How do we find the epidural
space? I pointed out to you
previously that the epidural space is really
almost an imaginary space. It exists but
it's not like a hole or an open area
in the body, it's an area that's packed full
of blood vessels and fat. But the interesting thing is,
that its major connection with the rest of the body is through
the, through the thorax, through the chest.
And because of that, it changes pressure
in the same way as the pressure in the chest
changes. And the pressure in the epidural
space is lower than the pressure
in surrounding tissues. So, we can insert
a needle, usually when we put it in we can feel the
Ligamentum flavum. It's usually a fairly
tough ligament, except in pregnant women where it can be
quite soft. But it's usually tougher than the surrounding tissues.
And you can almost feel the tip of the needle popping
through it. And while we're advancing at, we're putting
pressure on the syringe, which is full of saline,
and as you pop through the Ligamentum flavum,
you get what's called a Loss-of-Resistance, the syringe
just empties. And it's very sudden and very
dramatic. People use different solutions
in the syringe. They use saline or air, or
a mixture of the two. I used to use air, than I went
to a mixture of the two, I now always use saline.
And I get a very distinct feeling as you go through
the Ligamentum flavum and you lose
the resistance that you're feeling as you
advance the needle. You then disconnect
the syringe from the needle hub and you pass
a catheter through the syringe, through the, excuse
me, through the needle, which then passes
up into the epidural space. This is a typical
epidural needle, this is called a Tuohy needle.
And you can see it in the middle diagram
here. The Tuohy tip is quite
large. It's at a bit of an angle
and if you look very carefully, you can see that
it's got a cutting edge. It's quite a sharp needle.
It'll actually cut through that Ligamentum flavum. On
the right part of the diagram you can see the catheter
beside the needle. Once the needle's in place
and you've done your loss-of-resistance, you take
a stylet out of the needle and then you pass the catheter
through. So, what's the difference between
an epidural and a spinal. Well, the big difference
is 3 millimeters and it's a very small
distance when you're putting pressure on the back,
and you can feel the needle moving
forward. But epidurals produce
good anesthesia, but not
profound anesthesia. So, patients with
an epidural block will feel pressure
when the surgeon pushes
on their abdomen, may feel position
changes as the surgeon moves back
and forth, and may have some
sense of work going on. Sometimes
patients will actually describe that
they can feel the surgeon's
hand, but they don't have pain.
It's a very good technique for patients in labor, because
you can use it for an extended period of time, because
a catheter is placed and you can give very dilute
local anesthetics over a period of time that cause
excellent labor pain relief. And then, if you have to,
you can change it to an anesthetic for surgical
pain. You could adjust the rate of flow through
the catheter, you can adjust the dose
and strength of the local anesthetic
that's being used to modify
your block to the patient's needs. But
the big difference between epidural
and sub-arachnoid block, as I said is 3 millimeters.
But the difference in block is quite
profound. The first thing is that, when you
get into the spinal space, the sub-arachnoid
space, you're actually in the Central Nervous System. And you'll see
in a picture I'm going to show you in a moment, that you
get cerebral spinal fluid dripping out of the needle.
You do not place a catheter in this situation.
You merely inject local anesthetic, sometimes
with very dilute morphine or other
narcotic into the space. You get
a very profound block. The patients
have absolutely no sensation below the waist
with this block. So, those sensations
of pressure or movement that I described for the epidural
are absent in the case of a spinal. And
for the ideal surgical situation, a spinal
is actually superior to an epidural.
The negative about a spinal is, you can't adjust it.
You can't decrease or change the flow
of the drug. It's a one shot technique.