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PubMed2004

Increase in 6-hydroxymelatonin excretion in humans during ascent to high altitudes.

Melatonin (MLT), the pineal gland hormone involved in the regulation of circadian rhythms, shows characteristic diurnal variation. Its physiological role in humans is not clear. Exposure to high altitudes may disrupt the circadian rhythm and lead to various endocrine changes. MLT in humans has not been studied under these conditions. Urinary 6-hydroxy-MLT sulfate (aMT6s) excretion was analyzed during the day (0700-2200 h) and night (2200-0700 h) phases. A cohort of 33 healthy volunteers, aged 19-65 yr, was studied during an ascent to a high altitude in the Himalayas on three occasions (at a lower altitude, at 3400 m, and after reaching maximal altitudes of 5600-6100 m). aMT6s excretion during the daytime remained unchanged during exposure to high altitudes. As expected, nocturnal values were higher than diurnal values at each point in time. However, there was a significant increase in nocturnal MLT excretion after the ascent to high altitudes. Ascent to high altitudes is associated with increased nocturnal excretion of aMT6s. The mechanism and physiological significance of this MLT increase are unclear.

Adult↗
PubMed2001

Genetic approaches to understanding human adaptation to altitude in the Andes.

Despite the initial discomfort often experienced by visitors to high altitude, humans have occupied the Andean altiplano for more than 10000 years, and millions of people, indigenous and otherwise, currently live on these plains, high in the mountains of South America, at altitudes exceeding 3000 m. While, to some extent, acclimatization can accommodate the one-third decrease in oxygen availability, having been born and raised at altitude appears to confer a substantial advantage in high-altitude performance compared with having been born and raised at sea level. A number of characteristics have been postulated to contribute to a high-altitude Andean phenotype; however, the relative contributions of developmental adaptation (within the individual) and genetic adaptation (within the population of which the individual is part) to the acquisition of this phenotype have yet to be resolved. A complex trait is influenced by multiple genetic and environmental factors and, in humans, it is inherently very difficult to determine what proportion of the trait is dictated by an individual's genetic heritage and what proportion develops in response to the environment in which the person is born and raised. Looking for changes in putative adaptations in vertically migrant populations, determining the heritability of putative adaptive traits and genetic association analyses have all been used to evaluate the relative contributions of nurture and nature to the Andean phenotype. As the evidence for a genetic contribution to high-altitude adaptation in humans has been the subject of several recent reviews, this article instead focuses on the methodology that has been employed to isolate the effects of 'nature' from those of 'nurture' on the acquisition of the high-altitude phenotype in Andean natives (Quechua and Aymara). The principles and assumptions underlying the various approaches, as well as some of the inherent strengths and weaknesses of each, are briefly discussed.

Adaptation, Physiological↗
PubMed2004

Comparative response of EPO and soluble transferrin receptor at high altitude.

PURPOSE: Soluble transferrin receptor (sTfR) classically raises with increased erythropoiesis, along with the rise in erythropoietin (EPO). However, the specific effect of altitude-induced erythropoiesis on sTfR remains poorly documented. This study investigated the response of sTfR during high-altitude exposure in human and verified that sTfR was related to EPO response in this case. METHODS: EPO, sTfR, red cell volume (RCV), ferritin, and iron intake were measured during: 1) experiment A (N = 8, 31 d at 5000-8848 m), at sea level (SL), and at the simulated altitude of 5000, 6000, 7000, and 8000 m; and 2) during experiment B (N = 10, 7 d at 4350 m), at SL, after 3, 5, and 7 d at 4350 m and 1-2 d after return to SL (RSL). RESULTS: In experiment A, progressive decompression from SL to 8000 m induced a large parallel rise in EPO (33.8-fold) and sTfR (5.9-fold), whereas ferritin was dramatically decreased and iron intake reduced. RCV was increased after 31 d of decompression. In experiment B, EPO peaked at day 3 at 4350 m, then declined later at altitude and returned to baseline values at RSL, whereas sTfR progressively rose at altitude (+86%) and remained elevated during RSL (+64%). Ferritin progressively declined at 4350 m, whereas iron intake was unchanged. RCV was not enhanced after exposure to 4350 m. CONCLUSION: In summary, sTfR mirrors EPO response for a given level of altitude hypoxia but differs from EPO response during transitory phases, such as early acclimatization or reoxygenation. Analysis of sTfR may therefore account for altitude-related erythropoiesis, at a time when EPO is blunted.

