CABIN ATMOSPHERES9In the first U.S. manned space flight program, Project Mercury, and in the face of very severe weight limitations, a cabin atmosphere of pure oxygen at one-third atmospheric pressure was adopted. This choice probably represented the greatest simplification which could be achieved readily and, at the same time, provide protection against some of the risks of rapid decompression. Although breathing pure oxygen at higher pressures was known to be attended by some undesirable physiological effects, the short duration of the flights to be undertaken, and the low pressure employed, suggested that no harmful results would result in this case. That these expectations were generally borne out is now history. Preparations for space flights of longer duration—many weeks or months—present similar problems and require special attention to phenomena which may be either undetectable or of trivial significance on a time scale of a few days.Physiological Criteria in the Choice of Cabin AtmosphereIf maintenance of normal respiratory function were the only consideration, a cabin atmosphere of about sea-level composition and pressure might be an ideal and straightforward choice for manned spacecraft. In fact, this atmosphere has been used in the manned space flights conducted by the U.S.S.R. No other atmosphere has been shown to be more satisfactory from the physiological point of view, and the tedious respiratory studies which should accompany the use of other atmospheres can be avoided. Nevertheless, the formidable problems of spacecraft design and the necessary precautions for safeguarding the crew from accident require that other atmospheric compositions and pressures be considered. For example, if a cabin at 1-atm pressure were decompressed to space suit pressure (0.3 atm), the occupants would develop decompression sickness; i.e., "bends."Several engineering considerations argue for low cabin pressures and pure oxygen composition. Among these are structural design, weight of atmospheric gas storage and control equipment, and the difficulty of contriving pressure suits which allow operation at pressures near one atmosphere. Such departures from the normal human gaseous environment, however, require the demonstration of an acceptable level of safety and physiological performance.The limits of the composition and pressure of acceptable cabin atmospheres are then set by—A pure oxygen atmosphere at a pressure which will provide an alveolar oxygen partial pressure equal to that provided by air at sea levelA mixed gas (oxygen and inert gas) atmosphere having a pressure and composition that will allow decompression to the highest acceptable suit pressure without the risk of bendsA numerical value for the lower limit (1) is approximately 0.2 atm of pure oxygen. The upper limit (2) is determined by the operating pressure and composition of the space-suit atmosphere and may be of the order of 0.5 atm for a cabin atmosphere of 50 percent oxygen. It is necessary to determine the astronaut's ability to survive and perform his duties in any atmosphere selected.Atelectasis and Pulmonary EdemaLocalized or diffuse collapse of alveoli in the lungs may, if the condition persists, lead to arterial hypoxia which may be extremely undesirable under the stresses of space flight. The alveoli are probably unstable when pure oxygen is breathed; they tend to collapse if there is blockage of the airways, especially at low pressures. This collapse occurs becauseeach of the gases present in the alveoli (oxygen, water vapor, and carbon dioxide) is subject to prompt and complete absorption from the alveoli by the blood.The alveoli are normally stabilized against collapse by the presence of inert and relatively insoluble gas (nitrogen) and an internal coating of lipoprotein substances with low surface tension.Theoretical and experimental results strongly suggest the desirability of using oxygen-inert gas atmospheres for long missions to avoid atelectasis and other gas absorption phenomena, such as retraction of the eardrum. However, further experimental evidence is required both to confirm this point and to establish its upper limit of suitability of pure oxygen atmospheres.At Ohio State University in 1962, scientists studied the effect on young rats exposed for 27 days to 100 percent oxygen (with no nitrogen), at a reduced barometric pressure equivalent to 33 000 feet altitude. The rats showed no difference in growth rate, oxygen consumption, food and water intake, or behavior from control rats in air at 1 atm.Oxygen ToxicityIt has long been known that breathing pure oxygen at normal atmospheric pressure often produces pulmonary irritation and other toxic effects both in man and animals. This knowledge has occasioned concern over the use of pure oxygen atmospheres in spacecraft.The effect of 100 percent oxygen at a simulated altitude of 26 000 feet for 6 weeks was studied using white rats at Oklahoma City University under a NASA grant. Radioactive carbon techniques revealed a 15-percent reduction of metabolism in the 100-percent oxygen-exposed rats, compared with rats in air at 1 atmosphere. There was a 20-percent decrease in lipid metabolism in the liver compared with controls, but no decrease in heart metabolism. There was no gross change in body weight.The White Leghorn chick between 2 and 7 weeks old is markedly resistant to the toxic effects of 1 atm of O2. Continuous exposure (Ohio State University) for as long as 4 weeks did not cause deaths, obvious morbidity, or any signs of pulmonary damage on gross autopsy. Nevertheless, the hyperoxia had some adverse effects, primarily reducing the growth rate to between three-fourths to one-fourth of normal; reducing feed intake per unit body weight to three-fourths of normal; slowing respiratory rate by 30 percent; decreasing erythrocytes, hemoglobin, and hematocrit by 9 to 12 percent; and causing reversible histological changes in the lungs. Arterial O2tensions were elevated over 300-mm Hg, but arterial pCO2and blood pH were unaffected. No residual effects werenoted upon return to air breathing. It is possible that the anatomical peculiarities of the avian lung play some role in the chicks' resistance to hyperoxia, but it is also possible that this resistance is a function of age, similar to the tolerance shown by the young rat but not the adult.Carbon Dioxide ToleranceStudies of CO2tolerance in submarine crews indicate that no loss of performance is involved if the concentration in air at normal pressure does not exceed 1.5 percent with exposures of 30 to 40 days. However, biochemical adaptive changes were observed at this concentration.Inert-Gas ComponentsIf other investigations establish the need for an inert gas in manned spacecraft atmospheres, gases other than nitrogen may be considered. Compared