Inhalational anaesthetic agents
- Apart from nitrous oxide, inhalational anaesthetic agents are rarely used in the ED.
- Occasionally may be needed for gaseous induction with imminent upper airway obstruction (usually sevoflurane), and patients arrive using methoxyflurane.
- Inhalational agents tend to cause venodilatation which facilitates IV access. The halogenated hydrocarbons/ethers generally depress the myocardium to varying degrees, cause ↓ RR, raised PaCO2, ↓ hypoxic drive and carry a small risk of malignant hyperthermia.
- The exact site of action for these anaesthetics is debated. They likely have membrane altering properties which differ amongst neuronal groups, rather than specific receptor interactions.
Nitrous Oxide (N20)
- Schedule 4 drug – clear, odourless gas, not flammable but supports combustion.
- Actions: Weak anaesthetic – MAC 102%. Good for analgesia/sedation/amnesia. Carrier gas for other anaesthetic agents reducing their MACs by ~40%.
- Pharm: Poorly soluble in blood so rapidly absorbed by inhalation, onset/offset <5mins. Beware diffusion hypoxia at offset (alveoli filled with nitrous diffusing out from blood) so pt to breathe 100% O2 for ~5-10mins after N20 ceased. 35x more soluble in blood than N2 so can exchange for nitrogen in air cavities (see Cons).
- Usage: Mainly ED use is conscious sedation for painful procedures: Requires 2 hours fasting, age normally >1 year
- Pros: Airway reflexes preserved. Good analgesia, well tolerated.
- Cons: Closed HI, ICP or decreased LOC, airway obstruction/burns, pneumothorax, resp distress, recent middle ear surgery, suspected bowel obstruction. S/E: nausea, Vit.B12 oxidation/marrow depression (chronic use), behavioural
- Presentation: Neat cylinders for anaesthetic use. For ED use: usually mixed with O2 – cylinder Entonox (50%) or blending system Quantiflex (up to 70%). Mask/mouthpiece valve system requires patient to form seal and actively inhale.
- Halogenated ether
- Pharm: MAC 0.16% (i.e. very potent). Onset/offset is slow
- Pros/Usage: Powerful analgesic, in sub-anaesthetic doses
- Cons: Delirium, BP, arrhythmias, hepatic necrosis, Fluoride (nephrotoxic) produced by metabolism
- Presentation: Used by ambulances as patient controlled inhaler (green tube)
- Dosage: 6mg/inhaler – single dose only
Comparison table for other inhalational anaesthetic agents
|Drug||Blood:Gas partition coefficient (solubility)||Onset & Offset||Metabolism||Notes||MAC* %|
|Nitrous Oxide||0.47||Very rapid||None||See above||102|
|Halothane||2.3||Medium||>40% (20% by liver thus risk of hepatic necrosis 1:35000)||Respiratory & myocardial depression. Arrhythmias (heart sensitised to adrenaline)||0.75|
|Enflurane||1.8||Medium||8% (2% hepatic)||Seizure risk||1.7|
|Isoflurane||1.4||Medium||<2% (0.2% hepatic)||Less myocardial depression, ↓BP||1.4|
|Sevoflurane||0.69||Rapid||2-5% (produces fluoride)||Unstable in soda lime (→ nephrotoxic Compound A)||2.0|
|Desflurane||0.42||Very rapid||<0.05%||Needs heater vaporizer. Quite airway irritant – thus poor for induction. CO produced by soda lime.||6-7|
|Methoxyflurane||12||Slow||>70% (produces fluoride)||Not used for general anaesthesia||0.16|
- *MAC=Minimum alveolar concentration (conc. of anaesthetic that results in immobility of 50% pts exposed to a std noxious stimulus e.g. surgical incision.)
- Dunn R. The Emergency Medicine Manual 3rd Ed.
- Katung B. Basic & Clinical Pharmacology 8th Ed.
- Cameron P. et al. Textbook of Adult Emergency Medicine 2nd Ed.
- Mahadevan S. & Garmel G. An Introduction to Clinical Emergency Medicine
- Neal M. Medical Pharmacology at a Glance 3rd Ed.