Altitude↗
PubMed2005

Autonomic adaptations in andean trained participants to a 4220-m altitude marathon.

PURPOSE: Both training and chronic hypoxia act on the autonomic nervous system. Because trained Andean high-altitude natives could perform a high-altitude marathon (4220 m above sea level) in 02:27:23 h, we hypothesized that living in chronic hypoxia does not limit the training-induced benefits on the autonomic modulation of the heart. METHODS: Trained (N=13) and sedentary (N=11) Andean high-altitude natives performed an active orthostatic test. Eight of the trained subjects repeated the test 6-8 and 20-24 h after the end of a high-altitude marathon. Resting heart rate (HR) and the autonomic modulation of the heart were assessed by time domain and spectral analysis of HR variability (HRV): sympathetic (RR low frequency (LF)) and parasympathetic (RR high frequency (HF)) modulations, and sympathovagal balance (RR-LF:HF ratio). RESULTS: Trained subjects exhibited a higher total power of HRV and a lower resting HR (+30%, P<0.005) than sedentary subjects secondary to a higher and dominant parasympathetic modulation on sympathetic activity (RR-HF, RR-LF:HF ratio). At 6-8 h after the marathon, total power of HRV decreased (-69%), whereas resting HR increased from basal level (+22%), mainly because of a rise in sympathetic modulation (RR-LF, RR-LF:HF ratio). From 8 to 24 h of recovery, sympathetic modulation fell (RR-LF, RR-LF:HF ratio) and all HRV parameters were restored. Responses to the active standing position did not change between each recording session. CONCLUSION: Living in chronic hypoxia does not limit the training-induced benefits on the autonomic control of the cardiovascular system in Andean high-altitude natives. The sympathetic predominance on the heart observed 6-8 h after the high-altitude marathon disappeared after 1 d of recovery. Therefore, living at high altitude does not impair the autonomic response to training.

Adaptation, Physiological↗
PubMed1996

Urinary leukotriene E4 levels in high-altitude pulmonary edema. A possible role for inflammation.

STUDY OBJECTIVES: Inflammation may contribute to the pathogenesis of high-altitude pulmonary edema (HAPE). This study was designed to determine whether a marker of inflammation, urinary leukotriene E4 (LTE4), is elevated in patients with HAPE. DESIGN: We conducted a case-control study to collect clinical data and urine samples from HAPE patients and healthy control subjects at moderate altitude (> or = 2727 m), and follow-up urine samples from HAPE patients following their return to low altitude (< or = 1,600 m). SETTING: Five medical clinics in Summit County, Colorado. PATIENTS: Questionnaire data were evaluated in 71 HAPE patients and 36 control subjects. Urinary LTE4 levels were determined from a random subset of 38 HAPE patients and 10 control subjects presenting at moderate altitude, and on 5 HAPE patients who had returned to low altitude. MEASUREMENTS AND RESULTS: Using an enzyme immunoassay technique, urinary LTE4 levels were found to be significantly higher in HAPE patients (123 [16 to 468] pg/mg creatinine, geometric mean [range]) than in control subjects (69 [38 to 135]), p = 0.02. Following return to low altitude, urinary LTE4 levels fell significantly from 122 (41.8 to 309) to 53.6 (27.6 to 104) pg/mg creatinine (p = 0.05). Urinary LTE4 levels were not related to age, sex, time at altitude, physical condition or habitual exercise, recent use of alcohol or nonsteroidal anti-inflammatory drugs (NSAIDs), or oxygen saturation. Clinical factors associated with HAPE included male sex, regular exercise, and recent use of NSAIDs. CONCLUSIONS: We conclude that urinary LTE4 levels are elevated in patients with HAPE, supporting the view that HAPE involves inflammatory mechanisms.

Adult↗
PubMed1998

Rehabilitation of hypoxemic patients with COPD at low altitude at the Dead Sea, the lowest place on earth.