with nitrogen, the physical properties or helium and neon offer advantages with respect to solubility in body fluids, storage weight, and thermal properties.Studies at Ohio State University in 1964, under a NASA grant, showed that helium substituted for nitrogen in a closed container causes humans to feel "cold" at a normally comfortable temperature. Studies with animals have shown that in a helium atmosphere there is greater heat loss due to the increased conducting capacity and probably greater evaporative capacity. In 6 days at 21 percent oxygen and 79 percent helium at 1-atmosphere pressure, young rats grew at the same rate as controls, but drank more water, excreted more urine, and had a higher rate of food and oxygen consumption than controls in air at 1 atmosphere. Men are being tested on a bicycle ergometer in saturated and low relative humidity helium atmospheres to study heat balance.Mice were exposed to 80 percent argon and 20 percent oxygen continuously at 1-atmosphere pressure for 35 days at Oklahoma City University. Carbon 14 studies of metabolism showed a slight slowing and a twofold to threefold increase in fat deposition.BendsDecompression, whether accidental (due to damage of the spacecraft) or intentional (as in the use of the pressure suit outside the capsule), carries the risk of bends if the inert gases dissolved in the tissues and body fluids come out of solution. The magnitude of this risk is determined to a very considerable extent by—Individual susceptibilityThe extent to which the nitrogen (or other inert gas) concentrations of tissues and body fluids have been reducedThe magnitude and rate of the inert-gas, partial pressure change on decompressionThe probability of getting bends is reduced by—Selection of bends-resistant individualsThorough denitrogenation before flightLimitation of decompressive pressure changes by appropriate choice of cabin atmosphere pressure and compositionSpace-suit pressure settingIn some cases, further improvements might be obtained by using, in the cabin atmosphere, an inert-gas component which has a lower solubility in tissue and body fluids or less tendency than nitrogen to form bubbles.Fire HazardExperience indicates that fires in pure oxygen atmospheres, even at low pressures (e.g.,1/3atm), are extremely difficult to extinguish. While this phenomenon has nothing to do with respiratory physiology, the risk on flights of long duration may be so serious as to demand special measures. Unless effective countermeasures can be devised, this risk may argue very strongly against the use of such atmospheres in the future. Further experimental investigation is required.Acceleration Effects on the Lungs and Pulmonary CirculationForces produced by high acceleration overdistend one part and compress another part of the lungs. Blood flow diminishes in some parts of the lungs and increases in others. Fluid leaks from the blood into the tissues and into the air sacs in parts of the lungs. These effects cause difficulty in breathing, low arterial oxygen saturation, and impaired consciousness during high sustained acceleration and, to a lesser extent, after its cessation. They must be considered when selecting the best gas to be breathed, since a high partial pressure of oxygen is favorable for consciousness, but a low inert-gas concentration during acceleration is unfavorable for rapid lung recovery afterward.
CABIN ATMOSPHERES9In the first U.S. manned space flight program, Project Mercury, and in the face of very severe weight limitations, a cabin atmosphere of pure oxygen at one-third atmospheric pressure was adopted. This choice probably represented the greatest simplification which could be achieved readily and, at the same time, provide protection against some of the risks of rapid decompression. Although breathing pure oxygen at higher pressures was known to be attended by some undesirable physiological effects, the short duration of the flights to be undertaken, and the low pressure employed, suggested that no harmful results would result in this case. That these expectations were generally borne out is now history. Preparations for space flights of longer duration—many weeks or months—present similar problems and require special attention to phenomena which may be either undetectable or of trivial significance on a time scale of a few days.Physiological Criteria in the Choice of Cabin AtmosphereIf maintenance of normal respiratory function were the only consideration, a cabin atmosphere of about sea-level composition and pressure might be an ideal and straightforward choice for manned spacecraft. In fact, this atmosphere has been used in the manned space flights conducted by the U.S.S.R. No other atmosphere has been shown to be more satisfactory from the physiological point of view, and the tedious respiratory studies which should accompany the use of other atmospheres can be avoided. Nevertheless, the formidable problems of spacecraft design and the necessary precautions for safeguarding the crew from accident require that other atmospheric compositions and pressures be considered. For example, if a cabin at 1-atm pressure were decompressed to space suit pressure (0.3 atm), the occupants would develop decompression sickness; i.e., "bends."Several engineering considerations argue for low cabin pressures and pure oxygen composition. Among these are structural design, weight of atmospheric gas storage and control equipment, and the difficulty of contriving pressure suits which allow operation at pressures near one atmosphere. Such departures from the normal human gaseous environment, however, require the demonstration of an acceptable level of safety and physiological performance.The limits of the composition and pressure of acceptable cabin atmospheres are then set by—A pure oxygen atmosphere at a pressure which will provide an alveolar oxygen partial pressure equal to that provided by air at sea levelA mixed gas (oxygen and inert gas) atmosphere having a pressure and composition that will allow decompression to the highest acceptable suit pressure without the risk of bendsA numerical value for the lower limit (1) is approximately 0.2 atm of pure oxygen. The upper limit (2) is determined by the operating pressure and composition of the space-suit atmosphere and may be of the order of 0.5 atm for a cabin atmosphere of 50 percent oxygen. It is necessary to determine the astronaut's ability to survive and perform his duties in any atmosphere selected.Atelectasis and Pulmonary EdemaLocalized or diffuse collapse of alveoli in the lungs may, if the condition persists, lead to arterial hypoxia which may be extremely undesirable under the stresses of space flight. The alveoli are probably unstable when pure oxygen is breathed; they tend to collapse if there is blockage