Sleeping in moderate hypoxia at home for prevention of acute mountain sickness (AMS): a placebo-controlled, randomized double-blind study.

OBJECTIVE: Acclimatization at natural altitude effectively prevents acute mountain sickness (AMS). It is, however, unknown whether prevention of AMS is also possible by only sleeping in normobaric hypoxia. METHODS: In a placebo-controlled, double-blind study 76 healthy unacclimatized male subjects, aged 18 to 50 years, slept for 14 consecutive nights at either a fractional inspired oxygen (Fio2) of 0.14 to 0.15 (average target altitude 3043 m; treatment group) or 0.209 (control group). Four days later, AMS scores and incidence of AMS were assessed during a 20-hour exposure in normobaric hypoxia at Fio2 = 0.12 (equivalent to 4500 m). RESULTS: Because of technical problems with the nitrogen generators, target altitude was not achieved in the tents and only 21 of 37 subjects slept at an average altitude considered sufficient for acclimatization (>2200 m; average, 2600 m). Therefore, in a subgroup analysis these subjects were compared with the 21 subjects of the control group with the lowest sleeping altitude. This analysis showed a significantly lower AMS-C score (0.38; 95% CI, 0.21 to 0.54) vs 1.10; 95% CI, 0.57 to 1.62; P = .04) and lower Lake Louise Score (3.1; 95% CI, 2.2 to 4.1 vs 5.1; 95% CI, 3.6 to 6.6; P = .07) for the treatment subgroup. The incidence of AMS defined as an AMS-C score greater than 0.70 was also significantly lower (14% vs 52%; P < .01). CONCLUSIONS: Sleeping 14 consecutive nights in normobaric hypoxia (equivalent to 2600 m) reduced symptoms and incidence of AMS 4 days later on exposure to 4500 m.

Health risk for athletes at moderate altitude and normobaric hypoxia.

Altitudes at which athletes compete or train do usually not exceed 2000-2500 m. At these moderate altitudes acute mountain sickness (AMS) is mild, transient and affects at the most 25% of a tourist population at risk. Unpublished data included in this review paper demonstrate that more intense physical activity associated with high-altitude training or mountaineering does not increase prevalence or severity of AMS at these altitudes. These conclusions can also be extended to the use of normobaric hypoxia, as data in this paper suggest that the severity of AMS is not significantly different between hypobaric and normobaric hypoxia at the same ambient pO(2). Furthermore, high-altitude cerebral or pulmonary oedema do not occur at these altitudes and intermittent exposure to considerably higher altitudes (4000-6000 m) used by athletes for hypoxic training are too short to cause acute high-altitude illnesses. Even moderate altitude between 2000 and 3000 m can, however, exacerbate cardiovascular or pulmonary disease or lead to a first manifestation of undiagnosed illness in older people that may belong to the accompanying staff of athletes. Moderate altitudes may also lead to splenic infarctions in healthy athletes with sickle cell trait.

Transpulmonary plasma catecholamines in acute high-altitude pulmonary hypertension.

OBJECTIVE: High altitude leads to an increase in sympathetic nervous system (SNS) activity and pulmonary arterial pressure (PAP). We assessed whether the SNS contributes to this increase in PAP. METHODS: Sympathetic discharge to the pulmonary vasculature was assessed by measuring plasma norepinephrine concentrations in central venous blood entering the lung and systemic arterial blood leaving the lung (arterial-central venous difference; a - cv(diff)). Sympathetic activity in the adrenal gland was assessed by measuring systemic plasma epinephrine concentrations. The a - cv(diff) of epinephrine was assessed to investigate its metabolism across the lung. The measurements were performed in 34 mountaineers during both rest and exercise at low altitude and after 20 hours at high altitude (4559 m). Norepinehrine and epinephrine concentrations were measured by high-performance liquid chromatography. Pulmonary blood flow was assessed by inert gas rebreathing, and systolic PAP (PASP) by transthoracic Doppler-echocardiography. RESULTS: Exercise and high altitude increased PASP and increased arterial and central venous plasma norepinephrine. In contrast, exercise but not high altitude increased arterial and central venous epinephrine. There was no significant a - cv(diff) for norepinephrine and epinephrine during rest and exercise at low altitude, nor during rest at high altitude. However, during exercise at high altitude the a - cv(diff) for norepinephrine was positive. There was no correlation between the a - cv(diff) of both norepinephrine and epinephrine with PASP during exercise, high altitude or during a combination of both. CONCLUSIONS: The degree of pulmonary hypertension that occurs upon high-altitude exposure is largely independent of the SNS activity in the pulmonary vasculature and adrenal gland.

