Arterial oxygen saturation in Tibetan and Han infants born in Lhasa, Tibet

Epidemiology, Risk Factors, and Genetics of High-Altitude–Related Pulmonary Disease

High-altitude-related pulmonary disease is a spectrum of acute and chronic illnesses with a well-described epidemiology. The risk for these illnesses is related to well-known environmental risk factors and lesser-known but important genetic factors. Prevention of acute high-altitude illness is possible in most visitors from lower elevations. Chronic high-altitude illnesses have an important worldwide impact.

Saturación arterial de oxígeno a gran altitud. Estudio en montañeros no aclimatados y en habitantes de alta montaña

BACKGROUND AND OBJECTIVE

We decided to determine how arterial oxygen saturation (SaO2) diminishes with altitude in unacclimatized mountaineers and in mountain dwellers.

SUBJECTS AND METHOD

Pulseoximetric measurements in unacclimatized mountaineers (214 measurements in several Spanish mountains and in the Alps up to 4,164 m) and in mountain dwellers (209 measurements in several Spanish and Bolivian villages up to 4,230 m). We performed pulseoximetric measurements for three consecutive days in eight mountaineers on the summit of Aneto (3,404 m) to ascertain whether SaO2 increases or not during early acclimatization.

RESULTS

Equations describing the SaO2 reduction with altitude are as follows: a) for unacclimatized mountaineers, SaO2 = 98.8183 - 0.0001.h - 0.000001.h2, b) for mountain dwellers, SaO2 = 98.2171 + 0.0012.h - 0.0000008.h2. (SaO2 in %; h: altitude in m. Lower limit of 95% confidence intervals given in the text). SaO2 of mountain dwellers is higher than that of unacclimatized mountaineers studied at the same altitude (p < 0.05 for any altitude over 1,692 m). SaO2 of mountaineers increased during early acclimatization (p < 0.05) to reach in few days the SaO2 of mountain dwellers. Unacclimatized mountaineers who spent the previous night over 2,000 m had higher SaO2 in altitude than those who slept under 2,000 m (p < 0.05). Mountaineers who performed any high-mountain activity (i.e. over 2,500 m) in the previous 12 months had higher SaO2 on the summit of Aneto than those who have never been over 2,500 m before (p < 0.05).

CONCLUSION

SaO2 increases during the acclimatization process. Our equations serve to calculate the SaO2 which can be considered normal for healthy people for every altitude below 4,200 m, both before and after the acclimatization process.

Birth weight among Tibetans at different altitudes in India: Are Tibetans better protected from IUGR?

We report the variation in birth weight among the Tibetans at different altitudes in India to test the hypothesis of greater protection from intrauterine growth retardation (IUGR) among Tibetan compared with other high-altitude native populations. We found that the birth weight of Tibetans at Leh (3521 m, high altitude) is quite similar to what has been reported previously for Tibetans at similar altitudes and is significantly higher than the low-altitude native populations living at similar altitudes. Tibetan birth weights are greater than those of other ethnic groups, both at high and low altitudes. Compared with Tibetans at high altitude (Leh, India; 3521 m), Tibetans at low altitudes (Bylakuppe, India; 800 m and Chandragiri, India; 970 m) have heavier birth weights. This finding is similar to what has been observed previously for other high-altitude native populations. Greater protection from IUGR is not observed for Tibetans compared with other high-altitude native populations as was reported previously. Genetic potential for birth weight is seemingly manifested only at low altitude.

The effect of chronic or intermittent hypoxia on cognition in childhood: a review of the evidence

OBJECTIVE

A review of the evidence concerning the effect of chronic or intermittent hypoxia on cognition in childhood was performed by using both a systematic review of the literature and critical appraisal criteria of causality. Because of the significant impact of behavioral disorders such as attention-deficit/hyperactivity disorder on certain cognitive functions as well as academic achievement, the review also included articles that addressed behavioral outcomes.

METHODS

Both direct and indirect evidence were collected. A structured Medline search was conducted from the years 1966-2000 by using the OVID interface. Both English- and non-English-language citations were included. Significant articles identified by the reviewers up to 2003 were also included. To be included as direct evidence, an article needed to be an original report in a peer-reviewed journal with data on cognitive, behavioral, or academic outcomes in children up to 14 years old, with clinical conditions likely to be associated with exposure to chronic or intermittent hypoxia. Indirect evidence from other reviews and publications in closely related fields, including experimental studies in adults, was used to help formulate conclusions. Two reviewers screened abstracts and titles. Each article included as direct evidence received a structured evaluation by 2 reviewers. Adjudication of differences was performed by a group of 2 reviewers and a research consultant. After this review, tables of evidence were constructed that were used as the basis for group discussion and consensus development. Indirect evidence assigned by topic to specific reviewers was also presented as part of this process. A formal procedure was used to rank the studies by design strength. The critical appraisal criteria for causation described in Evidence Based Pediatrics and Child Health (Moyer V, Elliott E, Davis R, et al, eds. London, United Kingdom: BMJ Books; 2000:46-55) were used to develop consensus on causality.

RESULTS

A total of 788 literature citations were screened. For the final analysis, 55 articles met the criteria for inclusion in the direct evidence. Of these, 43 (78.2%) reported an adverse effect. Of the 37 controlled studies, 31 (83.8%) reported an adverse effect. Adverse effects were noted at every level of arterial oxygen saturation and for exposure at every age level except for premature newborns. The studies were classified into 5 clinical categories: congenital heart disease (CHD), sleep-disordered breathing (SDB), asthma, chronic ventilatory impairment, and respiratory instability in infants. Two of these categories, CHD and SDB, which accounted for 42 (76.4%) of the included articles, fulfilled the Evidence Based Pediatrics and Child Health criteria for causation. The indirect evidence included 8 reviews, 1 meta-analysis, and 10 original reports covering the fields of adult anoxia, animal research, SDB in adults, natural and experimental high-altitude studies, perinatal hypoxic-ischemic encephalopathy, anemia, and carbon-monoxide poisoning. The studies of high-altitude and carbon-monoxide poisoning provided evidence for causality.

CONCLUSIONS

Adverse impacts of chronic or intermittent hypoxia on development, behavior, and academic achievement have been reported in many well-designed and controlled studies in children with CHD and SDB as well as in a variety of experimental studies in adults. This should be taken into account in any situation that may expose children to hypoxia. Because adverse effects have been noted at even mild levels of oxygen desaturation, future research should include precisely defined data on exposure to all levels of desaturation.