of the airways, especially at low pressures. This collapse occurs becauseeach of the gases present in the alveoli (oxygen, water vapor, and carbon dioxide) is subject to prompt and complete absorption from the alveoli by the blood.The alveoli are normally stabilized against collapse by the presence of inert and relatively insoluble gas (nitrogen) and an internal coating of lipoprotein substances with low surface tension.Theoretical and experimental results strongly suggest the desirability of using oxygen-inert gas atmospheres for long missions to avoid atelectasis and other gas absorption phenomena, such as retraction of the eardrum. However, further experimental evidence is required both to confirm this point and to establish its upper limit of suitability of pure oxygen atmospheres.At Ohio State University in 1962, scientists studied the effect on young rats exposed for 27 days to 100 percent oxygen (with no nitrogen), at a reduced barometric pressure equivalent to 33 000 feet altitude. The rats showed no difference in growth rate, oxygen consumption, food and water intake, or behavior from control rats in air at 1 atm.Oxygen ToxicityIt has long been known that breathing pure oxygen at normal atmospheric pressure often produces pulmonary irritation and other toxic effects both in man and animals. This knowledge has occasioned concern over the use of pure oxygen atmospheres in spacecraft.The effect of 100 percent oxygen at a simulated altitude of 26 000 feet for 6 weeks was studied using white rats at Oklahoma City University under a NASA grant. Radioactive carbon techniques revealed a 15-percent reduction of metabolism in the 100-percent oxygen-exposed rats, compared with rats in air at 1 atmosphere. There was a 20-percent decrease in lipid metabolism in the liver compared with controls, but no decrease in heart metabolism. There was no gross change in body weight.The White Leghorn chick between 2 and 7 weeks old is markedly resistant to the toxic effects of 1 atm of O2. Continuous exposure (Ohio State University) for as long as 4 weeks did not cause deaths, obvious morbidity, or any signs of pulmonary damage on gross autopsy. Nevertheless, the hyperoxia had some adverse effects, primarily reducing the growth rate to between three-fourths to one-fourth of normal; reducing feed intake per unit body weight to three-fourths of normal; slowing respiratory rate by 30 percent; decreasing erythrocytes, hemoglobin, and hematocrit by 9 to 12 percent; and causing reversible histological changes in the lungs. Arterial O2tensions were elevated over 300-mm Hg, but arterial pCO2and blood pH were unaffected. No residual effects werenoted upon return to air breathing. It is possible that the anatomical peculiarities of the avian lung play some role in the chicks' resistance to hyperoxia, but it is also possible that this resistance is a function of age, similar to the tolerance shown by the young rat but not the adult.Carbon Dioxide ToleranceStudies of CO2tolerance in submarine crews indicate that no loss of performance is involved if the concentration in air at normal pressure does not exceed 1.5 percent with exposures of 30 to 40 days. However, biochemical adaptive changes were observed at this concentration.Inert-Gas ComponentsIf other investigations establish the need for an inert gas in manned spacecraft atmospheres, gases other than nitrogen may be considered. Compared with nitrogen, the physical properties or helium and neon offer advantages with respect to solubility in body fluids, storage weight, and thermal properties.Studies at Ohio State University in 1964, under a NASA grant, showed that helium substituted for nitrogen in a closed container causes humans to feel "cold" at a normally comfortable temperature. Studies with animals have shown that in a helium atmosphere there is greater heat loss due to the increased conducting capacity and probably greater evaporative capacity. In 6 days at 21 percent oxygen and 79 percent helium at 1-atmosphere pressure, young rats grew at the same rate as controls, but drank more water, excreted more urine, and had a higher rate of food and oxygen consumption than controls in air at 1 atmosphere. Men are being tested on a bicycle ergometer in saturated and low relative humidity helium atmospheres to study heat balance.Mice were exposed to 80 percent argon and 20 percent oxygen continuously at 1-atmosphere pressure for 35 days at Oklahoma City University. Carbon 14 studies of metabolism showed a slight slowing and a twofold to threefold increase in fat deposition.BendsDecompression, whether accidental (due to damage of the spacecraft) or intentional (as in the use of the pressure suit outside the capsule), carries the risk of bends if the inert gases dissolved in the tissues and body fluids come out of solution. The magnitude of this risk is determined to a very considerable extent by—Individual susceptibilityThe extent to which the nitrogen (or other inert gas) concentrations of tissues and body fluids have been reducedThe magnitude and rate of the inert-gas, partial pressure change on decompressionThe probability of getting bends is reduced by—Selection of bends-resistant individualsThorough denitrogenation before flightLimitation of decompressive pressure changes by appropriate choice of cabin atmosphere pressure and compositionSpace-suit pressure settingIn some cases, further improvements might be obtained by using, in the cabin atmosphere, an inert-gas component which has a lower solubility in tissue and body fluids or less tendency than nitrogen to form bubbles.Fire HazardExperience indicates that fires in pure oxygen atmospheres, even at low pressures (e.g.,1/3atm), are extremely difficult to extinguish. While this phenomenon has nothing to do with respiratory physiology, the risk on flights of long duration may be so serious as to demand special measures. Unless effective countermeasures can be devised, this risk may argue very strongly against the use of such atmospheres in the future. Further experimental investigation is required.Acceleration Effects on the Lungs and Pulmonary CirculationForces produced by high acceleration overdistend one part and compress another part of the lungs. Blood flow diminishes in some parts of the lungs and increases in others. Fluid leaks from the blood into the tissues and into the air sacs in parts of the lungs. These effects cause difficulty in breathing, low arterial oxygen saturation, and impaired consciousness during high sustained acceleration and, to a lesser extent, after its cessation. They must be considered when selecting the best gas to be breathed, since a high partial pressure of oxygen is favorable for consciousness, but a low inert-gas concentration during acceleration is unfavorable for rapid lung recovery afterward.
