Hypocapnia and sustained hypoxia blunt ventilation on arrival at high altitude

Swallow-breathing coordination during incremental ascent to altitude

Swallow and breathing are highly coordinated behaviors reliant on shared anatomical space and neural pathways. Incremental ascent to high altitudes results in hypoxia/hypocapnic conditions altering respiratory drive, however it is not known whether these changes also alter swallow. We examined the effect of incremental ascent (1045 m, 3440 m and 4371 m) on swallow motor pattern and swallow-breathing coordination in seven healthy adults. Submental surface electromyograms (sEMG) and spirometry were used to evaluate swallow triggered by saliva and water infusion. Swallow-breathing phase preference was different between swallows initiated by saliva versus water. With ascent, saliva swallows changed to a dominate pattern of occurrence during the transition from inspiration to expiration. Additionally, water swallows demonstrated a significant decrease in submental sEMG duration and a shift in submental activity to earlier in the apnea period, especially at 4371 m. Our results suggest that there are changes in swallow-breathing coordination and swallow production that likely increase airway protection with incremental ascent to high altitude. The adaptive changes in swallow were likely due to the exposure to hypoxia and hypocapnia, along with airway irritation.

Ventilatory acclimatisation is beneficial for high-intensity exercise at altitude in elite cyclists

AIM

The aim of this study was to examine the relationship between ventilatory adaptation and performance during altitude training at 2700 m.

METHODS

Seven elite cyclists (age: 21.2 ± 1.1 yr, body mass: 69.9 ± 5.6 kg, height 176.3 ± 4.9 cm) participated in this study. A hypoxic ventilatory response (HVR) test and a submaximal exercise test were performed at sea level prior to the training camp and again after 15 d at altitude (ALT15). Ventilation (VE), end-tidal carbon-dioxide partial pressure (PETCO2) and oxyhaemoglobin saturation via pulse oximetry (SpO2) were measured at rest and during submaximal cycling at 250 W. A hill climb (HC) performance test was conducted at sea level and after 14 d at altitude (ALT14) using a road of similar length (5.5-6 km) and gradient (4.8-5.3%). Power output was measured using SRM cranks. Average HC power at ALT14 was normalised to sea level power (HC%). Multiple regression was used to identify significant predictors of performance at altitude.

RESULTS

At ALT15, there was a significant increase in resting VE (10.3 ± 1.9 vs. 12.2 ± 2.4 L·min(-1)) and HVR (0.34 ± 0.24 vs. 0.71 ± 0.49 L·min(-1)·%(-1)), while PETCO2 (38.4 ± 2.3 vs. 32.1 ± 3.3 mmHg) and SpO2 (97.9 ± 0.7 vs. 94.0 ± 1.7%) were reduced (P < .05). Multiple regression revealed ΔHVR and exercise VE at altitude as significant predictors of HC% (adjusted r(2) = 0.913; P = 0.003).

CONCLUSIONS

Ventilatory acclimatisation occurred during a 2 wk altitude training camp in elite cyclists and a higher HVR was associated with better performance at altitude, relative to sea level. These results suggest that ventilatory acclimatisation is beneficial for cycling performance at altitude.

Differences in the control of breathing between Himalayan and sea-level residents

We compared the control of breathing of 12 male Himalayan highlanders with that of 21 male sea-level Caucasian lowlanders using isoxic hyperoxic ( = 150 mmHg) and hypoxic ( = 50 mmHg) Duffin's rebreathing tests. Highlanders had lower mean +/- s.e.m. ventilatory sensitivities to CO(2) than lowlanders at both isoxic tensions (hyperoxic: 2.3 +/- 0.3 vs. 4.2 +/- 0.3 l min(1) mmHg(1), P = 0.021; hypoxic: 2.8 +/- 0.3 vs. 7.1 +/- 0.6 l min(1) mmHg(1), P < 0.001), and the usual increase in ventilatory sensitivity to CO(2) induced by hypoxia in lowlanders was absent in highlanders (P = 0.361). Furthermore, the ventilatory recruitment threshold (VRT) CO(2) tensions in highlanders were lower than in lowlanders (hyperoxic: 33.8 +/- 0.9 vs. 48.9 +/- 0.7 mmHg, P < 0.001; hypoxic: 31.2 +/- 1.1 vs. 44.7 +/- 0.7 mmHg, P < 0.001). Both groups had reduced ventilatory recruitment thresholds with hypoxia (P < 0.001) and there were no differences in the sub-threshold ventilations (non-chemoreflex drives to breathe) between lowlanders and highlanders at both isoxic tensions (P = 0.982), with a trend for higher basal ventilation during hypoxia (P = 0.052). We conclude that control of breathing in Himalayan highlanders is distinctly different from that of sea-level lowlanders. Specifically, Himalayan highlanders have decreased central and absent peripheral sensitivities to CO(2). Their response to hypoxia was heterogeneous, with the majority decreasing their VRT indicating either a CO(2)-independent increase in activity of peripheral chemoreceptor or hypoxia-induced increase in [H(+)] at the central chemoreceptor. In some highlanders, the decrease in VRT was accompanied by an increase in sensitivity to CO(2), while in others VRT remained unchanged and their sub-threshold ventilations increased, although these were not statistically significant.

