Xenon is a monatomic colourless odourless and tasteless inert gas with the atomic number 54 which is five times heavier than air.


Some physico-chemical data on xenon:

Molecular weight:
131,290 (g/mol)

Density (in g/ml):
0.00589 (0 C, 1013 mbar)

Melting point (oC/1013 mbar):
111.9

Boiling point (oC/1013 mbar):
- 108.2

Solubility in water:
1.08 L/L

Vapour pressure:
33 bar (20oC)

Rel. vapour/gas density:
4.56

Critical temperature:
16.6oC

Critical pressure:
59.0 bar

Critical density:
1.10 g/cm3

Oil/gas coefficient:
1.9

Blood/gas coefficient:
0.14


Natural xenon contains the isotopes (incidence) 124 (0.01%), 126 (0.09%), 128 (1.91%), 129 (26.44%), 130 (4.08%), 131 (21.18%), 132 (26.89%), 134 (10.44%), 136 (8.87%). It also forms numerous artificial isotopes 114xenon - 142xenon with half-lives between 1.2 seconds and 36.3 days, of which the isotope 133xenon (half-life 5.25 days) is used in nuclear medicine in sterile isotonic saline solution or as a gas e.g. in investigating the perfusion of brain, muscle, skin and other organs.

The proportion by volume of xenon in air is small so that the air in a normal living room (50m3) contains 4 ml of xenon.

Xenon can be researched under the CAS Registry Number 7440-63-3.

2. Biological reactions and medical use
Xenon has no specific taste and smell and pure xenon does not have an irritant effect.
Xenon is currently used clinically as a routine diagnostic agent in the investigation of organ perfusion and pulmonary function. Xenon has been used for years by radiologists and nuclear medicine specialists intravenously, intra-arterially or by inhalation in patients following stroke, subarachnoid haemorrhage, head injury and in multiple sclerosis and brain tumours to assess cerebral blood flow both as the radioactive isotope 133xenon and also as the stable gas 131xenon without the occurrence of major side effects. Inhalation of 50% 131xenon for about five minutes leads to an increase in the density of the lung structure on computer tomography of up to 80 Houndsfield units. It has been known since the 1940s that the inert gas xenon is suitable for use as an inhalation anaesthetic .

Xenon offers many advantages from the medical aspect. In investigations performed using xenon as an inhalation anaesthetic, the advantages perceived by the patients compared to nitrous oxide included: sleep perceived as very pleasant, greater circulatory stability, reduced consumption of analgesics, lower adrenaline levels and advantages in the regional perfusion of individual organs . Xenon's anaesthetic effect is one and a half times greater than that of nitrous oxide and because of its lower blood/gas solubility (0.14 compared to 0.47 in the case of nitrous oxide) and the consequent faster ebb and flow in the body, it is more suited for anaesthesia than nitrous oxide. Xenon is therefore regarded as an ideal anaesthetic gas.

A few experimental results concerning the pharmacodynamics of xenon which are associated with the planned clinical use will be listed briefly. 32 patients undergoing an operation were randomised for anaesthesia with either nitrous oxide or xenon, 70% nitrous oxide or xenon and 30% oxygen. The following differences between the two anaesthetic procedures were noted: The additional use of analgesics was twice as high in the nitrous oxide group as in the xenon group.

There was a clear rise in the adrenaline concentration during the operative procedure in the nitrous oxide group compared to the xenon group. There was no difference in the laboratory chemical parameters of cortisol, growth hormone and electrolytes or in the haemodynamic parameters of heart rate and blood pressure. These results were confirmed in a larger number of patients.

In an animal study, the effects of a gas mixture of 70% and 80% respectively of xenon and oxygen in tracheotomized rabbits (n=5) on the depth of anaesthesia and the long-term effects of exposure to a 50-60% xenon mixture on rabbits were investigated. No macroscopic or microscopic changes were found in brain, lung, liver, kidney and adrenal after 48 h exposure to xenon 50%. One working group set out to establish the minimum alveolar concentration (MAC) for xenon. The MAC is defined as the concentration at which 50% of patients do not demonstrate a motor protective reaction to a surgical stimulus. Xenon was used in concentrations of 25%, 35% and 70% in a total of 68 patients; side effects are not described and the MAC was extablished as 71% xenon.

Xenon does not produce adequate depth of anaesthesia in dogs and monkeys under normal atmospheric conditions.

Using a pressure chamber, the partial pressures of xenon were increased and an appropriate depth of anaesthesia was achieved in tracheotomized dogs (n=7). During the xenon anaesthesia, an attempt was made to provoke abnormalities of heart rate by repeated intravenous administration of 10 g/kg of adrenaline but none occurred. A slowing of the heart rate during xenon anaesthesia can be interpreted as adequate anaesthesia as with all intravenous and volatile anaesthetics.






When xenon was first used as an inhalation anaesthetic in 2 patients, no side effects were described by the authors.

Xenon has already been used for a caesarean section. The infant showed no abnormalities post partum.


Toxicology
In an animal experiment, 4 groups of 32 gravid rats were exposed to the following gas mixtures for 24 h: Group 1 25-30% oxygen Group 2 25-30% oxygen and 70-75% nitrogen Group 3 25-30% oxygen and 70-75% xenon Group 4 25-30% oxygen and 70-75% nitrous oxide The animals were killed 20 days after exposure and the foetuses (1270 in total) were examined.
In groups 1 to 3 there were macroscopically visible organ anomalies such as hydrocephalus, anophthalmia and gastroschisis in 1 to 3% of cases, while this occurred in 15% in group 4 (p<0.0001) and there were skeletal anomalies in 37% of foetuses in group 4. On the basis of these results, the authors assume that nitrous oxide has a clear teratogenic potential, but not xenon.
Possible causes of the teratogenicity of nitrous oxide:
1. Nitrous oxide is chemically reactive and the metabolites which occur may be toxic; xenon, on the other hand, is stable.
2. Nitrous oxide can have a detrimental effect on blood flow within the uterus and foetus respectively and thus on development.
3. Inhibition of biosynthesis of vitamin B12 by nitrous oxide has been known for a long time. This has been used to explain negative effects on haematopoiesis, and a negative effect on organogenesis by inhibition of the biosynthesis of vitamin B12 is regarded as possible by the authors.
When a rabbit was exposed to a 50-70% xenon-oxygen mixture for 48 hours, there were no macro- or microscopic changes in brain, liver, lung, kidney and adrenal. Laboratory chemical investigations in 5 patients before and after xenon anaesthesia did not demonstrate any pathological changes.
There is no MAC for xenon.
In all previous uses of xenon, in animal studies and in patients, there has been no evidence of toxic side effects.
7. Summary
In numerous experimental animal studies and clinical studies, xenon is regarded as an optimum anaesthetic gas on the basis of the absence of biotransformation and cardiocirculatory side effects.
Nitrous oxide has been blamed for years for the frequent occurrence of miscarriage in operating room staff. Because of the absence of biotransformation and foetotoxicity, the use of xenon is an important contribution to reducing pollution within the operating room working area.
From the ecological aspect, it should be emphasized that xenon, unlike nitrous oxide, does not contribute to a reduction of the ozone layer.
The only disadvantage of xenon is its high cost.