TY - JOUR
AU - BSc, Eyal Peled,
AB - ABSTRACT Introduction: Humans may be accidentally trapped in a confined atmosphere in which oxygen availability is limited. If acclimation would extend survival, hypoxic acclimation in confined spaces would be recommended. Methods: After hypoxic acclimation, an immature pig was transferred into an experimental sealed chamber. The O2 CO2, chamber temperature, and pressure changes due to the animal's breathing were recorded. Six days acclimation (n = 3) and 3 weeks of acclimation (n = 3) were compared to control pigs (n = 3). Results: No signs of acute mountain sickness were noted in the pigs acclimated for 6 days, but some acute symptoms (which were resolved on the following day) were observed during the 3-week acclimation. The terminal partial pressure of inspired oxygen (PIO2; 3.5–3.6 kPa) was not affected by hypoxic acclimation. Oxygen consumption and CO2 production were similar in the three experimental groups. Conclusions: Our hypothesis that hypoxia acclimation would produce improved survival in a confined space was not supported by the results. It is possible that at very low inspired oxygen of 3.6 kPa, the oxygen consumption of critical life-supporting tissue reached the limit of viable cells in mammals. If this is right, no further improvement could be expected after hypoxic acclimation. INTRODUCTION Humans may be accidentally trapped in a confined atmosphere in which oxygen availability is limited. In our previous studies, we investigated the problems of survival in a confined space with limited oxygen content, as affected by CO2 accumulation, exposure time, hypothermia, and a few pharmaceutical treatments in the white rat and in the immature pig.1,12 To summarize the previous findings, we found that hypothermia protected the oxygen transfer from the lung to the blood and therefore extended the survival to a lower inspired PO2. However, no effect on hypoxic survival was related to CO2 accumulation, duration of development of the hypoxia, and various pharmacological treatments. Acclimation to hypoxia (similar to acclimation to high altitude) improves hypoxic performance. There were only small differences between acclimation to high altitude and acclimation in a hypobaric chamber.13 We therefore believed that acclimation to normobaric hypoxia would be similar. Humans who would suddenly be exposed to an altitude of 6,000 m will lose consciousness within 10 minutes; this will not happen with gradual climbing.14 It is possible to switch to hypoxic acclimation in confined spaces like a disabled submarine or a troubled spacecraft if acclimation would extend survival. Acclimation of humans to high altitudes consists of few phases.15 Heart frequency rises immediately and declines to baseline after 10 days. Minute ventilation starts to rise at 10 minutes and declines to baseline after 100 days. Hemoglobin concentration starts to rise after a day and reaches maximum at 18 days and stays high for at least 8 months. Other parameters like increased capillary density and increased hypoxic ventilatory drive start at 10 days. Hypoxic-induced factor increases to maximum at the third day and declines to baseline at day 18. As long as the acclimation progresses, the better is the fitness to hypoxia. We could not find studies on high-altitude acclimation of pigs. However, there are a few studies on acute mountain sickness (AMS) of the pig which was suggested as a good model for high-altitude pulmonary edema in humans.16,18 Our experimental protocol called for exposures lasting until the final hypoxic collapse. The terminal collapse is swift and unpredictable, and rats can be successfully resuscitated when almost at the terminal point.7 Humans accidentally exposed to severe hypoxia experience hallucinations and loss of consciousness. Prolonged exposure to hypoxia may produce AMS, but