Changes in Gas Composition during Low Flow Anaesthesia without Nitrous Oxide.

INTRODUCTION: Low flow anaesthesia utilising Oxygen (O2 ) and Nitrous Oxide (N2 O) mixture carries a risk of hypoxia, but avoiding N2 O results in increased analgesic and volatile anaesthetic agent requirement.

AIM: This study attempted to find the lowest Fraction of inspired Oxygen (FiO2 ) levels achieved with a mixture of 300 mL/min each of O2 and medical air over two hours and to compare the overall analgesic requirement and cost while using similar flows of N2 O and O2 , respectively.

MATERIALS AND METHODS: A prospective observational study was conducted between March 2015 and June 2016 at the Jawaharlal Institute of Postgraduate Medical Education and Research, Puducherry, India. Patients of American Society of Anaesthesiologists (ASA) Grade 1 and 2 undergoing surgery under general anaesthesia with an endotracheal tube were included in the study, in two groups of 40 each. In the initial ten minutes following induction of anaesthesia, both groups received high Fresh Gas Flows (FGF) of 3 L/min each (Group O: medical air and oxygen; Group N: N2 O and oxygen), following which the FGF rates were reduced to 300 ml/min each. Any value of FiO2 lesser than 0.3 during the duration of anaesthesia was considered to render the technique unsafe for clinical use. SPSS software version 20.0 was used to generate data and figures.

RESULTS: The lowest FiO2 recorded was 0.33 in Group O and 0.3 in Group N which occurred at the end of two hours. Mean analgesic requirement was significantly higher in Group O compared to Group N (151.85 μg, 124.85 μg; p-value=0.004) with a 62% increase in the cost incurred.

CONCLUSION: The use of medical air and oxygen in flows of 300 ml/min each following initial high flows of 3 L/min appears to be a safe technique. However, this combination was associated with an increase in the cost of anaesthesia and in the need for additional intra-operative analgesia.