CABIN ATMOSPHERES9In the first U.S. manned space flight program, Project Mercury, and in the face of very severe weight limitations, a cabin atmosphere of pure oxygen at one-third atmospheric pressure was adopted. This choice probably represented the greatest simplification which could be achieved readily and, at the same time, provide protection against some of the risks of rapid decompression. Although breathing pure oxygen at higher pressures was known to be attended by some undesirable physiological effects, the short duration of the flights to be undertaken, and the low pressure employed, suggested that no harmful results would result in this case. That these expectations were generally borne out is now history. Preparations for space flights of longer duration—many weeks or months—present similar problems and require special attention to phenomena which may be either undetectable or of trivial significance on a time scale of a few days.Physiological Criteria in the Choice of Cabin AtmosphereIf maintenance of normal respiratory function were the only consideration, a cabin atmosphere of about sea-level composition and pressure might be an ideal and straightforward choice for manned spacecraft. In fact, this atmosphere has been used in the manned space flights conducted by the U.S.S.R. No other atmosphere has been shown to be more satisfactory from the physiological point of view, and the tedious respiratory studies which should accompany the use of other atmospheres can be avoided. Nevertheless, the formidable problems of spacecraft design and the necessary precautions for safeguarding the crew from accident require that other atmospheric compositions and pressures be considered. For example, if a cabin at 1-atm pressure were decompressed to space suit pressure (0.3 atm), the occupants would develop decompression sickness; i.e., "bends."Several engineering considerations argue for low cabin pressures and pure oxygen composition. Among these are structural design, weight of atmospheric gas storage and control equipment, and the difficulty of contriving pressure suits which allow operation at pressures near one atmosphere. Such departures from the normal human gaseous environment, however, require the demonstration of an acceptable level of safety and physiological performance.The limits of the composition and pressure of acceptable cabin atmospheres are then set by—A pure oxygen atmosphere at a pressure which will provide an alveolar oxygen partial pressure equal to that provided by air at sea levelA mixed gas (oxygen and inert gas) atmosphere having a pressure and composition that will allow decompression to the highest acceptable suit pressure without the risk of bendsA numerical value for the lower limit (1) is approximately 0.2 atm of pure oxygen. The upper limit (2) is determined by the operating pressure and composition of the space-suit atmosphere and may be of the order of 0.5 atm for a cabin atmosphere of 50 percent oxygen. It is necessary to determine the astronaut's ability to survive and perform his duties in any atmosphere selected.Atelectasis and Pulmonary EdemaLocalized or diffuse collapse of alveoli in the lungs may, if the condition persists, lead to arterial hypoxia which may be extremely undesirable under the stresses of space flight. The alveoli are probably unstable when pure oxygen is breathed; they tend to collapse if there is blockage of the airways, especially at low pressures. This collapse occurs becauseeach of the gases present in the alveoli (oxygen, water vapor, and carbon dioxide) is subject to prompt and complete absorption from the alveoli by the blood.The alveoli are normally stabilized against collapse by the presence of inert and relatively insoluble gas (nitrogen) and an internal coating of lipoprotein substances with low surface tension.Theoretical and experimental results strongly suggest the desirability of using oxygen-inert gas atmospheres for long missions to avoid atelectasis and other gas absorption phenomena, such as retraction of the eardrum. However, further experimental evidence is required both to confirm this point and to establish its upper limit of suitability of pure oxygen atmospheres.At Ohio State University in 1962, scientists studied the effect on young rats exposed for 27 days to 100 percent oxygen (with no nitrogen), at a reduced barometric pressure equivalent to 33 000 feet altitude. The rats showed no difference in growth rate, oxygen consumption, food and water intake, or behavior from control rats in air at 1 atm.Oxygen ToxicityIt has long been known that breathing pure oxygen at normal atmospheric pressure often produces pulmonary irritation and other toxic effects both in man and animals. This knowledge has occasioned concern over the use of pure oxygen atmospheres in spacecraft.The effect of 100 percent oxygen at a simulated altitude of 26 000 feet for 6 weeks was studied using white rats at Oklahoma City University under a NASA grant. Radioactive carbon techniques revealed a 15-percent reduction of metabolism in the 100-percent oxygen-exposed rats, compared with rats in air at 1 atmosphere. There was a 20-percent decrease in lipid metabolism in the liver compared with controls, but no decrease in heart metabolism. There was no gross change in body weight.The White Leghorn chick between 2 and 7 weeks old is markedly resistant to the toxic effects of 1 atm of O2. Continuous exposure (Ohio State University) for as long as 4 weeks did not cause deaths, obvious morbidity, or any signs of pulmonary damage on gross autopsy. Nevertheless, the