this takes longer than our protocol. Reports of loss of consciousness and even death on early balloon flights do not describe subjective suffering from the extreme hypoxia and those who lost consciousness later told how they were unable to raise their hand to the flight controls.19 During our previous experiments in confined space when oxygen was consumed, the pig usually rested or slept down to a partial pressure of inspired oxygen (PIO2) of 9 to 8 kPa, at which point the animal began breathing heavily, probably not unlike the experience of high-altitude mountaineers. The pig did not emit distress calls during further hypoxic challenge. We believe that the importance of our study for human applications justifies the experimental protocol. The experimental protocol was approved previously by the Animal Care Committee of the Faculty of Medicine at the Technion and later by the Animal Care Committee of the Israel Defense Office. In our previous pig studies, we surgically prepared the animal a day before the experiment for monitoring of many physiological parameters. For the present study, the Animal Care Committee approved noninvasive study of three pigs per group; they would consider approval of invasive study only if the hypothesis would be supported. METHODS Experimental System Acclimation Chamber The acclimation chamber, 140 × 83 × 80 cm, was constructed from transparent polycarbonate. A tray below the perforated floor was used for urine collection. A door was fastened with bolts and sealed the chamber. When the door was opened, six removable bars kept the pig inside while the CO2 absorber (soda lime), food (pellets), and water were cleaned and replenished. A cooling device (water (C/H Temperature Controller Bath and Circulator 2067; Forma Scientific, Marietta, Ohio) pumped cold water to another device within the chamber that circulated cold air and kept the ambient temperature at 22 to 25°C. A humidity and temperature measuring device (EE20FT; EE Electronics, Engerwitzdorf, Austria) was inserted through the top of the cage. Gas from the chamber was pumped through a vapor absorption column (anhydrous calcium sulfate; Drierite; Vacumed, Ventura, California) to be sampled by an oxygen sensor (S3A/I Ametek; Applied Electrochemistry, Pittsburgh, Pennsylvania) and a CO2 sensor (Microcap Plus; Oridion Medical, Jerusalem, Israel) before returning back to the chamber. A hose from the back of the chamber enabled exit of the chamber gas to the outside of the laboratory. A control electronic device was specifically constructed (Emproko Ltd., Ashkelon, Israel) to control the level of oxygen in the chamber within ± 0.02%. After an input of the half percent of oxygen above the predetermined value, a switch was activated to allow pure nitrogen to flow into the chamber. The flow of nitrogen stopped automatically when the oxygen sensor signaled to the control device that the oxygen value has been reached. Thereafter, whenever the oxygen level dropped (due to the pig's oxygen consumption) by 0.02% below the predetermined value, a pump was activated to pump fresh air into the chamber. When the percentage of the oxygen reached 0.02% above the predetermined value, the pump was automatically shut down. The control system had few levels of response time to prevent overshooting in the O2 concentration. The output of the gas concentrations (CO2 and O2) and temperature were stored on a computer using a data logger (Codas; Dataq Instruments Inc., Acron, Ohio). Experimental Chamber The experimental system (details in Ref. 4) consisted of a plexiglas chamber, air cooler, air pump, air bank, analyzers, and a data logger. A door at the front of the chamber enabled the animal to be placed inside. The chamber had a number of ports: (1) Inlet and outlet for air from the air bank. (2) Inlet and outlet from the air cooler to the