High-altitude pulmonary hypertension is associated with a free radical-mediated reduction in pulmonary nitric oxide bioavailability.

High altitude (HA)-induced pulmonary hypertension may be due to a free radical-mediated reduction in pulmonary nitric oxide (NO) bioavailability. We hypothesised that the increase in pulmonary artery systolic pressure (PASP) at HA would be associated with a net transpulmonary output of free radicals and corresponding loss of bioactive NO metabolites. Twenty-six mountaineers provided central venous and radial arterial samples at low altitude (LA) and following active ascent to 4559 m (HA). PASP was determined by Doppler echocardiography, pulmonary blood flow by inert gas re-breathing, and vasoactive exchange via the Fick principle. Acute mountain sickness (AMS) and high-altitude pulmonary oedema (HAPE) were diagnosed using clinical questionnaires and chest radiography. Electron paramagnetic resonance spectroscopy, ozone-based chemiluminescence and ELISA were employed for plasma detection of the ascorbate free radical (A(·-)), NO metabolites and 3-nitrotyrosine (3-NT). Fourteen subjects were diagnosed with AMS and three of four HAPE-susceptible subjects developed HAPE. Ascent decreased the arterio-central venous concentration difference (a-cv(D)) resulting in a net transpulmonary loss of ascorbate, α-tocopherol and bioactive NO metabolites (P < 0.05 vs. LA). This was accompanied by an increased a-cv(D) and net output of A(·-) and lipid hydroperoxides (P < 0.05 vs. sea level, SL) that correlated against the rise in PASP (r = 0.56-0.62, P < 0.05) and arterial 3-NT (r = 0.48-0.63, P < 0.05) that was more pronounced in HAPE. These findings suggest that increased PASP and vascular resistance observed at HA are associated with a free radical-mediated reduction in pulmonary NO bioavailability.

Magnetic resonance imaging evidence of cytotoxic cerebral edema in acute mountain sickness.

The present study applied T2- and diffusion-weighted magnetic resonance imaging to examine if mild cerebral edema and subsequent brain swelling are implicated in the pathophysiology of acute mountain sickness (AMS). Twenty-two subjects were examined in normoxia (21% O2), after 16 hours passive exposure to normobaric hypoxia (12% O2) corresponding to a simulated altitude of 4,500 m and after 6 hours recovery in normoxia. Clinical AMS was diagnosed in 50% of subjects during hypoxia and corresponding headache scores were markedly elevated (P<0.05 versus non-AMS). Hypoxia was associated with a mild increase in brain volume (+7.0+/-4.8 ml, P<0.05 versus pre-exposure baseline) that resolved during normoxic recovery. Hypoxia was also associated with an increased T2 relaxation time (T2rt) and a general trend toward an increased apparent diffusion coefficient (ADC). During the normoxic recovery, brain volume and T2rt recovered to pre-exposure baseline values, whereas a more marked reduction in ADC in the splenium of the corpus callosum (SCC) was observed (P<0.05). While changes in brain volume and T2rt were not selectively different in AMS, ADC values were consistently lower (P<0.05 versus non-AMS) and associated with the severity of neurologic symptoms. Acute mountain sickness was also characterized by an increased brain to intracranial volume ratio (P<0.05 versus non-AMS). These findings indicate that mild extracellular vasogenic edema contributes to the generalized brain swelling observed at high altitude, independent of AMS. In contrast, intracellular cytotoxic edema combined with an anatomic predisposition to a 'tight-fit' brain may prove of pathophysiologic significance, although the increase in brain volume in hypoxia was only about 0.5% of total brain volume.

Unchanged anaerobic and aerobic performance after short-term intermittent hypoxia.

INTRODUCTION: Repeated short-term exposures to a severe degree of hypoxia, alternated with similar intervals of normoxia, are recommended for performance enhancement in sports. However, scientific evidence for the efficiency of this method is controversial with regard to anaerobic performance. Therefore, we conducted a randomized, double-blind, placebo-controlled study to investigate the effects of this new method on both anaerobic and aerobic performance. METHODS: During 15 consecutive days, 20 endurance-trained men (V O2max (mean +/- SD) 60.2 +/- 6.8 mL x kg(-1) x min(-1)) were exposed each day to breathing (through mouthpieces) either a gas mixture (11% O2 on days 1-7 and 10% O2 on days 8-15; hypoxia group, N = 10) or compressed air (control group, N = 10), six times for 6 min, followed by 4 min of breathing room air for a total of six consecutive cycles. Before and after the treatment, an incremental cycle ergometer test to exhaustion and the Wingate anaerobic test were performed to assess aerobic and anaerobic performance. RESULTS: Hypoxic treatment did not improve peak power or mean power during the Wingate anaerobic test, nor did it affect maximal oxygen uptake (V O2max), maximal power output (Pmax), lactate threshold or levels of heart rate (HR), minute ventilation (V E), oxygen uptake (V O2), or blood lactate concentration at the submaximal workloads during the ergometer test. Maximal lactate concentration (Lamax) after the tests and HRmax and maximal respiratory exchange ratio (RERmax) during the ergometer test were not significantly different between groups at any time. CONCLUSION: The results of this study demonstrated that 1 h of intermittent hypoxic exposure for 15 consecutive days has no effect on aerobic or anaerobic performance.

