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PubMed1995

Transesophageal echocardiographic study of decompression-induced venous gas emboli.

Transesophageal echocardiography was used to evaluate venous bubbles produced in nine anesthetized dogs following decompression from 2.84 bar after 120 min at pressure. In five dogs a pulsed Doppler cuff probe was placed around the inferior vena cava for bubble grade determination. The transesophageal echo images demonstrated several novel or less defined events. In each case where the pulmonary artery was clearly visualized, the venous bubbles were seen to oscillate back and forth several times, bringing into question the effect of coincidental counting in routine bubble grade analysis using precordial Doppler. A second finding was that in all cases, extensive bubbling occurred in the portal veins with complete extraction by the liver sinusoids, with one exception where a portal-to-hepatic venous anastomosis was observed. Compression of the bowel released copious numbers of bubbles into the portal veins, sometimes more than were released into the inferior vena cava. Finally, large masses of foam were routinely observed in the non-dependent regions of the inferior vena cava that not only delayed the appearance of bubbles in the pulmonary artery but also allowed additional opportunity for further reaction with blood products and for coalescence to occur before reaching the pulmonary microcirculation. These novel observations are discussed in relation to the decompression process.

Animals↗
PubMed1993

Breathing 100% oxygen compared with 50% oxygen: 50% nitrogen reduces altitude-induced venous gas emboli.

The risk of venous gas emboli (VGE) and decompression sickness (DCS) must be determined before selection of the lowest pressure for an extravehicular activity (EVA) pressure suit which eliminates the requirement for prebreathing. In earlier studies, use of a 50% oxygen:50% nitrogen breathing mixture (50:50 mix) during 139 zero-prebreathe decompressions of male subjects to 8.3-7.8 psia resulted in 51 instances of severe VGE and one case of DCS. Our current study investigated effects of 40 zero-prebreathe decompressions of male subjects to 8.3-6.8 psia for 6 h while breathing 100% oxygen and performing moderate exercise. No DCS symptoms were observed. Severe VGE were not detected at 8.3 psia, but were present during 10%, 20%, and 40% of the exposures at 7.8, 7.3, and 6.8 psia, respectively. Zero-prebreathe decompression while breathing 100% oxygen results in significantly lower VGE and DCS risk levels than while breathing a 50:50 mix. Our results show that 7.3 psia EVA pressure suits with 100% oxygen should be safer than 8.3 psia suits with a 50:50 mix.

Aerospace Medicine↗
PubMed1993

Body fluid alterations during head-down bed rest in men at moderate altitude.

To determine the effects of hypoxia on fluid balance responses to simulated zero-gravity, measurements were made in six subjects (acclimatized to 5,400 ft; 1,646 m) before and during -5 degrees continuous head-down bed rest (HDBR) over 8 d at 10,678 ft. The same subjects were studied again at this altitude without HDBR as a control (CON) using a cross-over design. During this time, they maintained normal upright day-time activities, sleeping in the horizontal position at night. Fluid balance changes during HDBR in hypoxia were more pronounced than similar measurements previously reported from HDBR studies at sea level. Plasma volume loss (-19% on day 6) was slightly greater and the diuresis and natriuresis were doubled in magnitude as compared to previous studies in normoxia and sustained for 4 d during hypoxia. These changes were associated with an immediate, but transient rise in plasma atrial natriuretic peptide (ANP) to day 4 of 140% in HDBR and 41% in CON (p < 0.005), followed by a decline towards baseline. Differences were less striking between HDBR and CON for plasma antidiuretic hormone and aldosterone, which were transiently reduced by HDBR. Plasma catecholamines showed a similar pattern to ANP (+122%) in both HDBR and CON, suggesting that elevated ANP and catecholamines together accounted for the enhanced fluid shifts with HDBR during hypoxia.

Adult↗
PubMed1996

Acute ventilatory response to simulated altitude, normobaric hypoxia, and hypobaria.