BACKGROUND: In patients with COPD, oxygen therapy has been shown to improve exercise capacity and survival. Increase in barometric pressure at low altitude can serve as a simple way to improve arterial oxygenation in hypoxemic patients. We have tried to evaluate the effect of staying at low altitude on arterial oxygenation and exercise performance in patients with COPD. PATIENTS AND METHOD: Eleven patients with COPD (9 male, 2 female) aged 38 to 79 years (mean FEV1, 0.96 L; 36% predicted) with hypoxemia (mean PaO2, 54.2+/-8.9 mm Hg) at Jerusalem (altitude 800 m above sea level) were taken down to the Dead Sea area (altitude 402 m below sea level) for 3 weeks. At both locations we tested arterial blood gases, spirometry, progressive exercise, 6-minute walking distance, and sleep oximetry. The study was repeated 2 weeks after returning to Jerusalem. RESULTS: Spirometry results were unchanged. Mean arterial PaO2 rose from 54.2+/-8.9 mm Hg to 69.5+/-11 at the first week and to 66.6+/-11 at the third week of stay (p<0.001). PaCO2 rose from 43.5+/-9.8 mm Hg to 47.7+/-9 and 49.5+/-8.4 (p<0.006). Six-minute walking distance rose from 337+/-107 m to 449+/-73 and 507+/-91 in the third week (p<0.005). Maximum oxygen consumption (VO2max) rose from 901+/-257 mL/min to 1,099+/-255 and 1,063+/-250 mL/min (p=0.01). Sleep oximetry showed an increase in mean sleep arterial oxygen saturation from 86.0+/-4.3% to 89.9+/-4.2% and 88.3+/-3.0 at 1 and 3 weeks, respectively (p<0.05). Following the return to Jerusalem, arterial gases returned to their baseline levels (PaO2, 52.9+/-9.4 mm Hg) but 6-min walking distance remained significantly high, 453+/-47 (p<0.02), and VO2max remained high as well (1,102+/-357 mL/min), although it did not reach statistical significance. CONCLUSIONS: Decline to low altitude or staving at high oxygen environment improves arterial oxygenation and exercise capacity in hypoxemic patients residing in moderate or high altitude. Low altitude (or pressurized wards) can improve pulmonary rehabilitation of hypoxemic patients with COPD.

Adult↗
PubMed2005

The effects of a 5-lipoxygenase inhibitor on acute mountain sickness and urinary leukotriene e4 after ascent to high altitude.

BACKGROUND: Elevated urine and blood leukotriene levels have been reported after ascent to high altitude in association with acute mountain sickness (AMS) and high-altitude pulmonary edema. Zileuton is an inhibitor of the enzyme 5-lipoxygenase that catalyzes conversion of arachidonic acid to leukotrienes. Study objectives and design: The objectives of this randomized, double-blind, placebo-controlled clinical trial were to determine whether zileuton (600 mg po qid) is effective prophylaxis for AMS, and to measure the effect of ascent to high altitude and zileuton on urinary leukotriene E(4) levels. SETTING AND PARTICIPANTS: The study group consisted of volunteers from among climbers on the West Buttress of Mt. McKinley (Denali), Alaska. After baseline urine samples at sea level, subjects flew by airplane to 2,300 m, and then ascended to the 4,200-m camp in 5 to 10 days. MEASUREMENTS AND RESULTS: Using an enzyme immunoassay, urinary leukotriene E(4) was found to decrease after ascent to high altitude in both the zileuton and placebo groups. Urinary leukotriene E(4) in the zileuton group (n = 9) decreased from 67 +/- 35 pg/mg creatinine at sea level to 33 +/- 22 pg/mg creatinine at high altitude (p = 0.003) [mean +/- SD]. Urinary leukotriene E(4) in the placebo group (n = 9) decreased from 97 +/- 82 pg/mg creatinine at sea level to 44 +/- 21 pg/mg creatinine at high altitude (p = 0.045). One subject in the zileuton group and three subjects in the placebo group met Lake Louise criteria for AMS after arriving at 4,200 m (p = 0.257). CONCLUSIONS: Elevated leukotrienes are not associated with ascent to high altitude. In subjects with AMS, urinary leukotrienes were not elevated, suggesting that leukotrienes may not be a component of the pathophysiology of AMS. The low incidence of AMS and the small sample size in this study prevented determination of whether zileuton is effective prophylaxis for AMS.

