Nitrous oxide (N₂O) is known to be a potent and persistent greenhouse gas and destroyer of stratospheric ozone. Nitrous oxide emissions continue to increase globally and are a major barrier to meeting the targets of the Paris Agreement. Although most N₂O is from agriculture, anaesthetic N₂O still accounts for a third of carbon emissions from anaesthetic gases within the UK National Health Service (NHS), and 0.67% of total NHS emissions [1]. The NHS Long Term Plan [2] and Delivering a ‘Net Zero’ NHS [1] specifically address N₂O reduction to align with net zero carbon emissions by 2050.

We examined 12-year trends in anaesthetic use of N₂O at University Hospital Southampton, a 1300-bed tertiary medical facility in the UK. Working with our institution’s medical gas supplier (BOC, Guildford, Surrey, UK), we obtained the size E (anaesthetic work station) and size G (cylinder manifold) medical gas cylinder return data for N₂O for each calendar year from 2008 to 2020. We used the universal gas equation to calculate the volume at 20°C, and Avogadro’s law and the molecular weight of nitrous oxide (44 g.mol^-1) to calculate the mass of N₂O from each cylinder. Using 265 as the global warming potential (GWP₁₀₀) of N₂O [3], carbon dioxide equivalence (CO₂e) was calculated as the product of the mass per cylinder used and GWP₁₀₀. The operating theatre information system was used to reference this to operating theatre activity. The absolute reduction in anaesthetic N₂O use was from 2125 tonnes CO₂e in 2008 to 448 tonnes CO₂e in 2020 (79%). The intensity of N₂O use decreased from 45.58 kg CO₂e.h^-1 to 12.96 kg CO₂e.h^-1 over the same period (Fig. 1).

We explored future drivers of and barriers to reduction by surveying the institution’s 212 anaesthetists regarding their experience and anaesthetic specialty areas, individual clinical use of N₂O and attitudes towards climate change. There were 95 responses, 23% from trainees. Clinical years of experience ranged from 2 to 37 (mean 17 y). A total of 80% of clinicians and all trainees use N₂O, with 75% of respondents using it for some cases or specific cases only, namely elderly and haemodynamically unstable patients, paediatric gas induction, obstetric general anaesthetics and short, surgically stimulating procedures. Of the respondents, 77% believed climate change to be very important or critical and 75% used less N₂O than in the past, the environment being their leading reason. Reasons for reduced use are shown in Table 1. A total of 60% of anaesthetists considered N₂O not to be essential. A majority (53%) believed N₂O should not be freely and immediately available.

Replacement of traditional Boyle’s machines with electronic semi-closed circle systems capable of low-flow anaesthesia between 2008 and 2010 may explain the first fall in the use of N₂O. The introduction of GE (Cleveland, OH, USA) Aisys CS Care Stations [4] between 2018 and 2020 with end-tidal control permitting ultra-low flow may explain the second fall. Changes in anaesthesia practice, including wider use of TIVA, sevoflurane, desflurane and regional anaesthesia may also have mitigated N₂O use. To our knowledge, there have been no identified leaks in the manifold or pipeline system. Four size G cylinders were stolen in 2015.

The respondents who considered climate change to be of critical importance were almost twice as likely to think that N₂O should be unavailable for use. The use of N₂O is not an outdated practice performed by older consultants as every trainee interviewed used it in their clinical practice. Clearly, there are benefits to the clinician, in the absence of evidence-based benefit to the patient [5]. It is probably difficult to replicate the convenience of N₂O while it can be delivered at the push of a button.

Ongoing reduction requires national strategies supported at all levels, the incorporation of sustainability into professional development and regular, transparent, emissions reporting. Alternatives to N₂O must be explored, confidence in them fostered and their ready availability improved. Ultra-low flow capability is important. Functional changes to the clinical physical environment, including disconnection of pipeline N₂O or capture and breakdown, should be considered.