Effects of intermittent hypoxic training on cycling performance in well-trained athletes

This study aimed to investigate the effects of a short-term period of intermittent hypoxic training (IHT) on cycling performance in athletes. Nineteen participants were randomly assigned to two groups: normoxic (NT, n = 9) and intermittent hypoxic training group (IHT, n = 10). A 3-week training program (5 x 1 h-1 h 30 min per week) was completed. Training sessions were performed in normoxia (approximately 30 m) or hypoxia (simulated altitude of 3,000 m) for NT and IHT group, respectively. Each subject performed before (W0) and after (W4) the training program, three cycling tests including an incremental test to exhaustion in normoxia and hypoxia for determination of maximal aerobic power (VO2max) and peak power output (PPO) as well as a 10-min cycle time trial in normoxia (TT) to measure the average power output (P(aver)). No significant difference in VO2max was observed between the two training groups before or after the training period. When measured in normoxia, the PPO significantly increased (P < 0.05) by 7.2 and 6.6% in NT and IHT groups, respectively. However, only the IHT group significantly improved (11.3%; P < 0.05) PPO when measured in hypoxia. The NT group improved (P < 0.05) P(aver) in TT by 8.1%, whereas IHT group did not show any significant difference. Intermittent training performed in hypoxia was less efficient for improving endurance performance at sea level than similar training performed in normoxia. However, IHT has the potential to assist athletes in preparation for competition at altitude.

Population genetic aspects and phenotypic plasticity of ventilatory responses in high altitude natives

Highland natives show unique breathing patterns and ventilatory responses at altitude, both at rest and during exercise. For many ventilatory traits, there is also significant variation between highland native groups, including indigenous populations in the Andes and Himalaya, and more recent altitude arrivals in places like Colorado. This review summarizes the literature in this area with some focus on partitioning putative population genetic differences from differences acquired through lifelong exposure to hypoxia. Current studies suggest that Tibetans have high resting ventilation (V (E)), and a high hypoxic ventilatory response (HVR), similar to altitude acclimatized lowlanders. Andeans, in contrast, show low resting V (E) and a low or "blunted" HVR, with little evidence that these traits are acquired via lifelong exposure. Resting V (E) of non-indigenous altitude natives is not well documented, but lifelong hypoxic exposure almost certainly blunts HVR in these groups through decreased chemosensitivity to hypoxia in a process known as hypoxic desensitization (HD). Together, these studies suggest that the time course of ventilatory response, and in particular the origin or absence of HD, depends on population genetic background i.e., the allele or haplotype frequencies that characterize a particular population. During exercise, altitude natives have lower V (E) compared to acclimatized lowland controls. Altitude natives also have smaller alveolar-arterial partial pressure differences P(AO2) - P(aO2) during exercise suggesting differences in gas exchange efficiency. Small P(AO2) - P(aO2) in highland natives of Colorado underscores the likely importance of developmental adaptation to hypoxia affecting structural/functional aspects of gas exchange with resultant changes in breathing pattern. However, in Andeans, at least, there is also evidence that low exercise V (E) is determined by genetic background affecting ventilatory control independent of gas exchange. Additional studies are needed to elucidate the effects of gene, environment, and gene-environment interaction on these traits, and these effects are likely to differ widely between altitude native populations.

Respiratory control in residents at high altitude: physiology and pathophysiology

Highland population (HA) from the Andes, living above 3000 m, have a blunted ventilatory response to increasing hypoxia, breathe less compared to acclimatized newcomers, but more, compared to sea-level natives at sea level. Subjects with chronic mountain sickness (CMS) breathe like sea-level natives and have excessive erythrocytosis (EE). The respiratory stimulation that arises through the peripheral chemoreflex is modestly less in the CMS group when compared with the HA group at the same P(ET(O2)). With regard to CO(2) sensitivity, CMS subjects seem to have reset their central CO(2) chemoreceptors to operate around the sea-level resting P(ET(CO2)). Acetazolamide, an acidifying drug that increases the chemosensitivity of regions in the brain stem that contain CO(2)/H(+) sensitive neurons, partially reverses this phenomenon, thus, providing CMS subjects with the possibility to have high CO(2) changes, despite small changes in ventilation. However, the same type of adjustments of the breathing pattern established for Andeans has not been found necessarily in Asian humans and/or domestic animals nor in the various high altitude species studied. The differing time frames of exposure to hypoxia among the populations, as well as the reversibility of the different components of the respiratory process at sea level, provide key concepts concerning the importance of time at high altitude in the evolution of an appropriate breathing pattern.

