Exhaled nitric oxide is associated with acute mountain sickness susceptibility during exposure to normobaric hypoxia

Exhaled Nitric Oxide Changes During Acclimatization to High Altitude: A Descriptive Study

Summerfield, Douglas T., Kirsten E. Coffman, Bryan J. Taylor, Amine N. Issa, and Bruce D. Johnson. Exhaled nitric oxide changes during acclimatization to high altitude: a descriptive study. High Alt Med Biol. 19:215-220, 2018.

AIMS

This study describes differences in the partial pressures of exhaled nitric oxide (PeNO) between subjects fully acclimatized (ACC) to 5300 m and those who have just arrived to high altitude.

METHODS

PeNO was determined in eight subjects newly exposed and nonacclimatized (non-ACC) to high altitude and compared with that in nine subjects who had ACC to high altitude for 1 month. In addition, systolic pulmonary artery pressure (sPAP) and arterial oxygen saturation (SaO₂) were measured in all participants. These measurements were repeated in the non-ACC group 5 and 9 days later.

RESULTS

PeNO levels on day 1 were significantly higher in the non-ACC versus ACC cohort (8.7 ± 3.5 vs. 3.9 ± 2.2 nmHg, p = 0.004). As the non-ACC group remained at altitude, PeNO levels fell and were not different when compared with those of the ACC group by day 9 (5.9 ± 2.4 vs. 3.9 ± 2.2 nmHg, p = 0.095). Higher sPAP was correlated with lower PeNO levels in all participants (R = -0.50, p = 0.043). PeNO levels were not correlated with SaO₂.

CONCLUSIONS

As individuals acclimatized to high altitude, PeNO levels decreased. Even after acclimatization, PeNO levels continued to play a role in pulmonary vascular tone.

Effects of dietary nitrate on respiratory physiology at high altitude - Results from the Xtreme Alps study

Nitric oxide (NO) production plays a central role in conferring tolerance to hypoxia. Tibetan highlanders, successful high-altitude dwellers for millennia, have higher circulating nitrate and exhaled NO (ENO) levels than native lowlanders. Since nitrate itself can reduce the oxygen cost of exercise in normoxia it may confer additional benefits at high altitude. Xtreme Alps was a double-blinded randomised placebo-controlled trial to investigate how dietary nitrate supplementation affects physiological responses to hypoxia in 28 healthy adult volunteers resident at 4559 m for 1 week; 14 receiving a beetroot-based high-nitrate supplement and 14 receiving a low-nitrate 'placebo' of matching appearance/taste. ENO, vital signs and acute mountain sickness (AMS) severity were recorded at sea level (SL) and daily at altitude. Moreover, standard spirometric values were recorded, and saliva and exhaled breath condensate (EBC) collected. There was no significant difference in resting cardiorespiratory variables, peripheral oxygen saturation or AMS score with nitrate supplementation at SL or altitude. Median ENO levels increased from 1.5/3.0  mPa at SL, to 3.5/7.4 mPa after 5 days at altitude (D5) in the low and high-nitrate groups, respectively (p = 0.02). EBC nitrite also rose significantly with dietary nitrate (p = 0.004), 1.7-5.1  μM at SL and 1.6-6.3 μM at D5, and this rise appeared to be associated with increased levels of ENO. However, no significant changes occurred to levels of EBC nitrate or nitrosation products (RXNO). Median salivary nitrite/nitrate concentrations increased from 56.5/786 μM to 333/5,194  μM  with nitrate supplementation at SL, and changed to 85.6/641 μM and 341/4,553 μM on D5. Salivary RXNO rose markedly with treatment at SL from 0.55 μM to 5.70 μM. At D5 placebo salivary RXNO had increased to 1.90 μM whilst treatment RXNO decreased to 3.26 μM. There was no association with changes in any observation variables or AMS score. In conclusion, dietary nitrate supplementation is well tolerated at altitude and significantly increases pulmonary NO availability and both salivary and EBC NO metabolite concentrations. Surprisingly, this is not associated with changes in hemodynamics, oxygen saturation or AMS development.