The effects of flight and altitude

Increasing numbers of infants and children journey by aeroplane, or travel to high altitude destinations, for example, on holiday or as part of a population migration. Most are healthy, although increasingly children may be transported by aeroplane or helicopter specifically to obtain treatment for severe illness or injury. It is therefore useful to review the effects of altitude, and their relevance to children who undertake flights or travel to, or at high altitudes, particularly those with acute and chronic medical conditions.

Syndromes of Subacute Mountain Sickness

Two clinical syndromes, acute and chronic mountain sickness, have traditionally been associated with high altitude. Recently, two separate entities of subacute nature have been described in infants and adults. In this paper, we review the published literature on these conditions. Subacute infantile mountain sickness is a condition seen predominantly in Han Chinese infants living in Tibet, although it has been described in other high altitude communities as well. It came into prominence only after the large-scale migration of Chinese population from the low altitude of mainland China to the high altitudes of the Qinghai-Tibetan plateau. The condition is characterized by features of severe hypoxic pulmonary hypertension and heart failure. Pulmonary histology is consistent with muscularization of the pulmonary arterioles, but no intimal proliferation or plexiform lesions are seen. The second syndrome, adult subacute mountain sickness, has been described almost exclusively in Indian soldiers living at extreme altitude for prolonged periods of time. In this condition also, hypoxic pulmonary hypertension appears to be the dominant factor responsible for severe congestive heart failure. Both these conditions have several similarities with brisket disease in cattle; hypoxic pulmonary vasoconstriction plays an important role in the pathogenesis, and removal from high altitude results in complete resolution. Thus, it appears that both these syndromes are human counterparts of brisket disease in cattle.

Iron deficiency and iron deficiency anaemia during infancy and childhood

The present definition of iron deficiency and iron deficiency anaemia is under debate. Our present figures for the prevalence of iron deficiency in infants and toddlers are thus disputed.

CONCLUSION

Better understanding of the regulation of iron metabolism and requirement during the critical early period of growth and development is needed for relevant control of iron deficiency anaemia.

Cardiopulmonary transition in the high altitude infant

The perinatal cardiopulmonary transition at high altitude differs from that at sea level because oxygen plays a fundamental role in the developmental changes from fetus to newborn infant. Under conditions of high altitude hypoxia, arterial oxygen saturations are lower, breathing patterns and maturation of respiratory control reflexes differ, and regression of fetal characteristics of the pulmonary vasculature proceeds more slowly. Several aspects of transition vary not only with postnatal age and altitude, but also with population group, suggesting an effect of genetic adaptation on perinatal physiology. Exposure to chronic high altitude hypoxia during the perinatal transition also results in apparent lifelong alterations in respiratory reflex responses and pulmonary vasoreactivity. Disruption of the normal process of cardiopulmonary transition can result in symptomatic high altitude pulmonary hypertension. The exaggerated hypoxemia associated with acute respiratory infections in young infants still undergoing transition contributes to infant mortality at high altitude.

Sleep structure and periodic breathing in Tibetans and Han at simulated altitude of 5000 m

Tibetans are the oldest population living permanently at high altitude. They possess several adaptations to low oxygen pressure that improve oxygen transport. We hypothesised that native Tibetans have mechanisms allowing them to maintain a better sleep structure and oxygenation during sleep at high altitude than newcomers from lower altitudes acclimatised to living at high altitude. We studied eight healthy young Tibetans, aged 26+/-7 years, and six healthy young Han aged 30.5+/-4 years. All subjects were living on the Tibetan plateau at an altitude of around 4000 m. Investigations were performed in Xining at an altitude of 2261 m, PB=581 mmHg. Two full polysomnographies (PSG) were performed in a hypobaric chamber, one at the ambient altitude, the second during acute exposure to the simulated altitude of 5000 m (PB=405 mmHg). Both PSG were done on the same night using split night design. At 2261 m no differences in sleep structure, breathing pattern during sleep or oxygenation were found, except a higher number of arousals and awakenings in Han (P<0.002). At 5000 m Tibetans had a longer sleep time (P=0.002), shorter stage 1 non-REM sleep (P<0.001) and longer stage 2 non-REM sleep than Han (P<0.001). Tibetans showed a trend to have more periodic breathing (PB) and higher mean arterial blood saturation than Han (NS). Our data suggest that Tibetans preserved better sleep structure and arterial blood oxygenation than Han during acute exposure to the simulated altitude of 5000 m.

Human physiological adaptation to pregnancy: inter- and intraspecific perspectives

Reproductive success requires successful maternal physiological adaptation to pregnancy. An interspecific perspective reveals that the human species has modified features of our haplorhine heritage affecting the uteroplacental circulation. We speculate that such modifications - including early implantation and deep, widespread invasion of fetal (trophoblast cells) into and resultant remodeling of maternal uterine vessels - are responses to or compensation for the biomechanical constraints imposed by bipedalism which, in turn, render our species susceptible to the pregnancy complication of preeclampsia. Preeclampsia is characterized by incomplete remodeling of maternal uterine vessels as the result of shallow trophoblast invasion, which in turn reduces uteroplacental blood flow and frequently leads to intrauterine growth restriction (IUGR). Using an intraspecific perspective, we consider the fitness-related consequences of variation in uteroplacental blood flow during high-altitude pregnancy. Although birth weights are reduced at high altitudes in Bolivia, multigenerational Andean residents are relatively protected from altitude-associated IUGR. Our preliminary data suggest that Andean women have greater uteroplacental oxygen delivery than European high-altitude residents due to more complete growth and remodeling of maternal uterine vessels. Identification of the physiological and genetic mechanisms involved in such inter- and intraspecific variations in pregnancy physiology will likely be useful for understanding human evolution and contemporary challenges to successful reproduction.

Characterization of high-altitude pulmonary hypertension in the Kyrgyz: association with angiotensin-converting enzyme genotype

Previous studies have suggested a genetic component in susceptibility to hypoxia-induced pulmonary hypertension. We therefore estimated the prevalence of high-altitude pulmonary hypertension (HAPH) in a Kyrgyz population and whether the insertion/deletion (I/D) polymorphism of the angiotensin-converting enzyme (ACE) gene associates with HAPH. An electrocardiographic survey of 741 highlanders demonstrated electrocardiogram signs of cor pulmonale in 14% of subjects. Pulmonary artery hemodynamics measured in an independent group of 136 male highlanders with symptoms of dyspnea at altitude revealed established pulmonary hypertension (mean pulmonary artery pressure [MPAP] > or = 25 mm Hg) in 20%. However, 26% of the normal subjects demonstrated an exaggerated response (twofold or greater increase in MPAP) to inhalation of 11% oxygen, and were classified as hyperresponsive. Ten-year follow-up of this group revealed increases in the MPAP, but not in normal subjects. Comparison of ACE I/D genotypes in the catheterized group revealed a threefold higher frequency of the I/I genotype in highlanders with HAPH, compared with normal highlanders (chi2 = 11.59, p = 0.003). In addition, MPAP was higher in highlanders with the I/I genotype (26.9 +/- 4.0 mm Hg) compared with the I/D genotype (20.6 +/- 1.2 mm Hg) or the D/D genotype (18.3 +/- 0.9 mm Hg) (p < 0.05). We conclude that HAPH is associated with ACE I/D genotype among Kyrgyz highlanders and the development of HAPH in this population and may be predicted by hyperresponsiveness to acute hypoxia.

