Q: At what level of FiO2 oxygen toxicity may occur? (select one)

A) more than 40% for more than four days
B)  more than 60% for more than four days
C)  more than 80% for more than two days
D) 100% beyond 24 hours 
E) There is no well-defined threshold FiO2 or duration for oxygen toxicity



Answer: E

Objective of above question is to enhance the awareness of supplemental oxygen in ICU. Conventionally 60% of FiO2 is used as threshold for oxygen toxicity. But this is a huge misconception. There is no well-defined threshold or duration of supplemental oxygen below which oxygen toxicity may or may not occur. Any level of or duration of  supplemental oxygen therapy may cause various effects like absorptive atelectasis, worsening of  hypercapnia in COPD patients, airway injury as well as parenchymal lung injury. Underlying lung disease, overall functional and nutritional status, and previous exposure to high risk lung medicines particularly bleomycin exposure may cause oxygen toxicity at any level and at any duration. PaO2 between 60 and 70 mmHg and pulse-ox oxygen saturation around 92% should be sufficient and beyond this add relatively little to the oxygen content of the blood. Most importantly, along with optimum FiO2, optimal PEEP should be provided in ventilated patients.