hyperoxia had some adverse effects, primarily reducing the growth rate to between three-fourths to one-fourth of normal; reducing feed intake per unit body weight to three-fourths of normal; slowing respiratory rate by 30 percent; decreasing erythrocytes, hemoglobin, and hematocrit by 9 to 12 percent; and causing reversible histological changes in the lungs. Arterial O2tensions were elevated over 300-mm Hg, but arterial pCO2and blood pH were unaffected. No residual effects werenoted upon return to air breathing. It is possible that the anatomical peculiarities of the avian lung play some role in the chicks' resistance to hyperoxia, but it is also possible that this resistance is a function of age, similar to the tolerance shown by the young rat but not the adult.Carbon Dioxide ToleranceStudies of CO2tolerance in submarine crews indicate that no loss of performance is involved if the concentration in air at normal pressure does not exceed 1.5 percent with exposures of 30 to 40 days. However, biochemical adaptive changes were observed at this concentration.Inert-Gas ComponentsIf other investigations establish the need for an inert gas in manned spacecraft atmospheres, gases other than nitrogen may be considered. Compared with nitrogen, the physical properties or helium and neon offer advantages with respect to solubility in body fluids, storage weight, and thermal properties.Studies at Ohio State University in 1964, under a NASA grant, showed that helium substituted for nitrogen in a closed container causes humans to feel "cold" at a normally comfortable temperature. Studies with animals have shown that in a helium atmosphere there is greater heat loss due to the increased conducting capacity and probably greater evaporative capacity. In 6 days at 21 percent oxygen and 79 percent helium at 1-atmosphere pressure, young rats grew at the same rate as controls, but drank more water, excreted more urine, and had a higher rate of food and oxygen consumption than controls in air at 1 atmosphere. Men are being tested on a bicycle ergometer in saturated and low relative humidity helium atmospheres to study heat balance.Mice were exposed to 80 percent argon and 20 percent oxygen continuously at 1-atmosphere pressure for 35 days at Oklahoma City University. Carbon 14 studies of metabolism showed a slight slowing and a twofold to threefold increase in fat deposition.BendsDecompression, whether accidental (due to damage of the spacecraft) or intentional (as in the use of the pressure suit outside the capsule), carries the risk of bends if the inert gases dissolved in the tissues and body fluids come out of solution. The magnitude of this risk is determined to a very considerable extent by—Individual susceptibilityThe extent to which the nitrogen (or other inert gas) concentrations of tissues and body fluids have been reducedThe magnitude and rate of the inert-gas, partial pressure change on decompressionThe probability of getting bends is reduced by—Selection of bends-resistant individualsThorough denitrogenation before flightLimitation of decompressive pressure changes by appropriate choice of cabin atmosphere pressure and compositionSpace-suit pressure settingIn some cases, further improvements might be obtained by using, in the cabin atmosphere, an inert-gas component which has a lower solubility in tissue and body fluids or less tendency than nitrogen to form bubbles.Fire HazardExperience indicates that fires in pure oxygen atmospheres, even at low pressures (e.g.,1/3atm), are extremely difficult to extinguish. While this phenomenon has nothing to do with respiratory physiology, the risk on flights of long duration may be so serious as to demand special measures. Unless effective countermeasures can be devised, this risk may argue very strongly against the use of such atmospheres in the future. Further experimental investigation is required.Acceleration Effects on the Lungs and Pulmonary CirculationForces produced by high acceleration overdistend one part and compress another part of the lungs. Blood flow diminishes in some parts of the lungs and increases in others. Fluid leaks from the blood into the tissues and into the air sacs in parts of the lungs. These effects cause difficulty in breathing, low arterial oxygen saturation, and impaired consciousness during high sustained acceleration and, to a lesser extent, after its cessation. They must be considered when selecting the best gas to be breathed, since a high partial pressure of oxygen is favorable for consciousness, but a low inert-gas concentration during acceleration is unfavorable for rapid lung recovery afterward.
In the first U.S. manned space flight program, Project Mercury, and in the face of very severe weight limitations, a cabin atmosphere of pure oxygen at one-third atmospheric pressure was adopted. This choice probably represented the greatest simplification which could be achieved readily and, at the same time, provide protection against some of the risks of rapid decompression. Although breathing pure oxygen at higher pressures was known to be attended by some undesirable physiological effects, the short duration of the flights to be undertaken, and the low pressure employed, suggested that no harmful results would result in this case. That these expectations were generally borne out is now history. Preparations for space flights of longer duration—many weeks or months—present similar problems and require special attention to phenomena which may be either undetectable or of trivial significance on a time scale of a few days.