BACKGROUND: Some reports claim that ventilation (VE) is greater in human subjects in normobaric hypoxia than at altitude following an equivalent drop in inspired PO2 (PIO2). It has been suggested that reduced barometric pressure (PB) may decrease chemoreceptor sensitivity and account for these results. In this pilot study we tested the hypothesis that VE and hypoxic chemoresponsiveness would not be different after 30 min of normobaric hypoxia and altitude. METHODS: We exposed three male and three female subjects to four conditions in an environmental chamber, varying the order. The four conditions were: air (PB = 640, FIO2 = 0.204), hypobaria (434, 0.298), hypoxia (640, 0.141) and altitude (434, 0.203). We measured VE, end-tidal O2 and CO2 and arterial O2 saturation (SpO2) after 30 min in each environment, and while breathing 100% O2 for 1 min immediately thereafter. RESULTS: The mean increase in VE relative to air was 14%, 20% and 26% for hypobaria, hypoxia and altitude, respectively, with corresponding reductions in PETCO2 in the three conditions. The reduction in VE with 100% O2 was inversely proportional to the rise in SpO2 in all cases, indicating that chemoresponsiveness was unchanged by PB. When hypobaria preceded altitude, the VE at altitude increased less, relative to air, than when altitude was given first (not significant). CONCLUSIONS: The VE and chemosensitivity are about the same after 30 min of altitude and equivalent hypoxia. However, when the drop in PIO2 is not synchronous with the drop in PB, like at altitude, the VE values may be altered. Air density, hypoxic pulmonary vasoconstriction and circulating microbubbles may interact to account for the observed findings.

Adult↗
PubMed1996

Left ventricular gas emboli in six cases of altitude-induced decompression sickness.

BACKGROUND: Ultrasonic techniques have demonstrated venous gas emboli (VGE) during exposure to high altitude. VGE per se have not been considered clinically hazardous. Arterial gas emboli (AGE), however, are viewed with great concern. The crossing-over of venous gas to the arterial circulation has not previously been seen in human subjects at altitude. This transfer may occur via either intracardiac defects, pulmonary shunts, or the pulmonary microcirculation. METHODS: A non-invasive ultrasonic echo imaging Doppler system was used to monitor volunteer human subjects for gas emboli simultaneously in the right and left sides of the heart at simulated altitude in a chamber. Subjects found to have gas cross-over were evaluated for septal defects with either transthoracic or transesophageal echocardiography. RESULTS: Previously unreported left ventricular gas emboli were observed with echo imaging in six subjects at altitude. In all six cases, at the time of AGE onset, the VGE scores were high from all monitored sites. Three subjects had no septal defect, another had a small sinus venosus defect, a third had a patent foramen ovale, and one was not available for evaluation. Five of the cases became symptomatic at the time of AGE onset. CONCLUSIONS: Operational altitude exposures known to elicit high VGE counts in the majority of people should be avoided because of an increased risk of right-to-left gas cross-over and resulting potential for severe cerebral symptomatology.

Aerospace Medicine↗
PubMed1996

The initial signs and symptoms of altitude decompression sickness.

BACKGROUND: With the potential for higher aircraft and cabin altitudes, the way in which altitude decompression sickness (DCS) presents continues to be of interest. The majority of previous papers on the symptomatology of DCS are retrospective reviews of patients treated hours or days post-exposure. The initial presentation while still at altitude is the form of DCS that aircrew must be able to recognize in order to respond correctly. This paper reports the initial manifestations of DCS that occurred during a series of prospective hypobaric chamber studies. These studies had been specifically designed to investigate DCS. METHODS: This paper presents a prospective analysis of DCS symptoms from 447 subjects, recorded over an 11-yr period at the Armstrong Laboratory (AL), and is an attempt to provide an accurate representation of the initial presentation of altitude DCS. RESULTS: Of the 447 cases, 83.2% had musculoskeletal involvement, 2.7% had chokes, 2.2% skin manifestations, 10.8% paresthesia, and 0.5% frank neurological features. CONCLUSIONS: The most common presenting feature was musculoskeletal, with knee pain predominating (occurring in 70% of these cases). A very low incidence of neurological features was seen in the AL database, which was in contrast to data from many other sources. Reasons for this difference may include the use of preoxygenation and the policy of prompt recompression upon symptom development at AL. There is also the possibility that individuals in the training and operational environments are more likely to report frank neurological involvement than other forms of DCS.