众所周知，一氧化二氮 (N ₂ O) 是一种强效且持久的温室气体，也是平流层臭氧的破坏者。一氧化二氮的排放量在全球范围内持续增加，是实现《巴黎协定》目标的主要障碍。尽管大多数 N ₂ O 来自农业，但麻醉剂 N ₂ O 仍占英国国民健康服务 (NHS) 内麻醉气体碳排放量的三分之一，以及 NHS 总排放量的 0.67% [ 1 ]。NHS长期计划[ 2 ] 和实现“净零” NHS [ 1 ] 专门解决 N ₂ O 减少问题，以与 2050 年的净零碳排放保持一致。

我们在英国拥有 1300 个床位的三级医疗机构南安普敦大学医院检查了 N ₂ O麻醉剂使用的 12 年趋势。与我们机构的医用气体供应商（英国萨里郡吉尔福德 BOC）合作，我们获得了从 2008 年到每个日历年的N ₂ O尺寸 E（麻醉工作站）和尺寸 G（气瓶歧管）医用气瓶返回数据。2020. 我们使用通用气体方程计算20°C时的体积，并使用阿伏伽德罗定律和一氧化二氮的分子量（44 g.mol ^-1）计算每个钢瓶中N ₂ O的质量。使用 265 作为N ₂ O的全球变暖潜能值 (GWP ₁₀₀ ) [ 3]，二氧化碳当量 (CO ₂ e) 计算为所使用的每缸质量与 GWP ₁₀₀的乘积。手术室信息系统用于将其与手术室活动相关联。麻醉剂 N ₂ O 使用的绝对减少从2008 年的2125 吨 CO ₂ e 减少到2020 年的448 吨 CO ₂ e (79%)。在同一时期，N ₂ O 的使用强度从 45.58 kg CO ₂ e.h ^-1下降到 12.96 kg CO ₂ e.h ^-1（图 1）。

我们通过调查该机构的 212 名麻醉师的经验和麻醉专业领域、N ₂ O 的个人临床使用以及对气候变化的态度，探索了未来减少的驱动因素和障碍。有 95 份回应，其中 23% 来自受训者。临床经验从 2 到 37 年不等（平均 17 年）。共有 80% 的临床医生和所有受训人员使用 N ₂ O，75% 的受访者仅将其用于某些病例或特定病例，即老年和血流动力学不稳定的患者、儿科气体诱导、产科全身麻醉和短期的手术刺激程序. 在受访者中，77% 的人认为气候变化非常重要或至关重要，75% 的人使用较少的 N ₂O 与过去相比，环境是他们的主要原因。减少使用的原因如表 1 所示。共有 60% 的麻醉师认为 N ₂ O 不是必需的。大多数人 (53%) 认为 N ₂ O 不应免费且立即可用。

2008 年至 2010 年间，用能够进行低流量麻醉的电子半闭环系统取代传统的 Boyle 机器可能解释了 N ₂ O使用量的首次下降。 GE（美国俄亥俄州克利夫兰市）Aisys CS 护理站的推出[ 4 ] 2018 年至 2020 年间，随着呼气末控制允许超低流量，可以解释第二次下降。麻醉实践的变化，包括更广泛地使用 TIVA、七氟醚、地氟醚和区域麻醉，也可能减少了 N ₂ O 的使用。据我们所知，歧管或管道系统中没有发现泄漏。2015 年有四个 G 号气瓶被盗。

认为气候变化至关重要的受访者认为应该无法使用N ₂ O 的可能性几乎是其两倍。使用 N ₂ O 并不是年长顾问所采用的过时做法，因为每位受访的受训者都在其临床实践中使用过它。很明显，在缺乏循证医学益处的情况下，对临床医生有好处 [ 5 ]。N ₂ O的便利性可能难以复制，而只需按一下按钮即可交付。

持续减排需要各级支持的国家战略，将可持续性纳入专业发展和定期、透明的排放报告。必须探索N ₂ O 的替代品，培养对它们的信心并提高它们的可用性。超低流量能力很重要。应考虑临床物理环境的功能变化，包括管道 N ₂ O 的断开或捕获和故障。