chamber. (3) Outlet and inlet for gas samples for CO2 and O2 analysis. (4) Inlet for an atmospheric balloon filled with nitrogen, which compensated for gas exchange by a valve, to equilibrate the inside pressure with the ambient atmosphere. There were two cables leaving the chamber: (1) The cable from the thermistor supplying information to the air cooler and for monitoring the chamber's ambient temperature. The chamber atmosphere was recirculated using a pump, and could be diverted via a CO2 absorber. A valve system enabled the atmosphere to be circulated via an air bank consisting of a barrel. The total volume of air was 431 L. The O2 and CO2 in the chamber atmosphere were read using the previously mentioned gas analyzers and the chamber temperature was monitored using a thermistor (Yellow Springs Instrument Co., Yellow Springs, Ohio). Pressure changes inside the chamber due to the animal's breathing were recorded by a pressure transducer (DP-103; Valydine, Northridge, California). All variables measured were stored on a computer. Hypoxic Acclimation Initially, we selected 6 days acclimation, because different navies keep, in submarine, an emergency store of oxygen and a CO2 absorber for 6 days. Because we found that this acclimation did not improve the survival, we also tested 3 weeks of acclimation. The hypoxic acclimation procedure was taken from the accepted high-altitude acclimation in humans15,20 for 6 days acclimation. For the 3-week acclimation, we follow approximately the protocol used for human acclimation to the condition at Mount Everest.13 After few signs of AMS, we adopted a somewhat milder protocol. The planned oxygen concentration during acclimation is given in Table I. TABLE I Hypoxic Acclimation Protocol Day  1  2  3  4  5  6  7  %O2  14.5  14.0  13.4  12.9  12.4  12.0  11.8  Day  8  9  10  11  12  13  14  %O2  11.6  11.4  11.2  11.0  10.8  10.6  10.4  Day  15  16  17  18  19  20  21  %O2  10.2  10.2  10.1  10.1  10.0  10.0  9.9  Day  1  2  3  4  5  6  7  %O2  14.5  14.0  13.4  12.9  12.4  12.0  11.8  Day  8  9  10  11  12  13  14  %O2  11.6  11.4  11.2  11.0  10.8  10.6  10.4  Day  15  16  17  18  19  20  21  %O2  10.2  10.2  10.1  10.1  10.0  10.0  9.9  View Large TABLE I Hypoxic Acclimation Protocol Day  1  2  3  4  5  6  7  %O2  14.5  14.0  13.4  12.9  12.4  12.0  11.8  Day  8  9  10  11  12  13  14  %O2  11.6  11.4  11.2  11.0  10.8  10.6  10.4  Day  15  16  17  18  19  20  21  %O2  10.2  10.2  10.1  10.1  10.0  10.0  9.9  Day  1  2  3  4  5  6  7  %O2  14.5  14.0  13.4  12.9  12.4  12.0  11.8  Day  8  9  10  11  12  13  14  %O2  11.6  11.4  11.2  11.0  10.8  10.6  10.4  Day  15  16  17  18  19  20  21  %O2  10.2  10.2  10.1  10.1  10.0  10.0  9.9  View Large Protocol Acclimation Every morning during the acclimation period, the door was opened, and the tray with the excretions and soda lime was cleaned as well as the perforated floor. Food and water were supplied. During the 3-week acclimation, once every few days the pig was weighed and returned back to the chamber. New soda lime (7.5 kg) was spread on the tray which was inserted back. The door was closed and the gas control system was activated to keep the designed oxygen concentration. This daily procedure lasted 2 hours. No signs of AMS were observed during the 6-day acclimation. When signs of AMS like lethargy, foam at the mouth, and anorexia appeared during the 3-week acclimation, the concentration of oxygen was raised by approximately 1% until the resumption of normal behavior. Hypoxic Survival The pig was placed into the experimental chamber and the door was closed. A pump circulated the confined atmosphere between the experimental chamber and one barrel from the air bank to yield a confined volume of 431 L. The pig consumed the oxygen, and a recording was made at approximately each 2% change in the inspired oxygen and at 1% intervals