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.     

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.     

Ventilatory acclimatization in awake guinea pigs raised at high altitude

To determine if laboratory strains of guinea pigs bred at sea level (SL) are "pre-adapted" to high altitude (HA), we raised litter-matched weanling Hartley guinea pigs for 4 months at SL, intermediate altitude (IA, 1250 m) or HA (3800 m) and exposed them acutely to 100, 21 and 12% inspired O2 at their respective altitude of residence. Control animals raised at SL were also exposed acutely to 10 and 8% inspired O2. In awake spontaneously breathing guinea pigs raised at SL, resting minute ventilation and tidal volume increased significantly when inspired O2 tension fell below about 60 mm Hg. In guinea pigs raised at IA or HA, ventilation was higher at any given inspired O2 tension in direct relationship to the altitude of residence. Resting hematocrit was also higher in animals raised at HA than at SL. We conclude that the pattern of ventilatory acclimatization to HA exposure in Hartley guinea pigs is similar to that in laboratory rats and human lowlanders; therefore laboratory guinea pigs are not pre-adapted and are suitable animals for the study of adaptation to high altitude.     

Control of erythropoiesis after high altitude acclimatization

Erythropoiesis was studied in 11 subjects submitted to a 4-h hypoxia (HH) in a hypobaric chamber (4,500 m, barometric pressure 58.9 kPa) both before and after a 3-week sojourn in the Andes. On return to sea level, increased red blood cells (+3.27%), packed cell volume (+4.76%), haemoglobin (+6.55%) (P<0.05), and increased arterial partial pressure of oxygen (+8.56%), arterial oxygen saturation (+7.40%) and arterial oxygen blood content (C_aO₂) (+12.93%) at the end of HH (P<0.05) attested high altitude acclimatization. Reticulocytes increased during HH after the sojourn only (+36.8% vs +17.9%, P<0.01) indicating a probable higher reticulocyte release and/or production despite decreased serum erythropoietin (EPO) concentrations (–46%, P<0.01). Hormones (thyroid, catecholamines and cortisol), iron status (serum iron, ferritin, transferrin and haptoglobin) and renal function (creatinine, renal, osmolar and free-water clearances) did not significantly vary (except for lower thyroid stimulating hormone at sea level, P<0.01). Levels of 2,3-diphosphoglycerate (2,3-DPG) increased throughout HH on return (+14.7%, P<0.05) and an inverse linear relationship was found between 2,3-DPG and EPO at the end of HH after the sojourn only (r=–0.66, P<0.03). Inverse linear relationships were also found between C_aO₂ and EPO at the end of HH before (r=–0.63, P<0.05) and after the sojourn (r=–0.60, P=0.05) with identical slopes but different ordinates at the origin, suggesting that the sensitivity but not the gain of the EPO response to hypoxia was modified by altitude acclimatization. Higher 2,3-DPG levels could partly explain this decreased sensitivity of the EPO response to hypoxia. In conclusion, we show that altitude acclimatization modifies the control of erythropoiesis not only at sea level, but also during a subsequent hypoxia.     

Enhancement of cold-induced vasodilatation following acclimatization to altitude

The present study evaluated the effect of high-altitude acclimatisation on the cold-induced vasodilatation (CIVD) response. A group of highly trained mountaineers (N = 9; Alpinists) were tested before and after a 3 week high-altitude Himalayan expedition (altitude ranging from 3,985 to 6,828 m). A control group (N = 7) with no mountaineering experience was tested at the same time points. During each test, subjects first immersed their hand to the styloid process in 37 degrees C water for 5 min and then in 10 degrees C water for 30 min. Upon completion of the hand immersion, the same procedure was repeated for the foot. Skin temperature of the pads of all immersed digits was measured throughout the immersion and for 10 min following the immersion. In the Alpinists, a significant increase in amplitude of CIVD and absolute maximum finger skin temperature during immersion was observed in the hand post-expedition. For the foot, peak time of CIVD was significantly shorter in the Alpinist group, and there were significant increases in minimum and maximum toe skin temperature during CIVD, mean toe skin temperature during immersion, absolute minimum and maximum toe skin temperature during immersion, and absolute amplitude during immersion. The results demonstrate a significant enhancement of the CIVD response as a consequence of a brief high altitude acclimatisation, and that these changes were especially prominent in the toes.     

