Hypobaric Environments Research Articles

Effects of CO₂ on the occurrence of decompression sickness: review of the literature.

Inhalation of high concentrations of carbon dioxide (CO₂) at atmospheric pressure can be toxic with dose-dependent effects on the cardiorespiratory system or the central nervous system. Exposure to both hyperbaric and hypobaric environments can result in decompression sickness (DCS). The effects of CO₂ on DCS are not well documented with conflicting results. The objective was to review the literature to clarify the effects of CO₂ inhalation on DCS in the context of hypobaric or hyperbaric exposure. The systematic review included experimental animal and human studies in hyper- and hypobaric conditions evaluating the effects of CO₂ on bubble formation, denitrogenation or the occurrence of DCS. The search was based on MEDLINE and PubMed articles with no language or date restrictions and also included articles from the underwater and aviation medicine literature. Out of 43 articles, only 11 articles were retained and classified according to the criteria of hypo- or hyperbaric exposure, taking into account the duration of CO₂ inhalation in relation to exposure and distinguishing experimental work from studies conducted in humans. Before or during a stay in hypobaric conditions, exposure to high concentrations of CO₂ favors bubble formation and the occurrence of DCS. In hyperbaric conditions, high CO₂ concentrations increase the occurrence of DCS when exposure occurs during the bottom phase at maximum pressure, whereas beneficial effects are observed when exposure occurs during decompression. These opposite effects depending on the timing of exposure could be related to 1) the physical properties of CO₂, a highly diffusible gas that can influence bubble formation, 2) vasomotor effects (vasodilation), and 3) anti-inflammatory effects (kinase-nuclear factor and heme oxygenase-1 pathways). The use of O₂-CO₂ breathing mixtures on the surface after diving may be an avenue worth exploring to prevent DCS.

Oxidative Stress Reaction to Hypobaric-Hyperoxic Civilian Flight Conditions.

In military flight operations, during flights, fighter pilots constantly work under hyperoxic breathing conditions with supplemental oxygen in varying hypobaric environments. These conditions are suspected to cause oxidative stress to neuronal organ tissues. For civilian flight operations, the Federal Aviation Administration (FAA) also recommends supplemental oxygen for flying under hypobaric conditions equivalent to higher than 3048 m altitude, and has made it mandatory for conditions equivalent to more than 3657 m altitude. We hypothesized that hypobaric-hyperoxic civilian commercial and private flight conditions with supplemental oxygen in a flight simulation in a hypobaric chamber at 2500 m and 4500 m equivalent altitude would cause significant oxidative stress in healthy individuals. Twelve healthy, COVID-19-vaccinated (third portion of vaccination 15 months before study onset) subjects (six male, six female, mean age 35.7 years) from a larger cohort were selected to perform a 3 h flight simulation in a hypobaric chamber with increasing supplemental oxygen levels (35%, 50%, 60%, and 100% fraction of inspired oxygen, FiO2, via venturi valve-equipped face mask), switching back and forth between simulated altitudes of 2500 m and 4500 m. Arterial blood pressure and oxygen saturation were constantly measured via radial catheter and blood samples for blood gases taken from the catheter at each altitude and oxygen level. Additional blood samples from the arterial catheter at baseline and 60% oxygen at both altitudes were centrifuged inside the chamber and the serum was frozen instantly at -21 °C for later analysis of the oxidative stress markers malondialdehyde low-density lipoprotein (M-LDL) and glutathione-peroxidase 1 (GPX1) via the ELISA test. Eleven subjects finished the study without adverse events. Whereas the partial pressure of oxygen (PO2) levels increased in the mean with increasing oxygen levels from baseline 96.2 mm mercury (mmHg) to 160.9 mmHg at 2500 m altitude and 60% FiO2 and 113.2 mmHg at 4500 m altitude and 60% FiO2, there was no significant increase in both oxidative markers from baseline to 60% FiO2 at these simulated altitudes. Some individuals had a slight increase, whereas some showed no increase at all or even a slight decrease. A moderate correlation (Pearson correlation coefficient 0.55) existed between subject age and glutathione peroxidase levels at 60% FiO2 at 4500 m altitude. Supplemental oxygen of 60% FiO2 in a flight simulation, compared to flying in cabin pressure levels equivalent to 2500 m-4500 m altitude, does not lead to a significant increase or decrease in the oxidative stress markers M-LDL and GPX1 in the serum of arterial blood.

