Saturday, March 19, 2011


In my experience, when you poison someone you usually intend for them to die.  If you would rather that person did not die, it would make sense for you to not administer the poison in the first place.  This is why the concept of LD50 - the dose of a poison at which half the subjects will cark it - makes no sense to me.  If I'm going to go to the trouble of trying to bump someone off I don't want there to be any doubt about the outcome.  Fifty percent odds are unacceptable.  A measure such as LD99 would be more useful.

The reason I bring this up is because in anaesthetics there is a commonly used measure of anaesthetic potency called MAC.  It is defined similarly to LD50.  At the MAC dosage, half of all subjects won't leap off the operating table when you cut them open.  What a stupid measure.  I would prefer that none of my patients leap off the table.  Yet all the monitors and equipment and doctors and research papers talk about MAC.  ( I won't even get into how outraged I was when I found out that MAC stands for "minimum alveolar concentration" even though it's actually a median value.  The War On Innumeracy is long since lost on this front.)

I went looking for information about what the inter-person variability is for MAC.  In other words, how much anaesthetic would you have to give to ensure that everyone went under? It turns out that this information is surprisingly hard to track down.  Eventually I found a paper giving the 95% bands for MAC for one specific anaesthetic gas (desflurane) when used on - wait for it - llamas and alpacas.  Great.

Well it turns out that this paper is more useful than I thought because MAC is pretty much the same for all mammals.  For all animals, actually, just in case your pet starfish needs a hysterectomy some time soon.  Which makes me wonder why somebody bothered to do the experiments on llamas in the first place, noisy stinking beasts that they are.  Why not just assume that MAC in llamas is the same as in every single other species ever examined?

I suppose the researchers were hoping that somehow llamas and alpacas would end up being radically different from every other living being on earth and that this, combined with their intimate knowledge of llamoid physiology would enable them to deduce how the heck it is that anaesthetics actually work. Which would be nice because nobody really knows yet.

Think about that.  Nobody knows how anaesthetics really work.  For all we know they may work by destroying you and replacing you with a sinister mind-clone.  Controlled by sunflowers.


Anonymous said...

Woah, leave the alpacas and llamas out of this! They're simply misunderstood animals...

Anonymous said...

Aren't they the same animal?

PTR said...

According to wikipedia, no.
Lama glama vs Vicugna pacos