NHS Lothian nitrous mitigation project: Assessing nitrous oxide usage at a district general hospital

ABSTRACT

The medical gas nitrous oxide (N2O) has historically played an important role in anaesthesia;however, its clinical use has declined with the advent of alternative agents and further understanding of its environmental impact [1]. It is a potent greenhouse gas with 298 times the global warming potential of carbon dioxide [2] and makes up a large proportion of the NHS carbon footprint. As part of a Lothian-wide nitrous mitigation project, we set out to establish the current clinical use of piped N2O at St John's Hospital (SJH). Methods Data were gathered regarding anaesthetic N2O use at SJH in December 2020 to February 2021. Working with site Estates we established the N2O manifold supplied theatres, radiology and ICU departments. We also determined that the radiology and ICU outlets were never used according to clinicians. Pharmacy provided us with the details of N2O manifold supply and returns data from BOC. Anaesthetic technique and speciality were self-reported by completion of a daily form for 5 consecutive weekdays, and anaesthetic machine logs were interrogated to provide the N2O use per case over a different 7-day period. Results Data from 103 self-reported theatre cases showed N2O was used for 10% of cases, eight of which were paediatric dental, one plastics emergency and one obstetric. One hundred and fifteen cases were logged by the anaesthetic machines, with 14% of cases using N2O, eight paediatric dental, one special needs dental, one gynaecology, one plastics and three obstetric. Total N2O use was calculated as 719 l over 7 days. Total anaesthetic case numbers were 6635 for 2020, significantly reduced with the impact of COVID-19. Assuming a similar case mix, annual use is estimated at 41,483 l for 2020. Manifold supply figures show that 342,000 l N2O was supplied in the last year, indicating an estimated 87.9% disparity in supply vs. demand. Discussion Our data suggest minimal clinical use of piped N2O at SJH. A large proportion of the manifold supply is not accounted for clinically and must be assumed as wastage, which may be a combination of leakage, theft and expired cylinders due to poor stock rotation. To address this, we were able to get the Estates team to shut down the pipelines to the intensive care unit and radiology departments, and pressure tests of the pipelines are underway to locate further leaks in the system. All sites using N2O should be looking for ways to deliver the leanest supply.