Acclimatization↗
PubMed2006

Effect of altitude on hospitalizations for respiratory syncytial virus infection.

OBJECTIVE: Respiratory syncytial virus (RSV) infection is the foremost cause of serious lower respiratory tract infection in young children and infants. Because higher rates of hospitalization for bronchiolitis and pneumonia have been noted in high-altitude regions, we hypothesized that physiologic responses to altitude would predispose children to more severe illness from RSV infection. This study examined the effect of residential altitude on hospitalizations for RSV infection in Colorado from 1998 through 2002. METHODS: A geographic information system was used to assemble data for altitude and demographic variables by zip code-tabulation areas. Data then were linked with hospital discharge data for RSV infections. Poisson regression models were developed to explore correlations between hospitalization rates and residential altitude, after adjustment for socioeconomic differences in the underlying population. RESULTS: RSV-associated hospitalizations averaged 15.9 per 1000 infants who were younger than 1 year and 1.8 per 1000 children who were 1 to 4 years of age per season. A multivariate analysis suggested that the rate of hospitalization for RSV-specific International Classification of Diseases, Ninth Revision, Clinical Modification codes increased 25% among infants who were younger than 1 year and 53% among children who were 1 to 4 years of age for every 1000-m increase in altitude. The risk for RSV-associated hospitalization was highest at elevations above 2500 m. CONCLUSIONS: High altitude above 2500 m is a modest predictor for RSV-associated hospitalization. Practitioners in these regions should consider additional efforts to educate parents about RSV infection and its prevention and the importance of early treatment.

Altitude↗
PubMed1997

Seizure and hemiparesis at high-altitude outside the setting of acute mountain sickness.

Neurologic problems at high altitudes are well known. What is probably less emphasized are neurologic problems at altitude outside the setting of high-altitude cerebral edema. Because neurologic symptoms for these kinds of problems at high altitude are often transient, neuroradiologic scanning for these problems is usually not done or reported. Furthermore, diagnostic testing facilities may be unavailable in these remote high-altitude settings. A patient is described here with transient seizure and right-sided hemiparesis at high altitude with no preceding symptoms of acute mountain sickness. Computed tomography of the head was obtained in a hospital at lower altitude where the patient was taken promptly. The findings of the scan revealed probable focal cerebral edema in the left parietal lobe in keeping with his temporary right-sided weakness. Possible treatment modalities on the mountain for this problem are also discussed.

Adult↗
PubMed2000

Disoriented and ataxic pilgrims: an epidemiological study of acute mountain sickness and high-altitude cerebral edema at a sacred lake at 4300 m in the Nepal Himalayas.

OBJECTIVE: To determine the incidence of high-altitude cerebral edema (HACE), acute mountain sickness (AMS), and high-altitude pulmonary edema (HAPE) in pilgrims. Although it is well known that western trekkers suffer from acute mountain sickness (AMS) in the Himalayas, not much is documented about the incidence of AMS in the local population of Nepal that go to high altitude. METHODS: The design was a randomized study set at a sacred high-altitude lake at 4300 m at Gosainkund in the Nepal Himalayas. There was a control study at 1300 m at Pashupatinath in Kathmandu, Nepal. The subjects were pilgrims of different ethnic Nepali backgrounds. The Lake Louise consensus for AMS, HACE, and HAPE was used, and oxygen saturation with a pulse oximeter was performed on HACE subjects. RESULTS: Out of 5000 pilgrims, 228 were randomly chosen. Sixty-eight percent had AMS, 31% had HACE, and 5% had HAPE. The mean oxygen saturation of HACE subjects at that altitude was 77%, 87% being normal for 4300 m altitude. Seventy-three percent of the study population were men, yet women had a significantly higher rate of AMS (odds ratio, 4.34; 95% confidence interval, 1.83-10.68), HACE (odds ratio 3.15, confidence interval 1.62-6.12), and HAPE (odds ratio, 5.2; 95% confidence interval, 1.24-24.73). CONCLUSIONS: Such a high incidence of HACE in an epidemiological study using the Lake Louise criteria has, to our knowledge, not been reported before. High-altitude pilgrims, especially women pilgrims in this study, seem to be a very susceptible group. Preventive measures in these pilgrims need to be adopted to avoid AMS, specifically life-threatening HACE and HAPE.