The Influence of 17 Hours of Normobaric Hypoxia on Parallel Adjustments in Exhaled Nitric Oxide and Airway Function in Lowland Healthy Adults

Van Iterson, Erik H., Eric M. Snyder, and Bruce D. Johnson. The influence of 17 hours of normobaric hypoxia on parallel adjustments in exhaled nitric oxide and airway function in lowland healthy adults. High Alt Med Biol. 18:1-10, 2017.-Currently, there is a disparate understanding of the role that normobaric hypoxia plays in affecting nitric oxide (NO) measured in exhaled air (eNO) and airway function in lowland healthy adults. Compared to normobaric normoxia, this study aimed to test the effect of 17 hours of normobaric hypoxia on relationships between eNO and airway function in healthy adults. In a crossover study including 2 separate visits, 26 lowland healthy Caucasian adults performed eNO and pulmonary function tests on visit 1 in normobaric normoxia, while repeating all tests on visit 2 following 17 hours of normobaric hypoxia (12.5% O₂). Compared to normobaric normoxia, eNO (29 ± 24 vs. 36 ± 28 ppb), forced expiratory volume in one second (FEV₁) (4.1 ± 0.7 vs. 4.3 ± 0.8 L), mean forced expiratory flow between 25% and 75% FVC (FEF_25-75) (3.9 ± 1.0 vs. 4.2 ± 1.2 L/s), and forced expiratory flow at 75% FVC (FEF₇₅) (2.0 ± 0.7 vs. 2.3 ± 0.8 L/s) increased in normobaric hypoxia, respectively (all p < 0.05). Correlations at normoxia between eNO and FEV₁ (r = 0.39 vs. 0.44), FEF_25-75 (r = 0.51 vs. 0.51), and FEF₇₅ (r = 0.53 vs. 0.55) persisted as both parameters increased in hypoxia, respectively. For the first time, these data suggest that 17 hours of hypoxic breathing in the absence of low ambient pressure contribute to increased eNO and airway function in lowland healthy adults.

Anti-hypoxia Activity of the Novel NO Donor Acetyl Ferulic Isosorbide Mononitrate in Acute High-Altitude Hypoxia Mice

Nitric oxide (NO) may act as either a pro-oxidant or an antioxidant in biological systems. Previous work has found inhalation of NO improved survival in a high altitude rat model. NO donor isosorbide mononitrate derivants might have a protective effect against hypoxia. We synthesized a series of isosorbide mononitrate derivant compounds to test their anti-hypoxia activities. Normobaric hypoxia and hypobaric hypoxia models were used to study the protective role of NO donor in mice. The results showed isosorbide mononitrate derivants had protective effects in hypoxia mice. Among those compounds, acetyl ferulic isosorbide mononitrate (AFIM) was the most effective. It prolonged the survival time during the normobaric hypoxia test. It decreased malondialdehyde (MDA) and H2O2 in hypobaric hypoxia mice. The antioxidase activities of superoxide dismutase (SOD), glutathione peroxidase (GSH-Px) and catalase (CAT) remained in normal ranges in the AFIM group. As a sign of mitochondrial dysfunction, the activities of ATPase were down regulated in mice under hypobaric hypoxia conditions. AFIM also protected ATPase activities. The protective effects of AFIM might come from a sustained NO supply and the release of acetyl ferulic acid with anti-oxidant activity.

Is poor sleep quality at high altitude separate from acute mountain sickness? Factor structure and internal consistency of the Lake Louise Score Questionnaire

BACKGROUND

The factor structure and internal consistency of the Lake Louise Score Questionnaire (LLSQ) have not been determined in a large population at high altitude; however, a single-factor structure and a high internal consistency are preferable for accurate clinical and research applications of the LLSQ.

METHODS

A large group of Nepalese pilgrims (n=491) were assessed for acute mountain sickness with a verbal Nepali translation of the LLSQ after rapidly ascending from 1950 m to 4380 m. The factor structure and internal consistency of the LLSQ were determined with a confirmatory factor analysis (CFA) and the ordinal alpha coefficient, respectively.

RESULTS

A one-factor structure with all five items of the LLSQ was accepted. Four items (headache, gastrointestinal upset, fatigue/weakness, and dizziness/lightheadedness) loaded strongly on this factor (>0.70), but sleep quality had a low factor loading (0.33). The internal consistency (ordinal alpha coefficient) was 0.79, but removing the sleep quality item improved this value to 0.84.

