Nitrous oxide has been used for analgesia and sedation in medicine for more than 150 years, and is well tolerated with manageable side effects. However, it is a potent greenhouse gas, and global efforts are underway to reduce its usage in a range of areas from agriculture to medicine.
One litre of nitrous oxide has approximately the same global warming potential as 0.5kg of CO2. A large hospital can use more than one million litres a year, resulting in a carbon footprint of 500 tons CO2e. Across an entire healthcare service, nitrous oxide can be responsible for up to 3-5% of the total carbon footprint. In modern hospitals, pipelines supply oxygen, medical air, and nitrous oxide from a central reservoir to their point of use. Recently, the UK’s National Health Service has shown that nitrous oxide pipelines can leak between 75-95% of the gas they carry.
Custom-made level flow meter built to measure leaks
Individual hospitals find it difficult to measure potential leaks during normal operations, because special testing interferes with clinical practice. Nitrous oxide is piped to locations of use in the hospital, and on occasion, delivered through individual cylinders. Leaks may be present in manifolds, pipelines, connections, and wards where nitrous is delivered to patients.
We designed and built a custom-made hospital level flow meter to precisely measure nitrous oxide flows at the Children’s Hospital at Westmead, to quantify how much was leaking, and what patterns of flow were associated with day-to-day operations. There were several specific requirements to be met:
- The ability to measure flow accurately and precisely across a wide range of possible flow rates. It was particularly important to accurately measure the lower range of flows, as this would help us quantify the size of any leakage.
- Have a good time resolution to enable us to detect changes in flows every minute.
- Placement at the actual source of nitrous oxide for the hospital, to try and capture all sources of leak.
- Data storage and transfer capabilities to speed up analysis and interventions.
Considerable leaks detected across the hospital
Our project has identified an approximate 50% leak of nitrous oxide at our institution over the period of monitoring. We are now in the process of identifying the locations responsible for leakage, together with preventive maintenance that includes regular checking of outlets, joints, manifolds and other areas where leaks may occur.
To aid in this leak localisation process, we are combining data from a custom anaesthesia database that tracks all gas usage in the anaesthesia department on a minutely basis, with the whole of hospital nitrous oxide flow data. This data science focussed approach substantially aids in identifying where individual administrations of nitrous oxide come from, and will make the identification of leaks, and the future maintenance of the pipeline system much easier.
This leak represents around 500 tons of CO2e, which is 0.5-1% of the carbon footprint of an average large hospital. Addressing these leaks, together with optimal clinical use of nitrous oxide, are part of the many actions needed to reduce the carbon footprint of healthcare. We are arranging testing of the mobility of the unit to other sites in NSW, as well as redesigning the flowmeters. The project is designed to scale across multiple institutions.