Sustained Acclimatization in Chilean Mine Workers Subjected to Chronic Intermittent Hypoxia

Farias, Jorge G., Jorge Osorio, Gustavo Soto, Julio Brito, Patricia Siques, and Juan G. Reyes. Sustained acclimatization in Chilean mine workers subjected to chronic intermittent hypoxia. High Alt. Med. Biol. 7:302-306, 2006--We wanted to know if sea-level mine workers exposed previously to chronic intermittent hypoxia reached a steady acclimatization at 36 months under hypobaric hypoxia. An intermittently exposed group of mine workers (IE, n = 25) were subjected to submaximal exercise (100 W) at 4500 m. Their systolic blood pressure (SBP), diastolic blood pressure (DBP), heart rate (HR), and hemoglobin oxygen saturation (HbSatO(2)) were monitored. Two comparison groups of unacclimatized sea-level workers (n = 17) were studied. A nonexposed group (NE) performed 5 min of submaximal exercise at sea level. Some kind of exercise was performed both by an acutely exposed group (AE) and IE group at 4500 m. No statistical differences were found for HR, SBP, and DBP (p > 0.05) during exercise between IE and AE groups. Resting HbSatO(2) of IE (87 +/- 6%) was lower than NE (97 +/- 3%) (p < 0.05), but was higher than AE (82 +/- 4%) (p < 0.05). In the exercise condition, HbSatO(2) of IE (85 +/- 5%) was lower than NE (95 +/- 3%) (p < 0.05), but was higher than AE (76 +/- 2%) (p < 0.05). These responses were maintained through the 6 months of the study period. Thus, mine workers subjected to intermittent hypobaric condition for 3 years showed a good degree of acclimatization that was maintained through time.

Effect of a repeated series of intermittent hypoxic exposures on ventilatory response in humans

The purpose of this study was to elucidate the magnitude and the time course of ventilatory changes resulting from a repeated series of hypoxic exposures. Eight healthy males participated in the present study. The subjects spent 1 h/day in normobaric hypoxia (12% inspired oxygen). Inspired minute ventilation (V(I)), end-tidal partial pressure of carbon dioxide (P(ET(CO2))), and arterial oxygen saturation (SaO2) were measured in a hypoxic tent. These measurements were taken for 10 consecutive days (series 1), and were taken again after the subjects had been away from hypoxic exposure for 1 month (series 2). P(ET(CO2)) decreased and SaO2 increased progressively in the hypoxic tent during the 10 days of intermittent hypoxia in series 1. At the onset of series 2 (days 1 to 3), P(ET(CO2)) was significantly lower and SaO2 was significantly higher than those on day 1 during series 1. These results suggest that humans who have had previous hypoxic exposure adapt sooner to hypoxic condition due to an increase in the magnitude of hyperventilation in the first few days of a series of reexposures to hypoxia.

Ancestry explains the blunted ventilatory response to sustained hypoxia and lower exercise ventilation of Quechua altitude natives

Andean high-altitude (HA) natives have a low (blunted) hypoxic ventilatory response (HVR), lower effective alveolar ventilation, and lower ventilation (VE) at rest and during exercise compared with acclimatized newcomers to HA. Despite blunted chemosensitivity and hypoventilation, Andeans maintain comparable arterial O(2) saturation (Sa(O(2))). This study was designed to evaluate the influence of ancestry on these trait differences. At sea level, we measured the HVR in both acute (HVR-A) and sustained (HVR-S) hypoxia in a sample of 32 male Peruvians of mainly Quechua and Spanish origins who were born and raised at sea level. We also measured resting and exercise VE after 10-12 h of exposure to altitude at 4,338 m. Native American ancestry proportion (NAAP) was assessed for each individual using a panel of 80 ancestry-informative molecular markers (AIMs). NAAP was inversely related to HVR-S after 10 min of isocapnic hypoxia (r = -0.36, P = 0.04) but was not associated with HVR-A. In addition, NAAP was inversely related to exercise VE (r = -0.50, P = 0.005) and ventilatory equivalent (VE/Vo(2), r = -0.51, P = 0.004) measured at 4,338 m. Thus Quechua ancestry may partly explain the well-known blunted HVR (10, 35, 36, 57, 62) at least to sustained hypoxia, and the relative exercise hypoventilation at altitude of Andeans compared with European controls. Lower HVR-S and exercise VE could reflect improved gas exchange and/or attenuated chemoreflex sensitivity with increasing NAAP. On the basis of these ancestry associations and on the fact that developmental effects were completely controlled by study design, we suggest both a genetic basis and an evolutionary origin for these traits in Quechua.

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.

Increased hypoxic ventilatory response during 8 weeks at 3800 m altitude

Acclimatization to chronic hypoxia (CH) increases ventilation (V(I)) and the isocapnic hypoxic ventilatory response (HVR) over 2-14 days but hypoxic desensitization blunts the HVR after years of CH. We tested for hypoxic desensitization during the first 2 months of CH by studying five normal subjects at sea level (SL) and for 8 weeks at 3800 m (CH, PI(O(2)) approximately 90 Torr). We measured the isocapnic HVR (Delta V(I)/Delta Sa(O(2)) and tested for hypoxic ventilatory decline (HVD) by stepping Sa(O(2)) to 80% after 14 min at 90%. The HVR increased significantly after 2 days and remained significantly elevated for 8 weeks of CH. HVD was similar at SL and during 8 weeks of CH. Hence, hypoxic desensitization of the HVR does not occur after only 8 weeks of hypoxia and the increased HVR during this time does not involve changes in HVD.

