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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