Xenon

Recent developments

Manufacture is by fractional distillation of air and costs 2000 times more than N ₂ O.

Commercial uses include lasers, high intensity lamps, flash bulbs, aerospace, X-ray tubes and medicine.

Physical properties

• Colourless, odourless, tasteless.
• Monatomic gas, atomic number = 54, molecular weight = 131,3
• Gas undr normal temperature and pressure
• 9 stable isotopes.
• Freezing point -111,9 ^o C, Boiling point -108,1 ^o C.
• 4 times more dense than air.
• Nonflammable and will not support combustion.
• Diffuses freely through rubber and silicone components.

Anaesthetic agent

• First used in 1951 by Cullen on an 81yr old man having an orchidectomy..
• Very close to the ‘ideal agent’
• The very large electron shell of xenon ca be distorted and polarised by nearby molecules, creating a dipole.
• Xenon inhibits the plasma membrane Ca ²⁺ pump, altering excitability.  It inhibits the nociceptive responsiveness of spinal dorsal horn neurons.
• MAC = 71%. (With more widespread usage the Russians have noticed the MAC to be closer to 60%)
• Minimal haemodynamic effects.
• Lowest blood/gas partition coefficient = 0,115 of currently available inhalational agents.
• Low oil/water partition co-efficient of 20. 
• Rapid induction and eduction regardless of duration of administration
• 4 stages of anaesthesia noted with 70% Xenon/ 30% oxygen.

1. Whole body paraesthesia & hypo-algesia.
2. Euphoria & increased psychomotor activity.
3. Analgesia with partial amnesia (after 3-4min).
4. Surgical anaesthesia with a degree of muscle relaxation.

• Equivalent analgesia when compared with equipotent doses of N ₂ O  The analgesia produced by both gases is not reversible by naloxone.
• No occupational/ environmental disadvantages.

Specific effects on the body

• Respiratory
  • Central depression causes a decrease in respiratory rate with a compensatory increase in tidal volume and can progress to apnoea
  • Higher density and viscosity (compared with oxygen, air and  N ₂ O) theoretically makes xenon more likely to increase airway resistance. Clinically the airway resistance is slightly less than that seen with N ₂ O and it can be used safely in lung disease
  • Diffusion hypoxia is very mild as the blood/gas partition of Nitrogen (0.014) is only 10 times less than that of Xenon (0.14) as opposed to the almost 40 times less than Nitrous Oxide (0.47)
• Cardiovascular
  • No inhibitory effects on cardiac ion channels i.e. calcium, sodium and inward potassium channels.
  • No significant change in contractility, blood pressure and systemic resistance.
  • Some reports of decrease in heart rate with variability in rhythm.
  • No sensitisation of the myocardium to adrenaline
  • May attenuate the  myocardial depressant effects of isoflurane.
  • In an animal study, xenon anaesthesia produced the highest regional blood flow to brain, liver, kidneys and GIT.  The control groups were 1% halothane in Nitrous oxide and thiopentone with fentanyl.
• Central nervous system
  • Xenon increases cerebral blood flow, increases intracranial pressure and decreases cerebral perfusion pressure in acute head injury patients.  This is not associated with cerebral oligaemia or ischaemia.
  • This increase in cerebral blood flow is reveresed by mild hyperventilation
  • At present it is not recommended for neurosurgery.
• Renal
  • No data available.
• Endocrine/neurohumoral
  • Attenuates surgical stress due to analgesia.  Does not have any short or long term cortisol suppresion effects.
• Toxicity
  • Platelet aggregation potentiated at 2atm (relevant to deep-sea divers).
  • No reported haematological toxicity.
• Malignant hyperthermia
  • Seems not to trigger malignant hyperthermia.
• Metabolism and elimination
  • Unlikely to be involved in any biochemical events in the body.
  • Eliminated via the lungs.
    • Under special conditions xenon is capable of forming compounds with very reactive elements e.g. clathrates, fluorides & chlorides

Potential ways to make xenon anaesthesia economically acceptable.

• Decreasing manufacturing costs.
  • This is only practical in large air separation plants.
  • Current price is 10 US$/ litre
  • Manufacture will increase as aerospace applications grow, but as this xenon is then lost to the atmosphere this will not contribute to decreasing the cost.
• Use in a fully closed breathing system
  • Use of semiclosed systems (facemasks/LMA/spontaneous breathing) cost £1200/hr.
  • Very low flow e.g. 0,3 l/min will cost £160-180/hr.
  • Fully closed automated systems are available that are suitable for xenon anaesthesia
    • Gas piston circle
    • physioflex device
    • Balanced circle systems.
    • Circle priming
• Recycling devices are the only way of guaranteeing the availability of a sufficient amount of xenon for routine clinical use.
  • Gas analysis
    • Xenon can be measured with mass spectrometry, piezoelectric absorption, thermal conductivity and ultra-sound.

Practical use

1. Nitrogen must be washed out by giving a high flow of pure oxygen for at least 5 minutes
2. Normal induction and muscle relaxation
3. After intubation connect the patient to an appropriate anaesthesia delivery system
4. The hypnotic concentration of 40-45%  is achieved after 1.5min
5. The anaesthetic concentration of 60-70% takes approximately 8 minutes.

Summary

• Colourless and odourless gas with no irritation to the respiratory tract.  Well tolerated with gas induction
• Low blood/gas and oil/water partition co-efficients allowing rapid induction and eduction
• Produces unconsciousness with analgesia and a degree of muscle relaxation
• MAC of 60-70% allows a reasonable inspired oxygen concentration
• It does cause respiratory depression, to the point of apnoea.
• It is cardiac stable.
• Not metabolised in the body and is eliminated rapidly and completely via the lungs.
• It is non toxic and is not associated with allergic reactions
• Stable in storage, no interaction with anaesthesia circuits or soda lime.  Should not be used with rubber anaesthesia circuits as there is a high loss through the rubber
• Non flammable
• Expensive - Routine usage will only be possible with a closed circuit delivery system that recycles xenon.

Owing to environmental concerns there may be no alternative but to use xenon even if it incurs an increase in cost.