Durability and microstructural evolution of high-performance ecological geopolymer concrete under low-pressure–salt-erosion–freeze–thaw cycling conditions

Environmental characteristics such as low pressure, high-frequency freeze–thaw cycles, and salt erosion in a plateau region have a significant impact on the mechanical properties and durability of concrete. To reveal the damage mechanism of high-performance ecological geopolymer concrete (HPEGPC) incorporating tunnel spoils, slag, and fly ash (FA) in extreme environments, this study used HPEGPC with 12 % alkali-activated binders (AABS) and a binding material to machine-made sand ratio of 0.36 and ordinary Portland cement concrete (OPCC) with 12.4 % cement and a water-cement ratio of 0.5 to simulate the deterioration process of mechanical properties and durability under the action of low-pressure–salt–erosion–freeze–thaw cycles. The mass, compressive strength, relative dynamic elastic modulus loss rate, microstructure, and pore evolution patterns of HPEGPC and OPCC were analysed using experimental and microscopic tests. The results showed that the HPEGPC was significantly more resistant to freezing and salt erosion than the OPCC in both standard and hypobaric environments. The interfaces of HPEGPC before and after the low-pressure–salt-erosion–freeze–thaw cycles underwent a developmental process from a dense structure with microcracks to microcracks expanding into fracture surfaces and the dispersion of hydration products. The cementitious materials of HPEGPC have a high percentage of non-hazardous and less hazardous pores, and their dense structure results in a high resistance to freezing and salt erosion. In addition, by establishing a damage decay model and damage accumulation model for the compressive strength and relative dynamic elastic modulus, this paper provides theoretical guidance for the application of HPEGPC under low pressure, salt erosion, and freeze–thaw cycle environments of the plateau.

Silica exposure among miners at high altitude. Inhaled dose and evaluation methods in hypobaric environments

To evaluate silica exposure among Chilean miners at high altitude, using different methodological approaches, for the purpose of determining the safest method to control exposures. Methods: The 46 miners in the sample worked at 3000 meters above sea level in nonstandard work shifts, consisting of four consecutive 12-hour days, followed by four consecutive days off. Silica samples were obtained in each of the jobs positions of these 46 high-altitude miners. The results of the concentrations are presented in mg/m3. Exposures were evaluated in compatison to the Threshold Limit Value (Method 1) and using two other methodologies that incorporate respiratory parameters (Methods 2 and 3). The proportion of miners at risk was determined with each of these methods and compared. Based on the Threshold Limit Value (Method 1), 43.48% of miners were classified as being at risk. With the other two methods that incorporate respiratory parameters, the proportion of overexposed miners was 82.61% with Method 2, and 73.91% with Method 3. Of the three methods analyzed, the one that considers the respiratory parameter minute volume, through the estimation of the inhaled dose, is the safest to define the group of miners at risk due to exposure to silica at high altitude.

Human thermoregulatory model for simulating thermal response in high-temperature and hypobaric environments

The main function of the human thermoregulation system is to maintain the stability of body temperature. When fighters and helicopters are performing high-altitude combat tasks, pilots are often exposed to hypobaric conditions and high-temperature environments, causing thermal discomfort and a reduction of physical and mental abilities. This research starts with the improvement of the previous thermoregulatory model for the human body by revising the convective and evaporative parameters of the environment, and the metabolic parameters of thermoregulation in hypobaric conditions. Then thermal physiology experiments were carried out on the ground and at 4000 m altitude. The sweating volume, local skin temperature, and subjective thermal sensation scales were measured to explore the human thermal regulation mechanism and validate the accuracy of simulation results. For comparison of simulation and experiments, the average skin temperature errors are 0.37 °C and 0.24 °C, and the relative errors of the sweating volume are 9 % and 8 % in the atmospheric and hypobaric environments, respectively. The results indicate that the improved thermoregulatory model for simulating thermal response could realize accurate predictions in high-temperature atmospheric and hypobaric environments.