Central and obstructive sleep apnoea during ascent to high altitude

OBJECTIVE

The aim of the study was to investigate the relationship between central sleep apnoea (CSA) at high altitude and arterial blood gas tensions, and by inference, ventilatory responsiveness.

METHODOLOGY

Fourteen normal adult volunteers were studied by polysomnography during sleep, and analysis of awake blood gases during ascent over 12 days from sealevel to 5050 m in the Nepal Himalayas.

RESULTS

Thirteen subjects developed CSA. Linear regression analysis showed tight negative correlations between mean CSA index and mean values for sleep SaO2, PaCO2 and PaO2 over the six altitudes (r2 > or = 0.74 for all, P < 0.03). Paradoxically there was poor correlation between the individual data for CSA index and those parameters at the highest altitude (5050-m) where CSA was worst (r2 < 0.12 for all, NS), possibly due to variation in degree of acclimatization between subjects. In addition, CSA replaced mild obstructive sleep apnoea during ascent. Obstructive sleep apnoea index fell from 5.5 +/- 6.9/h in rapid eye movement sleep at sealevel to 0.1 +/- 0.3/h at 5050 m (P < 0.001, analysis of variance), while CSA index rose from 0.1 +/- 0.3/h to 55.7 +/- 54.4/h (P < 0.001).

CONCLUSION

There was a general relationship between decreasing PaCO2 and CSA, but there were significant effects from variations in acclimatization that would make hypoxic ventilatory response an unreliable predictor of CSA in individuals.

Absence of Work Efficiency Differences During Cycle Ergometry Exercise in Bolivian Aymara

This study tested the hypothesis that Andean natives are adapted to high altitude (HA) via high work efficiency during exercise in hypoxia. A total of 186 young males and females were tested in Bolivia, comprising eight different subject groups. Groups were identified based on gender, ancestry (Aymara vs. European), altitude of birth (highlands vs. lowlands), and the altitude where tested (420, 3600, 3850 m). This design allows partitioning of ancestral (i.e., genetic) and developmental effects. To minimize measurement error, subjects were given two submaximal exercise tests on a cycle ergometer (on separate days). Each test consisted of four 5-min work bouts (levels), each separated by a 5-min rest period. For all groups, the oxygen consumption (V(O2))-work rate relationship was not different from the sea-level reference. Gross and net efficiencies (GE and NE) were not different between groups at any work level, with the exception of European men born in the lowlands and acclimatized and tested at 3600 m. These men showed slightly lower V(O2) at high work output, but this may be due to a nonsteady-state V(O2) kinetic, rather than to an altered steady-state V(O2)-work rate relationship per se. There were no significant group differences in delta efficiency (DE). In sum, these results provide no support for the hypothesis of energetic advantage during submaximal work in Andean HA natives. A review and analysis of the literature suggest that the same is true for HA natives in the Himalayas.

Modulating effects of the menstrual cycle on cardiorespiratory responses to exercise under acute hypobaric hypoxia

The purpose of this study was to examine the hypothesis that the menstrual cycle-induced modulation of the cardiorespiratory response to exercise might be altered by acute exposure to altitude. During both the luteal and follicular phases, 9 moderately trained female subjects with normal menstrual cycles performed incremental exercise to maximal effort on a cycle ergometer at sea level (SL) and under hypobaric hypoxia (HH) at the equivalent of 3,000 m altitude. Both at rest and during exercise, minute ventilation (.VE) and oxygen uptake (.VO(2)) did not differ between the luteal and follicular phases (either at SL or HH). However, the ratio of .VE to .VO(2) (.VE /.VO(2)), both at rest and during peak exercise, was greater in the luteal phase than in the follicular phase under HH conditions. Furthermore, the partial pressure of end-tidal carbon dioxide (PETCO(2)) during exercise was lower in the luteal phase than in the follicular phase in HH. These results suggest that the menstrual cycle-induced modulation of the ventilatory response to exercise may be altered under acute hypobaric-hypoxic conditions.

