hi everyone this is abc's of anesthesia and happy 2022. now this episode comes thanks to a comment i
got recently um from a medical student so from fh thanks so much for this comment by the way
because it's definitely from your comments that i get ideas and how and what to do videos on so this
particular comment and essentially said can you please do a similar video but this time drawing
the full cannula insertion process with all the specific steps for example advanced then lower etc
it's hard to wrap your head around it all and overwhelming as a medical student now i really
resonate with this as a medical student even as a junior doctor it was really hard to think about
exactly what the canyon was doing inside the vein and you know just how to visualize that so thanks
very much fh thanks very much for your comment and that's what we're going to do to get started
i also want to thank ali abdull fantastic youtube channel and just his advice on using notability
and how to communicate that way on youtube was pretty much the inspiration for me getting this
and having a go at it myself yep so apologies for the lack of drawing skills but let's get started
so in this video i'm going to go through a drawing or schematic representation of what the needle
looks like as you're achieving cannulation in a vein so we're going to go through seven
steps so these ones are firstly starting off at a shallow angle then entering very slowly getting
the flashback of blood in the chamber leveling out advancing the needle and cannula together
until you're in the right distance in the vein then pushing off the plastic cannula and taking
the needle out so that's pretty much exactly what we're going to go through now note that we're not
going to go through any of the preparation setup positioning aseptic non-touch technique or ants
all of that's for another video this is simply the needle and how it interacts with the vein now just
a bit of a refresher this is a 14 gauge needle probably the best way i can really demonstrate
but there's essentially a few components but the first thing to know is that you have a needle
a chamber and a plastic cannula so the needle is on the inside the plastic cannula goes
over the needle and that's the thing that's left inside the vein
and then if you look over here there is this plastic chamber so the needle itself
then feeds into this plastic chamber and that's where you'll see the first flashback as i call
it so the flashback of blood will appear in that chamber now this chamber you can actually take off
this cap and that chamber you can see will fill up with blood and will actually drip out blood
or liquid um without this cap and in a future video i'll show you about one of the advantages
of putting saline into that chamber instead of having air but that's for another video as well
so that's what it looks like um if i take it apart so you've got your safety needle there and here is
the plastic cannula that's what stays in the vein it is not sharp and that way you can give drugs
fluids whatever you need to and it stays inside the vein so i'm going to start by drawing a vein
so the first step is making sure that you have a shallow angle for this i'm just going to draw this
one line here and that's going to be your cannula so imagine that if you insert your cannula with a
pretty steep angle this is the distance that you'll be able to extend that cannula before
you go through the other side of the vein which is a problem now if you have a really shallow angle
imagine that so as you can see when you enter at a shallow angle this distance over here is
far better because if there's more distance that your needle has to travel before it goes
to the other side of the vein versus this one here it's a lot shallower versus the steep angle here
the distance that the needle can travel down through the vein is much shorter therefore
you have less room for error so it's better to go to shallow angle but that's the first step
so the next thing i want to mention is that it is far better to go slowly especially
when you're starting in my previous video you'll notice that i mentioned
there's like 39 different steps to getting your cannulation absolutely right so imagine you have
a really small vein now in a big vein it probably doesn't matter too much how fast you go as long as
your technique's pretty accurate because as soon as that needle is in the vein you'll experience
a flashback of blood in the chamber now in small veins small veins are a lot lower pressure
and the needle that you're going to be using is a very tiny needle so that blood might take at
least you know a second or so before it actually appears in the cannula chamber so that's why i'd
recommend for these really small veins that you just want to go really really slowly
and pausing you know for a little bit of time this is definitely for advanced cannulas in very small
veins using very small needles and so that's why i always enter slowly you just need time for that
blood to appear in the cannulation chamber now you hear people talking about a flashback so just to
demonstrate again flashback is what occurs uh when you get your needle into the vein and then you get
blood flashing back into this chamber here so that's what flashback is so if i'm to just draw
that needle and cannula structure you'll see what i mean so you've got a needle entering like this
and then you've got your cannula just across the outside of the needle as soon as your needle is
in the vessel you'll have blood traverse up the needle and go into the chamber and
collect there and that's what flashback means so imagine you enter the vein and you get flashback
if you don't sharpen your angle and you keep going straight down
you've only got this much distance before you hit the other side of the wall
now that's the reason why we shallow the angle imagine we get flashback as soon as we enter the
vein wall so now we've got a flashback of blood if i'm then to change the angle and make it shallower
i enter the wall and then i've got far more distance to travel
before i get a puncture on the deep side so that's why we should make the out make the angle shallow
and advance the needle at that point you might hear me saying the more you flatten that angle
the better it is and there's a reason why the iv cannula is designed in such a way
that the top surface is not very good at cutting but the bottom surface is very good at being cut
now you might hear me talk about flattening that angle and flattening it quite severely and the
reason is this so imagine that this is actually your vein this needle is very good at cutting
inside the vein and then very good at cutting below just like that as you can see
but it's very very bad at cutting the top surface now i'm pulling up quite high there
and it's just very difficult to cut the top surface just the way this bevel tip is designed
so what that looks like in this diagram once i've got flashback i really shallow that angle
that way there's very little chance of me cutting this bottom surface and it's very
unlikely that i'm going to cut this top surface and just remember there's only two ways that your
cannula fails once you have flashback first of all you don't advance it far enough or you advance it
too far and it punctures another wall in the vein now the next question is how far do you advance a
cannula and the answer is really depends on the size of the cannula a 22 gauge cannula
like this one the bevel is only about a millimeter or two that means that you only have to advance
it about you know three millimeters maybe four before you enter both the needle and the cannula
into the vein and therefore you can advance the cannula fully within within the vein but if you
look in a big 14 gauge the bevel itself is about seven millimeters and the cannula starts probably
almost nine millimeters therefore once you have flashback you probably need to advance at least
a centimeter so even more depending on how thick the vein vane wall is as well
so that's a 14 gauge you have to advance it far more so the way i think about it if i've got
a needle here and i've got a cannula here the distance i advance really depends on the distance
of the bevel plus a little bit more to account for the cannula where it starts and the vein wall so
roughly bevel plus a couple of millimeters usually accounts for all of those things but this is the
distance that you need to advance so imagine you've got the cannula in and you got flashback
now you flatten out the angle so it's really flat and you advance that cannula in so now
the cannula looks a bit like this you've got the needle in and you've got the cannula in now
you're able to advance that cannula in through the vein without any problem so you've pretty much got
the flashback you've advanced the distance which is a bevel plus a little bit um and then you know
you're in you flatten out the angle so you don't you've got very little chance of puncturing the
deep surface of the vein and you're almost aiming at the top surface of vein just because it's very
hard to cut the top surface so now your needle's in the vein your cannula is in the vein as well
now you're able to advance that plastic cannula off so again what it looks like is you're pushing
off the cannula with your pointer finger and that goes into the vein itself and then you pull this
needle out in that direction so what you're now left with is the cannula inside the vein and that
way you can give your drugs and diminish your drugs and your medications or whatever you need
so please forgive me for my terrible drawings but that basically is everything that you need to know
about advancing the cannula as it relates to the vein just in a diagrammatic format hopefully that
helps you understand exactly mechanics of what's going inside and i'll probably try and use these
kind of templates in the future to try and demonstrate things like arterial lines and
central beams line as well so thanks very much for watching abc's of anesthesia i hope that helped
i suspect there'll be a lot of criticism of my drawing but that's completely fine hopefully i'll
get better for the next one and again i'll see you in the next video thanks very much for watching
you