below 10% O2. The flow of the chamber's atmosphere was alternately directed through the CO2 absorber or by passing it. This allowed the calculation of CO2 production for the section of CO2 accumulation. At each sampling time, a recording was made of chamber atmosphere O2 and CO2, temperature, and pressure changes due to breathing. The experiment continued until breathing ceased. The ambient temperature was maintained at 22 to 25°C. Experimental Protocol Immature male pigs were used. Three animals were studied as control, three were acclimated for 6 days, and three were acclimated for 3 weeks. Calculations The following parameters were calculated from the data obtained: inspired oxygen pressure—PIO2 (using an assumed body temperature of 38°C, Ref. 4), oxygen consumption— , and CO2 production— (using the volume of gas in the chamber, the cooling system, and the bank, the change in concentration, and the time interval), and breathing frequency (from pressure recording of whole body plethysmography). RESULTS No special behavior or signs of AMS were noted in the pigs acclimated for 6 days. Smaller pigs were selected for the 3-week acclimation because of their expected growth during the 3 weeks. Their weight during acclimation with some remarks is depicted in Figure 1. They did not elevate their mass as was expected for a normoxic environment, but after initial reduced weight they regained the loss. Observed acute symptoms disappeared on the following day. FIGURE 1 View largeDownload slide Body mass (in percentage of the initial mass) is shown for three pigs (represented by different symbols) during 3 weeks of hypoxic acclimation. Phenomena that may be related to AMS, like convulsions, foam at the mouth, lethargy, and diarrhea, are written near the appropriate day for a specific pig. FIGURE 1 View largeDownload slide Body mass (in percentage of the initial mass) is shown for three pigs (represented by different symbols) during 3 weeks of hypoxic acclimation. Phenomena that may be related to AMS, like convulsions, foam at the mouth, lethargy, and diarrhea, are written near the appropriate day for a specific pig. A summary of the exposure to hypoxic challenge is given in Table II. Data are given in mean and range because of small number in each group (n = 3). The terminal PIO2 (3.5–3.6 kPa) was not affected by hypoxic acclimation. The greater survival time in the 3-week acclimated pigs is related to their smaller weight and constant initial volume of air. Thus, there was more oxygen per pig's body mass in the 3-week acclimation group. Breathing frequency in the control pigs was elevated when oxygen pressure was reduced from 15 to 7 kPa (Fig. 2). Breathing frequency in the acclimated pigs started to rise at 12 kPa and rose further on to a milder level compared to the control pigs. Oxygen consumption (Fig. 3) and CO2 production (data not shown) were similar in the three experimental groups. TABLE II Summary of the Results in the Sealed Chamber for the Three Groups: Mean (Range)   Weight (kg)  Survival Time (hours)  Terminal PIO2 (kPa)  Control  28 (23–31)  4.4 (4.0–4.6)  3.65 (3.21–3.80)  Six days acclimation  31 (29–33)  4.6 (3.7–5.8)  3.60 (3.49–3.80)  Three weeks acclimation  22 (17–25)  6.3 (6.0–6.6)  3.52 (3.15–3.92)    Weight (kg)  Survival Time (hours)  Terminal PIO2 (kPa)  Control  28 (23–31)  4.4 (4.0–4.6)  3.65 (3.21–3.80)  Six days acclimation  31 (29–33)  4.6 (3.7–5.8)  3.60 (3.49–3.80)  Three weeks acclimation  22 (17–25)  6.3 (6.0–6.6)  3.52 (3.15–3.92)  View Large TABLE II Summary of the Results in the Sealed Chamber for the Three Groups: Mean (Range)   Weight (kg)  Survival Time (hours)  Terminal PIO2 (kPa)  Control  28 (23–31)  4.4 (4.0–4.6)  3.65 (3.21–3.80)  Six days acclimation  31 (29–33)  4.6 (3.7–5.8)  3.60 (3.49–3.80)  Three weeks acclimation  22 (17–25)  6.3 (6.0–6.6)  3.52 (3.15–3.92)    