Reliability of methods to estimate the fraction of inspired oxygen in patients with acute respiratory failure breathing through non-rebreather reservoir bag oxygen mask

Severity of hypoxaemia can be assessed using the partial pressure of arterial oxygen to fraction of inspired oxygen ratio (FiO(2)). However, in patients breathing through non-rebreather reservoir bag oxygen mask, accuracy of bedside FiO(2) estimation methods remains to be tested. In a post-hoc analysis of a multicentre clinical trial, three FiO(2) estimation methods were compared with FiO(2)measured with a portable oxygen analyser introduced in the oxygen mask. Among 262 patients analysed, mean (SD) measured FiO(2) was 65% (13). The 3%-formula (21% + oxygen flow rate in L/min x 3) was the most accurate method to estimate FiO(2). Other methods overestimated FiO(2)and hypoxaemia severity, so they should be avoided.