Exhaled Nitric Oxide and Pulmonary Oxygen Toxicity Susceptibility

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 (F_ENO) 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 F_ENO 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. F_ENO 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 F_ENO, had the lowest post-dive F_ENO, and had clinical symptoms of PO₂tox. Individuals with low F_ENO 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 lungs may protect against the development of PO₂tox.