Acute Disease↗
PubMed1994

Nutrition and energetics of exercise at altitude. Theory and possible practical implications.

Altitude exposure may lead to considerable weight loss. Most reports, showing weight losses of 3% in 8 days at 4300m and up to 15% after 3 months at 5300 to 8000m, appear to indicate that this weight loss is a function of both absolute altitude and the duration of exposure. Based on the available scientific evidence to date, it is concluded that altitude weight loss is because of an initial loss of water and subsequent loss of fat and muscle mass due to malnutrition. Up to 5500m, malabsorption of macronutrients does not occur. Up to altitudes around 5000m, weight loss from a reduction of fat and muscle appears to be avoidable by maintaining adequate dietary intake. Primary anorexia, lack of comfort and palatable food, detraining, and possibly direct effects of hypoxia on protein metabolism seem inevitably to lead to weight loss during longer exposures at higher altitudes. To minimise losses, it is advisable to acclimatise properly, reduce the length of stay at extreme altitude as much as possible and maintain a high and varied nutrient intake. With sojourns at intermediate altitude for training purposes, adequate energy intake should be maintained taking into account the decrease in aerobic training intensity and the increase in basal metabolic rate that ensue from the hypoxic environment.

Altitude↗
PubMed2001

The effect of altitude on cycling performance: a challenge to traditional concepts.

Acute exposure to moderate altitude is likely to enhance cycling performance on flat terrain because the benefit of reduced aerodynamic drag outweighs the decrease in maximum aerobic power [maximal oxygen uptake (VO2max)]. In contrast, when the course is mountainous, cycling performance will be reduced at moderate altitude. Living and training at altitude, or living in an hypoxic environment (approximately 2500 m) but training near sea level, are popular practices among elite cyclists seeking enhanced performance at sea level. In an attempt to confirm or refute the efficacy of these practices, we reviewed studies conducted on highly-trained athletes and, where possible, on elite cyclists. To ensure relevance of the information to the conditions likely to be encountered by cyclists, we concentrated our literature survey on studies that have used 2- to 4-week exposures to moderate altitude (1500 to 3000 m). With acclimatisation there is strong evidence of decreased production or increased clearance of lactate in the muscle, moderate evidence of enhanced muscle buffering capacity (beta m) and tenuous evidence of improved mechanical efficiency (ME) of cycling. Our analysis of the relevant literature indicates that, in contrast to the existing paradigm, adaptation to natural or simulated moderate altitude does not stimulate red cell production sufficiently to increase red cell volume (RCV) and haemoglobin mass (Hb(mass)). Hypoxia does increase serum erthyropoietin levels but the next step in the erythropoietic cascade is not clearly established; there is only weak evidence of an increase in young red blood cells (reticulocytes). Moreover, the collective evidence from studies of highly-trained athletes indicates that adaptation to hypoxia is unlikely to enhance sea level VO2max. Such enhancement would be expected if RCV and Hb(mass) were elevated. The accumulated results of 5 different research groups that have used controlled study designs indicate that continuous living and training at moderate altitude does not improve sea level performance of high level athletes. However, recent studies from 3 independent laboratories have consistently shown small improvements after living in hypoxia and training near sea level. While other research groups have attributed the improved performance to increased RCV and VO2max, we cite evidence that changes at the muscle level (beta m and ME) could be the fundamental mechanism. While living at altitude but training near sea level may be optimal for enhancing the performance of competitive cyclists, much further research is required to confirm its benefit. If this benefit does exist, it probably varies between individuals and averages little more than 1%.

2,3-Diphosphoglycerate↗
PubMed1996

High altitude-induced albuminuria in normal man is enhanced by infusion of low-dose dopamine.

Renal function and the urinary excretion rate of albumin (Ualb) at rest and during infusion of dopamine (3 micrograms kg-1 min-1) were investigated in eight normal volunteers at sea level and 48 h after a rapid, passive ascent to an altitude of 4350 m. Oxygen saturation decreased to 81% (77-85) (median with quartiles in parentheses) at high altitude. High altitude hypoxia increased Ualb from 3.2 micrograms min-1 (2.7-3.5) to 5.0 micrograms min-1 (3.3-6.6) (p < 0.05); increased mean arterial blood pressure from 80 mmHg (73-95) to 102 mmHg (96 108) (p < 0.01); decreased the effective renal plasma flow (ERPF) from 465 ml min-1 (412-503) to 410 ml min-1 (385-451) (p < 0.05), and increased the filtration fraction from 24% (22-27) to 28% (26-29) (p < 0.01). Glomerular filtration rate (GFR), and the renal clearances of lithium (CLi) and sodium (CNa) remained unchanged at high altitude. Dopamine increased ERPF, GFR, CLi, CNa, and decreased the filtration fraction in both environments. Infusion of dopamine further increased Ualb to 10.5 micrograms min-1 (5.5-64.8) (p < 0.05) at high altitude, but had no effect on Ualb at sea level. In conclusion, high altitude hypoxia per se increases the urinary excretion rate of albumin, which is further increased by the renal vasodilating drug dopamine. This effect of dopamine at high altitude may result from combined effects of the increase in renal plasma flow and a hypoxia-induced increase in the glomerular capillary permeability to albumin.

Adult↗
PubMed1992

Active regulation of altitude as a function of optical texture.

Two empirical studies are reported that examine active regulation of altitude as a function of the type of ground texture. Three ground textures were examined: lines perpendicular to the direction of motion, lines parallel to the direction of motion, and the combination (i.e., square or checkerboard texture). Although subjects only controlled altitude, disturbances were introduced on three axes: vertical, lateral, and fore-aft. The results show a clear advantage for texture parallel to the direction of motion. However, in considering these results in the context of previous research on altitude control, the argument is made that there is no compelling evidence that suggests either parallel (splay) or perpendicular (density) texture is privileged with regard to altitude control. Rather, the most effective display for altitude control will be the one that best isolates the optical activity associated with changing altitude from the optical activity arising from other sources of disturbance (such as forward locomotion). Such a display will make it easier for the observer to distinguish and respond specifically to the disturbances of altitude.

Adult↗
PubMed2003

Pharmacokinetics of lithium in healthy volunteers after exposure to high altitude.

INTRODUCTION: Exposure of the human body to high altitude causes a number of physiological changes. In previous studies, we observed that these changes may alter the pharmacokinetics of drugs. The number of erythrocytes/mm3 increases both, after acute exposure to high altitude (HA), i.e. within 12 - 24 h after reaching high altitude (H), as well as in chronic exposure (HC) (> 10 months) to H. Also binding of drugs to biologic material may change with exposure to HA and/or HC. OBJECTIVE: Since lithium is transported into and out of erythrocytes and binds strongly to erythrocytes, but is not plasma protein-bound, we selected this drug as candidate for the present study. SUBJECTS, MATERIAL AND METHODS: Lithium carbonate 300 mg were administered orally to young healthy volunteers. One group residing at low altitude (Santiago, Chile, 600 m, group L), these same volunteers after 15 hours of exposure to high altitude (4,360 m, group HA), and volunteers living at high altitude for at least 10 months (group HC). RESULTS: We found a significant increase of both hematocrit and red blood cell count (RBC) after exposure to H, both, acute or chronic. Elimination half-life increased 64.1% in group HA and 111.4% in group HC in comparison to group L. We also found an increase in volume of distribution: + 18.9% in group HA, and + 35.8% in group HC when measured in plasma, and + 16.9% in group HA and + 18.8% in group HC when measured in whole blood. Lithium uptake by the erythrocytes increases: the value of 36.7 +/- 22.7% in Group L rose to 54.8 +/- 21.1% and to 54.6 +/- 24.2% in groups HA and HC, respectively. Total clearance decreases at high altitude, though the differences were significant only in group HC (37%). CONCLUSION: Results indicate that exposure to H produces alterations in the pharmacokinetics of lithium and that these variations may be clinically relevant.

