Within the context of space exploration, the lymphatic and venous systems and the related vascular endothelial glycocalyx (GCX) remain under-researched areas. Numerous biologic and physiologic pathologies are associated with exposure to extreme environments and may result from the response and adaptation of the venous and lymphatic systems as well as the GCX.7–10 Research is required in ground-based analogues for spaceflight, then should progress to true weightlessness in low Earth orbit (LEO) as we begin to understand the impact on human physiology in the context of venous, lymphatic and GCX functioning during exposure to extreme environments.11–19
Lymphatic function/contractility is noted to be regionally, organ- and tissue-function specific.20 The GCX lines the luminal surface of the 60–70,000 miles of the arterial, venous, and lymphatic vasculature.
On Earth, body fluid compartments are maintained and balanced by the nano-scaled architectural integration of the endothelial GCX, the vasculature, the integument, and the lymphatic system. The lymphatic system’s capability to transport fluid from the lower extremities and torso, against gravity and soft tissue gradients, occurs via a combination of lymphangion contractility, leg muscle contraction, and respiratory/chest wall function in the setting of “primed” local/regional subatmospheric tissues. This creates a “suction effect” within the subatmospheric tissue distribution zones for lymphatic fluid movement within the lymphatic vasculature, which is lined with lymphatic endothelial cells and the GCX (the Guyton principle).22–23 The head, neck, and upper torso, however, are gravity gradient-dependent for venous and lymphatic drainage, and must be balanced with arterial in-flow to this region.24 In the weightlessness of space, astronauts experience a dramatic fluid redistribution of ~ 2 liters from the legs to the head, neck and upper torso within the first 24–48 hours of flight;25 this may be due in part to the persistence of pedal to cephalad lymphatic fluid flow due to “Guyton forces” and the loss of gravity induced head and neck lymphatic drainage, resulting in an imbalance of soft tissue fluid gradients.
Research leading to countermeasure development to support the venous and lymphatic systems at pre-launch, in-flight and post-flight have the potential to support nominal human health in space. Additionally, an improved understanding of venous, lymphatic and GCX functional restoration and maintenance will support improved human health on Earth for the vast number of patients dealing with the many associated diseases and conditions related to dysfunction and dysregulation of these integral systems.
Innovation
Studies upon astronauts and cosmonauts in true weightlessness have been performed in all spaceflight eras, from Mercury to Apollo,26 to ISS, including the 25-month long NASA Twins Study,16 that compared monozygotic twin astronauts through extensive integrated longitudinal, multidimensional measures of physiological, telomeric, transcriptomic, epigenetic, proteomic, metabolomic, immune, microbiomic, cardiovascular, vision related and cognitive data, as one spent 340 days in space on ISS and his identical twin remained Earthbound. Ground based analogues for simulation of the impact of weightlessness on human physiology, providing for extensive research opportunities, exist in several forms including days/weeks of strict 6-degree head tilt down bed rest, dry immersion, and brief periods (20–40 seconds) of weightlessness during parabolic flights. 11, 27–28
Lymphatic research has experienced a renaissance in the past decade with the recognition and increasing knowledge of the dynamic nature of the endothelial glycocalyx and the resulting modification of the classical Starling function to recognize the true critical nature of lymphatic integrative function with arterial, venous, integument and immune health. The modified Starling principle is a recognition that most interstitial fluid resulting from arterial perfusion re-enters the central venous system via the vast lymphatic network, and not via venules.8–12, 29–33 Despite new significant milestones in research and clinical application, lymphatic education and recognition remains “paradoxically and unnecessarily ignored” at the medical school and residency levels.34 Lymphatic research in space and astronaut health may “paradoxically” elevate the subject on terra firma. Given that all venous ulcerations and venous hypertension have associated glycocalyx thinning or shedding,35 and associated lymphedema of venous etiology/phlebolymphedema is a common though vastly underrecognized and undertreated component,36–37 the potential for patient care improvement may be elevated with the continued focus on astronaut health, countermeasure development, the near-term space tourism era, and the ambitious near-term goals of moon habitation and Mars human exploration.6
Lymphatic research in low Earth orbit (LEO) is in the initial phases of development. Current published data has used mice and rats in studies involving tail elevation (simulates effects of weightlessness), space shuttle and satellites (Bion-M unmanned automated 30-day satellite mission with mice).38 A 2-week head down, tail suspension model simulating microgravity, was found to be a potent inhibitor of pressure/stretch-stimulated pumping in cervical, thoracic and to a lesser degree, mesenteric lymphatics. The greatest inhibition was found in cervical lymphatics. These findings presumably are correlated to the cephalic fluid shifts that occur in HDT suspended rats as well as those observed during spaceflight.39, 24
The opportunity to further the understanding and develop noninvasive imaging methods to assess fluid shifts may contribute to the development of restorative countermeasures for lymphangion contractility, overall lymphatic function, and associated immune function in the extreme environment of space. This carries high potential to improve our understanding and treatment of patients that we care for in 1G clinics, who have lymphedema and venous disease that negatively affects quality of life, outcomes, and economics. The consequences on crew behavior and performance due to these fluid shifts in microgravity has not yet been fully determined for long duration missions.6
This novel study will evaluate four non-invasive medical devices to assess venous and lymphatic flow patterns as well as temperature differentiation, during performance of a validated spaceflight analogue. Fluid shifts experienced in microgravity can contribute to alterations in the cerebral venous system and are hypothesized to increase susceptibility to ophthalmic pathologies experienced in weightlessness, such as spaceflight associated neuro ocular syndrome (SANS).6 Imaging in true weightlessness may contribute to an enhanced understanding of how to manage cephalad fluid shifts and the contribution to SANS development in astronauts, which has not been fully assessed in space medicine to date.
