Background: In military flight operations fighter pilots work during flights constantly under hyperoxic breathing conditions with supplemental oxygen in varying hypobaric environments. These conditions are suspected to cause oxidative stress to neuronal organ tissues. For civilian flight operations the Federal Aviation Administration (FAA) also recommends supplemental oxygen for flying under hypobaric conditions equivalent to higher than 3048m altitude and makes it mandatory for conditions equivalent of more than 3657m altitude. Aim: We hypothesized that hypobaric-hyperoxic civilian commercial and private flight conditions with supplemental oxygen in a flight simulation in a hypobaric chamber at 2500m and 4500m equivalent altitude would cause significant oxidative stress in healthy individuals. Methods: 12 healthy subjects (6m, 6f, mean age 35,7yrs) from a larger cohort were selected to perform a 3hrs flight simulation in a hypobaric chamber with increasing supplemental oxygen levels (35%,50%,60% and 100% FiO2 via venturi valve equipped face mask) switching back and force between simulated altitudes of 2500m and 4500m. Arterial blood pressure and oxygen saturation were constantly measured via radial catheter and blood samples for blood gases taken from the catheter at each altitude and oxygen level. Additional blood samples from the arterial catheter at baseline and 60% oxygen at both different altitudes were centrifuged inside the chamber and the serum frozen instantly at minus 21°Celsius for later analysis of the oxidative stress markers malondialdehyde low density lipoprotein (M-LDL) and glutathione-peroxidase (GLP) via elisa test. Results: 11 subjects finished the study without adverse events. Whereas PO2 levels increased in the mean with increasing oxygen levels from baseline 96.2 mmHg to 160.9 mmHg at 2500m altitude and 60% FiO2 and 113.2 mmHg at 4500m altitude and 60% FiO2, there was no significant increase in both oxidative markers from baseline to 60% FiO2 at these simulated altitudes. Some individuals had a slight increase, some not at all or even a slight decrease. A moderate correlation (Pearson correlation coefficient 0,55) existed between subject age and glutathione peroxidase levels at 60% FiO2 at 4500m altitude. Conclusion: Supplemental oxygen of 60% FiO2 in a flight simulation comparing to flying in cabin pressure levels equivalent to 2500m-4500m altitude does not lead to a significant increase or decrease in the oxidative stress markers M-LDL and GLP in serum of arterial blood.

Standard cell culture is routinely performed at supraphysiological oxygen concentrations (~18% O2). Conversely, oxygen levels in most tissues range from 1%–6% (physioxia). Such hyperoxic conditions can alter reactive oxygen species (ROS) production, energy metabolism, mitochondrial network dynamics, and response to drugs and hormones. The aim of this project was to investigate the transcriptional response to different oxygen levels and whether it is similar across cell lines, or cell-line specific. Using RNA-seq, we performed differential gene expression and functional enrichment analyses in four human cancer cell lines, LNCaP, Huh-7, PC-3, and SH-SY5Y cultured at either 5% or 18% oxygen for 14 days. We found that oxygen levels affected transcript abundance of hundreds of genes, with the affected genes having little overlap between cell lines. Functional enrichment analysis also revealed different processes and pathways being affected in each cell line. Interestingly, we found that the top differentially expressed genes are involved in cancer biology. Further, we observed several hypoxia-inducible factor (HIF) targets upregulated at 5% oxygen, suggesting a role of HIF at physiological oxygen conditions. Finally, oxygen strongly induced transcription of mitochondrial genes in most cell lines, in a cell-type specific manner too. We conclude that cellular response to oxygen is widely cell-type specific, emphasizing the importance of maintaining physioxia in cell culture.

Since 1991, there has been an alarming increase in the number of unexplained physiological events (UPEs) reported and experienced by pilots of jet fighters across different fleets. The UPEs have resulted in grounding some airframes, loss of aircraft, and even loss of life. There is no single agreed-upon root cause of UPEs that has been identified, and therefore there is no reliable corrective action. This author analyzed the literature related to other industries where artificial hyperoxic gas mixes are employed and where similar adverse reports have been reported. I hypothesize that UPEs are caused by high-dose oxygen delivery in excess of officially approved oxygen schedules while airflow rates are often inadequate, at a time when the positive pressure breathing feature of their oxygen regulator is not used. In a setting where pulmonary vital capacity is adversely affected by G-maneuvers and oxygen- and G-induced atelectasis, tidal volume is reduced by flight gear, and effective gas exchange is not supported by adequate ventilation, these factors combine to produce respiratory acidosis, followed by acute respiratory distress syndrome, CO₂ narcosis, and coma. Reports from field data related to incidents in F-18S/H, showing that emergency oxygen did not correct the hypoxia-like symptoms including long-lasting periods of incapacitation and prolonged headaches, lend support to our hypothesis.

