Desflurane and carbon‐based economics: a reply

We thank Slingo and Slingo [1] for their comments on our article [2]. They correctly note that global warming potential over a 100-year time span (GWP100) and carbon dioxide equivalents (CO2e) are an imperfect mechanism of creating a common currency for emitted greenhouse gases. We utilised GWP100 as it was commonly available in the literature and, notwithstanding its flaws, has been recognised previously by the United Nations Climate Change Committee as a mechanism of creating a currency of emissions to enable reporting of a country's emissions. It remains the metric that the UK Emissions Trading Scheme uses to create CO2e for nitrous oxide emissions with a plan to do similarly for the more short-lived methane emissions [3]. Measuring the global warming potential in CO2e over 100 years, or even 20 years, as opposed to over 1000 years, will tend to overemphasise the impact of shorter-lived gases, such as volatile anaesthetic agents, over those that will remain in the atmosphere for thousands of years, such as perfluorocarbons [4]. The GWP* metric discussed by Slingo and Slingo is a dynamic value depending on the current utilisation of a gas. Given the recent fall in desflurane utilisation, GWP* generates negative CO2e emissions. However, were desflurane utilisation to increase again, GWP* would become significantly positive. With this fluctuating entity, we feel that GWP* struggles to tell a story of the marginal cost, monetary and environmental, of releasing a new tonne of a greenhouse gas [5]. It requires an environmentally conscious anaesthetist to consider whether the country is using more desflurane this year than last to be able to calculate the putative climate impact of their drug utilisation. Additionally, the dynamic element of GWP* means that it would be difficult to integrate into an emission trading scheme as the values would be changing continuously [5]. By contrast, the choice of a 100-year time span emphasises nearer term impact across a timescale that is potentially salient to the current living population. Desflurane in our analysis is more expensive than both total intravenous anaesthesia (TIVA) and sevoflurane. Even if the GWP* figure of £797 (US$1080, €931) per equivalent tonne of carbon dioxide is entered for the carbon footprint of waste desflurane released to the environment, its high cost would mean that transitioning from TIVA to desflurane would not be expected to be cost-effective at a carbon price of £41.84 (US$56.68, €48.88). Additionally, because sevoflurane is also a short-lived gas, which is less costly and less carbon intensive to manufacture, sevoflurane would remain dominant over desflurane at constant use under the GWP* calculation. Therefore, in the absence of a clear clinical benefit, the relevant comparison remains sevoflurane versus TIVA, rather than desflurane versus TIVA. In our article, we recognise that because clinical outcome is valued at > £20,000 (US$27,095, €23,366) per quality adjusted life year, improvements in clinical outcome are likely to eclipse any difference in carbon footprint between modes of general anaesthesia. This remains the case regardless of whether GWP100 or GWP* are utilised. Our model finds that TIVA is more cost-effective compared with desflurane under both the established GWP100 and the emerging GWP* metrics. Moreover, calculated with either metric, sevoflurane is dominant over desflurane as it is both cheaper and less carbon intensive.