Effects of simulated high altitude on body temperature and heart rate in pilot reserves during the hot-dry exposure

Recent research has primarily focused on human thermoregulatory responses at high altitudes, but investigations involving extremely high-temperature and hypobaric compound environments are currently lacking. To address this gap in knowledge, this study aimed to enhance the prediction of human heat sensation for pilot reserves while operating in extremely hot cabins with decompression. To achieve this, an improved Predicted Heat Strain (PHS) model was developed by incorporating the influence of air pressure on metabolic rate. The model's validity was assessed through experiments conducted at different altitudes (0m, 5000m, and 8000m) and varying environmental conditions (26°C and 45 °C, 10% RH and 40% RH) within an environmental simulation cabin. During these experiments, local skin temperature, core temperature, heart rate, and blood oxygen saturation were measured. The findings revealed distinct variations in the skin temperature of the hand and foot segments across different experimental stages, and significant correlations were observed between heart rate, core temperature, and local skin temperatures. Furthermore, regression models were established to explore the relationship between heart rate and local skin temperatures. By comparing simulated and experimental core and local skin temperatures, the enhanced PHS model was successfully validated. The conclusions drawn from this study provide valuable insights for predicting thermal physiological indices accurately and conveniently in hot-dry and hypobaric environments.

Hearing Aids Performance in Hypobaric Environments.

OBJECTIVE: High altitudes imply exposure to a decreased ambient air pressure. Such a situation may also alter the performance of acoustic transducers using vibrating diaphragms due to air rarefaction. This study aimed at analyzing the performance at high altitude of hearing aids (HAs) where mechano-electric and electro-mechanic transducers are used. METHODS: A hypobaric chamber was used to perform two separated experimental sessions. In the first one two commercial models of HAs were exposed to a simulated altitude of 25,000 ft (7620 m) and to a subsequent rapid decompression profile, with a rapid climb (< 3 s) from 8000 (2438 m) to 25,000 ft. The second session separately analyzed the performance of microphone and receiver at an altitude of 9000 and 15,000 ft (2743 and 4572 m). Before and after the first session, the HAs were tested with an electronic ear while a dedicated recording system was used in the second session. RESULTS: No HA damage or dysfunction was detected during the first session. In the second one, the microphone showed a mild decrease of its output, while the receiver exhibited a much higher reduction of its output. CONCLUSION: Our findings highlight the safe use of HAs even under extreme environmental pressure changes. For altitudes exceeding 10,000 ft (3048 m), a recalibration of the HAs output via a dedicated program may be suggested. Lucertini M, Sanjust F, Manca R, Cerini L, Lucertini L, Sisto R. Hearing aids performance in hypobaric environments. Aerosp Med Hum Perform. 2021; 92(9):738743.

Resistance training exercises for obese and non-obese individuals living in high-altitude regions utilizing biochemical markers-A controlled trial.

High-altitude disease prevalence varies according to types of exposure and the effects of hypoxic and hypobaric environments, with the result that people at high altitudes present many different physiological responses. The research aims to analyze the effects of resistance training (RT) exercises at high altitudes on obese subjects and to explain the determinants that make obese people more susceptible to various chronic illnesses such as diabetes mellitus, hypertension, asthma, etc. This study recruited 225 subjects living in the high-altitude region of Aseer, Saudi Arabia, and divided them into three groups. The first two groups consisted of obese people, out of which one group received RT and one did not. The third group consisted of average-weight individuals, according to their BMI, who received RT. Biochemical parameters were checked for all three groups before commencing with the RT and at the 4th and 8th week to measure the effects of the exercise. Mean and standard deviations of the demographic variables: age was 34.2 ± 8.9 years, weight was 69.3 ± 8.5 kg, and height was 1.6 ± 0.06 meters. RT had a significant effect on the total levels of cholesterol, triglycerides, HDL-cholesterol, LDL-cholesterol, adiponectin, interleukin-6, and testosterone. Post-hoc comparisons using the Fisher's Least Significant Difference test indicated that the mean scores between the groups differed significantly. Our findings show that RT would be a useful and practical substitute to improve the health status of obese patients. It helps to decrease body fat and to improve lipid profiles and hormonal control.