Physiological Criteria in the Choice of Cabin AtmosphereIf maintenance of normal respiratory function were the only consideration, a cabin atmosphere of about sea-level composition and pressure might be an ideal and straightforward choice for manned spacecraft. In fact, this atmosphere has been used in the manned space flights conducted by the U.S.S.R. No other atmosphere has been shown to be more satisfactory from the physiological point of view, and the tedious respiratory studies which should accompany the use of other atmospheres can be avoided. Nevertheless, the formidable problems of spacecraft design and the necessary precautions for safeguarding the crew from accident require that other atmospheric compositions and pressures be considered. For example, if a cabin at 1-atm pressure were decompressed to space suit pressure (0.3 atm), the occupants would develop decompression sickness; i.e., "bends."Several engineering considerations argue for low cabin pressures and pure oxygen composition. Among these are structural design, weight of atmospheric gas storage and control equipment, and the difficulty of contriving pressure suits which allow operation at pressures near one atmosphere. Such departures from the normal human gaseous environment, however, require the demonstration of an acceptable level of safety and physiological performance.The limits of the composition and pressure of acceptable cabin atmospheres are then set by—A pure oxygen atmosphere at a pressure which will provide an alveolar oxygen partial pressure equal to that provided by air at sea levelA mixed gas (oxygen and inert gas) atmosphere having a pressure and composition that will allow decompression to the highest acceptable suit pressure without the risk of bendsA numerical value for the lower limit (1) is approximately 0.2 atm of pure oxygen. The upper limit (2) is determined by the operating pressure and composition of the space-suit atmosphere and may be of the order of 0.5 atm for a cabin atmosphere of 50 percent oxygen. It is necessary to determine the astronaut's ability to survive and perform his duties in any atmosphere selected.
If maintenance of normal respiratory function were the only consideration, a cabin atmosphere of about sea-level composition and pressure might be an ideal and straightforward choice for manned spacecraft. In fact, this atmosphere has been used in the manned space flights conducted by the U.S.S.R. No other atmosphere has been shown to be more satisfactory from the physiological point of view, and the tedious respiratory studies which should accompany the use of other atmospheres can be avoided. Nevertheless, the formidable problems of spacecraft design and the necessary precautions for safeguarding the crew from accident require that other atmospheric compositions and pressures be considered. For example, if a cabin at 1-atm pressure were decompressed to space suit pressure (0.3 atm), the occupants would develop decompression sickness; i.e., "bends."
Several engineering considerations argue for low cabin pressures and pure oxygen composition. Among these are structural design, weight of atmospheric gas storage and control equipment, and the difficulty of contriving pressure suits which allow operation at pressures near one atmosphere. Such departures from the normal human gaseous environment, however, require the demonstration of an acceptable level of safety and physiological performance.
The limits of the composition and pressure of acceptable cabin atmospheres are then set by—
A pure oxygen atmosphere at a pressure which will provide an alveolar oxygen partial pressure equal to that provided by air at sea level
A mixed gas (oxygen and inert gas) atmosphere having a pressure and composition that will allow decompression to the highest acceptable suit pressure without the risk of bends
A numerical value for the lower limit (1) is approximately 0.2 atm of pure oxygen. The upper limit (2) is determined by the operating pressure and composition of the space-suit atmosphere and may be of the order of 0.5 atm for a cabin atmosphere of 50 percent oxygen. It is necessary to determine the astronaut's ability to survive and perform his duties in any atmosphere selected.
Atelectasis and Pulmonary EdemaLocalized or diffuse collapse of alveoli in the lungs may, if the condition persists, lead to arterial hypoxia which may be extremely undesirable under the stresses of space flight. The alveoli are probably unstable when pure oxygen is breathed; they tend to collapse if there is blockage of the airways, especially at low pressures. This collapse occurs becauseeach of the gases present in the alveoli (oxygen, water vapor, and carbon dioxide) is subject to prompt and complete absorption from the alveoli by the blood.The alveoli are normally stabilized against collapse by the presence of inert and relatively insoluble gas (nitrogen) and an internal coating of lipoprotein substances with low surface tension.Theoretical and experimental results strongly suggest the desirability of using oxygen-inert gas atmospheres for long missions to avoid atelectasis and other gas absorption phenomena, such as retraction of the eardrum. However, further experimental evidence is required both to confirm this point and to establish its upper limit of suitability of pure oxygen atmospheres.At Ohio State University in 1962, scientists studied the effect on young rats exposed for 27 days to 100 percent oxygen (with no nitrogen), at a reduced barometric pressure equivalent to 33 000 feet altitude. The rats showed no difference in growth rate, oxygen consumption, food and water intake, or behavior from control rats in air at 1 atm.
Localized or diffuse collapse of alveoli in the lungs may, if the condition persists, lead to arterial hypoxia which may be extremely undesirable under the stresses of space flight. The alveoli are probably unstable when pure oxygen is breathed; they tend to collapse if there is blockage of the airways, especially at low pressures. This collapse occurs becauseeach of the gases present in the alveoli (oxygen, water vapor, and carbon dioxide) is subject to prompt and complete absorption from the alveoli by the blood.