Genetic and environmental adaptation in high altitude natives. Conceptual, methodological, and statistical concerns

A great number of physiological and anthropological studies have investigated Andean and Himalayan populations native to high altitude (HA). A non-scientific survey of the extant literature reveals a relatively liberal tradition of inferring genetic (evolutionary) adaptation to HA in these groups, often based on limited evidence and/or based on study designs insufficient to fully address the issue. Rather than review the evidence for or against genetic adaptation, and in order to provide some perspective, this paper will review relevant conceptual, methodological, and statistical issues that are germane to the study of HA native human groups. In particular, focus will be on the limitations of the most common research approach which bases evolutionary inference on the comparison of phenotypic mean differences between highland and lowland native populations. The migrant study approach is discussed, as is a relatively new approach based on genetic admixture in hybrid populations.

Are very low birth weight infants born at high altitude at greater risk for adverse outcomes?

OBJECTIVE

To determine if neonatal intensive care at higher altitudes was associated with any variation in mortality or morbidity.

METHODS

We reviewed demographic and outcome data on 5450 neonates with birth weights between 500 and 1500 g cared for in 76 different level II and III neonatal intensive care units (NICUs). The altitude break point of 4300 feet was prospectively chosen. Care was provided at 63 NICUs located below 4300 feet, "low-altitude," (n = 4534 neonates) and at 13 NICUs at or above 4300 feet, "high-altitude" (n = 916 neonates).

RESULTS

Compared with neonates cared for at low altitude, neonates cared for at high altitude were more often non-Hispanic white and exposed to prenatal steroids. Neonates born at high altitude were more often treated with surfactant (60% vs 53%, P <.01). At 28 days of age, neonates cared for at high altitude were less often in room air (33% vs 50%, P <.01) compared with neonates cared for at low altitude. However, when corrected for barometric pressure, the calculated partial pressure of inspired oxygen at 28 days of age was lower for neonates cared for at high altitude compared with low altitude (165 +/- 80 vs 183 +/- 57, P <.01). There were no differences in the rates of mortality, severe intraventricular hemorrhage (grades 3 and 4), severe retinopathy of prematurity (stages 3 and 4), or necrotizing enterocolitis requiring surgical treatment.

CONCLUSIONS

Being cared for at NICUs located above 4300 feet above sea level was not associated with any increase in adverse events compared with NICUs located below 4300 feet.

Oxygen saturation and heart rate in healthy school children and adolescents living at high altitude

This study was conducted to establish reference values for percent oxygen saturation of hemoglobin (SaO2, %) and heart rate (HR, bpm) in children living at high altitude (4,100 m) and to relate possible differences in the variables with ethnic origin. Healthy children from a mine-located school (Tintaya, n = 417), a nearby school (Marquiri, n = 474), and a rural Andean community (Nuñoa, n = 373) were investigated. The samples included different ethnic combinations, with the Nuñoa children having a predominant Quechua ancestry. Mean SaO2 for all ages was substantially lower in all high altitude children compared to values considered normal for sea level. Among the three samples, SaO2 was higher (91.3 +/- 2.7) and HR was lower (84.8 +/- 13.6) in Nuñoa than in Tintaya (SaO2, 89.8 +/- 2.5; HR, 91.7 +/- 14.9) and Marquiri (SaO2, 89.6 +/- 3.1; HR, 88.5 +/- 12.9) (P < 0.05). There was no sex difference and only a weak age-dependent trend for SaO2. Values considered abnormal at sea level were observed in all healthy high-altitude children. Higher SaO2 and lower HR in Nuñoa children may suggest a better degree of acclimatization to altitude.

Human genetic adaptation to high altitude

Some 140 million persons live permanently at high altitudes (>2500 m) in North, Central and South America, East Africa, and Asia. Reviewed here are recent studies which address the question as to whether genetic adaptation to high altitude has occurred. Common to these studies are the use of the oxygen transport system and the passage of time as organizing principles, and the recognition of the multifaceted ways in which genetic factors can influence physiological processes. They differ in terms of study approach and sources of evidence for judging duration of high altitude residence. Migrant, family set, and admixture study designs have been used for comparisons within populations. These collectively demonstrate the existence of genetic influences on physiological characteristics of oxygen transport. Differences in oxygen transport-related traits between Tibetan, Andean and European populations have been interpreted as having demonstrated the existence of genetic influences on high altitude adaptation but there is not consensus as to which groups are the best-adapted. Part of the controversy lies in the kinds of evidence used to assess duration of high altitude habitation. More other information is needed for a fuller appreciation of duration of residence and also features of population history (genetic drift, gene flow) but existing data are consistent with Tibetans having lived at high altitude longer than the other groups studied. Another issue surrounds usage of the term "adaptation." The definition should be based on evolutionary biology and physiological traits linked to indices of differential fertility and/or mortality. Two examples are developed to illustrate such linkages; intrauterine growth restriction (IUGR) at high altitude and the prevalence of Chronic Mountain Sickness (CMS). Interpopulational as well as intrapopulational variation exists in these conditions which appear linked to characteristics of oxygen transport. Both adversely influence survival and appear to be less severe (IUGR) or less common (CMS) among Tibetans than other groups. Thus available evidence suggest that Tibetans are better adapted. Needed, however, are studies which are better controlled for population ancestry, especially in South America, to determine the extent to which Tibetans differ from Andean highlanders. More precise information is also needed regarding the genetic factors underlying characteristics of oxygen transport. Such studies in Tibetan, Andean and Europeans as well as other high altitude populations offers a promising avenue for clarifying the adaptive value of physiological components of oxygen transport and the extent to which such factors differ among high altitude populations.