Balloon Dilation of the Eustachian Tube for Baro-Challenge-Induced Otologic Symptoms in Military Divers and Aviators: A Retrospective Analysis.

To evaluate the effectiveness of balloon dilation (tuboplasty) of the Eustachian tube (BET) in active duty military personnel working in hyper- and hypobaric environments suffering from baro-challenge-induced ETD using functional outcomes. Military divers and aviators diagnosed with persistent baro-challenge-induced ETD resulting in disqualification from performing flight and dive duties and who elected for treatment with BET were included for analysis. Posttreatment follow-up assessments were undertaken at 1, 6, and 12 months. Outcome measures included successful hyperbaric chamber testing or return to the hyper- or hypobaric environment without significant baro-challenge-induced ETD symptoms and pre- and postdilation Eustachian Tube Dysfunction Questionnaire (ETDQ-7) scores. Mean pretreatment duration of symptoms was 48 months (range: 3-120 months). Following treatment, 92% (12/13) of patients successfully returned to operational duties with resolution of limiting symptoms. Average return to duty time was 8.5 weeks (range: 6-24 weeks). The ETDQ-7 scores improved from a mean of 4.33 (2.57-6.57) predilation to 2.19 (1.00-4.43) postdilation (Z = 2.73, W = 70, P = .0063). Mean duration of follow-up was 38 weeks (range: 13-70 weeks). Eustachian tube balloon dilation appears to be a safe and highly effective treatment option for baro-challenge-induced ETD in affected military divers and aviators who work in hyper- and hypobaric environments. Further study is needed to determine whether similar results can be achieved in more diverse subject populations and to assess long-term effectiveness.

High Altitude Performance of Loudspeakers and Potential Impact on Audiometric Findings.

BACKGROUND: The evaluation of how air rarefaction can affect a loudspeaker performance at altitude implies the need for characterization of earphones during hypobaric conditions. The aim of this study was phonometric analysis at different altitudes of the acoustic output of a widely used earphone model, along with its consequences on audiological investigations conducted under such environmental conditions.METHODS: The transfer function of a TDH-39P earphone was analyzed with an artificial ear under nine different altitude levels, from sea level up to 35,000 ft, inside a hypobaric chamber. A specific phonometric system not sensitive to environmental pressure changes was used. Other potentially confounding factors, such as environmental temperature and humidity, were continuously monitored.RESULTS: No relevant temperature or humidity changes were detected. The sound pressure level generated by the earphone under hypobaric conditions was found considerably affected by air density changes. These data produced a correction table aiming at recalibrating the earphone's output at each audiometric octave test frequency within the 250-8000 Hz range. Quite different characteristics of response were observed at different audiometric frequencies. Such findings were particularly evident for altitudes exceeding 12,000 ft.DISCUSSION: The development of a frequency-selective and altitude-related correction factor for acoustic stimuli is an essential aspect when hearing threshold measurements in hypobaric environments are performed.Lucertini M, Botti T, Sanjust F, Cerini L, Autore A, Lucertini L, Sisto R. High altitude performance of loudspeakers and potential impact on audiometric findings. Aerosp Med Hum Perform. 2019; 90(7):655-659.

Hypobaric Conditions and Retention of Dental Crowns Luted with Manually or Automixed Dental Cements.

There is only scant information on the influence of the hypobaric environment on luting agents and their efficacy on dental crown cementation. The objective of this study was to provide data on the retentive characters of two cements commonly used on implant abutment surfaces both under normal and under hypobaric conditions. There were 56 implant abutments supplied with CAD/CAM milled zirconia oxide crowns. 1) A zinc phosphate cement (ZP), and 2) a resin-modified glass ionomer cement (RMGI), each mixed either A) manually or B) by means of automix capsules, were used for cementation. The cemented crowns of the 4 × 2 subgroups were either kept on the ground or were transported in an aircraft at altitudes up to 13,730 m (45,045.9 ft; N = 28 each), thus being subjected to the pressure changes (80×) every aircrew member or frequent flyer is exposed to. All cemented crowns were stored in climatized boxes during the experimental phase. Hand-mixing of ZP resulted in a significant reduction of mean (± SD) retention forces (581.6 ± 204.5 N) when compared to the control group on the ground (828.4 ± 147.9 N). Automixed ZP (931.9 ± 134.4 N in flight; 996.0 ± 107.4 N on the ground) and RMGI subgroups (ranging from 581.0 N ± 114.3 N to 662.4 N ± 92.5 N) were not affected by hypobaric conditions. When treating patients frequently exposed to hypobaric environments, automixing of ZP would seem favorable, while manual mixing should be avoided. RMGI is considered suitable and is not influenced by hand-mixing or barometric pressure changes.Kielbassa AM, Müller JAG. Hypobaric conditions and retention of dental crowns luted with manually or automixed dental cements. Aerosp Med Hum Perform. 2018; 89(5):446-452.