CONCLUSIONS

The sleep quality item of the LLSQ was weakly related to the other items of the LLSQ. Future research should further investigate whether impaired sleep at altitude should be considered separately from other symptoms of AMS.

Acute mountain sickness is not repeatable across two 12-hour normobaric hypoxia exposures

OBJECTIVE

The purposes of this experiment were to determine the repeatability of acute mountain sickness (AMS), AMS symptoms, and physiological responses across 2 identical hypoxic exposures.

METHODS

Subjects (n = 25) spent 3 nights at simulated altitude in a normobaric hypoxia chamber: twice at a partial pressure of inspired oxygen (PIO2) of 90mmHg (4000 m equivalent; "hypoxia") and once at a PIO2 of 132 mmHg (1000 m equivalent; "sham") with 14 or more days between exposures. The following variables were measured at hours 0 and 12 of each exposure: AMS severity (ie, Lake Louise score [LLS]), AMS incidence (LLS ≥3), heart rate, oxygen saturation, blood pressure, and the fraction of exhaled nitric oxide. Oxygen saturation and heart rate were also measured while subjects slept.

RESULTS

The incidence of AMS was not statistically different between the 2 exposures (84% vs 56%, P > .05), but the severity of AMS (ie, LLS) was significantly lower on the second hypoxic exposure (mean [SD], 3.1 [1.8]) relative to the first hypoxic exposure (4.8 [2.3]; P < .001). Headache was the only AMS symptom to have a significantly greater severity on both hypoxic exposures (relative to the sham exposure, P < .05). Physiological variables were moderately to strongly repeatable (intraclass correlation range 0.39 to 0.86) but were not associated with AMS susceptibility (P > .05).

CONCLUSIONS

The LLS was not repeatable across 2 identical hypoxic exposures. Increased familiarity with the environment (not acclimation) could explain the reduced AMS severity on the second hypoxic exposure. Headache was the most reliable AMS symptom.

Exhaled nitric oxide concentration upon acute exposure to moderate altitude

The purpose of this study was to assess immediate changes in the partial pressure of nitric oxide (NO) in exhaled gas (PE NO ) in healthy trained subjects who were acutely exposed to moderate altitude. One group of nine and another group of 20 healthy subjects were exposed to an ambient pressure of 728 hPa (546 mmHg) corresponding to an altitude of 2800 m for 5 and 90 min, respectively, in an altitude chamber. PE NO was measured offline by sampling exhaled gas in tight metal foil bags at 5, 30, 60, and 90 min. A correction for increased expiratory flow rate due to gas density effects at altitude was performed (PE NO corr). PE NO was significantly decreased by 13-16%, while the fraction of NO in exhaled gas (FE NO) was increased by 16-19% compared to sea level. There was no significant change in PE NO corr after exposure to altitude for 5, 30, 60, and 90 min. We conclude that there was no change in PENO upon arrival at altitude after correcting for gas density effects on expiratory flow rate. Corrections for altitude effects must be done before comparing measurements performed at different altitudes when using measurements of FENO to monitor athletes who have asthma during training at altitude.

Lung oxidative damage by hypoxia

One of the most important functions of lungs is to maintain an adequate oxygenation in the organism. This organ can be affected by hypoxia facing both physiological and pathological situations. Exposure to this condition favors the increase of reactive oxygen species from mitochondria, as from NADPH oxidase, xanthine oxidase/reductase, and nitric oxide synthase enzymes, as well as establishing an inflammatory process. In lungs, hypoxia also modifies the levels of antioxidant substances causing pulmonary oxidative damage. Imbalance of redox state in lungs induced by hypoxia has been suggested as a participant in the changes observed in lung function in the hypoxic context, such as hypoxic vasoconstriction and pulmonary edema, in addition to vascular remodeling and chronic pulmonary hypertension. In this work, experimental evidence that shows the implied mechanisms in pulmonary redox state by hypoxia is reviewed. Herein, studies of cultures of different lung cells and complete isolated lung and tests conducted in vivo in the different forms of hypoxia, conducted in both animal models and humans, are described.