Living high-training low increases hypoxic ventilatory response of well-trained endurance athletes

This study determined whether "living high-training low" (LHTL)-simulated altitude exposure increased the hypoxic ventilatory response (HVR) in well-trained endurance athletes. Thirty-three cyclists/triathletes were divided into three groups: 20 consecutive nights of hypoxic exposure (LHTLc, n = 12), 20 nights of intermittent hypoxic exposure (four 5-night blocks of hypoxia, each interspersed with 2 nights of normoxia, LHTLi, n = 10), or control (Con, n = 11). LHTLc and LHTLi slept 8-10 h/day overnight in normobaric hypoxia (approximately 2,650 m); Con slept under ambient conditions (600 m). Resting, isocapnic HVR (DeltaVE/DeltaSp(O(2)), where VE is minute ventilation and Sp(O(2)) is blood O(2) saturation) was measured in normoxia before hypoxia (Pre), after 1, 3, 10, and 15 nights of exposure (N1, N3, N10, and N15, respectively), and 2 nights after the exposure night 20 (Post). Before each HVR test, end-tidal PCO(2) (PET(CO(2))) and VE were measured during room air breathing at rest. HVR (l. min(-1). %(-1)) was higher (P < 0.05) in LHTLc than in Con at N1 (0.56 +/- 0.32 vs. 0.28 +/- 0.16), N3 (0.69 +/- 0.30 vs. 0.36 +/- 0.24), N10 (0.79 +/- 0.36 vs. 0.34 +/- 0.14), N15 (1.00 +/- 0.38 vs. 0.36 +/- 0.23), and Post (0.79 +/- 0.37 vs. 0.36 +/- 0.26). HVR at N15 was higher (P < 0.05) in LHTLi (0.67 +/- 0.33) than in Con and in LHTLc than in LHTLi. PET(CO(2)) was depressed in LHTLc and LHTLi compared with Con at all points after hypoxia (P < 0.05). No significant differences were observed for VE at any point. We conclude that LHTL increases HVR in endurance athletes in a time-dependent manner and decreases PET(CO(2)) in normoxia, without change in VE. Thus endurance athletes sleeping in mild hypoxia may experience changes to the respiratory control system.

[Respiratory changes during ascension to 8,000 meters mountain]

BACKGROUND

Our goal was to determine whether spirometric alterations occur during expeditions to 8,000-metre peaks, and whether these are modified by acclimatization or are related to acute mountain sickness, to arterial oxygen saturation (SaO2) or to muscular deterioration due to chronic hypoxic exposure.

SUBJECTS AND METHOD

Forced vital capacity (FVC), forced expiratory volume in the first second (FEV1), inspiratory (MIP) and expiratory (MEP) maximal static pressures, grip strength in both hands, and SaO2 at rest and exercise were measured in eight subjects during an expedition to Gasherbrum II (8,035 m).

RESULTS

Upon arrival at the base camp (5,200 m), both FVC and FEV1 decreased, with no changes in the FEV1/FVC ratio. FVC did not improve after a brief pressurisation in a portable hyperbaric chamber. A month later, FVC in the base camp returned to normal values. FVC fall correlated with both the severity of acute mountain sickness and weight loss. Resting SaO2 improved with acclimatisation and correlated with the previous hypoxic ventilatory response, both before and after acclimatisation. Acclimatisation led to a decrease in the exercise-induced SaO2 fall. Stay at a high altitude lowered body weight and grip strength, although MIP and MEP remained unchanged.

CONCLUSIONS

We observed a restrictive alteration was corrected by with acclimatisation. This phenomenon seems to be related to a subclinical high-altitude pulmonary oedema rather than to an increase in the pulmonary vascular volume. Despite the high-altitude muscular deterioration, respiratory muscle weakness was not

Women at altitude: ventilatory acclimatization at 4,300 m

Women living at low altitudes or acclimatized to high altitudes have greater effective ventilation in the luteal (L) compared with follicular (F) menstrual cycle phase and compared with men. We hypothesized that ventilatory acclimatization to high altitude would occur more quickly and to a greater degree in 1) women in their L compared with women in their F menstrual cycle phase, and 2) in women compared with men. Studies were conducted on 22 eumenorrheic, unacclimatized, sea-level (SL) residents. Indexes of ventilatory acclimatization [resting ventilatory parameters, hypoxic ventilatory response, hypercapnic ventilatory response (HCVR)] were measured in 14 women in the F phase and in 8 other women in the L phase of their menstrual cycle, both at SL and again during a 12-day residence at 4,300 m. At SL only, ventilatory studies were also completed in both menstrual cycle phases in 12 subjects (i.e., within-subject comparison). In these subjects, SL alveolar ventilation (expressed as end-tidal PCO(2)) was greater in the L vs. F phase. Yet the comparison between L- and F-phase groups found similar levels of resting end-tidal PCO(2), hypoxic ventilatory response parameter A, HCVR slope, and HCVR parameter B, both at SL and 4,300 m. Moreover, these indexes of ventilatory acclimatization were not significantly different from those previously measured in men. Thus female lowlanders rapidly ascending to 4,300 m in either the L or F menstrual cycle phase have similar levels of alveolar ventilation and a time course for ventilatory acclimatization that is nearly identical to that reported in male lowlanders.

