Nitrous oxide has a significant carbon footprint, and global initiatives are in progress to diminish its environmental impact in agriculture, industry and healthcare [1]. It is piped to locations in many healthcare environments, and in some hospitals, these systems can leak most of the gas they carry (75–95%) [1]. Leak detection traditionally relies on static pressure testing, a time-consuming and non-continuous method. This project aimed to measure the use and potential leakage of nitrous oxide at a large tertiary children's hospital using a custom-built, hospital-level flowmeter providing real-time, high-resolution, continuous measurements at the source of supply.

Local quality improvement approval was received for this project. We designed a customised flowmeter system with the following goals: the capability to gauge nitrous oxide flow within the range of 0.1–500 l.m^-1 under conditions of 100 kPa pressure and 20°C; high temporal resolution of 1 s; temperature compensated; connection between nitrous oxide supply packs and the hospital manifold, avoiding cutting into existing hospital pipework; and digital signal acquisition, storage and transmission capability with remote data access.

The system was built by Emerson Corporation (St Louis, MO, USA). It incorporates two mass flowmeters (Coriolis principle method, 3 mm and 1 mm pipe diameters), connection points for nitrous oxide, a bypass arm, control valves and a secure data transfer module. The unit was leak-tested and commissioned before connection to a standard cylinder pack and then to the main hospital manifold. The 1 mm calibre flowmeter was utilised for the measurement period, following testing of the 3 mm unit, to obtain the most precise quantification of the basal flow rate. Leakage flow rate was defined as that measured during time periods of very low clinical activity (03.00–04.00) with no flow peaks suggesting clinical use. Mean total flow rate was calculated over 1 week and compared with the average leakage flow rate to derive the leakage rate as a proportion. Data were visualised and analysed in MATLAB 2020b (Mathworks™ Natick, MA, USA).

Flowmeter readings for a 1-week period are shown in Figure 1. Average daily flow rates were 1.44 l.min^-1, and basal continuous flows representing leakage were 0.52 l.min^-1, representing a 50% leakage/wastage rate. Daily use patterns can clearly be identified in Figure 1, with peaks in flowrates between 15–25 l.min^-1. The hospital conducts approximately 15,000 anaesthetics per year across all specialities, has 300 beds and is a national-level paediatric burns unit. The paediatric emergency department is co-located with an adult emergency department in a separate, newer building and these figures do not include emergency department use.

This leak fraction is lower than those reported by investigators in the UK NHS but represents an annual carbon footprint of 621 tons of carbon dioxide equivalents based on total hospital nitrous oxide purchasing in 2019 of 2.3 million litres. Other methods of leak detection include analysis of purchase and use data, with larger uncertainties in measurement [2]; a weighing method using scales and G cylinders [3]; and the use of flowmeters. The described technique involves direct, high fidelity, hospital-wide measurement, building on a more localised flowmeter method described by Wong et al. [4], and may be a useful tool to identify and reduce nitrous oxide leak in hospital pipeline systems. This may be particularly useful in areas where clinical usage of nitrous oxide is still required, including maternity suites and paediatric environments. Utilising nitrous oxide destruction systems is an option in environments using only nitrous oxide (not volatile anaesthetics), but problems with their real-world efficacy and their inability to assist with nitrous leakage are noted issues [5]. There have been moves to supply nitrous oxide purely using cylinders in several healthcare systems, and active efforts to decommission pipelines are underway in several regions because of the discovery of these large leaks. However, these decisions assume that a well-maintained, low leak fraction pipeline system is very difficult or impossible to achieve, something high-fidelity monitoring systems may facilitate greatly in places with the continued need for pipeline systems. This system will now be used to identify leak locations using sequential isolation of pipeline networks, together with outlet testing and other methods.

一氧化二氮具有显着的碳足迹，全球正在采取行动减少其对农业、工业和医疗保健的环境影响[ 1 ]。它通过管道输送到许多医疗环境中的位置，在一些医院中，这些系统可能会泄漏其携带的大部分气体 (75–95%) [ 1 ]。传统上，泄漏检测依赖于静压测试，这是一种耗时且非连续的方法。该项目旨在使用定制的医院级流量计测量一氧化二氮的使用情况和潜在泄漏，该流量计可在供应源提供实时、高分辨率、连续测量。

该项目已获得当地质量改进批准。我们设计了定制的流量计系统，目标如下：能够在100 kPa压力和20°C条件下测量0.1-500 lm ^-1范围内的一氧化二氮流量； 1 秒的高时间分辨率；温度补偿；一氧化二氮供应包和医院歧管之间的连接，避免切割现有的医院管道；以及具有远程数据访问功能的数字信号采集、存储和传输能力。

该系统由艾默生公司（美国密苏里州圣路易斯）建造。它包含两个质量流量计（科里奥利原理方法，3 毫米和 1 毫米管道直径）、一氧化二氮连接点、旁路臂、控制阀和安全数据传输模块。该装置在连接到标准气瓶组和医院主歧管之前进行了泄漏测试和调试。在测试 3 毫米单位之后，在测量期间使用 1 毫米口径流量计，以获得最精确的基础流量量化。泄漏流量定义为在临床活动非常低的时间段（03.00-04.00）测量的流量，没有表明临床使用的流量峰值。计算 1 周内的平均总流量，并与平均泄漏流量进行比较，得出泄漏率的比例。数据在 MATLAB 2020b（Mathworks™ Natick，马萨诸塞州，美国）中进行可视化和分析。

1 周期间的流量计读数如图 1 所示。平均日流量为 1.44 l.min ^-1，代表泄漏的基础连续流量为 0.52 l.min ^-1，代表 50% 的泄漏/浪费率。图 1 可以清楚地识别日常使用模式，流量峰值在 15–25 l.min ^-1之间。该医院每年对所有专科进行约 15,000 次麻醉，拥有 300 张床位，是国家级小儿烧伤病房。儿科急诊科与成人急诊科位于一栋独立的较新大楼内，这些数字不包括急诊科的使用情况。

这一泄漏比例低于英国 NHS 调查人员报告的水平，但根据 2019 年医院一氧化二氮采购总量 230 万升计算，年度碳足迹为 621 吨二氧化碳当量。其他泄漏检测方法包括购买和使用数据分析，测量不确定度较大[ 2 ]；使用天平和G缸的称重方法[ 3 ]；以及流量计的使用。所描述的技术涉及直接、高保真、全医院范围的测量，建立在 Wong 等人描述的更局部化的流量计方法的基础上。 [ 4 ]，并且可能是识别和减少医院管道系统中一氧化二氮泄漏的有用工具。这在仍然需要临床使用一氧化二氮的领域可能特别有用，包​​括产房和儿科环境。在仅使用一氧化二氮（而不是挥发性麻醉剂）的环境中，使用一氧化二氮销毁系统是一种选择，但其实际功效和无法协助解决二氧化氮泄漏的问题是值得注意的问题[ 5 ]。在一些医疗保健系统中，已经采取了纯粹使用钢瓶供应一氧化二氮的举措，并且由于发现了这些大泄漏，一些地区正在积极努力停用管道。然而，这些决策假设维护良好、低泄漏分数的管道系统非常困难或不可能实现，而高保真监控系统可能会在持续需要管道系统的地方提供极大便利。该系统现在将用于通过管道网络的顺序隔离以及出口测试和其他方法来识别泄漏位置。