A recent systematic review and meta-analysis showed that liberal oxygen therapy increases the mortality rate of patients and does not improve other important patient clinical outcomes.8 The study used different oxygen delivery devices including nasal prongs, face masks, and invasive mechanical ventilators. Different from a previous study, our study used a high-flow and non-invasive oxygen device, NRM, before providing ventilator support. In EDs, EPs usually preoxygenate patients with NRM before induction and intubation to increase the duration of safe apnoea.16,17 EPs also use non-invasive but high-flow oxygen therapy in elderly patients considering the provision of hospice care rather than intensive resuscitation; however, this requires further communication and discussion among healthcare professionals.21–23 In Taiwan, NRM is commonly used. Several patients hesitate to receive intubation considering the possibility of delayed weaning and tracheostomy, subsequently delaying the provision of ventilator therapy. Considering that excess supplemental oxygen causes oxygen toxicity in critically ill patients18, we determined whether NRM use in patients with respiratory failure could increase their mortality rate.
According to a previous study, the 30-day mortality rate in patients with respiratory failure ranged from 29.7–41.7%.24 In this study, the mortality rates were 30.0% and 30.4% in patients using NRM and patients not using NRM, respectively, a result consistent with the results of the previous studies. However, according to clinical practice, patients using NRM as an alternative oxygen treatment are considered less critical than patients using ventilator support immediately. We found that the 72-hour mortality rate in patients using NRM was 4.5% lower than the mortality rate of patients not using NRM. This suggests that the initial conditions of patients using NRM were less critical than the initial conditions of patients not using NRM. Therefore, this study performed PSM and analysed the influence of NRM according to its duration of use. Patients using NRM for a prolonged period had a higher mortality rate than patients using NRM for a limited time. This is possibly attributed to the high oxygen toxicity as a result of high-concentration oxygen therapy reported in a previous study.18
Although liberal oxygen therapy has not been recommended considering that it results in an increased mortality rate, it is still unclear which groups of patients are evidently affected with the increase in mortality rate following liberal oxygen therapy. The recent systemic review enrolled patients with sepsis, critical illness, stroke, trauma, myocardial infarction, or cardiac arrest and patients undergoing emergency surgery.8 While liberal oxygen therapy might increase patients’ mortality rate, there was no association between liberal oxygen treatment and mortality in patients with sepsis, stroke, trauma, and myocardial infarction. In our study, after performing a stratified analysis, the influence of NRM use on mortality was observed only in patients with respiratory failure due to pulmonary disease. This result is relatively consistent with the results of a systemic review, although there was no clear definition of critically ill patients in the systemic review article. Clinically, high-concentration oxygen therapy was not recommended in patients with asthma and chronic obstructive pulmonary disease, as it might result in carbon dioxide retention and acidosis.25–27 Several studies have also reported that prolonged breathing with a significantly high fraction of inspired oxygen (FIO2) (FIO2 ≥ 0.9) uniformly causes severe hyperoxic acute lung injury, and when the FIO2 is not reduced, death may occur.28 Hence, patients with pulmonary disease who prolonged use NRM have higher mortality rates than patients with pulmonary disease who are not using NRM.