Sleep in older African Americans and Caucasians at risk for sleep-disordered breathing

This study explored differences in sleep between older African Americans (AA) and Caucasians (CA) at risk for sleep-disordered breathing. Seventy AA and 70 CA were compared on ambulatory monitoring sleep variables and on self-reports on health and socioeconomic status (SES). After controlling for SES and health covariates, CA woke up significantly more often than AA (p = .018), but there were no other differences in sleep variables between the two groups. Time awake at night was related to being male, more depression, less walking, and lower income, whereas having more awakenings during the night was related to being CA, higher apnea-hypopnea index, and higher periodic leg movement index. Importance of inclusion of SES, health, and other covariates in studies exploring racial differences in sleep are discussed.     

Metabolic consequences of reduced frequency breathing during submaximal exercise at moderate altitude

Summary The intention of this study was to determine the metabolic consequences of reduced frequency breathing (RFB) at total lung capacity (TLC) in competitive cyclists during submaximal exercise at moderate altitude (1520 m; barometric pressure, P _B=84.6 kPa; 635 mm Hg). Nine trained males performed an RFB exercise test (10 breaths · min ^–1) and a normal breathing exercise test at 75–85% of the ventilatory threshold intensity for 6 min on separate days. RFB exercise induced significant (P<0.05) decreases in ventilation (V _E), carbon dioxide production (VCO₂), respiratory exchange ratio. (RER), ventilatory equivalent for O₂ consumption (V _E/VO₂), arterial O₂ saturation and increases in heart rate and venous lactate concentration, while maintaining a similar OZ consumption (VO₂). During recovery from RFB exercise (spontaneous breathing) a significant (P< 0.05) decrease in blood pH was detected along with increases in V _E, VO₂, VCO₂, RER, and venous partial pressure of carbon dioxide. The results indicate that voluntary hypoventilation at TLC, during submaximal cycling exercise at moderate altitude, elicits systemic hypercapnia, arterial hypoxemia, tissue hypoxia and acidosis. These data suggest that RFB exercise at moderate altitude causes an increase in energy production from glycolytic pathways above that which occurs with normal breathing.     

Periodic breathing at high altitude and ventilatory responses to O2 and CO2

To determine the relationship between periodic breathing (PB) during sleep at high altitude and ventilatory chemosensitivities, we studied nine Japanese climbers who participated in the expedition to the Kunlun Mountains (7,167 m) in China in 1986. At sea level, ventilatory response to hypoxia (HVR) by isocapnic progressive hypoxia test and to hypercapnia (HCVR) by Read's method were examined. At altitude 5,360 m, respiratory movements of the chest and abdominal wall, SaO2, ECG, and HR were monitored. Seven climbers manifested PB during sleep. There was a significant correlation between PB during sleep and HVR and HCVR (p less than 0.05). All the climbers showed severe desaturation during sleep. There was a significant negative correlation between degree of desaturation during sleep and HVR (p less than 0.05). A negative correlation was also detected between PB and the degree of desaturation during sleep. We concluded that ventilatory chemosensitivities play an important role in eliciting PB and that climbers with high HVR can maintain their arterial oxygenation during sleep, due to hyperventilation induced by PB, which is considered an advantageous adaptation for lowland sojourners.     

Failure of long-term acclimatization in smokers moving to high altitude

It was found that several foreign experts recruited for an irrigation project in southern Peru could not cope with their work at an altitude of 3 200 m. When we analyzed the haematological acclimatization to high altitude in 109 adult males, we found that the moderate differences in haematocrit level (0.9) and haemoglobin concentration (0.2 g/100 ml) between male smokers and non-smokers at low altitude were almost quadrupled at 3 200 m height. The haemoglobin concentration increased in both groups during the first month, after which it remained constant. Of 51 Swedish employees, 25 smokers and 26 non-smokers, 14 were obliged to terminate their contracts. All 10 who had failed to cope with their work for other than purely medical reasons were smokers. This group of smoking men also had an overconsumption of alcohol. We therefore conclude that life style determines long-term acclimatization to high altitude.     

Sleep of Andean high altitude natives

Summary The structure of sleep in lowland visitors to altitudes greater than 4000 m is grossly disturbed. There are no data on sleep in long-term residents of high altitudes. This paper describes an electroencephalographic study of sleep in high altitude dwellers who were born in and are permanent residents of Cerro de Pasco in the Peruvian Andes, situated at 4330 m. Eight healthy male volunteers aged between 18 and 69 years were studied. Sleep was measured on three consecutive nights for each subject. Electroencephalographs, submental electromyographs and electro-oculograms were recorded. Only data from the third night were used in the analysis. The sleep patterns of these subjects resembled the normal sleep patterns described by others in lowlanders at sea level. There were significant amounts of slow wave sleep in the younger subjects and rapid eye movement sleep seemed unimpaired.