Optimal Fresh Gas Flow when Employing Total Intravenous Anesthesia

Fresh gas flow (FGF) during total intravenous anesthesia (TIVA) is uncoupled from anesthetic agent delivery. Higher FGFs reduce carbon dioxide absorber consumption, but the overall costs and environmental trade-offs are unclear. This study aimed to develop a context-specific calculator to estimate the "optimal" FGF during TIVA across different settings.

A life-cycle assessment was conducted of oxygen and air production, and carbon dioxide absorbent production and disposal, quantifying environmental impact as global warming potential in 100 yr (GWP 100 ). The component costs (oxygen, air, and carbon dioxide absorbers) were obtained from institutional data. Annual costs and total GWP 100 were modeled across FGFs of 1 to 10 l/min and F io2 of 30 to -60%. Subsequently, a web-based calculator was developed ( https://fgfcalculator.com ) that allows users to input context-specific parameters, including grid emissions factors, to identify a FGF that best meets local objectives. This study assessed the annual proportion of TIVA cases in an institution in Singapore and applied local inputs to determine the optimal range.

In the study's institution (grid emissions factor, 0.599 kilogram carbon dioxide equivalent [kgCO 2 e]/kWh; 36.4% TIVA), increasing FGF (F io2 30%) reduced annual cost from Singapore dollars (S$) 12,948 (1 l/min) to S$1,572 (10 l/min) but increased annual GWP 100 nearly linearly from 1,939 kgCO 2 e (1 l/min) to 5,910 kgCO 2 e (10 l/min). The largest cost reduction occurred between 2 and 4 l/min, beyond which savings plateaued. Globally (assuming landfill disposal), the FGF that minimized GWP 100 depended strongly on grid emissions factor: higher FGF decreased GWP 100 when grid emissions factor was less than or equal to 0.011 kgCO 2 e/kWh; was favorable up to 6 l/min when grid emissions factor was 0.011 to 0.080; up to 4 l/min when grid emissions factor was 0.080 to 0.20; and increased GWP100 at all higher-grid emissions factor settings (greater than or equal to 0.200).

The optimal FGF is context specific. It is highly dependent on a country's grid emissions factor and the method of carbon dioxide absorbent disposal. By factoring in these variables, the calculator provides an evidence-based suggestion for FGF during TIVA, balancing economic and environmental considerations across the globe.