Weight (kg)  Survival Time (hours)  Terminal PIO2 (kPa)  Control  28 (23–31)  4.4 (4.0–4.6)  3.65 (3.21–3.80)  Six days acclimation  31 (29–33)  4.6 (3.7–5.8)  3.60 (3.49–3.80)  Three weeks acclimation  22 (17–25)  6.3 (6.0–6.6)  3.52 (3.15–3.92)  View Large FIGURE 2 View largeDownload slide Breathing frequency (mean ± SD) as a function of decreased inspired oxygen pressure in the three experimental groups (different symbols). Data from only two pigs were recorded in the 3-week acclimation group, therefore, only the mean is shown. FIGURE 2 View largeDownload slide Breathing frequency (mean ± SD) as a function of decreased inspired oxygen pressure in the three experimental groups (different symbols). Data from only two pigs were recorded in the 3-week acclimation group, therefore, only the mean is shown. FIGURE 3 View largeDownload slide Oxygen consumption (mean + SD) as a function of decreased inspired oxygen pressure in the three experimental groups (different symbols). FIGURE 3 View largeDownload slide Oxygen consumption (mean + SD) as a function of decreased inspired oxygen pressure in the three experimental groups (different symbols). DISCUSSION Because of the noninvasive nature of the present study, we could not follow acclimation using markers in the blood. However, from observation of the breathing frequency response to hypoxia, we saw that the control pigs responded with highly elevated frequency when inspired oxygen was reduced from 15 to 6 kPa. The acclimated pigs responded with moderate elevation between the range of 10 to 5 kPa; this is typical to altitude acclimation. Our hypothesis that hypoxia acclimation would produce improved survival in a confined space was not supported by the results. Because of the disappointing results, the experiment was not continued into the invasive phase. The terminal inspired oxygen pressure of control pigs was similar to the experimental pigs. However, this value of 3.6 kPa is lower then the terminal PIO2 of 5.1, 5.1, and 5.7 kPa in previous invasive studies.5,10,11 This lower tolerance to hypoxia may be related to the anesthesia and surgical preparation of the pigs a day before the exposure in previous studies. A very intensive acclimation of humans to high altitude which consisted of 3 years of training and 60 days of control climbing on Mount Everest proved its effectiveness by less casualties and deaths compared to others.21 Such a long acclimation is not possible in troubled confined space. It is possible that at very low inspired oxygen of 3.6 kPa, the oxygen consumption of critical life-supporting tissue reached the limit of viable cells in mammals of approximately 0.02 L/(kg×hr).22 The measured oxygen consumption (Fig. 3) was calculated along a time interval and does not reflect the final value before the terminal collapse. If this is right, no further improvement could be expected after hypoxic acclimation. The terminal PIO2 of the pigs was not different from that of the hypoxic-adapted subterranean mole rat of 3.8 to 2.4 kPa.9 This also supports the idea that no further reduction of terminal PIO2 could be added after the hypoxic acclimation. ACKNOWLEDGMENTS We thank Mr. R. Lincoln for skillful editing. The research was supported by the Israel Ministry of Defense Research & Development. REFERENCES 1. Arieli R Do the exposure time and PCO2 affect the terminal PO2 in a confined atmosphere? Respir Physiol  1985; 62: 105– 15. Google Scholar CrossRef Search ADS PubMed  2. Arieli R, Ertracht O Latency to CNS oxygen toxicity in rats as a function of PCO2 and PO2. Eur J Appl Physiol  1999; 80: 598– 603. Google Scholar CrossRef Search ADS   3. Arieli R, Kerem D Independence of hypoxic death of inspiratory PCO2 in rats and fossorial mole rats. Undersea Biomed Res  1984; 11: 275– 85. Google Scholar PubMed  4. Arieli R, Kerem D Effect of accumulation of CO2 on the