, and right lateral E′.
After completing all the imaging components with maneuvers in Braslet-off and Braslet-on states, the cardiac probe was positioned to get a continuous four-chamber view. The operator crewmember rapidly released the Velcro straps on both Braslet cuffs, and at least 10 cardiac cycles were recorded.(see discussion). Unfortunately, the four-chamber view (n = 6) was difficult to maintain during the Braslet release, resulting in deviation of the imaging plane from its original position; therefore, it was replaced with the more stable TD view (n = 9) in later sessions.
Herault et al. (32) reported that despite extensive training of Mir cosmonauts before flight, the quality of the ultrasound data obtained in-flight may be inadequate. To assure precise execution of the study protocol and quality of the data of our study, both the ultrasound video and the cabin view video were streamed in real time to the NASA Telescience Center, enabling continuous control of the experiment and verbal guidance of the crew by investigators. Each image and each cine-loop used in the analysis were acquired by the astronauts only when the ground-based expert considered the image adequate and commanded acquisition. Thus an established system of balanced expertise distribution enabled data acquisition with quality that was acceptable for real-time assessment and thorough retrospective analysis.
TD was used for the first time in spaceflight during this investigation. As an excellent means to assess cardiac performance, TD method is of particular importance for long-duration spaceflight (Fig. 4). TD spectra were taken from the LV lateral wall, septum, and RV free wall. Furthermore, TD spectra are easier to obtain and less vulnerable to motion artifacts than two-dimensional echocardiographic views. In our subjects, LV lateral E′ decreased by >20% with the application of Braslet regardless of the maneuver, and LV Lateral A′ decreased only when Braslet was applied and a Valsalva maneuver was performed. RV isovolumic relaxation time decreased by >20% with the Braslet applied and by >30% when a Mueller was simultaneously performed. This indicates that preload was reduced with Braslet and that at these reduced end-diastolic volumes the reduced intrathoracic pressure from Mueller made the tricuspid valve open sooner. Mitral pulsed-wave Doppler velocity of the LV inflow is a load-dependent parameter and decreases in response to reduced preload (34, 41). By decreasing preload with Braslet, both Valsalva and Mueller seemed to have caused a modest reduction in LV preload as demonstrated by smaller velocities or extended relaxation slopes. Impaired LV filling is seen with Valsalva secondary to increased thoracic pressure while Mueller produces a similar result as a consequence of increased RV afterload from the subatmospheric intrathoracic pressure and the small end-diastolic cavitary and pericardial volumes created by the Braslet (Fig. 5).
Femoral vein area increased by 89% with the application of the Braslet. This change was less pronounced (51%) when a Valsalva maneuver was compared pre/post-Braslet, since without Braslet, Valsalva already increased the femoral vein area. This observation provides evidence that the Braslet, when worn and calibrated correctly, still allows for thoracic maneuvers to have a demonstrable effect on lower extremity venous filling. The pressure from occlusion cuffs compresses the superficial veins more than the deep circulation (14, 24, 35). Although Herault et al. (32) reported the IJV area decrease in spaceflight with Braslet applied, we could affect significant changes in IJV area only when Braslet was combined with thoracic maneuvers.
Cardiac output significantly decreased by 19% and stroke volume decreased by 12% when the Braslet was worn, and the heart rate compensation changed significantly in the increasing direction during a Valsalva maneuver. These findings are consistent with Pourcelot and Pottier and colleagues (44–46) who reported a decrease in stroke volume (∼20%) and cardiac output (∼20%) using echocardiography with thigh compression during a short-duration French-Soviet Salyut flight in 1982. On flight day 4, they used pneumatic thigh cuffs at 40 and 60 mmHg, which is similar to the constraining stress of the Braslet measured by Hamilton et al. (unpublished results) using balloon pressure transducers (29). Diridollou and Maillet (17) reported that the Braslet used in their HDT study provided the equivalent of 30 mmHg “counterpressure” over the area of its application on the upper thigh. Lindgren et al. (37) observed a 3% increase in leg volume with 12° HDT with thigh cuffs inflated to 50 mmHg for 15 min; however, these subjects would have been considered hypervolemic compared with microgravity.
Herault et al. (32) measured the effects of wearing the Braslet for 5 h on six cosmonauts during a 6-mo stay on the Mir space station and reported stroke volume and cardiac output reduction of 15 and 14%, respectively, after 1 mo of spaceflight. It is interesting to note that these changes became minuscule after 3 mo of spaceflight, and the Braslet actually increased stoke volume and cardiac output after 5 mo of spaceflight. They also report that femoral vein area increased by ∼20% with the Braslet applied at 1 and 3 mo, but 5 mo into the flight this increase was only 9%. We did not observe this trend on any subject. Muscle atrophy was more prevalent in pre-ISS missions, altering the fit and, therefore, the efficacy of the Braslet (26, 53); the ISS countermeasures system is far more effective at preventing muscle atrophy and exercise deconditioning. Hamilton et al. (30) performed a prospective echocardiographic study in six ISS crewmembers 152, 116, 149, 34, 190, and 41 days after launch and found no clinically significant differences between cardiovascular parameters acquired on-orbit compared with pre- and postmission data. We therefore believe that mission length up to 6 mo does not have a significant effect on the outcome of this study.
