Arterial cannulation technique

To maintain some level of attachment to core syllabus topics, this chapter should have been at least vaguely relevant to Section G7(iii)  of the 2017 CICM Primary Syllabus,  "describe the invasive and non-invasive measurement of blood pressure, including limitations and potential sources of error". At this stage there have been no attempts to introduce arterial cannulation technique into SAQs, viva stations or CICM workplace competency assessments. It is not inconceivable that the college will at some stage demand it as one of their training objectives, and it seems reasonable to futureproof against this possibility.

There is a satisfactory amount of literature dealing with this topic. The best references include "Chapter 3: Arterial line placement and care" in Irwin and Rippe's Intensive Care Medicine, and the 2012 article by Tiru et al. 

The notes attempt to remain site-agnostic, as Seldinger techinque does not vary substantially between sites and because specific site-related instructions are carried on in the chapters which answer CICM Syllabus Section X(ii), "describe the anatomy relevant to the insertion of an arterial line into a
brachial, axillary, posterior tibial, dorsalis pedis, radial or femoral artery." However, it is difficult not to acknowledge the fact that the radial artery is the most used site, and so a certain radio-centric bias can be felt throughout the text.  

Aseptic technique

Irwin and Rippe (Ch.3) recommend that

“The catheter over the needle approach (e.g., radial or brachial site) necessitates cap, mask, sterile gloves and a small fenestrated drape; whereas, the Seldinger technique (i.e., femoral approach) requires maximum barrier precautions.”

No references is offered to explain where they got this, but it certainly seems reflective of what happens in real life theatres and ICUs. However, if one digs deep enough one can discover that in fact this "small fenestrated drape" recommendation is derived from the CDC Guidelines for the Prevention of Intravascular Catheter-Related Infections (O'Grady et al, 2011). The CDC suggest that "a minimum of a cap, mask, sterile gloves and a small sterile fenestrated drape should be used during peripheral arterial catheter insertion".

Seldinger technique for arterial cannulation

The reader is again subjected to self-indulgent abuse of Adobe Illustrator:

In short, the process is as follows:

• The artery is palpated
• The needle is introduced into the artery until pulsatile blood flow is observed
• The needle is kept perfectly still while the flexible-tipped guidewire is inserted into the artery
• With the guidewire in the artery, the needle is withdrawn
• The needle must never be re-threaded over the guidewire as this has a high risk of severing or fraying the guidewire
• The arterial catheter is threaded over the guidewire, with the proceduralist maintaining a grip on the guidewire at all times
• The guidewire is removed and the pressure transducer set is connected to the arterial line.

There is a modification of this , known generally as the "transfixation" technique. It is usually used with “cannula over needle” kits, and involves intentionally puncturing both walls of the artery, skewering it “through and through”.

• The artery is accessed and pulsatile blood flow is confirmed
• The needle is advanced until blood flow ceases (i.e. the posterior wall is punctured)
• The needle is withdrawn, leaving the cannula in situ
• The cannula is withdrawn slowly until pulsatile flow is restored
• A guidewire is then threaded through the cannula, and the arterial catheter is then inserted using a conventional Seldinger technique.

Surprisingly there does not appear to be any major difference between thrombotic complication rates for either technique (Jones et al, 1981). Or rather, the thrombotic complication rate with radial punctures is already high (almost 20% end up having Doppler evidence of  occlusion) which could mean that about 20% of the time the posterior wall of the vessel ends up being lacerated to some extent anyway.

What to do when the guidewire will not advance

Generally, beyond a certain level of skill most junior doctors will have little difficulty hitting the pulse with a needle. The most common reason for "hey boss, can you give me a hand here" is the failure of the guidewire to advance beyond the needle tip. Classically, the guidewire comes out of the puncture looking like a dog's breakfast. They've clearly been at it for a while.

 

This could be happening for a variety of reasons:

• You're nowhere near the artery (maybe in one of the venae comitantes, or in some godforsaken wasteland of fatty soft tissue)
• You're catching on the arterial intima
• You've created a false passage (i.e. you've dissected the artery)
• You've chosen an artery which takes the scenic route. 
• There is excessive atheroma
• The artery has developed vasospasm because of your multiple stabbings

There are a few recommendations one can make to help prevent problems when arterial line insertion gets tricky:

• Wait for a clearly arterial blood flow. In the excitement of the moment, it is a common mistake to insert the guidewire at the first sign of blood. If the guidewire is inserted too early, the intima may offer resistance and the guidewire will not pass. To prevent this from happening, do not attempt to insert the guidewire until you find a needle position with robust pulsatile flow. Of course, one might offer the counterargument that the shocked crashing ICU patient doesn't have robust pulsatile flow anywhere in their circulatory system. 
• Transfixation technique as described above (or some modification of it) could help you find the maximal pulsatile blood flow
• Make sure you're using the flexible tip of the guidewire. It is a common mistake to respond to difficult guidewire insertion by reversing the guidewire and using the “stiff” end. This does nothing to improve the chances of successful arterial catheterisation, and can increase the risk of dissection. It is also easy to puncture the posterior wall of the artery and advance the stiff end of the guidewire through the tissue planes; there may be little resistance, which may give the impression of intravascular placement, but after the catheter is threaded over the guidewire there will be no blood flow.
• Reposition the needle or bevel  direction. Gently rotating the needle changes the direction of the bevel, which may redirect the guidewire enough to miss the intima and enter the lumen.
• A smaller cannula can sometimes rescue the situation. Eker et al (2009) demonstrated that changing from the usual 20G to a 22G cannula improved first-puncture success rates in patients with severe atherosclerotic disease.
• Pre-procedure bedside ultrasound may be used to confirm vessel patency, excluding anatomic variation (eg. tortuosity) which could give rise to difficult guidewire insertion. There's a surprising amount of anatomical variation in the public. Yokoyama et al (2000) found that 10% of patients undergoing coronary angiography had significant anatomical variation (tortuosity, stenosis, radioulnar loop origin) and a small minority (1.7%) had a hypoplastic or totally absent radial artery unilaterally.
• Use ultrasound to guide the procedure. 

Use of ultrasound guidance

Ultrasound-guided insertion of arterial lines has not been popularised to the same extent as central lines, because there is a lower rate of insertion-related complications with arterial lines, and the pulse makes a convenient palpable landmark.

The use of ultrasound guidance does not obviate the need for a sound knowledge of anatomy, or protect inexperienced practitioners from the risk of complications. If you don't know what you are doing, stabbing mindlessly at the pulsatile object on the screen will yield only self-doubt and disappointment. However, ultrasound guidance has been demonstrated to increase the success rate, decrease the number of “passes” and decrease procedure time for practitioners who are already proficient with the palpation method (Shiver et al, 2006). It has also been demonstrated to improve radial arterial access chances in situations where it is being used as a rescue technique, after failed “blind” attempts (Sandhu et al, 2006).

Achievement of competence

How many do you have to do before you're good at this? An old study by Konrad et al (1988) plotted learning curves for anaesthesia trainees, and found that after 5-7 attempts you have a better than 50% success rate. The competence seems to plateau after 20-30 attempts. The fact that the curve trends down towards failure once one becomes experienced suggests that either some of us become overconfident, or pick the difficult cases (leaving bounding pulses for the novices).