Effect of developmental and ancestral high altitude exposure on chest morphology and pulmonary function in Andean and European/North American natives

Chest depth, chest width, forced vital capacity (FVC), and forced expiratory volume (FEV1) were measured in 170 adult males differing by ancestral (genetic) and developmental exposure to high altitude (HA). A complete migrant study design was used to study HA natives (Aymara/Quechua ancestry, n = 88) and low altitude (LA) natives (European/North American ancestry, n = 82) at both altitude (La Paz, Bolivia, 3,600 m) and near sea level (Santa Cruz, Bolivia, 420 m). HAN and LAN migrant groups were classified as: N(th) generation migrants, born and raised in a non-native environment; child migrants who migrated during the period of growth and maturation (0-18 yrs); and adult migrants who migrated after 18 years of age. Chest depth, FVC, and FEV1 measures were larger with increasing developmental exposure in both HAN migrants at LA and LAN migrants at HA. Developmental responses were similar between HAN and LAN groups. FVC and FEV1 measures were larger in HANs vs LANs born and raised at HA to suggest a genetic effect, but were similar in HANs and LANs born and raised at LA. The similarity of HAN and LAN groups at LA suggests that the genetic potential for larger lung volumes at HA depends upon developmental exposure to HA. Additional data for females (HANs at HA, n = 20, and LAN adult migrants to HA, n = 17) show similar differences as those shown between male HAN and LAN groups. Am. J. Hum. Biol. 11:383-395, 1999. Copyright 1999 Wiley-Liss, Inc.

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.

Intermittent hypoxia increases ventilation and Sa(O2) during hypoxic exercise and hypoxic chemosensitivity

The purpose of this study was 1) to test the hypothesis that ventilation and arterial oxygen saturation (Sa(O2)) during acute hypoxia may increase during intermittent hypoxia and remain elevated for a week without hypoxic exposure and 2) to clarify whether the changes in ventilation and Sa(O2) during hypoxic exercise are correlated with the change in hypoxic chemosensitivity. Six subjects were exposed to a simulated altitude of 4,500 m altitude for 7 days (1 h/day). Oxygen uptake (VO2), expired minute ventilation (VE), and Sa(O2) were measured during maximal and submaximal exercise at 432 Torr before (Pre), after intermittent hypoxia (Post), and again after a week at sea level (De). Hypoxic ventilatory response (HVR) was also determined. At both Post and De, significant increases from Pre were found in HVR at rest and in ventilatory equivalent for O2 (VE/VO2) and Sa(O2) during submaximal exercise. There were significant correlations among the changes in HVR at rest and in VE/VO2 and Sa(O2) during hypoxic exercise during intermittent hypoxia. We conclude that 1 wk of daily exposure to 1 h of hypoxia significantly improved oxygenation in exercise during subsequent acute hypoxic exposures up to 1 wk after the conditioning, presumably caused by the enhanced hypoxic ventilatory chemosensitivity.

Naloxone improves arterial blood pressure and hypoxic ventilatory depression, but not survival, of rats during acute hypoxia

OBJECTIVE

To investigate the effects of naloxone and morphine during acute hypoxia.

DESIGN

Prospective, randomized animal study.

SETTING

University laboratory.

SUBJECTS

Twenty-eight adult male Sprague Dawley rats, weighing 300-350 g.

INTERVENTIONS

The rats were implanted with a femoral catheter and subcutaneous electrodes for electrocardiogram recording and were randomly assigned to receive morphine (5 mg/kg), naloxone (5 mg and 10 mg/kg), or normal saline (control) (n = 7 in each). Fifteen minutes after intraperitoneal injection of the drug, each rat was exposed to hypoxic gas (5% oxygen, 95% N2) for 70 mins. Hypoxic survival time was measured. Mean arterial pressure (MAP), arterial pH, Paco2, Pao2, and base excess were measured before injection (baseline), 14 mins after injection (H0), and 6 mins (H1), 33 mins (H2), and 48 mins (H3) after exposure to hypoxia.

MEASUREMENTS AND MAIN RESULTS

Hypoxic survival was similar between the naloxone 5 mg/kg and control groups (p = .183), significantly lower in the naloxone 10 mg/kg group (p < .01), and significantly higher in the morphine 5 mg/kg group (p < .05) compared with controls. MAP significantly decreased in all groups. However, at H2-H3, MAP was better preserved in both naloxone groups and was lower in the morphine group compared with controls. Paco2 was maintained higher at H0-H3 in the morphine group and lower at H2-H3 in both naloxone groups compared with controls.

CONCLUSION

During acute hypoxia, naloxone preserves arterial blood pressure and attenuates hypoxic ventilatory depression by antagonizing endogenous opiates, but it does not improve hypoxic survival. In contrast, morphine, which enhances the action of endogenous opiates, does improve hypoxic survival. The acute hypoxic tolerance of morphine may be partly attributable to a depression of oxygen consumption, increased cerebral blood flow secondary to high Paco2, and protective actions mediated by delta-opioid receptors.

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.