Individual susceptibility to pulmonary oxygen toxicity (PO2tox) is highly variable and currently lacks a reliable biomarker for predicting pulmonary hyperoxic stress. As nitric oxide (NO) is involved in many respiratory system processes and functions, we aimed to determine if expired nitric oxide (FENO) levels can provide an indication of PO2tox susceptibility in humans. Eight U.S. Navy trained divers volunteered as subjects. The hyperoxic exposures consisted of six- and eight-hour hyperbaric chamber dives conducted on consecutive days in which subjects breathed 100% oxygen at 202.65 kPa. Subjects’ individual variability in pulmonary function and FENO was measured twice daily over five days and compared with their post-dive values to assess susceptibility to PO2tox. Only subjects who showed no decrements in pulmonary function following the six-hour exposure conducted the eight-hour dive. FENO decreased by 55% immediately following the six-hour oxygen exposure (n=8, p<0.0001) and by 63% following the eight-hour exposure (n=4, p<0.0001). Four subjects showed significant decreases in pulmonary function immediately following the six-hour exposure. These subjects had the lowest baseline FENO and the lowest post-dive FENO and had clinical symptoms of PO2tox. Individuals with low FENO were the first to develop PO2tox symptoms and deficits in pulmonary function from the hyperoxic exposures. These data suggest that endogenous levels of NO in the lung may protect against the development of PO2tox.

Over the last 20 years, there has been a significant increase in the number of unexplained physiological events (UPEs) reported by pilots of fighter jets across different fleets. The UPEs have resulted in grounding some types of aircraft, loss of airplanes, and even loss of life. Despite considerable research, no single agreed-upon root cause has been found that explains UPEs, and therefore no reliable corrective actions exist. The purpose of this review was to analyze the literature related to other industries in which artificial hyperoxic gas mixes are employed and similar adverse reports have been reported. Based on analysis of the literature, it is hypothesized that UPEs are caused by unlimited delivery of high-dose oxygen in excess of officially approved oxygen schedules in the presence of inadequate airflow rates, at a time when the positive pressure breathing feature of their oxygen regulator system is not used. During flight maneuvers such as climbs, turns, and descents, pulmonary vital capacity is impaired by G-maneuvers and oxygen- and G-induced atelectasis. At the same time, tidal volume is reduced by flight gear, and effective gas exchange is not supported by adequate ventilation. These factors combine to produce hypercarbia, respiratory acidosis, acute respiratory distress syndrome, CO₂ narcosis, and coma. In fact, reports from field data related to incidents in F-18S/H, showing that emergency oxygen did not correct the hypoxia-like symptoms including long-lasting periods of incapacitation and prolonged headaches, lend support to this hypothesis.

Snoring gets no respect. It also gets little to no medical therapy. Why is this? How can something that is clearly pathological based on epidemiologic research not be diagnosed and treated with effective therapy? The problem is the lack of a credible, objective index of pathology during snoring. Pulse oximetry detects drops in arterial blood oxygen saturation (SpO₂) associated with obstructive sleep apnea and hypopnea events in polysomnographic (PSG) testing. When no desaturation is present, evidence of sleep disturbance is required to indicate the presence of pathology. However, obstruction at the mild end of the continuum of sleep disordered breathing (SDB) can occur without producing a drop in SpO₂ or sleep disturbance; in which case it is referred to as ‘primary snoring.’ Although statistically associated with co-morbidities of SDB, without there being a drop in SpO₂ or sleep disturbance, primary snoring is not thought to be pathologic enough to warrant diagnosis or treatment. One promising means of detecting the pathological processes associated with primary snoring is molecular tissue oximetry, which detects skin tissue oxygen need, vs. oxygen supply. In cases of hypoxic challenge, regulatory mechanisms restrict blood flow to the skin to preferentially maintain blood oxygen supply to more vital organs. As a result, molecular tissue oximetry of the skin is a more sensitive measure of inadequate breathing than pulse oximetry measuring blood oxygen saturation. In this article we review molecular tissue oximetry technology and methodology and make the case that it is a promising tool for identifying pathology occurring in association with primary snoring.