Utilising electronic anaesthesia records to audit nitrous oxide consumption and waste, and implementation of a loss monitoring system

Editor—Nitrous oxide (N₂O), a potent greenhouse gas and ozone layer depleter, is under scrutiny.¹ Hospitals internationally report 70–95% leakage from pipelines and manifolds,^2,3 prompting recommendations to decommission centralised systems in favour of cylinders mounted on anaesthesia machines to mitigate leakage and waste.3, 4, 5 At our institution, siloed clinical usage and procurement data prevented comparisons, and our anaesthesia machines did not record cumulative N₂O usage. From mid-2021 we transitioned to electronic health record (EHR) anaesthesia charting software,⁶ with automated interval recording of N₂O and oxygen gas flow rates and N₂O concentration every 1 min for all general anaesthesia cases. We aimed to investigate the magnitude of N₂O leakage from our hospital manifold and pipelines using EHR data to retrospectively calculate clinical use of N₂O for comparison with procurement.

Minute-by-minute N₂O and oxygen flow rates (L min⁻¹) and concentration (%) were obtained from EHR anaesthesia chart data from January 2022 to December 2023 at Changi General Hospital (Singapore), a tertiary 1000-bed nonobstetric teaching hospital with 14 operating theatres (OT).

N₂O cylinder procurement data were obtained from our hospital vendor, with confirmation from facilities management that only the OT complex utilised piped N₂O. N₂O loss was determined by calculating clinical usage for comparison with procured volume:

$$\text{Leakage}\text{volume}=\text{Procurement}\text{volume}-\text{Usage}\text{volume}-\text{Residual}\text{volume}$$

Differing methods to determine N₂O usage volume were used, as EHR data recorded varied by anaesthesia machine model and mode (Supplementary material 1). Where N₂O flow rates were available, minute-by-minute flow rates were added together to give volume. If unavailable, N₂O flow was calculated from inspired N₂O concentration and oxygen flow data, given O₂ concentration = 1−N₂O concentration. For a small subset (13%) of data, N₂O and oxygen flow rates were absent due to the Automated Gas Control mode (an automated target end-tidal inhalational programme) used in Maquet Flow-i anaesthesia machines.⁷ To account for the corresponding N₂O used during these missing data points, an adjusted total N₂O volume was extrapolated from the known proportion of N₂O used (100%–13%=87%), where:

$$\text{Adjusted}{\mathrm{N}}_2\mathrm{O}\text{volume}\text{used}=\frac{\text{Known}{\mathrm{N}}_2\mathrm{O}\text{volume}\text{used}}{1-\text{Proportion}\text{of}\text{missing}\text{datapoints}}$$

Our manifold cylinder pressures when depleted correspond to a 23% residual volume⁸ (Supplementary material 2). This corresponds to our N₂O vendor reporting that depleted returned cylinders contain 15–20% of residual N₂O, significantly higher than reported overseas.² We thus assumed residual volume as 20% of procurement volume. Although residual volume is vented to the atmosphere and contributes towards carbon emissions, we excluded it from calculations of leakage volumes to avoid inflation of the leakage volumes.

Yearly adjusted N₂O usage and residual volume were deducted from procurement volume to calculate the volume of N₂O leakage. Using the N₂O 100-year global warming potential of 273⁹ and a molar volume of 24.5 L, we calculated the leaked volume equivalent CO₂ emissions.

A total of 32 028 patients underwent surgical procedures from January 2022 to December 2023, with 2619 (16.2%) in 2022 and 1753 (11.1%) in 2023 receiving N₂O. A stacked area chart of percentage of total procurement volume against calendar month was plotted to visualise the change in N₂O wastage proportion over time. Figure 1 shows the N₂O leakage volume as a percentage of total N₂O procurement volume across time. The proportion of N₂O leakage increased towards the end of 2022 to start of 2023, then stayed relatively constant. The N₂O leakage volume was 451 214 L (63.2%) in 2022 and 821 182 L (73.2%) in 2023, with adjusted usage of 119 986 L (16.8%) in 2022 and 76 418 L (6.8%) in 2023. This leakage equates to 221 tonnes and 403 tonnes of carbon dioxide equivalents (CO₂e), respectively, and with residual volumes added equals a total of 804 tonnes of CO₂e in N₂O leaked or vented over 2 years, equivalent to driving 7 400 000 km in an average car.¹⁰ In 2023, the monthly adjusted usage per OT was a median (min, max) of 407.5 L (59.5, 945.6).

Fig 1.

Stacked area chart of percentage of total N₂O procurement volume against time. Residual N₂O was defined as 20% of total procurement volume.

While the proportion of N₂O leakage in our institution is slightly lower than internationally,² it is the primary driver for the hospital's high N₂O emissions. Although the proportion of patients receiving N₂O and its usage volumes decreased in 2023, leakage increased. The reduction in clinical usage likely reflects increased awareness from department education sessions on environmental concerns of N₂O.

As leakage is the main driver, reducing clinical use of N₂O does not always result in emissions reduction, prompting our institution to decommission our N₂O manifold in September 2024. Our 2023 monthly usage per OT demonstrates that portable 950 L N₂O cylinders will only require changing at most 4-weekly, an insignificant additional workload.

Prior publications on N₂O leakage were short-term audits requiring additional equipment or specific anaesthesia machines with N₂O usage data recording.^2,11,12 Our technique using EHR data allows trending of clinical usage against procurement for leak detection, consistent with recommended guidelines.⁵

Even after decommissioning manifolds, leakage can occur with portable cylinders,¹³ making ongoing monitoring prudent. To monitor ongoing N₂O usage and leakage, a monthly updating dashboard was created with the percentage stacked area chart of total procurement volume, adjusted N₂O usage and leakage volumes, monthly usage per OT, and percentage of missing data (Supplementary Fig. S1).

Unfortunately, the Automated Gas Control mode⁷ precludes N₂O flow rate calculations as N₂O and oxygen flow rate data are not recorded. We adjusted for missing data to account for the N₂O usage during this time. This calculated component constituted <15% of our data, but could result in significant error if it were a larger proportion. Residual volumes were assumed at 20%, but a lower 15% residual volume would yield higher leakage volumes. This technique cannot quantify leakage when the manifold supplies areas without anaesthesia machines.

Utilising anaesthesia EHR data we demonstrated significant N₂O leakage, prompting decommissioning of our N₂O manifold, with establishment of a monitoring system for clinical usage against procurement. This is replicable at hospitals with electronic charting systems, allowing unnecessary N₂O wastage to be identified and mitigated. Going forward, we will explore using EHR data to monitor clinical usage and carbon emissions from all inhalational anaesthetics.

Authors' contributions

Study design: MBHT, J-AY, WG, HMT, LWO

Study conduct and data analysis: MBHT, J-AY, WG, HMT, LWO

Data collection and statistical analysis: SS, DTGK, XHK

Writing of paper: MBHT, J-AY, WG, HMT, LWO, SS, DTGK, XHK

Declaration of interest

The authors declare that they have no conflicts of interest and have no relationships and activities to disclose.

Appendix A. Supplementary data

The following is the Supplementary data to this article: