Technique: Basic Tips

Consider the following:

Overview

Select the correct probe and machine settings as previously described. All invasive procedures should employ standard sterile techniques to diminish the risk of infection.

For central venous access using real-time ultrasound, a sterile probe cover should always be used. For peripheral venous access, a sterile probe cover should also ideally be used apart from in a true emergency or cardiac arrest situation. Alternatives to a sterile probe cover could include a sterile dressing (eg. Tegaderm) or sterile glove cuff, which may be quicker in an emergency.

Holding the probe

Some linear probes have their surfaces at right angles to the skin (image bottom left), while others are angled (image bottom right). In the case of the latter, it may be easier to hold the probe in the most natural way, and switch the marker from left to right on the screen.

You will soon discover the need to stabilise the probe by resting your hand or little and/or ring fingers on the patient’s skin (i.e. “pool cue” grip). Unless you do this, the slippery nature of the gel will inevitably mean that your probe slowly drifts and you lose the screen image.

In all invasive procedures it is important to learn to watch the screen, not your hands. This involves an appreciation of which way to move the cannula to achieve the desired result.

Additionally, using centre-line or the M-mode line can be helpful to keep the target vessel in the midline during cannulation.

Choosing an appropriate vein and cannula

The evidence suggests veins between 0.3 and 1.5cm deep and greater than 0.4cm in diameter are much more likely to be successfully cannulated (Gottlieb et al 2017, PMID: 29085536). This should be taken into account when choosing an appropriate vein, particularly when standard cannulas are being used. The depth markers on the right side of the screen are helpful in estimating this. If your institution stocks long lines or mid-line/PICC lines, these may be preferable with deeper veins or where longer-term access may be required.

Standard peripheral IV cannulas (eg. Venflon) may be used for US-guided access. Standard cannula size and lengths are shown in the image below (though there may be slight variations depending on brand). Standard 18G (green), 16G (grey) and 14G (orange) cannulas usually have the same length of 45mm. This is usually the minimum length required for US-guided peripheral venous access to ensure enough of the catheter is in the vein.

The vein in transverse section

The first skill to acquire is to be able to recognise the vein in transverse section, and to distinguish it from the artery by virtue of its easy compressibility, thin walls and lack of pulsatility. It is also important to appreciate the axis (or direction) of the vein, as not all veins will run parallel and might be at an angle to the limb. This is achieved by scanning up and down the vein to establish it’s axis. If the vein (in transverse/short-axis) is moving off to one side of the screen whilst scanning up and down, it suggests it is not parallel to the transducer. Rotating the probe slightly whilst scanning up or down will allow you to identify its axis.

The vein in longitudinal section

Secondly, learn to rotate the probe to view the vein in longitudinal section. This entails knowing which part of the probe corresponds to the marker on the screen. This is useful during cannulation of the vessel to ensure that the entire bevel of the needle is within the lumen of the vein prior to threaded over the cannula. This is known as a the “two-axis” technique and will be discussed later on.

Learning bite

Become skilled in visualising the vein in transverse section, then in longitudinal section.

Venous cannulation

Venous cannulation can be carried out using a single or dual operator technique. Though not mandatory, it may be helpful if initially the trainee works with the trainer in a dual operator technique, with the ultrasound carried out by the trainer, and the cannulation by the trainee.

Besides the obvious training benefit, this also frees up both hands for the trainee. In addition, it significantly reduces the risks of the probe sliding off the area of work and onto the floor. The consequences of dropping a probe are more than inconvenience and loss of sterility crystals may be broken in the probe resulting in permanent damage.

As the needle enters the vessel, it firsts tents the wall (image right).

Clot

Sufficient pressure is needed to enter the vessel, and at this stage the back wall can be perforated. If so, simply pull back until the bevel lies within the vessel.

The final tip is to be aware that soon after a vessel is entered, if successful cannulation does not occur, a small clot may form in the lumen of the cannula. This can result in subsequent failure to get any flash-back, even when the cannula is correctly placed. Priming the cannula with heparinised saline can minimise this, but will not prevent it.

In this image, the needle is breeching the posterior wall.

If successful cannulation does not occur immediately, a lumenal clot is likely to form.