survival of immature pigs in a confined atmosphere. A. Gas exchange. J Basic Clin Physiol Pharmacol  1995; 6: 1– 18. Google Scholar CrossRef Search ADS PubMed  5. Arieli R, Kerem D Effect of accumulation of CO2 on the survival of immature pigs in a confined atmosphere. B. Blood gas exchange. J Basic Clin Physiol Pharmacol  1995; 6: 19– 37. Google Scholar PubMed  6. Arieli R, Kwetny I Effect of diazepam on survival of the immature pig in a confined atmosphere. J Basic Clin Physiol Pharmacol  1999; 10: 15– 27. Google Scholar CrossRef Search ADS PubMed  7. Arieli R, Lehrer C, Gavriely N Hypothermia prolongs survival in a confined atmosphere. Milit Med  1993; 158: 798– 802. 8. Arieli R, Lehrer C, Gavriely N The effect of hypothermia on gas exchange in anaesthetized rats in a confined atmosphere. J Basic Clin Physiol Pharmacol  1994; 5: 277– 94. Google Scholar PubMed  9. Arieli R, Nevo E Hypoxic survival differs between two mole rat species (Spalax ehrenbergi) of humid and arid habitats. Comp Biochem Physiol  1991; 100A: 543– 5. Google Scholar CrossRef Search ADS   10. Arieli R, Reuveni A, Melnikov V Effect of hypothermia on the survival of the immature pig in a confined atmosphere. J Basic Clin Physiol Pharmacol  1997; 8: 91– 111. Google Scholar CrossRef Search ADS PubMed  11. Arieli R, Robicsek R Effect of exposure time on the survival of immature pigs in a confined atmosphere. J Basic Clin Physiol Pharmacol  1996; 7: 83– 96. Google Scholar CrossRef Search ADS PubMed  12. Ertracht O, Arieli R, Gavriely N, Shenedrey B, Youdim MBH Selegiline does not prolong survival in a confined atmosphere. Curr Top Pharmacol  2005; 9: 59– 69. 13. Houston CS, Sutton JR, Cymerman A, Reeves JT Operation Everest II: man at extreme altitude. J Appl Physiol  1987; 63: 877– 82. Google Scholar CrossRef Search ADS PubMed  14. Sharp CR Hypoxia and hyperventilation. In: Aviation Medicine Physiology and Human Factors.  Edited by Ernsting J London, UK, Tir-Med Books, 1978. 15. Ward MP, Milledge JS, West JB High Altitude Medicine and Physiology , Ed 3. New York, Oxford University Press, 2000. 16. Kleinsasser AD, Levin L, Loeckinger A, Hopkins SR A pig model of high altitude pulmonary edema. High Alt Med Biol  2003; 4: 465– 74. Google Scholar CrossRef Search ADS PubMed  17. Schoene RB, Goldberg S The quest for an animal model of high altitude pulmonary edema. Int J Sports Med  1992; 13(Suppl 1): S59– 61. Google Scholar CrossRef Search ADS PubMed  18. Tucker A, McMutry if, Reeves JT, Alexander AF, Will DH, Grover RF Lung vascular smooth muscle as a determinant of pulmonary hypertension at high altitude. Am J Physiol  1975; 228: 762– 7. Google Scholar PubMed  19. Pugh LGCE The effect of acute and chronic exposure to low oxygen supply on consciousness. In: Environmental Effects on Consciousness , pp 106– 16. Edited by Schaefer KE New York, MacMillan, 1962. 20. Johnson TS, Rock PB Current concepts acute mountain sickness. N Engl J Med  1988; 319: 841– 5. Google Scholar CrossRef Search ADS PubMed  21. Colby SP, McKenna J, Alan J Avoiding deaths on Everest. BMJ  2006; 333: 603. 22. Singer D, Bach F, Bretschneider HJ, Kuhn HJ Metabolic size allometry and the limits to beneficial metabolic reduction: hypothesis of a uniform specific minimal metabolic rate. In: Suviving Hypoxia: Mechanisms of Control and Adaptation , pp 447– 58. Edited by Hochachka PW, Lutz PL, Sick T, Rosental M, van den Thillart G London, U.K., CRC Press, 1993. Reprint & Copyright © Association of Military Surgeons of the U.S.
TI - Acclimation to Hypoxia Does Not Improve Hypoxic Survival of the Immature Pig in Confined Atmosphere
JF - Military Medicine
DO - 10.7205/MILMED.173.1.107
DA - 2008-01-01
UR - https://www.deepdyve.com/lp/oxford-university-press/acclimation-to-hypoxia-does-not-improve-hypoxic-survival-of-the-kXeqNI06Un
SP - 107
EP - 111
VL - 173
IS - 1
DP - DeepDyve
ER -