Assessment of the inflammatory effect of low-dose oxygen in mechanically ventilated patients

Although low doses of oxygen (FiO₂ <0.50) are considered nontoxic, recent studies have shown that even lower doses increase pulmonary inflammatory mediators. We aimed to evaluate the acute effects of reducing FiO₂ on pulmonary inflammation in mechanically ventilated patients without respiratory failure.

This study was a prospective, single-center crossover study in a medical/surgical intensive care unit at a university hospital. Hemodynamically stable patients under mechanical ventilation for >24 h without severe respiratory failure (PaO₂/FiO₂ >250). A basal FiO₂ of 0.40 was reduced to 0.21 provided SpO₂ remained higher than 90 %. Patients who could not tolerate the reduction in FiO₂ to 0.21 were excluded.

We screened 40 patients, but only 28 (70 %) tolerated FiO₂ 0.21. We measured common clinical variables and inflammatory mediators in plasma and in exhaled breath condensate (EBC) at the end of three 4-h periods: (1) basal (FiO₂ 0.40), (2) after FiO₂ reduction to 0.21, and (3) after returning FiO₂ 0.40. We used one-way ANOVA for repeated measurements with FiO₂ as the grouping variable. Median values of inflammatory mediators in EBC showed nonsignificant changes among the three periods: NO₂ + NO₃ 17.1, 14.1 and 11.0 μmol/L (p = 0.2), and 8-isoprostane 4.4, 8.2 and 5.3 pg/ml (p = 0.6) for the three periods, respectively. Plasma levels also showed nonsignificant changes during the period of the study: NO₂ + NO₃ 12.6, 16.3 and 15.0 μmol/L (p = 0.9), TNFα 13.5, 18.0 and 14.5 pg/ml (p = 0.8), IL-4 12.9, 18.7 and 23.9 pg/ml (p = 0.1), IL-6 50.9, 35.1 and 28.3 pg/ml (p = 0.6), and IL-10 15.2, 22.2 and 22.2 pg/ml (p = 0.7) for the three periods, respectively.

FiO₂ 0.40 in mechanically ventilated patients without severe respiratory failure did not increase systemic or pulmonary inflammation.