Jaye asks about whether anesthetic administered directly into the  
theca will get into the bloodstream.  The short answer is, it doesn't  
matter where in the body you administer things, sooner or later some  
of it will end up in blood.  Via the blood, all such things are  
carried to the part of the body that metabolizes them, usually the  
liver.  If there were no contact between the epidural space and the  
blood, or theca and blood, then anything administered there would have  
a permanent effect.  This would make it unnecessary to have top-ups of  
anesthetic because they would never wear off.

Factors that play a role in how quickly a substance gets from the  
administration site to the bloodstream include: the site, the  
pharmacokinetics of the substance, and the condition of the individual  
who is getting the substance.  Alcohol crosses all body membranes and  
equilibrates rapidly so the concentration is about the same  
everywhere.  Some substances concentrate in the central nervous system  
and are found there in greater amounts than in the blood, such as  
fentanyl.  Some things are not absorbed across the gut mucosa and must  
be given by injection, such as heparin.  Some things are not absorbed  
across the gut mucosa and this is good, because they are meant to work  
locally, such as some of the antihelmintics, and mycostatin.  Some  
places are hard to reach, because they are dense tissues with low  
vascularization, like tendons and joint capsules.  Some places are  
easier, like skin and mucous membranes.

Our bodies are not made of impermeable materials and we are not  
static.  I can't think of the English translation of the Greek saying  
about how everything is in a state of flux, can only remember 'alt  
flyter' in Norwegian, but it is right on.

Rachel Myr
Kristiansand, Norway

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