Limits on inferring genetic adaptation to high altitude in Himalayan and Andean populations

Many physiological and anthropological studies have investigated the unique Andean and Himalayan populations that have resided for many hundreds of generations at high altitude (HA). A nonscientific survey of the extant literature reveals a relatively liberal tradition of inferring genetic (evolutionary) adaptation to HA in these groups, often based on limited evidence and/or based on study designs insufficient to fully address the issue. In order to provide some perspective, I review relevant methodological issues that should be considered before evolutionary inference is made. On the whole, this paper takes a conservative stance and cautions against evolutionary inference based on the serious limitations of currently applied research approaches.

Tibetan protection from intrauterine growth restriction (IUGR) and reproductive loss at high altitude

Chronic hypoxia at high altitude restricts fetal growth, reducing birth weight and increasing infant mortality. We asked whether Tibetans, a long-resident high-altitude population, exhibit less altitude-associated intrauterine growth restriction (IUGR) and prenatal or postnatal reproductive loss than Han (ethnic Chinese), a group that has lived there for a shorter period of time. A population sample was obtained, comprising 485 deliveries to Tibetan or Han women over an 18-month period at 8 general hospitals or clinics located at 2,700-4,700 m in the Tibet Autonomous Region, China. Birth weight, gestational age, and other information were recorded for each delivery. Prenatal and postnatal mortality were calculated using information obtained from all pregnancies or babies born to study participants. Tibetan babies weighed more than the Han, averaging 310 g heavier at altitudes 2,700-3,000 m (95% CI = 126, 494 g; P < 0.01) and 530 g heavier at 3,000-3,800 m (210, 750 g; P < 0.01). More Han than Tibetan babies were born prematurely. Prenatal and postnatal mortality rose with increasing elevation and were 3-fold higher across all altitudes in the Han than the Tibetans (P < 0.05). Tibetans experience less altitude-associated IUGR than Han and have lower levels of prenatal and postnatal mortality. When the relationships between birth weight and altitude are compared among these and other high-altitude populations, those living at high altitude the longest have the least altitude-associated IUGR. This may suggest the occurrence of an evolutionary adaptation.

Pathophysiology of neonatal resuscitation: application in a global context

Despite the adoption of evidence-based guidelines for neonatal resuscitation, formulated with international consensus, the process of resuscitating a newly born infant remains a uniquely local activity. Variations in the physical environment, cultural and medical beliefs, and available resources mediate significant difference in practices worldwide. Yet, the universal nature of the physiology surrounding birth, and its disturbances, provides a common basis for reference. Recognition of the importance of assistance available at the moment of birth, management of the thermal environment, and establishment of adequate ventilation is nearly universal. Differences in specific practices arise from local differences in the risks and challenges to perinatal health, which, in turn, stem from the environment or the available resources. Valuable information can be learned through comparison and evaluation of different techniques. In such a way, the evidence base for neonatal resuscitation can be strengthened and infants around the world can share in the benefits realized.

Oxygen saturation increases during childhood and decreases during adulthood among high altitude native Tibetians residing at 3,800-4,200m

This report describes age differences in oxygen saturation throughout the life cycle in a sample of high altitude native Tibetans residing in villages at 3,800-4,200 m altitude in the Tibet Autonomous Region, China. Oxygen saturation of 3,812 Tibetans was measured by pulse oximetry and a subsample of 1,582 healthy, nonpregnant, nonsmokers from 1 week to 80 years of age was selected for analyses. Infants under 1 year of age had 5-6% lower oxygen saturation than the peak of 89.8% attained at 11 years of age. There was a steady increase in mean oxygen saturation-for-age during the first decade of life, but not during the second decade. Adult males exhibited a slight decrease starting in the 20-29 year age range. Adult females maintained the peak oxygen saturation through the 40-49 year age range, exhibiting a decrease in oxygen saturation beginning in the 50-59 year age range and as a result had higher oxygen saturation than males during the female reproductive span. Thus, developmental factors during infancy and childhood, but not adolescence, enhanced oxygen transfer in this high altitude native resident Tibetan sample. The age of onset of aging processes detrimental to oxygen transfer differed for females and males.

Comparative aspects of high-altitude adaptation in human populations

The conditions and duration of high-altitude residence differ among high-altitude populations. The Tibetan Plateau is larger, more geographically remote, and appears to have been occupied for a longer period of time than the Andean Altiplano and, certainly, the Rocky Mountain region as judged by archaeological, linguistic, genetic and historical data. In addition, the Tibetan gene pool is less likely to have been constricted by small numbers of initial migrants and/or severe population decline, and to have been less subject to genetic admixture with lowland groups. Comparing Tibetans to other high-altitude residents demonstrates that Tibetans have less intrauterine growth retardation better neonatal oxygenation higher ventilation and hypoxic ventilatory response lower pulmonary arterial pressure and resistance lower hemoglobin concentrations and less susceptibility to CMS These findings are consistent with the conclusion that "adaptation" to high altitude increases with time, considering time in generations of high-altitude exposure. Future research is needed to compare the extent of IUGR and neonatal oxygenation in South American high-altitude residents of Andean vs. European ancestry, controlling for gestational age and other characteristics. Another fruitful line of inquiry is likely to be determining whether persons with CMS or other altitude-associated problems experienced exaggerated hypoxia during prenatal or neonatal life. Finally, the comparison of high-altitude populations with respect to the frequencies of genes involved in oxygen sensing and physiologic response to hypoxia will be useful, once candidate genes have been identified.

Higher arterial oxygen saturation during submaximal exercise in Bolivian Aymara compared to European sojourners and Europeans born and raised at high altitude

Arterial oxygen saturation (SaO(2)) was measured at 3,600-3,850 m by pulse oximetry at rest and during submaximal exercise in three study groups: 1) highland Aymara natives of the Bolivian altiplano (n = 25); 2) lowland European/North American sojourners to the highlands with at least 2 months of acclimatization time to 3,600 m (n = 27); and 3) subjects of European ancestry born and raised at 3,600 m (n = 22). Aymara subjects maintained approximately 1 percentage point higher SaO(2) during submaximal work up to 70% of their maximal work capacity, and showed a smaller rate of decline in SaO(2) with increasing work compared to both European study groups. The higher-exercise SaO(2) of Aymara compared to Europeans born and raised at 3,600 m suggests genetic adaptation. The two European study groups, who differed by exposure to high altitude during their growth and development period, did not show any significant difference in either resting or exercise SaO(2). This suggests that the developmental mode of adaptation is less important than the genetic mode of adaptation in determining exercise SaO(2). A weak correlation was detected (across study groups only) between the residual forced vital capacity (FVC) and the residual SaO(2) measured at the highest level of submaximal work output (P = 0.024, R = 0.26). While firm conclusions based on this correlation are problematic, it is suggested that a part of the higher SaO(2) observed in Aymara natives is due to a larger lung volume and pulmonary diffusion capacity for oxygen. Results from this study are compared to similar studies conducted with Tibetan natives, and are interpreted in light of recent quantitative genetic analyses conducted in both the Andes and Himalayas.

