Article Notes
- Does peri-operative intravenous dexamethasone reduce pain and opioid consumption after caesarean delivery? YES
- Are the effects statistically significant? YES
- Are the effects clinically significant? Possibly, though pain scores are only modestly improved and the reduction in opioid use is only small.
- Are the findings applicable to my patient population? Possibly, though the majority of studies were performed in Middle East, Asian & South Asian hospitals, and with diverse post-operative analgesic regimes.
- Is peri-operative dexamethasone safe? Probably, though few studies were adequately powered to identify less-common potential side effects, such as infection or delayed wound healing.
- Quality of evidence is low to modest. Notably, the primary outcome for most studies was PONV reduction, not post-operative pain.
- Should this evidence result in routine practice change? Probably not at this stage. IV dexamethasone may however be an appropriate intervention in select patient groups.
Global Warming Potential vs Radiative Forcing
Global warming potential is "...the ratio of the cumulative radiative forcing (the energy being added to the planet, based on radiative efficiency and lifetime) over a time horizon from the instantaneous release of 1 kg of a given gas, such as desflurane, relative to that of 1 kg of carbon dioxide." A 100 year time horizon is commonly used as GWP100, reflecting the long lifetime of CO2, representing the warming potential of a single emission of a gas compared to the same mass of CO2. GWP is then used to derive CO2-equivalents.
Although GWP allows a simplistic comparison of greenhouse gases (eg. desflurane GWP 2530), it is misleading to use it to assess the climate impact of volatile anaesthetic agents primarily because of their short lifetimes, but also because "GWP does not consider the actual atmospheric abundance of a greenhouse gas, nor does it represent any of the physical complexity of the overall climate system..."
"Based on climate science, we assert that this is a physically unsound approach for determining the potential climate impact of volatile anaesthetic gases, and that it is the atmospheric concentration and radiative forcing of individual greenhouse gases that matter..." – Slingo & Slingo.
Instead climate scientists today prefer to use radiative forcing, "... the difference between the energy entering the planet and the energy leaving it..."
The percentage contribution of all volatile anaesthetic gases to radiative forcing is <0.01% compared to that from carbon dioxide. In comparison, the global emission reductions due to the COVID-19 lockdown were almost 1000x greater than the cumulative impact from anaesthetic volatile gases, and yet this reduction was undetectable above normal climate variability.
| Atmospheric concentration (ppt) | Atmospheric lifetime (y) | (Effective) radiative forcing (Wm-2) | |
|---|---|---|---|
| Carbon dioxide | 420,000,000 | 100+ | 2.16 |
| Methane | 1,920,000 | 12.4 | 0.54 |
| Nitrous oxide | 336,000 | 123 | 0.21 |
| Desflurane | 0.37 | 14.1 | 0.00017 |
| Sevoflurane | 0.16 | 1.4 | 0.00003 |
| Isoflurane | 0.11 | 3.5 | 0.00006 |
"On the basis of GWP, anaesthetic gases appear to be very 'damaging'. However, this conclusion is scientifically unsound: their lifetimes are short; their emissions, accumulation and resulting atmospheric concentrations are minute; and their actual radiative forcing is vanishingly small."
More complexity...
Beyond the over-simplification of GWP and CO2e, Slingo & Slingo also explore the challenge in linking the minute radiative forcing of volatile anaesthetic gases to actual climate change and impact.
"...future warming will be determined by how successful we are in controlling our direct carbon dioxide emissions. ... While changing a vaporiser feels achievable and tangible, the scientific reality is inconsequential when we consider all the steps from GWP to climate impact. ... It is carbon dioxide emissions that will define our future, not the use of volatile anaesthetic agents."
Daniel Jolley
Daniel Jolley