Adult↗
PubMed1991

The physiology of spacecraft and space suit atmosphere selection.

The majority of the environmental factors which comprise the spacecraft and space suit environments can be controlled at "Earth normal" values, at optimum values, or at other values decided upon by spacecraft designers. Factors which are considered in arriving at control values and control ranges of these parameters include physiological, engineering, operational cost, and safety considerations. Several of the physiological considerations, including hypoxia and hyperoxia, hypercapnia, temperature regulation, and decompression sickness are identified and their impact on spacecraft and space suit atmosphere selection are considered. The past experience in controlling these parameters in U.S. and Soviet spacecraft and space suits and the associated physiological responses are reviewed. Current areas of physiological investigation relating to environmental factors in spacecraft are discussed, particularly decompression sickness which can occur as a result of change in pressure from Earth to spacecraft or spacecraft to space suit. Physiological considerations for long-term lunar or Martian missions will have different impacts on atmosphere selection and may result in the selection of atmospheres different than those currently in use.

Atmospheric Pressure↗
PubMed1994

Pyrolysis-gas chromatography/mass spectrometry analyses of biological particulates collected during recent space shuttle missions.

Biological particulates collected on air filters during shuttle missions (STS-40 and STS-42) were identified using pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS). A method was developed for identifying the atmospheric particles and their sources through the analysis of standard materials and the selection of "marker" compounds specific to the particle type. Pyrolysis spectra of biological standards were compared with those of airborne particles collected during two space shuttle missions; marker compounds present in the shuttle particle spectra were matched with those of the standards to identify the source of particles. Particles of 0,5--1-mm diameter and weighing as little as 40 micrograms could be identified using this technique. The Py-GC/MS method identified rat food and soilless plant-growth media as two sources of particles collected from the shuttle atmosphere during flight.

Air Pollutants↗
PubMed1996

Cardiovascular deconditioning and venous air embolism in simulated microgravity in the rat.

BACKGROUND: Astronauts conducting extravehicular activities undergo decompression to a lower ambient pressure, potentially resulting in gas bubble formation within the tissues and venous circulation. Additionally, exposure to microgravity produces fluid shifts within the body leading to cardiovascular deconditioning. A lower incidence of decompression illness in actual spaceflight compared with that in ground-based altitude chamber flights suggests that there is a possible interaction between microgravity exposure and decompression illness. HYPOTHESIS: The purpose of this study was to evaluate the cardiovascular and pulmonary effects of simulated hypobaric decompression stress using a tail suspension (head-down tilt) model of microgravity to produce the fluid shifts associated with weightlessness in conscious, chronically instrumented rats. METHODS: Venous bubble formation resulting from altitude decompression illness was simulated by a 3-h intravenous air infusion. Cardiovascular deconditioning was simulated by 96 h of head-down tilt. Heart rate, mean arterial blood pressure, central venous pressure, left ventricular wall thickening and cardiac output were continuously recorded. Lung studies were performed to evaluate edema formation and compliance measurement. Blood and pleural fluid were examined for changes in white cell counts and protein concentration. RESULTS: Our data demonstrated that in tail-suspended rats subjected to venous air infusions, there was a reduction in pulmonary edema formation and less of a decrease in cardiac output than occurred following venous air infusion alone. Mean arterial blood pressure and myocardial wall thickening fractions were unchanged with either tail-suspension or venous air infusion. Heart rate decreased in both conditions while systemic vascular resistance increased. CONCLUSIONS: These differences may be due in part to a change or redistribution of pulmonary blood flow or to a diminished cellular response to the microvascular insult of the venous air embolization.