Effect of developmental and ancestral high-altitude exposure on VO(2)peak of Andean and European/North American natives

Peak oxygen consumption (VO(2)peak) was measured in 150 adult males (18-35 years old) in Bolivia, using a complete migrant study design to partition developmental from ancestral (genetic) effects of high-altitude (HA) exposure. High-altitude natives (HANs, Aymara/Quechua ancestry, n = 75) and low-altitude natives (LANs, European/North American ancestry, n = 75) were studied at high altitude (3,600-3,850 m) and near sea level (420 m). HAN and LAN migrant groups to a nonnative environment were classified as: multigeneration migrants, born and raised in a nonnative environment; child migrants who migrated to the nonnative environment during the period of growth and development (0-18 years old); and adult migrants who migrated after 18 years of age. Variability in VO(2)peak due to high-altitude adaptation was modeled by covariance analysis, adjusting for fat-free mass and physical activity (training) differences between groups. A trend for increased VO(2)peak with increasing developmental high-altitude exposure in migrant groups did not reach statistical significance, but low statistical power may have limited the ability to detect this effect. HANs and LANs born, raised, and tested at high altitude had similar VO(2)peak values, indicating no genetic effect, or an effect much smaller than that reported previously in the literature. There was no functional correlation between forced vital capacity and VO(2)peak, within or across groups. These results do not support the hypothesis that Andean HANs have been selected to express a greater physical work capacity in hypoxia.

Cerebral vasomotor reactivity at high altitude in humans

The purpose of this study was twofold: 1) to determine whether at high altitude cerebral blood flow (CBF) as assessed during CO2 inhalation and during hyperventilation in subjects with acute mountain sickness (AMS) was different from that in subjects without AMS and 2) to compare the CBF as assessed under similar conditions in Sherpas at high altitude and in subjects at sea level. Resting control values of blood flow velocity in the middle cerebral artery (VMCA), pulse oxygen saturation (SaO2), and transcutaneous PCO2 were measured at 4,243 m in 43 subjects without AMS, 17 subjects with AMS, 20 Sherpas, and 13 subjects at sea level. Responses of CO2 inhalation and hyperventilation on VMCA, SaO2, and transcutaneous PCO2 were measured, and the cerebral vasomotor reactivity (VMR = DeltaVMCA/PCO2) was calculated as the fractional change of VMCA per Torr change of PCO2, yielding a hypercapnic VMR and a hypocapnic VMR. AMS subjects showed a significantly higher resting control VMCA than did no-AMS subjects (74 +/- 22 and 56 +/- 14 cm/s, respectively; P < 0.001), and SaO2 was significantly lower (80 +/- 8 and 88 +/- 3%, respectively; P < 0.001). Resting control VMCA values in the sea-level group (60 +/- 15 cm/s), in the no-AMS group, and in Sherpas (59 +/- 13 cm/s) were not different. Hypercapnic VMR values in AMS subjects were 4.0 +/- 4.4, in no-AMS subjects were 5.5 +/- 4. 3, in Sherpas were 5.6 +/- 4.1, and in sea-level subjects were 5.6 +/- 2.5 (not significant). Hypocapnic VMR values were significantly higher in AMS subjects (5.9 +/- 1.5) compared with no-AMS subjects (4.8 +/- 1.4; P < 0.005) but were not significantly different between Sherpas (3.8 +/- 1.1) and the sea-level group (2.8 +/- 0.7). We conclude that AMS subjects have greater cerebral hemodynamic responses to hyperventilation, higher VMCA resting control values, and lower SaO2 compared with no-AMS subjects. Sherpas showed a cerebral hemodynamic pattern similar to that of normal subjects at sea level.

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.

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.

High-altitude medicine

This article discusses prevention, recognition, and treatment of altitude illnesses, especially acute mountain sickness, high-altitude pulmonary edema, and high-altitude cerebral edema. Physicians advising travelers and trekkers who will be visiting high-altitude areas will find an organized approach to giving pretravel advice. Physicians practicing in or visiting high-altitude areas will find guidelines for diagnosis and treatment. This article also addresses the issue of patients with underlying diseases who wish to travel to high-altitude destinations.

Cardiopulmonary function in bicycle racing over mountainous terrain at moderate altitude

To examine cardiopulmonary function during exercise in a mountainous region at moderate altitude, we measured cardiac frequency, oxygen consumption (VO2), and percentage arterial hemoglobin oxygen saturation (%SaO2) before and after a bicycle race with a starting point at 638 m and finishing point at 1980 m. The time required to ascend an elevation of 10 m was prolonged with increasing altitude, and heart rate also increased with altitude. The %SaO2 at the starting point and at the finishing point differed significantly (P < 0.01). Faster cyclists exhibited higher %SaO2 and lower VO2, while slower cyclists exhibited a reduction in %SaO2 and an increase in VO2 immediately after the race. The %SaO2 recovery time was significantly correlated with the racing time (r = 0.54, P < 0.001). Therefore, the faster cyclists' oxygen debt upon completion of the race may be small and recovery of cardiopulmonary function may be fast, while the slower cyclists' oxygen debt may be large and recovery of cardiopulmonary function may be slow.

