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/Ultrasound

This guide is for doctors who have reasonable skill in peripheral IV cannulation. The US guided technique is not a substitute or crutch for standard IV cannulation skills (basic micro/macro skills). As an emergency physician, I do not end up using ultrasound (as it takes 3-4 times longer to perform) a majority of the time I am requested to perform an ultrasound guided cannula.

1. ALWAYS USE A STERILE PROBE COVER AND GEL

The number of times I have seen people using unsanitary uncovered probes (i can guarantee linear US probes in ED do not go through sterilisation) is shocking. The use of a sterile probe cover and sterile gel is mandatory. If I saw someone use a dirty probe and non-sterile gel on me or a family member it would be an automatic complaint. A staph bacteraemia has a mortality of above 30%. If you continue to do this, it is almost guaranteed that at some point someone will die from it, because US guided cannulas are such a common procedure. In extreme resource limited settings – eg in the outback/bush, a tegaderm or cling wrap can be used in a pinch, but I wouldnt recommend it.

2. Know your depth! Needle never comes close to the probe.

A standard cannula is only 1″ (2.5cm) long. A bariatric cannula is about 1.88″ (4.7cm) long. Check your depth to the vessel on the screen. Remember that even at a steep 45 degree insertion angle, a 1cm deep vessel will require 1.41cm (sqrt of 2) of cannula to reach the vessel. Taking into account tenting of the vein and the elasticity of the skin (1cm deep to the tip of the probe – that tents the skin downwards), you realistically need about 2-2.5cm of cannula length just to get flashback. The realistic depth achievable with US guided technique with a standard 4.7cm bariatric cannula is about 1.8-2.0cm. Most people dont even insert at a 45deg angle – 20 to 30deg insertions are more common. You will not be able to reach a 1.2cm deep vessel with a 4.7cm bariatric cannula with a 20deg insertion angle. It’s not you! It’s Trigonometry!

pro-tip: When you visualize the trigonometry and 3D location of the vein, you realize that the skin insertion point of your cannula would be at least 1cm away from the probe.

3. Use the force! (VISUALISE)

Now that you have checked the depth, it’s now time to visualize the vein. Move your probe along the length of the vessel. Keep the centerline smack bang in the middle of the vessel as you move your prove proximal and distal. The probe should be at a perfect 90 degrees to the vessel and you should be able to visualize the direction of the vessel.

pro-tip: most point of care ultrasound machines have a focal point about 2/3rd the distance from the top of the screen. Adjust the depth until the vein is 2/3rd the way down the screen.

4. Check for bifurcations and valves

Continuing from the above, there should be a good 1.5-2cm of vein proximal to the insertion point for the plastic cannula/straw to slide into. It is painful and uncomfortable to have the tip of the plastic cannula sitting at a bifurcation point. It also causes the cannula to kink internally, making aspiration slow/impossible, while causing discomfort to the patient. If you see the vessel take a turn, or branch into different veins just proximal to where you are attempting to hit, move distally to get some space.

5. Less is more - The Toothpaste and Pea Rule

As a rule of thumb, you only need about two peas worth of gel on the tip of your probe. Think of applying toothpaste to your toothbrush. That’s it. Small amount of gel on the tip of your probe – then ‘grab’ the probe with your sterile probe cover. Use the rubber band to fix the distal end of the probe cover first – minimize air and have a nice thin bleb of gel at the tip. That’s all you need. Too much gel in the probe cover makes it hard to handle.

6. Avoid the double walled pulsing things!

In most people, arteries are quite distinct from veins. But there is a small group of people with no veins that have arteries looking like veins (after attenuation from overlying sc fat). Arteries are not as collapsible as veins but you may have no veins to compare to. If in doubt, turn up the gain on the ultrasound machine and look for the double wall sign. Veins do not have double walls. More advanced users will use color doppler to measure flow, but this is almost never needed.

7. Ergonomics - Probe tension reduction.

The probe needs to be in an ergonomic position without any tension from the cords. Various methods can be used. Some people wrap the cord around their wrist (much like sonographers). Some tuck it under the patient’s pillow. I prefer attaching the probe to the bed/pillow/drape with some tape. The US probe cover kits we have at Frankston Hospital is just about the best you can find anywhere – they come with these white sticky strips to attach the probe cable to a convenient location.

8. Do not be deceived by flashback.

I’ve lost count the number of times i’ve been told “the cannula tissued when i tried to insert it”. Flashback doesnt mean much when you are trying to cannulate a deeper 0.8-1.5cm deep non-palpable vein. Deeper veins are thicker and often ‘tent’ when the bevel of the introducer needle touches it. This elasticity is partly the reason why longer cannulas are generally necessary for deep vein cannulation. The skin can tent for as much as 0.5cm. Walking the bevel into the vein, you will notice the skin tenting, and be very tempted to start advancing the cannula as soon as you see flashback. This is when the cannula would usually tissue. What happens is that the tip of the bevel has penetrated the vein, but you have not actually inserted the plastic straw in the vein. Because it’s a deeper vessel, there is some elasticity to the skin, so when you try to advance the plastic straw, it literally pushes the vein out of the way – hence “cannula tissued”.

With the US guided technique, the entirety of the bevel should be in the lumen of the vessel. *gently* fan your probe prox/distal to ensure the tip of the bevel is in the center of the vein. Then flatten your cannula against the skin and advance the introducer needle a further 0.5cm into the vein under US guidance. Walk the bevel into the vein. When you are sure the plastic bit of the cannula is in the lumen, advance the cannula. This has a near 100% success rate. “The cannula tissued” will be a thing of the past.

9. Local Anaesthethic is good for patient and good for YOU!

For deeper veins > 0.5cm to the skin, local anaesthethic makes US guided insertion much easier for two reasons: (1) The patient is more comfortable and will flinch less. (2) more importantly, subcutaneous fluid/liquid conducts ultrasound much more than subcutaneous fat.

My default for any vein > 0.5cm is to use a bleb of 1-2ml of 1% lignocaine (no adrenaline). Larger cannulas (18g and up) can then be used without too much pain or wriggling. These larger cannulas are more visible under ultrasound, and the subcutaneous fluid bleb of lignocaine also transmits US waves better, improving view even more. This is how I would insert a drip into the cephalic or basilic veins.

10. Know your veins! + tips for each vein!

In order, my list of places I look for veins are as follows:
1) median cubital – but be wary of bifurcations and valves
2) basilic – patient needs to be positioned with palms facing up, taped to a bedside table, for better ergonomics for both you and the pt.
3) cephalic – can be hit proximal to the elbow. More commonly, the origin of the cephalic on the radial side of the forearm is where you will find a relatively superficial ~0.5cm deep large straight vessel ideal for cannulas.
4) vena committante of the brachial artery – you will often see two vessels alongside the brachial artery. Easily collapsible without a double lumen. With good technique, it can be easily accessed without much risk to the brachial artery. You need to be confident and skillful in walking the bevel of your needle into a vein before attempting this.
5) external jugular – in extreme circumstances only. With good technique, visualizing bevel every step of the way, this is an option. Not recommended for beginners. pro tip: some textbooks recommend against applying torniquet to the neck.