Animals↗
PubMed1997

Modeling the effects of exercise during 100% oxygen prebreathe on the risk of hypobaric decompression sickness.

BACKGROUND: Several previous studies indicated that exercise during prebreathe with 100% O2 decreased the incidence of hypobaric decompression sickness (DCS). We report a meta-analysis of these investigations combined with a new study in our laboratory to develop a statistical model as a predictive tool for DCS. HYPOTHESIS: Exercise during prebreathe increases N2 elimination in a theoretical 360-min half-time compartment decreasing the incidence of DCS. METHODS: A dose-response probability tissue ratio (TR) model with 95% confidence limits was created for two groups, prebreathe with exercise (n = 113) and resting prebreathe (n = 113), using nonlinear regression analysis with maximum likelihood optimization. RESULTS: The model predicted that prebreathe exercise would reduce the residual N2 in a 360-min half-time compartment to a level analogous to that in a 180-min compartment. This finding supported the hypothesis. The incidence of DCS for the exercise prebreathe group was significantly decreased (Chi-Square = 17.1, p < 0.0001) from the resting prebreathe group. CONCLUSIONS: The results suggested that exercise during prebreathe increases tissue perfusion and N2 elimination approximately 2-fold and markedly lowers the risk of DCS. Based on the model, the prebreathe duration may be reduced from 240 min to a predicted 91 min for the protocol in our study, but this remains to be verified. The model provides a useful planning tool to develop and test appropriate prebreathe exercise protocols and to predict DCS risks for astronauts.

Clinical Protocols↗
PubMed1997

Utility of Doppler-detectable microbubbles in the diagnosis and treatment of decompression sickness.

BACKGROUND: Doppler-detectable microbubbles (DMB) are frequently used to evaluate altitude decompression stress. However, the role of DMB in the therapy of decompression sickness (DCS) has not been examined. HYPOTHESIS: The ability of Doppler to detect microbubbles during decompression (Doppler test) may be used in the diagnosis of DCS, and to aid clinical decisions about treatment options for DCS. METHODS: We examined the data on DMB and symptoms from NASA Database on DCS (n = 516). The accuracy of Doppler test was obtained from the Receiver Operating Characteristic (ROC) for DMB (grades I through IV), and efficacy was obtained by calculating predictive or post-test probabilities. Threshold analysis was used to obtain the probabilities for testing and/or treatment decisions. RESULTS: The Doppler test was useful for both screening and confirming DCS, when different criteria (grade I for screening; grade IV for confirming) were used for a positive test. Calculation of predictive values and threshold analysis showed that: 1) early recompression was the therapy of choice when post-test probability of disease was > 0.25 in individuals with non-specific pain at altitude, and early recompression with 100% oxygen for 2 h at site level was optimal therapy when this probability was > 0.33; 2) hyperbaric therapy was optimal when post-test probability was > 0.04 in individuals with uncertain symptoms post-flight. CONCLUSIONS: The Doppler test was of greater utility in excluding DCS than confirming its presence, and was useful in making therapeutic decisions on DCS when confronted with non-specific symptoms at altitude.

Aerospace Medicine↗
PubMed1992

Space habitat contaminant growth models.

This paper outlines the need for and role of Contaminant Growth Models (CGM) in designing space habitats and space mission operations. The tremendous complexity of the contamination problem and the current lack of data suggests that a "layered" approach to CGM development and utilization be made in conjunction with medical/toxicological research and risk assessments. Two space habitat contaminant examples: thermodegradation and hydrazine, are utilized to show how such a model might be useful in analyzing the environment of a space habitat. The examples also depict how NASA's interdisciplinary Center for Space Environmental Health (CSEH) will combine medical/toxicological research results with engineering environmental system design tools to set standards and help in the design, prediction, monitoring and safe control of space habitats. Since the CSEH project is just beginning, the contents of the paper are descriptive of the goal and the approach being taken, rather than conclusive in nature.

Ammonia↗
PubMed1992

Polymer degradation and ultrafine particles: potential inhalation hazards for astronauts.