Ergogenic properties of metformin in simulated high altitude.

Metformin augments glucose/glycogen regulation and may acutely promote fatigue resistance during high-intensity exercise. In hypobaric environments, such as high altitude, the important contribution of carbohydrates to physiological function is accentuated as glucose/glycogen dependence is increased. Because hypoxia/hypobaria decreases insulin sensitivity, replenishing skeletal muscle glycogen in high altitude becomes challenging and subsequent physical performance may be compromised. We hypothesized that in conditions where glycogen repletion was critical to physical outcomes, metformin would attenuate hypoxia-mediated decrements in exercise performance. On three separate randomly ordered occasions, 13 healthy men performed glycogen-depleting exercise and ingested a low-carbohydrate dinner (1200kcals, <10% carbohydrate). The next morning, in either normoxia or hypoxia (FiO2 =0.15), they ingested a high-carbohydrate breakfast (1225kcals, 70% carbohydrate). Placebo (719mg maltodextrin) or metformin (500mg BID) was consumed 3days prior to each hypoxia visit. Subjects completed a 12.5km cycle ergometer time trial 3.5hours following breakfast. Hypoxia decreased resting and exercise oxyhemoglobin saturation (P<.001). Neither hypoxia nor metformin affected the glucose response to breakfast (P=.977), however, compared with placebo, metformin lowered insulin concentration in hypoxia 45minutes after breakfast (64.1±6.6μU/mL vs 48.5±7.8μU/mL; mean±SE; P<.001). Post-breakfast, pre-exercise vastus lateralis glycogen content increased in normoxia (+33%: P=.025) and in hypoxia with metformin (+81%; P=.006), but not in hypoxia with placebo (+27%; P=.167). Hypoxia decreased time trial performance compared with normoxia (P<.01). This decrement was similar with placebo (+2.6±0.8minutes) and metformin (+1.6±0.3minutes). These results indicate that metformin promotes glycogen synthesis but not endurance exercise performance in healthy men exposed to simulated high altitude.

Endotracheal Cuff Pressure Changes During Helicopter Transport: A Systematic Review

ObjectiveCuff pressures are important in ventilated patients undergoing helicopter transport. An altitude-related increase in endotracheal tube (ETT) intracuff pressure has been shown in simulated hypobaric environments, model tracheas, and animal studies and may not accurately reflect in vivo pressures. The aim of this study was to determine if ETT intracuff pressure increases above the critical perfusion pressure of the trachea in ventilated patients during helicopter transport. MethodsOvid Medline, CINAHL, Embase, Scopus, and the Cochrane Library were searched from their commencement to January 29, 2016. Google Scholar was searched, and reference lists of relevant articles were examined to identify additional studies. Articles were included if they reported on ETT intracuff pressure in ventilated patients during helicopter emergency medical service transport. ResultsA total of 330 articles were identified; only 2 prospective observational studies met the inclusion criteria. The studies reported a mean cuff pressure increase of 23 cm H2O and 33.9 cm H2O. Both studies reported ETT intracuff pressure to frequently exceed the critical perfusion pressure of the tracheal mucosa during helicopter transport. ConclusionFurther research with longitudinal follow-up is required to confirm these findings to determine if the effects of transient increased ETT intracuff pressure are clinically significant.