Comparative human ventilatory adaptation to high altitude

Studies of ventilatory response to high altitudes have occupied an important position in respiratory physiology. This review summarizes recent studies in Tibetan high-altitude residents that collectively challenge the prior consensus that lifelong high-altitude residents ventilate less than acclimatized newcomers do as the result of acquired 'blunting' of hypoxic ventilatory responsiveness. These studies indicate that Tibetans ventilate more than Andean high-altitude natives residing at the same or similar altitudes (PET[CO(2)]) in Tibetans=29.6+/-0.8 vs. Andeans=31.0+/-1.0, P<0.0002 at approximately 4200 m), a difference which approximates the change that occurs between the time of acute hypoxic exposure to once ventilatory acclimatization has been achieved. Tibetans ventilate as much as acclimatized newcomers whereas Andeans ventilate less. However, the extent to which differences in hypoxic ventilatory response (HVR) are responsible is uncertain from existing data. Tibetans have an HVR as high as those of acclimatized newcomers whereas Andeans generally do not, but HVR is not consistently greater in comparisons of Tibetan versus Andean highland residents. Human and experimental animal studies demonstrate that inter-individual and genetic factors affect acute HVR and likely modify acclimatization and hyperventilatory response to high altitude. But the mechanisms responsible for ventilatory roll-off, hyperoxic hyperventilation, and acquired blunting of HVR are poorly understood, especially as they pertain to high-altitude residents. Developmental factors affecting neonatal arterial oxygenation are likely important and may vary between populations. Functional significance has been investigated with respect to the occurrence of chronic mountain sickness and intrauterine growth restriction for which, in both cases, low HVR seems disadvantageous. Additional studies are needed to address the various components of ventilatory control in native Tibetan, Andean and other lifelong high-altitude residents to decide the factors responsible for blunting HVR and diminishing ventilation in some native high-altitude residents.

Haplogroup-associated differences in neonatal death and incidence of low birth weight at elevation: a preliminary assessment

OBJECTIVE

We sought to assess reproductive fitness differences between mitochondrial deoxyribonucleic acid haplogroups at high altitude.

STUDY DESIGN

This study considers differences in outcomes of conception, birth weight, and neonatal mortality rates for 62 women classified according to haplogroups (B or non-B).

RESULTS

The number of low-weight births (<2500 g) for the non-B group was significant (P =.019). Mothers in the non-B group reported more spontaneous abortions (P =.171) and stillbirths (P =.301). The difference in conceptions per woman between groups was significant (P =.036). However, no difference in infants alive at 1 month of age was evident. Neonatal death was significant (P =.017). The odds of an unsuccessful outcome among mothers in the B group was compared with mothers in the non-B group and was significant (P =.029). The chance of an adverse outcome, that is, fetal or infant death before 1 month, for mothers in the B group was between 11.1% and 88.7% lower than for mothers in the non-B group.

CONCLUSIONS

The neonatal mortality rate for the non-B group was significantly elevated relative to the B group. The molecular basis for these observations is not clear.

Gender differentiation of the chemoreflex during growth at high altitude: functional and neurochemical studies

The effect of chronic hypoxia on gender differences in physiology and neurochemistry of chemosensory pathways was studied in prepubertal and adult rats living at sea level (SL; Lyon, France) or at high altitude (HA; La Paz, Bolivia, 3,600 m). HA adult rats had higher hematocrit (Ht%), Hb concentration, resting ventilatory rate (Ve(100)), and higher tyrosine hydroxylase (TH) activity in carotid bodies (CB) than SL animals. At HA and SL, adult females had lower Ht% (46.0 +/- 0.8 vs. 50.4 +/- 0.6% at HA, P < 0.05 and 43.8 +/- 0.9 vs. 47.1 +/- 0.8% at SL, P < 0.05) and Hb (16.1 +/- 0.3 vs. 17.7 +/- 0.2 g/dl at HA, P < 0.05 and 14.5 +/- 0.3 vs. 15.6 +/- 0.1 g/dl at SL, P < 0.05) than males. Females had higher Ve(100) [170 +/- 19 vs. 109 +/- 7 ml. min(-1). 100 g(-1) at HA, P < 0.05 and 50 +/- 3 vs. 40 +/- 2 ml. min(-1). 100 g(-1) at SL, not significant (NS)] and lower CB-TH activity (1.40 +/- 0.2 vs. 3.87 +/- 0.6 pmol/20 min at HA, P < 0.05 and 0.52 +/- 0.1 vs. 0.68 +/- 0.1 pmol/20 min at SL; NS) than males at HA only. The onset of hypoxic ventilatory response during development was delayed at HA. Prepubertal HA females had higher Ve(100) than males (2 wk old, +47%) and higher CB-TH activity (3 wk old, +51%). Medullary noradrenergic groups were sex dimorphic during development at SL. Rats raised at HA had a drop of TH activity between the second and the third postnatal week in all medullary groups. In conclusion, our data support the hypothesis that the CB is the major site for sexual differentiation of the ventilatory control. Ventilatory differences appeared before puberty, and the animals bred at HA had profound alterations in the developmental process of the chemoreflex and its neural pathways. Some of these alterations are under dependence of the sex of the animal, and there is an important interaction between gender and the hypoxic environmental condition during the developmental period.

Hematological and pulmonary responses to high altitude in Quechuas: a multivariate approach

This study investigates the relationships among hematological variables, pulmonary function, and age in a sample of high-altitude natives. The following anthropometric and physiological variables were examined in 77 adult Quechua males from the Peruvian Central Andes (Huancavelica, 3,680 m): height, weight, sitting height, chest diameters, chest and abdominal circumferences, forced vital capacity (FVC), forced expiratory volume at 1 sec (FEV1), peak expiratory flow (PEF), hemoglobin concentration (Hb), red blood cells (RBC), hematocrit (Htc), diastolic and systolic blood pressure, body temperature, pulmonary rate, and pulse rate. The means of these variables for the Huancavelica sample fall within the range of variability previously observed in Andean populations. Principal components analysis and canonical correlation analysis suggest that in this native Andean population: 1) aging decreases lung function but does not affect hematological features, and 2) there is a negative age-independent correlation between lung function (FVC, FEV1, PEF) and hematological traits (Hb, RBC, Htc).

