Inhalational Anaesthetic Agents

Inhalational Anaesthetic Agents and Carrier Gases

• Gases used in everyday clinical anaesthetic practice can negatively impact the environment by increasing the concentration of greenhouse gases (GHGs) +/- depleting the ozone layer
• Ozone stops the majority of the sun’s harmful UV radiation from reaching the earth’s surface
• GHGs are transparent to visible light but opaque to infrared radiation
• GHGs can absorb the earth’s emitted longwave infrared radiation, thus heating the lower atmosphere by slowing the rate at which it can escape into space
• GHGs differ in their ability to absorb energy (radiative efficiency) and their duration (atmospheric lifetimes)

Radiative Efficiency

• Measure of ‘greenhouse strength’ for different GHG, defined as the change in radiative forcing per change in atmospheric concentration of a gas
• Radiative efficiency (watts per metre squared per part per billion)
  • Sevoflurane 0.351
  • Isoflurane 0.453
  • Desflurane 0.469
  • Nitrous Oxide 0.003

Atmospheric Lifetimes

• Atmospheric lifetime refers to the duration of time a greenhouse gas remains in the atmosphere before being decomposed by chemical processes
  • Sevoflurane 1.1
  • Isoflurane 3.2
  • Desflurane 14
  • Nitrous Oxide 110
• Does a gas with a long atmospheric lifetime but a low radiative efficiency have a larger or smaller effect on climate than a gas with a short atmospheric lifetime and a high radiative efficiency?

Global Warming Potential (GWP)

• GWP allows us to compare the impact of various GHGs over a specific period of time with respect to CO₂
• The most commonly employed measure is GWP over 100 years GWP₁₀₀
• ​Inhalational anaesthetic GWP variability is predominantly caused by differences in atmospheric lifetime rather than radiative efficiency
• GWP₁₀₀
  • Sevoflurane 130
  • Isoflurane 510
  • Desflurane 2540
  • Nitrous Oxide 310

Ozone Depleting Substances (ODS)

• These are chemicals that destroy the earth’s protective ozone layer. They are also very potent GHGs
• Ozone- depletion potential (ODP) is a measure of their destructiveness
  • global loss of ozone due to the given substance: global loss of ozone due to CFC-11 of the same mass
• Nitrous Oxide is currently the dominant anthropogenic ODS
  • ODP of 0.017
  • Anaesthetic nitrous oxide contributes to 1-3% of all global nitrous emissions
• Atmospheric bromine (Halothane) and chlorine atoms (Isoflurane and Enflurane) form ozone-depleting radicals
• ODP
  • Halothane 0.36 (obsolete in most countries now)
  • Isoflurane 0.01
  • Enflurane 0.02

Desflurane and Sevoflurane have no ODP as fluorine does not destroy the ozone layer. They do, however, have significant global warming potential.