高度換算1,500 mの周期的低圧低酸素環境が持久性運動中および運動後回復期の生理応答に及ぼす影響

高度換算1,500 mの周期的低圧低酸素環境が持久性運動中および運動後回復期の生理応答に及ぼす影響

Semiparametric analysis of clustered interval-censored survival data with a cure fraction

A generalization of the semiparametric Cox’s proportional hazards model by means of a random effect or frailty approach to accommodate clustered survival data with a cure fraction is considered. The frailty serves as a quantification of the health condition of the subjects under study and may depend on some observed covariates like age. One single individual-specific frailty that acts on the hazard function is adopted to determine the cure status of an individual and the heterogeneity on the time to event if the individual is not cured. Under this formulation, an individual who has a high propensity to be cured would tend to have a longer time to event if he is not cured. Within a cluster, both the cure statuses and the times to event of the individuals would be correlated. In contrast to some models proposed in the literature, the model accommodates the correlations among the observations in a more natural way. A multiple imputation estimation method is proposed for both right-censored and interval-censored data. Simulation studies show that the performance of the proposed estimation method is highly satisfactory. The proposed model and method are applied to the National Aeronautics and Space Administration’s hypobaric decompression sickness data to investigate the factors associated with the occurrence and the time to onset of grade IV venous gas emboli under hypobaric environments.

Fifty Years of Decompression Sickness Research at Brooks AFB, TX: 1960–2010

INTRODUCTION, FACILITIES, AND METHODS: Decompression sickness (DCS) occurring in hypobaric environments related to aviation or spaceflight was a major focus of research at Brooks AFB/City-Base, TX, throughout the period 1960-2010. Multiple hypobaric chambers and extensive support facilities were built for research on altitude DCS using both human subjects and animal models. Areas of study included symptomatology, incidence, prediction, and prevention of DCS. High-altitude aviation, spacecraft atmospheres, and pressure suits were evaluated with various decompression and prebreathing schedules to reduce DCS risk. FACTORS AFFECTING DCS INCIDENCE: The results from these efforts were recorded in an extensive Altitude DCS Research Database which served as a resource for developing reports and exploring relationships of various parameters such as altitude, time at altitude, prebreathe time, and mode of activity while decompressed. PREVENTION AND PREDICTION OF DCS: Individual susceptibility to DCS was also evaluated in an effort to tailor preventive measures and predict susceptibility. Completion of the 26 human-use protocols provided information which was incorporated into NASA and USAF operational practices to reduce DCS risk. DCS researchers working at Brooks throughout this period produced 177 papers documenting results of thousands of subject-exposures and other experiments. An Altitude DCS Risk Assessment Computer Model was fielded in 2005. This review centers on the results of research at Brooks and notes questions about operational DCS risk that have not yet been answered.

Bioluminescent enzyme assay for the indication of plant stress in enclosed life support systems

The application of bioluminescent sensors for monitoring key metabolites and enzymes that are indicators of stress in plants is demonstrated. The sensitivity of bioluminescent assay for NAD(P)H and NAD(P)(+) was about 0.5 and 1 nmol, respectively. The levels of NAD(P)H and NAD(P)(+) in radish (Raphanus sativus) root extracts from controls and from stress-induced conditions were compared. To induce environmental stress, the plants were grown in enclosed environmental chambers with low pressure (9 or 32 kPa), high humidity (>80%) and low oxygen partial pressure (down to 3.3-6.5 kPa). The concentrations of NAD(P)(+) and NAD(P)H in plants varied under stress conditions. Decreasing both total pressure from 101.5 to 32 or 9 kPa and partial pressure of oxygen increased the ratio of NAD(P)(+) /NAD(P)H from 0.2 to 4 or 6, respectively. The increase in this ratio suggests that plants are undergoing stress in these hypobaric environments. The developed bioluminescent assay for quantification of pyridine nucleotides in plant tissues is rapid, low-cost and easily performed.