Exercise with the intensity of the individual anaerobic threshold in acute hypoxia.

PURPOSE: The aim of the present study was to find out if the determination of the individual anaerobic threshold (IAT) during incremental treadmill tests in normoxia and acute normobaric hypoxia (FiO2 0.15) defines equivalent relative submaximal intensities in these environmental conditions. METHODS: 11 male middle and long distance runners performed a 1-h treadmill run in normoxia and hypoxia at the intensity of the IAT determined in the respective environment with measurement of lactate, glucose, heart rate, catecholamines, ventilatory parameters, and rate of perceived exertion (RPE). RESULTS: During the 1-h treadmill runs, speed was significantly reduced in hypoxia compared with normoxia (12.8 +/- 0.7 vs 14.7 +/- 0.7 km x h(-1)). Relative intensity expressed as a percentage of VO(2max) was similar in both environments (82-83% on the average) and elicited comparable lactate steady states [LaSS, 2.5 +/- 0.7 - 3.4 +/- 1.1 mmol x L(-1) (normoxia), 2.7 +/- 0.8 - 3.6 +/- 1.0 mmol x L(-1) (hypoxia) after 10 and 60 min, respectively] and glucose levels, but significantly reduced heart rate in hypoxia by 5 beats x min(-1) on the average. A steady state was also found for the ventilatory parameters. Plasma epinephrine and norepinephrine levels were similar in both environments. RPE was significantly lower after 40-60 min of exercise in hypoxia. CONCLUSIONS: Relative intensities in normoxia and acute hypoxia are equivalent when endurance exercise is performed with the running speed at the IAT determined in the respective environment. The heart rate-blood lactate relationship, however, is changed in hypoxia and relative submaximal exercise intensity is higher in acute hypoxia when training is performed with similar heart rate as in normoxia.

Exercise intensity typical of mountain climbing does not exacerbate acute mountain sickness in normobaric hypoxia.

Physical exertion is thought to exacerbate acute mountain sickness (AMS). In this prospective, randomized, crossover trial, we investigated whether moderate exercise worsens AMS in normobaric hypoxia (12% oxygen, equivalent to 4,500 m). Sixteen subjects were exposed to altitude twice: once with exercise [3 × 45 min within the first 4 h on a bicycle ergometer at 50% of their altitude-specific maximal workload (maximal oxygen uptake)], and once without. AMS was evaluated by the Lake Louise score and the AMS-C score of the Environmental Symptom Questionnaire. There was no significant difference in AMS between the exposures with and without exercise, neither after 5, 8, nor 18 h (incidence: 64 and 43%; LLS: 6.5 ± 0.7 and 5.1 ± 0.8; AMS-C score: 1.2 ± 0.3 and 1.1 ± 0.3 for exercise vs. rest at 18 h; all P > 0.05). Exercise decreased capillary Po(2) (from 36 ± 1 Torr at rest to 31 ± 1 Torr), capillary arterial oxygen saturation (from 72% at rest to 67 ± 2%), and cerebral oxygen saturation (from 49 ± 2% at rest to 42 ± 1%, as assessed by near-infrared spectroscopy; P < 0.05), and increased ventilation (capillary Pco(2) 27 ± 1 Torr; P < 0.05). After exercise, the increase in ventilation persisted for several hours and was associated with similar levels of capillary and cerebral oxygenation at the exercise and rest day. We conclude that moderate exercise at ~50% maximal oxygen uptake does not increase AMS in normobaric hypoxia. These data do not exclude that considerably higher exercise intensities exacerbate AMS.

Training in normobaric hypoxia and its effects on acute mountain sickness after rapid ascent to 4559 m.

In a randomized, placebo-controlled, double-blind study, we tested a 4-week program in normobaric hypoxia that is commercially offered for the prevention of acute mountain sickness (AMS). Twenty-two male and 18 female healthy subjects [mean age 33 +/- 7 (SD) years] exercised 70 min, 3 x /week for 3 weeks on a bicycle ergometer at workloads of 60% VO2max either in normoxia (normoxia group, NG) or in normobaric hypoxia (hypoxia group, HG), corresponding to altitudes of 2500, 3000, and 3500 m during weeks 1, 2, and 3, respectively. Four passive exposures of 90 min in normoxia (NG) or hypoxia corresponding to 4500 m (HG) followed in week 4. Five days after the last session, subjects ascended within 24 h from sea level to 4559 m (one overnight stay at 3611 m) and stayed there for 24 h. AMS was defined as LL (Lake Louise score) > or =5 and AMS-C > or =0.70. The AMS incidence (70% in NG vs. 60% in HG, p = 0.74), LL scores (7.1 +/- 4.3 vs. 5.9 +/- 3.4, p = 0.34), and AMS-C scores (1.50 +/- 1.22 vs. 0.93 +/- 0.81, p = 0.25) at the study endpoint were not significantly different between the groups. However, the incidence of AMS at 3611 m (6% vs. 47%, p = 0.01) and the functional LL score at 4559 m were lower in HG. SpO2 at 3611 m, heart rate during ascents, and arterial blood gases at 4559 m were not different between groups. We conclude that the tested program does not reduce the incidence of AMS within a rapid ascent to 4559 m, but our data show that it prevents AMS at lower altitudes. Whether such a program would prevent AMS at higher altitudes, but with slower ascent, remains to be tested.

Transpulmonary plasma ET-1 and nitrite differences in high altitude pulmonary hypertension.

Berger, Marc M., Christoph Dehnert, Damian M. Bailey, Andrew M. Luks, Elmar Menold, Christian Castell, Guido Schendler, Vitalie Faoro, Heimo Mairbäurl, Peter Bärtsch, and Eric R. Swenson. Transpulmonary plasma ET-1 and nitrite differences in high altitude pulmonary hypertension. High Alt. Med. Biol. 10:17-24, 2009.- Thirty-four mountaineers were studied at low (110 m) and high altitude (4559 m) to evaluate if increased pulmonary artery systolic pressure (PASP) at high altitude is associated with increased pulmonary endothelin-1 (ET-1) availability and alterations in nitrite metabolism across the lung. Blood samples were obtained using central venous and radial artery catheters for plasma ET-1 and nitrite. Pulmonary blood flow was measured by inert gas rebreathing to calculate transpulmonary exchange of plasma ET-1 and nitrite, and PASP was assessed by transthoracic Doppler echocardiography. After ascent to high altitude, PASP increased from 23 +/- 4 to 39 +/- 10 mmHg. Arterial and central venous plasma ET-1 increased, while plasma nitrite did not change significantly. At low altitude there was a transpulmonary loss of plasma ET-1, but a transpulmonary gain at high altitude. In contrast was a transpulmonary gain of plasma nitrite at low altitude and a transpulmonary loss at high altitude. PASP positively correlated with a transpulmonary gain of plasma ET-1 and negatively correlated with a transpulmonary loss of plasma nitrite. These results suggest that a transpulmonary gain of plasma ET- 1 is associated with higher PASP at high altitude. Transpulmonary loss of plasma nitrite indicates either less pulmonary nitric oxide (NO) production, which contributes to higher PASP, or increased NO bioavailability arising from nitrite reduction, which may oppose ET-1-mediated vasoconstriction.

Effects of acute moderate hypoxia on anaerobic capacity in endurance-trained runners.

While there is some controversy whether anaerobic capacity might be improved after altitude training little is known about changes in anaerobic capacity during hypoxic exposure in highly trained athletes. In order to analyze the effects of acute moderate normobaric hypoxia on anaerobic capacity, 18 male competitive triathletes, middle- and long-distance runners VO2max 67.4 +/- 3.8 ml kg min(-1) performed 2 supra-VO2max treadmill runs with the same speed, one in normoxia and one after 4 h exposure to normobaric hypoxia (FiO(2) 0.15), for estimation of their maximal accumulated oxygen deficit (MAOD) and measurement of peak capillary lactate and peak capillary ammonia concentration. MAOD was not significantly different in normoxia and in moderate hypoxia while time to exhaustion and accumulated O(2) uptake were significantly (P < 0.001) reduced in hypoxia compared to normoxia by 28 and 45%, respectively. The reduction in time to exhaustion was significantly correlated to the decrement in accumulated O(2) uptake (R = 0.730, P = 0.001). In hypoxia, there was a tendency for peak capillary lactate concentration to be decreased compared to normoxia (12.9 +/- 2.1 vs. 13.8 +/- 2.2 mmol l(-1), P = 0.082); peak capillary ammonia concentration was significantly decreased in hypoxia (97 +/- 52 vs. 121 +/- 44 micromol l(-1), P = 0.032). In conclusion, anaerobic capacity is not significantly changed during acute exposure to moderate hypoxia in endurance-trained athletes. The performance reduction during all-out exercise of short duration has to be attributed to the decrement in aerobic capacity.

Magnetic resonance imaging of uneven pulmonary perfusion in hypoxia in humans.

RATIONALE: Inhomogeneous hypoxic pulmonary vasoconstriction causing regional overperfusion and high capillary pressure is postulated for explaining how high pulmonary artery pressure leads to high-altitude pulmonary edema in susceptible (HAPE-S) individuals. OBJECTIVE: Because different species of animals also show inhomogeneous hypoxic pulmonary vasoconstriction, we hypothesized that inhomogeneity of lung perfusion in general increases in hypoxia, but is more pronounced in HAPE-S. For best temporal and spatial resolution, regional pulmonary perfusion was assessed by dynamic contrast-enhanced magnetic resonance imaging. METHODS: Dynamic contrast-enhanced magnetic resonance imaging and echocardiography were performed during normoxia and after 2 h of hypoxia (Fi(O2) = 0.12) in 11 HAPE-S individuals and 10 control subjects. As a measure for perfusion inhomogeneity, the coefficient of variation for two perfusion parameters (peak signal intensity, time-to-peak) was determined for the whole lung and isogravitational slices. RESULTS: There were no differences in perfusion inhomogeneity between the groups in normoxia. In hypoxia, analysis of coefficients of variation indicated a greater inhomogeneity in all subjects, which was more pronounced in HAPE-S compared with control subjects. Discrimination between HAPE-S and control subjects was best in gravity-dependent lung areas. Pulmonary artery pressure during hypoxia increased from 22 +/- 3 to 53 +/- 9 mm Hg in HAPE-S and 24 +/- 4 to 33 +/- 6 mm Hg in control subjects (mean +/- SD; p < 0.001), respectively. CONCLUSION: This study shows that hypoxic pulmonary vasoconstriction is inhomogeneous in hypoxia in humans, particularly in HAPE-S individuals where it is accompanied by a greater increase in pulmonary artery pressure compared with control subjects. These findings support the hypothesis of exaggerated and uneven hypoxic pulmonary vasoconstriction in HAPE-S individuals.

Cytochrome P450 enzyme-mediated drug metabolism at exposure to acute hypoxia (corresponding to an altitude of 4,500 m).

OBJECTIVE: To investigate the effect of acute hypoxia and concomitant changes in portal blood flow on the disposition of drugs mainly metabolized by the cytochrome P(450) enzymes (CYP) 3A4 (verapamil) and CYP1A2 (theophylline). METHODS: Twenty healthy male participants were studied on two 14-h study days in a normobaric hypoxic chamber and were allocated randomly to one of two groups receiving short infusions of either theophylline (6 mg kg (-1) body weight) or verapamil (5 mg) intravenously. According to a randomized, cross-over design, participants were once exposed to normoxia and once to hypoxia (12% oxygen corresponding to the ambient( P)O(2) at an altitude of 4,500 m above sea level). The concentrations of theophylline, 1,3-dimethyluric acid, verapamil, and norverapamil were determined in serial blood samples by means of liquid chromatography-mass spectrometry (LC/MS/MS). Portal blood flow was assessed by transabdominal duplex ultrasonography. RESULTS: Acute hypoxia did not alter the pharmacokinetics of theophylline [half-life+/-SD: 9.29+/-1.77 versus 9.39+/-1.40 (hypoxia)], 1,3-dimethyluric acid (12.9+/-4.72 versus 15.1+/-8.59), verapamil (2.00+/-0.98 versus 1.79+/-0.58), or norverapamil (7.98+/-2.94 versus 9.91+/-6.40). Individual changes of elimination half-life and changes in capillary oxygen saturation,( P)O(2), or portal vein flow were not correlated. Portal vein flow was unaffected by hypoxia. CONCLUSIONS: Acute hypoxia corresponding to hypoxia at altitudes of 4,500 m does not impair the metabolism mediated by CYP1A2 or CYP3A4. At rapid ascent to and short-term stay at altitudes up to 4,500 m, the doses of drugs metabolized by these CYPs do therefore not require dose modification, and major changes in the disposition of already administered drugs are not to be expected.