I came across an intriguing story on the BBC today about the use of inhaled Xenon as a therapy for babies born with brain trauma injuries. Children inhaled the gas while being cooled just a little below normal body temperature, and somehow, they just got better! It is an intriguing story and I read it twice searching for some hint of how the Xenon had any therapeutic effect. There was nothing! Not even a hint. Of course the article is really a collection of press releases and is not the product of any journalist prepared to put their name to it, so I should not really be surprised.
There was a link to Sparks, the charity that funded the research and that had a little bit more:
“Xenon is a very rare and chemically inert anaesthetic gas found in tiny quantities in the air that we breathe. Over the past eight years, we have shown in the laboratory that xenon doubles the protective effect of cooling on the brain; however we faced the challenge of how to safely and effectively deliver this rare and extremely expensive gas to newborn babies.”
Dr Dingley has been developing equipment in Swansea for xenon anaesthesia in adults for over 10 years and has invented a machine to successfully deliver the gas to babies. His machine takes the exhaled gas, removes any waste products from it and re-circulates it to be breathed again without any loss at all to the outside air. Some types of specialist military diving equipment work in this way but it is very unusual to build a system small enough to work reliably in newborn babies.
“A key design feature of this machine is that it is very efficient, using less than 200ml of xenon per hour – less than the volume of a soft drinks can. Xenon is a precious and finite resource and difficult to extract so it can cost up to £30 per litre. As ventilated newborns breathe many litres of air per minute, any xenon based treatment would be impossibly expensive without an economical delivery method.”
He continued: “Despite these challenges, the lack of side-effects and brain protecting properties of xenon make it uniquely attractive as a potential treatment to apply alongside cooling in these babies.
So xenon is an anaesthetic! I was gobsmacked. Finding out that xenon has any biological activity is like finding out that a maiden aunt is a pole dancer in here spare time. The defining feature of xenon and its noble gas brothers and sisters – helium, neon, argon, and krypton – is that they don’t react with things. So how could they have any biological activity, especially something so profound as anaesthesia? So to the web! And now with two mysteries to solve: Firstly, how does xenon perform this trick in the absence of any specific chemical interactions, and secondly: how does anaesthesia work anyway!
Wikipedia – always my first port of call when I have no idea at all about a topic – actually had an article about therapeutic uses of Xenon (which already included a link to the BBC story that appeared earlier today!). And there are actually a plethora of articles (e.g. 1 , 2, 3) about the use of Xenon to induce ‘medical sleep’ in a relatively safe (if expensive) manner. All the articles state that xenon has this effect, but none of them state how! Now xenon is a big fat atom – and it is highly polarizable (i.e. electric fields distort its electron cloud easily) and so it can be stuck electrostaticaly to a wide variety of charged molecules. But there isn’t anything specific about its general fatness that would cause it to stick one biological site rather than another. So this one remains a mystery!
So how does anaesthesia work anyway?
At its briefest, I was astonished to discover that ‘nobody knows’! Wow! Paul H Ting has a helpful summary, and there are several other articles hither and thither, but at the bottom of it all, we just don’t know. Searching for some deeper insight, I called upon my subscription to the archive of the Scientific American. There I read a long article which clarified some of the components of the anaesthetised state. Specifically it mentioned:
- Sedation: Reduced arousability, as evidenced by longer response times, slurred speech and decreased movement. Neuronal activity across brain cortical areas drops.
- Unconsciousness (also called hypnosis): Impaired perception of, and response to, stimuli. Cortical depression is deeper than in sedation. Activity in the thalamus, an area important for integrating brain processes, also falls signiﬁcantly.
- Immobility: Lack of movement in response to stimulation such as shaking or heat. Suppression of spinal cord neuronal activity is the main cause of this temporary paralysis, although the cerebellum, a motor control area, may also contribute.
- Amnesia: Lack of recall for the anesthetized period. Many brain structures involved in memory formation, including the hippocampus, amygdala, prefrontal cortex and sensory and motor areas, exhibit anesthetic-induced changes.
- Others: Muscle relaxation and lack of pain (analgesia) are sometimes included in deﬁnitions of the anesthetized state and are largely attributed to depression of spinal cord activity.