hi everyone this is lahiru from abc's of anesthesia
and today i'm going to go through the three ways you can actually miss or fail to cannulate someone
let's get straight into it so these are the three methods first of all
you don't even get the vein there's no flashback at all the second method is you go through one
of the other walls whether it's the deep wall or a sidewall and finally you don't advance the cannula
enough so what's happening is you're pushing the cannula into the wall of the vein and pushing
the vein away just to go through this first one so this is actually the easiest one if you don't
get the flashback that means that you haven't traumatized the vein so really you just have to
have another go at re-angling or reattempting i try all those things you know make sure your
patient's okay just reassure them you might even put some local anesthetic into the skin just to
make sure they're not feeling too much pain and do all of these things to make sure your
patient is comfortable but then it's really important to try and make sure your target is
exactly the way you want it so i'll do all the other things including ensuring the tourniquet is
at the right pressure putting the arm down to help gravity enlarge the vein tap on the vein as well
and then maybe even just choose another site because you never know maybe that
vein you thought was great seems to have disappeared and you wouldn't believe how
many times it actually happens when you think that that vein is absolutely amazing
and then next time you look at it for some reason this tiny vein has actually disappeared completely
sometimes you're going in and you think the vein is in one position but then as you're going in it
just seems to roll to the side there's no shame in re-angling or just reinserting a new cannula
at the right position now the second way of missing this vein is probably one of the
commonest things you'll come up with during your training you've already entered the vein so you've
got the flashback but then as you're advancing to try to get the whole cannula needle into
that vein you end up going through the other side now obviously when this happens it's you
know you fail the vein it's very hard to salvage this in adult veins they're not muscular enough
and what happens is that the vein will blow out now in a couple of other videos i'll show you
all the ways you can try to minimize this one of the biggest best techniques is to just level
out completely lift up your cannula and the vein as you're advancing and that way you just pass
it through just know that the cannula design is incredible it's very very hard to cut the
superficial or top surface of that vein but it's very easy because of the design of the cannula to
go through the other end i'll put some videos that show this technique really really clearly
now one of the other ways you might miss is by actually going through one of the side
walls imagine you have a tortuous vein and this is pretty common when we're in practice
now it's really important that you aim for the site that has the longest straight pathway of
that vein for example this vein here obviously there's a lot of curves it can be quite difficult
but when i look at this vein i look at this line across here that's probably the longest straight
line and so my insertion point will be here aiming in that direction not somewhere like here where
there's a shorter trajectory so that's really important the final way of missing this vein
is you've got a flashback so your needle's in the vein but your cannula isn't inside so you haven't
pushed and advanced the cannula needle far enough into the lumen now what happens then is if it's
not far enough you're gonna you're gonna push the cannula trying to advance it and what that does
is it pushes the vein away now the way to avoid this is know exactly how far you need to advance
this cannula before you thread the plastic cannula itself now that distance is roughly
the bevel distance plus a little bit more now this bevel distance changes for the different
size of cannula you have for example a 22 gauge cannula the bevel's about two millimeters so you
might just advance maybe three to four millimeters but in a large 14 gauge needle the bevel's about
seven millimeters and so if you make up for seven millimeters plus a little bit plus a thick vein
you might be advancing 10 to 15 millimeters before you just advance that plastic cannula through
so i found this knowledge really incredibly useful to make sure that if i miss i know exactly why i
miss and i don't make up any other reasons for it and then that way you can target the reasons
why you've missed and improve your cannulation technique now there's a lot of other reasons
that we go through in our heads that we think oh that's probably why we missed now some of these
reasons might be that you think that you have say a valve now a valve is an internal structure
if you're in the vein with a cannula it's not a reason that you'll miss all you have to do is
to pull back the cannula slightly out of the way of this valve and you'll still be able
to flush this cannula and it'll still be okay for temporary use another way that people say
you could miss is through vasospasm so maybe at one point the vein looks large and then it just
contracts if you're in the vein because you've got flashback that's really no reason that you'd miss
even if the vein has spasmed that's still not a problem as long as you follow this technique
now i hope those three mechanisms for missing the cannula were useful for you to know
again knowing how you could miss the cannula makes you a better anaesthetist makes you
a better doctor makes you better at cannulated overall thanks very much