Human adaptation to high altitude: regional and life-cycle perspectives

Studies of the ways in which persons respond to the adaptive challenges of life at high altitude have occupied an important place in anthropology. There are three major regions of the world where high-altitude studies have recently been performed: the Himalayas of Asia, the Andes of South America, and the Rocky Mountains of North America. Of these, the Himalayan region is larger, more geographically remote, and likely to have been occupied by humans for a longer period of time and to have been subject to less admixture or constriction of its gene pool. Recent studies of the physiological responses to hypoxia across the life cycle in these groups reveal several differences in adaptive success. Compared with acclimatized newcomers, lifelong residents of the Andes and/or Himalayas have less intrauterine growth retardation, better neonatal oxygenation, and more complete neonatal cardiopulmonary transition, enlarged lung volumes, decreased alveolar-arterial oxygen diffusion gradients, and higher maximal exercise capacity. In addition, Tibetans demonstrate a more sustained increase in cerebral blood flow during exercise, lower hemoglobin concentration, and less susceptibility to chronic mountain sickness (CMS) than acclimatized newcomers. Compared to Andean or Rocky Mountain high-altitude residents, Tibetans demonstrate less intrauterine growth retardation, greater reliance on redistribution of blood flow than elevated arterial oxygen content to increase uteroplacental oxygen delivery during pregnancy, higher levels of resting ventilation and hypoxic ventilatory responsiveness, less hypoxic pulmonary vasoconstriction, lower hemoglobin concentration, and less susceptibility to CMS. Several of the distinctions demonstrated by Tibetans parallel the differences between natives and newcomers, suggesting that the degree of protection or adaptive benefit relative to newcomers is enhanced for the Tibetans. We thus conclude that Tibetans have several physiological distinctions that confer adaptive benefit consistent with their probable greater generational length of high-altitude residence. Future progress is anticipated in achieving a more integrated view of high-altitude adaptation, incorporating a sophisticated understanding of the ways in which levels of biological organization are articulated and a recognition of the specific genetic variants contributing to differences among high-altitude groups.

Percent of oxygen saturation of arterial hemoglobin among Bolivian Aymara at 3,900-4,000 m

A range of variation in percent of oxygen saturation of arterial hemoglobin (SaO2) among healthy individuals at a given high altitude indicates differences in physiological hypoxemia despite uniform ambient hypoxic stress. In populations native to the Tibetan plateau, a significant portion of the variance is attributable to additive genetic factors, and there is a major gene influencing SaO2. To determine whether there is genetic variance in other high-altitude populations, we designed a study to test the hypothesis that additive genetic factors contribute to phenotypic variation in SaO2 among Aymara natives of the Andean plateau, a population geographically distant from the Tibetan plateau and with a long, separate history of high-altitude residence. The average SaO2 of 381 Aymara at 3,900-4,000 m was 92+/-0.15% (SEM) with a range of 84-99%. The average was 2.6% higher than the average SaO2 of a sample of Tibetans at 3,800-4,065 m measured with the same techniques. Quantitative genetic analyses of the Aymara sample detected no significant variance attributable to genetic factors. The presence of genetic variance in SaO2 in the Tibetan sample and its absence in the Aymara sample indicate there is potential for natural selection on this trait in the Tibetan but not the Aymara population.

Living at high altitude and risk of sudden infant death syndrome

OBJECTIVE

To investigate the association between altitude of residence and risk of sudden infant death syndrome (SIDS).

METHODS

A retrospective, case control study in the Tyrol, Austria enrolled 99 infants with SIDS occurring between 1984 and 1994, and 136 randomly selected control cases. Data on pregnancy, delivery, child care practice, and socio-demographic characteristics including altitude of residence were collected with a standardised questionnaire.

RESULTS

The risk of SIDS increased gradually with increasing altitude of residence. This relation remained independently significant when the analysis was adjusted for gestational age, birth weight, prenatal care, mother's age at delivery, educational level of parents, and cigarette smoking during pregnancy. The prone sleeping position emerged as an obligatory cofactor in this association. In the whole of Austria, a similar trend of association emerged between the average altitudes in the 99 political counties and the rates of SIDS.

CONCLUSIONS

This study identified altitude of residence as a significant risk predictor of SIDS, primarily in combination with the prone sleeping position. Respiratory disturbances, reduced oxygen saturation, and lower temperatures at high altitude might explain this association.

Response to cooling temperature in infants born at an altitude of 4,330 meters

The metabolic response to reduction in ambient temperature was studied in healthy, full-term, 1-d-old infants in Lima (50 m altitude, n = 20) and Cerro de Pasco (4,330 m, barometric pressure approximately 450 mm Hg, n = 20), Peru. Oxygen consumption (V O2) and carbon dioxide production (V CO2) were measured with an open-flow system as each infant rested quietly in a cylindrical humicrib, at wall temperatures of 35 degrees C (warm) and 26 degrees C (cool). The infants were exposed for 20 min to both temperatures, with the higher temperature followed by the lower, and oxygen consumption (V O2) and carbon dioxide production (V CO2) were measured over the last 8 min of each exposure. Average birth weight in Cerro de Pasco (2,933 +/- 77 g [mean +/- SE]) was less than in Lima (3,457 +/- 73 g). In warm conditions, infants born at high altitude had slightly yet significantly lower body and skin temperatures than did those born at low altitude, with similar values of V O2 and heart rate (HR). Neither body nor skin temperature changed in either group during cooling. At low altitude, cooling increased V O2 ( approximately 34%), whereas no significant increase occurred in the high-altitude group. A similar response occurred for HR. Among several possibilities, the most likely interpretation of the results would be that of a decreased thermogenic capacity in the high-altitude infants because of the correspondingly lower oxygen availability during gestation.

Normoxic ventilatory response in lowlander and Sherpa elite climbers

The differences in ventilatory response to exercise of some highland ethnic communities is a controversial issue. We have evaluated the differences in ventilatory response to exercise at sea level between two groups of elite climbers, four Himalayan Sherpas (S) and four Caucasian lowlanders (C), after descent from extreme altitude. All of them performed a progressive-intensity exercise test on a treadmill under normoxic conditions. Pulmonary gas exchange was obtained until exhaustion by means of an automatic gas-analyzer system. Significant differences in expired ventilation and carbon dioxide production were found between the two groups, the VE x VO2(-1) being lower in the S at rest (41.9 +/- 5) in comparison with C (48.7 +/- 9) (P < 0.05), higher at medium loads of the test (S = 28.2 +/- 4 vs. C = 25.7 +/- 2; P < 0.05) and reaching similar values at higher loads (S = 34.5 + 2 vs. C = 35.6 +/- 4; NS). We conclude that the special ventilatory response observed in these highlanders could explain their adaptation to altitude, allowing higher oxygen blood saturation at medium working loads and reducing the risk of neurological injury caused by a high ventilatory response when exercising at high intensity effort under extreme altitude environment.