Interval-Censored Data with Repeated Measurements and a Cured Subgroup

SummaryThe hypobaric decompression sickness data study was conducted by the National Aeronautics and Space Administration to investigate the risk of decompression sickness in hypobaric environments. The quantity of interest is the time to onset of grade IV venous gas emboli, which was mixed case interval censored because of measurement limitations. In the study, some subjects participated in multiple experiments, leading to repeated and correlated measurements on those subjects. In addition, it has been suggested that some subjects had a much lower risk of developing grade IV venous gas emboli than others, i.e. those subjects were immune from the event of interest (or ‘cured’). We propose to use two-part models, where the first part describes the probability of cure and the second part describes the survival for susceptible subjects. We use two random effects to account for the correlated nature of measurements. A leverage bootstrap approach is proposed for model diagnosis. A simulation study shows satisfactory performance of the estimation and diagnosis approaches proposed. Model estimation and evaluation of the hypobaric decompression sickness data are carefully investigated.

Exposure ofArabidopsis thalianato Hypobaric Environments: Implications for Low-Pressure Bioregenerative Life Support Systems for Human Exploration Missions and Terraforming on Mars

Understanding how hypobaria can affect net photosynthetic (P (net)) and net evapotranspiration rates of plants is important for the Mars Exploration Program because low-pressured environments may be used to reduce the equivalent system mass of near-term plant biology experiments on landers or future bioregenerative advanced life support systems. Furthermore, introductions of plants to the surface of a partially terraformed Mars will be constrained by the limits of sustainable growth and reproduction of plants to hypobaric conditions. To explore the effects of hypobaria on plant physiology, a low-pressure growth chamber (LPGC) was constructed that maintained hypobaric environments capable of supporting short-term plant physiological studies. Experiments were conducted on Arabidopsis thaliana maintained in the LPGC with total atmospheric pressures set at 101 (Earth sea-level control), 75, 50, 25 or 10 kPa. Plants were grown in a separate incubator at 101 kPa for 6 weeks, transferred to the LPGC, and acclimated to low-pressure atmospheres for either 1 or 16 h. After 1 or 16 h of acclimation, CO(2) levels were allowed to drawdown from 0.1 kPa to CO(2) compensation points to assess P (net) rates under different hypobaric conditions. Results showed that P (net) increased as the pressures decreased from 101 to 10 kPa when CO(2) partial pressure (pp) values were below 0.04 kPa (i.e., when ppCO2 was considered limiting). In contrast, when ppCO(2) was in the nonlimiting range from 0.10 to 0.07 kPa, the P (net) rates were insensitive to decreasing pressures. Thus, if CO(2 )concentrations can be kept elevated in hypobaric plant growth modules or on the surface of a partially terraformed Mars, P (net) rates may be relatively unaffected by hypobaria. Results support the conclusions that (i) hypobaric plant growth modules might be operated around 10 kPa without undue inhibition of photosynthesis and (ii) terraforming efforts on Mars might require a surface pressure of at least 10 kPa (100 mb) for normal growth of deployed plant species.

Resolution of a paradox: Hummingbird flight at high elevation does not come without a cost

Flight at high elevation is energetically demanding because of parallel reductions in air density and oxygen availability. The hovering flight of hummingbirds is one of the most energetically expensive forms of animal locomotion, but hummingbirds are nonetheless abundant at high elevations throughout the Americas. Two mechanisms enhance aerodynamic performance in high-elevation hummingbirds: increase in wing size and wing stroke amplitude during hovering. How do these changes in morphology, kinematics, and physical properties of air combine to influence the aerodynamic power requirements of flight across elevations? Here, we present data on the flight performance of 43 Andean hummingbird species as well as a 76-taxon multilocus molecular phylogeny that served as the historical framework for comparative analyses. Along a 4,000-m elevational transect, hummingbird body mass increased systematically, placing further aerodynamic demands on high-elevation taxa. However, we found that the minimum power requirements for hovering flight remain constant with respect to elevation because hummingbirds compensate sufficiently through increases in wing size and stroke amplitude. Thus, high-elevation hummingbirds are not limited in their capacity for hovering flight despite the challenges imposed by hypobaric environments. Other flight modes including vertical ascent and fast forward flight are more mechanically and energetically demanding, and we accordingly also tested for the maximum power available to hummingbirds by using a load-lifting assay. In contrast to hovering, excess power availability decreased substantially across elevations, thereby reducing the biomechanical potential for more complex flight such as competitive and escape maneuvers.