# Why are heavy gases anesthetics? **URL:** \ **Category:** Factual Questions\ **Created:** [April 15, 2014, 1:32pm UTC](https://boards.straightdope.com/t/why-are-heavy-gases-anesthetics/686174 "2014-04-15T13:32:38Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![Lumpy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lumpy/32/446_2.png) [@Lumpy](https://boards.straightdope.com/u/Lumpy)\ **Post date:** [April 15, 2014, 1:32pm UTC](https://boards.straightdope.com/t/why-are-heavy-gases-anesthetics/686174/1 "2014-04-15T13:32:38Z")
My understanding is that heavy gases like xenon and sulfur hexafluoride have an anesthetic effect. Since they’re more or less chemically inert, it must be somehow related to their molecular weight. Why does that have a biological effect? ---
**Author:** ![DrCube](https://avatars.discourse-cdn.com/v4/letter/d/a3d4f5/32.png) [@DrCube](https://boards.straightdope.com/u/DrCube)\ **Post date:** [April 15, 2014, 1:47pm UTC](https://boards.straightdope.com/t/why-are-heavy-gases-anesthetics/686174/2 "2014-04-15T13:47:56Z")
I think it’s because they displace oxygen. I think it’s a similar effect to carbon monoxide. You just fall asleep from too little oxygen and eventually die. I’m totally not a doctor or chemist though. ---
**Author:** ![minor\_details](https://avatars.discourse-cdn.com/v4/letter/m/74df32/32.png) [@minor\_details](https://boards.straightdope.com/u/minor_details)\ **Post date:** [April 15, 2014, 2:11pm UTC](https://boards.straightdope.com/t/why-are-heavy-gases-anesthetics/686174/3 "2014-04-15T14:11:57Z")
The efficiency of non polar anesthetics (He, Xe, N2, etc) is correlated with their polarizability, which means that it is also correlated with their size (molecular weight), their van der Waals “a” parameter (strength of interaction) and lipid solubility. See for example, p 14-19 of [Modern Anesthetics](http://books.google.com/books?id=JpkkWhPbh2QC&pg=PA15&lpg=PA15&dq=anesthetic+polarizability&source=bl&ots=HkG59JN73Q&sig=YgKYjO-pwQTBhTvRHOrtSet_998&hl=en&sa=X&ei=hjpNU97VDsjP0gG07IDIDg&ved=0CD0Q6AEwAw#v=onepage&q=anesthetic%20polarizability&f=false) > **[Modern Anesthetics](https://books.google.com/books?id=JpkkWhPbh2QC&pg=PA15&lpg=PA15&dq=anesthetic%2Bpolarizability&source=bl&ots=HkG59JN73Q&sig=YgKYjO-pwQTBhTvRHOrtSet_998&hl=en&sa=X&ei=hjpNU97VDsjP0gG07IDIDg&ved=0CD0Q6AEwAw#v=onepage&q=anesthetic%20polarizability&f=false)** > > Some important constraints of anesthesia must be taken into consideration when the pharmacological properties of modern anesthetics are discussed. The most imp- tant of these could be that the target effect be achieved preferably within seconds, at... Also (for the correlation): [http://www.ncbi.nlm.nih.gov/pubmed/13394957](http://www.ncbi.nlm.nih.gov/pubmed/13394957) The challenge is in going from these correlations to an actual mode of causation… here’s one try: “A consensus view holds that anesthetics act by van der Waals forces in hydrophobic pockets of select brain proteins to ablate consciousness.” [http://www.ncbi.nlm.nih.gov/pubmed/10049159](http://www.ncbi.nlm.nih.gov/pubmed/10049159) These anesthetics do NOT work by depriving the brain of oxygen. Their mode of action is VERY different from that of carbon monoxide. ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [April 15, 2014, 2:12pm UTC](https://boards.straightdope.com/t/why-are-heavy-gases-anesthetics/686174/4 "2014-04-15T14:12:13Z")
> [@DrCube](#): > > I think it’s because they displace oxygen. I think it’s a similar effect to carbon monoxide. You just fall asleep from too little oxygen and eventually die. Not true. [Xenon](http://en.wikipedia.org/wiki/Xenon#Anesthesia) has an anesthetic effect even when oxygen is present at life-sustaining concentrations. It takes a lot - you need a mix of at least 72% xenon - but the rest of the gas mix could be pure O2, and the xenon would still have an anesthetic effect. It’s considered a noble gas, but it is _slightly_ reactive, and its anesthetic effect is a consequence of that reactivity. [SF6 appears to have an anesthetic effect only at very high pressures (~100 psi).](http://www.nature.com/nature/journal/v221/n5179/abs/221468a0.html) Theories about its effect seem to focus on factors other than its high molecular weight. It seems that [most anesthetics aren’t as well understood as we would like.](http://en.wikipedia.org/wiki/Theories_of_general_anaesthetic_action) ---
**Author:** ![DrCube](https://avatars.discourse-cdn.com/v4/letter/d/a3d4f5/32.png) [@DrCube](https://boards.straightdope.com/u/DrCube)\ **Post date:** [April 15, 2014, 7:46pm UTC](https://boards.straightdope.com/t/why-are-heavy-gases-anesthetics/686174/5 "2014-04-15T19:46:04Z")
Thanks for setting me straight guys. I thought I’d hazard a guess to bump the thread before I realized it had only been posted 15 minutes before.