Administration, Oral↗
PubMed1999

Effects of acute prolonged exposure to high-altitude hypoxia on exercise-induced breathlessness.

The direct effects of hypoxia on exercise-induced breathlessness are unclear. Increased breathlessness on exercise is known to occur at high altitude, but it is not known whether this is related to the hypoxia per se, or to other ventilatory parameters. To examine the role of high-altitude hypoxia in exercise-induced breathlessness, studies were performed in 10 healthy, normal subjects at sea level and after acute exposure to an altitude of 4450 m. Although the perception of hand weights did not alter between sea level and high altitude, the intensity of exercise-induced breathlessness increased significantly at high altitude. This was associated with a higher minute ventilation and respiratory frequency for any given exercise level, whereas tidal volume was not significantly altered from sea level values. The increased intensity of breathlessness with exercise did not change significantly over the 5 days at high altitude. These results suggest that the increased intensity of exercise-induced breathlessness at high altitude is not related to peripheral mechanisms or the pattern of ventilation, or to the level of hypoxia per se, but to the level of reflexly increased ventilation.

Acute Disease↗
PubMed1999

Speech intelligibility in aircraft noise as a function of altitude.

UNLABELLED: Few studies have addressed physiological effects of altitude and noise combined, although these two factors are inherent parts of all aviation. Altitude effects on hearing function have mainly been studied using gas mixtures with reduced oxygen content. The results have been inconclusive. HYPOTHESIS: The present study was designed to assess the effect of altitude on speech intelligibility in aircraft noise. The primary hypothesis was that exposure to altitude would produce a detrimental, hypoxic effect on speech intelligibility in noise. METHODS: Eight male subjects with normal hearing were fitted with an aviation headset specially adapted for use with the audiometer. Pure-tone audiometry, as well as speech audiometry in noise, were performed at 0, 10,000, 13,000, and 16,000 ft simulated altitudes in a hypobaric chamber. The four test altitudes were performed double blind with respect to audiometry operator and test subject. Arterial blood gases where measured using an intra-arterial catheter. Tympanometric measurements verified full middle ear equilibration. Noise levels where monitored and logged throughout all experiments. RESULTS AND CONCLUSION: A substantial increase in speech intelligibility in noise due to altitude was evident in our study. The physical effect of barometric pressure on noise causing an increased signal-to-noise ratio was found to greatly outweigh any possible hypoxic detrimental effect.

Adult↗
PubMed1999

Commuting to high altitude. Recent studies of oxygen enrichment.

Further studies have been carried out on the potential value of oxygen enrichment of room air for commuters to high altitude. Novel ways of providing oxygen-enriched spaces for working and sleeping are being tested at the California Institute of Technology where a radiotelescope is being designed for an altitude of 5,000 m in north Chile. The modules are containers such as those used on container ships, and they are fitted out in California and then sealed and transported to the telescope site in Chile. The result is a turnkey facility which shows promise for field studies. The oxygen is provided by oxygen concentrators, and different modules are used for sleeping, living, and laboratory quarters. Two extensive experiments on oxygen enrichment were carried out at the University of California White Mountain Research Station, altitude 3,800 m, in the summer of 1998. The first study was devoted to the mechanism for the increase in arterial oxygen saturation on the day after sleeping in an oxygen-enriched atmosphere compared with sleeping in ambient air (5). Possible mechanisms include less fluid accumulation in the lung associated with acute mountain sickness, or a change in the control of ventilation. A double blind study was therefore carried out of the effects of sleeping in oxygen enrichment on both the ventilatory response to hypoxia and to carbon dioxide. In a related study, subjects who had been at an altitude of 3,800 m for two days, and were therefore partially acclimatized, were studied at a simulated altitude of 5,000 both breathing ambient air and 27% oxygen. The studies were done at 3800 m altitude by enriching the atmosphere of the test room with appropriate amounts of nitrogen or oxygen. An extensive series of neuropsychological tests were carried out with the objective of determining which features of CNS function were improved by oxygen enrichment at an altitude of 5,000 m.

Altitude↗