Research Plan
Thirty healthy college students (all within a healthy BMI and without lymphedema or known lymphatic dysfunction) were recruited for this study. There were 12 males and 18 females, ranging in age from 21 to 36. Each student was assessed for temperature and lymphatic/venous flow using three different non-invasive devices that can measure temperature, perfusion, and image the lymphatic and venous structures. Students were assessed in the sitting, supine, and 6-degree head down tilt (HDT) positions, with pre/post assessments for the HDT position. Venous and lymphatic flow patterns may vary upon position changes (sitting vs. supine vs. 6-degree head down tilt), and in response to MLD performance in the treatment group.
Sample Size and Randomization
The sample size for this exploratory pilot study was 30 participants (15 participants in the control group and 15 participants in the treatment group who received MLD). Participants were recruited and data collected over a 3-month period. Students were randomly assigned to either the control or treatment group, using a random number generator.
Overall Strategy
Participants were recruited to take part in this study through flyers and email announcements throughout the Health Professions Division at Nova Southeastern University. Once consented and randomized to a group, participants were sent a link to Zcal (an online scheduling system) where they could sign up for data collection days and times according to their individual schedules to accommodate duration of position time, the longest being up to 4 hours (Treatment group, HDT position). Participants arrived at a predesignated room for assessment and data collection. Participants acclimated to the room and ambient temperature by sitting quietly for 15 minutes prior to data collection. Baseline vitals (heart rate, blood pressure, respiration rate, and oxygen saturation) were taken and monitored throughout the data collection process. All participants were assessed in the sitting position for 30 minutes (gravity dependent), supine for 1.5 hours (gravity neutral) and HDT for 3 hours (validated simulated weightlessness). Each position was assessed on a different day to reduce any position-related carryover effects. Baseline image assessments using the devices were taken 1 minute after assuming the designated position. Subsequent image assessments were taken every 30 minutes thereafter. Each image acquisition requires ~ 15–30 seconds and was performed by 1 of 4 trained imagers. At the end of the established timepoints, reassessment using the devices was taken prior to moving out of designated position. Participants in the MLD group received 15 minutes of an established MLD protocol to the head, neck, and upper torso prior to moving out of position. Immediately post MLD, reassessment with the devices was taken. A follow up reassessment was taken with the devices 30 minutes after MLD to assess potential changes and resolution of any symptoms experienced in any of the positions. Our observations were designed to detect differences in flow patterns and temperature based upon position and with the implementation of MLD pre- and post HDT.
The devices being used include SnapShot to assess perfusion/flow by capturing total hemoglobin, total oxy- and deoxyhemoglobin, and superficial oxygenation levels as a near-infrared spectroscopy device, WoundVision Scout to assess physiological thermal temperature levels using long-wave infrared thermography, and LymphScanner to measure the tissue dielectric constant (TDC) which is representative of localized percent water content.
Data Analysis Plan
Summary statistics were calculated for study variables. We employed random effects, generalized linear models, to look for differences in Left and Right Temple TDC, Left and Right (sternocleidomastoid) SCM TDC, Left and Right SUBC (sub-clavicular) TDC, and Left and Right Ventromedial Bundle TDC and temperature. First, we looked for changes within the treatment group only. Second, we examined differences between the treatment and control groups for each visit. The fixed effects were visit (baseline, post-30-minutes, post-60-minutes, post-90-minutes, post-120-minutes, post-180-minutes, post-MLD, and post-30 minutes-MLD). The random effect was the subject. For all post-modeling comparisons, a false discovery adjustment (FDR) was used. The FDR method has higher power than the Bonferroni and Tukey HSD method and controls the type I error as well. This is important considering the project’s exploratory nature. R version 4.2.2 was used for all statistical modeling and statistical significance was found at p < 0.05.