Lower exhaled nitric oxide in hypobaric than in normobaric acute hypoxia

Analysis of exhaled nitric oxide (NO) has become an accepted complementary tool in the management of inflammatory airway diseases. Previous studies have demonstrated reduced exhaled NO at altitude and ascribed their findings to hypoxia. We studied exhaled NO partial pressures (PeNO) in eight healthy subjects at reduced ambient pressure down to 540 hPa (equivalent to 5000 m altitude) and at sea level, with equivalently hypoxic breathing gases (down to 11.3% O2 in N2). PeNO readings were corrected for gas density effects on the instrument performance. Sea level control values for PeNO at an exhaled flow of 50 ml s−1 averaged 2.4 mPa and were virtually unchanged with normobaric hypoxia down to an inspired PO2PO2 of 10.7 kPa. With the same degree of hypoxia, hypobaric PeNO was 1.4 mPa. The reduction in hypobaric PeNO of up to 33 ± 16% (mean ± SD) in comparison to normobaric PeNO, is likely to have been caused by enhanced axial backdiffusion of NO because of the reduced gas density and an associated increased alveolar NO uptake to the blood.