Effects of specific carotid body and brain hypoxia on respiratory muscle control in the awake goat

1. We assessed the effects of specific brain hypoxia on the control of inspiratory and expiratory muscle electromyographic (EMG) activities in response to specific carotid body hypoxia in seven awake goats. We used an isolated carotid body perfusion technique that permitted specific, physiological, steady-state stimulation of the carotid bodies or maintenance of normoxia and normocapnia at the carotid bodies while varying the level of systemic, and therefore, brain oxygenation. 2. Isolated brain normocapnic hypoxia of up to 1.5 h duration increased inspired minute ventilation (VI) by means of increases in both tidal volume (VT) and respiratory frequency (fR). Electromyographic activities of both inspiratory and expiratory muscles were augmented as well. These responses were similar to those produced by low levels of whole-body normoxic hypercapnia. We conclude that moderate levels of brain hypoxia (Pa,O2 approximately 40 mmHg) in awake goats caused a net stimulation of ventilatory motor output. 3. Hypoxic stimulation of the carotid bodies alone caused comparable increases in VT and fR, and EMG augmentation of both inspiratory and expiratory muscles whether the brain was hypoxic or normoxic. These responses were quite similar to those obtained over a wide range of whole-body normoxic hypercapnia. We conclude that the integration of carotid body afferent information is not affected by moderate brain hypoxia in awake goats. 4. We found no evidence for an asymmetrical recruitment pattern of inspiratory vs. expiratory muscles in response to carotid body hypoxia or in response to brain hypoxia alone. 5. Our data support the concept that moderate brain hypoxia results in a net stimulation of respiratory motor output. These findings question the significance of 'central hypoxic depression' to the regulation of breathing under physiological levels of hypoxaemia in the awake animal.

Lessons from high altitude

We have reviewed evidence that hypoxic chemosensitivity is variable and that this variation may be both endowed, partly through genetic mechanisms, and acquired, and may reflect fundamental changes in carotid body function. This variation may influence the nature and effectiveness of adaptation to high altitude and to hypoxic disease states such as chronic obstructive pulmonary disease. High chemosensitivity seems to be the choice for coping with the casual exposure to hypoxia; but fundamental, highly effective adaptations, presumably at the level of peripheral tissue, seem to be the strategy of choice for professionally adapted species.

Usual clinical dose of acetazolamide does not alter cerebral blood flow velocity

Prior reports indicate that acetazolamide, an inhibitor of carbonic anhydrase, in moderate doses reduces symptoms of acute mountain sickness, and in large doses increases cerebral blood flow. The effect on flow is not known for a moderate dose, but were flow to increase, then increased cerebral oxygen delivery would be one mechanism of benefit from acetazolamide at high altitude. We utilized Doppler ultrasound in 8 volunteers to determine whether a usual acetazolamide dose (250 mg three times daily) would increase flow velocities in internal carotid and vertebral arteries. Acetazolamide during normoxia decreased pHa, PaCO2, and PETCO2, but baseline flow velocity remained unchanged. In 2 subjects without acetazolamide, voluntary hyperventilation decreased both PETCO2 and flow velocity. Both hypoxia and hypercapnia caused increases in arterial velocities. The increases were not altered by acetazolamide administration. In one subject, 1 g acetazolamide by acute i.v. injection induced an increase in flow velocity (40%) concomitant with a 5 mm Hg decrease in PETCO2, confirming prior reports using similar intravenous dose. In doses employed for prevention of acute mountain sickness, acetazolamide induced metabolic acidosis and may have prevented the fall in velocity usually associated with hypocapnia, but it neither increased baseline cerebral blood flow velocity nor velocity responses to hypoxia and hypercapnia. Benefit of acetazolamide at high altitude may relate to mechanisms other than increased cerebral blood flow.

Increased metabolism contributes to increased resting ventilation at high altitude

Ventilatory acclimation to high altitude results in an increase in total or minute ventilation, and is associated with a fall in alveolar PCO2, i.e. alveolar hyperventilation. However, the extent to which the increase in total ventilation is matched by a greater metabolic rate (VO2, VCO2) vs alveolar hyperventilation is unclear. We sought to determine the contribution of changes in metabolic rate to the increase in minute ventilation observed during exposure to high altitude. In 12 healthy male subjects taken from Denver, Colorado (1600 m) to Pikes Peak, Colorado (4300 m) for 5 days, resting minute ventilation increased from low to high altitude (+ 26% for the 5 days) and arterialized PCO2 fell. Resting metabolic rate increased 16% for the 5 days and could account for more than half of the increase in minute ventilation. Among subjects the increases in ventilation on days 1, 2 and 4 were positively correlated with increased CO2 production; they were not correlated with arterial oxygen saturation on any day. During exercise at high altitude, PCO2 values were not different from those at rest and minute ventilation rose above low altitude values (+ 58% by day 5), but the increase could not be accounted for by an increased CO2 production. Thus at rest but not during exercise a substantial portion of the rise in minute ventilation could be attributed to increased metabolic rate.