Overview
In real-time guided cannulation there are two accepted approaches:
The short axis
The short axis, transverse approach allows only a cross section of the needle to be visualised by the ultrasound beam and may lead to errors in depth perception of the needle. The acoustic shadow can lead to confusion. The potential of the scan is not optimised by this approach. This is why it is vital to perform sequential needle tip tracking using this technique. This involves advancing the needle unless the needle tip is visualised on the screen, then advancing the probe followed by the needle to again visualise the needle tip. The process is continued until the needle tip is seen to enter the vessel lumen. This ensures that one does not puncture the posterior wall of the vessel (i.e. “back walling” the vessel).
The long axis
The long axis orientation allows the operator to trace the entire path and angle of the needle from the entry site at the skin and into the vessel. Whilst this may be preferable, it can be technically quite challenging particularly for smaller peripheral veins. It requires an extremely steady hand. Furthermore, the midline of the vessel may not be completely accurate due to beam width artefact.
Therefore, the short axis technique with sequential needle tip tracking may be easier to learn and the preferred approach for most for peripheral venous cannulation.
Two-axis technique
Combining both approaches may increase success rates by utilising the best of both techniques: the ease of probe handling and ability to identify the midline of the vessel in short axis, combined with the advantage of the long axis technique to ensure the bevel of needle is completely within the lumen and direct visualisation of the catheter being threaded into the vein.
This technique may require a bit of practice as one must be able to perform cannulation with one hand, whilst able to rotate the probe 90 degrees with the other hand. It is worth practicing this with a phantom model to become comfortable with it.
This link provides more detail on this technique.