The alveoli are normally stabilized against collapse by the presence of inert and relatively insoluble gas (nitrogen) and an internal coating of lipoprotein substances with low surface tension.
Theoretical and experimental results strongly suggest the desirability of using oxygen-inert gas atmospheres for long missions to avoid atelectasis and other gas absorption phenomena, such as retraction of the eardrum. However, further experimental evidence is required both to confirm this point and to establish its upper limit of suitability of pure oxygen atmospheres.
At Ohio State University in 1962, scientists studied the effect on young rats exposed for 27 days to 100 percent oxygen (with no nitrogen), at a reduced barometric pressure equivalent to 33 000 feet altitude. The rats showed no difference in growth rate, oxygen consumption, food and water intake, or behavior from control rats in air at 1 atm.
Oxygen ToxicityIt has long been known that breathing pure oxygen at normal atmospheric pressure often produces pulmonary irritation and other toxic effects both in man and animals. This knowledge has occasioned concern over the use of pure oxygen atmospheres in spacecraft.The effect of 100 percent oxygen at a simulated altitude of 26 000 feet for 6 weeks was studied using white rats at Oklahoma City University under a NASA grant. Radioactive carbon techniques revealed a 15-percent reduction of metabolism in the 100-percent oxygen-exposed rats, compared with rats in air at 1 atmosphere. There was a 20-percent decrease in lipid metabolism in the liver compared with controls, but no decrease in heart metabolism. There was no gross change in body weight.The White Leghorn chick between 2 and 7 weeks old is markedly resistant to the toxic effects of 1 atm of O2. Continuous exposure (Ohio State University) for as long as 4 weeks did not cause deaths, obvious morbidity, or any signs of pulmonary damage on gross autopsy. Nevertheless, the hyperoxia had some adverse effects, primarily reducing the growth rate to between three-fourths to one-fourth of normal; reducing feed intake per unit body weight to three-fourths of normal; slowing respiratory rate by 30 percent; decreasing erythrocytes, hemoglobin, and hematocrit by 9 to 12 percent; and causing reversible histological changes in the lungs. Arterial O2tensions were elevated over 300-mm Hg, but arterial pCO2and blood pH were unaffected. No residual effects werenoted upon return to air breathing. It is possible that the anatomical peculiarities of the avian lung play some role in the chicks' resistance to hyperoxia, but it is also possible that this resistance is a function of age, similar to the tolerance shown by the young rat but not the adult.
It has long been known that breathing pure oxygen at normal atmospheric pressure often produces pulmonary irritation and other toxic effects both in man and animals. This knowledge has occasioned concern over the use of pure oxygen atmospheres in spacecraft.
The effect of 100 percent oxygen at a simulated altitude of 26 000 feet for 6 weeks was studied using white rats at Oklahoma City University under a NASA grant. Radioactive carbon techniques revealed a 15-percent reduction of metabolism in the 100-percent oxygen-exposed rats, compared with rats in air at 1 atmosphere. There was a 20-percent decrease in lipid metabolism in the liver compared with controls, but no decrease in heart metabolism. There was no gross change in body weight.
The White Leghorn chick between 2 and 7 weeks old is markedly resistant to the toxic effects of 1 atm of O2. Continuous exposure (Ohio State University) for as long as 4 weeks did not cause deaths, obvious morbidity, or any signs of pulmonary damage on gross autopsy. Nevertheless, the hyperoxia had some adverse effects, primarily reducing the growth rate to between three-fourths to one-fourth of normal; reducing feed intake per unit body weight to three-fourths of normal; slowing respiratory rate by 30 percent; decreasing erythrocytes, hemoglobin, and hematocrit by 9 to 12 percent; and causing reversible histological changes in the lungs. Arterial O2tensions were elevated over 300-mm Hg, but arterial pCO2and blood pH were unaffected. No residual effects werenoted upon return to air breathing. It is possible that the anatomical peculiarities of the avian lung play some role in the chicks' resistance to hyperoxia, but it is also possible that this resistance is a function of age, similar to the tolerance shown by the young rat but not the adult.
Carbon Dioxide ToleranceStudies of CO2tolerance in submarine crews indicate that no loss of performance is involved if the concentration in air at normal pressure does not exceed 1.5 percent with exposures of 30 to 40 days. However, biochemical adaptive changes were observed at this concentration.
Studies of CO2tolerance in submarine crews indicate that no loss of performance is involved if the concentration in air at normal pressure does not exceed 1.5 percent with exposures of 30 to 40 days. However, biochemical adaptive changes were observed at this concentration.
Inert-Gas ComponentsIf other investigations establish the need for an inert gas in manned spacecraft atmospheres, gases other than nitrogen may be considered. Compared with nitrogen, the physical properties or helium and neon offer advantages with respect to solubility in body fluids, storage weight, and thermal properties.Studies at Ohio State University in 1964, under a NASA grant, showed that helium substituted for nitrogen in a closed container causes humans to feel "cold" at a normally comfortable temperature. Studies with animals have shown that in a helium atmosphere there is greater heat loss due to the increased conducting capacity and probably greater evaporative capacity. In 6 days at 21 percent oxygen and 79 percent helium at 1-atmosphere pressure, young rats grew at the same rate as controls, but drank more water, excreted more urine, and had a higher rate of food and oxygen consumption than controls in air at 1 atmosphere. Men are being tested on a bicycle ergometer in saturated and low relative humidity helium atmospheres to study heat balance.Mice were exposed to 80 percent argon and 20 percent oxygen continuously at 1-atmosphere pressure for 35 days at Oklahoma City University. Carbon 14 studies of metabolism showed a slight slowing and a twofold to threefold increase in fat deposition.