Reference values for pulse oximetry at high altitude

OBJECTIVE

To determine reference values for oxygen saturation (Sao2) in healthy children younger than 5 years living at high altitude.

DESIGN

One hundred and sixty eight children were examined for Sao2 at 4018 m during well child visits. Physiological state was also noted during the examination.

RESULTS

The mean Sao2 was 87.3% (95% confidence intervals (CI) 86.7%, 87.9%) with a median value of 87.7%. A significant difference was observed in Sao2 between children younger than 1 year compared with older children, although the difference was no longer demonstrable when sleeping children were excluded.

CONCLUSIONS

This study has provided a reference range of Sao2 values for healthy children under 5 years old so that pulse oximetry may be used as an adjunct in diagnosing acute respiratory infections. Younger children were also shown to have a lower mean Sao2 than older children living at high altitude, which suggests physiological adaptation to high altitude over time. In addition, sleep had a lowering effect on Sao2, although the clinical importance of this remains undetermined.

Effect of exposure to 15% oxygen on breathing patterns and oxygen saturation in infants: interventional study

OBJECTIVE

To assess the response of healthy infants to airway hypoxia (15% oxygen in nitrogen).

DESIGN

Interventional study.

SETTINGS

Infants' homes and paediatric ward.

SUBJECTS

34 healthy infants (20 boys) born at term; mean age at study 3.1 months. 13 of the infants had siblings whose deaths had been ascribed to the sudden infant death syndrome.

INTERVENTION

Respiratory variables were measured in room air (pre-challenge), while infants were exposed to 15% oxygen (challenge), and after infants were returned to room air (post-challenge).

MAIN OUTCOME MEASURES

Baseline oxygen saturation as measured by pulse oximetry, frequency of isolated and periodic apnoea, and frequency of desaturation (oxygen saturation < or = 80% for > or = 4 s). Exposure to 15% oxygen was terminated if oxygen saturation fell to < or = 80% for > or = 1 min.

RESULTS

Mean duration of exposure to 15% oxygen was 6.3 (SD 2.9) hours. Baseline oxygen saturation fell from a median of 97.6% (range 94.0% to 100%) in room air to 92.8% (84.7% to 100%) in 15% oxygen. There was no correlation between baseline oxygen saturation in room air and the extent of the fall in baseline oxygen saturation on exposure to 15% oxygen. During exposure to 15% oxygen there was a reduction in the proportion of time spent in regular breathing pattern and a 3.5-fold increase in the proportion of time spent in periodic apnoea (P < 0.001). There was an increase in the frequency of desaturation from 0 episodes per hour (range 0 to 0.2) to 0.4 episodes per hour (0 to 35) (P < 0.001). In 4 infants exposure to hypoxic conditions was ended early because of prolonged and severe falls in oxygen saturation.

CONCLUSIONS

A proportion of infants had episodes of prolonged (< or = 80% for > or = 1 min) or recurrent shorter (< or = 80% for > or = 4 s) desaturation, or both, when exposed to airway hypoxia. The quality and quantity of this response was unpredictable. These findings may explain why some infants with airway hypoxia caused by respiratory infection develop more severe hypoxaemia than others. Exposure to airway hypoxia similar to that experienced during air travel or on holiday at high altitude may be harmful to some infants.

Superior exercise performance in lifelong Tibetan residents of 4,400 m compared with Tibetan residents of 3,658 m

Few environments challenge human populations more than high altitude, since the accompanying low oxygen pressures (hypoxia) are pervasive and impervious to cultural modification. Work capacity is an important factor in a population's ability to thrive in such an environment. The performance of work or exercise is a measure of the integrated functioning of the O2 transport system, with maximal O2 uptake (.VO2max) a convenient index of that function. Hypoxia limits the ability to transport oxygen: maximal O2 uptake decreases with ascent to high altitude, and years of high altitude residence do not restore sea level .VO2max values. Since Tibetans live and work at some of the highest altitudes in the world, their ability to exercise at very high altitude (>4,000 m) may define the limits of human adaptation to hypoxia. We transported 20 Tibetan lifelong residents of > or =4,400 m down to 3,658 m in order to compare them with 16 previously studied Tibetan residents of Lhasa (3,658 m). The two groups of Tibetans were matched for age, weight, and height. All studies were performed in Lhasa within 3 days of the 4,400 m Tibetans' arrival. Standard test protocol and criteria were used for attaining .VO2max on a Monark bicycle ergometer, while measuring oxygen uptake (.VO2, ml/kg - min STPD), heart rate (bpm), minute ventilation (VE, 1/min BTPS), and arterial oxygen saturation (SaO2, %). The 4,400 m compared with 3,658 m residents had, at maximal effort, similar .VO2 (48.5 +/- 1.2 vs. 51.2 +/- 1.4 ml/kg - min, P = NS), higher workload attained (211 +/- 6 vs. 177 +/- 7 watts, P < 0.01), lower heart rate(176 +/- 2 vs. 191 +/- 2 bpm, P < 0.01), lower ventilation (127 +/- 5 vs. 149 +/- 5 l/min BTPS, P < 0.01), and similar SaO2(81.9 +/- 1.0 vs. 83.7 +/- 1.2%, P = NS). Furthermore, over the range of submaximal workloads, 4,400 m compared with 3,658 m Tibetans had lower .VO2 (P < 0.01), lower heart rates (P < 0.01), and lower ventilation (P < 0.01) and SaO2 (P < 0.05). We conclude that Tibetans living at 4,400 m compared with those residing at 3,658 m achieve greater work performance for a given .VO2 at submaximal and maximal workloads with less cardiorespiratory effort.

