This chapter bears minimal relationship to Section G7(iii) of the 2017 CICM Primary Syllabus, where the author misinterpreted "describe the invasive and non-invasive measurement of blood pressure" as "submerge into pointless minutae about arterial cannulation". To justify the increasing distance between this content and what is relevant for the CICM Part I, one can only make the feeble argument that early ICU training requires a person to insert hundreds of these, and that they are frequently the first Seldinger technique procedure anybody performs in their career. In that spirit, the information here is presented with the expectation that the target audience is maximally unfamiliar with the process of inserting anything over a guidewire; the sort of people who would benefit from this amazing arterial line simulator.
There is a massive number of different devices, kits and manufacturers. It is important to note that these are all unequal and there are better ones or poorer ones, depending on one's preferences and priorities. With that said, the piece of equipment depicted in this chapter is presented no because it is being touted as superior, but rather because it happened to expire shortly before the article was written. It happened to be an Argon 20G 6 inch kit.
The Seldinger needle
A needle is a needle, but there are two main design features which make this item particularly suited for its purpose.
The hub contains a channel which is conical in shape, so as to guide the wire towards the entrance of the steel tube. This may seem trivial until one tries to pass a guidewire through a non-Seldinger needle, which has a cylindrical hub.
These are usually at least three or four times as long as the catheter they are meant to pair with, constructed of a steel spring wire with an atraumatic tapered tip.
The flexibility of the guidewire is usually limited to its tip. The distal end tends to be quite stiff. It is important not to use the stiff end for arterial cannulation, lest you lacerate something delicate. Because the guidewire is quite thin, the stiff end will easily pass into the soft tissues, giving you the impression that you're in the artery.
The arterial catheter
Most of these devices will share the same characteristics.
The catheter will always be made of some sort of relatively inert material, will be relatively rigid and kink-resistant in its construction, and will have some sort of feature which enables the user to fasten it securely to the patient (in this case, little "wings" for two suture stitches on either side of the cannula). This happens to be an FEP (fluorinated ethylene propylene) catheter.
Factors which determine the choice of catheter
20G and 18G catheters are commonly available. The wider 18G catheters are reserved for use with larger arteries, i.e. they are only used to catheterise femoral arteries. This is because the ratio of vessel diameter to catheter diameter is a major determinant of ischaemic and thrombotic complications. The radial artery of an adult has the mean diameter of 2.2mm (cross-sectional area of 3.8 mm2 ) with the wrist in a neutral position; an 18G catheter has an external diameter of 1.27mm with a crossectional area of 1.26 mm2, i.e. it would occupy 33% of the lumen. In contrast, the 20G catheter has an external diameter of 0.9mm (crossectional area of 0.63mm2 ) and would only occupy 16% of the lumen area. The change in flow though the vessel would be significant.
There is little literature to guide catheter length selection. Short (32mm) catheters such as the commonly available peripheral IV cannulae are appropriate for short term periprocedural use - but you wouldn't want them for long term use in awake or lightly sedated patients because of the increased risk of dislodgement. However, no specific recommendations exist regarding the choice of 7.5cm vs. 15cm catheters.