If other investigations establish the need for an inert gas in manned spacecraft atmospheres, gases other than nitrogen may be considered. Compared with nitrogen, the physical properties or helium and neon offer advantages with respect to solubility in body fluids, storage weight, and thermal properties.
Studies at Ohio State University in 1964, under a NASA grant, showed that helium substituted for nitrogen in a closed container causes humans to feel "cold" at a normally comfortable temperature. Studies with animals have shown that in a helium atmosphere there is greater heat loss due to the increased conducting capacity and probably greater evaporative capacity. In 6 days at 21 percent oxygen and 79 percent helium at 1-atmosphere pressure, young rats grew at the same rate as controls, but drank more water, excreted more urine, and had a higher rate of food and oxygen consumption than controls in air at 1 atmosphere. Men are being tested on a bicycle ergometer in saturated and low relative humidity helium atmospheres to study heat balance.
Mice were exposed to 80 percent argon and 20 percent oxygen continuously at 1-atmosphere pressure for 35 days at Oklahoma City University. Carbon 14 studies of metabolism showed a slight slowing and a twofold to threefold increase in fat deposition.
BendsDecompression, whether accidental (due to damage of the spacecraft) or intentional (as in the use of the pressure suit outside the capsule), carries the risk of bends if the inert gases dissolved in the tissues and body fluids come out of solution. The magnitude of this risk is determined to a very considerable extent by—Individual susceptibilityThe extent to which the nitrogen (or other inert gas) concentrations of tissues and body fluids have been reducedThe magnitude and rate of the inert-gas, partial pressure change on decompressionThe probability of getting bends is reduced by—Selection of bends-resistant individualsThorough denitrogenation before flightLimitation of decompressive pressure changes by appropriate choice of cabin atmosphere pressure and compositionSpace-suit pressure settingIn some cases, further improvements might be obtained by using, in the cabin atmosphere, an inert-gas component which has a lower solubility in tissue and body fluids or less tendency than nitrogen to form bubbles.
Decompression, whether accidental (due to damage of the spacecraft) or intentional (as in the use of the pressure suit outside the capsule), carries the risk of bends if the inert gases dissolved in the tissues and body fluids come out of solution. The magnitude of this risk is determined to a very considerable extent by—
Individual susceptibility
The extent to which the nitrogen (or other inert gas) concentrations of tissues and body fluids have been reduced
The magnitude and rate of the inert-gas, partial pressure change on decompression
The probability of getting bends is reduced by—
Selection of bends-resistant individuals
Thorough denitrogenation before flight
Limitation of decompressive pressure changes by appropriate choice of cabin atmosphere pressure and composition
Space-suit pressure setting
In some cases, further improvements might be obtained by using, in the cabin atmosphere, an inert-gas component which has a lower solubility in tissue and body fluids or less tendency than nitrogen to form bubbles.
Fire HazardExperience indicates that fires in pure oxygen atmospheres, even at low pressures (e.g.,1/3atm), are extremely difficult to extinguish. While this phenomenon has nothing to do with respiratory physiology, the risk on flights of long duration may be so serious as to demand special measures. Unless effective countermeasures can be devised, this risk may argue very strongly against the use of such atmospheres in the future. Further experimental investigation is required.
Experience indicates that fires in pure oxygen atmospheres, even at low pressures (e.g.,1/3atm), are extremely difficult to extinguish. While this phenomenon has nothing to do with respiratory physiology, the risk on flights of long duration may be so serious as to demand special measures. Unless effective countermeasures can be devised, this risk may argue very strongly against the use of such atmospheres in the future. Further experimental investigation is required.
Acceleration Effects on the Lungs and Pulmonary CirculationForces produced by high acceleration overdistend one part and compress another part of the lungs. Blood flow diminishes in some parts of the lungs and increases in others. Fluid leaks from the blood into the tissues and into the air sacs in parts of the lungs. These effects cause difficulty in breathing, low arterial oxygen saturation, and impaired consciousness during high sustained acceleration and, to a lesser extent, after its cessation. They must be considered when selecting the best gas to be breathed, since a high partial pressure of oxygen is favorable for consciousness, but a low inert-gas concentration during acceleration is unfavorable for rapid lung recovery afterward.
Forces produced by high acceleration overdistend one part and compress another part of the lungs. Blood flow diminishes in some parts of the lungs and increases in others. Fluid leaks from the blood into the tissues and into the air sacs in parts of the lungs. These effects cause difficulty in breathing, low arterial oxygen saturation, and impaired consciousness during high sustained acceleration and, to a lesser extent, after its cessation. They must be considered when selecting the best gas to be breathed, since a high partial pressure of oxygen is favorable for consciousness, but a low inert-gas concentration during acceleration is unfavorable for rapid lung recovery afterward.