Human adaptation to high altitude: regional and life-cycle perspectives

Studies of the ways in which persons respond to the adaptive challenges of life at high altitude have occupied an important place in anthropology. There are three major regions of the world where high-altitude studies have recently been performed: the Himalayas of Asia, the Andes of South America, and the Rocky Mountains of North America. Of these, the Himalayan region is larger, more geographically remote, and likely to have been occupied by humans for a longer period of time and to have been subject to less admixture or constriction of its gene pool. Recent studies of the physiological responses to hypoxia across the life cycle in these groups reveal several differences in adaptive success. Compared with acclimatized newcomers, lifelong residents of the Andes and/or Himalayas have less intrauterine growth retardation, better neonatal oxygenation, and more complete neonatal cardiopulmonary transition, enlarged lung volumes, decreased alveolar-arterial oxygen diffusion gradients, and higher maximal exercise capacity. In addition, Tibetans demonstrate a more sustained increase in cerebral blood flow during exercise, lower hemoglobin concentration, and less susceptibility to chronic mountain sickness (CMS) than acclimatized newcomers. Compared to Andean or Rocky Mountain high-altitude residents, Tibetans demonstrate less intrauterine growth retardation, greater reliance on redistribution of blood flow than elevated arterial oxygen content to increase uteroplacental oxygen delivery during pregnancy, higher levels of resting ventilation and hypoxic ventilatory responsiveness, less hypoxic pulmonary vasoconstriction, lower hemoglobin concentration, and less susceptibility to CMS. Several of the distinctions demonstrated by Tibetans parallel the differences between natives and newcomers, suggesting that the degree of protection or adaptive benefit relative to newcomers is enhanced for the Tibetans. We thus conclude that Tibetans have several physiological distinctions that confer adaptive benefit consistent with their probable greater generational length of high-altitude residence. Future progress is anticipated in achieving a more integrated view of high-altitude adaptation, incorporating a sophisticated understanding of the ways in which levels of biological organization are articulated and a recognition of the specific genetic variants contributing to differences among high-altitude groups.

Ventilation and hypoxic ventilatory responsiveness in Chinese-Tibetan residents at 3,658 m

When breathing ambient air at rest at 3,658 m altitude, Tibetan lifelong residents of 3,658 m ventilate as much as newcomers acclimatized to high altitude; they also ventilate more and have greater hypoxic ventilatory responses (HVRs) than do Han ("Chinese") long-term residents at 3,658 m. This suggests that Tibetan ancestry is advantageous in protecting resting ventilation levels during years of hypoxic exposure and is of interest in light of the permissive role of hypoventilation in the development of chronic mountain sickness, which is nearly absent among Tibetans. The existence of individuals with mixed Tibetan-Chinese ancestry (Han-Tibetans) residing at 3,658 m affords an opportunity to test this hypothesis. Eighteen men born in Lhasa, Tibet, China (3,658 m) to Tibetan mothers and Han fathers were compared with 27 Tibetan men and 30 Han men residing at 3,658 m who were previously studied. We used the same study procedures (minute ventilation was measured with a dry-gas flowmeter during room air breathing and hyperoxia and with a 13-liter spirometer-rebreathing system during the hypoxic and hypercapnic tests). During room air breathing at 3,658 m (inspired O2 pressure = 93 Torr), Han-Tibetans resembled Tibetans in ventilation (12.1 +/- 0.6 vs. 11.5+/- 0.5 l/min BTPS, respectively) but had HVR that were blunted (63 +/- 16 vs. 121 +/- 13, respectively, for HVR shape parameter A) and declined with increasing duration of high-altitude residence. During administered hyperoxia (inspired O2 pressure = 310 Torr) at 3,658 m, the paradoxical hyperventilation previously seen in Tibetan but not Han residents at 3,658 m (11.8 +/- 0.5 vs. 10.1 +/- 0.5 l/min BTPS) was absent in these Han-Tibetans (9.8 +/- 0.6 l/min BTPS). Thus, although longer duration of high-altitude residence appears to progressively blunt HVR among Han-Tibetans born and residing at 3, 658 m, their Tibetan ancestry appears protective in their maintenance of high resting ventilation levels despite diminished chemosensitivity.

Physiological implications of altitude training for endurance performance at sea level: a review

Acclimatisation to environmental hypoxia initiates a series of metabolic and musculocardio-respiratory adaptations that influence oxygen transport and utilisation, or better still, being born and raised at altitude, is necessary to achieve optimal physical performance at altitude, scientific evidence to support the potentiating effects after return to sea level is at present equivocal. Despite this, elite athletes continue to spend considerable time and resources training at altitude, misled by subjective coaching opinion and the inconclusive findings of a large number of uncontrolled studies. Scientific investigation has focused on the optimisation of the theoretically beneficial aspects of altitude acclimatisation, which include increases in blood haemoglobin concentration, elevated buffering capacity, and improvements in the structural and biochemical properties of skeletal muscle. However, not all aspects of altitude acclimatisation are beneficial; cardiac output and blood flow to skeletal muscles decrease, and preliminary evidence has shown that hypoxia in itself is responsible for a depression of immune function and increased tissue damage mediated by oxidative stress. Future research needs to focus on these less beneficial aspects of altitude training, the implications of which pose a threat to both the fitness and the health of the elite competitor. Paul Bert was the first investigator to show that acclimatisation to a chronically reduced inspiratory partial pressure of oxygen (P1O2) invoked a series of central and peripheral adaptations that served to maintain adequate tissue oxygenation in healthy skeletal muscle, physiological adaptations that have been subsequently implicated in the improvement in exercise performance during altitude acclimatisation. However, it was not until half a century later that scientists suggested that the additive stimulus of environmental hypoxia could potentially compound the normal physiological adaptations to endurance training and accelerate performance improvements after return to sea level. This has stimulated an exponential increase in scientific research, and, since 1984, 22 major reviews have summarised the physiological implications of altitude training for both aerobic and anaerobic performance at altitude and after return to sea level. Of these reviews, only eight have specifically focused on physical performance changes after return to sea level, the most comprehensive of which was recently written by Wolski et al. Few reviews have considered the potentially less favourable physiological responses to moderate altitude exposure, which include decreases in absolute training intensity, decreased plasma volume, depression of haemopoiesis and increased haemolysis, increases in sympathetically mediated glycogen depletion at altitude, and increased respiratory muscle work after return to sea level. In addition, there is a risk of developing more serious medical complications at altitude, which include acute mountain sickness, pulmonary oedema, cardiac arrhythmias, and cerebral hypoxia. The possible implications of changes in immune function at altitude have also been largely ignored, despite accumulating evidence of hypoxia mediated immunosuppression. In general, altitude training has been shown to improve performance at altitude, whereas no unequivocal evidence exists to support the claim that performance at sea level is improved. Table 1 summarises the theoretical advantages and disadvantages of altitude training for sea level performance. This review summarises the physiological rationale for altitude training as a means of enhancing endurance performance after return to sea level. Factors that have been shown to affect the acclimatisation process and the subsequent implications for exercise performance at sea level will also be discussed. Studies were located using five major database searches, which included Medline, Embase, Science Citation Index, Sports Discus, and Sport, in