The myocardial performance index [Tei index (50)] is calculated using diastolic and systolic time intervals derived from TD spectra as a combined measure of myocardial performance. The RV Tei index exceeded the normal terrestrial value of <0.3 (27) in all but one subject.
Diridollou and Maillet (17) used high-frequency ultrasound to demonstrate the increase in forehead skin thickness (2.8%) and decrease in tibial skin thickness (3.5%) when subjects are placed in 6° HDT. When Braslet was applied during 7 days of HDT, the forehead thickness decreased 0.6% from baseline compared with the control group, which increased by 6.4%. Arbeille et al. (4, 8) measured IJV distension and facial edema in a HDT study. Compared with supine values they found that facial skin thickness increased by 5% after 7 days of HDT but, after wearing the Braslet for 8 h, the skin thickness was reduced by 5%. This agrees with the subjective comments by Herault et al. (32), which document that all astronauts who wore the Braslet during their study in space claim to have had a “sensation of comfort.” Matsnev et al. (39) reported that when Braslet was employed on Soyuz-38 the cosmonauts reported a reduction in space adaptation syndrome symptoms (dizziness, congestion, and headaches).
Kirsch et al. (36) measured CVP after 22 h of microgravity during the Spacelab 1 mission and found it to be less than the preflight supine levels in two subjects. They repeated this during the Spacelab D1 missions and found that the CVP again fell to levels below preflight supine 20–40 min after liftoff. This is consistent with the findings by Buckey et al. (12), which found that invasive CVP fell to 2.5 cmH2O immediately at the transition to microgravity on the Space Life Sciences 1 mission. Foldager et al. (20) reported that CVP decreased to 6.5–2.0 mmHg after 3 h of microgravity exposure during the Spacelab D2 (n = 4) and Space Life Sciences 2 (n = 2) missions.
Although Braslet did not induce a profound change in IJV area despite the obvious reduced cardiac preload, thoracic maneuvers seemed to have a profound additive effect when the Braslet was applied. This implies that the IJV is close to the pressure required to maintain its unstressed volume (i.e., to saturate its filling capacity) and that a Mueller maneuver will decrease its cross-sectional area. The Mueller − Valsalva difference is instructive since Valsalva caused a small increase in IJV area, implying the vein was close to being maximally distended despite the reduced cardiac preload caused by the Braslet. Nonetheless, the IJV area decreases significantly when a Mueller maneuver is performed (Fig. 6). This finding is consistent with the IJV, which takes very little CVP to distend it maximally and has a low enough CVP to be manipulated with limited thoracic pressures when Braslet is applied. This can be replicated at the bedside on a healthy patient by observing the change in fullness of the jugular venous pulse when raising or lowering the patient's neck by only 1 in. Therefore, a Mueller maneuver with Braslet applied in microgravity seems to decrease the CVP acutely to <2 mmHg. This is consistent with the maintenance of RV preload under normal terrestrial conditions where RV transmural pressures are ∼1.5 mmHg in humans (28).
The monitoring of the IJV using thoracic maneuvers may replace the terrestrial bedside JVP to determine volume status of the microgravity patient. We agree with the hypothesis of Tyberg et al. (54) suggesting a mechanism by which blood volume changes might explain the hypotension seen following return to normal gravity after spaceflight. On entering microgravity, lower-extremity peripheral veins significantly reduce in volume and, because of volume redistribution, distend the right atrium even though CVP is decreased. After some time in space, homeostatic mechanisms may cause a decrease in vascular volume and reduce the pressure in the central venous compartment to values below what is found on Earth but adequate to provide the necessary preload for the heart. The new set point for central venous pressure may be a combination of the pressure that provides the external constraint to the lower extremity venous system or the pericardial constraint of the heart under long-duration microgravity conditions. The RV end-diastolic pressure, which approximates the CVP, is mostly determined by pericardial constraint even at low filling pressures (22). Nonetheless, large changes in intrathoracic pressure manifest themselves more profoundly in microgravity when the Braslet is applied, which is similar to their effect on a hypovolemic patient. We propose that a human in space who is euvolemic by microgravity standards has less hemodynamic volume reserve compared with their terrestrial counterpart and would be more susceptible to hypovolemic shock for the same intravascular volume loss.