When Teflon is heated the developing fumes produce in exposed human an influenza-like syndrome (polymer fume fever) or also severe toxic effects like pulmonary edema, pneumonitis and death. The decomposition products and the resulting health effects are temperature-dependent. The toxic effects seem to be related to the ultrafine particulate fraction of the fume. To test the hypothesis that exposure to ultrafine particles results in an increased interstitialization of the particles which is accompanied by an acute pathological inflammation, rats were exposed to titanium dioxide (TiO2) particles by intratracheal instillation and by inhalation. Both acute intratracheal instillation and subchronic inhalation studies on rats show that ultrafine TiO2 particles (approximately 20 nm diameter) access the pulmonary interstitium to a larger extent than fine particles (approximately 250 nm diameter) and that they elicit an inflammatory response as indicated by PMN increase in lavaged cells. The release of ultrafine particles into the air of an enclosed environment from a thermodegradation event or from other sources is a potential hazard for astronauts. Knowing the mechanisms of action is a prerequisite for technical or medical countermeasures.

Administration, Inhalation↗
PubMed1990

Activity and adaptation of nitrilotriacetate (NTA)-degrading bacteria: field and laboratory studies.

Adaptation of bacterial activity for the degradation of nitrilotriacetate (NTA) was studied using natural sediment samples and an NTA-degrading bacterium (strain ATCC 29600). Sediment samples from a river with persistent levels of NTA had much higher NTA-degradative activity than comparable samples from a less contaminated control site. When sediment from the control site was exposed to high levels of NTA a 5 day lag preceded an abrupt increase in NTA degradation while strain 29600 colonized on sand and grown in the absence of NTA became induced within eight hours. The induction of strain 29600 was compared between bacteria in suspension and cells attached to sand. The sand-associated bacteria became induced 4 to 5 h before the planktonic suspension and displayed over threefold greater specific activity. Suspensions of strain 29600 became adapted within 8 h when placed in membrane diffusion chambers that were immersed within a municipal wastewater reactor containing NTA. These findings support the concept that induction is a part of the process of bacterial adaptation to degrade NTA and sand-associated bacteria can adapt more quickly to and have a greater degradative activity for NTA than planktonic cells.

Adaptation, Biological↗
PubMed1992

Toxicological implications of extended space flights.

The dominant reason for exposing humans to the risks of space flight is their ability to perform complex tasks and make complex decisions. To fulfill such a role, crews must be shielded against even incipient degradation of performance capacity. The space environment contains potential hazards ranging from microgravity to infectious microorganisms to chemical toxicants. An extensive literature indicates that incipient disruptions of function may occur at low levels of exposure to toxic agents and degrade performance. Such questions need to be pursued before irreversible decisions are made about space vehicle design.

Air Pollutants, Occupational↗
PubMed1989

Rapid enumeration of viable bacteria by image analysis.

A direct viable counting method for enumerating viable bacteria was modified and made compatible with image analysis. A comparison was made between viable cell counts determined by the spread plate method and direct viable counts obtained using epifluorescence microscopy either manually or by automatic image analysis. Cultures of Escherichia coli, Salmonella typhimurium, Vibrio cholerae, Yersinia enterocolitica and Pseudomonas aeruginosa were incubated at 35 degrees C in a dilute nutrient medium containing nalidixic acid. Filtered samples were stained for epifluorescence microscopy and analysed manually as well as by image analysis. Cells enlarged after incubation were considered viable. The viable cell counts determined using image analysis were higher than those obtained by either the direct manual count of viable cells or spread plate methods. The volume of sample filtered or the number of cells in the original sample did not influence the efficiency of the method. However, the optimal concentration of nalidixic acid (2.5-20 micrograms ml-1) and length of incubation (4-8 h) varied with the culture tested. The results of this study showed that under optimal conditions, the modification of the direct viable count method in combination with image analysis microscopy provided an efficient and quantitative technique for counting viable bacteria in a short time.

Colony Count, Microbial↗