During the Braslet release the four-chamber images consistently show an immediate increase in LV stroke volume, which oscillated in magnitude over several beats and stabilized to a value that was close to the pre-Braslet application. The increased stroke volume after the Braslet release stabilized within 10 beats, which suggests that a significant amount of the volume in the leg must have been sequestered in the extravascular space (Fig. 7). These results may be consistent with Fujimoto et al. (22), who showed that ventricular interaction modulates LV performance even at low filling pressures. They performed lower-body negative pressure (LBNP; −30 mmHg) on humans and observed a reduction in stroke volume of 3% despite a transient increase in RV preload. Perhaps the oscillatory nature of the response following Braslet release observed in our subjects may be due to pericardial-ventricular interaction or a baroreflex response (22). Hinghofer-Szalkay et al. (33) demonstrated that 35 mmHg of LBNP could be “counterbalanced” with 27.5° of HDT. Given that the Braslet is calibrated to ameliorate the effect of 30° HDT, it may have an effect on the lower extremities similar to −30 mmHg of LBNP.
The capability of Braslet to acutely reduce the persisting IJV distension suggests its possible use in a therapeutic capability, likely in combination with other measures, to control certain medical hazards in space. For example, the chronically dilated venous system of the neck, and presumably other cranial venous structures, may play a role in the pathophysiology responsible for occasionally noted moderate elevation of intracranial pressure. Some crewmembers report changes in their visual acuity requiring corrections of more than 2 diopters. In some crewmembers, postflight visual acuity changes remain abnormal for months and fundoscopic examinations revealed papilledema and choroidal folds. MRI and ultrasound examinations of the eye in these crewmembers also demonstrated optic disc edema, posterior flattening of the globe, or dilated optic nerve sheaths, which is similar to patients with idiopathic intracranial hypertension (1, 25, 38). The results of this study suggest that the application of Braslet in combination with thoracic maneuvers may play a role in understanding, and possibly mitigating, this spaceflight concern that is not fully understood at this time.
Braslet device in microgravity would likely be effective for the treatment of high-pressure pulmonary edema, since in microgravity all regions of the lung are susceptible to pulmonary edema at the same left atrial pressure. It may stand to reason, in microgravity, that when a higher initial pressure of 30 cmH2O is achieved, sudden global alveolar flooding will most likely occur. Appearance of crackles anywhere in the thorax in the setting of fluid overload may then indicate impending fulminant pulmonary edema. The Braslet could help control this condition in a manner similar to nitrates or thigh cuffs on Earth until diuretics can be used.
The Braslet device acutely reduces the effective circulating volume by sequestering fluid in the lower extremities, as directly observed by vascular ultrasound and supported by the reduced preload indexes measured by echocardiography. Vascular ultrasound confirmed reduced distention of the jugular venous system and increased sensitivity of the jugular vein area to thoracic maneuvers. These findings combined with subjective comments from crewmembers suggest that the relative cranial venous insufficiency caused by microgravity is partially alleviated by the Braslet. The hazards secondary to wearing the occlusive Braslet cuffs in microgravity for an extended time (>1 h) are unknown, although in some studies the device was worn for longer than 6 h.
Following conservative safety considerations, this experiment established an effective set of modifications to the widely used respiratory maneuvers, introducing their open-glottis, limited-pressure modifications. These physiologically gentle and unequivocal respiratory maneuvers were shown to cause statistically significant hemodynamic effects.
A statistically significant reduction in jugular venous filling was also observed during Braslet application, which coupled with negative airway pressure to consistently collapse the IJV. This effect has important diagnostic and therapeutic implications for space medicine and should be further studied from both perspectives.
Remotely guided ultrasound provides an effective and objective means of measuring the physiological effects of the Braslet and produces data quality that is superior to previous investigations in space. Valsalva and Mueller maneuvers appear to enhance the ability at the microgravity bedside to determine volume status.
Primary funding for this project was provided by the Exploration Medical Capability element of NASA's Human Research Program through the Bioastronautics Contract to Wyle Integrated Science and Engineering Group (NAS9-02078).
No conflicts of interest, financial or otherwise, are declared by the authors.
Author contributions: D.R.H., A.E.S., P.A.W., I.V.A., S.A.D., V.P.M., V.V.B., and J.M.D. conception and design of research; D.R.H., A.E.S., K.G., D.J.E., P.A.W., I.V.A., S.A.D., V.P.M., and V.V.B. performed experiments; D.R.H., A.E.S., K.G., D.J.E., and A.H.F. analyzed data; D.R.H., A.E.S., K.G., D.J.E., and A.H.F. interpreted results of experiments; D.R.H., A.E.S., K.G., D.J.E., and A.H.F. prepared figures; D.R.H., A.E.S., and D.J.E. drafted manuscript; D.R.H., A.E.S., D.J.E., S.A.D., and J.M.D. edited and revised manuscript; D.R.H., A.E.S., P.A.W., I.V.A., S.A.D., V.P.M., V.V.B., and J.M.D. approved final version of manuscript.
- Copyright © 2012 the American Physiological Society